Optical Evidence of an Enhanced Electronic Effective Mass in the Anomalous Pb_1-xTl_xTe 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 Pb_1-xTl_xTe over a very broad spectral range and contrast its normal-state, complete excitation spectrum with the optical response of the non-superconducting analog Na-doped PbTe [1]. 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≠ 0, we advance its relevance for the onset of superconductivity.
[1] A. Pal et al., Phys. Rev. Materials 3, 054801 (2019).