Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H22: Spectroscopy of Majorana States in SolidsInvited
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Sponsoring Units: DCMP Chair: Ali Yazdani, Princeton University Room: New Orleans Theater A |
Tuesday, March 14, 2017 2:30PM - 3:06PM |
H22.00001: Experimental progress on Majoranas in semiconductors Invited Speaker: Leo Kouwenhoven Majoranas in semiconductor nanowires can be probed via various electrical measurements. Tunnel spectroscopy reveals zero-bias peaks in the differential conductance. These zero-bias peaks have a particular dependence on magnetic field (amplitude and direction) and electron density, which determine the topological phase diagram. This phase diagram allows to falsify alternative theories for our observations. New challenges include quantum superpositions of Majorana states leading, for instance, to a 4pi current phase relation or a fractional Josephson effect. When the existence of Majoranas is firmly established, the next challenge is to build Majorana qubits. We discuss the different qubit schemes and report on our first building blocks. Recent papers: https://arxiv.org/pdf/1603.04069.pdf, https://arxiv.org/pdf/1609.00333.pdf [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:42PM |
H22.00002: Gapless Andreev bound states in a topological junction on the Quantum Spin Hall insulator HgTe Invited Speaker: Erwann Bocquillon Two dimensional topological insulators coupled to superconducting and ferromagnetic electrodes are a candidate system for the realization of Majorana fermions. Majorana physics has recently attracted considerable attention in both theoretical and experimental studies due to the prospects for new physics stemming from non-Abelian exchange statistics and the associated applications to topological quantum computation. However, experimental studies unveiling the interplay between superconductivity and topological electronic transport remain scarce. Here we report the observations of signatures of topological superconductivity induced in a HgTe quantum well, a system that exhibits the quantum spin Hall effect. Namely, we observe the fractional Josephson effect, in two different manners. When an rf excitation is applied, a doubling of the Shapiro steps is observed [1]. Besides, a clear emission line can be detected at half the Josephson frequency under dc voltage bias conditions [2]. Both features appear more clearly when the sample is gated towards the quantum spin Hall regime, in a regime where the current flows mostly along the edges of the device. These signatures thus strongly point towards induced topological superconductivity in the quantum spin Hall edge states. [1] E. Bocquillon et al., Nature Nanotechnology 10.1038/nnano.2016.159 [2] R.S. Deacon et al., arXiv 1603.09611 [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 4:18PM |
H22.00003: Observation of Majorana fermions in the vortex on topological insulator-superconductor heterostructure Bi$_{2}$Te$_{3}$/NbSe$_{2}$ Invited Speaker: Jinfeng Jia Majorana fermion (MF) zero modes have been predicted in a wide variety of condensed matter systems and proposed as a potential building block for fault-tolerant quantum computer. Signatures of the MFs have been reported in the form of zero-energy conductance peak in various systems. As predicted, MFs appear as zero-energy vortex core modes with distinctive spatial profile in proximity-induced superconducting surface states of topological insulators. Furthermore, MFs can induce spin selective Andreev reflection (SSAR), a unique signature of MFs. We report the observation of all the three features for the MFs inside vortices in Bi$_{2}$Te$_{3}$/NbSe$_{2}$ hetero-structure [1-4], in which proximity-induced superconducting gap on topological surface states was previously established [2,3]. Especially, by using spin-polarized scanning tunneling microscopy/spectroscopy (STM/STS), we observed the spin dependent tunneling effect, and fully supported by theoretical analyses, which is a direct evidence for the SSAR from MFs [4]. More importantly, all evidences are self-consistent. Our work provides definitive evidences of MFs and will stimulate the MFs research on their novel physical properties, hence a step towards their non-Abelian statistics and application in quantum computing. References: 1. M. X. Wang, et al., Science 336, 52-55 (2012). 2. J. P. Xu, et al., Phys. Rev. Lett. 112, 217001 (2014). 3. J. P. Xu, et al., Phys. Rev. Lett. 114, 017001 (2015). 4. H. H Sun, et al., Phys. Rev. Lett. 116, 257003 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:54PM |
H22.00004: Zero Modes in Single and Double Majorana Islands Invited Speaker: Charles Marcus This talk presents recent experiments on InAs nanowires with epitaxial superconducting Al that are electrostatically gated to produce single and double nanowire segments with controllable density and tunnel barriers. These hybrid semiconductor-superconductor devices support Andreev bound states, which coalesce into Majorana zero modes in an applied axial magnetic field. Here, we focus on the regime of Coulomb blockade, where the combined effects of charging energy and zero modes yield distinct and characteristic signatures in transport. Length and coupling-strength dependence of single islands, and interdot coupling dependence of double islands will be presented and compared to recent theory. Prospects for coherent control (qubit initialization, manipulation, and readout) based on these and related devices hybrid Coulomb-blockade structures will also be discussed. [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:30PM |
H22.00005: Conductance of a proximitized nanowire in the Coulomb blockade regime Invited Speaker: Leonid Glazman We build a quantitative theory of two-terminal conductance across finite-length segments of nanowires which are made superconducting by the proximity effect. In the presence of spin-orbit interaction, a proximitized nanowire can be tuned across the topological transition point by an applied magnetic field. Due to a finite segment length, electron transport is controlled by the discrete-charging effect, which gives rise to the Coulomb blockade. As the result, the linear conductance strongly depends on the gate voltage applied to the segment, exhibiting a periodic structure of maxima, known as Coulomb blockade peaks. Upon increasing of the magnetic field, the shape and magnitude of the Coulomb blockade peaks is defined, respectively, by Andreev reflection, single-electron tunneling, and -- after the topological transition occurs -- by resonant tunneling through the Majorana modes emerging after the transition. Our theory provides the framework for the analysis of recent experiments with proximitized nanowires, and identifies the signatures of the topological transition in the two-terminal conductance. The talk is based on research performed in collaboration with Bernard van Heck (Yale University) and Roman Lutchyn (Station Q, Microsoft Research). [Preview Abstract] |
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