Publications
2022

(2022) Physical Review X. 12, 2, 021057. Abstract
We construct highquality graphenebased van der Waals devices with narrow superconducting niobium nitride (NbN) electrodes, in which superconductivity and a robust fractional quantum Hall (FQH) state coexist. We find a possible signature for crossed Andreev reflection (CAR) across the superconductor separating two FQH edges. Our observed CAR probabilities in the particlelike fractional fillings are markedly higher than those in the integer and holeconjugate fractional fillings and depend strongly on temperature and magnetic field unlike the other fillings. Further, we find a fillingindependent CAR probability in integer fillings, which we attribute to spinorbit coupling in NbN allowing for Andreev reflection between spinpolarized edges. These results provide a route to realize novel topological superconducting phases in FQHsuperconductor hybrid devices based on graphene and NbN.

(2022) Nature Communications. 13, 1, 3032. Abstract
In a Josephson junction (JJ) at zero bias, Cooper pairs are transported between two superconducting contacts via the Andreev bound states (ABSs) formed in the Josephson channel. Extending JJs to multiple superconducting contacts, the ABSs in the Josephson channel can coherently hybridize Cooper pairs among different superconducting electrodes. Biasing threeterminal JJs with antisymmetric voltages, for example, results in a direct current (DC) of Cooper quartet (CQ), which involves a fourfermion entanglement. Here, we report half a flux periodicity in the interference of CQ formed in graphene based multiterminal (MT) JJs with a magnetic flux loop. We observe that the quartet differential conductance associated with supercurrent exhibits magnetooscillations associated with a charge of 4e, thereby presenting evidence for interference between different CQ processes. The CQ critical current shows nonmonotonic bias dependent behavior, which can be modeled by transitions between FloquetABSs. Our experimental observation for voltagetunable nonequilibrium CQABS in fluxloopJJs significantly extends our understanding of MTJJs, enabling future design of topologically unique ABS spectrum.
2021

(2021) Nature Nanotechnology. 16, 5, p. 563569 Abstract
Interferometers probe the wavenature and exchange statistics of indistinguishable particles—for example, electrons in the chiral onedimensional edge channels of the quantum Hall effect (QHE). Quantum point contacts can split and recombine these channels, enabling interference of charged particles. Such quantum Hall interferometers (QHIs) can unveil the exchange statistics of anyonic quasiparticles in the fractional quantum Hall effect (FQHE). Here, we present a fabrication technique for QHIs in van der Waals (vdW) materials and realize a tunable, graphenebased Fabry–Pérot (FP) QHI. The graphiteencapsulated architecture allows observation of FQHE at a magnetic field of 3T and precise partitioning of integer and fractional edge modes. We measure pure Aharonov–Bohm interference in the integer QHE, a major technical challenge in small FP interferometers, and find that edge modes exhibit highvisibility interference due to large velocities. Our results establish vdW heterostructures as a versatile alternative to GaAsbased interferometers for future experiments targeting anyonic quasiparticles.

Induced superconductivity in the fractional quantum Hall edge(2021) arXiv. Abstract
Topological superconductors represent a phase of matter with nonlocal properties which cannot smoothly change from one phase to another, providing a robustness suitable for quantum computing. Substantial progress has been made towards a qubit based on Majorana modes, nonAbelian anyons of Ising (Z2) topological order whose exchange−braiding−produces topologically protected logic operations. However, because braiding Ising anyons does not offer a universal quantum gate set, Majorana qubits are computationally limited. This drawback can be overcome by introducing parafermions, a novel generalized set of nonAbelian modes (Zn), an array of which supports universal topological quantum computation. The primary route to synthesize parafermions involves inducing superconductivity in the fractional quantum Hall (fqH) edge. Here we use highquality graphenebased van der Waals devices with narrow superconducting niobium nitride (NbN) electrodes, in which superconductivity and robust fqH coexist. We find crossed Andreev reflection (CAR) across the superconductor separating two counterpropagating fqH edges which demonstrates their superconducting pairing. Our observed CAR probability of the integer edges is insensitive to magnetic field, temperature, and filling, which provides evidence for spinorbit coupling inherited from NbN enabling the pairing of the otherwise spinpolarized edges. FqH edges notably exhibit a CAR probability higher than that of integer edges once fully developed. This fqH CAR probability remains nonzero down to our lowest accessible temperature, suggesting superconducting pairing of fractional charges. These results provide a route to realize novel topological superconducting phases with universal braiding statistics in fqHsuperconductor hybrid devices based on graphene and NbN.
2020

Interference of Cooper quartet Andreev bound states in a multiterminal graphenebased Josephson junction(2020) arXiv. Abstract
In a Josephson junction (JJ), Cooper pairs are transported via Andreev bound states (ABSs) between superconductors. The ABSs in the weak link of multiterminal (MT) JJs can coherently hybridize two Cooper pairs among different superconducting electrodes, resulting in the Cooper quartet (CQ) involving four fermions entanglement. The energy spectrum of these CQABS can be controlled by biasing MTJJs due to the AC Josephson effect. Here, using gate tunable fourterminal graphene JJs complemented with a flux loop, we construct CQs with a tunable spectrum. The critical quartet supercurrent exhibits magnetooscillation associated with a charge of 4e; thereby presenting the evidence for interference between entangled CQABS. At a finite bias voltage, we find the DC quartet supercurrent shows nonmonotonic bias dependent behavior, attributed to LandauZener transitions between different Floquet bands. Our experimental demonstration of coherent nonequilibrium CQABS sets a path for design of artificial topological materials based on MTJJs.

(2020) Nature (London). 583, 7815, p. 221225 Abstract
Reducing the energy bandwidth of electrons in a lattice below the longrange Coulomb interaction energy promotes correlation effects. Moiré superlattices—which are created by stacking van der Waals heterostructures with a controlled twist angle1,2,3—enable the engineering of electron band structure. Exotic quantum phases can emerge in an engineered moiré flat band. The recent discovery of correlated insulator states, superconductivity and the quantum anomalous Hall effect in the flat band of magicangle twisted bilayer graphene4,5,6,7,8 has sparked the exploration of correlated electron states in other moiré systems9,10,11. The electronic properties of van der Waals moiré superlattices can further be tuned by adjusting the interlayer coupling6 or the band structure of constituent layers9. Here, using van der Waals heterostructures of twisted double bilayer graphene (TDBG), we demonstrate a flat electron band that is tunable by perpendicular electric fields in a range of twist angles. Similarly to magicangle twisted bilayer graphene, TDBG shows energy gaps at the half and quarterfilled flat bands, indicating the emergence of correlated insulator states. We find that the gaps of these insulator states increase with inplane magnetic field, suggesting a ferromagnetic order. On doping the halffilled insulator, a sudden drop in resistivity is observed with decreasing temperature. This critical behaviour is confined to a small area in the density–electricfield plane, and is attributed to a phase transition from a normal metal to a spinpolarized correlated state. The discovery of spinpolarized correlated states in electricfieldtunable TDBG provides a new route to engineering interactiondriven quantum phases.
2019

(2019) Nature Communications. 10, 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.
2018

(2018) Physical Review B. 98, 3, 035418. Abstract
The unique zeroenergy Landau level of graphene has a particlehole symmetry in the bulk, which is lifted at the boundary leading to a splitting into two chiral edge modes. It has long been theoretically predicted that the splitting of the zeroenergy Landau level inside the bulk can lead to many interesting physics, such as quantum spin Hall effect, Diractype singular points of the chiral edge modes, and others. However, so far the obtained splitting with high magnetic field even on a hBN substrate is not amenable to experimental detection, and functionality. Guided by theoretical calculations, here we produce a largegap zeroenergy Landaulevel splitting (similar to 150 meV) with the usage of a onedimensional (1D) superlattice potential. We have created tunable 1D superlattice in a hBN encapsulated graphene device using an array of metal gates with a period of similar to 100 nm and carried out magnetocapacitance spectroscopy as a function of superlattice potential. At zero magnetic field we observe the modification of the density of states in our capacitance measurement which is consistent with the existing literature. At finite perpendicular magnetic field, we monitor the splitting of the zeroth Landau level as a function of superlattice potential. The observed splitting energy is an order higher in magnitude compared to the previous studies of splitting due to the symmetry breaking in pristine graphene. The origin of such large Landaulevel splitting in 1D potential is explained with a degenerate perturbation theory. We find that owing to the periodic potential, the Landau level becomes dispersive, and acquires sharp peaks at the tunable band edges. Our study will pave the way to create the tunable 1D periodic structure for multifunctionalization and device application like graphene electronic circuits from appropriately engineered periodic patterns in near future.

(2018) Proceedings Of The National Academy Of Sciences Of The United States Of AmericaBiological Sciences. 115, 27, p. 69916994 Abstract
A novel nonlocal supercurrent, carried by quartets, each consisting of four electrons, is expected to appear in a voltagebiased threeterminal Josephson junction. This supercurrent results from a nonlocal Andreev bound state (ABS), formed among three superconducting terminals. While in a twoterminal Josephson junction the usual ABS, and thus the dc Josephson current, exists only in equilibrium, the ABS, which gives rise to the quartet supercurrent, persists in the nonlinear regime. In this work, we report such resonance in a highly coherent threeterminal Josephson junction made in an InAs nanowire in proximity to an aluminum superconductor. In addition to nonlocal conductance measurements, crosscorrelation measurements of current fluctuations provided a distinctive signature of the quartet supercurrent. Multiple device geometries had been tested, allowing us to rule out competing mechanisms and to establish the underlying microscopic origin of this coherent nondissipative current.

(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.
2017

(2017) Nano Letters. 17, 12, p. 75207527 Abstract
It was recently shown that in situ epitaxial aluminum coating of indium arsenide nanowires is possible and yields superior properties relative to exsitu evaporation of aluminum (Nat. Mater. 2015, 14, 400406). We demonstrate a robust and adaptive epitaxial growth protocol satisfying the need for producing an intimate contact between the aluminum superconductor and the indium arsenide nanowire. We show that the (001) indium arsenide substrate allows successful aluminum side coating of reclined indium arsenide nanowires that emerge from (111)B microfacets. A robust, induced hard superconducting gap in the obtained indium arsenide/aluminum core/partial shell nanowires is clearly demonstrated. We compare epitaxial sidecoating of round and hexagonal crosssection nanowires and find the surface roughness of the round nanowires to induce a more uniform aluminum profile. Consequently, the extended aluminum grains result in increased strain at the interface with the indium arsenide nanowire, which is found to induce dislocations penetrating into round nanowires only. A unique feature of proposed growth protocol is that it supports in situ epitaxial deposition of aluminum on all three arms of indium arsenide nanowire intersections in a single growth step. Such aluminum coated intersections play a key role in engineering topologically superconducting networks required for Majorana based quantum computation schemes.
2016

(2016) Semiconductor Science and Technology. 31, 11, 115005. Abstract
Selfassisted growth of InAs nanowires on graphene by molecular beam epitaxy is reported. Nanowires with diameter of similar to 50 nm and aspect ratio of up to 100 were achieved. The morphological and structural properties of the nanowires were carefully studied by changing the substrate from bilayer graphene through buffer layer to quasifreestanding monolayer graphene. The positional relation of the InAs NWs with the graphene substrate was determined. A 30 degrees orientation configuration of some of the InAs NWs is shown to be related to the surface corrugation of the graphene substrate. InAs NWbased devices for transport measurements were fabricated, and the conductance measurements showed a semiballistic behavior. In Josephson junction measurements in the nonlinear regime, multiple Andreev reflections were observed, and an inelastic scattering length of about 900 nm was derived.

(2016) Nature. 539, 7629, p. 407410 Abstract
Energy dissipation is a fundamental process governing the dynamics of physical, chemical and biological systems. It is also one of the main characteristics that distinguish quantum from classical phenomena. In particular, in condensed matter physics, scattering mechanisms, loss of quantum information or breakdown of topological protection are deeply rooted in the intricate details of how and where the dissipation occurs. Yet the microscopic behaviour of a system is usually not formulated in terms of dissipation because energy dissipation is not a readily measurable quantity on the micrometre scale. Although nanoscale thermometry has gained much recent interest(115), existing thermal imaging methods are not sensitive enough for the study of quantum systems and are also unsuitable for the lowtemperature operation that is required. Here we report a nanothermometer based on a superconducting quantum interference device with a diameter of less than 50 nanometres that resides at the apex of a sharp pipette: it provides scanning cryogenic thermal sensing that is four orders of magnitude more sensitive than previous devicesbelow 1 mu K Hz(1/2). This non contact, noninvasive thermometry allows thermal imaging of very low intensity, nanoscale energy dissipation down to the fundamental Landauer limit(1618) of 40 femtowatts for continuous readout of a single qubit at one gigahertz at 4.2 kelvin. These advances enable the observation of changes in dissipation due to singleelectron charging of individual quantum dots in carbon nanotubes. They also reveal a dissipation mechanism attributable to resonant localized states in graphene encapsulated within hexagonal boron nitride, opening the door to direct thermal imaging of nanoscale dissipation processes in quantum matter.

(2016) Proceedings of the National Academy of Sciences of the United States of America. 113, 7, p. 17431748 Abstract
Nonlinear charge transport in superconductorinsulatorsuperconductor (SIS) Josephson junctions has a unique signature in the shuttled charge quantum between the two superconductors. In the zerobias limit Cooper pairs, each with twice the electron charge, carry the Josephson current. An applied bias VSD leads to multiple Andreev reflections (MAR), which in the limit of weak tunneling probability should lead to integer multiples of the electron charge ne traversing the junction, with n integer larger than 2 Delta/eV(SD) and Delta the superconducting order parameter. Exceptionally, just above the gap eV(SD) >= 2 Delta, with Andreev reflections suppressed, one would expect the current to be carried by partitioned quasiparticles, each with energydependent charge, being a superposition of an electron and a hole. Using shotnoise measurements in an SIS junction induced in an InAs nanowire (with noise proportional to the partitioned charge), we first observed quantization of the partitioned charge q = e*/e = n, with n = 14, thus reaffirming the validity of our charge interpretation. Concentrating next on the bias region eV(SD) similar to 2 Delta, we found a reproducible and clear dip in the extracted charge to q similar to 0.6, which, after excluding other possibilities, we attribute to the partitioned quasiparticle charge. Such dip is supported by numerical simulations of our SIS structure.
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 channelsthe hallmark of density reconstruction. Here, we provide evidence for such nonequilibrated counterpropagating current channels, in short regions (l = 4 mu m and l = 0.4 mu m) 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 (G(Q) = e(2)/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, 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 = 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.
2013

(2013) Nano Letters. 13, 11, p. 51905196 Abstract
Molecular beam epitaxy growth of merging InAs nanowire intersections, that is, a first step toward the realization of a network of such nanowires, is reported. While InAs nanowires play already a leading role in the search for Majorana fermions, a network of these nanowires is expected to promote their exchange and allow for further development of this field. The structural properties of merged InAs nanowire intersections have been investigated using scanning and transmission electron microscope imaging. At the heart of the intersection, a sharp change of the crystal structure from wurtzite to perfect zinc blende is observed. The performed lowtemperature conductance measurements demonstrate that the intersection does not impose an obstacle to current transport.
2012

(2012) Nature Physics. 8, 12, p. 887895 Abstract
Majorana fermions are the only fermionic particles that are expected to be their own antiparticles. Although elementary particles of the Majorana type have not been identified yet, quasiparticles with Majoranalike properties, born from interacting electrons in the solid, have been predicted to exist. Here, we present thorough experimental studies, backed by numerical simulations, of a system composed of an aluminium superconductor in proximity to an indium arsenide nanowire, with the latter possessing strong spinorbit coupling and Zeeman splitting. An induced onedimensional topological superconductor, supporting Majorana fermions at both ends, is expected to form. We concentrate on the characteristics of a distinct zerobias conductance peak and its splitting in energyboth appearing only with a small magnetic field applied along the wire. The zerobias conductance peak was found to be robustly tied to the Fermi energy over a wide range of system parameters. Although not providing definite proof of a Majorana state, the presented data and the simulations support its existence.

(2012) Nature Communications. 3, Abstract
Entanglement is at the heart of the EinsteinPodolskyRosen paradox, where the nonlocality is a necessary ingredient. Cooper pairs in superconductors can be split adiabatically, thus forming entangled electrons. Here, we fabricate such an electron splitter by contacting an aluminium superconductor strip at the centre of a suspended InAs nanowire. The nanowire is terminated at both ends with two normal metallic drains. Dividing each half of the nanowire by a gateinduced Coulomb blockaded quantum dot strongly impeds the flow of Cooper pairs due to the large charging energy, while still permitting passage of single electrons. We provide conclusive evidence of extremely high efficiency Cooper pair splitting via observing positive twoparticle correlations of the conductance and the shot noise of the split electrons in the two opposite drains of the nanowire. Moreover, the actual charge of the injected quasiparticles is verified by shot noise measurements.