Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y27: Superconductivity and Spin Orbit Coupling |
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Sponsoring Units: DCMP Chair: Ke Chen, Temple University Room: 204B |
Friday, March 6, 2015 8:00AM - 8:12AM |
Y27.00001: Fraunhofer pattern arising from an edge-stepped topological surface Josephson current distribution Jae Hyeong Lee, Gil-Ho Lee, Janghee Lee, Joonbum Park, Seung-Geol Nam, Yun-Sok Shin, Jun Sung Kim, Hu-Jong Lee We report a surface-dominant Josephson effect in superconductor-topological insulator-superconductor (S--TI--S) devices, where a Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ (BSTS) crystal flake was adopted as an intervening TI between Al superconducting electrodes. We observed a Fraunhofer critical current modulation in a perpendicular magnetic field in an Al--TI--Al junction for both local and nonlocal current biasing. For the local biasing, the Fraunhofer signal was highly robust to the magnetic field up to the critical field of the Al electrodes, corresponding to the edge-stepped nonuniform supercurrent distribution arising from the top and rough side surfaces of the BSTS flake. A Fraunhofer-like pattern was also observed in a neighboring Au-TI-Au normal junction when it was nonlocally biased by the Al--TI--Al junction. All these strongly suggest Josephson coupling established via topologically robust conducting channels present on the surface of the 3D topological insulator. [Preview Abstract] |
Friday, March 6, 2015 8:12AM - 8:24AM |
Y27.00002: Josephson junctions between topological and conventional superconductors Pavel Ioselevich, Mikhail Feigelman Topological superconductors host protected gapless modes at their boundaries. This leads to various robust phenomena when a topological superconductor is contacted to a topologically trivial lead, like the fractional ($4\pi$-periodic) Josephson effect or the zero-bias conductance peak in a tunneling NS-junction. Here we discuss phenomena arising in a Josephson contact between a topological and a trivial superconductor. The Josephson effect in this case has an asymmetric dependence on the order parameters involved, and the critical current is suppressed very strongly in a typical situation, so that the product $I_cR \ll e\Delta$ even in a short contact. [Preview Abstract] |
Friday, March 6, 2015 8:24AM - 8:36AM |
Y27.00003: Microwave spectroscopy of Majorana bound states in S/TI/S junctions Jukka Vayrynen, Gianluca Rastelli, Wolfgang Belzig, Leonid Glazman We study the effects of microwave irradiation in a phase-biased topological Josephson junction, consisting of two s-wave superconductors linked by a quantum-spin-Hall insulator edge. A long topological junction supports multiple Andreev bound states, one of which (Majorana mode) has zero energy at phase difference $\phi = \pi$. We consider weak time-periodic modulation of the phase difference to study transitions between the discrete sub-gap levels of the junction. In a generic disordered junction all the degeneracies of single-particle levels are lifted but the zero-energy state remains and leads to a ground state degeneracy at $\phi = \pi$. Upon sweeping $\phi$ across the ground state degeneracy point, we show that the lowest excitation energy of the junction displays a prominent kink which can be observed in the absorption spectrum. To study this feature, we calculate the absorption power as a function of frequency, phase difference, and at finite temperature. [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 8:48AM |
Y27.00004: Zero-bias peak in InSb nanowires Peng Yu, Jun Chen, Mo\"Ira Hocevar, S\'ebastien Plissard, Diana Car, Erik Bakkers, Sergey Frolov Zero-bias conductance peaks(ZBP) in InSb nanowires has been reported as a strong signature of Majorana bound states in semiconductors. We made similar superconductor-InSb nanowire-normal contact hybrid devices with NbTiN on bottom gates and found some features that may corresponding to Majorana bound states. By setting a barrier and tuning gates under the nanowire that are in proximity of superconductors, ZBPs appear at finite magnetic field and usually persist for several hundred miliTesla. In different devices, ZBPs appear at different magnetic field, which may result from different chemical potentials. To achieve a so-called hard induced gap and cleaner devices, we are trying various contact materials and etching methods. [Preview Abstract] |
Friday, March 6, 2015 8:48AM - 9:00AM |
Y27.00005: Anomalous Josephson Effect in Junctions with Rashba Spin-Orbit Coupling Konstantin Nesterov, Manuel Houzet, Julia Meyer We study two-dimensional double-barrier SINIS Josephson junctions in which the inversion symmetry in the normal part is broken by Rashba spin-orbit coupling. In the presence of a suitably oriented Zeeman field in the normal part, the system displays the anomalous Josephson effect: the current is nonzero even at zero phase difference between two superconductors.\footnote{A. Buzdin, Phys. Rev. Lett. \textbf{101}, 107005 (2008).} We investigate this effect by means of the Ginzburg-Landau formalism and microscopic Green's functions approach in the clean limit. [Preview Abstract] |
Friday, March 6, 2015 9:00AM - 9:12AM |
Y27.00006: Superconducting contacts to Ge/Si core/shell nanowires Zhaoen Su, Azarin Zarassi, Dharamraj Patil, Sergey Frolov, Moira Hocevar, Minh Nguyen, Jinkyoung Yoo, Shadi Dayeh Ge/Si core/shell nanowires are hosts to one dimensional hole gas. The spin-orbit interaction is expected to be much larger than that in electron systems such as InSb and InAs. Therefore, Ge/Si nanowires have great potential to demonstrate helical liquid. When strong superconductivity is induced in the nanowire, robust topological superconductivity may form in the system. We will show how to achieve semiconductor-superconductor contacts to the nanowire. The effects of a few surface cleaning methods and annealing process on the contact resistance will be shown. Superconducting contacts of NbTiN, Al, Ti and their combinations are studied. NbTiN may be suitable for hybrid device carrying Majorana fermions for its high critical temperature and magnetic field. Supercurrent through Josephson junctions with these contacts is measured. [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:24AM |
Y27.00007: Design and Implementation of a Josephson Junction Spectrometer Caglar Girit, Marcelo Goffman, Hugues Pothier, Cristi\'an Urbina, Daniel Esteve A Josephson tunnel junction can be used as an on-chip absorption spectrometer at frequencies up to several hundred gigahertz. As a result of the AC Josephson effect, a voltage biased junction acts as a microwave source. When emitted photons are absorbed in the junction's electromagnetic environment, a dc Cooper pair current flows (inelastic Cooper pair tunneling). By measuring this dc current as a function of applied voltage---the junction's current-voltage characteristic---one obtains a spectrum of the electromagnetic environment. We describe the design of a Josephson junction spectrometer which seeks to optimize bandwidth, sensitivity, coupling and linewidth. We present measurements of the spectra of miniature on-chip $LC$ circuits with resonant frequencies in the 25-100 GHz range. Our Josephson junction spectrometer will be used to study level transitions in mesoscopic systems. [Preview Abstract] |
Friday, March 6, 2015 9:24AM - 9:36AM |
Y27.00008: Domain walls in tetragonal superconductors: Andreev bound states and tunneling features Soumya Mukherjee, Kirill Samokhin Domain walls can be formed in superconductors with a discrete degeneracy of the ground state, which breaks time reversal symmetry or a point group symmetry. We study all cases where the formation of domain walls is possible for the tetragonal point group symmetry D$_{4h}$. We discuss both triplet and mixed singlet order parameters. It is found that in all cases domain walls support subgap Andreev bound states, whose energies strongly depend on the direction of semiclassical propagation. We also study the density of states of these bound states and show that the formation of bound states near the domain wall strongly affects the tunneling conductance. [Preview Abstract] |
Friday, March 6, 2015 9:36AM - 9:48AM |
Y27.00009: Anisotropic Andreev reflection in ferromagnet/s-wave superconductors (FS) Petra Hoegl, Alex Matos Abiague, Igor Zutic, Jaroslav Fabian Andreev reflection in FS junctions is a sensitive probe of the junction interface as well as of the spin polarization of the F region. By performing analytical and numerical calculations on widely accepted model systems, with interfacial Rashba and Dresselhaus spin-orbit fields, we show that Andreev reflection spectroscopy is also a sensitive tool of the interfacial spin-orbit coupling. In particular, we find a finite subgap conductance even in half-metallic systems due to the spin-flip Andreev reflection, imposing a triplet proximity effect. Furthermore, we predict a giant magnetic anisotropy of the Andreev reflection---anisotropic Andreev reflection (AAR)---with respect to the orientation of the F magnetization. We analyze the effects of the tunnel barrier strength, the F spin polarization, and the effective mass and Fermi wave vector mismatch. Our results should also have implications for designing Majorana states in semiconductor junctions with superconductors. [Preview Abstract] |
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