Optical Evidence of an Enhanced Electronic Effective Mass in the Anomalous Pb1−xTlxTe Superconductor

The narrow band-gap semiconductor PbTe exhibits a number of striking properties upon thallium (Tl)-doping, including the onset of superconductivity at temperatures T-c which are substantially higher than for materials with equivalent charge carrier concentration. Here we provide a thorough optical investigation of Pb1-xTlxTe over a very broad spectral range and contrast its normal-state, complete excitation spectrum with the optical response of the nonsuperconducting analog Na-doped PbTe. We capture the relevant energy scales shaping their electronic structure and uncover the formation of an impurity band upon doping with Tl, which evolves into a resonant state for large doping. This implies a large density of states and an enhancement of the optical effective mass m*/m(e) of the itinerant charge carriers, which is stronger for Tl- than for Na-doping. Since the enhancement of m*/m(e) particularly occurs upon crossing a critical concentration x(c) in Tl-doped PbTe for which T-c not equal 0, we advance its relevance for the onset of superconductivity.