Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L51: Invited Session: Topological Superconductors |
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Sponsoring Units: DCMP Chair: Ady Stern, Weizmann Institute of Science Room: Grand Ballroom C1 |
Wednesday, March 4, 2015 8:00AM - 8:36AM |
L51.00001: Time reversal invariant topological superconductors in one dimension: how to realize them and what can they do for you Invited Speaker: Erez Berg Time-reversal invariant (TRI) topological superconductors are exotic superconductors that support anomalous protected edge states. These states are electronic analogues of the B phase of superfluid $^{3}$He. In one spatial dimension, a TRI superconductor carries a Kramers pair of Majorana zero modes at each end. In this talk, I will discuss setups to realize this phase in spin-orbit coupled quantum wires, in proximity to conventional superconductors. The topologically non-trivial phase can be stabilized either by coupling the wire to two superconductors with a phase difference of $\pi$ between them, or spontaneously, due to repulsive interactions in the wire. In the former case, the system is a natural realization of a fermion parity pump, switching the local fermion parity of both edges when the relative phase between the superconductors is changed adiabatically by $2\pi$. I will show that a gapless TRI topological phase with exponentially localized edge states can exist even if the superconductor used to induce pairing is one-dimensional, and superconducting long-range order is destroyed by long-wavelength fluctuations. If time allows, I will talk about the signatures of the TRI phase in noise correlation experiments, and compare it to the the case of a time reversal breaking phase with a single Majorana zero mode at the ends. \\ References: A. Keselman, L. Fu, A. Stern, and E. Berg, PRL 111, 116402 (2013); A. Haim, A. Keselman, E. Berg, and Y. Oreg, PRB 89, 220504(R) (2014); A. Haim, E. Berg, F. von Oppen and Y. Oreg, arXiv:1411.0673. [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 9:12AM |
L51.00002: Induced superconductivity in the quantum spin Hall edge Invited Speaker: Sean Hart The quantum spin Hall insulator is an example of a two-dimensional topological insulator, a phase of matter in which protected gapless surface states surround a gapped bulk. Inducing superconducting pairing within the helical edge states of the quantum spin Hall effect has been proposed as an avenue towards topological superconductivity. In this talk we present measurements of superconductivity induced in two-dimensional HgTe/HgCdTe quantum wells, a material which becomes a quantum spin Hall insulator when the well width exceeds $d_{C}=6.3$ nm. In wells that are 7.5 nm wide, we find that supercurrents are confined to the one-dimensional sample edges as the bulk density is depleted. However, when the well width is decreased to 4.5 nm the edge supercurrents cannot be distinguished from those in the bulk. Our Josephson interferometry measurements provide evidence for supercurrents carried by the helical edges of the quantum spin Hall effect, one of the crucial ingredients for one-dimensional topological superconductivity. These results also directly yield information about the microscopic structure of the edge modes. In particular we find that the widths of the edge channels range from 180 nm to 408 nmÂ. [Preview Abstract] |
Wednesday, March 4, 2015 9:12AM - 9:48AM |
L51.00003: Local Adiabatic Mixing of Kramers Pairs of Majorana Bound States Invited Speaker: Karsten Flensberg In this talk adiabatic time evolution of Kramers pairs of Majorana bound states and the prospects of using such bound states as parity qubits are discussed. It is shown that local adiabatic perturbations can cause a rotation in the space spanned by the Kramers pair and that the quantum information is therefore unprotected against local perturbations. This is in contrast to the case of single localized Majorana bound states in systems with broken time reversal symmetry. However, under certain conditions such mixing does not occurs. A general scheme for determining when these conditions are satisfied is explained and exemplified with a quasi-1D model of a time reversal symmetric topological superconductor. [Preview Abstract] |
Wednesday, March 4, 2015 9:48AM - 10:24AM |
L51.00004: Phase coherent transport in hybrid superconductor-topological insulator devices Invited Speaker: Aaron Finck Heterostructures of superconductors and topological insulators are predicted to host unusual zero energy bound states known as Majorana fermions, which can robustly store and process quantum information. Here, I will discuss our studies of such heterostructures through phase-coherent transport, which can act as a unique probe of Majorana fermions. We have extensively explored topological insulator Josephson junctions through SQUID and single-junction diffraction patterns, whose unusual behavior give evidence for low-energy Andreev bound states. In topological insulator devices with closely spaced normal and superconducting leads, we observe prominent Fabry-Perot oscillations, signifying gate-tunable, quasi-ballistic transport that can elegantly interact with Andreev reflection. Superconducting disks deposited on the surface of a topological insulator generate Aharonov-Bohm-like oscillations, giving evidence for unusual states lying near the interface between the superconductor and topological insulator surface. Our results point the way towards sophisticated interferometers that can detect and read out the state of Majorana fermions in topological systems. This work was done in collaboration with Cihan Kurter, Yew San Hor, and Dale Van Harlingen. [Preview Abstract] |
Wednesday, March 4, 2015 10:24AM - 11:00AM |
L51.00005: Selective Equal Spin Andreev Reflections induced by Majorana Fermions Invited Speaker: Kam Tuen Law The search for Majorana fermions has become an important subject in recent years. Zero-bias conductance peaks (ZBCPs) possibly induced by Majorana fermions have been reported in tunneling spectroscopy experiments. However, the origin of these ZBCPs is still under debate. In this talk, I would like to point out that Majorana fermions induce a special type of Andreev reflection processes called selective equal spin Andreev reflections (SESARs) [1]. For SESAR processes at a normal lead/topological superconductor interface, incoming electrons with certain spin polarization in the lead are reflected as counter-propagating holes with the same spin. The spin polarization direction of the electrons of this Andreev reflected channel is selected by the Majorana fermions. Moreover, electrons with opposite spin polarization are always reflected as electrons with unchanged spin. We show that the tunneling current from a ferromagnetic lead to a topological superconductor is strongly affected by the spin polarization direction of the lead due to SESARs. This property can be used to detect Majorana fermions in several candidate topological superconductors. The application of SESARs in generating giant spin currents in nodal topological superconductors will also be discussed. \\[4pt] [1] James J. He, T. K. Ng, Patrick A. Lee and K. T. Law, Phys. Rev. Lett. 112, 037001 (2014). [Preview Abstract] |
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