Fully gapped superconductivity in In-doped topological crystalline insulator Pb$_{0.5}$Sn$_{0.5}$Te

TL;DR

This study reports scanning tunneling spectroscopy on In-doped Pb$_{0.5}$Sn$_{0.5}$Te, revealing a fully gapped superconducting state with anisotropic s-wave symmetry, challenging the expectation of topological superconductivity.

Contribution

First measurement of the superconducting gap in In-doped Pb$_{0.5}$Sn$_{0.5}$Te showing a fully gapped state, providing new insights into its superconducting nature.

Findings

Superconducting gap fitted by anisotropic s-wave model

No in-gap states observed, indicating a fully gapped superconductor

Contradicts the expectation of topological superconductivity in this material

Abstract

Superconductors derived from topological insulators and topological crystalline insulators by doping have long been considered to be candidates as topological superconductors. Pb$_{0.5}$Sn$_{0.5}$Te is a topological crystalline insulator with mirror symmetry protected surface states on (001), (011) and (111) oriented surfaces. The superconductor (Pb$_{0.5}$Sn$_{0.5}$)$_{0.7}$In$_{0.3}$Te is induced by In doping in Pb$_{0.5}$Sn$_{0.5}$Te, and is thought to be a topological superconductor. Here we report the first scanning tunneling spectroscopy measurement of the superconducting state as well as the superconducting energy gap in (Pb$_{0.5}$Sn$_{0.5}$)$_{0.7}$In$_{0.3}$Te on a (001)-oriented surface. The spectrum can be well fitted by an anisotropic $s$-wave gap function of $\Delta=0.72+0.18\cos4\theta$ meV using Dynes model. The results show that the quasi-particle density of states seem to be fully gapped without any in-gap states, in contradiction with the expectation of a topological superconductor.