Enhanced electronic effective mass in the anomalous Pb_1-xTl_xTe superconductor

The thermoelectric material PbTe, besides providing an alternative for power generation and refrigeration, also presents a series of unusual and poorly understood low-temperature electronic properties when appropriately doped. Of particular relevance to the present work, Tl substitution yields a metallic state, with no carrier freeze out to the lowest temperatures, and a superconducting ground-state for concentrations x > x_c ~ 0.3%. The critical temperature increases linearly with the concentration of Tl dopants, with maximum values (T_c ~ 1.4 K) that are anomalously high for the given carrier density of about 10²⁰ holes/cm³. Significantly, Tl is the only dopant to cause superconductivity in this material, so, for example, the analogous case of hole doping via Na substitution does not lead to superconductivity down to the lowest investigated temperature. It has therefore been conjectured that the special nature of the Tl impurities plays a prominent role.

We perform 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. We address 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 (Fig. 1.12).