Publications

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.

Studies of electronic interferometers, based on edge-channel transport in the quantum Hall effect regime, have been stimulated by the search for evidence of abelian and non-abelian anyonic statistics of fractional charges. In particular, the electronic Fabry-Pérot interferometer has been found to be Coulomb dominated, thus masking coherent Aharonov-Bohm 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.

This Rapid Communication was motivated by the quest for observing interference of fractionally charged quasiparticles. Here, we study the behavior of an electronic Mach-Zehnder 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. Shot-noise measurements suggest that phase averaging (due to phase randomization), rather than single-particle decoherence, is likely to be the cause of the dephasing in the fractional regime.

Nanoscale device fabrication has enabled remarkable scientific advances. Yet a single broken electrode can render a complex device useless. The authors consider local electron-beam-induced 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.

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 hole-conjugate states and in v 1/2=5/2, flow upstream. It is believed that the latter transport results from multiple counter-propagating channels-mixed by disorder that is accompanied by Coulomb interaction. Here we report on sensitive shot noise measurements that reveal unexpected presence of neutral modes in non-hole-conjugate 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 quasiparticles-preventing observation of fractional statistics.

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.

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 rule-which relates a system's occupation to its scattering phases-proves 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.

Counterpropagating (upstream) chiral neutral edge modes, which were predicted to be present in hole-conjugate 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 state-as it sheds light on the non-Abelian 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.

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.

We present an approach that allows forming a nanometric double dot single electron device. It uses chemical synthesis of metallic nanoparticles to form dimeric structures, e-beam lithography to define electrodes and gates, and electrostatic trapping to place the dimers in between the electrodes. We demonstrate a control of its charge configuration and conductance properties over a wide range of external voltages. This approach can be straightforwardly generalized to other material systems and may allow realizing quantum information devices.

A highly durable and versatile micro scale biocatalytic reactor has been designed and investigated in the heterogeneous enzyme-catalytic oxidation of phenols. It consisted of optical or electron beam lithographic gold deposited layer with different thickness from nano- to micro-scale on a semiconductor surface. Horseradish peroxidase (H RP) enzyme was immobilized on the gold-surface using two different approaches (i.e., physical adsorption and self-assembled monolayer). Characterization of those HRP-gold surfaces was made using AFM (atomic force microscopy) coupled with ellipsometry measurements, SEM (scanning electron microscopy), STM (scanning tunneling microscopy) and spectrophotometric technique. The catalytic performances of the developed structures were investigated after their incorporation in microreactors using a recycling-flow system dedicated to phenols oxidation. In order to optimize the efficiency of the oxidation process it was checked the effect of the successively addition of the reagent, the influence of the reaction time, the stability and the reuse of the biocatalytic micro-system. Maximum phenol conversion was of 35.1%. In addition to phenol the system has also been investigated in oxidation of other phenolic compounds such as cathecol, hydroquinone, rezorcine, beta-naphtol, o/p-aminophenol and 4-metoxyphenol. (C) 2011 Elsevier B.V. All rights reserved.

We report on the first experimental observation of neutral modes in particle-hole 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 non-abelian nature.

We report on observation of coherent electron transport in suspended high-quality InAs nanowire-based devices. The InAs nanowires were grown by low-temperature gold-assisted vapor-liquid-solid molecular-beam-epitaxy. The high quality of the nanowires was achieved by removing the typically found stacking faults and reducing possibility of Au incorporation. Minimizing substrate-induced scattering in the device was achieved by suspending the nanowires over predefined grooves. Coherent transport involving more than a single one-dimensional mode transport was observed in the experiment and manifested by Fabry-Pérot conductance oscillations. The length of the Fabry-Pérot interferometer, deduced from the period of the conductance oscillations, was found to be close to the physical length of the device. The high oscillations visibility imply nearly ballistic electron transport through the nanowire.

New nano applications, like T-gates, bridges, mirror arrays, blazed gratings, 3D zone plates, 3D holograms and MEMS devices require highly accurate 3D patterning of resist. De facto, Electron-beam (EB) lithography is a process for high resolution patterning in lateral dimensions. The aforementioned 3D applications, imply accurate control of resist thickness, after development, thus critically depending on proximity effect corrections (PEG) in the third dimension. We show a feasibility test for a new 3D PEC approach, using Layout BEAMER e-beam lithography software. (C) 2009 Elsevier B.V. All rights reserved.

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 two-electron backscattering processes dominate over single-electron backscattering processes near the unitary limit, with an average backscattered charge e* ∼5e 3.

The complementarity principle demands that a particle reveals wave-like 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 two-path solid-state interferometers with phase-coherent electrons. In the latter experiment, a charge detector embedded near one path of a two-path electron interferometer provided which-path information. Here, we report on electron dephasing in an Aharonov-Bohm ring interferometer via a charge detector adjacent to the ring. In contrast to the two-path 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 which-path information is more fundamental than the back-action in understanding quantum mechanical complementarity.

One of the points at issue with closed-loop-type interferometers is beating in the Aharonov-Bohm (AB) oscillations. Recent observations suggest the possibility that the beating results from the Berry-phase pickup by the conducting electrons in materials with the strong spin-orbit interaction (SOI). In this study, we also observed beats in the AB oscillations in a gate-defined closed-loop interferometer fabricated on a GaAs Al0.3 Ga0.7 As two-dimensional electron-gas heterostructure. Since this heterostructure has very small SOI, the picture of the Berry-phase 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 gate-defined interferometer, also rules out the often-claimed 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 two-dimensional multiple longitudinal modes in a single transverse subband. The Fourier spectrum of measured conductance, despite showing multiple h e peaks with the magnetic-field dependence that are very similar to that from strong-SOI materials, can also be interpreted as the two-dimensional multiple-longitudinal-modes effect.

First-order phase transitions are known to be accompanied by hysteresis due to the formation of domains with different order parameters. As the transition is approached, the domain sizes increase to macroscopic dimensions. It is therefore natural to expect that hysteresis will be absent from samples of microscopic size. Here, we explore from a microscopic standpoint the hysteretic behaviour across the spin phase transition that occurs at filling nu = 2/3 in the fractional-quantum- Hall regime(1-5). Using a single-electron transistor, we follow the evolution of localized states across the spin transition by measuring the local compressibility. Localized-state spectra clearly reveal the hysteretic behaviour accompanying the transition. Using electrostatic gating we continuously vary the size of the sample undergoing the phase transition. For submicrometre dimensions the hysteresis disappears, indicating domain sizes in excess of 500 nm.

A method for fabricating atom chips with a lithographic lift-off process was discussed. Wires that can tolerate high current densities of >10 ⁷ A/cm² were produced with this method. It was found that the fabrication process leads to very accurate edge and bulk features, limited by the grain size of 50-80 nm. Among the materials tested, silicon was found to be the best suited substrate for atom chips.

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.

We present a novel method for fabrication of contacts to nanosize particles. The method is based on conventional optical lithography of GaAs/AlGaAs molecular beam epitaxy grown structures. Taking advantage of the difference in etch rate between GaAs and AlGaAs a narrow gap is formed between metal contacts deposited on the side of a mesa structure. We demonstrate electrostatic trapping of charged metal clusters into these structures using alternating electric fields.

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 high-potential 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 high-potential barriers originally thought to be opaque for them.

How localized electrons interact with delocalized electrons is a question central to many of the problems at the forefront of solid state physics. The simplest example, the Kondo effect, occurs when an impurity atom with an unpaired electron is placed in a metal, and the energy of the unpaired electron is far below the Fermi energy. At low temperatures a spin singlet state is formed between the unpaired localized electron and delocalized electrons at the Fermi energy. The consequences of this singlet formation were first observed over 60 years ago in metals with magnetic impurities, but full theoretical understanding was slow to come. Today, the situation is reversed: scaling theories and recent renormalization group calculations (T.A. Costi, A.C. Hewson (1994) J. Phys.: Cond. Mat. 6, 2519) can predict quantitatively the bonding strength of the singlet state, and the singlet's effect on the conduction electrons at all temperatures. The detailed dependence of these properties on parameters such as the energy of the localized electron cannot be tested experimentally in the classic Kondo systems, since the relevant parameters cannot easily be tuned for impurities in a metal. Recently it has become possible to test these predictions with a new experimental approach creating an artificial Kondo system by nanofabrication (D. Goldhaber-Gordon et al. (1998), Nature 391, 156). The confined droplet of electrons interacting with the leads of a single electron transistor (SET) is closely analogous to an impurity atom interacting with the delocalized electrons in a metal, as described in the Anderson model (Y. Meir, N.S. Wingreen, P.A. Lee, Phys. Rev. Lett. (1993) 70 2601-2604). We review here measurements on a new generation of SETs that display all the aspects of the Kondo effect: the spin singlet forms and causes an enhancement of the zero-bias conductance when the number of electrons on the artificial atom is odd but not when it is even. The singlet is altered by applying

A single electron transistor is used as a local electrostatic probe to study the underlying spatial structure of the metal-insulator transition in two dimensions. The measurements show that as we approach the transition from the metallic side, a new phase emerges that consists of weakly coupled fragments of the two-dimensional system. These fragments consist of localized charge that coexists with the surrounding metallic phase. As the density is lowered into the insulating phase, the number of fragments increases on account of the disappearing metallic phase. The measurements reveal that the metal-insulator transition is a result of the microscopic restructuring that occurs in the system.

Keywords: QUANTUM-DOT; ANDERSON MODEL; TRANSPORT

We have measured the local electronic compressibility of a two-dimensional hole gas as it crosses the B = 0 metal-insulator transition. In the metallic phase, the compressibility follows the mean-field Hartree-Fock (HF) theory and is found to be spatially homogeneous. In the insulating phase it deviates by more than an order of magnitude from the HF predictions and is spatially inhomogeneous. The crossover density between the two types of behavior agrees quantitatively with the transport critical density, suggesting that the system undergoes a thermodynamic change at the transition.

Wave-particle duality, as manifest in the two-slit experiment, provides perhaps the most vivid illustration of Bohr's complementarity principle: wave-like behaviour (interference) occurs only when the different possible paths a particle can take are indistinguishable, even in principle. The introduction of a which-path (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 single-photon interference. Here we report the dephasing effects of a which path detector on electrons traversing a double-path 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.

We study the magnitude of the Weiss magnetoresistance commensurability oscillations in surface superlattices, in samples with a long sequence of such features. The high index oscillations are suppressed faster than the conventional exponential term—apparently by a term of the form (Formula presented). We discuss the role of small-angle scattering in the suppression of these oscillations and suggest a heuristic derivation of this envelope function.

Since Millikan's famous oil-drop 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) effect-is very counterintuitive. The FQH effect is a phenomenon observed in the conduction properties of a two-dimensional 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 two-dimensional electron gas in the FQH regime is carried by fractional charges-e/3 in the present case-in agreement with Laughlin's prediction.

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 dots-the 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 double-slit interference experiment which permits the measurement of the phase-shift 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.

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.

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.

Wideband shot noise, associated with dc current flow through a quantum point contact (QPC), is measured in the microwave frequency range of 8-18 GHz. As the number of conducting channels in the QPC changes the noise power oscillates. Consistent with existing theories, the noise peaks depend linearly on the dc current. Surprisingly, however, in the pinch off region, where QPC is expected to behave as a classical injector, we find strong noise suppression, possibly mediated by the Coulomb interaction.

We present a controlled interference experiment of ballistic electrons in a two-dimensional 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.