Charge Kondo Effect in Thermoelectric Properties of Lead Telluride Doped with Thallium Impurities

We investigate the thermoelectric properties of PbTe doped with a small concentration x of Tl impurities acting as acceptors and described by Anderson impurities with negative on-site correlation energy. The resulting charge Kondo effect naturally accounts for a number of the low temperature anomalies in this system, including the unusual doping dependence of the carrier concentration, the Fermi level pinning and the self-compensation effect. The Kondo anomalies in the low temperature resistivity at temperatures T <= 10K and the x-dependence of the residual resistivity are also in good agreement with experiment. Our model also captures the qualitative aspects of the thermopower at higher temperatures T > 300K for high dopings (x > 0.6%) where transport is expected to be largely dominated by carriers in the heavy hole band of PbTe.