Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V13: Majorana Bound States IIFocus

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Sponsoring Units: DMP Chair: Roman Lutchyn, Microsoft Station Q Room: LACC 304A 
Thursday, March 8, 2018 2:30PM  3:06PM 
V13.00001: Quantized Signature of Majorana Fermion:
Particle being its own Antiparticle Invited Speaker: Kang Wang In 1937, Ettore Majorana proposed a particle being its antiparticle. Since its inception, Majorana has been under intensive pursuit both theoretically and in experiments. Recent interest in robust topologically protected quantum computing has accelerated the experimental quest of Majorana. Among various proposals, I will discuss the scenario when a topological insulator meets a superconductor. This system offers a possible host for Majorana. The talk will begin from the experimental efforts of the quest of dissipationless transport: quantum Hall without magnetic field, quantum spin Hall to quantum anomalous Hall (QAH). The latter was enabled by a long term effort in the materials growth of topological insulator  magnetic (Cr) doped BiSbTe to achieve reliably QAH. I will discuss the topological transitions of Dirac electrons for TI in QAH. When the QAH edge states interface with a superconductor, the Dirac electron space is transformed to the Nambu space, hosting Majorana fermions via pairing energy. We will describe our experimental efforts to show the convincing evidence of quantized signature of the onedimensional chiral Majorana fermion [1]. A halfinteger quantized conductance plateau (0.5 e^{2}/h) gives a firm signature of the elusive Majorana fermion for the first time by scanning topological phase transitions under the reversal of the magnetization. This finding gives a new direction for topological quantum computing, for which I will discuss several possible paths for realizing the elemental qubits and operations. 
Thursday, March 8, 2018 3:06PM  3:18PM 
V13.00002: Intrinsic ac anomalous Hall effect of nonsymmorphic chiral superconductors with an application to UPt_{3} Zhiqiang Wang , John Berlinsky , Gertrud Zwicknagl , Catherine Kallin We identify an intrinsic mechanism of the anomalous Hall effect for nonsymmorphic chiral superconductors. This mechanism relies on both a nontrivial multiband chiral superconducting order parameter, which is a mixture of pairings of even and odd angular momentum channels, and a complex normal state intersublattice hopping, both of which are consequences of the nonsymmorphic group symmetry of the underlying lattice. We apply this mechanism to the putative chiral superconducting phase of the heavyfermion superconductor UPt_{3} and calculate the anomalous ac Hall conductivity in a simplified twoband model. From the ac Hall conductivity and optical data we estimate the polar Kerr rotation angle and compare it to the measured results for UPt_{3} [E. R. Schemm et al., Science 345,190(2014)]. 
Thursday, March 8, 2018 3:18PM  3:30PM 
V13.00003: Quasionedimensional Quantum Anomalous Hall Systems as New Platforms for Scalable Topological Quantum Computation ChuiZhen Chen , Yingming XIE , Jie Liu , Patrick Lee , Kam Tuen Law Quantum anomalous Hall insulator/superconductor heterostructures emerged as a competitive platform to realize topological superconductors with chiral Majorana edge states as shown in recent experiments [He et al. Science 357, 294 (2017)]. However, chiral Majorana modes, being extended, cannot be used for topological quantum computation. In this work, we show that quasionedimensional quantum anomalous Hall structures exhibit a large topological regime (much larger than the twodimensional case) which supports localized Majorana zero energy modes. The nonAbelian properties of a crossshaped quantum anomalous Hall junction is shown explicitly by timedependent calculations. We believe that networks of such quasionedimensional quantum anomalousHall systems can be easily fabricated for scalable topological quantum computation. 
Thursday, March 8, 2018 3:30PM  3:42PM 
V13.00004: Disorderinduced halfinteger quantized conductance plateau in quantum anomalous Hall insulator–superconductor structures Yingyi Huang , F. Setiawan , Jay Sau Weak superconducting proximity effect in the vicinity of the topological transition of a quantum anomalous Hall (QAH) system has been proposed as a venue to realize a topological superconductor (TSC) with chiral Majorana edge modes(CMEMs). Recent experiment [He, et al., Science 357, 294 (2017)] claimed to have observed such CMEMs in the form of a halfinteger quantized conductance plateau in the twoterminal transport measurement of a QAHsuperconductor junction. While the presence of a superconducting proximity effect generically splits the quantum Hall transition into two phase transitions with a gapped TSC in between, in this work we propose that a nearly flat conductance plateau, similar to that expected from CMEMs, can also arise from the percolation of quantum Hall edges well before the onset of the TSC or at temperatures much above the TSC gap. Our work, therefore, suggests that in order to confirm the TSC, it is necessary to supplement the observation of the halfquantized conductance plateau with a hard superconducting gap (which is unlikely for a disordered system) from the conductance measurements or the heat transport gap. Alternatively, the halfquantized thermal conductance would also serve as a smokinggun signature of the TSC. 
Thursday, March 8, 2018 3:42PM  3:54PM 
V13.00005: Search for topological superconductivity in the proximitized, quantumspinHall edge state of bismuth bilayers Berthold Jaeck , Yonglong Xie , Sangjun Jeon , Aris Alexandradinata , Andrei Bernevig , Ali Yazdani Twodimensional topological insulators host helical, onedimensional (1D) modes that are protected by timereversal symmetry. Proximity induced superconductivity on such edge modes is predicted to be topological in nature and provide a platform for realization of Majorana zero modes. Previously, scanning tunneling microscopy (STM) studies have found evidence for the presence of topological edge modes in bismuth (Bi) bilayers on the surface of bulk Bi(111) [1]. By epitaxially growing thin films on the surface of Nb, we have successfully induced superconductivity into (111) oriented Bi films and bilayers . Using highresolution STM spectroscopy and quasiparticle interference mapping, we characterize the nature of both the normal and the superconducting properties of Bi bilayer edge modes. We find the 1D edge mode to develop a hard superconducting gap that appears to be spectroscopically distinct from the gap of the 2D surface state of the bilayer islands. We will describe these and other experiments designed to elucidate the nature of superconductivity and search for signatures of topological superconductivity developing in these 1D topological edge states. 
Thursday, March 8, 2018 3:54PM  4:06PM 
V13.00006: Supercurrent as a probe for topological superconductivity in magnetic adatom chains Narayan Mohanta , Arno Kampf , Thilo Kopp A magnetic adatom chain, proximity coupled to a conventional superconductor with spinorbit coupling, is a candidate system that exhibits topological superconductivity. Such a composite system exhibits locally a spintriplet pairing amplitude. We show that the singlettriplet junction, thus formed, leads to a net spin accumulation in the near vicinity of the chain. The accumulated spins are polarized along the direction of the local $\mathbf{d}$vector for triplet pairing and generate an enhanced persistent current flowing around the chain. The spin polarization and the “supercurrent” reverse their directions beyond a critical exchange coupling strength at which the singlet order parameter changes its sign on the chain. The current is strongly enhanced in the topological superconducting regime where Majorana bound states appear at the chain ends. The current and the spin profile offer alternative routes to characterize the topological superconducting state in adatom chains and islands. 
Thursday, March 8, 2018 4:06PM  4:18PM 
V13.00007: Topological superconductor in quasionedimensional Tl_{2−x}Mo_{6}Se_{6} ShinMing Huang , ChuangHan Hsu , Suyang Xu , ChiCheng Lee , ShiueYuan Shiau , Hsin Lin , Arun Bansil Search for topological superconductors would be the most attractive subject among all the topological materials in part because the Majorana fermions they allow at boundaries are of potential applications in quantum computation. In this work, we propose that the quasionedimensional molybdenum selenide compound Tl_{2−x}Mo_{6}Se_{6} is a timereversalinvariant topological superconductor induced by intersublattice pairing. Weak spinorbit coupling will pin the triplet d vector lying perpendicular to the chain direction and results in the twodimensional E_{2u} superconductivity with a concurrent nematic order. We estimate the locking energy of the d vector to be weak and hence the proof of its direction would rely on tunnelling or phasesensitive measurements. 
Thursday, March 8, 2018 4:18PM  4:30PM 
V13.00008: Topological Majorana TwoChannel Kondo Effect Fan Zhang , Zhiqiang Bao A onedimensional timereversalinvariant topological superconductor hosts a Majorana Kramers pair at each end, where timereversal symmetry acts as a supersymmetry that flips local fermion parity. We examine the transport anomaly of such a superconductor, floating and tunnelcoupled to normal leads at its two ends. We demonstrate the realization of a topologically protected, channelsymmetric, twochannel Kondo effect without finetuning. Whereas the nonlocal teleportation vanishes, a lead present at one end telecontrols the universal transport through the other end. Reference: PRL 119, 187701 (2017). 
Thursday, March 8, 2018 4:30PM  4:42PM 
V13.00009: Topology and Quantum Criticality: their interplay in one dimension Ruben Verresen , Nick Jones , Roderich Moessner , Frank Pollmann Critical points between onedimensional symmetryprotected topological (SPT) phases are explored. We classify these for noninteracting superconducting spinless fermions with timereversal symmetry (BDI): the gapless phases are labeled by the central charge (of the conformal field theory) and a topological invariant. The latter invariant protects exponentially localized edge modes, despite there being no gapped degrees of freedom in the bulk. Moreover, the central charge of the critical point obeys a topological lower bound, determined by the nearby gapped phases. We conjecture such a lower bound to hold for all transitions between onedimensional SPT phases, as supported by analytical and numerical examples. 
Thursday, March 8, 2018 4:42PM  4:54PM 
V13.00010: Even to Odd Fermion Parity Transition in Majorana Islands based on AlInSb Hybrid System Jie Shen , Sebastian Heedt , Francesco Borsoi , Bernard Van Heck , Sasa Gazibegovic , Diana Car , Roy L. M. Op het Veld , Daniël Bouman , John Logan , Chris Palmstrom , Attila Geresdi , Erik P. A. M. Bakkers , Leo Kouwenhoven Different topological qubit schemes, based on onedimensional nanowires with proximityinduced superconductivity, have been proposed. Most of them consist of multiple superconducting islands in the Coulomb blockade regime, each hosting pairs of Majorana zero modes (MZMs). We build such Majorana islands in InSb nanowires with epitaxial Al shells. The measurement of Coulomb diamonds reveals 2e periodicity with respect to the gate voltage, originating from the even fermion parity being protected by a hard superconducting gap. When a magnetic field is applied parallel to the nanowire, the 2e periodicity gradually changes to 1eperiodic oscillations with alternating regions of even and odd fermion parity. This behavior is related to the appearance of lowenergy states, signalling either the appearance of MZMs in the topological regime or the presence of nontopological Andreev bound states. By sweeping across large ranges of plunger gate and tunneling gate voltages, we found that some features are very sensitive to the gate settings, and thus easily construed in terms of localized Andreev bound states. In other cases, the 1e oscillations are more robust and accompanied by an excitation gap in finite bias measurements, which could be interpreted as due to the presence of MZMs. 
Thursday, March 8, 2018 4:54PM  5:06PM 
V13.00011: Electric Field Tunable Superconductorsemiconductor Coupling in Majorana Nanowires Michiel De Moor , Di Xu , Jouri Bommer , Hao Zhang , Arno Bargerbos , Georg Winkler , Andrey Antipov , Guanzhong Wang , Nick van Loo , Roy Op het Veld , Sasa Gazibegovic , Diana Car , John Logan , Chris Palmstrom , Roman Lutchyn , Erik Bakkers , Leo Kouwenhoven Hybrid semiconductor nanowiresuperconductor systems have attracted interest in recent years as possible platforms for hosting Majorana Zero Modes (MZM). Although this system has been extensively studied both theoretically and experimentally, it has recently become clear that the effective parameters of the system depend strongly on the electrostatic environment. We use InSb nanowires with epitaxially grown Al superconductor to study the effect of electrostatic gating on device parameters such as the superconducting gap and the effective gfactor. We find that the induced superconducting gap as well as the effective Zeeman gfactor changes as the device is tuned from strong coupling to weak coupling, which is corroborated by SchrödingerPoisson simulations of the device geometry. We further investigate the effect this has on the phase diagram of Zero Bias Conductance Peaks (ZBCP), which are generally considered as a signature of MZM. 
Thursday, March 8, 2018 5:06PM  5:18PM 
V13.00012: Fate of Topological Superconductivity in Epitaxial ThinFilm Majorana Setups with Disorder Christopher Reeg , Daniel Loss , Jelena Klinovaja We study the proximity effect in a quantum wire strongly coupled to a superconductor with a thickness which is much shorter than its coherence length. Such geometries have become increasingly relevant in recent years in the experimental search for Majorana fermions with the development of thin epitaxial Al layers which form a very strong contact with either InAs or InSb. So far, however, no theoretical treatment of the proximity effect in these systems has accounted for the finite size of the superconducting film. We show that the finitesize effects essentially render the topological phase inaccessible when the level spacing of the superconductor greatly exceeds its energy gap. Without any finetuning of the size of the superconductor, the “hard gap” regime which is seen ubiquitously in the experiments is accompanied by a large shift in the effective chemical potential of the nanowire that pushes the topological phase transition to magnetic field strengths which greatly exceed the critical field of Al. We also show that this result is robust with respect to disorder scattering within the thin superconducting film. Our results demonstrate the significant importance of treating the parent superconductor on a microscopic level. 
Thursday, March 8, 2018 5:18PM  5:30PM 
V13.00013: The Quantum Anomalous Hall Majorana Platform Yongxin Zeng , Chao Lei , Gaurav Chaudhary , Allan MacDonald We show that quasionedimensional (1D) quantum wires can be written onto the surface of magnetic topological insulator (MTI) thin films by gate arrays. When the MTI is in a quantum anomalous Hall state, MTI/superconductor quantum wires have especially broad stability regions for both topological and nontopological states, facilitating creation and manipulation of Majorana particles on the MTI surface. 
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