Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session D65: Topological Superconductivity in NanowiresFocus Recordings Available

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Chair: Christian Volk, RWTH Aachen University Room: Hyatt Regency Hotel Grant Park C 
Monday, March 14, 2022 3:00PM  3:36PM 
D65.00001: Disorder and topological superconductivity Invited Speaker: Jay D Sau Topological superconductors have the potential to be one of the simplest platforms to demonstrate topologically protected degeneracy because they are effectively noninteracting phases. Proposals such as those based on semiconductorsuperconductor hybrids, topological superconductors, quantum Hall gases and iron superconductors provide a range of options to engineer such platforms. Unfortunately, despite the nominal stability of topological superconductors to weak disorder, realizing a topological phase often requires an effective one dimensional scattering rate below the induced superconducting gap. In this talk I will review how this principle restricts various platforms such as those based on semiconductor nanowires, quantum Hall systems and line Josephson junctions. I will also discuss transport experiment proposals to identify device issues from disorder and device imperfections. 
Monday, March 14, 2022 3:36PM  3:48PM Withdrawn 
D65.00002: Fullshell hybrid nanowires, a promising semiconducting nanowire design in the search for Majorana modes Elsa Prada Hybrid superconductorsemiconductor nanowires are being intensively studied in an effort to detect and manipulate Majorana bound states. Traditionally, a superconductor is deposited or grown epitaxially over some of the facets of the semiconducting wire and a Zeeman field is applied to drive the system into the topological regime. One of the main drawbacks of this platform is the need to subject the wire to strong magnetic fields that degrade the superconducting state of the parent superconductor. In the last couple of years, new nanowire configurations are being explored that could present some advantages with respect to previous designs. In this talk, I will discuss theoretical and experimental progress on one the most promising proposals: the socalled fullshell nanowires, where the semiconducting nanowire is fully coated with a superconducting shell. These wires could develop Majorana modes in the presence of small external magnetic fields not driven by the Zeeman effect, but by the winding of the superconducting phase around the semiconducting core. 
Monday, March 14, 2022 3:48PM  4:00PM 
D65.00003: Steering Majorana braiding via skyrmionvortex pairs: a scalable platform Kjetil Hals, Tobias Meng, Matteo Rizzi, Karin EverschorSitte, Jonas Nothhelfer, Stephan Kessler, Sebastian A Diaz Majorana zero modes are quasiparticles that hold promise as building blocks for topological quantum computing. However, the litmus test for their detection, the observation of exotic nonabelian statistics revealed by braiding, has so far eluded experimental efforts. Here we take advantage of the fact that skyrmionvortex pairs in superconductorferromagnet heterostructures harboring Majorana zero modes can be easily manipulated in two spatial dimensions. We adiabatically braid the hybrid topological structures and explicitly confirm the nonabelian statistics of the Majorana zero modes numerically using a selfconsistent calculation of the superconducting order parameter. Our proposal of controlling skyrmionvortex pairs provides the necessary leeway toward a scalable topological quantum computing platform. 
Monday, March 14, 2022 4:00PM  4:12PM 
D65.00004: Manipulation of Majorana bound states in proximity to a quantum ring with Rashba coupling Álvaro Díaz Fernández, Fabian G Medina, Francisco DominguezAdame, Dunkan Martínez, Pedro A Orellana, Luis Rosales The quest for Majorana zero modes in the laboratory is an active field of research in condensed matter physics. In this regard, there have been many theoretical proposals; however, their experimental detection remains elusive. In this article, we present a realistic setting by considering a quantum ring with Rashba spinorbit coupling and threaded by a magnetic flux, in contact with a topological superconducting nanowire. We focus on spinpolarized persistent currents to assess the existence of Majorana zero modes. We find that the Rashba spinorbit coupling allows for tuning the position of the zero modes in flux space and has sizable effects on spinpolarized persistent currents. Our results pave the way towards probing the existence of Majorana zero modes. 
Monday, March 14, 2022 4:12PM  4:24PM 
D65.00005: Tunable interdot coupling via Andreev levels ChunXiao Liu Quantum dotsuperconductorquantum dot is a widely used mesoscopic system for studying fundamental physics, such as Cooper pair splitting, spin qubit coupling via superconductor among others. However, when the superconductor is of metallic nature, the interdot tunneling strength decays with $k_F L$ in a power law and is difficult to finetune by system parameters. In this work we consider a short semiconductorsuperconductor hybrid nanowire for which the lowenergy physics is a pair of Andreev levels. We find that the Andreevlevelmediated interdot tunneling strength is tunable and has a universal dependence on the nanowire parameters, such as chemical potential, spinorbit interaction and induced Zeeman spin splitting. Moreover, we show that in tunnel spectroscopy resonant current flowing through the system is proportional to the square of interdot tunneling strength when dots are connected to external normal leads. Our findings will facilitate the device characterization and optimization in related experiments. 
Monday, March 14, 2022 4:24PM  4:36PM 
D65.00006: Extended quantummechanical supersymmetry in Majorana nanowires Pasquale Marra, Daisuke Inotani, Muneto Nitta Extended supersymmetry (SUSY) with central charges has been one of the most important notions in quantum field theory and string theory since the second string revolution in the 90s and the revolution of quantum field theory by Seiberg and Witten. In spite of its great importance in formal aspects of quantum field theory and string theory, all high energy theorists regard it as a useful tool which is not directly related to reality, because the extended N=2 SUSY does not allow chiral fermions relevant for elementary particles such as quarks and leptons. In high energy phenomenology, only N=1 SUSY and its breaking are considered. 
Monday, March 14, 2022 4:36PM  4:48PM 
D65.00007: Gatecontrolled Elastic CoTunneling and Crossed Andreev Reflection mediated by Andreev Bound States Alberto Bordin, Guanzhong Wang, Tom Dvir, Bas ten Haaf, David van Driel, Francesco Zatelli, Nick van Loo, Grzegorz P Mazur, ChunXiao Liu, Ghada Badawy, Sasa Gazibegovic, Erik P. A. M. Bakkers, Michael Wimmer, Gijs De Lange Elastic cotunneling (ECT) and crossed Andreev reflection (CAR) are complementary processes that take place when two normal leads are separated by a thin superconductor. In the former, an electron hops from one lead to another, and in the latter, two correlated electrons from both sides enter or leave the superconducting segment. Controlling the ECT and CAR amplitudes and extending the range over which they can occur is desirable to couple spin qubits and form artificial topological structures such as the Kitaev chain. We fabricate and measure CAR and ECT in an InSb nanowire device, where two quantum dots are separated by a superconducting segment. We show by comparison to a theoretical model that both ECT and CAR are mediated by an Andreev bound state (ABS) residing under the superconducting segment. Since ECT and CAR are mediated by the ABS, we show that they extend beyond the superconducting coherence length, and their amplitudes are tuned by external plunger gate and magnetic field. This tunability is an essential tool for the future engineering of topological superconductivity in metamaterials. 
Monday, March 14, 2022 4:48PM  5:00PM 
D65.00008: Delocalized Andreev bound states and nontopological Majorana excitations in InSb nanowiresc Tom Dvir, Guanzhong Wang, Grzegorz P Mazur, Alberto Bordin, Bas Ten Haaf, Ghada Badawy, Sasa Gazibegovic, Erik P. A. M. Bakkers, Gijs De Lange Majorana bound states are zeroenergy modes that are created and annihilated by the same operator. They are highly sought after mainly due to their nonabelian statistics, a property desirable both for fundamental research and possible applications in the field of quantum computing. Majorana zeromodes are necessarily spinless, which directed the attention of their pursuit on the boundary of topological superconductors. Andreev bound states (ABS), however, are spinfull excitations formed when a quantum level is coupled to a superconductor and thus cannot create Majorana modes. A recent proposal showed that it is possible to create a nonlocal ABS when two spinless quantum dots are coupled via a superconductor. Following this proposal, we fabricate an InSb nanowire device, in which a short Al strip separates two spinsplit quantum dots. 
Monday, March 14, 2022 5:00PM  5:12PM 
D65.00009: Tunneling spectroscopy in InSbAsAl multiprobe devices Qingzhen Wang, ChungTing Ke, Saurabh Karwal, Jacob Dalle, Yining Zhang, Christian M Moehle, Candice Thomas, Di Xiao, Geoffrey C Gardner, Srijit Goswami Majorana zero modes can emerge as edge states in a onedimensional topological superconductor.The primary experimental approach for identification is an emerging zerobias peaks (ZBPs) in tunneing spectroscopy at the edge of the system, which, however, can also be due to many trivial origins. A more unique (and thus strong) accompanying signature is the reopening of the bulk superconducting gap. Some initial attempts have been done on the mostly studied nanowire platform, where it requires some sophisticated fabrication steps .Here we study the electron transport of the quasi1D structures made from the new InSbAs2DEGsAl system, where the semiconductor shows very strong spinorbit interaction. In addition, the inherent design flexibility allows for a straightforward realization of multiple tunnel junctions along the 1D strip. By using tunneling spectroscopy, we study both the local energy spectra simultaneously at different positions, and also the nonlocal responses of individual sections. At zero magnetic field, locally we have observed the hard superconducting gap with same gap size along the wire, indicating a uniform proximity over μmlong length scale. The superconducting gap becomes smaller and eventually close at inplane field B≈1.1 T, after which zero bias peaks emerge at tunnel probes located at the edges, while this is less prominent for probes in the bulk. At similar magnetic field, a sign flip of the nonlocal conductance has also been repeatedly observed. 
Monday, March 14, 2022 5:12PM  5:24PM 
D65.00010: Vortexinduced pairing suppression and nearzero modes in quantum dots coupled to fullshell nanowires Samuel D. D Escribano, Pablo SanJose, Elsa Prada, Ramon Aguado, Alfredo Levy Yeyati We analyze the subgap excitations and phase diagram of a quantum dot (QD) coupled to a semiconducting nanowire fully wrapped by a superconducting (S) shell. We take into account how a LittleParks (LP) pairing vortex in the shell influences the proximity effect on the dot. We find that under axially symmetric QDS coupling, shell vortices cause the induced pairing to vanish, producing instead a level renormalization that pushes subgap levels closer to zero energy and flattens parity crossings as the coupling strength increases. This vortexinduced stabilization mechanism has analogues in symmetric SQDS Josephson junctions at phase π, and can naturally lead to patterns of nearzero modes weakly dispersing with parameters in all but the zeroth lobe of the LP spectrum. We also show that, under certain conditions, this mechanism could give rise to states that mimic some of the features of Majorana bound states in fullshell wires, despite of the fact that they are topologically trivial. 
Monday, March 14, 2022 5:24PM  5:36PM 
D65.00011: Structural investigation of cryogenicgrown Sn thin films on InSb substrates Moïra Hocevar, AnHsi Chen, Mihir Pendharkar, Connor Dempsey, Bomin Zhang, Chris J Palmstrom, Sergey M Frolov, Edith BelletAmalric Superconductorsemiconductor hybrid structures fabricated by optimizing interface formation recipes contribute to better topological superconducting devices. Lately, we measured superconductivity and 2e parity charging in crystalline tin islands grown at cryogenic temperatures on InSb nanowire facets. To further understand the structural properties of those heterostructures, we deposited tin layers on InSb (110) and (001) substrates using different growth recipes, including the one used for nanowires. First of all, Xray diffraction spectroscopy performed at room temperature shows that tin thin films grown on InSb are epitaxial and in the cubic alpha phase. Depending on the capping strategy to prevent thin film dewetting, some films may contain beta tin grains that are superconducting. We finally propose a mechanism responsible for the formation of the beta grains in the thin films. 
Monday, March 14, 2022 5:36PM  5:48PM 
D65.00012: Distinguishing topological Majorana zero modes: nonlocal conductance versus the von Neumann entropy Abhishek Kejriwal, Bhaskaran Muralidharan The topological origin of the zero bias conductance signatures of Majorana zero modes has been a topic of intense debate. We propose to differentiate between true Majorana bound states and trivial Andreev bound states using the von Neumann entanglement entropy. The topological von Neumann entropy provides a distinct signature of nonlocal correlations of topological origin. We also establish its connection with recently proposed ideas on the lines of the nonlocal conductance spectra in a threeterminal normaltopological superconductornormal hybrid system. We show that both entanglement entropy and nonlocal conductance exhibit a clear topological phase transition signature for long enough system sizes. However, we observe that the nonlocal conductance fails to signal a topological phase transition for small system sizes. Moreover, we demonstrate the robustness of entanglement entropy to system parameters as well as its size, establishing it as an ideal metric for distinguishing topological phases. 
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