Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D42: Topological Superconductors: Experiment |
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Sponsoring Units: DCMP Chair: Yoshinori Okada, WPI Advanced Institute for Materials Research Room: Mile High Ballroom 4A |
Monday, March 3, 2014 2:30PM - 2:42PM |
D42.00001: Andreev Reflection Spectroscopy on Single Crystals of Bismuth-Chalcogenide Topological Insulators Chris Granstrom, Igor Fridman, J.Y.T. Wei, Hechang Lei, Cedomir Petrovic Topological insulators have received great research interest in recent years. One salient feature of these materials is the helical spin polarization of their electronic surface states. Andreev reflection, a fundamental process that occurs between a superconductor and conducting material, has often been used to probe the spin polarization of various magnetic materials [1,2]. In this work, we use superconducting Nb tips to make cryomagnetic Andreev reflection spectroscopy measurements on bismuth-chalcogenide single crystals. We analyze our spectral data, which show Andreev-like features, in the context of both calculated and measured spin-dependent band structures of these topological insulators. [1] B. Nadgorny, Handbook of Spin Transport and Magnetism (Taylor and Francis, New York, 2011), p. 531. [2] C. S. Turel et al., Appl. Phys. Lett. 99, 192508 (2011) [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D42.00002: Scanning SQUID Measurements of Superconducting Proximity Effect in Bi2Se3-Nb Heterojunctions Philip Kratz, Ilya Sochnikov, Phillip Wu, Jung Ho Yu, Kristie Koski, Yi Cui, Robert Hammond, Malcolm R. Beasley, John R. Kirtley, Kathryn A. Moler In superconductivity induced on the surface of a 3D topological insulator, in contrast to conventional s-wave superconductivity, each vortex core theoretically carries a nondegenerate zero energy state with the properties of a Majorana fermion. The local superfluid density and its characteristic magnetic field penetration depth, critical current and temperature are sensitive metrics for placing limits on the relative contributions of the bulk and surface to a proximitized supercurrent in a topological insulator. Using a scanning SQUID microscope integrated with a quartz tuning fork sensor in a force-sensitive phase-locked loop for simultaneous topography characterization, we study the local superfluid density in Sb-doped Bi2Se3-Nb heterojunctions, prepared by Nb growth through molecular beam epitaxy on solvothermally synthesized Bi2Se3 nanoplates. We observe a suppression of the superconducting diamagnetic susceptibility, consistent with a superconducting proximity effect. We also explore the dependence of the local superfluid density on back gate voltage and temperature. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D42.00003: Probing the Superconducting Proximity Effect in a Topological Insulator Using Scanning Tunneling Microscopy Ian Dayton, Matthias Muenks, Eric Goodwin, Duck-Young Chung, Alex Levchenko, 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 distant quantum states. In this talk, we present cryogenic Scanning Tunneling Microscopy measurements to probe the local density of states in proximity to Pb/Bi2Se3 interfaces. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D42.00004: Vortices and gate-tunable bound states in a topological insulator coupled to superconducting leads Aaron Finck, C. Kurter, Y.S. Hor, D.J. Van Harlingen It has been predicted that zero energy Majorana bound states can be found in the core of vortices within topological superconductors. Here, we report on Andreev spectroscopy measurements of the topological insulator Bi$_2$Se$_3$ with a normal metal lead and one or more niobium leads. The niobium induces superconductivity in the Bi$_2$Se$_3$ through the proximity effect, leading to both signatures of Andreev reflection and a prominent re-entrant resistance effect. When a large magnetic field is applied perpendicular to the surface of the Bi$_2$Se$_3$, we observe multiple abrupt changes in the subgap conductance that are accompanied by sharp peaks in the dynamical resistance. These peaks are very sensitive to changes in magnetic field and disappear at temperatures associated with the critical temperature of the induced superconductivity. The appearance of the transitions and peaks can be tuned by a top gate. At high magnetic fields, we also find evidence of gate-tunable states, which can lead to stable zero-bias conductance peaks. We interpret our results in terms of a transition occurring within the proximity effect region of the topological insulator, likely due to the formation of vortices. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D42.00005: Tunable Anomalous Supercurrent in a topological tri-junction SQUID C. Kurter, A.D.K. Finck, P. Ghaemi, Y.S. Hor, D.J. Van Harlingen There has been intense interest in realizing Majorana fermions (MFs) in solid-state systems. Circuits of Josephson junctions (JJs) made of closely spaced s-wave superconductors on 3D topological insulators have been proposed to host zero energy Andreev bound states (ABSs) that act like MFs. Here, we present signatures of an anomalous supercurrent carried by topologically non-trivial low energy ABSs in a Nb/Bi$_2$Se$_3$/Nb tri-junction SQUID where two of the three superconducting leads are connected by a loop. An electrostatic top gate allows strong modulation of the supercurrent despite a high bulk contribution to the normal state conductance. In response to a magnetic field threading flux within the superconducting loop, we find unconventional SQUID oscillations enclosed by an envelope associated with a clear diffraction pattern, indicating spatially uniform and symmetric JJs. At a critical gate voltage, when the trivial 2DEG at the surface is nearly depleted, we observe a sharp drop in the critical current, signaling a topological phase transition in which the nature of the supercurrent-carrying states is transformed. This transition is accompanied by qualitative changes in the SQUID oscillations, magnetic diffraction pattern, and temperature dependence of the critical current. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D42.00006: Proximity induced superconductivity in the 3D topological insulator HgTe probed with scanning SQUID microscope Ilya Sochnikov, John R. Kirtley, Kathryn A. Moler, Luis Maier, Christoph Bruene, Hartmut Buhmann, Laurens W. Molenkamp Inducing superconductivity on the surface of a 3D topological insulator is important for novel broken symmetry phases. However, it is difficult to assess the existence of the surface superconductivity with a single experimental technique. We have used a scanning SQUID microscope to characterize the magnetic properties of hybrid structures made of the 3D topological insulator HgTe and superconducting Nb. The magnetic response of superconducting rings with exotic Josephson junctions reveals the current-phase relation, while measurements of bilayer HgTe/Nb disks reveal the total superfluid density of the hybrid structure. We analyze the degree of skew in the current-phase relation to determine the relative contribution of surface states, and discuss other contributions to the current-phase relation. This work sets an agenda for discussion of the prospects for detection of new broken symmetry phases in 3D topological insulators. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D42.00007: Induced Superconductivity in the Quantum Spin Hall Edge Hechen Ren, Sean Hart, Timo Wagner, Philipp Leubner, Mathias Muehlbauer, Christoph Bruene, Hartmut Buhmann, Laurens Molenkamp, Amir Yacoby Two-dimensional topological insulators have a gapped bulk and helical edge states, making it a quantum spin Hall insulator. Combining such edge states with superconductivity can be an excellent platform for observing and manipulating localized Majorana fermions. In the context of condensed matter, these are emergent electronic states that obey non-Abelian statistics and hence support fault-tolerant quantum computing. To realize such theoretical constructions, an essential step is to show these edge channels are capable of carrying coherent supercurrent. In our experiment, we fabricate Josephson junctions with HgTe/HgCdTe quantum wells, a two-dimensional material that becomes a quantum spin Hall insulator when the quantum well is thicker than 6.3 nm and the bulk density is depleted. In this regime, we observe supercurrents whose densities are confined to the edges of the junctions, with edge widths ranging from 180 nm to 408 nm. To verify the topological nature of these edges, we measure identical junctions with HgTe/HgCdTe quantum wells thinner than 6.3 nm and observe only uniform supercurrent density across the junctions. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D42.00008: Momentum space Cooper pairing in a spin-momentum locked Dirac gap on the surface of a topological insulator Su-Yang Xu Superconductivity in Dirac systems is one of the central theoretical themes in modern physics. In particular, a helical superconductor is a theoretically predicted exotic topological phase of matter, which can be experimentally realized if superconductivity can be induced in an odd number of spin-helical Dirac electronic states. By spectroscopically momentum-resolving the superconducting proximity effect at the boundary of a topological insulator ultra-thin film, we experimentally present direct experimental evidence for a helical topological superconductor via the observation of superconductivity in an odd number of spin-momentum locked topological surface states. Observation of helical superconductivity opens the door to a number of novel topological phenomena such as supersymmetry and Abelian Majorana modes in a condensed matter context. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D42.00009: Anomalous Cooper pair interference on Bi$_{2}$Te$_{3}$surface Li Lu, Jie Shen, Yue Ding, Yuan Pan, Fan Yang, Fanming Qu, Zhongqing Ji, Xiunian Jing, Jie Fan, Guangtong Liu, Changli Yang, Genghua Chen We have performed phase-sensitive measurements on particularly designed superconducting quantum interference devices constructed on the surface of topological insulators Bi$_{2}$Te$_{3}$ in such a way that a substantial portion of the interference loop is built on the proximity-effect-induced superconducting surface. Two types of Cooper interference patterns have been recognized at low temperatures. One is s-wave like and is contributed by a zero-phase loop inhabited in the bulk of Bi$_{2}$Te$_{3}$. The other, being identified to relate to the surface states, is anomalous for that there is a phase shift between the positive and negative bias current directions. The results support that the Cooper pairs on the surface of Bi$_{2}$Te$_{3}$ have a 2$\pi $ Berry phase which makes the superconductivity p-wave-like. Mesoscopic hybrid rings as constructed in this experiment are presumably arbitrary-phase loops suitable for studying topological quantum phenomena. \\[4pt] [1] J. Shen, et al., arXiv:1303.5598v3 [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D42.00010: Proximity Effect in a Topological Insulator on a Cuprate d-wave Superconductor Tonica Valla, Turgut Yilmaz, Ivo Pletikosic, Andrew Weber, Genda Gu, Elio Vescovo, Boris Sinkovic Proximity induced $s$-wave superconductivity in a 3D topological insulator (TI) represents a new avenue for observing zero-energy Majorana fermions inside the vortex cores. A relatively small gap and a low transition temperature of conventional $s$-wave superconductors put the hard constraints on these experiments. Larger gaps and higher transition temperatures in cuprate superconductors would significantly relax these constraints, but with intrinsic zero-energy modes in vortex cores, it is not clear if Majorana fermions could be resolved. Here, we present our angle-resolved photoemission studies of thin TI films grown $in$-$situ$ on optimally doped Bi2212 substrates. We discuss the obtained thickness dependence and the symmetry of the gap induced in the topological surface state on the prospects of detecting Majorana modes in such systems. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D42.00011: New Excitation at the Interface between High-Temperature Superconductors and Topological Insulators Parisa Zareapour, Alex Hayat, Shu Yang Frank Zhao, Michael Kreshchuk, Yong Kiat Lee, Anjan Reijnders, Achint Jain, Zhijun Xu, Alina Yang, G.D. Gu, Shuang Jia, Robert Cava, Kenneth Burch There has been an increased interest in the interplay between d-wave superconducting order parameter and helical surface states of a topological insulator, due to the recent theoretical proposals predicting the emergence of novel excitations at these interfaces. Motivated by these intriguing proposals, we fabricated high-temperature superconductor/topological insulator junctions by the mechanical bonding method [1]. We report the observation of a zero-bias conductance peak (ZBCP) at temperatures below the critical temperature of the bulk superconductor. The ZBCP in our data indicates the emergence of a new excitation in our devices. I will present a detailed study of the differential conductance measurement of our samples at various temperatures and magnetic fields.\\[4pt] [1] P. Zareapour, et al., Nature Communications 3, 1056 (2012). [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D42.00012: Two-dimensional superconductivity realized in an MBE-grown Bi2Te3/FeTe heterostructure Qing Lin He, Hongchao Liu, Mingquan He, Ying Hoi Lai, Hongtao He, Gan Wang, Kam Tuen Law, Rolf Lortz, Jiannong Wang, Iam Keong Sou We report a superconductivity realized at the interface of a Bi2Te3/FeTe heterostructure fabricated via van der Waals epitaxy using the molecular beam epitaxy technique, which appears even when the thickness of Bi2Te3 is as thin as one quintuple layer. The two-dimensional nature of the observed superconductivity with the highest transition temperature around 12 K was verified by the existence of a Berezinsky-Kosterlitz-Thouless transition and the diverging ratio of in-plane to out-plane upper critical field on approaching the superconducting transition temperature. The underlying mechanism of this interfacial superconductivity will be discussed. The heterostructure studied in this work provides an ideal platform with unconventional superconductivity for hosting Majorana fermions and studying their exotic physics. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D42.00013: Simulations and characterization of arrays of Josephson junctions on the surface of a topological insulator Erik Huemiller, Cihan Kurter, Aaron Finck, Dale Van Harlingen Topological insulators (TI) have drawn a great deal of interest due to their unique surface states protected by time-reversal symmetry and strong spin-orbit coupling. Josephson junctions made by proximity coupling of s-wave superconductors (S) through the surface states of 3D TI have been predicted to produce excitations of Majorana fermions, which modify the usual current-phase relationship (CPR). In this talk, we present simulations of arrays of superconducting islands connected by Josephson junctions with a CPR of the form of $I_1 \sin{\phi} + I_2 \sin{\phi/2}$. We calculate the energy of the metastable states of the array and the resistance in dynamical states as a function of external magnetic field, and junction critical current for different array sizes and geometries. The 4$\pi$-periodic component of the CPR lifts the degeneracy to create additional metastable states and a modulation of the energy and resistance that depends on whether the number of vortices per cell is even or odd. We discuss experimental progress towards the fabrication of superconducting islands connected by S/TI/S junctions and their characterization by transport and imaging. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D42.00014: Probing the local environment of a superconductor-proximitized nanowire using single electron transistors Fei Pei, Maja Cassidy, Sebastien Plissard, Diana Car, Erik Bakkers, Leo Kouwenhoven Majorana bound states are predicted to arise in semiconducting nanowires with strong spin-orbit coupling that are proximity-coupled to a s-wave superconductor and exposed to a magnetic field. Recent tunneling spectroscopy experiments have shown signatures of Majorana bound states through the existence of a peak in conductance that remains fixed to zero bias over a wide range in magnetic fields. Observation of the delocalized nature of these states remains an outstanding challenge. Here we present measurements of a InSb nanowire proximitized by a central superconducting contact. Normal metal leads allow tunneling spectroscopy from each end of the wire, while nearby single electron transistors provide simultaneous information on the local environment both within the proximitized wire and at each end. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D42.00015: Superconducting transport through InAs/GaSb heterostructures Vlad Pribiag, Christophe Charpentier, Werner Wegscheider, Leo Kouwenhoven Type-II InAs/GaSb heterostructures have recently attracted interest as a two-dimensional topological insulator that can be tuned between the normal and topological quantum phases by means of electrostatic gating. In proximity to a superconductor, 2D topological insulators are predicted to host Majorana zero-modes, a consequence of the helical nature of their edge conduction modes. Here, we present transport measurements through SNS junctions based on InAs/GaSb with NbTiN superconducting contacts. We observe induced supercurrents and investigate the effects of gating and applied magnetic fields, highlighting the potential for Majorana-detection experiments in this system. [Preview Abstract] |
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