Evidence of indium impurity band in superconducting (Sn,In)Te thin films

Sn1-xInxTe has been synthesized and studied recently as a candidate topological superconductor. Its superconducting critical temperature increases with indium concentration. However, the role of indium in altering the normal-state band structure and generating superconductivity is not well understood. Here, we explore this question in Sn1-xInxTe (0 < x < 0.3) thin films, characterized by magnetotransport, infrared transmission, and photoemission spectroscopy measurement. We show that indium is forming an impurity band below the valence-band edge which pins the Fermi energy and effectively generates electron doping. An enhanced density of states due to this impurity band leads to the enhancement of T-c measured in multiple previous studies. The existence of the In impurity band and the role of In as a resonant impurity should be more carefully considered when discussing the topological nature of Sn1-xInxTe.