Unusual nature of fully gapped superconductivity in In-doped SnTe

Abstract

The superconductor Sn${}_{1\ensuremath{-}x}$In${}_{x}$Te is a doped topological crystalline insulator and has become important as a candidate topological superconductor, but its superconducting phase diagram is poorly understood. By measuring about 50 samples of high-quality, vapor-grown single crystals, we found that the dependence of the superconducting transition temperature ${T}_{c}$ on the In content $x$ presents a qualitative change across the critical doping ${x}_{c}\ensuremath{\simeq}3.8%$, at which a structural phase transition takes place. Intriguingly, in the ferroelectric rhombohedral phase below the critical doping, ${T}_{c}$ is found to be strongly enhanced with impurity scattering. It appears that the nature of electron pairing changes across ${x}_{c}$ in Sn${}_{1\ensuremath{-}x}$In${}_{x}$Te.