Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A05: Topological Superconductivity: MajoranaFocus

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Sponsoring Units: DMP Chair: Zhiqiang Mao Room: BCEC 108 
Monday, March 4, 2019 8:00AM  8:36AM 
A05.00001: Majorana in Chains and Hinges Invited Speaker: Ali Yazdani In recent years, following pioneering theoretical work of Kitaev and others, we have learned how to engineer materials that harbor quasiparticles that behave similar to fermions that Majorana had first envisioned. In particular, there has been a focus on onedimensional topological superconductor that harbor Majorana zero modes (MZM) that can potentially be used to make faulttolerant topological quantum computation possible. We have proposed and implemented a platform for realization of topological superconductivity and MZM in chains of magnetic atoms on the surface of a superconductor. In this talk, I will describe the series of experiments on this platform that we have performed to establish the presence of these exotic quasiparticle using spectroscopic mapping with the scanning tunneling microscope (STM). These include the most recent study of the unique spin signature of MZM. I will also describe work on a new platform where we use the onedimensional helical hinge states of a higher order topological insulators. In paritcular, I will show experiment demonstrating how combination of magentism and superconductivity on such onedimensional states can also give rise to MZM that can be detected with an STM. Overall these experiments, illustrate how the power of spectroscopic imaging with the STM can be used to characterize novel quantum states of matter and visualize their exotic quasiparticles. 
Monday, March 4, 2019 8:36AM  8:48AM 
A05.00002: Experimental signatures of Majorana bound states in STIS lateral Josephson junctions Guang Yue, Can Zhang, Erik Huemiller, Maryam Salehi, Nikesh Koirala, Seongshik Oh, Alexey Bezryadin, Dale J Van Harlingen We present experiments designed to search for features of Majorana bound states (MBS) in lateral Josephson junctions formed by depositing swave superconductors onto the surface of topological insulator thin films. In a vertical magnetic field that induces a phase gradient across the junction, the localized MBS are expected to be stable at the cores of Josephson vortices where the phase difference across the junction is an odd multiple of π. To test this picture, we have fabricated NbBi_{2}Se_{3}Nb Josephson junctions with different geometries and carried out extensive measurements of their Josephson supercurrent vs. magnetic field diffraction patterns and the statistical critical current switching distribution in hysteretic junctions. Our results show a number of expected features of MBS, including: (1) finite critical current at the location of oddnumbered nodes of the diffraction pattern, (2) anomalous critical current distributions characterized by either a double peak in the switching distribution or an increase in the standard deviation of the distribution changes at values of magnetic field at which MBS are expected to be present in the junction. We will present a compilation of our data and analysis of these experiments. 
Monday, March 4, 2019 8:48AM  9:00AM 
A05.00003: Effects of gateinduced electric fields on semiconductor Majorana nanowires Andrey Antipov, Arno Bargerbos, Georg W. Winkler, Bela Bauer, Enrico Rossi, Roman Lutchyn We study the effect of gateinduced electric fields on the properties of semiconductorsuperconductor hybrid nanowires which represent a promising platform for realizing topological superconductivity and Majorana zero modes. Using a selfconsistent Schr\"odingerPoisson approach that describes the semiconductor and the superconductor on equal footing, we are able to access the strong tunneling regime and identify the impact of an applied gate voltage on the coupling between semiconductor and superconductor. We discuss how physical parameters such as the induced superconducting gap and Land\'e gfactor in the semiconductor are modified by redistributing the density of states across the interface upon application of an external gate voltage. Finally, we map out the topological phase diagram as a function of magnetic field and gate voltage for InAs/Al nanowires. 
Monday, March 4, 2019 9:00AM  9:12AM 
A05.00004: Universal Optical Control of Chiral Superconductors and Majorana Modes Martin Claassen, Dante Kennes, Manuel Zingl, Michael Sentef, Angel Rubio Chiral superconductors are a novel class of unconventional superconductors that host topologically protected chiral Majorana fermions at interfaces and domain walls, with great potential for topological quantum computing. Here we show that the outofequilibrium superconducting state in such materials is itself described by a Bloch vector in analogy to a qubit, which can be controlled alloptically on ultrafast time scales [1]. The mechanism is universal and permits a dynamical change of handedness of the condensate, relying on transient dynamical breaking of lattice rotation, mirror or timereversal symmetries via choice of pump pulse polarization to enable arbitrary rotations of the Bloch vector. The underlying physics can be intuitively understood in terms of transient Floquet dynamics, however the mechanism extends to ultrafast time scales, and importantly the engineered state persists after the pump is switched off. We demonstrate that these novel phenomena should appear in graphene and magicangle twisted bilayer graphene (TBG), as well as Sr_{2}RuO_{4}, as candidate chiral d+id and p+ip superconductors, and show that chiral superconductivity can be detected in timeresolved pumpprobe measurements. 
Monday, March 4, 2019 9:12AM  9:24AM 
A05.00005: HighTemperature Majorana Corner States Qiyue Wang, ChengCheng Liu, YuanMing Lu, Fan Zhang Majorana bound states often occur at the end of a 1D topological superconductor. Validated by a new bulk invariant and an intuitive edge argument, we show the emergence of one Majorana Kramers pair at each corner of a squareshaped 2D topological insulator proximitized by an extended swave (e.g., Febased) superconductor. We obtain a phase diagram that addresses the relaxation of crystal symmetry and edge orientation. We propose two experimental realizations in candidate materials. Our scheme offers a higherorder and highertemperature route for exploring nonAbelian quasiparticles. (arXiv:1804.04711, PRL 2018) 
Monday, March 4, 2019 9:24AM  9:36AM 
A05.00006: Majorana modes as electrical switches: computing the conductance of a junction between a Topological superconductor and a multichannel Luttinger liquid Alberto Nocera, Armin Rahmani, Adrian Feiguin, Marcel Franz, Ian Affleck Motivated by the experimental confirmation of topological superconductivity in semiconducting nanowires, I study the transport properties of a junction of three wires (Tjunction), which provides the simplest nontrivial example of wire network, as well as an important building block for topological quantum computing architectures. I show how to compute the electrical conductance of a Tjunction between a Topological superconductor and two normal interacting nanowires using the Density Matrix Renormalization Group (DMRG) method [1] that allows one to extract the conductance in the thermodynamic limit from static groundstate DMRG computations in closed finite systems [2]. As main result, I numerically demonstrate that the Tjunction can act as an electrical switch in the presence of a localized Majorana mode in agreement with field theory predictions [3]. I finally focus on the regime where the tunnel couplings between the superconductor and the nanowires is fine tuned to the same value, and compare the numerical results to the field theory calculations that predict a nontrivial critical point with an unusual conductance tensor. 
Monday, March 4, 2019 9:36AM  9:48AM 
A05.00007: Chiral Majorana modes on electrically gated high Tc Topological Superconductors Nima Djavid, Roger Lake Majorana edge modes were detected by observation of halfinteger conductivity in a magnetic topological insulator/superconductor (MTI/SC) heterostructure [1]. Recently proposed systems which are compatible with standard semiconductor processing are the most promising schemes for braiding Majorana modes. However, identifying Majorana zero modes experimentally has been challenging mostly because of the small pairing gap of swave SCs. A heterostructure between an anomalous quantum Hall insulator and a high Tc dwave superconductor with a large pairing gap may offer a more feasible approach for implementing Majorana zero modes. 
Monday, March 4, 2019 9:48AM  10:00AM 
A05.00008: Majorana Zero Modes and Braiding in Realistic Nanowire YJunctions Fenner Harper, M A Mueed, Benjamin Madon, Fabrizio Nichele, Markus Ritter, Noel Arellano, Heinz Schmid, Siegfried Karg, Heike Riel, Aakash Pushp, Rahul Roy Under the right conditions, semiconductor nanowires are believed to host zeroenergy Majorana fermions, particles with nonabelian properties suitable for topological quantum computation. While recent experiments have demonstrated encouraging local signatures of Majorana fermions, evidence for their nonabelian nature remains elusive. Motivated by ongoing experimental work at IBM, we build a realistic model of a nanowire Yjunction and study the properties of the resulting Majorana fermions numerically. We make quantitative predictions about the device parameters and geometry that are conducive to realising Majorana zero modes, and study the corresponding experimental signatures that would be expected. We go on to simulate braiding operations between multiple Majorana fermions, and suggest experimental gating protocols that may lead to signatures of their nonabelian nature. 
Monday, March 4, 2019 10:00AM  10:12AM 
A05.00009: Majorana double nanowires in the presence of magnetic field, interactions, and disorder Manisha Thakurathi, Pascal simon, Jelena Klinovaja, Daniel Loss We study double Rashba nanowires (NWs) coupled to an swave superconductor, 
Monday, March 4, 2019 10:12AM  10:24AM 
A05.00010: Multiband Physics in Inhomogeneous Majorana Nanowires Benjamin Woods, Tudor Dan Stanescu Semiconductor nanowires proximity coupled to superconductors are a promising platform to detect and manipulate Majorana zero energy modes. Single band models are used ubiquitously throughout the literature to describe these systems. In practice, however, it is quite likely that multiple confinement bands determine the properties of these devices, which can lead to new features not predicted by the single band models. We consider different types of inhomogeneity, such as a quantum dot defined by a region of the nanowire not covered by the superconductor. As previously shown, the dot region can give rise to Andreev bound states that mimic some of the signatures of Majorana zero modes. We find an increased pinning of Andreev bound states to zero energy within the multiband model due to strong interband coupling induced by an inhomogeneous electrostatic potential. Understanding in detail the multiband effects are important for the practical realization of Majorana zero modes in semiconductorsuperconductor nanowire devices. 
Monday, March 4, 2019 10:24AM  10:36AM 
A05.00011: Nonhermitian topology: a unifying framework for the Andreev versus Majorana states controversy Ramon Aguado, Pablo SanJose, Elsa Prada, Fernando Peñaranda, Jesus Avila Andreev bound states (ABSs) in hybrid semiconductorsuperconductor nanowires can have nearzero energy in parameter regions where band topology predicts trivial phases. This surprising fact has been used to challenge the interpretation of a number of transport experiments in terms of nontrivial topology with Majorana zero modes (MZMs). 
Monday, March 4, 2019 10:36AM  10:48AM 
A05.00012: Metamorphosis of Andreev bound states into Majorana bound states in pristine nanowires Yingyi Huang, Haining Pan, ChunXiao Liu, Jay Sau, Tudor Dan Stanescu We show theoretically that a clean superconducting spinorbitcoupled nanowire with finite chemical potential has two distinct nontopological regimes as a function of Zeeman splitting: one is characterized by finiteenergy ingap Andreev bound states (ABS), while the other has only extended bulk states. The Andreev bound state regime is characterized by strong features in the tunneling spectra creating a “gap closure” signature. However, no “gap reopening” signature should be apparent above the topological quantum phase transition (TQPT), in agreement with the most recent experimental observations. The gap closure feature is not a signature of the trivial gap of extended bulk states closing at the transition, but rather reflects the coming together of the ABS in systems with high chemical potential. Our theoretical finding establishes the generic intrinsic ABS on the trivial side of the topological quantum phase transition as the main contributors to the tunneling conductance spectra, providing a generic interpretation of existing experiments in clean Majorana nanowires. Our work also explains why experimental tunnel conductance spectra generically have gap closing features below the TQPT, but no gap opening features above it. 
Monday, March 4, 2019 10:48AM  11:00AM 
A05.00013: PhononAssisted Andreev Reection at Majorana Zero Mode Ning Dai, QingFeng Sun One of the typical features of Majorana zero mode (MZM) at the edge of topological superconductor is a zerobias peak in the tunneling spectroscopy of the normalsuperconductor (NS) junction. We study on a model with one phonon mode coupling to the superconductor lead of the NS junction, which can be viewed as an electronlead/phononcoupledMZM/holelead structure. The phononcoupled MZM acts as a series of channels in which electron can turn into hole by transmitting phonons to itself. These channels present in the local density of states(LDOS) as a series of stripes, generating the corresponding peaks in the tunneling spectroscopy. In LDOS, the electronphonon interaction narrows and redistributes the weight among stripes. In the tunneling spectroscopy, the heights of peaks present a feature of the multiphonon process. With these investigations, our work illuminates the mechanism of phononassisted Andreev reflection at a Majorana zero mode. 
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