Signatures of Topological Superconductivity in Bulk Insulating Topological Insulator BiSbTe$_{1.25}$Se$_{1.75}$ in Proximity with Superconducting NbSe$_2$

TL;DR

This study demonstrates that proximity-induced superconductivity in bulk insulating topological insulators with NbSe2 can exhibit signatures consistent with topological p-wave superconductivity, advancing the pursuit of Majorana fermions.

Contribution

It provides experimental evidence of proximity-induced superconductivity in bulk insulating TIs and discusses potential topological nature of the induced states, overcoming previous material limitations.

Findings

Observation of double-gap spectra and conductance asymmetries.

Unusual conductance ripples at high biases.

Evidence supporting topological superconductivity in bulk insulators.

Abstract

The combination of superconductivity and spin-momentum locking at the interface between an s-wave superconductor and a three-dimensional topological insulator (3D-TI) is predicted to generate exotic p-wave topological superconducting phases that can host Majorana fermions. However, large bulk conductivities of previously investigated 3D-TI samples and Fermi level mismatches between 3D bulk superconductors and 2D topological surface states have thwarted significant progress. Here we employ bulk insulating topological insulators in proximity with two-dimensional superconductor NbSe$_2$ assembled via Van der Waals epitaxy. Experimentally measured differential conductance yields unusual features including a double-gap spectrum, an intrinsic asymmetry that vanishes with small in-plane magnetic fields and differential conductance ripples at biases significantly larger than the superconducting gap. We explain our results on the basis of proximity induced superconductivity of topological surface states, while also considering possibilities of topologically trivial superconductivity arising from Rashba-type surface states. Our work demonstrates the possibility of obtaining p-wave superconductors by proximity effects on bulk insulating TIs.