Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R60: Topological Superconductivity with 1D ModesFocus

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Sponsoring Units: DMP Chair: Enrico Rossi, William & Mary College Room: Mile High Ballroom 4A 
Thursday, March 5, 2020 8:00AM  8:36AM 
R60.00001: Interference effects in quanum Hall  superconductor hybrid devices Invited Speaker: Gleb Finkelstein I plan to discuss our group’s two recent results: realization of the quantum Hallbased SQUID in sidegated samples; and observation of the chiral Andreev edge states – hybrid electron/hole single particle modes running along the superconductor  quantum Hall interfaces. 
Thursday, March 5, 2020 8:36AM  8:48AM 
R60.00002: Chiral ElectronHole Hybrid Edge Modes from the Lowest Landau Level of Proximitized Graphene Alexey Bondarev, Lingfei Zhao, Harold U Baranger We study a tightbinding model of a graphene nanoribbon in the integer quantum Hall state with one edge coupled to an swave superconductor. Along the superconducting edge at high field, we find two gapless electronhole hybrid modes related by particlehole symmetry. These modes have nearly equal electron and hole content and propagate more slowly than the corresponding pure vacuumquantum Hall edge modes. The momentum difference between these modes leads to accumulation of a relative phase between them. This phase is tunable via gate voltage, magnetic field, or applied bias. We show that such phase variation gives rise to oscillations of the longitudinal resistance between electronlike and holelike transport similar to those measured recently by Zhao et al. [1]. Armchair and zigzag nanoribbons lead to similar results because the valley degeneracy is broken by the superconductor. 
Thursday, March 5, 2020 8:48AM  9:00AM 
R60.00003: Hybrid quantum circuits in vertical magnetic fields Kaveh Delfanazari, Llorens Serra, Pengcheng Ma, Ian Farrer, David A Ritchie, Hannah J Joyce, Michael Joseph Kelly, Charles G Smith Topological superconductors, that host Majorana zero modes and parafermions, can also be 
Thursday, March 5, 2020 9:00AM  9:12AM 
R60.00004: Induced superconductivity in fractional quantum Hall edge Onder Gul, Yuval Ronen, Si Young Lee, Jonathan Zauberman, Hassan Shapourian, Brennan Dizdar, Young Jae Shin, Danial Haei Najafabadi, Kenji Watanabe, Takashi Taniguchi, Ashvin Vishwanath, Amir Yacoby, Philip Kim Topological superconductors (TS) represent a phase of matter whose properties are insensitive to local perturbations. This robustness renders TS suitable for application in quantum computing. The past decade has witnessed substantial progress towards a quantum bit using Majorana modes, the wellknown nonAbelian modes in TS. However, because Majoranas lack a universal logic gate set, Majorana quantum bits are computationally limited. This important drawback can be overcome by parafermions, a novel set of nonAbelian modes whose array supports universal topological quantum computation. A primary route to synthesize parafermions involves inducing superconductivity in fractional quantum Hall (FQH) edge. Here we use highquality van der Waals devices coupled to a narrow NbN which remains superconducting in the magnetic fields required for robust FQH. We find crossed Andreev reflection (CAR) across the narrow NbN that separates two counterpropagating FQH edges. Control experiments show that CAR vanishes with increasing temperature, excitation and magnetic field as expected from the theory. These results lay the groundwork for experimental parafermion research in condensed matter. 
Thursday, March 5, 2020 9:12AM  9:24AM 
R60.00005: Platform of chiral Majorana edge modes and its quantum transport phenomena James Jun He, Tian Liang, Yukio Tanaka, Naoto Nagaosa We propose a method to create twodimensional topological superconductors with a heterostructure of ferromagnet (FM), topological insulator (TI) thin film and superconductor, in which the two surfaces of the TI thin film are treated as a twodimensional system. One of surfaces is superconducting due to proximity effect and the other feels an exchange field from the FM. We show that there is a topological phase with single chiral Majorana edge mode that exists in readily achievable parameter regions and does not require magnetization to be small. An experimental setup is proposed based on our model to uniquely determine the existence of Majorana chiral modes using a Josephson junction. Also, we show that multiple chiral Majorana edge modes may appear when unconventional superconductors are used. 
Thursday, March 5, 2020 9:24AM  9:36AM 
R60.00006: Chiral Majorana Mode in Topological Superconducting Element Hcp Thallium Motoaki Hirayama, Takuya Nomoto, Ryotaro Arita The chiral Majorana fermion is an exotic particle that is its own antiparticle. It can arise in a onedimensional edge of topological materials, and especially that in a topological superconductor can be exploited in nonAbelian quantum computation. While the chiral Majorana mode (CMM) remains elusive, a promising situation is realized when superconductivity coexists with a topologically nontrivial surface state. Here, we perform the fully nonempirical calculation for the CMM considering superconductivity and surface relaxation and show that hexagonal closepacked thallium (Tl) has an ideal electronic state that harbors the CMM [1]. The k_{z} = 0 plane corresponds to the TCI with mirror Chern number N_{M} = 2. One of the Dirac points on the (0110) surface is located almost at the Fermi level. Tl is a textbooklike swave superconductor and the gap function has no significant wavenumber dependence. Only one of the two Dirac points is relevant for the gap opening due to the superconducting transition, and the CMM appears at the hinge under the Zeeman field. Our calculation indicates that Tl will provide a new platform of the Majorana fermion and quantum computation. 
Thursday, March 5, 2020 9:36AM  9:48AM 
R60.00007: Spontaneous thermal Hall conductance in superconductors with broken timereversal symmetry Firat Yilmaz, Sungkit Yip The thermal Hall conductivities(THCs), $\kappa_{ij}$s have extensively been studied in recent condensed matter experiments. THC can spontaneously become nonzero for a timereversal symmetry (TRS) broken system, and have a contribution from topologically protected edge states. In this talk, we focus on an additional bulk effect, the impurity pair breaking mechanism(IPM) in superconductors (SCs). Previously, the THCs were calculated for the chiral pwave[12] SCs for point impurities. Motivated by dwave TRS broken SCs; URu$_2$Si$_2$, SrPtAs including Sr$_2$RuO$_4$ which is recently suggested to be also possibbly, we calculate THCs at finite temperatures and for finite size impurities using the nonequilibrium quasiclassical Green's functions. 
Thursday, March 5, 2020 9:48AM  10:00AM 
R60.00008: Microwave spectroscopy of semiconductornanowirebased superconducting qubits with Majorana bound states Ramon Aguado, Jesus Avila, Elsa Prada, Pablo SanJose Recent experimental efforts have focused on replacing the weak link in the Josephson Junction (JJ) of a superconducting qubit by electrostaticallygateable technologies compatible with high magnetic fields. Such alternatives are crucial in order to reach a regime relevant for readout of topological qubits based on Majorana bound states (MBS). We here focus on a system where a semiconducting nanowire forming the JJ is driven to a topological superconductor phase with MBS which coherently interact with the superconducting qubit degrees of freedom. Our fully microscopic theoretical description of this nanowirebased superconducting qubit allows to unveil new physics. This includes the magnetic field dependence of the qubit frequency, which follows the gap closing and the emergence of MBS in the topological phase. In this phase, the periodicity of the qubit spectrum with respect to n_{g} can be either that of Cooper pairs 2e or single electrons e, depending on microscopic parameters such as the degree of Majorana energy splitting or the ratio of Josephson to charging energy E_{J}/E_{C}. Overall, the corresponding microwave spectroscopy presents nontrivial features, including a full mapping of zero energy crossings and fermionic parity switches in the nanowire owing to Majorana oscillations. 
Thursday, March 5, 2020 10:00AM  10:12AM 
R60.00009: Decays of Majorana or Andreev Oscillations Induced by Steplike SpinOrbit Coupling Zhan Cao, Hao Zhang, HaiFeng L\''{u}, WanXiu He, Haizhou Lu, Xincheng Xie The Majorana zero mode in the semiconductorsuperconductor nanowire is one of the promising candidates for topological quantum computing. Recently, in islands of nanowires, subgapstate energies have been experimentally observed to oscillate as a function of the magnetic field, showing a signature of overlapped Majorana bound states. However, the oscillation amplitude either dies away after an overshoot or decays, sharply opposite to the theoretically predicted enhanced oscillations for Majorana bound states. We reveal that a steplike distribution of spinorbit coupling in realistic devices can induce the decaying Majorana oscillations, resulting from the couplinginduced energy repulsion between the quasiparticle spectra on the two sides of the step. This steplike spinorbit coupling can also lead to decaying oscillations in the spectrum of the Andreev bound states. For Coulombblockade peaks mediated by the Majorana bound states, the peak spacings have been predicted to correlate with peak heights by a π/2 phase shift, which was ambiguous in recent experiments and may be explained by the steplike spinorbit coupling. Our work will inspire more works to reexamine effects of the nonuniform spinorbit coupling in experimental devices. 
Thursday, March 5, 2020 10:12AM  10:24AM 
R60.00010: Nonlocal conductance in threeterminal hybrid Al/InSb nanowire devices Sebastian Heedt, Francesco Borsoi, Marina Quintero Perez, Alexandra Fursina, Nick van Loo, Ghada Badawy, Sasa Gazibegovic, Kevin Van Hoogdalem, Erik Bakkers, Leo P Kouwenhoven Majorana zeroenergy modes (MZMs) emerge at the edges of a onedimensional topological segment. Signatures of these modes are correlated zerobias peaks (ZBPs) in the local conductance that emerge simultaneously at both edges. The energy gap that separates them from the continuum  the bulk topological gap  can be identified in the nonlocal conductance, which is insensitive to local Andreev levels or transmission resonances. Thus, it is a vital tool to distinguish nontopological zeroenergy modes and MZMs. Here, we have realized hybrid threeterminal devices based on InSb nanowires. The proximityinduced superconductivity is achieved via selective shadowwall deposition of Al thin films. We demonstrate a hard induced gap using voltagebias spectroscopy and study the appearance of zerobias peaks in the local conductance. Harnessing the threeterminal geometry, we investigate the evolution of the nonlocal conductance in a magnetic field before the transition to the normal state. 
Thursday, March 5, 2020 10:24AM  10:36AM 
R60.00011: Engineering Majorana Bound States with Periodic Structures Benjamin Woods, Tudor Dan Stanescu Majorana bound states within semiconductorsuperconductor heterostructures are a promising platform for future topological quantum computation. Here we theoretically explore the effects of periodic potentials and periodic structures on the emergence and properties of topological superconductivity and Majorana bound states within 1D and quasi1D systems. Three key findings suggest that periodic structures may possess some advantages over homogeneous systems in supporting topological superconductivity. The three findings to be discussed are (1) an increased region of parameter space in which the topological phase and Majorana bound states are realized, (2) an increasing topological gap as one moves to higher energy minibands, and (3) an increased robustness against disorder compared to homogeneous systems. We also explore several physical systems in which the periodic potential/structure can be engineered and discuss their outlook. 
Thursday, March 5, 2020 10:36AM  10:48AM 
R60.00012: Topological Josephson effect in gatetunable qubit devices Andrey Antipov, Cameron King, Kewei Li, John Gamble, Bela Bauer, Georg Winkler, Roman Lutchyn Gatetunable superconducting qubits that host Majorana zero modes realize an intermediate step to topologically protected qubits by storing the quantum state in topological degrees of freedom. In this talk I will present numerical simulations of such devices that combine superconducting proximity effect, orbital effects, electrostatic environment and realistic geometry. I will show the phase diagram and the key properties of these topological Josephson junctions, in particular the Josephson energy and Majorana coupling energy. Based on these I identify regimes of the surrounding electrostatic environment most favorable for realizing qubits and relate this to observable experimental features. 
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