Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V28: Josephson and Superconducting Proximity Effect |
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Sponsoring Units: DCMP Room: 327 |
Thursday, March 17, 2016 2:30PM - 2:42PM |
V28.00001: Topological phase transition of a Josephson junction and its dynamics Jimmy Hutasoit, Marco Marciani, Brian Tarasinski, Carlo Beenakker A Josephson junction formed by a superconducting ring interrupted by a semiconductor nanowire can realize a zero-dimensional class D topological superconductor. By coupling the Josephson junction to a ballistic wire and altering the strength of the coupling, one can drive this topological superconductor through a topological phase transition. We study the compressibility of the junction as a probe of the topological phase transition. We also study the dynamics of the phase transition by studying the current pulse injected into the wire. [Preview Abstract] |
Thursday, March 17, 2016 2:42PM - 2:54PM |
V28.00002: Combined gate-tunable Josephson junctions and normal state transport in Bi$_2$Te$_3$ topological insulator thin films Prosper Ngabonziza, Martin, P Stehno, Hiroaki Myoren, Alexander Brinkman In recent years, extensive efforts have been made to improve the coupling between topological insulators and s-wave superconductors in topological insulator Josephson devices (TIJDs). Despite significant progress, essential questions remain open such as the bulk contribution to the Josephson critical current or the existence (and number) of $4\pi$-periodic bound states (Majoranas) in TIJDs. To address these issues, we fabricated Nb/Bi$_2$Te$_3$/Nb Josephson junctions alongside Hall bar devices on MBE-grown Bi$_2$Te$_3$ topological insulator thin films. Using the SrTiO$_3$ [111] substrate as a gate dielectric, we tuned the carrier density electrostatically and measured the Josephson supercurrent and the normal state transport properties of our thin film devices. We identify three gate voltage ranges with distinct behavior: A region of intermediate gate bias where the measured quantities change rapidly with the applied electric field, and two saturation regions for large bias of either polarity. We discuss carrier distribution and band alignment in the material as well as implications for the effective Josephson coupling in TIJDs. [Preview Abstract] |
Thursday, March 17, 2016 2:54PM - 3:06PM |
V28.00003: Quantum Phase Slips in Topological Josephson Junction Rings Rosa Rodriguez Mota, Smitha Vishveshwara, Tami Pereg-Barnea We study quantum phase slip processes (QPS) in a ring of N topological superconducting islands joined by Josephson junctions and threaded by magnetic flux. In this array, neighboring islands interact through the usual charge 2e Josephson tunneling and the Majorana assisted charge e tunneling.\footnote{A.Y. Kitaev, Phys.-Usp. 44, 131 (2001).}. When the charging energy associated with the island's capacitance is zero, the energy vs. flux relation of the system is characterized by parabolas centered around even or odd multiples of the superconducting flux quantum, depending on the parity of the system. For small but non-zero charging energy, quantum fluctuations can lead to tunneling between these classical states.\footnote{K. A. Matveev, A. I. Larkin, and L. I. Glazman., Phys. Rev. Lett. 89, 096802 (2002).} In this work, we calculate the amplitude of these tunneling processes, commonly known as quantum phase slips. We also add gate voltages to our system and study how the amplitude of QPS in these topological Josephson array is modified by Aharanov-Casher interference effects. [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V28.00004: Detecting evidence for chiral superconductivity in Sr$_2$RuO$_4$ through the use of Josephson junctions Brian Zakrzewski, Y. A. Ying, Xinxin Cai, Shaun Mills, N. E. Staley, Y. Xin, David Fobes, Tijiang Liu, Zhi-Qiang Mao, Ying Liu Sr$_2$RuO$_4$ is predicted to be an odd-parity, spin-triplet superconductor, possibly featuring a doubly degenerate chiral order parameter, which leads to the presence of chiral edge currents, domains, and domain walls. We fabricated Josephson junctions on ramps cut by focused ion beam as well as on naturally cleaved edges of micron thick crystals of Sr$_2$RuO$_4$ using Al as the conventional superconductor electrode. The sensitivity of these Josephson junctions to a magnetic flux penetrating the junction and the domain dependent intrinsic phase of the superconducting order parameter make them a powerful tool for probing the effects of chiral superconductivity mentioned above. We will present the methodology as well as preliminary measurements and discuss the implications of our results. [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V28.00005: Switching Current Distributions in Superconductor--Topological Insulator--Superconductor Junctions Andrew Murphy, Can Zhang, Erik Huemiller, Seongshik Oh, James Eckstein, Dale Van Harlingen, Alexey Bezryadin It has been proposed that localized Majorana fermion (MF) modes can exist in lateral Josephson junctions with a 3D-topological insulator barrier at locations at which the phase difference across the junction is an odd multiple of $\pi $. These states enter the junctions bound to the nodes of the Josephson vortices as a perpendicular magnetic field is increased. Each mode contributes a local 4$\pi $-periodic sin($\varphi $/2)-component to the junction's current-phase relation, adding to the usual sin($\varphi )$ dependence. The sign of this new term encodes the parity of the Majorana pair. As a way to detect these states and measure their parity, we study the distribution of switching currents in Nb-Bi2Se3-Nb junctions fabricated on thin Bi2Se3 films in which the superconductivity is induced by a pair of closely spaced Nb electrodes. We expect that such measurements will be sensitive to the parity of the MFs, yielding a splitting of the distribution. Preliminary measurements of the critical current distributions show the onset of unusual features when the magnetic field is increased which we are analyzing to determine if they may arise from Majorana fermions in the junctions. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V28.00006: Signature of topological transition in InAs nanowire Josephson junctions Elia Strambini, J. Paajaste, M. Amado, S. Roddaro, P. San-Jose, R. Aguado, S. Bergeret, D. Ercolani, L. Sorba, F. Giazotto The coupling of a conventional s-wave superconductors to semiconductors with strong spin-orbit (SO) coupling, like e. g. InAs or InSb nanowires (NWs), gives rise to unconventional p-wave superconductivity that may become a topological superconductor (TS), which is a natural host for exotic edge modes with Majorana character. Recently the enhancement of the critical supercurrent $I_c$ in a strong SO semiconducting Josephson junction (JJ) have been proposed as a new evidence of the sought-after Majorana bound states.\footnote{Phys.Rev.Lett. \bf{112}, 137007 (2014)} Here we report on the first observation of the colossal $I_c$ enhancement induced by an external magnetic field on a mesoscopic JJ formed by InAs NWs and Ti/Al leads. This anomalous enhancement appears precisely above a threshold magnetic field $B_{th}$ orthogonal to the substrate and in junctions of different lengths, suggesting that the origin of the enhancement is intrinsic, i.e. it is not related to geometrical resonances in the junction. None of the standard phenomenon known in JJ, including e. g. Fraunhofer patterns or $\pi$-junction behavior, can explain this colossal enhancement while a topological transition at $B_{th}$ is qualitatively compatible with the observed phenomenology. [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V28.00007: Band structure of topological insulators from noise measurements in tunnel junctions Juan Pedro Cascales Sandoval, Isidoro Martinez, Ruben Guerrero, Cui-zu Chang, Ferhat Katmis, Jagadeesh Moodera, Farkhad Aliev The unique properties of spin-polarized surface or edge states in topological insulators (TIs) make these quantum coherent systems interesting from the point of view of both fundamental physics and their implementation in low power spintronic devices. Here we present such a study in TIs, through tunnelling and noise spectroscopy utilizing TI/Al2O3/Co tunnel junctions with bottom TI electrodes of either Bi2Te3 or Bi2Se3. We demonstrate that features related to the band structure of the TI materials show up in the tunnelling conductance and even more clearly through low frequency noise measurements. The bias dependence of 1/f noise reveals peaks at specific energies corresponding to band structure features of the TI. TI tunnel junctions could thus simplify the study of the properties of such quantum coherent systems that can further lead to the manipulation of their spin-polarized properties for technological purposes. [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V28.00008: Quantum interferences and edge states in Bismuth based Josephson junctions Anil Murani, Shamashis Sengupta, Alik Kasumov, Sophie Gueron, Hélène Bouchiat We have investigated proximity induced superconductivity in single crystal bismuth nanowires connected to superconducting electrodes with a high critical field. I will specially report recent results on nanowires whose crystalline orientation could be determined by electron diffraction. \\ At low temperature a supercurrent is measured which persists up to the critical field of the electrodes and exhibits sample dependent fast squid-like oscillations (period one to few hundred gauss) modulated by slower (few thousand Gauss) oscillations. We attribute this striking result to the appearance of 1D topological edge channels on special surfaces of Bi due to its strong spin-orbit coupling, in addition to a strong Zeeman effect caused by an unusually high g-factor. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V28.00009: Quantum transport in topological insulator nanoribbon field effect and Josephson devices Morteza Kayyalha, Luis Jauregui, Aleksander Kazakov, Michael Pettes, Ireneusz Miotkowski, Li Shi, Leonid Rokhinson, Yong Chen The spin-helical topological surface states (TSS) of topological insulators have attracted great attention in the past few years as an excellent platform to study topological transport and other exotic physics such as Majorana fermions. Here we present experiments studying quantum transport of TSS in topological insulator nanoribbon (TINR) field effect devices with normal as well as superconducting contacts. In Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ NRs with normal contacts, we observe that the conductance vs. axial magnetic field exhibits Aharonov-Bohm (AB) oscillations with an alternating phase of zero and $\pi $, depending periodically on the Fermi momentum k$_{\mathrm{F}}$ tuned by an applied back-gate voltage, consistent with the 1D sub-band structure formed by circumferentially quantized TSS [1]. We also investigated the Josephson effects in BiSbTeSe$_{\mathrm{2}}$ TINRs with superconducting Nb contacts. We measured the gate voltage and temperature dependence of the supercurrent and multiple Andreev reflections (MAR), to probe phase coherent transport via TSS. [1] L. A. Jauregui et al., arxiv:1503.00685. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V28.00010: Multi-terminal Josephson junctions as topological matter Roman-Pascal Riwar, Manuel Houzet, Julia S. Meyer, Yuli V. Nazarov Topological materials and their unusual transport properties are now at the focus of modern experimental and theoretical research. Their topological properties arise from the bandstructure determined by the atomic composition of a material and as such are difficult to tune and naturally restricted to $\leq 3$ dimensions. Here we demonstrate that $n$-terminal Josephson junctions with conventional superconductors may provide novel realizations of topology in $n-1$ dimensions, which have similarities, but also marked differences with existing 2D or 3D topological materials. For $n\geq 4$, the Andreev subgap spectrum of the junction can accommodate Weyl singularities in the space of the $n-1$ independent superconducting phases, which play the role of bandstructure quasimomenta. The presence of these Weyl singularities enables topological transitions that are manifested experimentally as changes of the quantized transconductance between two voltage-biased leads, the quantization unit being $4e^2/h$. [Preview Abstract] |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V28.00011: Towards all electrical control of topological Josephson junctions and Majorana zero modes via spin-orbit interactions Xin Liu, Xiaopeng Li, Xiong-Jun Liu, Dong-Ling Deng We study the current-phase relation of topological Josephson junctions with spin-orbit interactions, and show that the coupling between Majorana zero modes (MZMs) can be controlled via gate tunable spin-orbit couplings (SOCs). The spin-triplet pairings in the presence of MZMs at the two ends of a one-dimensional topological superconductor, are shown to have a $\pi$ phase difference, from which a Josephson $\pi$-junction can be created. This $\pi$ phase is unambiguously manifested to be a spin-dependent superconducting phase, dubbed spin-phase. We demonstrate that SOC can induce such spin-phase in spin-triplet superconducting condensates which can tune the MZM coupling energy and allow a finite topological Josephson current without a magnetic flux in superconducting circuits. We further establish the linkage between this Josephson current and the fermion parity in a topological Josephson junction and propose an all-electronically controlled superconductor-semiconductor hybrid circuit to detect the non-Ableian nature of MZMs. [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V28.00012: Mean-field description of topological charge 4e superconductors Victoria Gabriele, Jing Luo, Jeffrey C. Y. Teo BCS superconductors can be understood by a mean-field approximation of two-body interacting Hamiltonians, whose ground states break charge conservation spontaneously by allowing non-vanishing expectation values of charge 2e Cooper pairs. Topological superconductors, such as one-dimensional $p$-wave wires, have non-trivial ground states that support robust gapless boundary excitations. We construct a four-body Hamiltonian in one dimension and perform a mean-field analysis. The mean-field Hamiltonian is now quartic in fermions but is still exactly solvable. The ground state exhibits 4-fermion expectation values instead of Cooper pair ones. There also exists a topological phase, where the charge 4e superconductor carries exotic zero energy boundary excitations. [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V28.00013: Scattering states of the Majorana-bound-state-supporting vortices at the interface of a topological insulator and an s-wave superconductor Adam Durst We consider an isolated vortex in the 2D proximity-induced superconducting state formed at the interface of a 3D topological insulator (TI) and an s-wave superconductor (sSC). Prior calculations of the bound states of this system famously revealed a zero-energy state that is its own conjugate, a Majorana fermion bound to the vortex core. We calculate, not the bound states, but the scattering states of this system, and ask how the spin-momentum-locked massless Dirac form of the single-particle Hamiltonian, inherited from the TI surface, affects the cross section for scattering Bogoliubov quasiparticles from the vortex. As in the case of an ordinary superconductor, this is a two-channel problem with the vortex mixing particle-like and hole-like excitations. And as in the ordinary case, the same-channel differential cross section diverges in the forward direction due to the Aharonov-Bohm effect, resulting in an infinite total cross section but finite transport and skew cross sections. We calculate the transport and skew cross sections numerically, via a partial wave analysis, as a function of both quasiparticle excitation energy and chemical potential. Novel effects emerge as particle-like or hole-like excitations are tuned through the Dirac point. [Preview Abstract] |
Thursday, March 17, 2016 5:06PM - 5:18PM |
V28.00014: Observation of superconductivity induced by a point contact on 3D Dirac semimetal Cd3As2 crystals. He Wang, Huichao Wang, Haiwen Liu, Hong Lu, Wuhao Yang, Shuang Jia, Xiongjun Liu, Xincheng Xie, Jian Wei, Jian Wang The 3D Dirac semimetal state is located at the topological phase boundary and can potentially be driven into other topological phases including topological insulator, topological metal and the long-pursuit topological superconductor states. Crystalline Cd3As2 has been proposed and proved to be one of 3D Dirac semimetals which can survive in atmosphere. By precisely controlled point contact (PC) measurements, we observe the exotic superconductivity in the vicinity of the point contact region on the surface of Cd3As2 crystal, which might be induced by the local pressure in the out-of-plane direction from the metallic tip for PC. The observation of zero bias conductance peak (ZBCP) and double conductance peaks (DCPs) symmetric to zero bias further reveals p-wave like unconventional superconductivity in Cd3As2. Considering the special topological property of the 3D Dirac semimetal, our findings may indicate that the Cd3As2 crystal under certain conditions is a candidate of topological superconductor, which is predicted to support Majorana zero modes or gapless Majorana edge/surface modes on the boundary depending on the dimensionality of the material. [Preview Abstract] |
Thursday, March 17, 2016 5:18PM - 5:30PM |
V28.00015: Induced gap in topological materials from the superconducting proximity effect Ching-Kai Chiu, William Cole Topological superconductivity has been of considerable interest lately, with several proposed experimental realizations in solid state systems. A heterostructure of s-wave superconductor and 3D topological insulator is one of the more promising platforms, with topological superconductivity realized on the "naked" surface of the topological insulator through the superconducting proximity effect. We theoretically study the induced superconducting gap on the naked surface. Adjusting the Fermi level above the bulk gap (which is the case in experiments), our results for the induced superconducting gap are in agreement with that probed in thin topological insulators (\textless 10nm) in the experiments (Nat. Phys. 10, 943-950 (2014) and Phys. Rev. Lett. 112, 217001 (2014)). We further predict the gap in thick topological insulators (\textgreater 10nm). This work is supported by LPS-MPO-CMTC, Microsoft Q, and JQI-NSF-PFC. [Preview Abstract] |
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