Publications

57Hindex

12569Citations

241Publications
2024

(2024) Nature Communications. 15, 1, 3428. Abstract
Despite its ubiquity in quantum computation and quantum information, a universally applicable definition of quantum entanglement remains elusive. The challenge is further accentuated when entanglement is associated with other key themes, e.g., quantum interference and quantum statistics. Here, we introduce two novel motifs that characterize the interplay of entanglement and quantum statistics: an 'entanglement pointer' and a 'statisticsinduced entanglement entropy'. The two provide a quantitative description of the statisticsinduced entanglement: (i) they are finite only in the presence of quantum entanglement underlined by quantum statistics; (ii) their explicit form depends on the quantum statistics of the particles (e.g., fermions, bosons, anyons). We have experimentally implemented these ideas by employing an electronic HongOuMandel interferometer fed by two highly diluted electron beams in an integer quantum Hall platform. Performing measurements of autocorrelation and crosscorrelation of current fluctuations of the scattered beams (following 'collisions'), we quantify the statisticsinduced entanglement by experimentally accessing the entanglement pointer and the statisticsinduced entanglement entropy. Our theoretical and experimental approaches pave the way to study entanglement in various correlated platforms, e.g., those involving anyonic Abelian and nonAbelian states.

(2024) Physical review letters. 132, 7, 076301. Abstract
Interferometry is a vital tool for studying fundamental features in the quantum Hall effect. For instance, AharonovBohm interference in a quantum Hall interferometer can probe the waveparticle duality of electrons and quasiparticles. Here, we report an unusual AharonovBohm interference of the outermost edge mode in a quantum Hall FabryPérot interferometer, whose Coulomb interactions were suppressed with a grounded drain in the interior bulk of the interferometer. In a descending bulk filling factor from νb=3 to νb≈(5/3), the magnetic field periodicity, which corresponded to a single "flux quantum,"agreed accurately with the enclosed area of the interferometer. However, in the filling range, νb≈(5/3) to νb=1, the field periodicity increased markedly, a priori suggesting a drastic shrinkage of the AharonovBohm area. Moreover, the modulation gate voltage periodicity decreased abruptly at this range. We attribute these unexpected observations to edge reconstruction, leading to area changing with the field and a modified modulation gateedge capacitance. These reproducible results support future interference experiments with a quantum Hall FabryPérot interferometer.

(2024) Nature. 625, 7995, p. 489493 Abstract
The quantum Hall effect is a prototypical realization of a topological state of matter. It emerges from a subtle interplay between topology, interactions and disorder^{1–9}. The disorder enables the formation of localized states in the bulk that stabilize the quantum Hall states with respect to the magnetic field and carrier density^{3}. Still, the details of the localized states and their contribution to transport remain beyond the reach of most experimental techniques^{10–31}. Here we describe an extensive study of the bulk’s heat conductance. Using a novel ‘multiterminal’ short device (on a scale of 10 µm), we separate the longitudinal thermal conductance, κ_{x}_{x}T (owing to the bulk’s contribution), from the topological transverse value κ_{x}_{y}T by eliminating the contribution of the edge modes^{24}. When the magnetic field is tuned away from the conductance plateau centre, the localized states in the bulk conduct heat efficiently (κ_{x}_{x}T∝ T), whereas the bulk remains electrically insulating. Fractional states in the first excited Landau level, such as the ν= 7 / 3 and ν= 5 / 2 , conduct heat throughout the plateau with a finite κ_{x}_{x}T . We propose a theoretical model that identifies the localized states as the cause of the finite heat conductance, agreeing qualitatively with our experimental findings.
2023

(2023) Nature Communications. 14, 1, 415. Abstract
The resemblance between electrons and optical waves has strongly driven the advancement of mesoscopic physics, evidenced by the widespread use of terms such as fermion or electron optics. However, electron waves have yet to be understood in open cavity structures which have provided contemporary optics with rich insight towards nonHermitian systems and complex interactions between resonance modes. Here, we report the realization of an open cavity resonator in a twodimensional electronic system. We studied the resonant electron modes within the cavity and resolved the signatures of longitudinal and transverse quantization, showing that the modes are robust despite the cavity being highly coupled to the open background continuum. The transverse modes were investigated by applying a controlled deformation to the cavity, and their spatial distributions were further analyzed using magnetoconductance measurements and numerical simulation. These results lay the groundwork to exploring matter waves in the context of modern optical frameworks.

Topological Thermal Hall Conductance of Even Denominator Fractional States(2023) arXiv.org. Abstract
The even denominator fractional quantum Hall (FQH) states ν=5/2 and ν=7/2, have been long predicted to host nonabelian quasiparticles (QPs). The presence of energycarrying neutral modes cripples customary conductance measurements and thus motivates thermal transport measurements, which already proved to be sensitive to all energycarrying modes. Each state has a different capacity to carry quanta of heat  as expressed by the socalled: 'central charge'  identifying the state's topological order. While the 'twoterminal' thermal conductance measurements identified the topological orders of abelian and nonabelian QH states, they are prone to partial thermal equilibration among counterpropagating modes. Here, we report a 'fourterminal' thermal Hall conductance measurement, which separately measures the heat carried by the downstream and upstream chiral modes. This measurement is insensitive to thermal equilibration among modes. We verify that the ν=5/2 and ν=7/2 states are nonabelian, supporting a single upstream Majorana mode, thus obeying the ParticleHole Pfaffian topological order. While current numerical works predict a different central charge, this contribution should motivate further theoretical work.

(2023) Physical review letters. 131, 9, 096302. Abstract
The remarkable Cooperlike pairing phenomenon in the AharonovBohm interference of a FabryPerot interferometer  operating in the integer quantum Hall regime  remains baffling. Here, we report the interference of paired electrons employing "interface edge modes."These modes are born at the interface between the bulk of the FabryPerot interferometer and an outer gated region tuned to a lower filling factor. Such a configuration allows toggling the spin and the orbital of the Landau level of the edge modes at the interface. We find that electron pairing occurs only when the two modes (the interfering outer and the first inner) belong to the same spinless Landau level.

(2023) Physica ELowDimensional Systems & Nanostructures. 153, 115780. Abstract
Here we discuss the effect of topology on the quantum Hall effect taking into account the direct Coulomb interactions, considering two distinct geometries, namely the Hall bar and the Corbino disc. The consequences of interactions are underestimated in the standard approaches to explain the quantized Hall effect. However, the local distributions of the electron number density, the electrochemical potential, and current distributions depend on electronelectron interactions. Accounting for the direct Coulomb interaction and realistic boundary conditions results in local variations of compressibilitynamely metallike compressible and (topological) insulatorlike incompressible regions. Within the framework of the screening theory, we show in the coordinate space that for both geometries, the bulk is compressible within most of the magnetic field interval corresponding to a quantized Hall plateau. The nonincompressible bulk throughout the plateau directly contrasts the standard explanation of the quantized Hall effect but is confirmed by our transport experiments. Our experimental results with two inner contacts within the Hall bar imply that the QHE plateaus scattering free transport along the sample edges even if the bulk of the sample is clearly in a compressible state. The scattering free transport is thereby supported by incompressible stripes. Our results confirm that the often promoted analogy in coordinate space between the quantized Hall effect and topological insulators is invalid throughout the entire plateau. We conclude that the equivalence of Hall and Corbino geometries is questionable. In addition, family relations of quantized Hall effect and topological insulators are doubtful. Finally, we propose experiments which will enable us to distinguish the topological properties of the two geometries in the coordinate space.

(2023) JETP Letters. 118, 6, p. 455459 Abstract
An ensemble of neutral excitations in a 1/3 Laughlin liquid is studied experimentally. It has been found that excitations induce a nonlinear optical response in the form of a quadratic dependence of the reflection signal on the pump power. The reported experimental results have shown that the observed effect is due to the contribution of the coherent antiStokes–Stokes scattering from the excited Laughlin liquid.

(2023) JETP Letters. 117, 12, p. 938944 Abstract
In samples of fieldeffect transistors based on GaAs/AlGaAs heterostructures with an electron system in a single 50nmwide GaAs quantum well, a transition stimulated by a quantizing magnetic field has been detected from a bilayer state of the system in zero magnetic field to a singlelayer state when only the lowest Landau level is filled. In contrast to the results for the 60nmwide quantum well obtained in [S. I. Dorozhkin, A. A. Kapustin, I. V. Fedorov, V. Umansky, and J. H. Smet, Phys. Rev. V 102, 235307 (2020)], the singlelayer state is observed not only in incompressible quantum Hall effect states of the electron system at filling factors of 1 and 2, but also in compressible states between these filling factors. The spatial location of the singlelayer system in the quantum well has been established; it appears to be independent of the electron distribution over the layers in a low magnetic field. A possible qualitative explanation for this observation has been proposed. The detected transition is supposedly due to the negative compressibility of twodimensional electron systems caused by exchangecorrelation contributions to the electronelectron interaction.

(2023) Nature Communications. 14, 1, 2948. Abstract
Multielectron semiconductor quantum dots (QDs) provide a novel platform to study the Coulomb interactiondriven, spatially localized electron states of Wigner molecules (WMs). Although Wignermolecularization has been confirmed by realspace imaging and coherent spectroscopy, the open system dynamics of the strongly correlated states with the environment are not yet well understood. Here, we demonstrate efficient control of spin transfer between an artificial threeelectron WM and the nuclear environment in a GaAs double QD. A Landau–Zener sweepbased polarization sequence and lowlying anticrossings of spin multiplet states enabled by Wignermolecularization are utilized. Combined with coherent control of spin states, we achieve control of magnitude, polarity, and site dependence of the nuclear field. We demonstrate that the same level of control cannot be achieved in the noninteracting regime. Thus, we confirm the spin structure of a WM, paving the way for active control of correlated electron states for application in mesoscopic environment engineering.

(2023) Nature. 617, 7960, p. 277281 Abstract
Correlations of partitioned particles carry essential information about their quantumness1. Partitioning full beams of charged particles leads to current fluctuations, with their autocorrelation (namely, shot noise) revealing the particles’ charge2,3. This is not the case when a highly diluted beam is partitioned. Bosons or fermions will exhibit particle antibunching (owing to their sparsity and discreteness)4,5,6. However, when diluted anyons, such as quasiparticles in fractional quantum Hall states, are partitioned in a narrow constriction, their autocorrelation reveals an essential aspect of their quantum exchange statistics: their braiding phase7. Here we describe detailed measurements of weakly partitioned, highly diluted, onedimensionlike edge modes of the onethird filling fractional quantum Hall state. The measured autocorrelation agrees with our theory of braiding anyons in the time domain (instead of braiding in space); with a braiding phase of 2θ = 2π/3, without any fitting parameters. Our work offers a relatively straightforward and simple method to observe the braiding statistics of exotic anyonic states, such as nonabelian states8, without resorting to complex interference experiments9.

(2023) Nature Physics. 19, 4, p. 515521 Abstract
Fractional quantum Hall states have long been predicted to be a testing ground of fractional—anyonic—exchange statistics. These topological states, which can have either an Abelian or nonAbelian character, harbour quasiparticles with fractional charges. The charge of the quasiparticles can be measured by shot noise measurements, whereas their quantum statistics can be revealed by appropriate interference experiments. The multipath Fabry–Pérot electronic interferometer is easier to fabricate, but it is often plagued by Coulomb interactions, area breathing with the magnetic field and fluctuating charges. Yet, recent experiments with an adequately screened Fabry–Pérot interferometer allowed the observation of anyonic interference at a bulk filling factor of ν = 1/3. Here we demonstrate the interference and braiding of anyons in an interactionfree twopath Mach–Zehnder interferometer tuned to bulk filling of ν = 2/5 with an outermost ν = 1/3 edge mode. Interference with this mode reveals a phase dependence that corresponds to the predicted anyonic braiding. This proves that a Mach–Zehnder interferometer is a powerful tool that probes the quantum statistics of complex anyonic states.

(2023) Nature Physics. 19, 3, p. 327332 Abstract
Thermal conductance measurements are sensitive to both charge and chargeless energy flow and are an essential measurement technique in condensedmatter physics. For twodimensional topological insulators, the determination of thermal Hall (transverse) conductance and thermal longitudinal conductance is crucial for the understanding of topological order in the underlying state. Such measurements have not been accomplished, even in the extensively studied quantum Hall effect regime. Here we report a local power measurement technique that we use to reveal the topological thermal Hall conductance, going beyond the ubiquitous twoterminal conductance. For example, we show that the thermal Hall conductance is approximately zero in the v = 2/3 particle\u2013hole conjugated state. This is in contrast to the twoterminal thermal conductance that gives a nonuniversal value that depends on the extent of thermal equilibration between the counterpropagating edge modes. Moreover, we demonstrate the utility of this technique in studying the power carried by the current fluctuations of a partitioned edge mode with an outofequilibrium distribution.

(2023) JETP Letters. 117, 1, p. 6874 Abstract
A bilayer electron system that is formed in a 60nmwide GaAs quantum well and has a large difference of the electron densities in the layers has been studied. It has been found that, when a magnetic field is tilted from the normal to the plane of the system, integer quantum Hall effect states at the filling factors of Landau levels of 1 and 2 disappear; instead, fractional quantum Hall effect states in the interval between these filling factors appear at the filling factors ν_{F} = 4/3, 10/7, and 6/5 with odd denominators and at the filling factor ν_{F} = 5/4. Several different states can be observed under the variation of the magnetic field. The detected fractional quantum Hall effect states are interpreted as combined states with the same filling factor 1 in the layer with the higher density and with the filling factors ν_{F} – 1 in the layer with the lower density. These states are formed because of the redistribution of electrons between the layers, which occurs under the variation of the magnetic field. The appearance of the state with the filling factor ν_{F} = 5/4 with the even denominator is presumably attributed to the dominance of the interlayer electron–electron interaction over the intralayer one for electrons in the layer with the lower density.
2022

(2022) Nature Communications. 13, 1, 376. Abstract
Twodimensional topological insulators, and in particular quantum Hall states, are characterized by an insulating bulk and a conducting edge. Fractional states may host both downstream (dictated by the magnetic field) and upstream propagating edge modes, which leads to complex transport behavior. Here, we combine two measurement techniques, local noise thermometry and thermal conductance, to study thermal properties of states with counterpropagating edge modes. We find that, while charge equilibration between counterpropagating edge modes is very fast, the equilibration of heat is extremely inefficient, leading to an almost ballistic heat transport over macroscopic distances. Moreover, we observe an emergent quantization of the heat conductance associated with a strong interaction fixed point of the edge modes. Such understanding of the thermal equilibration on edges with counterpropagating modes is a natural route towards extracting the topological order of the exotic 5/2 state.

(2022) Nature Physics. 18, 12, p. 14761481 Abstract
The fractional charge of quasiparticles is a fundamental feature of quantum Hall effect states. The charge—important in characterizing the state and in interference experiments—has long been measured via shot noise at moderate temperatures, with the Fano factor revealing the charge of the quasiparticles. However, at sufficiently low temperatures of ~10 mK, we previously found that the Fano factor is instead equal to the bulk filling factor. Noise with this pattern was also observed on intermediate conductance plateaux in the transmission of the quantum point contact, where shot noise is not expected. Here, we extend this lowtemperature behaviour of the Fano factor to a situation where the edge modes do not sit at the physical edge of the device but instead reside in an artificially constructed interface at the boundary between two adjoining quantum Hall effect states: the tested state and a different state. We attribute the unexpected shot noise behaviour to upstream neutral modes that proliferate at the lowest spinless Landau level. We present a theoretical approach based on an interplay between charge and neutral modes that hints at the origin of the universal Fano factor.

(2022) Science. 377, 6611, p. 11981201 Abstract
The topological order of a quantum Hall state is mirrored by the gapless edge modes owing to bulkedge correspondence. The state at the filling of ν = 5/2, predicted to host nonabelian anyons, supports a variety of edge modes (integer, fractional, neutral). To ensure thermal equilibration between the edge modes and thus accurately determine the state’s nature, it is advantageous to isolate the fractional channel (1/2 and neutral modes). In this study, we gapped out the integer modes by interfacing the ν = 5/2 state with integer states ν = 2 and ν = 3 and measured the thermal conductance of the isolatedinterface channel. Our measured halfquantized thermal conductance confirms the nonabelian nature of the ν = 5/2 state and its particlehole Pfaffian topological order. Such an isolated channel may be more amenable to braiding experiments.

(2022) Proceedings of the National Academy of Sciences  PNAS. 119, 32, e220353111. Abstract
We show that a Bose–Einstein condensate consisting of dark excitons forms in GaAs coupled quantum wells at low temperatures. We find that the condensate extends over hundreds of micrometers, well beyond the optical excitation region, and is limited only by the boundaries of the mesa. We show that the condensate density is determined by spinflipping collisions among the excitons, which convert dark excitons into bright ones. The suppression of this process at low temperature yields a density buildup, manifested as a temperaturedependent blueshift of the exciton emission line. Measurements under an inplane magnetic field allow us to preferentially modify the bright exciton density and determine their role in the system dynamics. We find that their interaction with the condensate leads to its depletion. We present a simple rateequations model, which well reproduces the observed temperature, power, and magneticfield dependence of the exciton density.

(2022) Science (American Association for the Advancement of Science). 375, 6577, p. 193197 Abstract
Quantum Hall states can harbor exotic quantum phases. The nature of these states is reflected in the gapless edge modes owing to ‘bulkedge’ correspondence. The moststudied putative nonabelian state is the spinpolarized filling factor ν = 5/2, which permits different topological orders that can be abelian or nonabelian. We develop a method that interfaces the studied quantum state with another state, and employ it to identify the topological order of the ν = 5/2 state. The interface between two halfplanes, one hosting the ν = 5/2 state and the other an integer ν = 3 state, supports a fractional ν = 1/2 charge mode and a neutral Majorana mode. The counterpropagating chirality of the Majorana mode, probed by measuring partition noise, is consistent with the particlehole Pfaffian (PHPf) topological order and rules out the antiPfaffian order.
2021

(2021) Nature Communications. 12, 1, 6477. Abstract
Twodimensional electron systems in a quantizing magnetic field are regarded as of exceptional interest, considering the possible role of anyons—quasiparticles with nonboson and nonfermion statistics—in applied physics. To this day, essentially none but the fractional states of the quantum Hall effect (FQHE) have been experimentally realized as a system with anyonic statistics. In determining the thermodynamic properties of anyon matter, it is crucial to gain insight into the physics of its neutral excitations. We form a macroscopic quasiequilibrium ensemble of neutral excitations  spin one anyon complexes in the Laughlin state ν = 1/3, experimentally, where ν is the electron filling factor. The ensemble is found to have such a long lifetime that it can be considered the new state of anyon matter. The properties of this state are investigated by optical techniques to reveal its Bose properties.

(2021) Physical Review Applied. 16, 4, 044001. Abstract
A highelectronmobility GaAs/Al0.36Ga0.64As heterostructure is processed by electronbeam lithography to fabricate samples with a separation of the currentsupplying Ohmic contacts of between 28μm and 1.3 mm. Voltage pulses of 20ms duration, with a 4% filling factor and an amplitude up to 30 V, stimulate terahertz emission from samples cooled to 4 K. The radiation spectrum, centered at about 390 GHz (about 1.6 meV), registered with a FabryPerot interferometer, shows almost no dependence on the applied voltage or on the shape of the sample. The emission appears in a threshold manner and is accompanied by a jump of current by more than 2 orders of magnitude. The energy of the emitted photons is about three times smaller than in the case of any previously reported impurityrelated emission from either bulk GaAs or GaAsbased quantum wells. The emission is interpreted as resulting from a twostep ionizationrecombination process involving shallow donors in the (Al,Ga)As barrier, which create bound states of electrons in the GaAs quantum well. Thus, the frequency of emission is argued to be tuned by the width of the spacer layer. We present these as small convenient sources that can be used for calibration and checkup operation of cooled bolometric systems used in astronomical observations.

(2021) Nano Letters. 21, 12, p. 49995005 Abstract
We report a singleshotbased projective readout of a semiconductor hybrid qubit formed by three electrons in a GaAs double quantum dot. Voltagecontrolled adiabatic transitions between the qubit operations and readout conditions allow highfidelity mapping of quantum states. We show that a large ratio both in relaxation time vs tunneling time (≈50) and singlettriplet splitting vs thermal energy (≈20) allows energyselective tunnelingbased spintocharge conversion with a readout visibility of ≈92.6%. Combined with ac driving, we demonstrate high visibility coherent Rabi and Ramsey oscillations of a hybrid qubit in GaAs. Further, we discuss the generality of the method for use in other materials, including silicon.

(2021) JETP Letters. 113, 10, p. 670675 Abstract
The magnetic field dependence of the amplitudes of Shubnikov–de Haas oscillations in GaAs/AlGaAs heterostructure samples with a twodimensional electron system irradiated by microwave radiation in the range of 130–170 GHz has been studied. Two features of the radiationinduced suppression of amplitudes of oscillations having a fieldresonance character have been revealed. One of the resonances appears in a magnetic field corresponding to the second harmonic of the cyclotron resonance, whereas the dependence of the existence, position, and amplitude of the second resonance on the radiation frequency is more complex. The detected resonance absorption of radiation at the second harmonic is apparently responsible for an anomalous peak of the magnetoresistance recently observed near this harmonic. The detected resonances can be explained by the excitation of standing magnetoplasma waves in a confined sample with the same the wavenumber but corresponding to different regions of their dispersion relation: an almost dispersionless region of the Bernstein mode and a cyclotron magnetoplasma mode.
2020

(2020) Physical review letters. 125, 25, 256803. Abstract
Studies of energy flow in quantum systems complement the information provided by common conductance measurements. The quantum limit of heat flow in one dimensional (1D) ballistic modes was predicted, and experimentally demonstrated, to have a universal value for bosons, fermions, and fractionally charged anyons. A fraction of this value is expected in nonabelian states. Nevertheless, open questions about energy relaxation along the propagation length in 1D modes remain. Here, we introduce a novel experimental setup that measures the energy relaxation in chiral 1D modes of the quantum Hall effect (QHE). Edge modes, emanating from a heated reservoir, are partitioned by a quantum point contact (QPC) located at their path. The resulting noise allows a determination of the 'effective temperature' at the location of the QPC. We found energy relaxation in all the tested QHE states, being integers or fractional. However, the relaxation was found to be mild in particlelike states, and prominent in holeconjugate states.

(2020) Applied Physics Letters. 117, 23, 234001. Abstract
The individual confinement and twoaxis qubit operations of twoelectron spin qubits in a GaAs gatedefined sextuple quantum dot array with an integrated micromagnet are reported in this study. As a first step toward multiplequbit operations, we demonstrate coherent manipulations of three singlettriplet qubits showing underdamped Larmor and Ramsey oscillations in all double dot sites. An accurate measurement of sitedependent field gradients as well as rootmeansquared electric and magnetic noise is provided, which is followed by a discussion of the adequacy of a simple rectangular micromagnet for practical use in multiple quantum dot arrays. Current limitations and possible strategies for achieving simultaneous multiqubit operations in extended linear arrays are also presented.

(2020) Physical Review B. 102, 23, 235307. Abstract
Ground states that appear in a quantizing magnetic field in an imbalanced bilayer electron system hosted by a dualgated, wide GaAs quantum well are explored with a magnetocapacitance technique that enables detection of the compressibility of each layer separately, the characterization of the charge distribution, as well as the distinction of single or doublelayerlike behavior. Magnetic field induced reentrant quantum phase transitions are observed between a compressible doublelayer ground state and a singlelayerlike incompressible phase for both total fillings 1 and 2. The transitions are accompanied by a charge redistribution across the well. Our observations indicate for both incompressible states easyplane pseudospin ferromagnetism as the origin.

(2020) Physical Review Letters. 125, 10, 107701. Abstract
The realization of integrated quantum circuits requires precise onchip control of charge carriers. Aiming at the coherent coupling of distant nanostructures at zero magnetic field, here we study the ballistic electron transport through two quantum point contacts (QPCs) in series in a three terminal configuration. We enhance the coupling between the QPCs by electrostatic focusing using a field effect lens. To study the emission and collection properties of QPCs in detail we combine the electrostatic focusing with magnetic deflection. Comparing our measurements with quantum mechanical and classical calculations we discuss generic features of the quantum circuit and demonstrate how the coherent and ballistic dynamics depend on the details of the QPC confinement potentials.

(2020) npj Quantum Information. 6, 1, 64. Abstract
Fast and highfidelity quantum state detection is essential for building robust spinbased quantum information processing platforms in semiconductors. The Pauli spin blockade (PSB)based spintocharge conversion and its variants are widely used for the spin state discrimination of twoelectron singlettriplet (ST0) qubits; however, the singleshot measurement fidelity is limited by either the low signal contrast, or the short lifetime of the triplet state at the PSB energy detuning, especially due to strong mixing with singlet states at large magnetic field gradients. Ultimately, the limited singleshot measurement fidelity leads to low visibility of quantum operations. Here, we demonstrate an alternative method to achieve spintocharge conversion of ST(0)qubit states using energyselective tunneling between doubly occupied quantum dots (QDs) and electron reservoirs. We demonstrate a singleshot measurement fidelity of 90% and an ST(0)oscillation visibility of 81% at a field gradient of 100 mT (similar to 500MHz h (g*center dot mu(B))(1)); this allows singleshot readout with full electron charge signal contrast and, at the same time, long and tunable measurement time with negligible effect of relaxation even at strong magnetic field gradients. Using an rfsensor positioned opposite to the QD array, we apply this method to two ST(0)qubits and show highvisibility readout of two individual singlequbit gate operations is possible with a single rf singleelectron transistor sensor. We expect our measurement scheme for twoelectron spin states can be applied to various hosting materials and provides a simplified and complementary route for multiple qubit state detection with high accuracy in QDbased quantum computing platforms.

(2020) Physical Review Research. 2, 3, 033123. Abstract
We studied neutral excitations in a twodimensional electron gas with an orbital momentum ΔM=1 and spin projection over the magnetic field axis ΔSz=1 in the vicinity of a filling factor of 3/2. It is shown that the 3/2 state is a singular point in the filling factor dependence of the spin ordering of the twodimensional electron gas. In the vicinity of ν=3/2, a significant increase in the relaxation time (τ=13μs) for the excitations to the ground state is exhibited even though the number of vacancies in the lowest energy level is macroscopically large. The decrease in the relaxation rate is related to the spin texture transformation in the ground state induced by spin flips and electron density rearrangement. Based on the experimental data we believe that the 3/2 state is an example of locally incompressible fractional quantum Hall state.

(2020) JETP Letters. 111, 10, p. 562567 Abstract
The observation of microwaveinduced changes in the charge of a field effect transistor with a channel formed by a bilayer electron system has confirmed that microwave radiation induces a nonequilibrium electron energy distribution function, which generates magnetooscillations of the resistance of twodimensional electron systems. The observed periodicity and beating of magnetooscillations of the charge have been explained by the redistribution of electrons between the layers, which occurs because of the corresponding nonequilibrium occupation of electronic states.

(2020) Physical Review B. 101, 16, 165429. Abstract
Quantum point contacts (QPCs) are fundamental building blocks of nanoelectronic circuits. For their emission dynamics as well as for interaction effects such as the 0.7 anomaly the details of the electrostatic potential are important, but the precise potential shapes are usually unknown. Here, we measure the onedimensional subband spacings of various QPCs as a function of their conductance and compare our findings with models of lateral parabolic versus hardwall confinement. We find that a gatedefined QPC near pinchoff is compatible with the parabolic saddlepoint scenario. However, as the number of populated subbands is increased, Coulomb screening flattens the potential bottom and a description in terms of a finite hardwall potential becomes more realistic.

(2020) Physical Review Letters. 124, 11, 117601. Abstract
Surface acoustic waves (SAW) have been utilized to investigate the properties of a twodimensional electron system subjected to a perpendicular magnetic field and monochromatic microwave radiation in the regime where the socalled microwaveinduced zeroresistance states form. Contrary to conventional magnetotransport in Hall bar and van der Pauw geometries, the collimated SAW beam probes only the bulk of the electronic system exposed to this wave. Clear signatures appear in the SAW propagation velocity, corroborating that neither contacts nor sample edges are a root source for their emergence. By virtue of the directional nature of this probing method and with the assistance of theoretical modeling, we were able to demonstrate that the SAW response depends on the angle between its propagation vector and the orientation of domains that spontaneously form when zeroresistance is observed in transport. This confirms in unprecedented manner the formation of an inhomogeneous phase under these nonequilibrium conditions.

(2020) Sensors. 20, 3, 829. Abstract
We propose a new design microwave radiation sensor based on a selectively doped semiconductor structure of asymmetrical shape (socalled bowtie diode). The novelty of the design comes down to the gating of the active layer of the diode above different regions of the twodimensional electron channel. The gate influences the sensing properties of the bowtie diode depending on the nature of voltage detected across the ungated one as well as on the location of the gate in regard to the diode contacts. When the gate is located by the wide contact, the voltage sensitivity increases ten times as compared to the case of the ungated diode, and the detected voltage holds the same polarity of the thermoelectric electromotive force of hot electrons in an asymmetrically shaped nn(+) junction. Another remarkable effect of the gate placed by the wide contact is weak dependence of the detected voltage on frequency which makes such a microwave diode to be a proper candidate for the detection of electromagnetic radiation in the microwave and subterahertz frequency range. When the gate is situated beside the narrow contact, the two orders of sensitivity magnitude increase are valid in the microwaves but the voltage sensitivity is strongly frequencydependent for higher frequencies.

(2020) Scientific Reports. 10, 1, 2270. Abstract
An experimental technique based on timeresolved Kerr rotation allows a comparison of the spin stiffnesses of different spinpolarized and depolarized states in a twodimensional electron system. With this technique, a new spincorrelated phase that has no known analogues was discovered. The new spindepolarized phase is characterized by high spin stiffness equal to that of a spinpolarized quantum Hall ferromagnet.
2019

(2019) JETP Letters. 110, 6, p. 424429 Abstract
The compressibility of electrons in a bilayer electron system implemented in a GaAs double quantum well is investigated. Manifestations of the negative compressibility of a lowdensity twodimensional electron system in zero and quantizing magnetic fields are observed. It is found that the magnetic field ranges where incompressible phases at the spinresolved Landau level filling factors of 2 and 1 exist in the layer with the higher electron density are broadened considerably upon the filling of the other layer. The effect is explained by the stabilization of the quantum Hall effect states owing to the transfer of electrons from the layer with the lower density. The magnitude of jumps in the chemical potential for the corresponding quantum Hall effect states is estimated.

(2019) Physical Review Letters. 122, 24, 246801. Abstract
We attempted to measure interference of the outer edge mode in the fractional quantum hall regime with an electronic Machzehnder interferometer. The visibility of the interferometer wore off as we approached νB ¼ 1 and the transmission of the quantum point contacts (QPCs) of the interferometer simultaneously developed a v ¼ 1=3 conductance plateau accompanied by shot noise. The appearance of shot noise on this plateau indicates the appearance of nontopological neutral modes resulting from edge reconstruction. We have confirmed the presence of upstream neutral modes measuring upstream noise emanating from the QPC. The lack of interference throughout the lowest Landau level was correlated with a proliferation of neutral modes.

(2019) Applied Physics Letters. 114, 24, 242102. Abstract
We report raster scan multiplexed chargestability diagram measurements for tuning multiple gatedefined quantum dots in GaAs/AlGaAs heterostructures. We evaluate the charge sensitivity of the quantum point contact (QPC) in both radio frequency (rf)reflectometry and direct currenttransport modes, where we measure the signaltonoise ratio (SNR) of 40 for rfQPC with an integration time per pixel of10ms, corresponding to1.14ms for resolving single electron transition in the few electron regime. The high SNR for reasonable integration time allows fast twodimensional (2D) scanning, which we use to facilitate double and triple quantum dot (TQD) tuning processes. We configure a highly stable raster scan multiplexed quantum dot tuning platform using a switching matrix and transformercoupled alternating current ramp sources with software control. As an example of highthroughput multiple quantum dot tuning, we demonstrate systematic TQD formation using this platform in which a multiplexed combination of 2D scans enables the identification of the few electron regime in multiple quantum dots in just a few minutes. The method presented here is general, and we expect that the tuning platform is applicable to more complex multiple quantum dot arrays, allowing efficient quantum dot system Hamiltonian parameter calibration.

(2019) SolidState Electronics. 155, p. 117122 Abstract
Three different methods of experimental mesoscopic physics, namely, weak antilocalization effects, universal conductance fluctuations, and AharonovBohm oscillations, have been used to extract the electron phasecoherence scattering rate in twodimensional gas of InGaAs/AlInAs heterostructures. The AharonovBohm oscillations reveal strong beating effects, indicating the existence of two similar periodicities of the flux dependencies. As suggested by certain theoretical models, such a behavior might be expected from the socalled Berry phase acquired by electrons with different spin orientations in the presence of strong spinorbit coupling and Zeeman splitting. In our paper we deduce the experimental values of the dephasing length and try to compare the observed beating pattern with possible scenarios for the appearance of the Berry phase.

(2019) JETP Letters. 109, 3, p. 185190 Abstract
The temperature dependence of microwave absorption minima observed at cyclotron resonance harmonics in a twodimensional electron system implemented in a GaAs/AlGaAs heterostructure has been studied. The coexistence of this effect with the temperaturedependent giant negative magnetoresistance has been revealed. A possible explanation of the results by the effect of electronelectron scattering on the nonMarkovian electron kinetics has been discussed.

(2019) Nature Communications. 10, 1, 1920. Abstract
Topological edgereconstruction occurs in holeconjugate states of the fractional quantum Hall effect. The frequently studied filling factor, ν = 2/3, was originally proposed to harbor two counterpropagating modes: a downstream v = 1 and an upstream v = 1/3. However, charge equilibration between these two modes always led to an observed downstream v = 2/3 charge mode accompanied by an upstream neutral mode. Here, we present an approach to synthetize a v = 2/3 edge mode from its basic counterpropagating charged constituents, allowing a controlled equilibration between the two counterpropagating charge modes. This platform is based on a carefully designed doublequantumwell, which hosts two populated electronic subbands (lower and upper), with corresponding filling factors, v _{l} and v _{u} . By separating the 2D plane to two gated intersecting halves, each with different fillings, counterpropagating chiral modes can be formed along the intersection line. Equilibration between these modes can be controlled with the top gates’ voltage and the magnetic field.

(2019) Science. 363, 6422, p. 5457 363. Abstract
The quantum Hall effect, observed in a twodimensional (2D) electron gas subjected to a perpendicular magnetic field, imposes a 1Dlike chiral, downstream, transport of charge carriers along the sample edges. Although this picture remains valid for electrons and Laughlin's fractional quasiparticles, it no longer holds for quasiparticles in the socalled holeconjugate states. These states are expected, when disorder and interactions are weak, to harbor upstream charge modes. However, so far, charge currents were observed to flow exclusively downstream in the quantum Hall regime. Studying the canonical spinpolarized and spinunpolarized v = 2/3 holelike states in GaAsAlGaAs heterostructures, we observed a significant upstream charge current at short propagation distances in the spin unpolarized state.
2018

(2018) Physical Review Applied. 10, 5, 054026. Abstract
We present efficient methods to reliably characterize and tune gatedefined semiconductor spin qubits. Our methods are developed for double quantum dots in GaAs heterostructures, but they can easily be adapted to other quantumdotbased qubit systems. These tuning procedures include the characterization of the interdot tunnel coupling, the tunnel coupling to the surrounding leads, and the identification of various fast initialization points for the operation of the qubit. Since semiconductorbased spin qubits are compatible with standard semiconductor process technology and hence promise good prospects of scalability, the challenge of efficiently tuning the dot's parameters will only grow in the near future, once the multiqubit stage is reached. With the anticipation of being used as the basis for future automated tuning protocols, all measurements presented here are fasttoexecute and easytoanalyze characterization methods. They result in quantitative measures of the relevant qubit parameters within a couple of seconds and require almost no human interference.

(2018) JETP Letters. 108, 7, p. 465470 Abstract
The effect of microwave radiation on the amplitudes of the quantum magnetocapacitance oscillations in fieldeffect transistors has been studied. It has been found that the microwaveinduced variation of the oscillation amplitude depends nonmonotonically on the magnetic field including the amplitude enhancement effect. It has been shown that this behavior is due to the band bending near the edge of the gate. The effect has been attributed to the interference of quantum oscillations with radiationinduced magnetooscillations of the screening length of a static electric field.

(2018) Acta Physica Polonica A. 134, 4, p. 978980 Abstract
A subterahertz emission from a gridgated structure lithographically fabricated on a GaAs/AlGaAs heterostructure was observed at liquid helium temperatures. The frequency of observed emission was equal to about 75 GHz and was almost independent of the voltage biasing the emitter. The Gunn effect is proposed as a possible explanation of the emission. This hypothesis, however, is critically discussed and further experiments are indicated which could lead to a definite identification of the physical mechanism of emission.

(2018) Nature. 562, 7726, p. E6E6 Abstract
In this Article, the publication details for references 33, 34 and 40 have been corrected online.

(2018) JETP Letters. 108, 3, p. 215219 Abstract
The temperature dependence of the microwave photovoltage has been studied in microwaveinduced states of a twodimensional electron system, which are characterized by an almost dissipationless flow of a lowfrequency current. At decreasing temperature, a smooth transition has been found from a bistable state, where the photovoltage demonstrates switching between two levels, which are due to reversals of the spontaneous electric field in a domain structure, to a steady state. The transition occurs as the shift of one of the levels of the bistable photovoltage to the other level accompanied by a decrease in the switching frequency. The results indicate the freezing of the dynamic domain structure in the state corresponding to the more stable configuration of the electric field.

(2018) Nature. 559, 7713, p. 205210 Abstract
Topological states of matter are characterized by topological invariants, which are physical quantities whose values are quantized and do not depend on the details of the system (such as its shape, size and impurities). Of these quantities, the easiest to probe is the electrical Hall conductance, and fractional values (in units of e ^{2}/h, where e is the electronic charge and h is the Planck constant) of this quantity attest to topologically ordered states, which carry quasiparticles with fractional charge and anyonic statistics. Another topological invariant is the thermal Hall conductance, which is harder to measure. For the quantized thermal Hall conductance, a fractional value in units of κ _{0} (κ _{0} = π ^{2}k _{B}^{2}/(3h), where k _{B} is the Boltzmann constant) proves that the state of matter is nonAbelian. Such nonAbelian states lead to groundstate degeneracy and perform topological unitary transformations when braided, which can be useful for topological quantum computation. Here we report measurements of the thermal Hall conductance of several quantum Hall states in the first excited Landau level and find that the thermal Hall conductance of the 5/2 state is compatible with a halfinteger value of 2.5κ _{0}, demonstrating its nonAbelian nature.

(2018) Physical Review B. 97, 24, 241115. Abstract
Electron pairing due to a repulsive Coulomb interaction in a triple quantum dot (TQD) is experimentally studied. It is found that electron pairing in two dots of a TQD is mediated by the third dot, when the third dot strongly couples with the other two via Coulomb repulsion so that the TQD is in the twofold degenerate ground states of (1,0,0) and (0,1,1) charge configurations. Using the transport spectroscopy that monitors electron transport through each individual dot of a TQD, we analyze how to achieve the degeneracy in experiments, how the degeneracy is related to electron pairing, and the resulting nontrivial behavior of electron transport. Our findings may be used to design a system with nontrivial electron correlations and functionalities.

(2018) Nature Physics. 14, 4, p. 411416 Abstract
Electronic systems harboring onedimensional helical modes, where spin and momentum are locked, have lately become an important field of its own. When coupled to a conventional superconductor, such systems are expected to manifest topological superconductivity; a unique phase hosting exotic Majorana zero modes. Even more interesting are fractional helical modes, yet to be observed, which open the route for realizing generalized parafermions. Possessing nonabelian exchange statistics, these quasiparticles may serve as building blocks in topological quantum computing. Here, we present a new approach to form protected onedimensional helical edge modes in the quantum Hall regime. The novel platform is based on a carefully designed doublequantumwell structure in a GaAs based system hosting two electronic subbands; each tuned to the quantum Hall effect regime. By electrostatic gating of different areas of the structure, counterpropagating integer, as well as fractional, edge modes with opposite spins are formed. We demonstrate that due to spinprotection, these helical modes remain ballistic for large distances. In addition to the formation of helical modes, this platform can serve as a rich playground for artificial induction of compounded fractional edge modes, and for construction of edge modes based interferometers.

(2018) Physical Review Letters. 120, 13, 137603. Abstract
The spontaneous ordering of spins and charges in geometric patterns is currently under scrutiny in a number of different material systems. A topic of particular interest is the interaction of such ordered phases with itinerant electrons driven by an externally imposed current. It not only provides important information on the charge ordering itself but potentially also allows manipulating the shape and symmetry of the underlying pattern if current flow is strong enough. Unfortunately, conventional transport methods probing the macroscopic resistance suffer from the fact that the voltage drop along the sample edges provides only indirect information on the bulk properties because a complex current distribution is elicited by the inhomogeneous ground state. Here, we promote the use of surface acoustic waves to study these brokensymmetry phases and specifically address the bubble and stripe phases emerging in highquality twodimensional electron systems in GaAs/AlGaAs heterostructures as prototypical examples. When driving a unidirectional current, we find a surprising discrepancy between the sound propagation probing the bulk of the sample and the voltage drop along the sample edges. Our results prove that the currentinduced modifications observed in resistive transport measurements are in fact a local phenomenon only, leaving the majority of the sample unaltered. More generally, our findings shed new light on the extent to which these ordered electron phases are impacted by an external current and underline the intrinsic advantages of acoustic measurements for the study of such inhomogeneous phases.

(2018) Physical Review B. 97, 12, 125405. Abstract
In recent interference experiments with an electronic FabryPérot interferometer (FPI), implemented in the integer quantum Hall effect regime, a flux periodicity of h/2e was observed at bulk fillings νB>2.5. The halved periodicity was accompanied by an interfering charge e∗=2e, determined by shotnoise measurements. Here, we present measurements demonstrating that, counterintuitively, the coherence and the interference periodicity of the interfering chiral edge channel are solely determined by the coherence and the enclosed flux of the adjacent edge channel. Our results elucidate the important role of the latter and suggest that a neutral chiral edge mode plays a crucial role in the pairing phenomenon. Our findings reveal that the observed pairing of electrons is not a curious isolated phenomenon, but one of many manifestations of unexpected edge physics in the quantum Hall effect regime.

(2018) Journal of Applied Physics. 123, 8, 084301. Abstract
Here, we report on a transparent method to characterize individual layers in a doublelayer electron system, which forms in a wide quantum well, and to determine their electron densities. The technique relies on the simultaneous measurement of the capacitances between the electron system and gates located on either side of the well. Modifications to the electron wave function due to the population of the second subband and the appearance of an additional electron layer can be detected. The magnetic field dependence of these capacitances is dominated by quantum corrections caused by the occupation of Landau levels in the nearest electron layer. The technique should be equally applicable to other implementations of a double layer electron system. Published by AIP Publishing.

(2018) Physical Review Letters. 120, 4, 047402. Abstract
We study the exciton gasliquid transition in GaAs/AlGaAs coupled quantum wells. Below a critical temperature, TC = 4.8 K, and above a threshold laser power density the system undergoes a phase transition into a liquid state. We determine the densitytemperature phase diagram over the temperature range 0.14.8 K. We find that the latent heat increases linearly with temperature at T less than or similar to 1.1 K, similarly to a BoseEinstein condensate transition, and becomes constant at 1.1 less than or similar to T

(2018) JETP Letters. 107, 1, p. 6165 Abstract
The temperature dependence of the switching frequency of a microwaveinduced spontaneous electric field forming a domain structure and the conductance of a doping layer that provides electrons to the twodimensional electron system have been measured on samples fabricated from the same GaAs/AlGaAs heterostructure. Both quantities have been found to obey the thermally activated dependence (Arrhenius law) with close activation energies. This result indicates a relation between the quantities and confirms a hypothesis that the observed dynamics of the domain structure originates from the dynamical screening of the spontaneous electric field of domains by charges in the doping layer.
2017

(2017) Nature Communications. 8, 2251. Abstract
Controlling the transmission of electrical current using a quantum point contact constriction paved a way to a large variety of experiments in mesoscopic physics. The increasing interest in heat transfer in such systems fosters questions about possible manipulations of quantum heat modes that do not carry net charge (neutral modes). Here we study the transmission of upstream neutral modes through a quantum point contact in fractional holeconjugate quantum Hall states. Employing two different measurement techniques, we were able to render the relative spatial distribution of these chargeless modes with their charged counterparts. In these states, which were found to harbor more than one downstream charge mode, the upstream neutral modes are found to flow with the inner charge modeas theoretically predicted. These results unveil a universal upstream heat current structure and open the path for more complex engineering of heat flows and cooling mechanisms in quantum nanoelectronic devices.

(2017) Nature Physics. 13, 11, p. 1124+ Abstract
The physics of itinerant electrons in condensed matter is by and large governed by repulsive Coulomb forces. However, attractive interactions may emerge and prevail in determining the ground state despite the pervasive Coulomb repulsion. A notable example is electron pairing and superconductivity. The interplay of attractive and repulsive interactions may also instigate spontaneous symmetry lowering and clustering of charges in geometric patterns even without net attraction. Both types of attractive interaction triggered physicspairing and charge orderingare at play in twodimensional electron systems exposed to a quantizing magnetic field. The charge ordering has been concluded indirectly from transport behaviour. Here we report the observation of negative permittivity present solely when bubble and stripe phases form. In conjunction with a theoretical model, the negative permittivity provides evidence for the underlying attractive exchangecorrelation energy which suffciently countervails Coulomb repulsion at small distances to enable and mediate charge clustering.

(2017) Physical Review B. 96, 20, 205431. Abstract
Transmission through a quantum point contact (QPC) in the quantum Hall regime usually exhibits multiple resonances as a function of gate voltage and high nonlinearity in bias. Such behavior is unpredictable and changes sample by sample. Here, we report the observation of a sharp transition of the transmission through an open QPC at finite bias, which was observed consistently for all the tested QPCs. It is found that the bias dependence of the transition can be fitted to the FermiDirac distribution function through universal scaling. The fitted temperature matches quite nicely to the electron temperature measured via shotnoise thermometry. While the origin of the transition is unclear, we propose a phenomenological model based on our experimental results that may help to understand such a sharp transition. Similar transitions are observed in the fractional quantum Hall regime, and it is found that the temperature of the system can be measured by rescaling the quasiparticle energy with the effective charge (e* = e/3). We believe that the observed phenomena can be exploited as a tool for measuring the electron temperature of the system and for studying the quasiparticle charges of the fractional quantum Hall states.

(2017) Physical Review B. 96, 15, 155306. Abstract
We have studied the absorption of monochromatic microwave radiation in highquality twodimensional electron systems for the frequency span from 10 to 380 GHz using a bolometric method. For frequencies above 100 GHz the absorption exhibits an anomalous magnetic field dependence. Minima form at harmonics of the cyclotron resonance frequency. The results contrast previously reported data for other frequency ranges. Quasiclassical memory effects originating from the nonMarkovian dynamics of electrons in a disorder potential containing shortrange scatterers on top of a smooth potential background favorably account for the observed behavior.

(2017) Physical Review B. 96, 12, 121301. Abstract
We report on Hall fieldinduced resistance oscillations (HIROs) in a 60nmwide GaAs/AlGaAs quantum well with an in situ grown back gate, which allows tuning the carrier density n. At low n, when all electrons are confined to the lowest subband (SB1), the HIRO frequency, proportional to the product of the cyclotron diameter and the Hall field, scales with n(1/2) as expected. Remarkably, the population of the second subband (SB2) significantly enhances the HIROs, whereas their frequency now scales as n(1). We demonstrate that in this twosubband regime HIROs still originate solely from backscattering of SB1 electrons. The unusual density dependence occurs because the population of SB2 steadily increases, whereas that of SB1 remains essentially unchanged. The enhancement of the HIROs manifests an unexpected steplike increase in the quantum lifetime of SB1 electrons, which reaches a record value of 52 ps in the twosubband regime.

(2017) Nature Physics. 13, 5, p. 491496 Abstract
The nature of edge reconstruction in the quantum Hall effect (QHE) and the issue of where the current flows have been debated for years. Moreover, the recent observation of proliferation of 'upstream' neutral modes in the fractional QHE has raised doubts about the present models of edge channels. Here, we present a new picture of the edge reconstruction in two of the holeconjugate states. For example, while the present model for 1/2 = (2/3) consists of a single downstream chiral charge channel with conductance (2/3)(e 2 /h) and an upstream neutral mode, we show that the current is carried by two separate downstream chiral edge channels, each with conductance (1/3)(e 2 /h). We uncover a novel mechanism of fragmentation of upstream neutral modes into downstream propagating charge modes that induces current fluctuations with zero net current. Our unexpected results underline the need for better understanding of edge reconstruction and energy transport in all fractional QHE states.

(2017) Nature. 545, 7652, p. 7579 Abstract
The quantum of thermal conductance of ballistic (collisionless) onedimensional channels is a unique fundamental constant1. Although the quantization of the electrical conductance of onedimensional ballistic conductors has long been experimentally established2, demonstrating the quantization of thermal conductance has been challenging as it necessitated an accurate measurement of very small temperature increase. It has been accomplished for weakly interacting systems of phonons(3,4), photons(5) and electronic Fermi liquids(68); however, it should theoretically also hold in strongly interacting systems, such as those in which the fractional quantum Hall effect is observed. This effect describes the fractionalization of electrons into anyons and chargeless quasiparticles, which in some cases can be Majorana fermions(2). Because the bulk is incompressible in the fractional quantum Hall regime, it is not expected to contribute substantially to the thermal conductance, which is instead determined by chiral, onedimensional edge modes. The thermal conductance thus reflects the topological properties of the fractional quantum Hall electronic system, to which measurements of the electrical conductance give no access(912). Here we report measurements of thermal conductance in particlelike (LaughlinJain series) states and the more complex (and less studied) holelike states in a highmobility twodimensional electron gas in GaAsAlGaAs heterostructures. Holelike states, which have fractional Landaulevel fillings of 1/2 to 1, support downstream charged modes as well as upstream neutral modes(13), and are expected to have a thermal conductance that is determined by the net chirality of all of their downstream and upstream edge modes. Our results establish the universality of the quantization of thermal conductance for fractionally charged and neutral modes. Measurements of anyonic heat flow provide access to information that is not easily accessible from measurements of conductance

(2017) Journal Of Vacuum Science & Technology B. 35, 2, ARTN 02B11. Abstract
The authors compare four methods to investigate the threading dislocations (TDs) observed in metamorphic buffers used in the growth of InSb quantum well on GaAs (001) substrates. Three types of buffers with varying number of Al0.24In0.76Sb interlayers (N = 0, 1, and 3) were studied. Crosssectional scanning transmission electron microscopy (STEM) revealed an effective dislocation filtering by the interlayers. Individual TDs were identified with atomicforce microscopy (AFM) as distinct morphological features of dislocation outcrops on the surface. Threading dislocation density (TDD) is reduced by 1 order of magnitude with three interlayers, consistent with the STEM observation. TDD measured with a scanning electron microscope in electron channeling contrast imaging (ECCI) mode agrees closely with the AFM analysis, except for the N = 0 buffer where the ECCI gives TDD lower by more than a factor of two. The etch pit density of N = 3 buffer, measured with a Nomarski differential interference contrast microscope after defect selective etching (DSE), is almost 1 order of magnitude lower than the TDD measured by AFM and ECCI. Due to the large pit size, the used etching recipe only works well for samples with TDD lower than 10(7) cm(2). AFM, ECCI, and DSE are excellent alternatives to transmission electron microscopy in the process of metamorphic buffer optimization. The AFM technique offers the additional advantage of high vertical resolution morphology mapping. Such capability is of great importance for the optimization of metamorphic buffers from the perspective of surface smoothness improvement. (C) 2017 American Vacuum Society.

(2017) Physical Review B. 95, 3, 035306. Abstract
We present experimental data and associated theory for correlations in a series of experiments involving repeated LandauZener sweeps through the crossing point of a singlet state and a spinaligned triplet state in a GaAs double quantum dot containing two conduction electrons, which are loaded in the singlet state before each sweep, and the final spin is recorded after each sweep. The experiments reported here measure correlations on time scales from 4 μs to 2 ms. When the magnetic field is aligned in a direction such that spinorbit coupling cannot cause spin flips, the correlation spectrum has prominent peaks centered at zero frequency and at the differences of the Larmor frequencies of the nuclei, on top of a frequencyindependent background. When the spinorbit field is relevant, there are additional peaks, centered at the frequencies of the individual species. A theoretical model which neglects the effects of highfrequency charge noise correctly predicts the positions of the observed peaks, and gives a reasonably accurate prediction of the size of the frequencyindependent background, but gives peak areas that are larger than the observed areas by a factor of 2 or more. The observed peak widths are roughly consistent with predictions based on nuclear dephasing times of the order of 60 μs. However, there is extra weight at the lowest observed frequencies, which suggests the existence of residual correlations on the scale of 2 ms. We speculate on the source of these discrepancies.

(2017) Nature Communications. 8, 14082. Abstract
Since the experimental realization of the integer quantum Hall effect in a twodimensional electron system, the interrelation between the conductance quantization and the topological properties of the system has been investigated. Assuming that the twodimensional electron system is described by a Bloch Hamiltonian, system is insulating in the bulk of sample throughout the quantum Hall plateau due to a magnetic field induced energy gap. Meanwhile, the system is conducting at the edges resembling a 2+1 dimensional topological insulator without timereversal symmetry. Here, by our magnetotransport measurements performed on GaAs/AlGaAs high purity Hall bars with two inner contacts we show that incompressible strips formed at the edges result in Hall quantization, even if the bulk is compressible. Consequently, the relationship between the quantum Hall effect and topological bulk insulator breaks for specific field intervals within the plateaus. The measurement of conducting bulk, strongly challenges all existing singleparticle theories.
2016

(2016) JETP Letters. 104, 10, p. 721725 Abstract
It has been found on a sample of the GaAs/AlGaAs heterostructure with the twodimensional electron system that different configurations of domains of a spontaneous electric field are possible within one microwave induced state with the resistance tending to zero. Transitions between such configurations are observed at the variation of the radiation power and magnetic field. In the general case, the configuration of domains is more complicated than existing models. The fragment of the distribution of the electric field in the sample for one of the observed configurations is in agreement with the rhombic domain structure considered by I. G. Finkler and B. I. Halperin, Phys. Rev. B 79, 085315 (2009).

(2016) Physical review letters. 117, 17, 176801. Abstract
In a twodimensional electron system, microwave radiation may induce giant resistance oscillations. Their origin has been debated controversially and numerous mechanisms based on very different physical phenomena have been invoked. However, none of them have been unambiguously experimentally identified, since they produce similar effects in transport studies. The capacitance of a twosubband system is sensitive to a redistribution of electrons over energy states, since it entails a shift of the electron charge perpendicular to the plane. In such a system, microwaveinduced magnetocapacitance oscillations have been observed. They can only be accounted for by an electron distribution function oscillating with energy due to Landau quantization, one of the quantum mechanisms proposed for the resistance oscillations.

(2016) Nature Communications. 7, 12184. Abstract
Studies of electronic interferometers, based on edgechannel transport in the quantum Hall effect regime, have been stimulated by the search for evidence of abelian and nonabelian anyonic statistics of fractional charges. In particular, the electronic FabryPérot interferometer has been found to be Coulomb dominated, thus masking coherent AharonovBohm interference patterns: the flux trapped within the interferometer remains unchanged as the applied magnetic field is varied, barring unobservable modulations of the interference area. Here we report on conductance measurements indicative of the interferometer's area 'breathing' with the variation of the magnetic field, associated with observable (a fraction of a flux quantum) variations of the trapped flux. This is the result of partial (controlled) screening of Coulomb interactions. Our results introduce a novel experimental tool for probing anyonic statistics.

(2016) Physical Review B. 93, 12, 121412. Abstract
This Rapid Communication was motivated by the quest for observing interference of fractionally charged quasiparticles. Here, we study the behavior of an electronic MachZehnder interferometer at the integer quantum Hall effect regime at filling factors greater than 1. Both the visibility and the velocity were measured and found to be highly correlated as a function of the filling factor. As the filling factor approached unity, the visibility quenched, not to recover for filling factors smaller than unity. Alternatively, the velocity saturated around a minimal value at the unity filling factor. We highlight the significant role interactions between the interfering edge and the bulk play as well as that of the defining potential at the edge. Shotnoise measurements suggest that phase averaging (due to phase randomization), rather than singleparticle decoherence, is likely to be the cause of the dephasing in the fractional regime.

(2016) Physical Review B. 93, 3, 035302. Abstract
Optical pumping can increase the polarization of nuclear spins in semiconductors, such as GaAs, by many orders of magnitude, improving the sensitivity in conventionally detected nuclear magnetic resonance (NMR) experiments. Optical detection of these NMR transitions may further increase the sensitivity, allowing an alloptical NMR which is free of radiofrequency fields. Here we report alloptical NMR experiments in a GaAs/Al0.35Ga0.65As twodimensional electron system. We observed multiple magnetic resonance of nuclear species in the sample when the modulation frequency of the pump light was 12m+1 of the fundamental NMR frequency (multiple NMR), where m is an integer. Such multiple NMRs arise from interaction between modulated spins of polarized electrons and nuclear spins.
2015

(2015) Nature. 526, 7572, p. 237240 Abstract
Quantum critical systems derive their finitetemperature properties from the influence of a zerotemperature quantum phase transition. The paradigm is essential for understanding unconventional highT c superconductors and the nonFermi liquid properties of heavy fermion compounds. However, the microscopic origins of quantum phase transitions in complex materials are often debated. Here we demonstrate experimentally, with support from numerical renormalization group calculations, a universal crossover from quantum critical nonFermi liquid behaviour to distinct Fermi liquid ground states in a highly controllable quantum dot device. Our device realizes the nonFermi liquid twochannel Kondo state, based on a spin1/2 impurity exchangecoupled equally to two independent electronic reservoirs. On detuning the exchange couplings we observe the Fermi liquid scale T∗, at energies below which the spin is screened conventionally by the more strongly coupled channel. We extract a quadratic dependence of T∗ on gate voltage close to criticality, and validate an asymptotically exact description of the universal crossover between strongly correlated nonFermi liquid and Fermi liquid states.

(2015) Journal Of Vacuum Science & Technology B. 33, 5, 051803. Abstract
Nanoscale device fabrication has enabled remarkable scientific advances. Yet a single broken electrode can render a complex device useless. The authors consider local electronbeaminduced deposition (EBID) of platinum as a method for restoring function to devices with damaged gate electrodes. The authors find that platinum deposits written with EBID at low acceleration voltage (350V) remain conductive down to millikelvin temperatures, if they are annealed after deposition in the presence of oxygen. The authors apply this technique to a complex quantum dot device based on a GaAs/AlGaAs heterostructure.

(2015) Physical Review B  Condensed Matter and Materials Physics. 92, 12, 125402. Abstract
Nuclear spins are an important source of dephasing for electron spin qubits in GaAs quantum dots. Most studies of their dynamics have focused on the relatively slow longitudinal polarization. We present a semiclassical model and experimental data showing that the dynamics of the transverse hyperfine field can be probed by correlating individual LandauZener sweeps across the ST+ transition of a twoelectron spin qubit. The relative Larmor precession of different nuclear spin species leads to oscillations in these correlations, which decay due to dephasing of the nuclei. In the presence of spinorbit coupling, oscillations with the absolute Larmor frequencies whose amplitude depends on the spinorbit coupling strength are expected. These oscillations reflect rapid dynamics of the transverse hyperfine field, which are relevant for several qubit control schemes.

(2015) Nature Communications. 6, 7682. Abstract
The centralspin problem is a widely studied model of quantum decoherence. Dynamic nuclear polarization occurs in centralspin systems when electronic angular momentum is transferred to nuclear spins and is exploited in quantum information processing for coherent spin manipulation. However, the mechanisms limiting this process remain only partially understood. Here we show that spinorbit coupling can quench dynamic nuclear polarization in a GaAs quantum dot, because spin conservation is violated in the electronnuclear system, despite weak spinorbit coupling in GaAs. Using LandauZener sweeps to measure static and dynamic properties of the electron spinflip probability, we observe that the size of the spinorbit and hyperfine interactions depends on the magnitude and direction of applied magnetic field. We find that dynamic nuclear polarization is quenched when the spinorbit contribution exceeds the hyperfine, in agreement with a theoretical model. Our results shed light on the surprisingly strong effect of spinorbit coupling in centralspin systems.

(2015) Chinese Physics B. 24, 6, 067302. Abstract
The nuclear spin relaxation rate (1/T_{1}) is measured for GaAs twodimensional (2D) electron systems in the quantum Hall regime with an allelectrical technique for agitating and probing the nuclear spins. A "tilted plateau" feature is observed near the Landau level filling factor v = 1 in 1/T_{1} versus v. Both the width and magnitude of the plateau increase with decreasing electron density. At low temperatures, 1/T_{1} exhibits an Arrhenius temperature dependence within the tilted plateau regime. The extracted energy gaps are up to two orders of magnitude smaller than the corresponding charge transport gaps. These results point to a nontrivial mechanism for the disorderenhanced nuclear spin relaxation, in which microscopic inhomogeneities play a key role for the low energy spin excitations related to skyrmions.

(2015) Nature Communications. 6, 7435. Abstract
Electron pairing is a rare phenomenon appearing only in a few unique physical systems; for example, superconductors and Kondocorrelated quantum dots. Here, we report on an unexpected electron pairing in the integer quantum Hall effect regime. The pairing takes place within an interfering edge channel in an electronic FabryPerot interferometer at a wide range of bulk filling factors, between 2 and 5. We report on three main observations: highvisibility AharonovBohm conductance oscillations with magnetic flux periodicity equal to half the magnetic flux quantum; an interfering quasiparticle charge equal to twice the elementary electron charge as revealed by quantum shot noise measurements, and full dephasing of the pairs' interference by induced dephasing of the adjacent inner edge channel  a manifestation of interchannel entanglement. Although this pairing phenomenon clearly results from interchannel interaction, the exact mechanism that leads to electronelectron attraction within a single edge channel is not clear. We believe that substantial efforts are needed in order to clarify these intriguing and unexpected findings.

(2015) Physical review letters. 114, 17, 176808. Abstract
In a twodimensional electron system subject to microwaves and a magnetic field, photovoltages emerge. They can be separated into two components originating from builtin electric fields and electric field domains arising from spontaneous symmetry breaking. The latter occurs in the zero resistance regime only and manifests itself in pulsed behavior, synchronous across the sample. The pulses show sign reversal. This implies a flip of the field in each domain, consistent with the existence of two equally probable electric field domain configurations due to the spontaneous symmetry breaking.

(2015) Physical Review B. 91, 4, 045437. Abstract
Terahertz spectroscopy experiments at magnetic fields and low temperatures were carried out on samples of different gate shapes processed on a high electron mobility GaAs/AlGaAs heterostructure. For a given radiation frequency, multiple magnetoplasmon resonances were observed with a dispersion relation described within a local approximation of the magnetoconductivity tensor. The second harmonic of the cyclotron resonance was observed and its appearance was interpreted as resulting from a highfrequency, inhomogeneous electromagnetic field on the border of a twodimensional electron gas with a metallic gate and/or an ohmic contact.

(2015) Optical Engineering. 54, 1, 017101. Abstract
Terahertz (THz) detector based on a GaAs/AlGaAs heterostructure was investigated at low temperatures and high magnetic fields. The response of the detector showed a feature caused by a cyclotron resonance that was accompanied by several peaks originated from excitations of magnetoplasmons. Illumination with a visible light (VIS) caused an increase of a plasma concentration and resulted in a change of a magnetoplasmon spectrum. An analysis of spectra allowed to determine changes in the plasmon dispersion with a VIS, which gives a tool to tune the THz response of a plasmonic detector.
2014

(2014) Physical review letters. 113, 26, 266803. Abstract
It is well established that density reconstruction at the edge of a twodimensional electron gas takes place for holeconjugate states in the fractional quantum Hall effect (such as v=2/3, 3/5, etc.). Such reconstruction leads, after equilibration between counterpropagating edge channels, to a downstream chiral current edge mode accompanied by upstream chiral neutral modes (carrying energy without net charge). Short equilibration length prevented thus far observation of the counterpropagating current channels  the hallmark of density reconstruction. Here, we provide evidence for such nonequilibrated counterpropagating current channels, in short regions (l=4μm and l=0.4μm) of fractional filling v=2/3 and, unexpectedly, v=1/3, sandwiched between two regions of integer filling v=1. Rather than a twoterminal fractional conductance, the conductance exhibited a significant ascension towards unity quantum conductance (GQ=e2/h) at or near the fractional plateaus. We attribute this conductance rise to the presence of a nonequilibrated channel in the fractional short regions.

(2014) Nature Communications. 5, 5156. Abstract
Unwanted interaction between a quantum system and its fluctuating environment leads to decoherence and is the primary obstacle to establishing a scalable quantum information processing architecture. Strategies such as environmental and materials engineering, quantum error correction and dynamical decoupling can mitigate decoherence, but generally increase experimental complexity. Here we improve coherence in a qubit using realtime Hamiltonian parameter estimation. Using a rapidly converging Bayesian approach, we precisely measure the splitting in a singlettriplet spin qubit faster than the surrounding nuclear bath fluctuates. We continuously adjust qubit control parameters based on this information, thereby improving the inhomogenously broadened coherence time (T∗_{2}) from tens of nanoseconds to >2 μs. Because the technique demonstrated here is compatible with arbitrary qubit operations, it is a natural complement to quantum error correction and can be used to improve the performance of a wide variety of qubits in both meteorological and quantum information processing applications.

(2014) Physical review letters. 113, 7, 076803. Abstract
A prominent manifestation of the competition between repulsive and attractive interactions acting on different length scales is the selforganized ordering of electrons in a stripelike fashion in material systems such as highTc superconductors. Such stripe phases are also believed to occur in twodimensional electron systems exposed to a perpendicular magnetic field, where they cause a strong anisotropy in transport. The addition of an inplane field even enables us to expel fractional quantum Hall states, to the benefit of such anisotropic phases. An important example represents the disappearance of the 5/2 fractional state. Here, we report the use of nuclear magnetic resonance spectroscopy to probe the electron density distribution of this emergent anisotropic phase. A surprisingly strong spatial density modulation was found. The observed behavior suggests a stripe pattern with a period of 2.6±0.6 magnetic lengths and an amplitude as large as 20% relative to the total density.

(2014) Journal of Applied Physics. 115, 21, 214503. Abstract
In order to characterize magnetic field (B) tunable THz plasmonic detectors, spectroscopy experiments were carried out at liquid helium temperatures and high magnetic fields on devices fabricated on a high electron mobility GaAs/AlGaAs heterostructure. The samples were either gated (the gate of a meander shape) or ungated. Spectra of a photovoltage generated by THz radiation were obtained as a function of B at a fixed THz excitation from a THz laser or as a function of THz photon frequency at a fixed B with a Fourier spectrometer. In the first type of measurements, the wave vector of magnetoplasmons excited was defined by geometrical features of samples. It was also found that the magnetoplasmon spectrum depended on the gate geometry which gives an additional parameter to control plasma excitations in THz detectors. Fourier spectra showed a strong dependence of the magnetoplasmon resonance amplitude on the conductionband electron filling factor which was explained within a model of the electron gas heating with THz radiation. The study allows to define both the advantages and limitations of plasmonic devices based on highmobility GaAs/AlGaAs heterostructures for THz detection at low temperatures and high magnetic fields.

(2014) Applied Physics Letters. 104, 26, 263514. Abstract
Low temperature, high magnetic field experiments were carried out with monochromatic terahertz (THz) sources to reveal multimode spectra of magnetoplasmons excited in gated and ungated samples processed on a high electron mobility GaAs/AlGaAs heterostructure. We show that playing with the geometry and thickness of the gate one can control both the plasmon dispersion relation and selection rules for plasmon excitation, giving a tool to a better control of plasmon resonances in THz detectors.

(2014) Nature Communications. 5, 4067. Abstract
The fractional quantum Hall effect is a canonical example of topological phases. While electric currents flow downstream in edge modes, neutral edge modes, observed only in holeconjugate states and in v 1/2=5/2, flow upstream. It is believed that the latter transport results from multiple counterpropagating channelsmixed by disorder that is accompanied by Coulomb interaction. Here we report on sensitive shot noise measurements that reveal unexpected presence of neutral modes in nonholeconjugate fractional states; however, not in the integer states. Furthermore, the incompressible bulk is also found to allow energy transport. While density reconstructions along the edge may account for the energy carrying edge modes, the origin of the bulk energy modes is unidentified. The proliferation of neutral modes changes drastically the accepted transport picture of the fractional quantum Hall effects. Their apparent ubiquitous presence may explain the lack of interference of fractional quasiparticlespreventing observation of fractional statistics.

(2014) Physical Review Letters. 112, 18, 189902. Abstract
This Letter was published online on 25 April 2014 without all the author’s corrections incorporated into the published article. The article has been corrected as of 29 April 2014. The text is correct in the printed version of the journal.

(2014) Physical review letters. 112, 16, 166801. Abstract
We report an observation, via sensitive shot noise measurements, of charge fractionalization of chiral edge electrons in the integer quantum Hall effect regime. Such fractionalization results solely from interchannel Coulomb interaction, leading electrons to decompose to excitations carrying fractional charges. The experiment was performed by guiding a partitioned current carrying edge channel in proximity to another unbiased edge channel, leading to shot noise in the unbiased edge channel without net current, which exhibited an unconventional dependence on the partitioning. The determination of the fractional excitations, as well as the relative velocities of the two original (prior to the interaction) channels, relied on a recent theory pertaining to this measurement. Our result exemplifies the correlated nature of multiple chiral edge channels in the integer quantum Hall effect regime.

(2014) Science. 343, 6166, p. 5557 Abstract
Excitons in semiconductors may form correlated phases at low temperatures. We report the observation of an exciton liquid in gallium arsenide/aluminum gallium arsenidecoupled quantum wells. Above a critical density and below a critical temperature, the photogenerated electrons and holes separate into two phases: an electronhole plasma and an exciton liquid, with a clear sharp boundary between them. The two phases are characterized by distinct photoluminescence spectra and by different electrical conductance. The liquid phase is formed by the repulsive interaction between the dipolar excitons and exhibits a shortrange order, which is manifested in the photoluminescence line shape.

(2014) Science. 344, 6190, p. 13631366 Abstract
The quantum eraser is a device that illustrates the quantum principle of complementarity and shows how a dephased system can regain its lost quantum behavior by erasing the "whichpath" information already obtained about it. Thus far, quantum erasers were constructed predominantly in optical systems. Here, we present a realization of a quantum eraser in a mesoscopic electronic device. The use of interacting electrons, instead of noninteracting photons, allows control over the extracted information and a smooth variation of the degree of quantum erasure. The demonstrated system can serve as a first step toward a variety of more complex setups.

(2014) TERAHERTZ EMITTERS, RECEIVERS, AND APPLICATIONS V. Baranov AN., Pavlidis D., Zavada JM. & Razeghi M.(eds.). (trueProceedings of SPIE). Abstract
Magneticfield tunable semiconductor detectors are used in THz spectroscopy due to their sensitivity and possibility to respond to photons in a broad frequency range. We compare THz detectors processed on high electron mobility GaAs/GaAlAs and CdTe/CdMgTe quantum wells. Transmission, photocurrent and photovoltage measurements were carried out as a function of the magnetic field at a constant energy of incident THz photons from a THz laser. The samples investigated were gridgated and gridfree. The spectra show features resulting from excitation of the cyclotron resonance and magnetoplasmons. Theoretical models allow to analyze quantitatively the frequency of observed excitations and determine plasmon dispersion relations. This study allows to point at advantages and disadvantages of THz cyclotronresonance and plasmonic detectors fabricated on GaAsand CdTebased quantum wells as well as to compare these two types of devices.

(2014) Terahertz Emitters, Receivers, and Applications V. Baranov AN., Pavlidis D., Zavada JM. & Razeghi M.(eds.). (trueProceedings of SPIE). Abstract
Terahertz detectors based on GaAs/AlGaAs heterostructure were investigated at low temperatures and high magnetic fields. A response of detectors showed a line caused by a cyclotron resonance transition which was accompanied by several peaks originated form excitation of magnetopasmons. Illumination with a visible light caused an increase of the plasma concentration and resulted in a change of the magnetoplasmon spectrum. An analysis of spectra allowed to determine changes in the plasmon dispersion relation with a visible light which gives a tool to tune a THz response of a plasmonic detector.

(2014) Scientific Reports. 4, 3806. Abstract
Multivalued logic gates, which can handle quaternary numbers as inputs, are developed by exploiting the ballistic transport properties of quantum point contacts in series. The principle of a logic gate that finds the minimum of two quaternary number inputs is demonstrated. The device is scalable to allow multiple inputs, which makes it possible to find the minimum of multiple inputs in a single gate operation. Also, the principle of a halfadder for quaternary number inputs is demonstrated. First, an adder that adds up two quaternary numbers and outputs the sum of inputs is demonstrated. Second, a device to express the sum of the adder into two quaternary digits [Carry (first digit) and Sum (second digit)] is demonstrated. All the logic gates presented in this paper can in principle be extended to allow decimal number inputs with high quality QPCs.
2013

(2013) Physical review letters. 110, 14, 146804. Abstract
Two level systems that can be reliably controlled and measured hold promise as qubits both for metrology and for quantum information science. Since a fluctuating environment limits the performance of qubits in both capacities, understanding environmental coupling and dynamics is key to improving qubit performance. We show measurements of the level splitting and dephasing due to the voltage noise of a GaAs singlettriplet qubit during exchange oscillations. Unexpectedly, the voltage fluctuations are nonMarkovian even at high frequencies and exhibit a strong temperature dependence. This finding has impacts beyond singlettriplet qubits since nearly all solid state qubits suffer from some kind of charge noise. The magnitude of the fluctuations allows the qubit to be used as a charge sensor with a sensitivity of 2×10 ^{}8e/√Hz, 2 orders of magnitude better than a quantumlimited rf single electron transistor. Based on these measurements, we provide recommendations for improving qubit coherence, allowing for higher fidelity operations and improved charge sensitivity.

(2013) Physical review letters. 110, 4, 046803. Abstract
We experimentally investigate the charge (isospin) frustration induced by a geometrical symmetry in a triangular triple quantum dot. We observe the groundstate charge configurations of sixfold degeneracy, the manifestation of the frustration. The frustration results in omnidirectional charge transport, and it is accompanied by nearby nontrivial triple degenerate states in the charge stability diagram. The findings agree with a capacitive interaction model. We also observe unusual transport by the frustration, which might be related to elastic cotunneling and the interference of trajectories through the dot. This work demonstrates a unique way of studying geometrical frustration in a controllable way.

(2013) Molecular Beam Epitaxy. p. 121137 Abstract
We describe in some detail the desired characteristics of an exceedingly clean MBE system, its optimal growth conditions, and the main parameters of structures design needed in order to obtain extremely high purity AlGaAsGaAs heterostructures embedding low disorder twodimensional (2D) electron systems. A significant part of the chapter deals with the main scattering mechanisms and, consequently, with a variety of methods of ‘modulation doping’, as it governs the detailed disorder in the 2D electron gas. We discuss the limited applicability of the well known electron mobility, being thus far the main figure of merit for the quality of 2D electron gas, to an observed system behavior in the fractional quantum Hall effect. The implications on basic science as well as on applied one are also put in perspective.
2012

(2012) Acta Physica Polonica A. 122, 6, p. 10961098 Abstract
We have observed a multimode spectrum of magnetoplasmons in the Hall bars processed on a high electron mobility GaAs/AlGaAs heterostructure. We have found that the dispersion relation of these excitation follows square root dependence. Calculated wavelength of the fundamental magnetoplasmon mode fits to the width of sample.

(2012) Physical review letters. 109, 25, 250401. Abstract
Controlled dephasing of electrons, via "which path" detection, involves, in general, coupling a coherent system to a current driven noise source. However, here we present a case in which a nearly isolated electron puddle within a quantum dot, at thermal equilibrium and in millikelvin range temperature, fully dephases the interference in a nearby electronic interferometer. Moreover, the complete dephasing is accompanied by an abrupt π phase slip, which is robust and nearly independent of system parameters. Attributing the robustness of the phenomenon to the Friedel sum rulewhich relates a system's occupation to its scattering phasesproves the universality of this powerful rule. The experiment allows us to peek into a nearly isolated quantum dot, which cannot be accessed via conductance measurements.

(2012) Physical Review B. 86, 11, 115421. Abstract
In this work, we demonstrate that significant changes in electron temperature and nuclear spin polarization can be created by applying an electric current in a twodimensional electron system at Landau level filling factor ν=1/2. The current induced effects on nuclear spins can be attributed to electron heating and the efficient coupling between the nuclear and electron spin systems at ν=1/2. The electron temperature, elevated by the current, can be measured with a thermometer based on the measurement of the nuclear spin relaxation rate. The electron temperature is found to be proportional to the square root of the current density at ν=1/2. Electron spin transitions at ν=2/3 and 1/2 are utilized for the measurement of the current induced changes in nuclear spin polarization. Consistent results are obtained from these two different methods of nuclear magnetometry. The finite thickness of the electron wave function is found to be important for evaluation of the nuclear spin polarization even in a narrow quantum well. The nuclear spin polarization follows a Curie law dependence on the electron temperature. This work also allows us to evaluate the electron g factor in high magnetic fields as well as the polarization mass of composite fermions.

(2012) Nature. 488, 7409, p. 6569 Abstract
Metamaterials with negative refractive indices can manipulate electromagnetic waves in unusual ways, and can be used to achieve, for example, subdiffractionlimit focusing, the bending of light in the ‘wrong’ direction, and reversed Doppler and Cerenkov effects. These counterintuitive and technologically useful behaviours have spurred considerable efforts to synthesize a broad array of negativeindex metamaterials with engineered electric, magnetic or optical properties. Here we demonstrate another route to negative refraction by exploiting the inertia of electrons in semiconductor twodimensional electron gases, collectively accelerated by electromagnetic waves according to Newton’s second law of motion, where this acceleration effect manifests as kinetic inductance. Using kinetic inductance to attain negative refraction was theoretically proposed for threedimensional metallic nanoparticles and seen experimentally with surface plasmons on the surface of a threedimensional metal. The twodimensional electron gas that we use at cryogenic temperatures has a larger kinetic inductance than threedimensional metals, leading to extraordinarily strong negative refraction at gigahertz frequencies, with an index as large as 700. This pronounced negative refractive index and the corresponding reduction in the effective wavelength opens a path to miniaturization in the science and technology of negative refraction.

(2012) Physical Review B. 85, 19, 195329. Abstract
We demonstrate that branching of the electron flow in semiconductor nanostructures can strongly affect macroscopic transport quantities and can significantly change their dependence on external parameters compared to the ideal ballistic case, even when the system size is much smaller than the mean free path. In a cornershaped ballistic device based on a GaAs/AlGaAs twodimensional electron gas, we observe a splitting of the commensurability peaks in the magnetoresistance curve. We show that a model which includes a random disorder potential of the twodimensional electron gas can account for the random splitting of the peaks that result from the collimation of the electron beam. The shape of the splitting depends on the particular realization of the disorder potential. At the same time, magnetic focusing peaks are largely unaffected by the disorder potential.

(2012) Physical review letters. 108, 22, 226801. Abstract
Counterpropagating (upstream) chiral neutral edge modes, which were predicted to be present in holeconjugate states, were observed recently in a variety of fractional quantum Hall states (ν=2/3, ν=3/5, ν=8/3, and ν=5/2), by measuring the charge noise that resulted after partitioning the neutral mode by a constriction (denoted, as N→C). Particularly noticeable was the observation of such modes in the ν=5/2 fractional stateas it sheds light on the nonAbelian nature of the state's wave function. Yet, the nature of these unique, upstream, chargeless modes and the microscopic process in which they generate shot noise, are not understood. Here, we study the ubiquitous ν=2/3 state and report of two main observations: First, the nature of the neutral modes was tested by "colliding" two modes, emanating from two opposing sources, in a narrow constriction. The resultant charge noise was consistent with local heating of the partitioned quasiparticles. Second, partitioning of a downstream charge mode by a constriction gave birth to a dual process, namely, the appearance of an upstream neutral mode (C→N). In other words, splitting "hole conjugated" type quasiparticles will lead to an energy loss and decoherence, with energy carried away by neutral modes.

(2012) Applied Physics Letters. 100, 18, 183502. Abstract
Quantum point contact (QPC) with an extra metallic gate in between the split gates of a conventional QPC was fabricated and studied. Clear conductance quantization was observed at 4.2 K when a proper positive voltage was set to the middle gate of the QPC. The maximum energy spacing between the ground and the first exited state of the QPC was around 7 meV which is at least a few times larger than that of conventional QPCs. Using same approach, a possibility of making a relatively clean and long 1D wire has been tested.

(2012) Science. 336, 6078, p. 202205 Abstract
Quantum computers have the potential to solve certain problems faster than classical computers. To exploit their power, it is necessary to perform interqubit operations and generate entangled states. Spin qubits are a promising candidate for implementing a quantum processor because of their potential for scalability and miniaturization. However, their weak interactions with the environment, which lead to their long coherence times, make interqubit operations challenging. We performed a controlled twoqubit operation between singlettriplet qubits using a dynamically decoupled sequence that maintains the twoqubit coupling while decoupling each qubit from its fluctuating environment. Using state tomography, we measured the full density matrix of the system and determined the concurrence and the fidelity of the generated state, providing proof of entanglement.

(2012) Physical review letters. 108, 6, 066810. Abstract
Resistively detected nuclear magnetic resonance is used to measure the Knight shift of the As75 nuclei and determine the electron spin polarization of the fractional quantum Hall states of the second Landau level. We show that the 5/2 state is fully polarized within experimental error, thus confirming a fundamental assumption of the MooreRead theory. We measure the electron heating under radio frequency excitation and show that we are able to detect NMR at electron temperatures down to 30 mK.

(2012) Nature Communications. 3, 1289. Abstract
Upstream neutral modes, counter propagating to charge modes and carrying energy without net charge, had been predicted to exist in some of the fractional quantum Hall states and were recently observed via noise measurements. Understanding such modes will assist in identifying the wavefunction of these states, as well as shedding light on the role of Coulomb interactions within edge modes. Here, operating mainly in the ν=2/3 state, we place a quantum dot a few micrometres upstream of an ohmic contact, which serves as a 'neutral modes source'. We show that the neutral modes heat the input of the dot, causing a net thermoelectric current to flow through it. Heating of the electrons leads to a decay of the neutral mode, manifested in the vanishing of the thermoelectric current at T>110 mK. This setup provides a straightforward method to investigate upstream neutral modes without turning to the more cumbersome noise measurements.

(2012) Physical Review Letters. 108, 4, Abstract
The evolution of the fractional quantum Hall state at filling 5/2 is studied in density tunable twodimensional electron systems formed in wide wells in which it is possible to induce a transition from single to twosubband occupancy. In 80 and 60 nm wells, the quantum Hall state at 5/2 filling of the lowest subband is observed even when the second subband is occupied. In a 50 nm well, the 5/2 state vanishes upon second subband population. We attribute this distinct behavior to the width dependence of the capacitive energy for intersubband charge transfer and of the overlap of the subband probability densities.
2011

(2011) Physical Review B. 84, 24, 245320. Abstract
The local properties of a highmobility, twodimensional electron gas (2DEG) subjected to an increasing Hall electric field are studied by imaging photoluminescence spectroscopy. It is observed that as the Hall electric field increases, the distribution of a 2DEG density across the sample becomes spatially nonuniform. This nonuniformity is associated with the "gating effect" of the Hall electric field that is screened by lowmobility charges accumulating in the layers parallel to the 2DEG. We consider two mechanisms to explain the 2DEG density redistribution induced by the Hall electric field. The first involves inplane electron transport that results in a linear 2DEG density variation across the Hall bar. The second is activated at a high Hall voltage (>50 mV) and involves vertical electron tunneling out of the 2DEG layer. We conclude that the 2DEG density redistribution can affect the nonlinear magnetotransport phenomena recently studied in GaAs/Al _{x}Ga _{1x}As heterostructures containing a highmobility 2DEG.

(2011) Physical review letters. 107, 3, 036805. Abstract
Fractionally charged quasiparticles, which obey nonAbelian statistics, were predicted to exist in the ν=8/3, ν=5/2, and ν=7/3 fractional quantum Hall states (in the second Landau level). Here we present measurements of charge and neutral modes in these states. For both ν=7/3 and ν=8/3 states, we found a quasiparticle charge e=1/3 and an upstream neutral mode only in ν=8/3excluding the possibility of nonAbelian ReadRezayi states and supporting Laughlinlike states. The absence of an upstream neutral mode in the ν=7/3 state also proves that edge reconstruction was not present in the ν=7/3 state, suggesting its absence also in ν=5/2 state, and thus may provide further support for the nonAbelian antiPfaffian nature of the ν=5/2 state.

(2011) Physical Review B. 83, 24, 245325. Abstract
Coulomb exchange interactions of electrons in the ν=3 quantum Hall state are determined from two interLandaulevel spinflip excitations measured by resonant inelastic light scattering. The two coupled collective excitations are linked to interLandaulevel spinflip transitions arising from the N=0 and N=1 Landau levels. The strong repulsion between the two spinflip modes in the longwavelength limit is clearly manifested in spectra displaying Coulomb exchange contributions that are comparable to the exchange energy for the quantum Hall state at ν=1. Theoretical calculations within the HartreeFock approximation are in a good agreement with measured energies of spinflip collective excitations.

(2011) Physical Review Letters. 106, 20, 206804. Abstract
We investigate the quantum Hall stripe phase at filling factor 9/2 at the microscopic level by probing the dispersion of its collective modes with the help of surface acoustic waves with wavelengths down to 60 nm. The dispersion is strongly anisotropic. It is highly dispersive and exhibits a roton minimum for wave vectors aligned along the easy transport direction. In the perpendicular direction, however, the dispersion is featureless, although not flat as predicted by theory. Oscillatory behavior in the absorption intensity of the collective mode with a wave vector perpendicular to the stripes is attributed to a commensurability effect. It allows us to extract the periodicity of the quantum Hall stripes.

(2011) Nature Physics. 7, 2, p. 109113 Abstract
Qubits, the quantum mechanical bits required for quantum computing, must retain their quantum states for times long enough to allow the information contained in them to be processed. In many types of electronspin qubits, the primary source of information loss is decoherence due to the interaction with nuclear spins of the host lattice. For electrons in gatedefined GaAs quantum dots, spinecho measurements have revealed coherence times of about 1μs at magnetic fields below 100mT (refs 1, 2). Here, we show that coherence in such devices can survive much longer, and provide a detailed understanding of the measured nuclearspininduced decoherence. At fields above a few hundred millitesla, the coherence time measured using a singlepulse spin echo is 30μs. At lower fields, the echo first collapses, but then revives at times determined by the relative Larmor precession of different nuclear species. This behaviour was recently predicted ^{3,4} and can, as we show, be quantitatively accounted for by a semiclassical model for the dynamics of electron and nuclear spins. Using a multiplepulse Carr Gillecho sequence, the decoherence time can be extended to more than 200μs, an improvement by two orders of magnitude compared with previous measurements ^{1,2,5} .

(2011) Physics of Semiconductors: 30th International Conference on the Physics of Semiconductors. Cheong H. & Ihm J.(eds.). p. 927 928 (trueAIP Conference Proceedings). Abstract
We report on investigation of oscillations periodic in the magnetic field observed in magnetoresistence, photocurrent and photovoltage measurements in GaAs/AlGaAs high electron mobility fieldeffect transistor subjected to subterahertz radiation. The spectra show edge magnetoplasmons and cyclotron resonances which can be a basis of tunable resonant subterahertz detectors.

(2011) AIP Conference Proceedings. Vol. 1399. (truePhysics Of Semiconductors: 30Th International Conference On The Physics Of Semiconductors). Abstract
We report on the first experimental observation of neutral modes in particlehole conjugate fractional quantum Hall states using shot noise measurements. The presence of the neutral modes, in addition to producing excess noise in a quantum point contact (QPC), was seen to affect strongly the charge of the tunneling quasiparticles of the charge mode in the QPC and to increase their apparent temperature. The observation of an upstream neutral mode in the 5/2 state may constitute an added indication of its nonabelian nature.
2010

(2010) Japanese Journal of Applied Physics. 49, 11, 114001. Abstract
We present the study of DC currents of an open dot generated from two time dependent electric fields in the absence of external bias. Two electrical setups were applied. In one configuration, two fast oscillating voltages were applied on two side gates; in the other, one of the oscillating biases was directly applied to the source lead. The DC current as a function of frequency, coupling strength, and magnetic field was investigated. The current is sinusoidally dependent with the phase shift and bilinearly dependent with the excitation voltage for both configurations. However, the current as a function of frequency, coupling strength, and magnetic fields behaves differently in these two setups. The results indicate that the currents generated in different setups originate from different mechanisms, and moreover, not from any classical circuitry effect.

(2010) Physical review letters. 105, 21, 216803. Abstract
In many realizations of electron spin qubits the dominant source of decoherence is the fluctuating nuclear spin bath of the host material. The slowness of this bath lends itself to a promising mitigation strategy where the nuclear spin bath is prepared in a narrowed state with suppressed fluctuations. Here, this approach is realized for a twoelectron spin qubit in a GaAs double quantum dot and a nearly tenfold increase in the inhomogeneous dephasing time T2* is demonstrated. Between subsequent measurements, the bath is prepared by using the qubit as a feedback loop that first measures its nuclear environment by coherent precession, and then polarizes it depending on the final state. This procedure results in a stable fixed point at a nonzero polarization gradient between the two dots, which enables fast universal qubit control.

(2010) Journal Of PhysicsCondensed Matter. 22, 39, 395303. Abstract
We have studied the linear conductance and sourcedrain bias spectroscopies of clean and disordered quantum wires (QWs) against thermal cycling and lateral shifting, which change the impurity configuration. Conductance quantization and the zero bias anomaly (ZBA) are robust in clean QWs. In contrast, disordered QWs show complexities in the ways of conductance resonance, peak splitting and trace crossing in sourcedrain bias spectroscopies. The experimental results and theoretical predictions are in congruence. Moreover, the resonant state arising from the impurities results in either a single peak or doublesplitting peaks in the spectroscopies from the detailed impurity configurations. The resonant splitting peaks are found to influence the ZBA, indicating that a clean QW is crucial for investigating the intrinsic characteristics of the ZBA of QWs.

(2010) Journal of Applied Physics. 108, 6, 063522. Abstract
Experimental results examining the photoluminescence spectra of selectively Sidoped GaAs/ Al_{x} Ga_{1x} As heterostructures is presented. Possible mechanisms of carrier recombination are discussed with a special emphasis on the peculiarities of excitonic photoluminescence. Strong intensity lines in photoluminescence spectra are associated with the formation and enhancement of free exciton and excitonpolariton emission in the flat band region of an active i GaAs layer. The excitonic PL intensity is sensitive to the excitation intensity indicating high nonlinear behavior of spectralintegrated photoluminescence intensity and exciton line narrowing. These observed phenomena may be related to the collective interaction of excitons and the interaction of excitons with emitted electromagnetic waves. The gain of the amplification of the excitonic photoluminescence intensity in the heterostructure was found to be more than 1000 times larger than the intensity of i GaAs active layer. The quality factor of the exciton line emission and the excitonpolariton line was found to be 3800 and 7600, respectively.

(2010) Physical review letters. 105, 9, 096801. Abstract
We apply polarization resolved photoluminescence spectroscopy to measure the spin polarization of a two dimensional electron gas in perpendicular magnetic field. We find that the splitting between the σ^{+} and σ^{} polarizations exhibits a sharp drop at ν=5/2 and is equal to the bare Zeeman energy, which resembles the behavior at even filling factors. We show that this behavior is consistent with filling factor ν=5/2 being unpolarized.

(2010) Nature. 466, 7306, p. 585590 Abstract
The quantum Hall effect takes place in a twodimensional electron gas under a strong magnetic field and involves current flow along the edges of the sample. For some particleĝ\u20ac"hole conjugate states of the fractional regime (for example, with fillings between 1/2 and 1 of the lowest Landau level), early predictions suggested the presence of counterpropagating edge currents in addition to the expected ones. When this did not agree with the measured conductance, it was suggested that disorder and interactions will lead to counterpropagating modes that carry only energyĝ\u20ac"the so called neutral modes. In addition, a neutral upstream mode (the Majorana mode) was expected for selected wavefunctions proposed for the evendenominator filling 5/2. Here we report the direct observation of counterpropagating neutral modes for fillings of 2/3, 3/5 and 5/2. The basis of our approach is that, if such modes impinge on a narrow constriction, the neutral quasiparticles will be partly reflected and fragmented into charge carriers, which can be detected through shot noise measurements. We find that the resultant shot noise is proportional to the injected current. Moreover, when we simultaneously inject a charge mode, the presence of the neutral mode was found to significantly affect the Fano factor and the temperature of the backscattered charge mode. In particular, such observations for filling 5/2 may single out the nonAbelian wavefunctions for the state.

(2010) Physical Review B. 81, 23, 235316. Abstract
We present a systematic study on the zerobias conductance peak and its dependences on the carrier density and structural geometry in quasionedimensional quantum wires (QWs). This zerobias anomaly (ZBA) is suppressed by either decreasing the carrier density or increasing the QW length. The differential conductance at zero bias decreases with increasing temperature in accordance with a thermalactivation model up to a welldefined cutoff temperature. We demonstrate that the activation energy, cutoff temperature, and width of the ZBA are correlated, and suggest that these features are controlled by electron scattering in QWs.

(2010) Physical Review B. 81, 16, 161303. Abstract
Charged excitations in the fractional quantum Hall effect are known to carry fractional charges, as theoretically predicted and experimentally verified. Here we report on the dependence of the tunneling quasiparticle charge, as determined via highly sensitive shot noise measurements, on the measurement conditions, in the odd denominators states v=1/3 and v=7/3, and in the even denominator state v=5/2. In particular, for very weak backscattering probability and sufficiently small excitation energies (temperature and applied voltage), tunneling charges across a constriction were found to be significantly higher than the theoretically predicted fundamental quasiparticle charges.

(2010) Physical Review B. 81, 12, 125320. Abstract
We report the observation of inverse magnetic field periodic, radiationinduced magnetoresistance oscillations in GaAs/AlGaAs heterostructures prepared in W. Wegscheider's group, compare their characteristics with similar oscillations in V. Umansky's material, and describe the lineshape variation vs the radiation power, P, in the two systems. We find that the radiationinduced oscillatory resistance, ΔR_{xx}, in both materials, can be described by ΔR_{xx}=A exp(λ/B)sin(2πF/B), where A is the amplitude, λ is the damping parameter, and F is the oscillation frequency. Both λ and F turn out to be insensitive to P. On the other hand, A grows nonlinearly with P.

(2010) Proceedings of the National Academy of Sciences of the United States of America. 107, 12, p. 52765281 Abstract
Interference of edge channels is expected to be a prominent tool for studying statistics of charged quasiparticles in the quantum Hall effect (QHE). We present here a detailed study of an electronic FabryPerot interferometer (FPI) operating in the QHE regime [C. Chamon, et al. (1997) Phys Rev B 55:23312334], with the phase of the interfering quasiparticles controlled by the AharonovBohm effect. Our main finding is that Coulomb interactions among the electrons dominate the interference, even in a relatively large area FPI, leading to a strong dependence of the area enclosed by the interference loop on the magnetic field. In particular, for a composite edge structure, with a few independent edge channels propagating along the edge, interference of the outmost edge channel (belonging to the lowest Landau level) was insensitive to magnetic field  suggesting a constant enclosed flux. However, when any of the inner edge channels interfered, the enclosed flux decreased when the magnetic field increased. By intentionally varying the enclosed area with a biased metallic gate and observing the periodicity of the interference pattern, charges e (for integer filling factors) and e/3 (for a fractional filling factor) were found to be expelled from the FPI. Moreover, these observations provided also a novel way of detecting the charge of the interfering quasiparticles.

(2010) Physica ELowDimensional Systems & Nanostructures. 42, 4, p. 11221125 Abstract
We have measured the electric transport of double quantum point contacts in series at low temperatures. Two pairs of metal gates are placed longitudinally and sequentially with an edgetoedge distance of 600 nm. They are used to form two quantum point contacts in a GaAs / Al_{x} Ga_{1  x} As heterostructure. Isolating from an insulating layer, a top gate is also fabricated on top of the quantum point contacts to modify the electron densities in the quantum point contacts and the two dimensional electron gas as well. The transport is characterized by the direct transmission probability T_{d} which represents the portions of electrons travelling ballistically from one quantum point contact to the other. Our results show that the parameter T_{d} decreases with decreasing carrier density. The transport is partially adiabatic in high 2D electron densities and transits to completely ohmic regimes in low densities. Because of the correlation between the coherence length and transmission probability, we attribute the result to the reduction of the coherence length and mean free path in the unconstricted electron gas between quantum point contacts.

(2010) Physics of Semiconductors. Caldas MJ. & Studart N.(eds.). p. 145146 (trueAIP Conference Proceedings). Abstract
The radiative decay of collective plasma oscillations as a new mechanism for THz emission is studied. This phenomenon is based on the attractive interaction of two intersubband plasmons. This interaction can be viewed as a collective ee scattering phenomenon. The emission results fit very well to the results of the current voltage measurements indicating that the conditions for a plasma instability are reached.

(2010) Physical Review B. 81, 3, 035316. Abstract
A densitytunable GaAsAlGaAs heterostructure is used to study the density dependence of the filling factor ν= 5 2 and other fractional and reentrant integer quantumHall states in the second Landau level. The activation energy at ν= 5 2 can be determined for densities between 1.3 and 2.7× 1011 / cm2 and reaches up to 310 mK. The 5/2 energy gap is calculated numerically, including finite width and Landaulevelmixing corrections, both as a function of electron density. The discrepancy between theory and experiment increases moderately with density and reaches about 1.5 K at the highest density. We argue that the activation energy is strongly influenced by disorder from ionized donors and attribute this to the surprisingly large size of the 5/2 quasiparticles. We find that the quasiparticles have a diameter of at least 12 times the magnetic length or 150 nm at a magnetic field of 4 T. Implications for heterostructure design are discussed.

(2010) Physical Review B. 81, 4, 045316. Abstract
The effect of the surface treatments on the transport properties of a twodimensional electron gas was studied at the quantum limit. The surface of the Al0.36 Ga0.64 As/GaAs heterostructure was either coated with gold or etched with HCl solution, or etched and then coated by a selfassembled monolayer (SAM) of either phosphonated (ODPC18 H39 PO3) or thiolated (ODTC18 H37 S) molecules. The etching process was found to reduce significantly both the mobility and the charge density. This effect was reversed upon sequential adsorption of the phosphonated SAM. We propose fine tuning of the device performance by the flexible chemistry of the assembled molecules, two of them demonstrated here. The results indicate that the surface oxidation does not necessarily play the dominant role in this respect and, in particular, that octadecane phosphonic acid (ODP) can protect the substrate from both oxidation and the formation of a passivating carbon layer. In contrast, octadecanethiol (ODT) is not stable enough and is not effective in eliminating surface states, as a result devices covered with ODT behave like those with etched surfaces.
2009

(2009) Nature Physics. 5, 12, p. 903908 Abstract
One fundamental requirement for quantum computation is to carry out universal manipulations of quantum bits at rates much faster than the qubits rate of decoherence. Recently, fast gate operations have been demonstrated in logical spin qubits composed of two electron spins where the rapid exchange of the two electrons permits electrically controllable rotations around one axis of the qubit. However, universal control of the qubit requires arbitrary rotations around at least two axes. Here, we show that by subjecting each electron spin to a magnetic field of different magnitude, we achieve full quantum control of the twoelectron logical spin qubit with nanosecond operation times. Using a single device, a magneticfield gradient of several hundred millitesla is generated and sustained using dynamic nuclear polarization of the underlying Ga and As nuclei. Universal control of the twoelectron qubit is then demonstrated using quantum state tomography. The presented technique provides the basis for single and potentially multiplequbit operations with gate times that approach the threshold required for quantum error correction.

(2009) Physical review letters. 103, 23, 236802. Abstract
The exact structure of edge modes in "hole conjugate" fractional quantum Hall states remains an unsolved issue despite significant experimental and theoretical efforts devoted to their understanding. Recently, there has been a surge of interest in such studies led by the search for neutral modes, which in some cases may lead to exotic statistical properties of the excitations. In this Letter, we report on detailed measurements of shot noise, produced by partitioning of the more familiar 2/3 state. We find a fractional charge of (2/3)e at the lowest temperature, decreasing to e/3 at an elevated temperature. Surprisingly, strong shot noise had been measured on a clear 1/3 plateau upon partitioning the 2/3 state. This behavior suggests an uncommon picture of the composite edge channels quite different from the accepted one.

(2009) Physical Review B. 80, 19, 193301. Abstract
We report on timeresolved Kerr rotation measurements of spin coherence of electrons in the first excited subband of a highmobility lowdensity twodimensional electron system in a GaAs/ Al0.35 Ga0.65 As heterostructure. While the transverse spin lifetime (T2) of electrons decreases monotonically with increasing magnetic field, it has a nonmonotonic dependence on the temperature and reaches a peak value of 596 ps at 36 K, indicating the effect of intersubband electronelectron scattering on the electronspin relaxation.

(2009) Physical Review B. 79, 20, 205320. Abstract
We report the experimental results from a dark study and a photoexcited study of the highmobility GaAs/AlGaAs system at large filling factors, ν. At large ν, the dark study indicates several distinct phase relations ("type 1," "type 2," and "type 3") between the oscillatory diagonal and Hall resistances, as the canonical integral quantum Hall effect (IQHE) is manifested in the type 1 case of approximately orthogonal diagonal and Hall resistance oscillations. Surprisingly, the investigation indicates quantum Hall plateaus also in the type 3 case characterized by approximately "antiphase" Hall and diagonal resistance oscillations, suggesting an unfamiliar and distinct class of IQHE. Transport studies under microwave photoexcitation exhibit radiationinduced magnetoresistance oscillations in both the diagonal, Rxx, and offdiagonal, Rxy, resistances. Further, when the radiationinduced magnetoresistance oscillations extend into the quantum Hall regime, there occurs a radiationinduced nonmonotonic variation in the amplitude of Shubnikovde Haas (SdH) oscillations in Rxx vs B, and a nonmonotonic variation in the width of the quantum Hall plateaus in Rxy. The latter effect leads into the vanishing of IQHE at the minima of the radiationinduced Rxx oscillations with increased photoexcitation. We reason that the mechanism which is responsible for producing the nonmonotonic variation in the amplitude of SdH oscillations in Rxx under photoexcitation is also responsible for eliminating, under photoexcitation, the type 3 associated IQHE in the highmobility specimen.

(2009) Science. 324, 5930, p. 10441047 Abstract
The rich correlation physics in twodimensional (2D) electron systems is governed by the dispersion of its excitations. In the fractional quantum Hall regime, excitations involve fractionally charged quasi particles, which exhibit dispersion minima at large momenta referred to as rotons. These rotons are difficult to access with conventional techniques because of the lack of penetration depth or sample volume. Our method overcomes the limitations of conventional methods and traces the dispersion of excitations across momentum space for buried systems involving small material volume. We used surface acoustic waves, launched across the 2D system, to allow incident radiation to trigger these excitations at large momenta. Optics probed their resonant absorption. Our technique unveils the full dispersion of such excitations of several prominent correlated ground states of the 2D electron system, which has so far been inaccessible for experimentation.

(2009) Journal of Crystal Growth. 311, 7, p. 16581661 Abstract
Twodimensional electron gas (2DEG) in AlGaAs/GaAs heterostructures, grown by molecular beam epitaxy (MBE), has been a favorite template for numerous research in a field of quantum physics during last several decades. While in the early stages the main efforts were devoted to fabricate extremely highmobility 2DEG by concentrating on the purity of the grown material, nowadays it became clear that the further progress in the field requires new approaches of heterostructures design and the growth procedure. Here we report on the MBE growth of AlGaAs/GaAs heterostructures using a shortperiod superlattice (SPSL) doping instead of the more standard nAlGaAs doping. Such doping process allows the use of a low AlAs mole fraction spacer which, in turn, leads to a lower background of impurities as well as a better interface quality. Mobility exceeding 35×10^{6} cm^{2}/V s was measured in samples with doping introduced on both sides of a quantum well (QW) where the 2DEG was imbedded in. Most importantly the SPSL doping allows introducing "correlations" between ionized donors and allows controlling the potential disorder landscape that governs the appearance of various fractional quantum Hall states.

(2009) Physical review letters. 102, 12, 126806. Abstract
Optical absorption measurements are used to probe the spin polarization in the integer and fractional quantum Hall effect regimes. The system is fully spin polarized only at filling factor ν=1 and at very low temperatures (∼40mK). A small change in filling factor (δν ±0.01) leads to a significant depolarization. This suggests that the itinerant quantum Hall ferromagnet at ν=1 is surprisingly fragile against increasing temperature, or against small changes in filling factor.

(2009) Applied Physics Letters. 94, 19, 192107. Abstract
We report experimental results of the effect of Kaband microwave on the spin dynamics of electrons in a twodimensional electron system (2DES) in a GaAs/ Al_{0.35} Ga_{0.65} As heterostructure via timeresolved Kerr rotation measurements. While the microwave reduces the transverse spin lifetime of electrons in the bulk GaAs, it significantly increases that in the 2DES, from 745 to 1213 ps, when its frequency is close to the Zeeman splitting of the electrons in the magnetic field. Such a microwaveenhanced spin lifetime is ascribed to the microwaveinduced electron scattering which leads to a "motional narrowing" of spins via D'yakonovPerel' mechanism.

(2009) Physical review letters. 102, 3, 036602. Abstract
We observe microwaveinduced photocurrent and photovoltage oscillations around zero as a function of the applied magnetic field in high mobility GaAs 2D electron systems. The photosignals pass zero whenever the microwave frequency is close to a multiple of the cyclotron resonance frequency. They originate from builtin electric fields due to for instance band bending at contacts. The oscillations correspond to a suppression (screening) or an enhancement ("antiscreening") of these fields by the photoexcited electrons.

(2009) 25th International Conference on Low Temperature Physics (LT25). Kes P. & Jochemsen R.(eds.). (trueJournal of Physics Conference Series). Abstract
The electron transport in a short quasi1D quantum wire is studied. In addition to the conductance quantization, several resonances are observed. Two of the resonances appear as extra plateaus below 2e(2)/h. A gate voltage offset is used to tune the local potential of the quantum wire. A robust resonance is seen and the resonances evolve continuously with respect to the offset. The conductance has opposite responses to temperature in successive regions of gate voltage Vg. In the sourcedrain bias spectroscopy, two more conductance peaks are observed in addition to the ZeroBiasAnomaly. The locations of the extra peaks evolve with respect to Vg showing a pattern similar to that in a quantum dot. We suggest that a bound state forms in the quantum wire. The prominent 0.7 anomally in our quantum wire is recognized as the residue of conductance resonance.

(2009) Physical review letters. 102, 4, 046803. Abstract
We report transport and nuclear spin relaxation studies of a density tunable twodimensional electron system at filling v = 1/2 in tilted magnetic fields. The transition from partial to full spin polarization with an inplane field leaves a clear signature in the resistance. Nuclear spin relaxation studies suggest that puddles of minority spins are responsible for an observed nonKorringa temperature dependence. This inhomogeneous spin polarization, similarly encountered in manganites where it strongly affects resistance, may help with understanding the spin dependent transport at v =1/2.
2008

(2008) Physical review letters. 101, 25, 257402. Abstract
In this Letter, we study the diffusion properties of photoexcited carriers in coupled quantum wells around the Mott transition. We find that the diffusion of unbound electrons and holes is ambipolar and is characterized by a large diffusion coefficient, similar to that found in pin junctions. Correlation effects in the excitonic phase are found to significantly suppress the carriers' diffusion. We show that this difference in diffusion properties gives rise to the appearance of a photoluminescence ring pattern around the excitation spot at the Mott transition.

(2008) Physical review letters. 100, 22, 226601. Abstract
We report on the phase measurements on a quantum dot containing a single electron in the Kondo regime. Transport takes place through a single orbital state. Although the conductance is far from the unitary limit, we measure directly, for the first time, a transmission phase as theoretically predicted of π/2. As the dot's coupling to the leads is decreased, with the dot entering the Coulomb blockade regime, the phase reaches a value of π. Temperature shows little effect on the phase behavior in the range 30600 mK, even though both the twoterminal conductance and amplitude of the AharonovBohm oscillations are strongly affected. These results also suggest that previous phase measurements involved transport through more than a single level.

(2008) Physical review letters. 100, 25, 256402. Abstract
In this work we study the phase diagram of indirect excitons in coupled quantum wells and show that the system undergoes a phase transition to an unbound electronhole plasma. This transition is manifested as an abrupt change in the photoluminescence linewidth and peak energy at some critical power density and temperature. By measuring the exciton diamagnetism, we show that the transition is associated with an abrupt increase in the exciton radius. We find that the transition is stimulated by the presence of direct excitons in one of the wells and show that they serve as a catalyst of the transition.

(2008) Physical Review B. 77, 24, 241303. Abstract
We report on noise measurements in a quantum dot in the presence of Kondo correlations. Close to the unitary limit, with the conductance reaching 1.8 e2 h, we observed an average backscattered charge of e* ∼5e 3, while weakly biasing the quantum dot. This result held to bias voltages up to half the Kondo temperature. Away from the unitary limit, the charge was measured to be e as expected. These results confirm and extend theoretical predictions that suggested that twoelectron backscattering processes dominate over singleelectron backscattering processes near the unitary limit, with an average backscattered charge e* ∼5e 3.

(2008) Physical Review B. 77, 19, Abstract
By utilizing timeresolved Kerr rotation techniques, we have investigated the spin dynamics of a highmobility low density twodimensional electron gas in a GaAs/Al(0.35)Ga(0.65)As heterostructure in the dependence on temperature from 1.5 to 30 K. It is found that the spin relaxation/dephasing time under a magnetic field of 0.5 T exhibits a maximum of 3.12 ns around 14 K, which is superimposed on an increasing background with rising temperature. The appearance of the maximum is ascribed to that at the temperature where the crossover from the degenerate to the nondegenerate regime takes place, electronelectron Coulomb scattering becomes strongest, and thus inhomogeneous precession broadening due to the D'yakonovPerel' mechanism becomes weakest. These results agree with the recent theoretical predictions [J. Zhou et al., Phys. Rev. B 15, 045305 (2007)], which verify the importance of electronelectron Coulomb scattering to electron spin relaxation/dephasing.

(2008) Physical Review B. 77, 15, Abstract
We report on an inelastic light scattering study in the twodimensional electron system at filling factor nu=1. The energy and the inelastic cross section of the cyclotron spinflip mode are analyzed. The exchange enhanced electronic g factor in the quantum Hall ferromagnet is measured. In addition to the magnetoplasmon and cyclotron spinflip modes, inelastic scattering lines associated with spinsinglet and spintriplet D complexes are observed. The inelastic light scattering is shown to probe the thermodynamics of the quantum Hall ferromagnet and gives insight into the formation of ferromagnetic order in two dimensions. It is established that at temperatures above the bare Zeeman energy, the ferromagnet breaks up into domains, whose size and number change with temperature. The experimental data allow us to construct a stability diagram of the nu=1 quantum Hall ferromagnet in the temperature versus magnetic field plane.

(2008) Nature. 452, 7189, p. 829834 Abstract
The fractional quantum Hall effect, where plateaus in the Hall resistance at values of h/νe^{2} coexist with zeros in the longitudinal resistance, results from electron correlations in two dimensions under a strong magnetic field. (Here h is Planck's constant, ν the filling factor and e the electron charge.) Current flows along the sample edges and is carried by charged excitations (quasiparticles) whose charge is a fraction of the electron charge. Although earlier research concentrated on odd denominator fractional values of ν, the observation of the even denominator ν = 5/2 state sparked much interest. This state is conjectured to be characterized by quasiparticles of charge e/4, whose statistics are 'nonabelian'in other words, interchanging two quasiparticles may modify the state of the system into a different one, rather than just adding a phase as is the case for fermions or bosons. As such, these quasiparticles may be useful for the construction of a topological quantum computer. Here we report data on shot noise generated by partitioning edge currents in the ν = 5/2 state, consistent with the charge of the quasiparticle being e/4, and inconsistent with other possible values, such as e/2 and e. Although this finding does not prove the nonabelian nature of the ν = 5/2 state, it is the first step towards a full understanding of these new fractional charges.

(2008) Nature Physics. 4, 3, p. 205209 Abstract
The complementarity principle demands that a particle reveals wavelike properties only when the different paths that it can take are indistinguishable. The complementarity has been demonstrated in optics with pairs of correlated photons and in twopath solidstate interferometers with phasecoherent electrons. In the latter experiment, a charge detector embedded near one path of a twopath electron interferometer provided whichpath information. Here, we report on electron dephasing in an AharonovBohm ring interferometer via a charge detector adjacent to the ring. In contrast to the twopath interferometer, charge detection in the ring does not always provide path information. The interference was suppressed only when path information could be acquired, even if only in principle. Thisconfirms that dephasing is not always induced by disturbingthe interfering particle through the interferometer environment interactions: path information of the particle must be available too. Our experiment suggests that acquisition of whichpath information is more fundamental than the backaction in understanding quantum mechanical complementarity.

(2008) Optics InfoBase Conference Papers. QTuC2. Abstract
We couple electrons passing through a twoslit interferometer to electrons tunneling through a FabryPerot interferometer (a quantum dot) at resonance, and demonstrate that the mutual detection of these interferometer currents dephases and suppresses the resonance.
2007

(2007) Physical Review Letters. 99, 24, Abstract
The correlated phases in a twodimensional electron system with a high index partially filled Landau level are studied in transport under nonequilibrium conditions by imposing a dccurrent drive. At filling 1/4 and 3/4 of these Landau levels, where the charge density wave picture predicts an isotropic bubble phase, the dc drive induces anisotropic transport behavior consistent with stripe order. The easy axis of the emerging anisotropic phase is perpendicular to the drive. At half filling the anisotropic stripe phase is stabilized by the dc drive provided drive and easyaxis directions coincide.

(2007) Nature Physics. 3, 8, p. 534537 Abstract
In a controlled dephasing experiment, an interferometer loses its coherence owing to entanglement of the interfering electron with a controlled quantum system, which effectively is equivalent to path detection. In previous experiments, only partial dephasing was achieved owing to weak interactions between many detector electrons and the interfering electron, leading to a gaussianphase randomizing process. Here, we report the opposite extreme, where interference is completely destroyed by a few (that is, one to three) detector electrons, each of which has a strong randomizing effect on the phase. We observe quenching of the interference pattern in a periodic, lobetype fashion as the detector current is varied, and with a peculiar Vshaped dependence on the detectors partitioning. We ascribe these features to the nongaussian nature of the noise, which is also important for qubit decoherence. In other words, the interference seems to be highly sensitive to the full counting statistics of the detectors shot noise.

(2007) Physical Review B. 76, 3, 035316. Abstract
A consistent approach in forming the 0.7 structure by using a quantum dot rather than a quantum point contact is demonstrated. With this scheme, it was possible to tune on and off the 0.7 structure. The 0.7 structure continuously evolved into a normal integer conductance plateau by varying the tuning condition. Unlike the conventional 0.7 plateau, the new 0.7 structure was observed even at low electron temperatures down to 100 mK, with unprecedented flatness. From our results, it is concluded that electron interference should be taken into consideration to explain the 0.7 structure.

(2007) Physical Review B. 76, 3, 035110. Abstract
One of the points at issue with closedlooptype interferometers is beating in the AharonovBohm (AB) oscillations. Recent observations suggest the possibility that the beating results from the Berryphase pickup by the conducting electrons in materials with the strong spinorbit interaction (SOI). In this study, we also observed beats in the AB oscillations in a gatedefined closedloop interferometer fabricated on a GaAs Al0.3 Ga0.7 As twodimensional electrongas heterostructure. Since this heterostructure has very small SOI, the picture of the Berryphase pickup is ruled out. The observation of beats in this study, with the controllability of forming a single transverse subband mode in both arms of our gatedefined interferometer, also rules out the oftenclaimed multiple transverse subband effect. It is observed that nodes of the beats with an h 2e period exhibit a parabolic distribution for varying the side gate. These results are shown to be well interpreted, without resorting to the SOI effect, by the existence of twodimensional multiple longitudinal modes in a single transverse subband. The Fourier spectrum of measured conductance, despite showing multiple h e peaks with the magneticfield dependence that are very similar to that from strongSOI materials, can also be interpreted as the twodimensional multiplelongitudinalmodes effect.

(2007) Nature. 448, 7151, p. 333337 Abstract
Very much like the ubiquitous quantum interference of a single particle with itself, quantum interference of two independent, but indistinguishable, particles is also possible. For a single particle, the interference is between the amplitudes of the particle's wavefunctions, whereas the interference between two particles is a direct result of quantum exchange statistics. Such interference is observed only in the joint probability of finding the particles in two separated detectors, after they were injected from two spatially separated and independent sources. Experimental realizations of twoparticle interferometers have been proposed; in these proposals it was shown that such correlations are a direct signature of quantum entanglement between the spatial degrees of freedom of the two particles ('orbital entanglement'), even though they do not interact with each other. In optics, experiments using indistinguishable pairs of photons encountered difficulties in generating pairs of independent photons and synchronizing their arrival times; thus they have concentrated on detecting bunching of photons (bosons) by coincidence measurements. Similar experiments with electrons are rather scarce. Crosscorrelation measurements between partitioned currents, emanating from one source, yielded similar information to that obtained from autocorrelation (shot noise) measurements. The proposal of ref. 3 is an electronic analogue to the historical Hanbury Brown and Twiss experiment with classical light. It is based on the electronic MachZehnder interferometer that uses edge channels in the quantum Hall effect regime. Here we implement such an interferometer. We partitioned two independent and mutually incoherent electron beams into two trajectories, so that the combined four trajectories enclosed an AharonovBohm flux. Although individual currents and their fluctuations (shot noise measured by autocorrelation) were found to be independent of the AharonovBohm flux, the crosscorrelation between current fluctuations at two opposite points across the device exhibited strong AharonovBohm oscillations, suggesting orbital entanglement between the two electron beams.

(2007) Physical Review B. 75, 19, Abstract
The microwave response of a twodimensional electron system with a nearby in situ grown back gate has been investigated in transport. Several resonant absorption peaks are detected in the magnetoresistance and can be assigned to the excitation of collective magnetoplasma modes. The results are compared with data measured on a similar electron system but without gate. The twodimensional plasma spectrum is drastically altered by the gate and exhibits a linear dispersion instead of the conventional squareroot dependence anticipated from theoretical considerations.

(2007) Advanced Optical Materials, Technologies, and Devices. (trueProceedings of SPIE  The International Society for Optical Engineering). Abstract
Planar microwave detectors on the base of modulation doped AlGaAs/GaAs structures with δdoped layer were investigated in (26÷120) GHz frequency range. Comparison of the features of the microwave diodes on the base of modulation doped structures with δ and smoothlydistributed doping impurities in the AlGaAs barrier is presented. Influence of the layers composing the modulation doped structure onto detective properties of the microwave diodes is ascertained both theoretically and experimentally. In the case of the structure with δdoping this influence was less, especially, in the case of symmetrically shaped structure with nn^{+} and homogeneous asymmetrically shaped modulation doped structure.

(2007) Physical review letters. 98, 18, 186810. Abstract
We study the absorption spectrum of a twodimensional electron gas (2DEG) in a magnetic field. We find that at low temperatures, when the 2DEG is spin polarized, the absorption spectra, which correspond to the creation of spin up or spin down electrons, differ in magnitude, linewidth, and filling factor dependence. We show that these differences can be explained as resulting from the creation of a Mahan exciton in one case, and of a power law Fermiedge singularity in the other.

(2007) Physical review letters. 98, 15, 156803. Abstract
We present measurements of optical interband absorption in the fractional quantum Hall regime in a GaAs quantum well in the range 0

(2007) Physical review letters. 98, 9, 096803. Abstract
Resonant tunneling through two identical potential barriers renders them transparent, as particle trajectories interfere coherently. Here we realize resonant tunneling in a quantum dot (QD), and show that detection of electron trajectories renders the dot nearly insulating. Measurements were made in the integer quantum Hall regime, with the tunneling electrons in an inner edge channel coupled to detector electrons in a neighboring outer channel, which was partitioned. Quantitative analysis indicates that just a few detector electrons completely dephase the QD.

(2007) Physical review letters. 98, 6, 066801. Abstract
Shot noise measurements provide information on particle charge and its correlations. We report on shot noise measurements in a generic quantum dot under a quantized magnetic field. The measured noise at the peaks of a sequence of conductance resonances was some 9 times higher than expected, suggesting bunching of electrons as they traverse through the dot. This enhancement might be mediated by an additional level, weakly coupled to the leads or an excited state. Note that in the absence of a magnetic filed no bunching had been observed.

(2007) Physical review letters. 98, 3, 036803. Abstract
Determination of the path taken by a quantum particle leads to a suppression of interference and to a classical behavior. We employ here a quantum "which path" detector to perform accurate path determination in a twopath MachZehnder electron interferometer, leading to full suppression of the interference. Following the dephasing process we recover the interference by measuring the cross correlation between the interferometer and detector currents. Under our measurement conditions every interfering electron is dephased by approximately a single electron in the detectorleading to mutual entanglement of approximately single pairs of electrons.
2006

(2006) Physica ELowDimensional Systems & Nanostructures. 35, 2, p. 315319 Abstract
We have investigated the influence of the polarization of the incident radiation on the recently discovered microwaveinduced resistance oscillations in stateoftheart highest purity 2D electron systems. A quasioptical setup allows us to tune in situ between different circular as well as linear polarization states. We find that the microwaveinduced zero resistance and the resistance oscillations are notably immune to changes in the polarization. This observation is discrepant with a number of proposed theories. Deviations for different polarizations only occur for a bolometric contribution to the resistance associated with the resonant heating at the cyclotron resonance itself.

(2006) Physical review letters. 96, 1, 016804. Abstract
We report the observation of an unpredictable behavior of a simple, twopath, electron interferometer. Utilizing an electronic analog of the wellknown optical MachZehnder interferometer, with current carrying edge channels in the quantum Hall effect regime, we measured high contrast AharonovBohm (AB) oscillations. Surprisingly, the amplitude of the oscillations varied with energy in a lobe fashion, namely, with distinct maxima and zeros (namely, no AB oscillations) in between. Moreover, the phase of the AB oscillations was constant throughout each lobe period but slipped abruptly by Ï€ at each zero. The periodicity of the lobes defines a new energy scale, which may be a general characteristic of quantum coherence of interfering electrons.
2005

(2005) Nature. 436, 7050, p. 529533 Abstract
The measurement of phase in coherent electron systemsthat is, 'mesoscopic' systems such as quantum dotscan yield information about fundamental transport properties that is not readily apparent from conductance measurements. Phase measurements on relatively large quantum dots recently revealed that the phase evolution for electrons traversing the dots exhibits a 'universal' behaviour, independent of dot size, shape, and electron occupancy. Specifically, for quantum dots in the Coulomb blockade regime, the transmission phase increases monotonically by π throughout each conductance peak; in the conductance valleys, the phase returns sharply to its starting value. The expected mesoscopic features in the phase evolutionrelated to the dot's shape, spin degeneracy or to exchange effectshave not been observed, and there is at present no satisfactory explanation for the observed universality in phase behaviour. Here we report the results of phase measurements on a series of small quantum dots, having occupancies of between only 120 electrons, where the phase behaviour for electron transmission should in principle be easier to interpret. In contrast to the universal behaviour observed thus far only in the larger dots, we see clear mesoscopic features in the phase measurements when the dot occupancy is less than ∼10 electrons. As the occupancy increases, the manner of phase evolution changes and universal behaviour is recovered for some 14 electrons or more. The identification of a transition from the expected mesoscopic behaviour to universal phase evolution should help to direct and constrain theoretical models for the latter.

(2005) Physical Review B. 72, 4, 045320. Abstract
We investigate commensurability oscillations in the magnetoresistances of unstressed ungated rectangular twodimensional superlattices of different periods. The amplitude of the commensurability oscillations in these systems exhibits a nonmonotonic dependence on the applied magnetic field, which is not present in 1D or square superlattices. Furthermore, the high and low resistance directions switch between the two axial directions of the superlattices depending on the magnetic field. Our observations are explained by the drift of the cyclotron motion guiding center along contours of a magneticfielddependent effective potential as put forward in a recent theory. Comparison of the data with the theoretical predictions shows good agreement. For a larger modulation amplitude, we observe a flattening of commensurability oscillation minima, which is also predicted by the calculation.

(2005) Physical Review B. 71, 20, 201306. Abstract
The influence of microwave radiation on the dc magnetoresistance of twodimensional electrons is studied in the regime beyond the recently discovered zero resistance states when the cyclotron frequency exceeds the radiation frequency. Radiation below 30 GHz causes a strong suppression of the resistance over a wide magnetic field range, whereas higher frequencies produce a nonmonotonic behavior in the damping of the Shubnikovde Haas oscillations. These observations are explained by the creation of a nonequilibrium electron distribution function by microwave induced intraLandaulevel transitions.
2004

(2004) Physica ELowDimensional Systems & Nanostructures. 25, 3Feb, p. 227232 Abstract
The spectrum of twodimensional 2D electrons subjected to a weak 2D potential and a perpendicular magnetic field is composed of Landau bands with a fractal internal pattern of subbands and minigaps referred to as Hofstadter's butterfly. The Hall conductance may serve as a spectroscopic tool as each filled subband contributes a specific quantized value. Advances in sample fabrication now finally offer access to the regime away from the limiting case of a very weak potential. Complex behavior of the Hall conductance is observed and assigned to Landau band coupling induced rearrangements within the butterfly. (C) 2004 M.C. Geisler. Published by Elsevier B.V. All rights reserved.

(2004) Physica ELowDimensional Systems & Nanostructures. 25, 23 SPEC.ISS., p. 219226 Abstract
Using a scanning single electron transistor we probe the individual localized states in the integer quantum Hall regime. We find that the observed states are utterly different than those predicted by the singleparticle theory and are mainly determined by Coulomb interactions. They appear only when quantization of kinetic energy limits the screening ability of electrons. Consequently, the QH effect becomes more diverse acquiring new regimes and phase transitions, absent in the singleparticle framework. Our experiments suggest a unified picture of localization within the QH phenomena, in which the validity of the singleparticle model is constrained only to the limit of strong disorder.

(2004) Physical Review B. 70, 15, p. 15531011553105 155310. Abstract
High mobility twodimensional electron systems exhibit vanishing resistance over broad magnetic field intervals upon excitation with microwaves, with a characteristic reduction of the resistance with increasing radiation intensity at the resistance minima. Here, we report experimental results examining the voltagecurrent characteristics, and the resistance at the minima versus the microwave power. The findings indicate that a nonlinear VI curve in the absence of microwave excitation becomes linearized under irradiation, unlike expectations, and they suggest a similarity between the roles of the radiation intensity and the inverse temperature.

(2004) Science. 305, 5686, p. 980983 Abstract
An outstanding question pertaining to the microscopic properties of the fractional quantum Hall effect is understanding the nature of the particles that participate in the localization but that do not contribute to electronic transport. By using a scanning single electron transistor, we imaged the individual localized states in the fractional quantum Hall regime and determined the charge of the localizing particles. Highlighting the symmetry between filling factors 1/3 and 2/3, our measurements show that quasiparticles with fractional charge e*= e/3 localize in space to submicrometer dimensions, where e is the electron charge.

(2004) Physical review letters. 93, 9, 096802. Abstract
We measure the absorption spectrum of a twodimensional electron system (2DES) in a GaAs quantum well in the presence of a perpendicular magnetic field. We focus on the absorption spectrum into the lowest Landau level around [Formula presented]. We find that the spectrum consists of bound electronhole complexes, trionlike and excitonlike. We show that their oscillator strength is a powerful probe of the 2DES spatial correlations. We find that near [Formula presented] the 2DES ground state consists of Skyrmions of small size (a few magnetic lengths).

(2004) Journal of Applied Physics. 95, 11 II, p. 67086710 Abstract
The magnetic vortex pinning at single point defects in submicronsized Permalloy disks was analyzed. The effect of defects on magnetic vortex structures was investigated using microHall magnetometry and micromagnetic simulations. An image reversal electron beam lithography process was used for the fabrication of ferromagnetic particles with artificial point defects. The asymmetry, which was observed in the hysteresis trace with respect to the origin, was due to asymmetric position of the defect on the the disk. The results show that point defects in the ferromagnetic disks alters the corresponding hysteresis traces.

(2004) Physical review letters. 92, 25 I, p. 25680112568014 256801. Abstract
The spectrum of two dimensional electrons subject to a weak two dimensional potential and a perpendicular magnetic field was discussed. The study suggests that it was composed of Landau bands with a fractal internal pattern of subbands and minigaps referred as a Hofstadter's butterfly. It was observed that due to advance in sampling quality and fabrication, it was possible to resolve higher order minigaps in Hofstadter's energy spectrum with quantum Hall effect. The results shows that complex behavior of the Hall conductance assigned to Landau bandcouplinginduced rearrangements within the butterfly.

(2004) Physical Review B. 69, 19, p. 19330411933044 193304. Abstract
We suggest an approach for characterizing the zerofield spin splitting of high mobility twodimensional electron systems, when beats are not readily observable in the Shubnikovde Haas effect. The zerofield spin splitting and the effective magnetic field seen in the reference frame of the electron are evaluated from a quantitative study of beats observed in radiationinduced magnetoresistance oscillations.

(2004) Physical Review B. 69, 16, p. 16130611613064 161306. Abstract
We examine the radiation induced modification of the Hall effect in highmobility GaAs/Al_{x}Ga_{1x}As devices that exhibit vanishing resistance under microwave excitation. The modification in the Hall effect upon irradiation is characterized by (a) a small reduction in the slope of the Hall resistance curve with respect to the dark value, (b) a periodic reduction in the magnitude of the Hall resistance R_{xy} that correlates with an increase in the diagonal resistance R_{xx}, and (c) a Hall resistance correction that disappears as the diagonal resistance vanishes.

(2004) Physical review letters. 92, 14, p. 14680111468014 146801. Abstract
A findependent 1/4cycle phase shift of the extrema in the radiationinduced oscillary magnetoresistance in GsAs/AlGaAs devices was demonstrated. The phase and the period of the magnetoresistance was examined by utilizing in situ magnetic field calibration by electron spin resonance of diphenylpicrylhydrazal. The phase of the R_{xx} oscillation were consistence with a 1/4 cycle phase shifts only at low B. The small reduction in the effective mas ration with respect to the standard value for GaAs/AlGaAs devices.

(2004) Physical review letters. 92, 15, p. 15680111568014 156801. Abstract
The dephasing experiment of a quantum dot in the Kondo regime was performed. Dephasing was induced by a capacitively coupled quantum point contact (QPC), which serves as a which path detector. The qualitative dephasing is similar to that of a QD in the Coulomb blockade regime. The results show that the suppression strength is inversely proportional to the Kondo temperature.

(2004) Nature. 427, 6972, p. 328332 Abstract
The quantum Hall effect arises from the interplay between localized and extended states that form when electrons, confined to two dimensions, are subject to a perpendicular magnetic field. The effect involves exact quantization of all the electronic transport properties owing to particle localization. In the conventional theory of the quantum Hall effect, strongfield localization is associated with a singlepartide drift motion of electrons along contours of constant disorder potential. Transport experiments that probe the extended states in the transition regions between quantum Hall phases have been used to test both the theory and its implications for quantum Hall phase transitions. Although several experiments on highly disordered samples have affirmed the validity of the singleparticle picture, other experiments and some recent theories have found deviations from the predicted universal behaviour. Here we use a scanning singleelectron transistor to probe the individual localized states, which we find to be strikingly different from the predictions of singleparticle theory. The states are mainly determined by Coulomb interactions, and appear only when quantization of kinetic energy limits the screening ability of electrons. We conclude that the quantum Hall effect has a greater diversity of regimes and phase transitions than predicted by the singleparticle framework. Our experiments suggest a unified picture of localization in which the singleparticle model is valid only in the limit of strong disorder.
2003

(2003) Physica ELowDimensional Systems & Nanostructures. 20, 12, p. 143148 Abstract
The energy spectrum of a twodimensional electron system in a perpendicular homogeneous magnetic field and a weak lateral superlattice potential with square symmetry is composed of Landau bands with recursive internal subband structure. The Hall conductance in the minigaps is anticipated to be quantized in integer multiples of e^{2}/h, that vary erratically from minigap to minigap in accordance with a diophantic equation. Hall measurements on samples with the requisite properties uncover this longsearchedfor evidence of Hofstadter's butterflylike energy spectrum.

(2003) Physical review letters. 91, 1, Abstract
We report measurements of spin transitions for GaAs quantum dots in the Coulomb blockade regime and compare ground and excited state transport spectroscopy to direct measurements of the spin polarization of emitted current. Transport spectroscopy reveals both spinincreasing and spindecreasing transitions, as well as higherspin ground states, and allows [Formula presented] factors to be measured down to a single electron. The spin of emitted current in the Coulomb blockade regime, measured using spinsensitive electron focusing, is found to be polarized along the direction of the applied magnetic field regardless of the ground state spin transition.

(2003) Applied Physics Letters. 82, 23, p. 41104112 Abstract
Vortex nucleation in submicrometer ferromagnetic disks was investigated. MicroHall sensors fabricated from GaAs/AlGaAs heterojunction material were used to measure stray field hysteresis loops of individual disks. The analysis showed that vortex formation can be reached via different precursor states.

(2003) Physical Review B. 67, 20, Abstract
Fractionally charged quasiparticles in edge states, are expected to condense to a chiral Luttinger liquid (CLL). We studied their condensation by measuring the conductance and shot noise due to an artificial backscatterer embedded in their path. At sufficiently lowtemperatures backscattering events were found to be strongly correlated, producing a highly nonlinear currentvoltage characteristic and a nonclassical shot noise—both are expected in a CLL. When, however, the impinging beam of quasiparticles was made dilute, either artificially via an additional weak backscatterer or by increasing the temperature, the resultant outgoing noise was classical, indicating the scattering of independent quasiparticles. Here, we study in some detail this surprising crossover from correlated particle behavior to an independent behavior, as a function of beam dilution and temperature.

(2003) Journal of Applied Physics. 93, 10 2, p. 74297431 Abstract
The interaction between magnetic vortices and artificial point defects by using microHall magnetometry was studied. The examination of diskshaped Permalloy particles with thickness from 20 to 60 nm and diameters between 300 and 800 nm, which contain a single lithographically defined defect was performed. For different inplane directions of the applied field, magnetization reversal curves were measured.

(2003) Physical review letters. 91, 21, Abstract
We report the unexpected bunching of Laughlin’s quasiparticles, induced by an extremely weak backscattering potential at exceptionally low electron temperatures ([Formula presented]), deduced from shot noise measurements. Backscattered charges [Formula presented], specifically, [Formula presented], [Formula presented], and [Formula presented], in the respective filling factors, were measured. For the same settings but at a slightly higher electron temperature, the measured backscattered charges were [Formula presented], [Formula presented], and [Formula presented], as expected. Moreover, the backscattered current exhibited distinct temperature dependence that was correlated to the backscattered charge and the filling factor. This observation suggests the existence of “low” and “high” temperature backscattering states, each with its characteristic charge and energy.

(2003) Science. 299, 5607, p. 679682 Abstract
We demonstrate a quantum coherent electron spin filter by directly measuring the spin polarization of emitted current. The spin filter consists of an open quantum dot in an inplane magnetic field; the inplane field gives the two spin directions different Fermi wavelengths resulting in spindependent quantum interference of transport through the device. The gate voltage is used to select the preferentially transmitted spin, thus setting the polarity of the filter. This provides a fully electrical method for the creation and detection of spinpolarized currents. Polarizations of emitted current as high as 70% for both spin directions (either aligned or antialigned with the external field) are observed.

(2003) Physical review letters. 91, 25, Abstract
We demonstrate the operation of a quantum spin pump based on cyclic radiofrequency excitation of a GaAs quantum dot, including the ability to pump pure spin without pumping charge. The device takes advantage of bidirectional mesoscopic fluctuations of pumped current, made spin dependent by the application of an inplane Zeeman field. Spin currents are measured by placing the pump in a focusing geometry with a spinselective collector.
2002

(2002) Nature. 420, 6916, p. 646650 Abstract
The observation of vanishing electrical resistance in condensed matter has led to the discovery of new phenomena such as, for example, superconductivity, where a zeroresistance state can be detected in a metal below a transition temperature T_{c} (ref. 1). More recently, quantum Hall effects were discovered from investigations of zeroresistance states at low temperatures and high magnetic fields in twodimensional electron systems (2DESs). In quantum Hall systems and superconductors, zeroresistance states often coincide with the appearance of a gap in the energy spectrum. Here we report the observation of zeroresistance states and energy gaps in a surprising setting: ultrahighmobility GaAs/AIGaAs heterostructures that contain a 2DES exhibit vanishing diagonal resistance without Hall resistance quantization at low temperatures and low magnetic fields when the specimen is subjected to electromagnetic wave excitation. Zeroresistancestates occur about magnetic fields B = 4/5B_{f} and B = 4/9 B_{f}, where B_{f} = 2πfm*/e,m* is the electron mass, e is the electron charge, and f is the electromagneticwave frequency. Activated transport measurements on the resistance minima also indicate an energy gap at the Fermi level. The results suggest an unexpected radiationinduced, electronicstatetransition in the GaAs/AIGaAs 2DES.

(2002) Physical review letters. 89, 15, p. 157402/1157402/4 157402. Abstract
Significant longrange correlations which exist in the disorder potential is shown. It focuses on broad quantum wells (QWs) in which the shortrange behavior is averaged by the scanning nearfield optical microscopy (SNOM) tip and studies in detail the vertical and lateral longrange order of the exciton energy distribution. It is found that the seemingly random exciton energy fluctuations exhibit a welldefined order.

Anisotropy and periodicity in the density distribution of electrons in a quantum well(2002) Physical Review B. 66, 3, p. 333101333104 033310. Abstract
We use low temperature nearfield optical spectroscopy to image the electron density distribution in the plane of a high mobility GaAs quantum well. We find that the electrons are not randomly distributed in the plane, but rather form narrow stripes (width smaller than 150 nm) of higher electron density. The stripes are oriented along the [110] crystal direction, and are arranged in a quasiperiodic structure. We show that elongated structural mounds, which are intrinsic to molecular beam epitaxy, are responsible for the creation of this electron density texture.

(2002) Physical review letters. 88, 22, p. 226805/1226805/4 226805. Abstract
An overview is given on the experimental evidence that at low temperature the unpaired spin associated with the 0.7 structure forms a Kondolike manybody state. The evidence presented includes: 1) a narrow conductance peak at zero sourcedrain bias that forms at low temperature, 2) collapse of conductance data onto a single function, 3) correspondence between the Kondo scaling factor and the width of the zerobias peak, and 4) splitting of the zerobias peak in a magnetic field.

(2002) Physica Status Solidi AApplied Research. 190, 3, p. 607614 Abstract
We present a microscopic understanding of the underlying physics that governs the photoluminescence spectrum at low electron densities. By performing near and farfield measurements we show how the various characteristics of the spectrum (intensity, energy, width) are affected by the background electron density and the potential fluctuations due to the remote ionized donors.

(2002) Nature. 416, 6880, p. 515518 Abstract
Shot noise measurements have been used to measure the charge of quasiparticles in the fractional quantum Hall (FQH) regime. To induce shot noise in an otherwise noiseless current of quasiparticles, a barrier is placed in its path to cause weak backscattering. The measured shot noise is proportional to the charge of the quasiparticles; for example, at filling factor v = 1/3, noise corresponding to q = e/3 appears. For increasingly opaque barriers, the measured charge increases monotonically, approaching q = e asymptotically. It was therefore believed that only electrons, or alternatively, three bunched quasiparticles can tunnel through highpotential barriers encountered by a noiseless current of quasiparticles. Here we investigate the interaction of e/3 quasiparticles with a strong barrier in FQH samples and find that bunching of quasiparticles in the strong backscattering limit depends on the average dilution of the quasiparticle current. For a very dilute current, bunching ceases altogether and the transferred charge approaches q = e/3. This surprising result demonstrates that quasiparticles can tunnel individually through highpotential barriers originally thought to be opaque for them.

(2002) Physical Review B. 65, 7, Abstract
The evolution of photoluminescence (PL) spectra with increasing magnetic field (Bless than or equal to7 T) was studied in highmobility, wide GaAs/Al0.3Ga0.7As heterojunctions (HJ's) at lattice temperatures TL = 1.925 K. The twodimensional electrongas (2DEG) density in the studied samples is n(2D)(o) =(0.93) x 10(11) cm(2), and it was varied with HeNe laser illumination by optical depletion. For B = 0 the PL is completely dominated by exciton recombination in the undoped GaAs layer. As B increases this band shows a typical exciton diamagnetic shift. For a filling factor nuless than or equal to2 a strong PL transformation is observed: the exciton PL intensity decreases and a new, lowenergy PL line abruptly appears and gains intensity at the expense of the exciton PL. We attribute this line to the 2DEGfree hole recombination, and propose that its appearance in the HJ's results from an increased freeexciton dissociation near the 2DEG at nuless than or equal to2. Thus, the evolution of the bulk free exciton to 2DEGfree hole PL with increasing B is due to "condensation" of the bulk excitons on the magnetized 2DEG layer at nuless than or equal to2.

(2002) Physica ELowDimensional Systems & Nanostructures. 13, 1, p. 8993 Abstract
A new concept to form ballistic quantum wires based on a triple split gate structure on top of a GaAs/AlGaAs heterostructure is presented. Due to the flexibility in the design we propose this method, which would allow one to check the predictions of the Luttinger liquid model. The currentvoltage characteristic of an embedded tunneling barrier in a 2 μm long ballistic quantum wire is also addressed in some detail.

(2002) Physical review letters. 88, 5, p. 568011568014 056801. Abstract
A study of internal transitions of quasitwodimensional, negatively charged magnetoexcitons (X^{}) and their evolution with excess electron density, in GaAs/AlGaAs quantum wells, was presented. The optically detected resonance spectra were dominated by boundtocontinuum bands, in dilute electron limit, in contrast to negatively charged donor system due to magnetic translational invariance. Magnetoplasmons bound to a mobile valence band hole was used to explain the blueshifted transitions.

(2002) Physica ELowDimensional Systems & Nanostructures. 12, 14, p. 216219 Abstract
We have investigated the magnetoresistance of high mobility twodimensional electron gases (2DEG) under the influence of periodically arranged ferromagnetic nanopillars. A huge positive magnetoresistance and commensurability oscillations (COs) indicate that the 2DEG is subjected to a surprisingly strong modulation. Angulardependent magnetoresistance measurements as well as the phase of the COs suggest that the modulation is dominated by electrostatic rather than by magnetic contributions.

(2002) Physical review letters. 89, 6, Abstract
We employ shot noise measurements to characterize the effective charge of quasiparticles, at filling factor [Formula presented] of the fractional quantum Hall regime, as they scatter from an array of identical weak backscatterers. Upon scattering, quasiparticles are known to bunch, e.g., only three [Formula presented] charges, or “electrons” are found to traverse a rather opaque potential barrier. We find here that the effective charge scattered by an array of scatterers is determined by the scattering strength of an individual scatterer and not by the combined scattering strength of the array, which can be very small. Moreover, we also rule out intraedge equilibration of [Formula presented] quasiparticles over a length scale of hundreds of microns.

(2002) Physica ELowDimensional Systems & Nanostructures. 12, 14, p. 524527 Abstract
We studied the evolution of the PL spectra in high mobility wide GaAs/AlGaAs heterojunctions (HJ) at1.9 K with increasing magnetic field (B≤7 T) that was applied perpendicularly to the HJ interface. For different samples the 2DEG density ranges from (0.73) × 10^{11} cm^{2}, and it can be varied by HeNe laser illumination due to optical depletion. At B=0, the PL is completely dominated by free exciton recombination in the undoped GaAs layer. For a filling factor ν≤2 a strong PL transformation is observed: the exciton PL intensity decreases and a lowenergy PL due to the 2DEGfree hole recombination sharply appears and quickly gains intensity at the expense of the exciton PL. We propose that this is due to a "condensation" of the bulk excitons on the magnetized 2DEG layer at ν≤2 where the free exciton dissociates into a 2Delectron and hole.

(2002) Physica ELowDimensional Systems & Nanostructures. 12, 14, p. 574577 Abstract
The cyclotron spinflip excitations of a twodimensional electron system have been investigated by inelastic light scattering (ILS). At small electron filling factors, ν ∼ 0.1, cyclotron spinsinglet and spintriplet excitations have been observed, which are the magnetoplasmon and the three possible spinflip modes that differ by their spin orientations along the axis of the quantizing magnetic field. At larger electron filling factors, ν > 0.2, the electron system is fully spinpolarized. The cyclotron spinwave (CSW) mode reenters in the ILS spectra at an electron filling factor ν > 1. The CSW energy is insensitive to both the experimentally accessible inplane momenta and the electron concentration, whereas its ILS efficiency is strongly influenced by the spin polarization of the electron system.

(2002) Physica ELowDimensional Systems & Nanostructures. 12, 14, p. 208211 Abstract
Twodimensional electron systems were laterally modulated using the method of in situ interferometric illumination. Magnetotransport measurements on 1D and 2D modulated systems revealed a phase change of the commensurability oscillations depending on temperature. This behaviour is surprising and cannot be explained by existing theories.

(2002) Physica ELowDimensional Systems & Nanostructures. 12, 14, p. 97100 Abstract
The compressibility of composite fermion metallic state in twodimensional electron system (2DES) was measured. A comparative study of the derivative of the chemical potential with respect to the carrier density at zero magnetic field was performed. The derivatives were calculated from the bottom of the ground subband. This derivative was inversely proportional to the 2DES compressibility. The contribution of electronelectron interaction was found to be identical for metallic states of electrons at zero magnetic field and of composite fermions at Landau level filling 1/2.

(2002) Physical review letters. 89, 26, Abstract
We demonstrate a mesoscopic spin polarizer/analyzer system that allows the spin polarization of current from a quantum point contact in a large inplane magnetic field to be measured. A transverse electron focusing geometry is used to couple current from an emitter point contact into a collector point contact. At large inplane fields, with the point contacts biased to transmit only a single spin ([Formula presented]), the voltage across the collector depends on the spin polarization of the current incident on it. Spin polarizations of [Formula presented] are found for both emitter and collector at [Formula presented] and [Formula presented] inplane field.
2001

(2001) JETP Letters. 74, 5, p. 270273 Abstract
The cyclotron spinwave mode of a twodimensional electron system have been investigated by inelastic light scattering. It is observed at small electron filling factors, ν ∼ 0.1, when the electron system is spindepolarized. As long as the electron system becomes fully spinpolarized (ν > 0.2), the cyclotron spinwave disappears from the inelastic light scattering spectra. It reenters at electron filling factors ν > 1. Over the range of electron filling factors of 1

(2001) Physical Review B. 63, 12, Abstract
We have studied the capacitance between a twodimensional electron system and a gate of fieldeffect transistors, produced from three different wafers with a single remotely doped GaAs/AlxGa1xAs heterojunction. In the vicinity of the Landaulevel filling factor v = 1/2, the increment of the capacitance relative to its zeromagneticfield value, deltaC(1/2), was found to be insensitive to the carrier density, and close to the value as if the particles are noninteracting. This observation implies that electronelectron interaction affects the compressibility of the zerofield and compositefermion metallic states in a very similar manner.

(2001) Physical Review B  Condensed Matter and Materials Physics. 64, 8, Abstract
The near and farfield photoluminescence (PL) spectra of a gated twodimensional electron gas have been measured in a GaAs quantum well. Scanning nearfield measurements reveal the microscopic origin of the different line shapes of the neutral (X) and negatively charged (formula presented) exciton. We find a new broadening mechanism of the exciton: local density fluctuations give rise to spatial fluctuations of the local X peak energy, and hence to inhomogeneous broadening of the farfield X line. The X linewidth is therefore proportional to the width of the electron density distribution. On the other hand, we find that the (formula presented) is homogeneously broadened, and the numerator of its Lorentzian line shape is linearly proportional to the electron density. We present a simple method to determine low electron densities from the PL spectrum.

(2001) Physical review letters. 86, 1, p. 147150 Abstract
The Hall resistance, the longitudinal resistance, and its temperature dependence are combined to identify minima and even weak shoulder structures modifying the Shubnikovde Haas (SdH) peaks. A transition from monotonous to nonmonotonous behavior of the Hall resistance is confirmed. As a result, clearcut evidence for the internal Landau band structure induced by the potential of the underlying artificial crystal is established.

(2001) Applied Physics Letters. 78, 4, p. 472474 Abstract
We demonstrate the feasibility of monolithic integration of a quantumwell infrared detector and a readout circuit on the same GaAs/AlGaAs crystal. Charge storage capability of 2 × 10^{7} electrons in a 50 × 50 μm^{2} pixel is obtained. The operation of a 5 × 5 test array is reported, performing all the basic functions of a practical focal plane array.
2000

(2000) Physical review letters. 85, 18, p. 39183921 Abstract
A study was carried out, with the aim of extending the range of quantum point contact (QPC) reflection to the strong backscattering limit. In particular, the evolution of an opaque barrier was followed. As a case, the e/3 and e/5 quasiparticles were considered.

(2000) Physical Review B. 62, 15, p. 1031010317 Abstract
We studied the effect of microwave (mw) irradiation on the lowtemperature photoluminescence (PL) of highquality, modulationdoped, wide GaAs/AlxGa1xAs heterojunctions (HJ's) containing a twodimensional electron gas (2DEG), in the density range of (0.94)x 10(11) cm(2). The PL arises from excitons that recombine radiatively in the GaAs buffer layer, far from the 2DEG which is confined close to the GaAs/AlxGa1xAs interface. We observe that the exciton PL is affected by a mw heating of the 2DEG: the mwinduced PL intensity change increases with increasing 2DEG density as well as under a perpendicular magnetic field that corresponds to the 2DEG dimensional magnetoplasma resonance (DMPR) condition. Moreover, the exciton PL intensity shows a bistability at magnetic field strengths that are close to those observed in the DMPR mw absorption. The mwinduced PL modulation effects are interpreted as being due to the interaction of the excitons with lowenergy, ballistically propagating acoustic phonons that are emitted by the mwheated 2DEG. The exciton PL quenching is associated with an exciton drag by the phonon flux towards the opposite lIJ interface where the excitons recombine nonradiatively. The rate of phonon emission is determined by the 2DEG state, and thus the exciton PL responds to the changes of the 2DEG parameters.

(2000) Physica E. 6, 1, p. 8790 Abstract
We investigate the magnetoresistance of a weakly density modulated high mobility twodimensional electron system around filling factor ν = 1/2 . The density modulation with periods a between 300 nm and 500 nm was generated by in situ interferometric illumination. At low B we found commensurability oscillations of the magnetoresistance characteristic for transport in weak electrostatic potentials with minima positions on the magnetic field axis given by 2R_{C} = (λ1/4)a. Here 2R_{C} is the electron cyclotron diameter at the Fermienergy and λ an integer. In contrast, we find clear magnetic commensurability features around filling factor ν = 1/2 with a pronounced minimum for a composite fermion cyclotron radius R_{C,CF} matching 1.25a. This gives direct evidence that the density modulation results in a modulated effective magnetic field for composite fermions. The experimental ρ_{xx}traces around ν = 1/2 are well described by novel model calculations, based on a semiclassical solution of the Boltzmann equation, taking into account anisotropic scattering.
1999

(1999) Physical Review Letters. 83, 11, p. 22342237 Abstract
We present magnetotransport experiments on high mobility twodimensional electron systems subjected to a weak, short period superlattice potential. The imposed periodic potential modifies the contours of constant energy of the free electrons such that new closed kspace trajectories, involving magnetic breakdown, become possible. Their existence is heralded by a novel type of lowfield quantum oscillations.

(1999) Nature. 399, 6733, p. 238241 Abstract
The fractional quantum Hall effect occurs in the conduction properties of a twodimensional electron gas subjected to a strong perpendicular magnetic field. In this regime, the Hall conductance shows plateaux, or fractional states, at rational fractional multiples of e^{2}/h, where e is the charge of an electron and h is Planck's constant. The explanation of this behaviour invokes strong Coulomb interactions among the electrons that give rise to fractionally charged quasiparticles which can be regarded as noninteracting current carriers. Previous studies have demonstrated the existence of quasiparticles with onethird of an electron's charge, the same fraction as that of the respective fractional state. An outstanding ambiguity is therefore whether these studies measured the charge or the conductance. Here we report the observation of quasiparticles with a charge of e/5 in the 2/5 fractional state, from measurements of shot noise in a twodimensional electron gas. Our results imply that charge can be measured independently of conductance in the fractional quantum Hall regime, generalizing previous observations of fractionally charged quasiparticles.

(1999) Physical review letters. 83, 13, p. 26202623 Abstract
Within the composite fermion model, it is anticipated that a periodic variation of the density in a twodimensional electron system at half filling produces a periodic fictitious magnetic field. In regions of smaller density fictitious flux quanta are missing to compensate the external magnetic field. The observation of magnetic commensurability minima in the dctransport response furnishes evidence for this fundamental aspect of the composite fermion picture. In general, composite fermions in these modulated systems behave largely equivalent to electrons moving in a periodic magnetic field.
1998

(1998) Physica BCondensed Matter. 256258, p. 466469 Abstract
We report on timeresolved photoluminescence studies of charged and neutral excitons in a modulation doped GaAs quantum well under resonant excitation and high magnetic field. The radiative lifetime of the charged exciton is rather short, 60 ps at zero field, and is found to increase by a factor of ∼2 up to 7 T. The short lifetimes suggest that, under magnetic field, the exciton bound in the trion is delocalized.

(1998) Physica B. 249251, p. 914917 Abstract
We have investigated magnetotransport through twodimensional lateral superlattices with periods a of 100 and 120 nm prepared on a shallow, high mobility heterojunction. Measurements of the longitudinal and Hall resistance were carried out at a temperature of 50 mK with the bias voltage of the top gate as parameter. The longitudinal resistance displays an unprecedented richness in oscillatory features of both semiclassical and quantum mechanical origin. The results are discussed on the basis of the zero field miniband energy spectrum and the subband splitting of single Landaulevels.

(1998) Physica B. 249251, p. 435439 Abstract
We report phase sensitive measurements of microwave propagation through a high mobility two dimensional electron gas subjected to a perpendicular magnetic field. Two types of configurations were used, one that allows all wave vectors q, and one that selects only specific q values. The spectrum of edge excitations was studied over a broad frequency span, which allowed us to observe the logarithmic dispersion of edge magnetoplasmons.

(1998) Physica B. 251, p. 395400 Abstract
The charge of quasiparticles in the fractional quantum Hall regime was determined via quantum shot noise measurements. The noise is generated by a current flow through a partially transmitting quantum point contact in a two dimensional electron gas and is directly proportional to the charge of the quasiparticles. We measured quantum shot noise at a filling factor of 1/3 and found that the charge of the quasiparticles is e/3 as predicted by Laughlin. (C) 1998 Elsevier Science B.V. All rights reserved.

(1998) Chemical Physics Letters. 283, 56, p. 301306 Abstract
The interactions between adsorbed organic molecules and the electronic charge carriers in specially made GaAs structures are studied by time and wavelengthdependent measurements of the photocurrent. The adsorption of the molecules modifies the photocurrent decay time by orders of magnitude. The effects are molecularly specific, as they depend on the electronic properties and absorption spectrum of the molecules. These observations are rationalized by assuming that new surface states are created upon adsorption of the molecules and that the character of these states is controlled by the relative electronegativity of the substrates and the adsorbed molecules. The relevance for surface passivation and for construction of semiconductorbased sensors is indicated.

(1998) Nature. 391, 6670, p. 871874 Abstract
Waveparticle duality, as manifest in the twoslit experiment, provides perhaps the most vivid illustration of Bohr's complementarity principle: wavelike behaviour (interference) occurs only when the different possible paths a particle can take are indistinguishable, even in principle. The introduction of a whichpath (welcher Weg) detector for determining the actual path taken by the particle inevitably involved coupling the particle to a measuring environment, which in turn results in dephasing (suppression of interference). In other words, simultaneous observations of wave and particle behaviour is prohibited. Such a manifestation of the complementarity principle was demonstrated recently using a pair of correlated photons, with measurement of one photon being used to determine the path taken by the other and so prevent singlephoton interference. Here we report the dephasing effects of a which path detector on electrons traversing a doublepath interferometer. We find that by varying the sensitivity of the detector we can affect the visibility of the oscillatory interference signal, thereby verifying the complementarity principle for fermions.

(1998) Physical Review B. 58, 19, p. 1263712640 Abstract
We study the dynamics of the charged and neutral excitons in a modulationdoped GaAs quantum well by timeresolved photoluminescence under a resonant excitation. The radiative lifetime of the charged exciton is found to be surprisingly short, 60 ps. This time is temperature independent between 2 and 10 K, and increases by a factor of 2 at 6 T. We discuss our findings in view of present theories of exciton radiative decay.
1997

(1997) Applied Physics Letters. 71, 5, p. 683685 Abstract
The results of an organized study of low temperature twodimensional electron gas (2DEG) mobility in high purity selectively doped GaAs/AlGaAs heterostructures are presented. Both technological factors and structure design that determine the mobility are discussed. By varying structure parameters, the various scattering mechanisms are identified. Optimizing the structure leads to a record 2DEG mobility of 14.4 × 10^{6} cm^{2}/Vs at 0.1 K.

Effect of organic molecules on a GaAs transistor(1997) Proceedings Of The Symposium On Chemical And Biological Sensors And Analytical Electrochemical Methods. 97, 19, p. 704710 Abstract
The effects of chemisorbed molecules on the electronic transport through an ungated high electron mobility transistor and through an ungated field effect transistor are examined, Current versus voltage measurements in the dark reveal that the adsorbed molecules and their chemical nature have a pronounced effect on the structures' performance, as they reduce the current by up to one order of magnitude. The molecular specificity of the devices is expressed in the wavelength dependence of photo current decay. The decay time, which increases by several orders of magnitude upon adsorption of the molecules, changes drastically when the excitation wavelength matches the absorption of the adsorbed molecules. Effect of Cu ions caught by adsorbed organic molecules on the photo current decay is clearly demonstrated. The observations open new possibilities in constructing semiconductor based light and chemical sensors.

(1997) Physical Review B  Condensed Matter and Materials Physics. 55, 23, p. 1542715430 Abstract
We study the effect of spatial correlation of ionized donors on the singleparticle scattering time and on spin splitting in a twodimensional electron gas (2DEG). As the correlation is being reduced we observe a reduction in the scattering time and a collapse of the spinsplitted peaks into a single peak. We find these electronic properties to be much more sensitive than the momentum relaxation time (or mobility) in high mobility 2DEG. We compare our results with Monte Carlo simulations and find them to be in partial agreement.

(1997) Nature. 389, 6647, p. 162164 Abstract
Since Millikan's famous oildrop experiments, it has been well known that electrical charge is quantized in units of the charge of an electron, e. For this reason, the theoretical prediction by Laughlin of the existence of fractionally charged 'quasiparticles'proposed as an explanation for the fractional quantum Hall (FQH) effectis very counterintuitive. The FQH effect is a phenomenon observed in the conduction properties of a twodimensional electron gas subjected to a strong perpendicular magnetic field. This effect results from the strong interaction between electrons, brought about by the magnetic field, giving rise to the aforementioned fractionally charged quasiparticles which carry the current. Here we report the direct observation of these counterintuitive entities by using measurements of quantum shot noise. Quantum shot noise results from the discreteness of the current carrying charges and so is proportional to both the charge of the quasiparticles and the average current. Our measurements of quantum shot noise show unambiguously that current in a twodimensional electron gas in the FQH regime is carried by fractional chargese/3 in the present casein agreement with Laughlin's prediction.

(1997) Superlattices and Microstructures. 21, 1, p. 137142 Abstract
Transport measurements were performed on surface superlattices, formed by a grating gate on a twodimensional electron gas. We find a suppression of the differential conductance with increasing electric fields, on a scale of a few V cm^{1}. Even more remarkable is a strong suppression of conductance with increasing temperature T, where the Tdependence is quadratic. We attribute these observations to electronelectron (ee) scattering which, in the presence of an external modulated potential, can have a significant influence on the conductance. We also discuss the role of disorder in these systems.

(1997) Nature. 385, 6615, p. 417420 Abstract
The transport properties of electronic devices are usually characterized on the basis of conductance measurements. Such measurements are adequate for devices in which transport occurs incoherently, but for very small devices such as quantum dotsthe wave nature of the electrons plays an important role. Because the phase of an electron's wavefunction changes as it passes through such a device, phase measurements are required to characterize the transport properties fully. Here we report the results of a doubleslit interference experiment which permits the measurement of the phaseshift of an electron traversing a quantum dot. This is accomplished by inserting the quantum dot into one arm of an interferometer, thereby introducing a measurable phase shift between the arms. We find that the phase evolution within a resonance of the quantum dot can be accounted for qualitatively by a model that ignores the interactions between the electrons within the dot. Although these electrons must interact strongly, such interactions apparently have no observable effect on the phase. On the other hand, we also find that the phase behaviour is identical for all resonances, and that there is a sharp jump of the phase between successive resonance peaks. Adequate explanation of these features may require a model that includes interactions between electrons.

(1997) Physical review letters. 78, 4, p. 705708 Abstract
We present transport measurements on surface superlattices fabricated on a GaAs/AlGaAs twodimensional electron gas. Significant suppression of the conductance is found with increasing electric fields and with increasing temperature T. We attribute these effects to ee scattering, which can significantly affect the resistance in the presence of a spatially modulated static potential.

(1997) Physical Review B  Condensed Matter and Materials Physics. 55, 20, p. R13397R13400 Abstract
We report phasesensitive measurements of microwave propagation through a highmobility twodimensional electron gas subjected to a perpendicular magnetic field. Two types of configurations were used, one that allows all wave vectors q, and one that selects only specific q values. The spectrum of edge excitations was studied over a broad frequency span, which allowed us to observe the logarithmic dispersion of edge magnetoplasmons.

(1997) IEEE Transactions on Electron Devices. 44, 11, p. 18071812 Abstract
We report the fabrication and testing of an allGaAs/AlGaAs hybrid readout circuit operating at 77 K designated for use with an GaAs/AlGaAs backgroundlimited quantumwell infrared photodetector focal plane array (QWIP FPA). The circuit is based on a direct injection scheme, using specially designed cryogenic GaAs/AlGaAs MODFET's and a novel n+GaAs/A!GaAs/n+GaAs semiconductor capacitor, which is able to store more than 15 000 electrons/jum2 in a voltage range of ±0.7 V. The semiconductor capacitor shows little voltage dependence, small frequency dispersion, and no hysteresis. We have eliminated the problem of lowtemperature degradation of the MODFET IV characteristics and achieved very low gate leakage current of about 100 fA in the subthreshold regime. The MODFET electrical properties including inputreferred noise voltage and subthreshold transconductance were thoroughly tested. Inputreferred noise voltage as low as 0.5 /uV/vTlz at 10 Hz was measured for a 2 x 30 fim2 gate MODFET. We discuss further possibilities for monolithic integration of the developed devices.
1996

(1996) Physical review letters. 77, 22, p. 46644667 Abstract
We measure the phase and magnitude of the reflection coefficient of a quantum dot (QD) in the integer quantum Hall regime. This was done by coupling the QD under study to a large QD, serving as an interferometer, and monitoring the phase of the magnetoconductance oscillations of the coupled system. As the Coulomb blockade resonances of the QD are scanned we find two distinct and qualitatively different behaviors of the phase. Our results agree for the most part with the theoretical predictions for resonant tunneling in a noninteracting system.

(1996) Physical Review B  Condensed Matter and Materials Physics. 54, 8, p. R5247R5250 Abstract
We study conductance through a system of weakly coupled narrow wires formed in the plane of a (Formula presented) heterojunction, where a voltage (Formula presented) is applied perpendicular to the wires, in the plane. We find an abrupt onset of current as (Formula presented) is increased beyond a certain critical value. In contrast, when (Formula presented) is swept down there is a gradual decrease in current which extends well below the critical (Formula presented). The abrupt onset and the hysteresis are interpreted in terms of a bistability of the system, where the conducting state is associated with dynamic depopulation of the wires. Additional features in the conductance are also observed and discussed.
1994

(1994) Semiconductor Science and Technology. 9, 5, p. 907910 137. Abstract
Dephasing of ballistic electrons is measured as a function of both temperature and Fermi energy in a highmobility twodimensional electron gas. We find a qualitative agreement between the measured phasebreaking length and the theoretical prediction for the electronelectron scattering length using the value of E(F) measured with largearea Hall bars. A good quantitative agreement is obtained when a local value of E(F), measured via onchip magnetic focusing, is used. The good agreement between the measured phasebreaking length and the theoretical electronelectron scattering length strongly suggests that these two quantities are the same in the ballistic regime.

(1994) Physical review letters. 73, 23, p. 31493152 Abstract
We present a controlled interference experiment of ballistic electrons in a twodimensional electron gas. While the phase along one interfering path is kept constant, the phase along the second interfering path is varied using a biased metallic gate, thereby enabling a direct measurement of the phase accumulated underneath this gate. Surprisingly, in addition to the expected oscillatory signal measured as a function of the gate bias, we observe a longer period signal with approximately half the expected frequency.
1993

(1993) Semiconductor Science and Technology. 8, 7, p. 14901492 Abstract
Electron beam lithography (EBL) With a low accelerating voltage (approximately 2 kV) was utilized for the fabrication of nanostructures. A resolution of 30 nm was achieved for both sparse and dense lines. The high resolution resulted from the low aberrations of the electron optics system of the field emission scanning electron microscope used as an EBL machine and from the preferred smallangle forward scattering characteristic of the lowenergy exposing electrons. By comparison with 50 kV EBL, we show a large reduction in the proximity effect and demonstrate a 60 nm spacing between two large exposed areas. Moreover, it is shown that the critical dose at 2 kV is more than an order of magnitude less than that at 50 kV exposures.