Abstract
We present a combined experimental and the oretical study of the evolution of the Fermi surface of the anomalous superconductor Pb1−x TlxTe as a function of thallium concentration, drawing on a combination of magneto transport measurements Shubnikov de Haas oscillations and Hall coefficient), Angle Resolved Photoemission Spectroscopy (ARPES), and density functional theory (DFT) calculations of the electronic structure. Our results indicate that for Tl concentrations beyond a critical value the Fermi energy coincides with resonant impurity states in Pb1−x TlxTe, and we rule out the presence of an additional valence band maximum at the Fermi energy. Comparison tonon-superconducting Pb1−x NaxTe implies that the presence of these impurity states at the Fermienergy provides the enhanced pairing interaction and thus also the anomalously high temperatures uper conductivity in this material.