Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S13: Majorana Bound States IFocus

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Sponsoring Units: DMP Chair: Gen Yin, University of California, Los Angeles Room: LACC 304A 
Thursday, March 8, 2018 11:15AM  11:51AM 
S13.00001: Chiral Majorana Modes Invited Speaker: Jing Xia An emerging approach to quantum computing seeks to utilize topologically protected quantum states as Qubits to solve the errorcorrection problem, as the information encoded in such a "topological quantum computer" cannot be easily corrupted. A recent focus in condensed matter physics has been finding and fabricating such topological materials. In this talk, I will discuss two material systems that could host chiral Majorana modes and may have potential applications in topological quantum computing. The first system is the interface between a magnetically doped topological insulator and a superconductor, where we found experimental transport evidence for a chiral edge state of Majorana Fermions, which were proposed theoretically by Ettore Majorana in the 1930s by remained elusive. The second system is the ultrathin bilayer film of bismuth and nickel, where we found experimental optical evidence for a superconducting state that breaks timereversal symmetry, pointing to a 'd+id' superconducting state. Theories suggest that this state may have two chiral Majorana edge modes propagating around the sample edge, either clockwise or counterclockwise. These Majorana edge states, with further engineering and manipulation, could be useful for topological quantum computing. The works presented here were performed in collaboration with Qing Lin He, Lei Pan, Alexander L. Stern, Edward Burks, Xiaoyu Che, Gen Yin, Jing Wang, Biao Lian, Quan Zhou, Eun Sang Choi, Koichi Murata, Xufeng Kou, Tianxiao Nie, Qiming Shao, Yabin Fan, ShouCheng Zhang, Kai Liu, Kang L. Wang (Science, 357(6348), 294–299 (2017)) , and in collaboration with Xinxin Gong, Mehdi Kargarian, Alex Stern, Di Yue, Hexin Zhou, Xiaofeng Jin, Victor M. Galitski, Victor M. Yakovenko. (Science Advances, 3, 3, e1602579 (2017)) 
Thursday, March 8, 2018 11:51AM  12:03PM 
S13.00002: Towards unveiling elusive Majorana fermions in gold nanowires via STS Sujit Manna, Peng Wei, Patrick Lee, Jagadeesh Moodera Onedimensional topological superconductor has inspired extensive theoretical work and spurred several experimental thrusts to explore Majorana fermions (MFs). A particularly promising theory involves combining the surface state of gold to an swave superconductor subjected to an applied field [1]. This system promises robust MFs protected by an intense Rashba spinorbit coupling much larger than in semiconductor systems. Spatially well separated MFs are expected in long nanowires of Au, for an unambiguous detection of MFs. We studied scanning tunneling spectroscopy (STS) of Au nanowires long compared to the MF coupling length. STS shows clean superconducting gap induced in the surface state of Au via proximity effect. Using spatially resolved STS at ~ 400mK with a magnetic field applied along the wire, we demonstrate a pair of zerobias bound states emerging at the ends of nanowire. While the observed nonlocal features nicely agree with the earlier theoretical prediction, our temperature and energy resolution is not sufficient to distinguish between MF and minigap state that are expected to exist at the end of a multichannel wire. [1] Potter, A. C. and Lee, P. A., Phys Rev Lett 105, 227003 (2010). 
Thursday, March 8, 2018 12:03PM  12:15PM 
S13.00003: 2eperiodic Switching Current Modulation in Nanowire Single Cooper Pair Transistors Jasper Van Veen, Alex Proutski, Dmitry Pikulin, Torsten Karzig, Roman Lutchyn, Peter Krogstrup, Jesper Nygard, Attila Geresdi, Leo Kouwenhoven, John Watson Over the past years, many studies have reported on signatures of Majorana modes via zerobias peaks. However, to date there have been no experiments demonstrating their nonAbelian statistics. Recently, Aasen et al. proposed that the Majorana fusion channels, intimately related to their nonAbelian statistics, could be probed in a singlewire geometry [1]. An outstanding issue for the fusion experiment, however, is quasiparticle poisoning, because it can lead to decoherence and readout errors. 
Thursday, March 8, 2018 12:15PM  12:27PM 
S13.00004: Signatures of Majorana Fermions in Hybrid SuperconductorTopological Insulator Josephson Junctions Guang Yue, Can Zhang, Erik Huemiller, Maryam Salehi, Nikesh Koirala, Seongshik Oh, Alexey Bezryadin, Dale Van Harlingen We are studying the transport properties of lateral superconductortopological insulatorsuperconductor Josephson junctions. In a magnetic field perpendicular to the TI surface, it is expected that localized zeroenergy Majorana bound states will be stabilized in the junction at locations at which the phase difference is an oddmultiple of π, i.e. at the core of Josephson vortices. Our measurements of the critical current modulation patterns have shown lifting of odd nodes, which we interpret as evidence for a 4πperiodic sin(φ/2)component in the Josephson currentphase relation (CPR), and motivated a model for the nucleation and manipulation of these Majorana states. In this talk, we summarize the results of a series of experiments designed to test in detail specific features of this model: (1) testing whether there is an evenodd pattern of nodelifting, (2) identifying irregular features in the diffraction pattern that could indicate the abrupt entry of Majorana states, (3) looking for changes in the noise distribution of critical currents that would indicate parity transitions of Majorana pairs, and (4) searching for nonsinusoidal components in direct measurements of the CPR using an asymmetric SQUID technique. 
Thursday, March 8, 2018 12:27PM  12:39PM 
S13.00005: Comparing Majorana ZBP with trivial Andreev bound states and super hard superconducting gap in InSb nanowire hybrid devices Peng Yu, Jun Chen, Moira Hocevar, Diana Car, Sébastien Plissard, Erik Bakkers, Sergey Frolov Majorana bound states(MBS) has been predicted to emerge in 1D nanowire with induced superconductivity, spinorbital coupling and external magnetic field. Tunneling spectroscopy can be used to probe the signal of MBS, namely Zero bias conductance peaks(ZBP) appearing at finite field with suitable chemical potential. However, trivial Andreev bound states due to quantum confinement can also give rise to ZBPs with very similar manner. We compare Majorana ZBPs with Andreev bound states in different aspects: ZBP phase diagram, onset magnetic field and field angle dependence. We will also show super hard induced superconducting gap in our system with further improved contact recipe. 
Thursday, March 8, 2018 12:39PM  12:51PM 
S13.00006: Replicas of Andreev spectra in InSb nanowires Azarin Zarassi, Zhaoen Su, JenFeng Hsu, Pablo SanJose, Elsa Prada, Ramon Aguado, Eduardo Jian Hua Lee, Diana Car, Sasa Gazibegovic, Roy Op het Veld, Sébastien Plissard, John Logan, Mihir Pendharkar, Dan Pennachio, Joon Sue Lee, Moira Hocevar, Chris Palmstrom, Erik P. A. M. Bakkers, Sergey Frolov A superconducting contact to a quantum dot can induce electronhole correlations known as Andreev bound states (ABSs) inside the dot. We study transport through ABSs in InSb nanowires with superconducting contacts as both source and drain. Anomalous transport resonances are observed in these systems, namely one or more (up to 4) replicas of Andreev spectra at higher sourcedrain biases, as well as negative differential conductance shadows of the resonances inside the induced gap. We develop a model that captures these features by considering the effect of a softgap superconducting probe, and of additional ABSs induced in the dot, and in the nanowire segments adjacent to the dot. Our findings extend the framework for the interpretation of tunneling experiments in mesoscopic systems, and specifically of Majorana bound states which are studied with semiconductor nanowire probes similar to those used here. 
Thursday, March 8, 2018 12:51PM  1:03PM 
S13.00007: Effective gfactor in Majorana Wires Saulius Vaitiekenas, Ming Deng, Jesper Nygard, Peter Krogstrup, Charles Marcus We use the effective gfactor of subgap states, g*, in hybrid InAs nanowires with an epitaxial Al shell to investigate how the superconducting density of states is distributed between the nanowire core and the shell. We find a steplike reduction of g* and improved hard gap with reduced carrier density in the nanowire, controlled by gate voltage. These observations are relevant for Majorana devices, which require tunable carrier density and g* exceeding the gfactor of the proximitizing superconductor. Additionally, we observe the closing and reopening of a gap in the subgap spectrum coincident with the appearance of a zerobias conductance peak. Finally, more recent results from different device geometries are discussed. 
Thursday, March 8, 2018 1:03PM  1:15PM 
S13.00008: Longrange Topological Superconductors: Majorana & Nonlocal Dirac fermions Oscar Viyuela, Liang Fu, Miguel Angel MartinDelgado Longrange interactions can dramatically modify the phase diagram of condensed matter systems. Novel topological superconducting phases can be induced in both wireshaped and planar structures by including longrange effects. In 1D, massless Majorana modes at the edge pair into a new topological quasiparticle: a massive Dirac fermion localized at both edges of the wire. This new topological phase has fractional quantum numbers as a consequence of the longrange couplings. Moreover, we remarkably observe a staircase of higherorder topological phase transitions. In 2D, we show how longrange interactions may greatly enhance the formation of chiral topological superconductivity. The inclusion of longrange effects is potentially applicable to recent experiments with magnetic impurities and islands in superconductors. 
Thursday, March 8, 2018 1:15PM  1:27PM 
S13.00009: Fermionizing Parafermions Aaron Chew, David Mross, Jason Alicea Parafermion zero modes are Majoranafermion generalizations that exhibit comparatively rich nonAbeliananyon properties. We introduce exact mappings that connect parafermion chains, which can emerge in 2D fractionalized media, to strictly 1D fermionic systems. Most notably, Z_{4} parafermions map onto physical electrons enjoying timereversal symmetry. Phases for the former translate into a variety of interesting electronic states, including a timereversalinvariant topological superconductor (TRITOPS) hosting Kramers pairs of edge Majorana zero modes. We further show that nontrivial parafermionic fusion rules survive in their electronic counterparts, and can be detected via a novel pumping cycle for the TRITOPS phase that yields an edge magnetization with quadrupled periodicity. We generalize these results to arbitrary Z_{2N} parafermion chains. Our work highlights new avenues for exploring 'beyondMajorana' physics in experimentally realistic, weakly interacting 1D electronic platforms. 
Thursday, March 8, 2018 1:27PM  1:39PM 
S13.00010: Parafermionic Bound States in 1D Lattices of Spinful Fermions Alessio Calzona, Tobias Meng, Maura Sassetti, Thomas Schmidt Parafermions are emergent excitations, potentially relevant to topological quantum information, which generalize Majorana fermions. Many features of the latter are well captured by the seminal Kitaev model, a 1D lattice of spinless fermions with pwave superconductivity. Here we present a nontrivial generalization of this model to the Ζ_{4} parafermionic case. In particular, we identify and characterize a whole family of spinful fermionic Hamiltonians on a 1D lattice which support zeroenergy parafermions, localized at the two ends of the system. Their properties are discussed as well as their robustness with respect to variations of parameters in the fermionic Hamiltonian. Our study provides a starting point to make a connection between fermionic lattice models and the low energy field theory description of 1D interacting systems featuring parafermionic bound states. 
Thursday, March 8, 2018 1:39PM  1:51PM 
S13.00011: Efficient realspace parameter optimization algorithm for Majorana nanowires Samuel Boutin, Ion Garate We present an efficient gradientbased method for the optimization of realspace parameter profiles in quasi1D systems such as Majorana nanowires. Inspired by an analogy with the quantum optimal control algorithm GRAPE (Gradient Ascent Pulse Engineering), we can efficiently evaluate gradients of discretized realspace parameter profiles using analytical derivatives of recursive Green functions expressions. This new approach leads to a polynomial speedup over a simpler finite difference calculation. As an application, we optimize magnetic textures for the creation and stabilization of topological superconducting phases in nanowires without spinorbit coupling. The optimization allows to go beyond intuitive and analytical results, leading to potentially new parameter regimes of experimental relevance in Majorana nanowires. 
Thursday, March 8, 2018 1:51PM  2:03PM 
S13.00012: A New Type of ManyBody Majorana Zero Modes in Fermionic Flux Ladder Model Chun Chen, Wei Yan, ChinSen Ting, Yan Chen, Fiona Burnell One promising avenue to study onedimensional (1D) topological phases is to realize them in synthetic materials such as cold atomic gases. Intriguingly, it is possible to realize Majorana boundary modes in a 1D numberconserving system consisting of two fermionic chains coupled only by pairhopping processes. It is commonly believed that significant interchain singleparticle tunneling necessarily destroys these Majorana modes, as it spoils the Z_2 fermionparity symmetry that protects them. In this talk, we present a new mechanism to overcome this obstacle, by piercing a (synthetic) magnetic \piflux through each plaquette of the Fermi ladder. Using bosonization, we show that in this case there exists an exact leginterchange symmetry that is robust to interchain hopping, and acts as fermion parity at long wavelengths. We utilize density matrix renormalization group and exact diagonalization to verify that the resulting model exhibits Majorana boundary modes up to large singleparticle tunnelings, comparable to the intrachain hopping strength. Our work highlights the unusual impacts of different topologically trivial band structures on these interactiondriven topological phases, and identifies a distinct route to stabilizing Majorana boundary modes in 1D fermionic ladders. 
Thursday, March 8, 2018 2:03PM  2:15PM 
S13.00013: Conductance of a multiMajorana island in the strong tunneling regime Jukka Vayrynen, Roman Lutchyn We consider a mesoscopic superconducting island in a Coulomb blockade regime hosting more than two Majorana zero modes which are coupled to normalmetal leads. It has been predicted that such a system may exhibit a novel phenomenon dubbed as topological Kondo effect. We study transport properties of such an island in the strong tunneling limit and calculate the conductance matrix as a function of temperature and the gate voltage applied to the island. We construct a microscopic model explicitly incorporating the superconductor underlying the island, which we then study using the bosonization technique. This approach provides a more microscopic starting point than previous studies. 
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