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Topological Surface States and Superconductivity in [Tl$_4$](Tl$_{1-x}$Sn$_{x}$)Te$_3$ Perovskites

Materials with strong spin-orbit coupling have attracted attention following the prediction and subsequent discovery of strong two- and three-dimensional topological insulators in which a topological property of the bulk band structure of an insulator results in metallic surface states with Dirac-like dispersion. Here we report the discovery of Dirac-like surface states in the perovskite superconductor [Tl$_4$]TlTe$_3$ (Tl5Te3) and its non-superconducting tin-doped derivative, [Tl4](Tl$_{0.4}$Sn$_{0.6}$)Te$_3$, as observed by angle-resolved photoemission spectroscopy (ARPES). Density functional theory (DFT) calculations predict a single spin-orbit driven band parity inversion at the $Z$ point above the Fermi level of Tl5Te3, suggesting the surface states are protected by Z$_2$ topology. Calculations on [Tl$_4$]SnTe$_3$ show no parity inversions, implying that a topological transition from non-trivial to trivial must occur upon doping with tin, i.e., [Tl$_4$](Tl$_{1-x}$Sn$_{x}$)Te$_3$. Thus [Tl$_4$]{\it M}Te$_3$ perovskites are a possible new, non-trigonal class of Z$_2$ topological compounds. Additionally, as Tl5Te3 is a stoichiometric bulk superconductor, these perovskites are ideal materials in which to study the interplay between surface states and bulk superconductivity.