Magnetoresistance and gating effects in ultrathin NbN-$\rm Bi_2Se_3$ bilayers

TL;DR

This study investigates magnetoresistance and gating effects in ultrathin NbN-$\rm Bi_2Se_3$ bilayers, revealing complex, gate-sensitive MR peaks linked to vortex physics, proximity-induced superconductivity, and potential pseudogap behavior.

Contribution

It demonstrates the influence of gating on magnetoresistance in topological insulator-superconductor bilayers, highlighting flux-flow MR and proximity effects in ultrathin NbN and $\rm Bi_2Se_3$ layers.

Findings

Gate-sensitive MR peaks extend up to 30 K.

MR peaks attributed to flux-flow in NbN islands and proximity regions.

Evidence of enhanced proximity-induced superconductivity at topological edges.

Abstract

Ultrathin $\rm Bi_2Se_3$-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 $\rm Bi_2Se_3$ 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 $\pm2$ 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 $\rm Bi_2Se_3$ 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.