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 iron-based 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 Fe-based 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 low-temperature STM on this maeteial, we clearly observe a pristine Majorana bound state inside a vortex core, well separated from non-topological 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 spin-orbit 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 SQUID-like 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 Magnetically-Doped 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 Non-Abelian 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 flux-controlled 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 non-Abelian rotations, and ii) measuring non-local 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 parity-based qubit states by Josephson current measurements and charge-sensitive single-electron 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 Three-Dimensional Josephson Junctions Fnu Setiawan, Chien-Te 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 s-wave superconductors (separated by an insulator) induce superconductivity by proximity coupling onto a substrate. This substrate effectively contains spin-orbit 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 Bogoliubov-de Gennes equation to determine the energy dispersion of the system and find multiple zero energy crossings which are the signatures of parity-changing quantum phase transitions. Important to our work is to clarify the extent to which these zero-energy crossings are associated with the topological phases in the system. Additional insight is provided through simple one-dimensional models of SIS junctions which host a variety of Shiba-like bound states. |
Friday, March 9, 2018 9:24AM - 9:36AM |
X13.00006: Non-Abelian Braiding of Majorana zero modes in a topological Josephson Junction Sang-Jun Choi, Heung-Sun 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 non-Abelian 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 SmB6 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 s-wave 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 Bi-based TIs. Recently, TI SmB6 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 in-situ deposited SC-TI Nb/SmB6 heterostructures as a result of Nb superconductivity induced in the surface state of SmB6. In this work, we show transport characteristics of Nb-SmB6-Nb Josephson junctions, including DC I-V characteristics, their magnetic field dependence, and the AC Josephson effect. Epitaxial SmB6 thin films are grown by a co-sputtering process. To secure high interfacial quality between Nb and SmB6, Nb layers are deposited on the SmB6 layers in-situ, and a top-down process including e-beam 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 high-transparency interface and the true insulating bulk nature of SmB6. |
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 Pereg-Barnea 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 Second-Order Topological Superconductors Qiyue Wang, Fan Zhang Majorana bound states often emerge at the ends of one-dimensional topological superconductors. Here we show that they can also appear at the corners of finite-size proximitized two-dimensional superconductors. We obtain a phase diagram that emphasizes the roles of chemical potential, order parameters, and edge orientations to achieve such second-order topological superconductors. Our scheme offers a unique platform and opens a new perspective for exploring non-Abelian quasiparticles and topological quantum computing. |
Friday, March 9, 2018 10:12AM - 10:24AM |
X13.00010: Evolution of the Transmission Phase Through a Coulomb-blockaded Majorana Wire Casper Drukier, Bernd Rosenow, Heinrich-Gregor Zirnstein, Ady Stern, Yuval Oreg We present a study of the transmission of electrons through a Coulomb-blockaded semiconductor wire with strong spin-orbit coupling in proximity to an s-wave 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 Bogoliubov-de-Gennes wave functions. Together with a modified scattering matrix formalism this allows us to study the transmission including electron-electron interactions. We finally discuss the role of spin polarization and breaking of effective time-reversal symmetry. |
Friday, March 9, 2018 10:24AM - 10:36AM |
X13.00011: Conductance interference in a superconducting Coulomb blockaded Majorana ring Ching-Kai Chiu By tuning the magnetic flux, the two ends of a 1D topological superconductor weakly coupled to a normal metal as a ring-shaped 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 two-terminal interference conductance can be experimentally measured as a function of gate voltage and magnetic flux through the ring. However, a 4π periodicity in the conductance-phase 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, Chun-Xiao 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 zero-modes hold great promise for topological quantum computing. A semiconductor nanowire coupled to a superconductor can be tuned into a topological superconductor with two Majorana zero-modes localized at the wire ends. Tunneling spectroscopy in electrical transport is the primary tool to identify the presence of Majorana zero-modes, which manifests itself as a zero-bias peak (ZBP) in the differential-conductance. The Majorana ZBP-height is predicted to be quantized at the universal conductance value of 2e2/h at zero temperature. Previous experiments, however, have shown ZBPs much smaller than 2e2/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 zero-bias peak at 2e2/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, Chun-Xiao 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 zero-bias-peak (ZBP) in the differential conductance. This tunneling process is a so-called Andreev reflection, where an incoming electron is reflected as a hole. Particle-hole symmetry dictates that the zero-energy tunneling amplitudes of electrons and holes are equal, resulting in a perfect resonant transmission with a ZBP-height quantized at 2e2/h, irrespective of the precise tunneling strength. The Majorana-nature of this perfect Andreev reflection is a direct result of the well-known Majorana symmetry property “particle equals antiparticle”. Here we show the observed ZBP-height remains as a constant at 2e2/h, against the change of the tunneling strength, i.e. a quantized Majorana plateau. This quantized tunnel-conductance plateau can uniquely identify a topological Majorana zero-mode as far as tunneling spectroscopy is concerned. |
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