Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G12: Proximity Induced Superconductivity in Topological Insulators |
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Sponsoring Units: DCMP Chair: Aaron Finck, University of Illinois Room: 007C |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G12.00001: Gate-tuned superconducting proximity effect in 3D topological insulator (Bi,Sb)2Se3-Nb bilayer heterostructures Philip Kratz, Ilya Sochnikov, Phillip Wu, Jung Ho Yu, Kristie Koski, Yi Cui, Robert Hammond, Malcolm Beasley, John Kirtley, Kathryn Moler Interfacial superconductivity in three-dimensional topological insulators (3DTIs) is a promising avenue for topological superconductivity. We have used scanning SQUID microscopy to measure the diamagnetic susceptibility as a function of temperature in few QL (Bi,Sb)2Se3-Nb bilayer heterostructures. Local suppression of both the diamagnetic susceptibility and the critical temperature are consistent with an inverse proximity effect. We can model the relative contribution of the surface states to the overall superconductivity of the hybrid structures through gated susceptibility measurements and compare the fitted parameters to clean and dirty limit models within the general framework of Eilenberger's quasiclassical theory. Our work may contribute to understanding the complex interplay between topological and trivial surface states in real 3DTI interfaces. [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G12.00002: Universal transport signatures of topological superconductivity in quantum spin Hall architectures Shu-Ping Lee, David Aasen, Torsten Karzig, Jason Alicea Interfacing s-wave superconductors with quantum spin Hall systems provides a promising route to ``engineered'' topological superconductivity. Given exciting recent progress on the fabrication side, identifying experiments that definitively expose the topological superconducting phase (and clearly distinguish it from a trivial state) raises an increasingly important problem. With this goal in mind we use renormalization group methods to extract universal transport characteristics of superconductor/quantum spin Hall heterostructures where the native edge states serve as a lead. Interestingly, arbitrarily weak interactions induce qualitative changes in the behavior relative to the free-fermion limit, leading to a sharp dichotomy in conductance for the trivial (narrow superconductor) and topological (wide superconductor) cases. Furthermore, we find that strong interactions can in principle induce power-law-localized ``parafermion'' excitations at a superconductor/quantum spin Hall junction. [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G12.00003: Probing proximity-induced superconductivity in superconductor-topological Insulator devices via Josephson Interferometry Erik D. Huemiller, Aaron D. K. Finck, Cihan Kurter, Dale J. Van Harlingen We are exploring the nature of the proximity-induced order in 3D topological insulators in contact with an s-wave superconductor by phase-sensitive Josephson interferometry. It is predicted that the proximity region should have p-wave pairing symmetry with spin-dependent chiral components. To test this, we compare the behavior of edge and corner junctions on a patterned bilayer of Nb and Bi$_2$Se$_3$ to determine the phase anisotropy that should reflect a mixture of the s and p components. The alternate approach discussed will be measurement of the supercurrent properties in arrays of superconducting islands patterned on top of the topological insulator, which is sensitive to both the current-phase relation of the junctions and the array geometry. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:03PM |
G12.00004: Universal parity-crossing statistics in dirty hybrid normal-superconductor nanostructures Inanc Adagideli We focus on topologically protected crossings of Andreev bound states in normal-superconductor hybrid structures [1]. Such crossings, signaling a change in the ground state fermion parity, became the focus of recent attention as they are regarded to be precursors to Majorana fermions that appear in the long-wire limit. In recent work, we showed how a topological state can be induced from regular or irregular scattering in (i) p-wave superconducting wires and (ii) Rashba wires in proximity to an s-wave superconductor. We also related the topological properties of such nanowires to their normal state properties such as conductance [2]. In the present work, we build on these results and study the correlation between parity-crossings in the superconducting state and the normal state properties of a hybrid nanostructure. Surprisingly, we find that the crossing points as well as their statistics are universal and are described solely by their normal-state properties. We obtain formulae for mean spacing between parity crossings as well as crossing statistics in disordered wires/cavities. We finally discuss under what conditions these crossings signal Majorana fermions. [1] I. Adagideli et al . [2] I. Adagideli, M. Wimmer, A. Teker, Phys Rev B 89, 144506 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 12:03PM - 12:15PM |
G12.00005: Probing the Superconducting Proximity Effect in a Topological Insultor using Scanning Tunneling Microscopy Ian Dayton, Duck-Young Chung, Thomas Chasapis, Eric Goodwin, Reza Loloee, Mercouri Kanatzidis, Stuart Tessmer Topological insulators (TI) embody a new state of quantum matter characterized by topological invariants; this contrasts with superconductors (S), as superconductivity arises from a spontaneously broken symmetry of the underlying electron system. When a superconductor is placed on the surface of a topological insulator, the behavior of the superconducting condensate across the S/TI interface offers the opportunity to study the interplay between these two distinct quantum states. In this talk, we present our progress in applying cryogenic Scanning Tunneling Microscopy measurements to probe the local density of states in proximity to Pb/Bi$_2$Se$_3$ interfaces. [Preview Abstract] |
Tuesday, March 3, 2015 12:15PM - 12:27PM |
G12.00006: Non-contact current-phase measurements of topological weak links with scanning SQUID C.A. Watson, I. Sochnikov, J.R. Kirtley, K.A. Moler, M. Deng, W. Chang, P. Krogstrup, T.S. Jespersen, J. Nygard, C.M. Marcus, L. Maier, C. Gould, G. Tkachov, E.M. Hankiewicz, C. Br\"une, H. Buhmann, L.W. Molenkamp Topological superconductivity has recently generated substantial interest as a pathway to Majorana physics in the solid state. Experimental efforts have focused on the superconducting proximity effect in topologically non-trivial junctions, but proof of the topological nature of the induced superconductivity remains elusive. We employ scanning superconducting quantum interference device (SQUID) susceptometry to study conventional superconducting Nb rings interrupted by weak links of 3D topological insulator HgTe and Al rings with InAs nanowire junctions. Varying the flux through each ring, we directly measure the current-phase relation (CPR) of the junction. Forward skewness in the CPR of 3D-HgTe which persists even in junctions long compared to the mean free path suggests that helicity may play a role in the high transmittance of Andreev Bound States that carry the Josephson current. Progress in InAs nanowire junction CPR measurements is also discussed. These measurements showcase the CPR as a fundamental characteristic of superconducting weak links and establish scanning SQUID microscopy as a powerful probe for performing such measurements. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G12.00007: Observation of anomalous current phase relation on Pb-Bi$_{2}$Te$_{3}$ Josephson devices Yuan Pang, Jie Shen, Junhua Wang, Junya Feng, Fanming Qu, Zhaozhen Lyu, Jie Fan, Guangtong Liu, Zhongqing Ji, Xiunian Jing, Changli Yang, Qingfeng Sun, X. C. Xie, Liang Fu, Li Lu Josephson devices based on s-wave superconductor Pb and 3D topological insulator Bi2Te3 have been fabricated and investigated down to low temperatures. Anomalous current phase relation (CPR) was observed, indicating the existence of an unconventional component of superconductivity. Our experiment demonstrates that hybrid devices based on s-wave superconductor and 3D topological insulator might provide a platform for searching for and manipulating Majorana bound state. [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G12.00008: Anomalous Resistance Increase on Topological Insulator Bi$_{2}$Te$_{3}$ Nanotubes with Superconductor Contacts Renzhong Du, Yuewei Yin, Ludi Miao, Aokui Sun, Qi Li Topological superconductivity has gained much attention in recent years since it has been predicted to exhibit exotic behaviors and host the Majorana states. We report transport studies on topological insulator Bi$_{2}$Te$_{3}$ nanotube with Nb superconductor contacts. The Bi$_{2}$Te$_{3}$ nanotubes were synthesized by solution phase method and have previously been proved to possess robust surface states with long phase coherency against strong bulk disorders. Superconducting Nb contacts on the nanotubes were fabricated by electron beam lithography process. An anomalous resistance upturn of the resistance in the nanotube was observed when the contacts became superconducting. When applying a magnetic field, the resistance upturn is reduced gradually and disappears when the magnetic field exceeds the H$_{c2}$ of Nb. From both the temperature and magnetic field dependence of the resistance upturn, we conclude that the resistance upturn is associated superconducting transition of the contact leads. Combination of Au and Nb leads have been fabricated with different channel length and the detailed experimental results will be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G12.00009: Phase diagram for topological superconductivity in topological insulator nanowires Fernando de Juan, Jens Bardarson, Roni Ilan A topological insulator nanowire can be used as a platform to produce one dimensional topological superconductivity in the presence of magnetic fields and the proximity effect from a nearby superconductor. In this work, we discuss the conditions under which this can happen by computing the topological invariant of the system from both a continuum Dirac fermion model and a lattice realization of a TI. We demonstrate that the presence of a vortex in the order parameter winding around the wire is essential to have a non-trivial invariant. In addition, we present a full phase diagram of the model as a function of chemical potential, flux and superconducting pairing, emphasizing that not all regions present a fully gapped superconducting state. Implications for transport in normal-superconductor junctions in this system will be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G12.00010: Superconducting proximity effect in a synthetic helical liquid Mariana Malard, George Japaridze, Henrik Johannesson We explore the possibility of a superconducting proximity effect in a synthetic helical liquid, formed in a quantum wire in the presence of a spatially periodic Rashba interaction [1]. The practicality and robustness of this novel scheme for producing a 1D p-wave superconductor is discussed [2].\\[4pt] [1] G. I. Japaridze, H. Johannesson, and M. Malard, Phys Rev B 89, 201403(R) (2014).\\[0pt] [2] M. Malard, G. I. Japaridze, and H. Johannesson, in preparation. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G12.00011: InAs/GaSb quantum wells: quantum spin Hall effect and topological superconductivity Matthias Sitte, Karin Everschor-Sitte, Allan MacDonald Topological insulators have attracted a great deal of attention as a new quantum state of matter in the last decade. The first realizations of 2D TIs were HgTe/CdTe quantum well heterostructures, but in recent years another class of semiconductor heterostructures --- namely InAs/GaSb quantum wells --- was shown to yield 2D TIs as well. Compared to the HgTe/CdTe-based systems they have many advantages, most prominently a continuously tunable band structure via external electric fields and stronger proximity coupling to superconductors. We perform empirical tight-binding calculations on these systems to study how topological properties are changed by varying external control parameters such as electric fields or well thicknesses. Since proximity coupling of a 2D TI and an ordinary s-wave superconductor gives rise to 1D topological superconductivity, these systems also support Majorana fermions as non-local excitations. We will present preliminary results on the proximity effects when InAs/GaSb quantum wells are coupled to a superconductor. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G12.00012: Fermion Fractionalization to Majorana Fermions in Dimerized Kitaev Superconductor Ryohei Wakatsuki, Motohiko Ezawa, Yukio Tanaka, Naoto Nagaosa We study theoretically a one-dimensional dimerized Kitaev superconductor model which belongs to BDI class with time-reversal, particle-hole, and chiral symmetries.\footnote{R. Wakatsuki, M. Ezawa, Y. Tanaka, and N. Nagaosa, Phys. Rev. B, \textbf{90}, 014505 (2014).} There are two sources of the particle-hole symmetry, i.e., the sublattice symmetry and superconductivity. Accordingly, we define two types of topological numbers with respect to the chiral indices of normal and Majorana fermions, which offers an ideal laboratory to examine the interference between the two different physics within the same symmetry class. Phase diagram, zero-energy bound states, and conductance at normal metal/superconductor junction of this model are unveiled from this viewpoint. Especially, the electron fractionalization to the Majorana fermions showing the splitting of the local density of states is realized at the soliton of the dimerization in this model. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G12.00013: Majorana edge states in single layer graphene Lin Wang, Mingwei Wu We investigate the Majorana edge states in single layer graphene near the Dirac point with the Rashba spin-orbit coupling and an externally induced out-of-plane Zeeman splitting in the proximity of an s-wave superconductor. By calculating the topological invariant, we show the topological phase diagram. In the topological nontrivial regime, we study the Majorana edge states in the case of zigzag or armchair ribbon. For these two cases, both have two Majorana edge states along one edge. However, there exist strong anisotropies in the localization length, the group velocity and the momentum of the Majorana fermions in two ribbons. In addition, the effects of the in-plane Zeeman splitting and the disorder on the Majorana edge states are also discussed. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G12.00014: Time-Reversal-Invariant $Z_4$ Fractional Josephson Effect Fan Zhang, Charles Kane We study the Josephson junction mediated by the quantum spin Hall edge states and show that electron-electron interactions lead to a dissipationless fractional Josephson effect in the presence of time-reversal symmetry. Surprisingly, the periodicity is $8\pi$, corresponding to a Josephson frequency $eV/2\hbar$. We estimate the magnitude of interaction-induced many-body level splitting responsible for this effect and argue that it can be measured by using tunneling spectroscopy. For strong interactions we show that the Josephson effect is associated with the weak tunneling of charge $e/2$ quasiparticles between the superconductors. Our theory describes a fourfold ground state degeneracy that is similar to that of coupled ``fractional'' Majorana modes but is protected by time-reversal symmetry. [Reference: PRL 113, 036401 (2014).] [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G12.00015: Magnetotransport response in the 3D topological insulator Bi2Te3 with indium superconducting electrodes Zhuo Wang, Tianyu Ye, Ramesh Mani 3D Topological insulators (TIs) include novel surface states which are protected by time reversal symmetry from backscattering by impurities. Recently, the superconducting proximity effect at the interface between a TI and a superconductor has been a focus of attention. Hence, our study explores the magnetotransport behavior of thin Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ flakes with superconducting electrodes in a Hall bar configuration. Such specimens exhibit a magnetoresistance anomaly resulting from the proximity effect. To better understand this magnetoresistance anomaly, we examine here the effect of biasing thin Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ flakes samples of different thicknesses simultaneously with both a dc and a low-frequency ac current through the same pair of contacts at the ends of the device. Here, we report the role of finite bias and electron heating on the observed effects. [Preview Abstract] |
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