Andreev reflection spectroscopy on Bi$_{2}$X$_{3}$ (X = Se, Te) topological insulators: Implications for the c-axis superconducting proximity effect

TL;DR

This study uses Andreev reflection spectroscopy to evaluate the topological nature of superconductivity induced in Bi2X3 topological insulators via the c-axis proximity effect, finding it to be predominantly non-topological.

Contribution

It provides experimental evidence and analysis showing that the proximity-induced superconductivity in Bi2X3 is mainly non-topological, using advanced spectroscopy and theoretical modeling.

Findings

Superconductivity induced in Bi2X3 is predominantly non-topological.

Andreev reflection data analyzed with Blonder-Tinkham-Klapwijk theory.

Fermi level located relative to Dirac point using tunneling spectroscopy.

Abstract

Using Andreev reflection (AR) as an experimental gauge of the superconducting proximity effect (PE), we assess the topological purity of the superconductivity that is induced by the c-axis PE between an s-wave superconductor and the topological insulators Bi$_{2}$X$_{3}$ (X=Se,Te). Point-contact AR spectroscopy is performed with Nb tips on Bi$_{2}$X$_{3}$ single crystals at 4.2 K. Scanning tunneling spectroscopy is also used, to locate the Fermi level $E_F$ relative to the Dirac point in the crystals. The AR data is analyzed with Blonder-Tinkham-Klapwijk theory, taking into account tip-induced spin-orbit coupling, Fermi-surface mismatch, and the co-presence of bulk band and topological surface states at $E_F$. Our results indicate that the superconductivity that can be proximity-induced into Bi$_{2}$X$_{3}$ is predominantly non-topological.