Anomalous composition dependence of the superconductivity in In-doped SnTe

We report a reinvestigation of superconducting Sn1-xInxTe at both low and high In doping levels. Considering the system over a broad composition range in a single study allows us to characterize a significant change in the properties as a function of x: The system evolves from a weakly coupled p-type superconductor to a strongly coupled n-type superconductor with increasing indium content. Hall effect measurements show that the carrier density does not vary monotonically with indium content; a change from p-type to n-type is observed near 10% In doping. This is contrary to expectations dictating that In should be a p-type dopant in semiconducting SnTe, because it has one less valance electron than Sn. A crystallographic search for point defects at high x indicates that the material remains ideal NaCl-type over a wide composition range. Density functional theory calculations for In-doped SnTe support a picture where In does not act as a trivial hole dopant but instead forms a distinct, partly filled In 5s-Te5p hybridized state centered around EF, which is very different from what is seen for other nominal hole dopants such as Na, Ag, and vacant Sn sites.