Abstract

Ultrathin Bi 2 Se 3 -NbN bilayers comprise a simple proximity system of a topological insulator and an s-wave superconductor for studying gating effects on topological superconductors. Here we report on 3 nm thick NbN layers of weakly connected superconducting islands, overlayed with 10 nm thick Bi 2 Se 3 film which facilitates enhanced proximity coupling between them. Resistance versus temperature of the most resistive bilayers shows insulating behavior but with signs of superconductivity. We measured the magnetoresistance (MR) of these bilayers versus temperature with and without a magnetic field H normal to the wafer (MR = [R(H) − R(0)]/{[R(H) + R(0)]/2}), and under three electric gate-fields of 0 and ± 2 MV/cm. The MR results showed a complex set of gate sensitive peaks which extended up to about 30 K. The results are discussed in terms of vortex physics, and the origin of the different MR peaks is identified and attributed to flux-flow MR in the isolated NbN islands and the different proximity regions in the Bi 2 Se 3 cap-layer. The dominant MR peak was found to be consistent with enhanced proximity induced superconductivity in the topological edge currents regions. The high temperature MR data suggest a possible pseudogap phase or a highly extended fluctuation regime.

Highlights

  • Surface edge states of topological superconductors (TOS) are expected to support zero energy modes or Majorana fermions which are robust against disorder and decoherence [1,2]

  • The small decrease of R below 2.5 K disappears under 1 T, and we conclude that it originates in superconductivity of the NbN islands which are very weakly linked

  • The gap-like feature appearing at about 70 meV, is a result of several weak-links connected in series between the voltage contacts

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Summary

Introduction

Surface edge states of topological superconductors (TOS) are expected to support zero energy modes or Majorana fermions which are robust against disorder and decoherence [1,2]. An alternative way for realizing TOS is by inducing superconductivity in a topological insulator or in semiconductor-nanowires with strong spin-orbit interaction via the proximity effect (PE) [8,9,10,11]. Unconventional superconductivity in these systems, such as revealed by the presence of zero bias conductance peaks (ZBCP), indicates zero energy bound states that might be due to Majorana zero energy modes, but could originate in zero energy Andreev bound states in an unconventional superconductor. It is hard to distinguish between these two different phenomena, and efforts are ongoing in order to achieve this goal [12,13]

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