Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W35: Topological Insulators: Superconductivity |
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Sponsoring Units: DCMP Chair: Taylor Hughes, University of Illinois Room: C140 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W35.00001: New forms of superconductivity and magnetism in a doped topological insulator L. Andrew Wray, Suyang Xu, Hsin Lin, M. Zahid Hasan Topological insulators achieve a phase of matter characterized by the quantum topology of electron kinetics rather than by broken symmetries. The topological insulator state gives rise to spin-helical surface states that dramatically alter the surface physics and allow new phenomena in the presence of purturbations such as superconductivity and magnetism. We have used angle resolved photoemission spectroscopy to map electron dynamics at the surface of a topological insulator in the presence of magnetic surface ions and with doping compositions that superconduct. Our measurements establish that bulk Cu-doped Cu$_{0.12}$Bi$_2$Se$_3$ realizes a new form of superconductivity (Wray et al, Nat. Phys. 6, 855 (2010)) and is likely to host localized non-Abelian Majorana fermions on the crystal surface. We observe that surface-deposited ions lead to the formation of new topoligically-derived surface Dirac bands, and our data suggest that magnetic moments of deposited Fe undergo a phase transition to align along the out-of-plane axis (arXiv:1009.6216). [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W35.00002: Coupling superconductors to topological insulators Menno Veldhorst, Marcel Hoek, Marieke Snelder, Xiaolin Wang, Hans Hilgenkamp, Alexander Brinkman The recent discovery of the topological insulators sparked an enormous attention. The experimental investigation of topological insulators with surface sensitive spectroscopic techniques evidently exposed the helical Dirac fermions living at the surface of a bulk insulator. In transport experiments, intrinsic bulk conduction challenges the observation of the topological surface states. We have observed electrical transport through the surface states of Bi$_{2}$Te3 by utilizing the electrical field effect and by utilizing intrinsically transparant interfaces to metallic electrodes. Of particular interest is coupling topogical insulators to superconductors, where at the interface the elusive Majorana fermion is predicted to recide. We fabricated superconductor -- topological insulator -- superconductor Nb-Bi$_{2}$Te$_{3}$-Nb junctions and observed a supercurrent over a length scale of more than a micrometer. Shapiro steps appear under microwave irradiation. The supercurrent is found to be surprisingly robust in large magnetic fields. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W35.00003: Stability of surface Majorana modes in superconducting doped topological insulators Ashvin Vishwanath, Pavan Hosur, Pouyan Ghaemi, Roger Mong In recent years, several model condensed matter systems have been predicted to harbor Majorana fermion zero modes. One such system is the surface of a strong topological insulator with proximity-induced superconductivity. A vortex in this surface superconductor was shown to host a topologically protected Majorana mode. Since then, bulk superconductivity has been induced in several strong topological insulators via doping or application of pressure. Here, we address the question of whether a vortex in these superconductors will trap Majorana zero modes at the surface. Viewed as a 1D system, the vortex can be characterized by a $Z_2$ topological invariant which denotes the presence or absence of a Majorana mode at its end. For weak pairing, we find that, the transition point between the two topological phases is determined by a Fermi surface property in the normal state. Hence, the phase transition can be achieved by simply varying the Fermi level. At the transition, the vortex supports gapless Majorana excitations along its length. Using this criterion, we discuss whether surface Majorana modes exist in the experimentally established superconductors Cu$_x$Bi$_2$Se$_3$, p-doped TlBiTe$_2$ and Pd$_x$Bi$_2$Te$_3$. Interestingly, the Fermi surface criterion also allows superconducting vortices in systems with non-topological band structures to be associated with surface Majorana modes. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W35.00004: Surface Spectral Function in the Superconducting State of a Topological Insulator Ting-kuo Lee, Lei Hao We discuss the surface spectral function of superconductors realized from a topological insulator, such as the copper-intercalated Bi$_{2}$Se$_{3}$. These functions are calculated by projecting bulk states to the surface for two different models proposed previously for the topological insulator. Dependence of the surface spectra on the symmetry of the bulk pairing order parameter will be discussed with particular emphasis on the odd-parity pairing. Exotic spectra like an Andreev bound state connected to the topological surface states will be presented. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W35.00005: Superconductivity in Topological Parent Compound Induced via Pressure C.Q. Jin, J.L. Zhang, S.J. Zhang, H.M. Gong, W. Zhang, P.P. Kong, J. Zhu, R.Z. Yu, L.X. Yang, S.M. Feng, Q.Q. Liu, X.C. Wang, R.C. Yu, W.G. Yang, L. Wang, S.C. Zhang, X. Dai, Z. Fang We report successful observation of pressure induced superconductivity in topological compound of Bi$_{2}$Te$_{3}$ single crystal induced via pressure [1]. The combined high pressure structure investigations with first-principles calculations indicated that the superconductivity occurs at the ambient phase of topologically nontrivial. The results suggest topological superconductivity can be realized in the parent state of Bi$_{2}$Te$_{3}$ topological material. Ref: [1] J. L. Zhang et al., ``Pressure induced superconductivity in the parent compound of Bi$_{2}$Te$_{3}$'' (submitted).. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W35.00006: Detecting Majorana Bound States Colin Benjamin, Jiannis Pachos We propose a set of interferometric methods on how to detect Majorana bound states induced by a topological insulator. The existence of these states can be easily determined by the conductance oscillations as function of magnetic flux and/or electric voltage. We study the system in the presence and absence of Majorana bound states and observe strikingly different behaviors. Importantly, we show that the presence of coupled Majorana bound states can induce a persistent current in absence of any external magnetic field. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W35.00007: Chain of Majorana States from Superconducting Dirac Fermions at a Magnetic DomainWall Titus Neupert, Shigeki Onoda, Akira Furusaki We study theoretically a strongly type-II s-wave superconducting state of two-dimensional Dirac fermions in proximity to a ferromagnet having in-plane magnetization. It is shown that a magnetic domain wall can host a chain of equally spaced vortices in the superconducting order parameter, each of which binds a Majorana-fermion state. The overlap integral of neighboring Majorana states is sensitive to the position of the chemical potential of the Dirac fermions. Thermal transport and scanning tunneling microscopy experiments to probe the Majorana fermions are discussed. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W35.00008: Global Properties of 3D Topological Insulator Surface/Superconductor Junctions Matthew Gilbert, Taylor Hughes, Ching-Kai Chiu, Andrei Bernevig The ability to precisely find and manipulate non-Abelian anyons\footnote{A. Kitaev, \textit{Ann. Phys (N.Y.)} \textbf{303}, 2 (2003).} has long been sought after as a potential means for the realization of robust quantum information processing. The simplest of these particles, Majorana fermions, have been predicted to exist in a new class of materials commonly referred to as topological insulators\footnote{B. A. Bernevig and S. C. Zhang, \textit{Phys. Rev. Lett.} \textbf{96}, 106802 (2006).} when they are coupled with s-wave superconducting contacts.\footnote{L. Fu and C. L. Kane, \textit{Phys. Rev. Lett.} \textbf{100}, 096407 (2008).} This proposal is the focus of intense experimental research whose aim is to prove the existence of Majorana fermions trapped at the surface of topological insulators paired with superconductors. We will present our initial theoretical investigations into the surface properties of 3D topological insulator/superconductor junctions. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W35.00009: Order parameter and triplet correlations near a superconductor-topological insulator interface Mahmoud Lababidi, Erhai Zhao At the interface between a 3D topological band insulator and an s- wave superconductor forms a remarkable 2D superconductor that can host Majorana fermions at vortex cores. Going beyond the original work of Fu and Kane [Phys. Rev. Lett. 100, 096407 (2008)], we present a microscopic theory for the proximity effect near the TI-S interface. We compute the superconducting order parameter as a function of the distance away from the interface by self-consistently solving the Bogoliubov-de Gennes equation. We discuss the suppression of the order parameter by the topological insulator. Moreover, we show that triplet superconducting correlations of $p_x\pm ip_y$ orbital symmetry are induced near the interface by the spin-orbit coupling inside the topological insulator. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W35.00010: Superconductivity and Majorana fermion creation at the quantum spin Hall insulator edge Annica M. Black-Schaffer We focus on properties related to Majorana fermion creation in a self-consistent study of a microscopic interface between a quantum spin Hall insulator (QSHI) and a superconductor (SC). For a spin-singlet $s$-wave SC we show that large odd-in-momentum, or $p$-wave, order parameters exist for all doping levels of the QSHI and that they can be related to different spinless Cooper pair amplitudes. This result demonstrates that it is natural that the Majorana mode at a SC-ferromagnet (SF) interface survives even at zero doping. Despite the induced $p$-wave order parameters, the induced superconducting gap in the QSHI always retains its $s$-wave character, thus validating the commonly used effective model for superconductivity in a topological insulator. We also self-consistently model an SFS Josephson junction along the QSHI edge and report on Majorana mode occurrence in the junction, the current-phase relation, and novel odd-frequency spin-triplet correlations. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W35.00011: Measurements of Electrical Noise at the Interface between Bi2Se3 and a Superconductor J.S. Lee, D.M. Zhang, A.R. Richardella, Nitin Samarth The narrow band gap semiconductor Bi$_2$Se$_3$ is a promising candidate for exploring exotic quantum states that might arise at the interface between topological insulators and superconductors (Phys. Rev. Lett. {\bf 100}, 096407 (2008)). Motivated by proposed approaches for detecting such states (Phys. Rev. B {\bf 79}, R161408 (2009)), we have embarked on measurements of electrical noise in mesoscopic Bi$_2$Se$_3$ devices with superconducting electrodes. Present measurements focus on CVD-grown Bi$_2$Se$_3$ nanoribbons which show proximity-induced superconductivity below $\sim 5$ K when contacted by (dirty) W electrodes. The measurements are carried out using a balanced bridge technique over a temperature range of $0.5 \rm{K} \leq T \leq 40$ K and in magnetic fields up to 80 kOe. We observe 1/f noise over a wide range of temperature and discuss the variation in noise spectral density across the normal-superconductor transition as well as its dependence upon excitation current and magnetic field. We will also describe extensions of these studies to mesoscopic devices lithographically patterned from thin films of Bi$_2$Se$_3$ grown by molecular beam epitaxy. Supported by NSF and ONR. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W35.00012: Study of the interface of supercondutor and topological insulator Dong Qian, Xiaomei Wang, Chunlei Gao, Canhua Liu, Jinfeng Jia State-of-art Molecular beam epitaxy (MBE) has been carried out to grow high quality topological insulator (TI) films on some BCS s-wave superconductor (SC) substrates. The growth dynamics and the electronics structures of the SC/TI interface were studied using high energy reflected electron diffraction and ultralow-temperature scanning tunneling microscopy (STM). Electronic structure, superconducting gap, vortex dynamics would be reported in this presentation. The superconducting state of the topological insulator due to approximate effect and the formation of Majorana Fermion would be discussed. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W35.00013: Self-consistent study at the superconductor-3D topological insulator interface Xiaoting Zhou, Chen Fang, Jiangping Hu We perform a theoretical study of the interface between a 3D topological insulator (TI) and an s-wave conventional superconductor (both in normal and superconducting state) using the BdG self-consistent formulation. The robustness of the TI surface state when in contact with a metal is discussed and the topological superconductivity on the interface induced by the proximity effect is studied in detail. We find that in general a mixed singlet and triplet pairing can be observed on the interface as the spin-orbit coupling breaks the SU(2) symmetry in TI. The Majorana edge states of such a mixed pairing 2D topological superconductor are also studied. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W35.00014: Proximity induced superconductivity in Bi$_2$Se$_3$ nanoribbons D.M. Zhang, J. Wang, J.S. Lee, H.R. Gutierrez, M.H.W. Chan, N. Samarth Proposals for possible realizations of Majorana fermions in condensed matter provide a strong motivation for interfacing superconductors with topological insulators (PRL {\bf 100}, 096407 (2008)). We describe experiments that accomplish an important first step in this context: the realization of proximity-induced superconductivity in a candidate topological insulator. We have measured the bias-dependent differential conductance in Bi$_2$Se$_3$ nanoribbons contacted with superconducting electrodes over a temperature range 0.5 K $\leq T \leq$ 6 K in magnetic fields up to 8 T. We observe signatures of both proximity-induced superconductivity and incoherent multiple Andreev reflections in these mesoscale devices. In addition, we find periodic magneto- resistance oscillations for magnetic field perpendicular to both the nanoribbon axis and the superconducting contacts. The temperature- and field-dependence of the magneto-resistance oscillation amplitude and period are suggestive of dissipative vortex dynamics in the vicinity of the contacts. Supported by NSF-MRSEC, NSF-NNIN and ONR. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W35.00015: Majorana Fermions in a Spin Ladder System Wade DeGottardi, Smitha Vishveshwara, Ditpiman Sen We consider a two-legged spin chain version of Kitaev's honeycomb model. Like its parent, this model supports $Z_2$ vortices at every plaquette. The topological phase of the system is sensitive to, {\it inter alia}, the spatial pattern of these vortices. The topological phases are gapped in the bulk but possess isolated zero energy Majorana modes at each end. The existence of such Majorana modes can be inferred from a new topological invariant. We show that there is an intimate relation between the existence of Majorana modes and the spontaneous breaking of the global $Z_2$ symmetry. [Preview Abstract] |
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