Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session B18: Spinorbit and Superconductivity: Majorana ModesFocus Industry

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Sponsoring Units: GMAG DMP FIAP Chair: Alex MatosAbiague, University at Buffalo Room: 317 
Monday, March 14, 2016 11:15AM  11:27AM 
B18.00001: Quantized Conductance in InSb nanowires at zero magnetic field Jakob Kammhuber, Maja Cassidy, Hao Zhang, {\"O}nder G{\"u}l, Fei Pei, Michiel de Moor, Kenji Watanabe, Takashi Taniguchi, Diana Car, Erik Bakkers, Leo Kouwenhoven We present measurements of InSb nanowires in the ballistic transport regime. In 1D materials such as nanowires, electron scattering has an increased chance of backreflection, obscuring the observation of quantized conductance at low magnetic fields. By improving the contacts to the nanowire as well as its dielectric environment backscattering events are minimized and conductance quantization is observable at zero magnetic field with high device yield. We study the evolution of individual subbands in an external magnetic field, observing a degeneracy between the 2\textsuperscript{nd} and 3\textsuperscript{rd} subband when the magnetic field is orientated perpendicular to the nanowire axis. [Preview Abstract] 
Monday, March 14, 2016 11:27AM  11:39AM 
B18.00002: Majorana fermions in charge carrier hole quantum wires Jingcheng Liang, Yuli LyandaGeller In order to realize Majorana fermions in a hybrid semiconductor superconductor structure, spin helical order is needed, which prevents “fermion doubling”. A natural proposal for Majarana fermion setting is to utilize charge carrier hole systems, which have strong spin orbit couplings that can result in a spin helix. In this work, we demonstrate that transformation of heavy holes into light holes and vice versa upon reflection from the heterostructure boundaries crucially affects Luttinger hole spectrum in low dimensions, and most importantly, spinorbit interactions in the ground subband of a hole quantum wire. Spin orbit interactions are dominated by several terms linear in the hole momentum. We show that the criterion for realizing proximityinduced topologically nontrivial superconducting phase and zero Majorana modes in a hole wire is similar to that for electrons, but an extra constraint should be satisfied. Due to their stronger spinorbit coupling, the hole systems can be promising settings to study Majorana fermions experimentally. [Preview Abstract] 
Monday, March 14, 2016 11:39AM  11:51AM 
B18.00003: Majorana modes in InSb nanowires (II): resolving the topological phase diagram Hao Zhang, \"Onder G\"ul, Michiel de Moor, Fokko de Vries, Jasper van Veen, David van Woerkom, Kun Zuo, Vincent Mourik, Maja Cassidy, Attila Geresdi, Diana Car, Erik Bakkers, Srijit Goswami, Kenji Watanabe, Takashi Taniguchi, Leo Kouwenhoven Majorana modes in hybrid superconductorsemiconductor nanowire devices can be probed via tunnelling spectroscopy which shows a zero bias peak (ZBP) in differential conductance (1). Majoranas are formed when the Zeeman energy $E_Z$ and the chemical potential $\mu$ satisfy the condition $E_Z>\sqrt{\Delta^2+\mu^2}$, with $\Delta$ the superconducting gap. This Majorana condition outlines the topologically nontrivial phase and predicts a particular dependence of ZBPs on the gate voltage and the external magnetic field. In this talk we show that the magnetic field range of ZBPs can be tuned by gate voltage and vice versa, consistent with these Majorana predictions. Supported by measurements in different external magnetic field orientations, these observations pave the way for exploring the topological phase diagram of spinorbit coupled semiconductor nanowires with induced superconductivity. \\ (1) V. Mourik, K. Zuo et al, \emph{Science} \textbf{336}, 1003 (2012) [Preview Abstract] 
Monday, March 14, 2016 11:51AM  12:03PM 
B18.00004: Majorana modes in InSb nanowires (I): zero bias peaks in hybrid devices with lowdisorder and hard induced superconducting gap \"O. G\"ul, H. Zhang, M.W.A. de Moor, F. de Vries, J. van Veen, D.J. van Woerkom, K. Zuo, V. Mourik, M. Cassidy, A. Geresdi, D. Car, E.P.A.M. Bakkers, S. Goswami, K. Watanabe, T. Taniguchi, L.P. Kouwenhoven Majorana modes in hybrid superconductorsemiconductor nanowire devices can be probed via tunnelling spectroscopy which shows a zero bias peak (ZBP) in differential conductance (1). However, alternative mechanisms such as disorder or formation of quantum dots can also give rise to ZBPs, and obscure experimental studies of Majoranas. Further, a soft induced superconducting gap commonly observed in experiments presents an outstanding challenge for the demonstration of their topological protection. In this talk we show that with device improvements, we reach lowdisorder transport regime with clear quantized conductance plateaus and Andreev enhancement approaching the theoretical limit. Tunnelling spectroscopy shows a hard induced superconducting gap and no formation of quantum dots. Together with extremely stable ZBPs observed in large gate voltage and magnetic field ranges, we exclude various alternative theories besides the formation of localized Majorana modes for our observations. \\ (1) V. Mourik, K. Zuo et al, \emph{Science} \textbf{336}, 1003 (2012) [Preview Abstract] 
Monday, March 14, 2016 12:03PM  12:15PM 
B18.00005: Investigation of semiconductor nanowires with shadowevaporated epitaxial superconducting shells John Watson, Maja Cassidy, Jakob Kammhuber, Michiel de Moor, Leo Kouwenhoven, Peter Krogstrup, Mingtang Deng, Thomas Jespersen, Jesper Nygard, Charles Marcus We report progress on epitaxially grown InAs/Al core/shell nanowire heterostructures by molecular beam epitaxy with junctions in the Al shells formed insitu by a crossed wire shadowing method during growth. Such wires allow the creation of superconductornormalsuperconductor (SNS) junctions with high quality superconductorsemiconductor interfaces without introducing damage in the junction by etching the Al. Shadowing is accomplished by a twostep growth process in which the nanowire growth direction is changed resulting in crossed networks of nanowires which shadow one another from the Al flux. We observe hard superconducting gaps and supercurrents in excess of 50 nA with inplane critical fields above 1 T. We compare our results with shadowed devices to previous data from SNS junctions with wetetched shells. Our experiments indicate that this crossed wire shadowing technique provides an interesting route to investigating induced superconductivity in semiconductor nanowires. [Preview Abstract] 
Monday, March 14, 2016 12:15PM  12:27PM 
B18.00006: Wireless Majorana Fermions: From Magnetic Tunability to Braiding Geoffrey Fatin, Alex MatosAbiague, Benedikt Scharf, Igor Zutic We propose a versatile platform to investigate the existence of zeroenergy Majorana fermions (MFs) and their nonAbelian statistics through braiding [1]. This implementation combines a twodimensional electron gas formed in a semiconductor quantum well grown on the surface of an $s$wave superconductor, with a nearby array of magnetic tunnel junctions (MTJs). The underlying magnetic textures produced by MTJs provide highlycontrollable topological phase transitions to confine and transport MFs in two dimensions, overcoming the requirement for a network of wires.\\ &[1&] G. Fatin, A. MatosAbiague, B. Scharf, and I. \v{Z}uti\'c, arXiv eprints (2015), arXiv:1510.08182v1. [Preview Abstract] 
Monday, March 14, 2016 12:27PM  1:03PM 
B18.00007: SpinOrbit Coupling in Hybrid Semiconductor Structures: From Majorana Fermions to Topological Insulators Invited Speaker: Benedikt Scharf Hybrid semiconductor structures with strong spinorbit coupling are responsible for many fascinating phenomena. Topological states in systems of reduced dimensionality, in particular, offer many intriguing possibilities, both for fundamental research as well as for potential applications. In this talk, we describe the importance of the interplay of spinorbit coupling (SOC) and the sample geometry in realizing exotic Majorana fermions (MFs) in quantum dots and rings and discuss several schemes to detect MFs [1]. An effective SOC from the magnetic textures provided by magnetic tunnel junctions could enable a versatile control of MFs and their adiabatic exchange [2]. We show that in 2D topological insulators (TIs), such as inverted HgTe/CdTe QWs, helical quantum spin Hall (QSH) states persist even at finite magnetic fields below a critical magnetic field above which only quantum Hall (QH) states can be found [3]. We propose magnetooptical absorption measurements to probe the magneticfield induced transition between the QSH and QH regimes. This measurement scheme is robust against perturbations such as additional SOC due to bulk or structureinversion asymmetry [4]. Finally, tunnel junctions based on the surfaces of 3D TIs are presented. These junctions can exhibit giant tunneling anomalous Hall (TAH) currents and negative differential TAH conductance, which makes them an attractive and versatile system for spintronic applications [5]. [1] B. Scharf and I. Zutic, PRB \textbf{91}, 144505 (2015). [2] G. L. Fatin et al., arXiv:1510.08182. [3] B. Scharf et al., PRB \textbf{86}, 075418 (2012). [4] B. Scharf, et al., PRB \textbf{91}, 235433 (2015). [5] B. Scharf et al, preprint. [Preview Abstract] 
Monday, March 14, 2016 1:03PM  1:15PM 
B18.00008: Wavefunction oscillations and fermion parity crossings in disordered Majorana wire Suraj Hegde, Smitha Vishveshwara We study aspects of decay and oscillations of Majorana wavefunctions in one dimensional topological superconducting chains, by employing Majorana transfer matrix technique. The phase transition separating the trivial phase and the topological phase associated with the Majorana end modes can be traced to the cancellation of the two parts (‘superconducting’ and ‘normal’) of the Lyapunov exponent of the transfer matrix. We find that the Majorana oscillations and related fermion parity flips can be completely determined by an underlying nonsuperconducting tightbinding model. Using this observation we pinpoint the behavior of Majorana mode oscillations within the topological phase diagram. For a disordered wire, these band oscillations are completely washed out, leading to a second localization length for the Majorana mode. The remnant oscillations are however manifested and completely randomized by disorder effects. As a result, the associated fermion parity flips depend heavily on the average of the disorder distribution and the number of lattice sites of the chain. We show that the transfer matrix technique offers a simple way of understanding the known lognormal distribution of midgap Majorana states. [Preview Abstract] 
Monday, March 14, 2016 1:15PM  1:27PM 
B18.00009: Interfacial spinorbit fields in ferromagnet/normal metal (FN) and ferromagnet/superconductor (FS) systems Petra Hoegl, Alex MatosAbiague, Igor Zutic, Jaroslav Fabian Breaking of spaceinversion symmetry at interfaces induces spinorbit fields as an emergent phenomenon. Interfacial spinorbit fields are believed to enable a wealth of new phenomena, not existent or fragile in the bulk, such as the tunneling anisotropic magnetoresistance (TAMR), interfacial spinorbit torques, Skyrmions, or possible realization of topological superconductors. We theoretically investigate spinpolarized transport in FN and FS junctions in the presence of Rashba and Dresselhaus interfacial spinorbit fields. The interplay of magnetism and spinorbit fields leads to a marked magnetoanisotropy of the conductances. Remarkably, the anisotropy in FS systemsmagnetoanisotropic Andreev reflection (MAAR)is giant compared to TAMR, its normalstate counterpart in FN junctions [1]. We further report on the dependence of spinflip probability currents on characteristic system parameters [2].\newline [1] P. H\"{o}gl, A. MatosAbiague, I. \v{Z}uti\'c, J. Fabian, Phys. Rev. Lett. \textbf{115}, 116601 (2015)\newline [2] A. M. Kamerbeek, P. H\"{o}gl, J. Fabian, T. Banerjee, Phys. Rev. Lett. \textbf{115}, 136601 (2015) [Preview Abstract] 
Monday, March 14, 2016 1:27PM  1:39PM 
B18.00010: An exactly solvable model for a strongly spinorbitcoupled nanowire quantum dot Rui Li, LianAo Wu, Xuedong Hu, J.Q. You In the presence of spinorbit coupling, quantum models for semiconductor materials are generally not exactly solvable. As a result, understanding of the strong spinorbit coupling effects in these systems remains incomplete. Here we develop a method to solve exactly the onedimensional hardwall quantum dot problem for a single electron in the presence of a strong spinorbit coupling and a finite magnetic field. This method allows us to obtain the exact eigenenergies and eigenstates for the single electron. With the help of this solution, we demonstrate unique effects from the strong spinorbit coupling in a semiconductor quantum dot, in particular the anisotropy of the electron gfactor and its tunability. [Preview Abstract] 
Monday, March 14, 2016 1:39PM  1:51PM 
B18.00011: Charge instability in double quantum dots in Ge/Si core/shell nanowires Azarin Zarassi, Zhaoen Su, Jens Schwenderling, Sergey M. Frolov, Moïra Hocevar, BinhMinh Nguyen, Jinkyoung Yoo, Shadi A. Dayeh Controlling dephasing times are of great challenge in the studies of spin qubit. Reported long spin coherence time and predicted strong spinorbit interaction of holes in Ge/Si core/shell nanowires, as well as their weak coupling to very few nuclear spins of these group IV semiconductors, persuade electrical spin control. We have established Pauli spin blockade in gatetunable quantum dots formed in these nanowires. The gfactor has been measured and evidence of spinorbit interaction has been observed in the presence of magnetic field. However, electrical control of spins requires considerable stability in the double dot configuration, and imperfectly these dots suffer from poor stability. We report on fabrication modifications on Ge/Si core/shell nanowires, as well as measurement techniques to suppress the charge instabilities and ease the way to study spinorbit coupling and resolve electric dipole spin resonance. [Preview Abstract] 

B18.00012: ABSTRACT WITHDRAWN 
Monday, March 14, 2016 2:03PM  2:15PM 
B18.00013: Spin transistor based on pure nonlocal Andreev reflection in EuOgraphene/superconductor/EuOgraphene nanostructure Yee Sin Ang, Lay Kee Ang, Chao Zhang, Zhongshui Ma In graphenemagneticinsulator hybrid structure such as grapheneEuropiumoxide, proximity induced exchange interaction opens up a spindependent bandgap and spin splitting in the Dirac band. We show that such band topology allows pure crossed Andreev reflection to be generated exclusively without the parasitic local Andreev reflection and elastic cotunnelling over a wide range of bias and Fermi levels. We model the charge transport in an EuOgraphene/superconductor/EuOgraphene threeterminal device and found that the pure nonlocal conductance exhibits rapid on/off switching characteristic with a minimal subthreshold swing of $\sim 20$ mV. Nonlocal conductance oscillation is observed when the Fermi levels in the superconducting lead is varied. The oscillatory behavior is directly related to the quasiparticle propagation in the superconducting lead and hence can be used as a tool to probe the subgap quasiparticle mode in superconducting graphene. The nonlocal current is 100\% spinpolarized and is highly tunable in our proposed device. This opens up the possibility of highly tunable graphenebased spin transistor that operates purely in the nonlocal transport regime. [Preview Abstract] 
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