Resonance-state-induced superconductivity at high Indium contents in In-doped SnTe

We report a reinvestigation of superconducting Sn${}_{1-x}$In${}_x$Te at both low and high In doping levels. Analysis of the superconductivity reveals a fundamental change as a function of $\mathit{\text{x}}$: the system evolves from a weakly coupled to a strongly coupled superconductor with increasing indium content. Hall Effect measurements further show that the carrier density does not vary linearly with Indium content; indeed at high Indium content, the samples are overall $\mathit{\text{n}}$-type, which is contrary to expectations of the standard picture of In${}^{1+}$ replacing Sn${}^{2+}$ in this material. Density functional theory calculations probing the electronic state of In in SnTe show that it does not act as a trivial hole dopant, but instead forms a distinct, partly filled In 5$\mathit{\text{s}}$ - Te 5$\mathit{\text{p}}$ hybridized state centered around E${}_F$, very different from what is seen for other nominal hole dopants such as Na, Ag, and vacant Sn sites. We conclude that superconducting In-doped SnTe therefore cannot be considered as a simple hole doped semiconductor.