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CM-AMO Seminar<br>Coherent Transport and Proximity Induced Superconductivity in Mesoscopic Bi2Se3 Channels<br><b>Speaker: Nitin Samarth (Pennsylvania State University, Physics)</br></b>

Tuesday, October 25, 2011
12:00 AM
335 West Hall

Speaker: Nitin Samarth (Pennsylvania State University, Physics)

A possible route to the realization of exotic Majorana fermions is to interface a 3D topological insulator with a superconductor and to search for a zero energy bound state in vortices. However such studies are still in their infancy, in part due to the non-ideal nature of samples and difficulties encountered in inducing superconductivity. We describe recent studies [1] of proximity-induced superconductivity in nanoribbons of Bi2Se3 , a narrow gap semiconductor which is also a "candidate" 3D topological insulator. We observe signatures of the proximity effect, including a supercurrent and multiple Andreev reflections. Surprisingly, the proximity effect occurs for channel lengths much longer than anticipated from normal state transport, suggesting preferential coupling of superconductivity to a ballistic surface channel even in non-ideal samples. We also report an unexpected phenomenon in a perpendicular magnetic field: periodic magnetoresistance oscillations when the contacts are in the superconducting state. These oscillations cannot be explained using conventional scenarios based on Aharonov-Bohm or Shubnikov-de Haas physics, but are consistent with a “vortex blockade” model (proposed recently in Physical Review Letters by David Pekker and colleagues at Caltech and Illinois) wherein the number of vortices along a superconducting nanoribbon increases in unit steps with increasing magnetic field, with resistance peaks produced at the degenerate points. If confirmed by future studies, the experimental geometry used here would be a promising direction in the search for Majorana fermions.

1. Duming Zhang et al., arxiv: 1106.3605 (in press, Phys. Rev. B).