Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session F46: Integer Quantum Hall Effect: Coupled Layers, Superconductivity, and InterferometryLive
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Sponsoring Units: DCMP Chair: Inti Sodemann, Max Planck Institute for the Physics of Complex Systems |
Tuesday, March 16, 2021 11:30AM - 11:42AM Live |
F46.00001: Screening current enabled Andreev reflection in a chiral quantum Hall edge state Andreas Bock Michelsen, Patrik Recher, Bernd H. Braunecker, Thomas L Schmidt The phenomenon of induced superconductivity in a spin-polarized chiral quantum Hall edge state through the superconducting proximity effect is currently under investigation due to the macroscopy and stability of the involved quantum phenomena, which has lead to predicted applications in quantum technology. Recent experiments have observed the phenomenon through signatures of the mediating process of Andreev reflection, but lack support in theoretical works. We address this by modelling the system with a many-body Hamiltonian motivated by experiments, and integrate out the superconductor to get an effective pairing Hamiltonian in the quantum Hall edge state. In addition to clarifying the qualitative appearance of nonlocal superconducting correlations in a chiral edge state, we predict the effect of quantitative experimental conditions on these correlations. In particular, it follows how the induced screening current induced by the Meissner effect at the surface of the superconductor plays an essential role for the chiral Andreev reflection. |
Tuesday, March 16, 2021 11:42AM - 11:54AM Live |
F46.00002: Magneto-transport in near surface InAs quantum wells in contact with NbTiN Mehdi Hatefipour, William a Mayer, Noah Kurt Goss, William Strickland, Joseph Yuan, Kaushini S Wickramasinghe, Kasra Sardashti, Tzu-Ming Lu, Javad Shabani Indium Arsenide (InAs) near surface quantum wells have become the recent focus in topological superconductivity for their use in heterostructures with superconductors. An interface between a superconductor and a quantum Hall edge is predicted to exhibit excitations with non-abelian statistics. A promising candidate for this phenomenon is NbTiN, which can sustain strong magnetic fields in proximity to InAs, which hosts integer quantum Hall states. In this work, we study the proximity effect by monitoring the current flow along the superconductor-semiconductor interface. We find that enhanced conductance could be due to Andreev reflections in the quantum hall regime. In further characterization, we fabricate a hybrid device containing both a superconductor-semiconductor interface and a normal-semiconductor interface, in order to separate the normal and Andreev reflections in presence of strong magnetic fields. |
Tuesday, March 16, 2021 11:54AM - 12:06PM Live |
F46.00003: Role of interactions on topological flat bands in twisted bilayer graphene at high magnetic field Muhammad Imran, Yafis Barlas We study the role of interactions in small angle twisted bilayer graphene in the presence of high |
Tuesday, March 16, 2021 12:06PM - 12:18PM Live |
F46.00004: DMRG study of strongly interacting Z2 flatbands: a toy model inspired by twisted bilayer graphene Paul Eugenio, Ceren Dag Strong interactions between electrons occupying bands of opposite (or like) topological quantum numbers (Chern=±1), and with flat dispersion, are studied by using lowest Landau level (LLL) wavefunctions. More precisely, we determine the ground states for two scenarios at half-filling: (i) LLL's with opposite sign of magnetic field, and therefore opposite Chern number; and (ii) LLL's with the same magnetic field. In the first scenario -- which we argue to be a toy model inspired by the chirally symmetric continuum model for twisted bilayer graphene -- the opposite Chern LLL's are Kramer pairs, and thus there exists time-reversal symmetry (Z2). Turning on repulsive interactions drives the system to spontaneously break time-reversal symmetry -- a quantum anomalous Hall state described by one particle per LLL orbital, either all positive Chern |++...+> or all negative |--...+>. The ground states and some of their excitations for both of these scenarios are argued analytically, and further complimented by density matrix renormalization group (DMRG) and exact diagonalization. |
Tuesday, March 16, 2021 12:18PM - 12:30PM Live |
F46.00005: Density dependence of the excitation gap in Si/SiGe bilayers Davis Chen, Suyang Cai, Nai-Wen Hsu, Shi-Hsien Huang, Yen Chuang, Jiun-Yun Li, Chee Wee Liu, Tzu-Ming Lu, Dominique Laroche We report low-temperature magneto-transport measurements of an undoped Si/SiGe antisymmetric double quantum well heterostructure . The density in both layers is tuned independently utilizing a top and a bottom gate, allowing the investigation of both density matched and mis-matched quantum wells. A cross-over density, distinguishing the single-layer from the bi-layer regime, is clearly observed from the mobility versus density curve. Additionally, the integer quantum Hall states at total filling factor νT=1,2 are observed at both matched and mis-matched densities. These states arise from inter-layer effects; either through inter-layer coherence, or through the symmetric-antisymmetric tunneling gap. To disentangle these two mechanisms, the evolution of the filling fraction’s excitation gap is studied as a function of density. |
Tuesday, March 16, 2021 12:30PM - 12:42PM Live |
F46.00006: Quantum Transport in n-type Tellurium Quantum Well Chang Niu, Gang Qiu, Yixiu Wang, Wenzhuo Wu, Peide (Peter) Ye Tellurium (Te) is a narrow bandgap semiconductor with a unique chiral crystal structure. Quantum transport on n-type two-dimensional (2D) Te, realized by the atomic layer deposition doping technique, reveals its topological nature with Berry phase in the conduction band. In this work, we fabricated double-gated n-type Te Hall–bar devices, which can operate as two separate or coupled top and bottom layers controlled by the top gate and back gate independently. Profound Shubnikov-de Haas (SdH) oscillations are observed in both top and bottom layers. Temperature and density dependence of Landau levels of both layers are measured and analyzed. For example, under high magnetic field of 31T, we can identify two composite states such as ν=6 or 8, which are made of two quantum Hall states νb=4 from the bottom layer and νt=2 or 4 from the top layer. |
Tuesday, March 16, 2021 12:42PM - 12:54PM Live |
F46.00007: A tunable Fabry-Pérot quantum Hall interferometer in graphene Corentin Déprez, Louis Veyrat, Hadrien Vignaud, Goutham Nayak, Kenji Watanabe, Takashi Taniguchi, Frédéric Gay, Hermann Sellier, Benjamin Sacepe Electron interferometry with quantum Hall edge channels holds promise to probe and harness exotic exchange statistics of anyons. In semiconductor heterostructures, however, quantum Hall interferometry has proven challenging and often obscured by charging effects. In this talk, I will show that high-mobility monolayer graphene equipped with a series of gate-tunable quantum point contacts provides a model system to perform Fabry-Pérot quantum Hall interferometry. In the integer quantum Hall regime, we observe high-visibility Aharonov-Bohm interference free of charging effects and widely tunable through electrostatic gating or magnetic field in remarkable agreement with theory. I will also show that the coherence length of 10 μm allows us to further achieve coherently-coupled double Fabry-Pérot interferometry, as envisioned in braiding schemes of non-Abelian anyons. |
Tuesday, March 16, 2021 12:54PM - 1:06PM Live |
F46.00008: Modeling the local signatures of edge magnetoplasmons in a quantum Hall insulator Julien Alfaro, Taige Wang, Alex Cauchon, Monica Allen The quantum Hall effect is a 2D topological state of matter characterized by a quantized Hall resistivity and vanishing longitudinal resistivity. This project utilizes COMSOL finite element simulations to model the experimental signatures of quantum Hall edge currents that would be detectable using microwave impedance microscopy (MIM). Microwaves are emitted from a metallic probe tip at a distance from the sample shorter than the wavelength. The reflection coefficient of microwaves, which provides information on the complex impedance of the system, reveal topological plasmonic excitations that are confined to the sample boundaries. |
Tuesday, March 16, 2021 1:06PM - 1:18PM Live |
F46.00009: Emergence of spin-active channels at a quantum Hall interface Amartya Saha, Suman Jyoti De, Sumathi Rao, Yuval Gefen, Ganpathy N Murthy We study the ground state of a system with an interface between ν=4 and ν=3 in the quantum Hall regime. Far from the interface, for a range of interaction strengths, the ν=3 region is fully polarized but ν=4 region is locally a singlet. Upon varying the strength of the interactions and the width of the interface, the system chooses one of two distinct edge/interface phases. In phase A, stabilized for wide interfaces, spin is a good quantum number, and there are no gapless long-wavelength spin fluctuations. In phase B, stabilized for narrow interfaces, spin symmetry is spontaneously broken at the Hartree-Fock level. Going beyond Hartree-Fock, we argue that phase B is distinguished by the emergence of gapless long-wavelength spin excitations bound to the interface, which can, in principle, be detected by a measurement of the relaxation time T2 in nuclear magnetic resonance. |
Tuesday, March 16, 2021 1:18PM - 1:30PM Live |
F46.00010: Gapless Spin Wave Transport Through a Quantum Canted-Antiferromagnet Hailong Fu, Ke Huang, Kenji Watanabe, Takashi Taniguchi, Jun Zhu The Landau levels of graphene and magic-angle van der Waals heterostructures support quantum Hall ferromagnetism, where magnetism spontaneously develops in the spin and pseudospin degrees of freedom due to strong interactions. A quantum Hall ferromagnet can support a rich variety of gapped and gapless spin wave excitations, some of which may be useful as quantum information carriers. In this work, we demonstrate an all-electrical approach to obtain the dispersion relation ω(k) of spin wave excitations in bilayer graphene using transport devices that integrate a Fabry-Pérot cavity to resonantly select discrete wave vectors of the spin wave. We report on the observation of gapless, linearly dispersed spin wave excitations in the ν = 0 Landau level of bilayer graphene. The gapless spin wave propagates with a high group velocity of several tens of km/s and maintains long-distance phase coherence. This result provides direct experimental evidence for a predicted easy-plane, canted anti-ferromagnetic order in this material and lays the foundation for the exploration of spin superfluidity. The resonant cavity technique we developed can be generalized to investigate other collective excitation of spin and pseudospin symmetry-broken ground states. |
Tuesday, March 16, 2021 1:30PM - 1:42PM Live |
F46.00011: Current correlations of Cooper-pair tunneling into a quantum Hall system Andreas Michelsen, Thomas L Schmidt, Edvin Idrisov We study Cooper pair transport through a quantum point contact between a superconductor and a quantum Hall edge state at integer and fractional filling factors. We calculate the tunnelling current and its finite-frequency noise to the leading order in the tunneling amplitude for dc and ac bias voltage in the limit of low temperatures. At zero temperature and in case of tunnelling into a single edge channel both the conductance and differential shot noise vanish as a result of Pauli exclusion principle. In contrast, in the presence of two edge channels, this Pauli blockade is softened and a non-zero conductance and shot noise are revealed. |
Tuesday, March 16, 2021 1:42PM - 1:54PM Live |
F46.00012: Probing Andreev edge state with shot noise Manas Ranjan Sahu, Arup Kumar Paul, Jagannath Sutradhar, Kenji Watanabe, Takashi Taniguchi, Vibhor Singh, Subroto Mukerjee, Sumilan Banerjee, Anindya Das The existence of Andreev edge states (AESs) is one of the prerequisites to realize emergent excitations like Majorana fermion at the interface of normal or anomalous quantum Hall (QH) insulators and superconductor (SC). At a QH-SC interface the hybridized electron-hole states called AESs carry the current in the zero bias limit, whereas normal quasiparticles carry the current when the bias energy is higher than the superconducting gap. Here, we report on measurements of both electrical conductance and shot noise in a graphene QH and SC junction at integer filling ν=2. Remarkably, the Fano factor of the shot noise approaches to half when the bias energy is less than the superconducting gap, whereas it is close to zero above the superconducting gap. This is striking, given that, at the same time the electrical conductance remains at 2e2/h within and above the superconducting gap. These observations are in accord with our theoretical analysis based on non-equilibrium Green’s function (NEGF) calculation with disorders, pointing to complete de-phasing of interference of AESs along the QH-SC interface. Our results with unambiguous evidence of AESs at the QH-SC interface pave the way forward to explore the exotic topological excitations in QH-SC hybrids. |
Tuesday, March 16, 2021 1:54PM - 2:06PM Live |
F46.00013: Three-layer coherent quantum Hall effect Yihang Zeng, Cory R Dean The \nu = 1 quantum Hall state in closely spaced double layer system is demonstrated to be layer-coherent, i.e. electrons in two layers behave as if they are in one, which leads to the superfluid of indirect excitons. In this work, we report for the first time a triple-layer system with two hBN tunnel barriers between three graphene sheets. The combination of voltage bias on the top, bottom gate electrodes and on the bottom graphene renders the independent control the carrier density in each layer. By mapping out the total compressibility of all three graphene layers in the three-dimensional density space, we observe incompressible quantum Hall states on the two-dimensional plane of total filling fraction \nu = 1 and 2. The robustness of this quantum Hall state against charge distribution among all three layers suggests phase coherence across all three layers. The energy gap as a function of filling fraction in each layer matches well with the expectation from theory. Incompressible states at total filling fraction \nu = 4/3 and 5/3 are also observed in part of the density space, hinting at a fractional three-layer coherent state. |
Tuesday, March 16, 2021 2:06PM - 2:18PM Live |
F46.00014: Fabry-Perot interferometer in bilayer graphene Hailong Fu, Ke Huang, Kenji Watanabe, Takashi Taniguchi, Jun Zhu Quantum Hall edge state interferometry has been an important tool in probing the charge and statistics of elementary excitations of the quantum Hall and fractional quantum Hall effect of a two-dimensional electron gas. Decades of effort and the recent observation of fractional statistics in screened GaAs quantum wells highlight the importance of a small Coulomb charging energy and a sharply defined edge confinement potential [1]. High-quality bilayer graphene is an appealing platform because it exhibits a plethora of odd and even-denominator fractional quantum Hall states with large energy gaps and a dual-gated graphene/BN heterostructure is conducive to the reduction of Coulomb charging energy and edge reconstruction. In this work, we report on the fabrication and measurements of interferometer devices in bilayer graphene using different designs. We determine relevant energy and length scales revealed in our measurements and discuss the progress and challenges towards detecting fractional and non-Abelian statistics in this platform. |
Tuesday, March 16, 2021 2:18PM - 2:30PM Not Participating |
F46.00015: Numerical evidence for marginal scaling at the integer quantum Hall transition Elizabeth J Dresselhaus, Bjoern Sbierski, Ilya Gruzberg The integer quantum Hall transition (IQHT) is one of the most mysterious members of the family of Anderson transitions. Since the 1980s, the scaling flow close to the critical point in the parameter plane spanned by longitudinal and transversal conductivity has been studied vigorously both in experiment and numerical simulations. Despite all efforts, the IQHT is notoriously difficult for pinning down the precise value of critical exponents, which seem to vary with model details, challenging the principle of universality. Recently, M. Zirnbauer [Nucl. Phys. B 941, 458 (2019)] has conjectured a conformal field theory for the transition, in which the fixed point is exactly marginal, leading |
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