Unusual nature of fully gapped superconductivity in In-doped SnTe

The superconductor Sn1-xInxTe 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) similar or equal to 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 Sn1-xInxTe.