Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Z8: Topological Insulators: Transport and interfaces |
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Sponsoring Units: DCMP Chair: Rolando Valdes Aguilar, Los Alamos National Laboratory Room: 307 |
Friday, March 22, 2013 11:15AM - 11:27AM |
Z8.00001: Time-reversal anomaly and Josephson effect in time-reversal invariant topological superconductors Suk Bum Chung, Joshua Horowitz, Xiao-Liang Qi Topological superconductors are gapped superconductors with protected Majorana surface/edge states on the boundary. In this paper, we study the Josephson coupling between time-reversal invariant topological superconductors and $s$-wave superconductors. The Majorana edge/surface states of time-reversal invariant topological superconductors in all physical dimensions 1, 2, 3 have a generic topological property which we named as time-reversal anomaly. Due to the time-reversal anomaly, the Josephson coupling prefers a nonzero phase difference between topological and trivial superconductors. The nontrivial Josephson coupling leads to a current-flux relation with a half period in a SQUID geometry, and also a half period Fraunhofer effect in dimension higher than one. We also show that an in-plane magnetic field restores the ordinary Josephson coupling, as a sharp signature that the proposed effect is a consequence of the unique time-reversal property of the topological edge/surface states. Our proposal provides a general approach to experimentally verify whether a superconductor is topological or not. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z8.00002: Symmetry Protected Josephson Supercurrents in Three-Dimensional Topological Insulators Sungjae Cho, Brian Dellabetta, Alina Yang, John Schneeloch, Zhijun Xu, Tonica Valla, Genda Gu, Matthew Gilbert, Nadya Mason Coupling the surface state of a topological insulator (TI) to an s-wave superconductor is predicted to produce the long-sought Majorana quasiparticle excitations. However, superconductivity has not been measured in surface states when the bulk charge carriers are fully depleted, i.e., in the true topological regime relevant for investigating Majorana modes. Here, we report measurements of DC Josephson effects in TI-superconductor junctions as the chemical potential is moved through the true topological regime characterized by the presence of only surface currents. We compare our results to 3D quantum transport simulations, and determine the effects of bulk/surface mixing, disorder, and magnetic field; in particular, we show that the supercurrent is largely carried by surface states, due to the inherent topology of the bands, and that it is robust against disorder. Our results thus clarify key open issues regarding the nature of supercurrents in TIs. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z8.00003: Majorana fermions in a superconductor quantum wire connected to normal leads Edson Vernek, Ant\^onio C.F. Serid\^onio, Jos\'e C. Egues We study the appearance of Majorana fermions in a quantum wire connected to a normal lead. We employ a Kitaev model for the wire with induced superconductivity with a full coupling with a normal wire. In comparison with previous study of this problem, our approach has the advantage of allowing us to fine tune the Kitaev Hamiltonian model all the way from its normal to its superconducting topological phase. By developing an exact Green's function calculation scheme, we are to explore the full parameter space of the model via analysis of the electron and the Majorana density of states. Our results show clearly that the main effect of a particle-hole symmetric lead is the broadening of the Majorana density of states at the end of the wire, while particle-hole asymmetric leads are detrimental to the Majorana bound states. We also study the transmission through a quantum dot connected to two normal leads and to a superconducting wire. We show that by driving the wire from its normal to its topological phase, a great change in the transmission function through the dot is observed, clearly indicating the emergence of a Majorana mode in the wire. Although such a signature has already been predicted in recent works, our model leads to substantially different results. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z8.00004: Majorana Fermions Under Stress Ming Gong, Li Mao, Sumanta Tewari, Chuanwei Zhang Spin-orbit coupled semiconductor nanowires with Zeeman splitting in proximity contact with bulk s-wave superconductivity have recently been proposed as a promising platform for realizing Majorana fermions. However, in this setup the chemical potential of the nanowire is generally pinned by the Fermi surface of the superconductor. This makes the tuning of the chemical potential by external electrical gates, a crucial requirement for unambiguous detection of Majorana fermions, very challenging in experiments. Here we show that tunable topological superconducting regime supporting Majorana fermions can be realized in semiconductor nanowires using uniaxial stress. For n-type nanowires the uniaxial stress tunes the effective chemical potential, while for p-type systems the effective pairing may also be modified by stress, thus significantly enhancing the topological minigap. The required stress is within current experimental reach using conventional piezo crystals. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z8.00005: The soft superconducting gap in semiconductor Majorana nanowires So Takei, Benjamin M. Fregoso, Hoi-Yin Hui, Alejandro M. Lobos, Sankar Das Sarma We theoretically consider the mysterious topic of the soft gap in the tunneling conductance of the proximity-induced superconductivity in a semiconductor-superconductor hybrid structure, where the observation of a zero-bias conductance peak has created considerable excitement because of its possible connection with the elusive zero-energy Majorana mode. The observed experimental superconducting tunneling gap in the semiconductor nanowire looks v-shaped with considerable subgap conductance even at very low temperatures in sharp contrast to the theoretically expected hard BCS gap with exponentially suppressed subgap conductance. We systematically study, by solving the appropriate BdG equations both numerically and analytically, a number of physical mechanisms (e.g. magnetic and non-magnetic disorder, finite temperature, dissipative Cooper pair breaking, interface fluctuations), which could, in principle, lead to a soft gap, finding that only the interface fluctuation effect is a quantitatively and qualitatively viable mechanism that is consistent with the experimental observations. Our work indicates that improving the quality of the superconductor-semiconductor interface would go a long way in enhancing the gap in the hybrid structures being used for studying the Majorana mode. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z8.00006: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z8.00007: Majorana edge modes of topological exciton condensate with superconductors Babak Seradjeh I study the edge states of the topological exciton condensate formed by a Coulomb interaction between two parallel surfaces of a strong topological insulator. When the condensate is contacted by superconductors with a $\pi$ phase shift across the two surfaces, a pair of counterpropagating Majorana modes close the gap at the boundary. I propose a nanostructured system of topological insulators and superconductors that hosts unpaired Majorana fermions when and only when the exciton condensate forms. Therefore, measuring the Majorana signal in this structure provides a way of detecting the topological exciton condensate that is uniquely related to its topological nature. The relevant experimental signatures as well as implications for related systems are discussed. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z8.00008: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z8.00009: Entanglement in a Cooper-pair Splitter based on a Topological Insulator Koji Sato We theoretically study a solid state device producing entangled electron pairs that are spatially separated by coupling a superconductor to the helical edge states of a two-dimensional topological insulator. The interacting regions of the edge states are taken to be finite length around the tunneling region to capture the effect of non-interacting leads, and the ends of a given edge are further connected to a beam splitter. By controlling the scattering through such beam splitters, we show that Bell test can be performed via measurement of the current-current correlations. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z8.00010: Current-phase relationship of planar Josephson junctions mediated by the surface states of a topological insulator C. Kurter, A.D.K. Finck, C.D. English, Y.S. Hor, D.J. Van Harlingen It is predicted that the presence of Majorana fermions manifests itself with a $4\pi$ periodic current-phase relation (CPR) in planar Josephson junctions formed with topological weak links. To test this proposal, we have fabricated planar junctions by depositing Nb leads on exfoliated Bi$_{2}$Se$_{3}$ single crystals. The temperature and magnetic field dependence of the proximity-induced supercurrent have been examined in various doping regimes accessed via top gating. The critical current modulation with magnetic field deviates from the usual Fraunhofer diffraction pattern, suggesting modifications to a sinusoidal CPR consistent with a $sin(2\phi)$ component. We are corroborating those results with direct measurements of the CPR using a phase-sensitive SQUID interferometry technique. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z8.00011: Signatures of Majorana Fermions in Topological Insulator Josephson Junction Devices Benjamin Wieder, Fan Zhang, Charles Kane We study theoretically the electrical current and low-frequency noise for a linear Josephson junction structure on a topological insulator, in which the superconductor forms a closed ring, and currents are injected from normal regions inside and outside the ring. We find that this geometry offers a unique signature for the presence of gapless 1D Majorana fermion modes that are predicted to exist in the channel when the phase difference $\phi$, controlled by the magnetic flux through the ring, is $\pi$. We show that for low temperature, the linear conductance jumps by $2e^2/h$ when $\phi$ passes through $\pi$, accompanied by non-local correlations between the currents from the leads inside and outside of the ring. We compute the dependence of these features on temperature, voltage, and linear dimensions, and discuss the implications for experiments. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z8.00012: Spinful Majorana fermions and magnetoelectricity in junctions of semiconductor / superconductor heterostructures Panagiotis Kotetes, Alexander Shnirman, Gerd Schoen Recently, the interest in topological quantum computing has grown due to the appearance of promising platforms for realizing Majorana fermions. The most prominent proposal involves a 1D semiconducting quantum wire in proximity to a bulk s-wave superconductor, where in addition a Zeeman field is applied. Here we investigate the Josephson effect in TNT and NTN junctions, consisting of topological (T) and non-topological (N) phases of semiconductor-superconductor 1D heterostructures in the presence of a Zeeman field [1]. A key feature of our setup is that, in addition to the variation of the phase of the superconducting order parameter, we allow the orientation of the magnetic field to change along the junction. We find a novel magnetic contribution to the Majorana Josephson coupling that permits the Josephson current to be tuned by changing the orientation of the magnetic field along the junction. We also predict that a spin current can be generated and additionally controlled by a finite superconducting phase difference. This new type of coupling not only constitutes a unique fingerprint of Majorana fermions but also provides an alternative pathway for manipulating and braiding topological qubits.\\[4pt] [1] P. Kotetes, A. Shnirman, G. Sch\"{o}n, arXiv:1207.2691. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z8.00013: Proximity-effect-induced superconductivity in Bi$_2$Se$_3$ and Bi$_2$Te$_3$ Li Lu, Jie Shen, Yue Ding, Fanming Qu, Fan Yang, Jun Chen, Zhongqing Ji, Guangtong Liu, Jie Fan, Xiunian Jing, Changli Yang In this talk I will present our experimental investigations on the proximity effect between conventional superconductors such as Sn, Pb and the strong spin-orbit coupling materials Bi$_2$Se$_3$ or Bi$_2$Te$_3$ [1-3]. Several types of hybrid devices were fabricated, and their electron transport properties were measured down to $\sim$10 milli-Kelvin temperatures. The results show that a superconducting phase can be easily induced in Bi$_2$Se$_3$ and Bi$_2$Te$_3$ single crystals by superconducting Pb electrodes that are deposited on the surface of the former. The induced superconducting phase can be regarded as a true superconducting phase, i.e., it has an energy gap of the order 0.1 meV, and carries a Josephson supercurrent over a distance as far as several microns. The conductance spectrum of the interface between the induced superconducting phase and the normal phase of Bi$_2$Se$_3$ or Bi$_2$Te$_3$ exhibits a zero-bias peak. Based on the induced superconducting phase, single Josephson junction devices and superconducting quantum interference devices (SQUIDs) were constructied, and their critical supercurrent were investigated as a function of applied magnetic flux. We will discuss the implication of the results in terms of the pairing symmetry of the induced superconducting phase.\\[4pt] [1] F. Yang, et al., Phys. Rev. B 85, 104508 (2012).\\[0pt] [2] F. M. Qu, et al., Scientific Reports 2, 339 (2012). \\[0pt] [3] F. Yang, et al., Phys. Rev. B 86, 134504 (2012). [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z8.00014: Spin-polarized tunneling current through a thin film of a topological insulator in a parallel magnetic field Victor Yakovenko, Sergey Pershoguba We calculate the tunneling conductance between the surface states on the opposite sides of an ultra-thin film of a topological insulator in a parallel magnetic field $B$. The parallel magnetic field produces a relative shift of the in-plane momenta of the two surfaces states. An overlap between the shifted Fermi circles and spinor wave functions result in unusual non-monotonic dependence of the tunneling conductance $\sigma(B)$ on the magnetic field $B$. The conductance $\sigma(B)$ grows with the magnetic field $B$, which corresponds to a negative magnetoresistance observed in an experiment [2], until it drops down abruptly to zero at the critical magnetic field $B_{\rm cr}$. Because spin orientation of the electronic surface states in topological insulators is locked to momentum, spin polarization of the tunneling current can be controlled by the magnetic field.\\[4pt] [1] Sergey S. Pershoguba and Victor M. Yakovenko, Phys. Rev. B {\bf 86}, 165404 (2012).\\[0pt] [2] H. B. Zhang et al., Adv. Mater. {\bf 24}, 132 (2012). [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z8.00015: Polarization selective micro-Raman spectroscopy of gated 3D topological insulators Jeff Secor, Milan Begliarbekov, Lukas Zhao, Haiming Deng, Lia Krusin-Elbaum One of the majors challenges to understanding the behavior of the quantum states in 3D topological insulators (TIs) is a significant carrier conduction in the bulk. Understanding phonons and electron-phonon interactions can shed light on the link between surfaces and the bulk and are critical in potential applications based on TIs. Raman scattering is a fast nondestructive technique used to analyze electron lattice interactions. In this work we study micro-Raman scattering of few quintuple layer thin $2^{nd}$ generation excellent crystalline quality 3D TIs, such as Sb$_2$Te$_3$, Be$_2$Te$_3$, and Bi$_2$Se$_3$ in the 15-300~K temperature range in order to probe the interaction of circularly polarized light between the lattice phonon modes and helical surface states of TI's. Circularly and linearly polarized light combined with an applied gate bias and the temperature dependence is used to examine the helicity dependence of Raman scatter to analyze the strength of electron-phonon coupling in these systems. [Preview Abstract] |
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