Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X13: Majorana Bound States IIIFocus

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Sponsoring Units: DMP Chair: Jing Xia, University of California, Irvine Room: LACC 304A 
Friday, March 9, 2018 8:00AM  8:36AM 
X13.00001: Majorana bound state in ironbased superconductor Fe(Te,Se) Invited Speaker: Hong Ding In this talk I will report our recent discoveries of topological superconductivity and Majorana bound state in Febased superconductor Fe(Te, Se). We have obtained convincing ARPES evidence of superconducting topological surface state of Fe(Te, Se) single crystal with Tc ~ 14.5K. By using lowtemperature STM on this maeteial, we clearly observe a pristine Majorana bound state inside a vortex core, well separated from nontopological bound states away from zero energy due to the high ratio between the superconducting gap and the Fermi energy in this material. This observation offers a new, robust platform for realizing and manipulating Majorana bound states at a relatively high temperature. 
Friday, March 9, 2018 8:36AM  8:48AM 
X13.00002: Engineering topological superconductivity in InAs Josephson junctions Folkert De Vries, Tom Timmerman, Viacheslav Ostroukh, Jasper van Veen, Arjan Beukman, Fanming Qu, Michael Wimmer, Minh Nguyen, Andrey Kiselev, Wei Yi, Marko Sokolich, Sergei Gronin, Geoffrey Gardner, Candice Thomas, Michael Manfra, Srijit Goswami, Charles Marcus, Leo Kouwenhoven Josephson junctions can be used to engineer topological superconductivity in semiconductors with strong spinorbit coupling. Proposals to achieve this include proximitizing the edge of the material or applying a superconducting phase bias across the junction. Using superconducting quantum interference measurements on an InAs junction we demonstrate that in the trivial regime edge state transport gives rise to a SQUIDlike pattern. More strikingly, we find that SQUID signal is h/e (not h/2e) periodic. While this is considered a hallmark of the topological regime, we explain how crossed Andreev states in the trivial regime can give rise to the same effect. To avoid complications arising from such trivial edges we also explore the possibility of phase biasing InAs Josephson junctions. This is predicted to significantly bring down the Zeeman field needed for a topological phase transition. We study the response to phase bias and develop tunnel probes to measure the local density of states in the junction. 
Friday, March 9, 2018 8:48AM  9:00AM 
X13.00003: Topological Josephson Junctions on MagneticallyDoped Topological Insulators Rikizo Yano, Masao Koyanagi, Hiromi Kashiwaya, Kohei Tsumura, Hishiro Hirose, Takao Sasagawa, Satoshi Kashiwaya Topological superconductors (TSCs) hosting Majorana fermions at their edges have been a topic of intense debate in condensed matter physics. A large number of experimental trials to detect the evidence of TSCs have been performed using topological Josephson junctions composed of nanowires and topological insulators (TIs). In contrast to successful observation of the signatures of TSCs in InSb nanowires, the results are rather unclear in 3D TI junctions so far. 
Friday, March 9, 2018 9:00AM  9:12AM 
X13.00004: A topological Josephson junction platform for realizing Majorana modes and NonAbelian rotations Suraj Hegde, Yuxuan Wang, Erik Huemiller, Guang Yue, Dale Van Harlingen, Smitha Vishveshwara As part of the intense effort towards realizing Majorana mode platforms that perform qubit operations, we present a proposed topological Josephson junction architecture whose experimental implementation we are working towards in conjunction. The architecture consists of superconducting electrodes deposited on a topological insulator film to form networks of lateral Josephson junctions. Magnetic fields piercing the junction enable fluxcontrolled nucleation of multiple Majorana bound state modes localized in the junction at locations where the local phase difference is an odd multiple of p, i.e. attached to the cores of Josephson vortices. We describe protocols for i) tuning the coupling between the Majorana modes to affect nonAbelian rotations, and ii) measuring nonlocal parity correlations induced by such a rotation. The platform makes use of local magnetic fields and supercurrent pulses to manipulate the Majorana modes and enables the detection of their paritybased qubit states by Josephson current measurements and chargesensitive singleelectron transistors. We report our progress in the experimental realization of this architecture. 
Friday, March 9, 2018 9:12AM  9:24AM 
X13.00005: Quantum Phase Transitions in Proximity Coupled ThreeDimensional Josephson Junctions Fnu Setiawan, ChienTe Wu, Brandon Anderson, Weihan Hsiao, Kathryn Levin Josephson junctions are becoming of greater interest in the search for and confirmation of topological superconductors. Here we address a realistic configuration of a three dimensional SIS junction where two conventional swave superconductors (separated by an insulator) induce superconductivity by proximity coupling onto a substrate. This substrate effectively contains spinorbit and Zeeman coupling and can host a topological phase [Phys. Rev. B 95, 014519]. While the substrate here is taken to be a conical magnet, our findings should apply more generally. We solve the Bogoliubovde Gennes equation to determine the energy dispersion of the system and find multiple zero energy crossings which are the signatures of paritychanging quantum phase transitions. Important to our work is to clarify the extent to which these zeroenergy crossings are associated with the topological phases in the system. Additional insight is provided through simple onedimensional models of SIS junctions which host a variety of Shibalike bound states. 
Friday, March 9, 2018 9:24AM  9:36AM 
X13.00006: NonAbelian Braiding of Majorana zero modes in a topological Josephson Junction SangJun Choi, HeungSun Sim We theoretically study a Josephson junction which is formed of finite size superconductors on a topological insulator under a magnetic field. An extended chiral Majorana mode emerges surrounding two superconductors, in addition to localized Majorana zero modes along the junction under a certain number of magnetic flux quanta piercing the junction. We find that the qubit state composed of the Majorana zero modes can be manipulated by a bias voltage across the superconductors. The nonAbelian braiding of Majorana zero modes can be detected by the Josephson current. 
Friday, March 9, 2018 9:36AM  9:48AM 
X13.00007: Josephson Junctions with epitaxial topological insulator SmB_{6} thin films Seunghun Lee, Rodney Snyder, Xiaohang Zhang, Sheng Dai, Xiaoqing Pan, Joshua Higgins, Richard Greene, James Williams, Ichiro Takeuchi Since Josephson junctions with an swave superconductor (SC) and a topological insulator (TI) has been predicted to be a possible venue for hosting elusive Majorana Fermions, many experimental efforts have been made with HgTe and Bibased TIs. Recently, TI SmB_{6} has gained attention as a promising material for such applications because its true bulk insulating state can eliminate undesirable bulk contribution to the Josephson effect. We have previously demonstrated the superconducting proximity effect in insitu deposited SCTI Nb/SmB_{6} heterostructures as a result of Nb superconductivity induced in the surface state of SmB_{6}. In this work, we show transport characteristics of NbSmB_{6}Nb Josephson junctions, including DC IV characteristics, their magnetic field dependence, and the AC Josephson effect. Epitaxial SmB_{6} thin films are grown by a cosputtering process. To secure high interfacial quality between Nb and SmB_{6}, Nb layers are deposited on the SmB_{6} layers insitu, and a topdown process including ebeam lithography and ion milling was utilized to define Josephson junction structures with a gap width of 50 nm and the lateral dimension of 1 µm. We discuss the observed Josephson effect in terms of the hightransparency interface and the true insulating bulk nature of SmB_{6}. 
Friday, March 9, 2018 9:48AM  10:00AM 
X13.00008: Revisiting 2π phase slip suppression in topological Josephson junctions. Rosa Rodriguez Mota, Smitha Vishveshwara, Tami PeregBarnea Current state of the art devices to detect and manipulate Majorana fermions commonly consist of networks of Majorana wires. We study a key ingredient of these networks  a topological Josephson junction in the presence of charging energy. The phase dependent tunneling contains 4π periodic single particle tunneling, allowed due to the Majorana modes at the edges of the junction, and the usual 2π periodic Josephson tunneling. For small values of the charging energy, the low energy physics of conventional Josephson junctions is described by 2π phase slips. In a topological junction, we expect 2π phase slips to be suppressed. However, we find that if the ratio between the strengths of the Majorana assisted tunneling and the Josephson tunneling is small — as is likely to be the case for many setups — 2π phase slips may still occur. We provide effective descriptions of the system in terms of 2π and 4π phase slips valid throughout the parameter space. Finally, we discuss the implications of our results on the dissipative phase transitions expected in this system and in assessing the viability of certain Majorana schemes. 
Friday, March 9, 2018 10:00AM  10:12AM 
X13.00009: Majorana Corner States of SecondOrder Topological Superconductors Qiyue Wang, Fan Zhang Majorana bound states often emerge at the ends of onedimensional topological superconductors. Here we show that they can also appear at the corners of finitesize proximitized twodimensional superconductors. We obtain a phase diagram that emphasizes the roles of chemical potential, order parameters, and edge orientations to achieve such secondorder topological superconductors. Our scheme offers a unique platform and opens a new perspective for exploring nonAbelian quasiparticles and topological quantum computing. 
Friday, March 9, 2018 10:12AM  10:24AM 
X13.00010: Evolution of the Transmission Phase Through a Coulombblockaded Majorana Wire Casper Drukier, Bernd Rosenow, HeinrichGregor Zirnstein, Ady Stern, Yuval Oreg We present a study of the transmission of electrons through a Coulombblockaded semiconductor wire with strong spinorbit coupling in proximity to an swave superconductor. Such systems support Majorana bound states in the presence of an external magnetic field and are of interest both due to recent experimental progress and as candidates for realizations of quantum computing. While phase lapses in the transmission phase are expected to occur in the absence of superconductivity we find that they vanish completely in the topological regime. Our calculation is based on a model which expresses tunnelling through the wire, through effective matrix elements which depend on both the fermion parity of the wire and the overlap with BogoliubovdeGennes wave functions. Together with a modified scattering matrix formalism this allows us to study the transmission including electronelectron interactions. We finally discuss the role of spin polarization and breaking of effective timereversal symmetry. 
Friday, March 9, 2018 10:24AM  10:36AM 
X13.00011: Conductance interference in a superconducting Coulomb blockaded Majorana ring ChingKai Chiu By tuning the magnetic flux, the two ends of a 1D topological superconductor weakly coupled to a normal metal as a ringshaped junction can host split Majorana zero modes (MZMs). When this ring geometry becomes Coulomb blockaded, and the two leads come into contact with the two wire ends, the current moves through the superconductor or the normal metal as an interferometer. The twoterminal interference conductance can be experimentally measured as a function of gate voltage and magnetic flux through the ring. However, a 4π periodicity in the conductancephase relation (often considered the hallmark of MZMs), which can arise both in a topological superconductor and in a trivial metal, cannot establish the existence of MZMs. We show that the trivial metal phase can be ruled out in favor of a topological superconductor by studying persistent conductance distribution patterns. In particular, in the presence of MZMs, the conductance peak spacings of the Coulomb blockaded junction would manifest line crossings as the magnetic flux varies. The locations of the line crossings can distinguish similar line crossings stemming from the trivial metal. 
Friday, March 9, 2018 10:36AM  10:48AM 
X13.00012: Quantized Majorana conductance (I): hard gap and quantized zero bias peaks Di Xu, Hao Zhang, ChunXiao Liu, Guanzhong Wang, Nick van Loo, Jouri Bommer, Michiel de Moor, Sasa Gazibegovic, Diana Car, Roy Op het Veld, Petrus van Veldhoven, Sebastian Koelling, Marcel Verheijen, John Logan, Mihir Pendharkar, Daniel Pennachio, Borzoyeh Shojaei, Joon Sue Lee, Chris Palmstrom, Erik Bakkers, Sankar Das Sarma, Leo Kouwenhoven Majorana zeromodes hold great promise for topological quantum computing. A semiconductor nanowire coupled to a superconductor can be tuned into a topological superconductor with two Majorana zeromodes localized at the wire ends. Tunneling spectroscopy in electrical transport is the primary tool to identify the presence of Majorana zeromodes, which manifests itself as a zerobias peak (ZBP) in the differentialconductance. The Majorana ZBPheight is predicted to be quantized at the universal conductance value of 2e^{2}/h at zero temperature. Previous experiments, however, have shown ZBPs much smaller than 2e^{2}/h. The primary reason is due to dissipation from the soft gap. Here, we demonstrate that we have solved the soft gap problem [1], which leads to the observation of a quantized zerobias peak at 2e^{2}/h [2]. 
Friday, March 9, 2018 10:48AM  11:00AM 
X13.00013: Quantized Majorana Conductance (II): Zero Bias Peak as a Quantized Plateau Hao Zhang, Di Xu, ChunXiao Liu, Guanzhong Wang, Nick van Loo, Jouri Bommer, Michiel de Moor, Sasa Gazibegovic, Diana Car, Roy Op het Veld, Petrus van Veldhoven, Sebastian Koelling, Marcel Verheijen, John Logan, Mihir Pendharkar, Daniel Pennachio, Borzoyeh Shojaei, Joon Sue Lee, Chris Palmstrom, Erik Bakkers, Sankar Das Sarma, Leo Kouwenhoven Tunneling into a Majorana state will resolve a zerobiaspeak (ZBP) in the differential conductance. This tunneling process is a socalled Andreev reflection, where an incoming electron is reflected as a hole. Particlehole symmetry dictates that the zeroenergy tunneling amplitudes of electrons and holes are equal, resulting in a perfect resonant transmission with a ZBPheight quantized at 2e^{2}/h, irrespective of the precise tunneling strength. The Majorananature of this perfect Andreev reflection is a direct result of the wellknown Majorana symmetry property “particle equals antiparticle”. Here we show the observed ZBPheight remains as a constant at 2e^{2}/h, against the change of the tunneling strength, i.e. a quantized Majorana plateau. This quantized tunnelconductance plateau can uniquely identify a topological Majorana zeromode as far as tunneling spectroscopy is concerned. 
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