| Tl-doped PbTe (Pb1-xTl xTe) is a degenerate semiconductor exhibiting superconductivity at a remarkably high Tc given its relatively low carrier concentration of ∼1020 cm-3. For Tl concentrations x beyond a critical value x c∼0.3%, it is observed to superconduct with T c rapidly increasing with x up to a maximum Tc of 1.5 K for the highest Tl doping of 1.5%. In comparison, other similar superconducting semiconductors can achieve comparable Tc values with much higher carrier concentrations. Since thallium is the only impurity known to cause superconductivity in PbTe, there has been considerable discussion as to the role of the Tl impurities in this material. In particular, it has been suggested that the superconductivity can be linked to the presence of a mixed Tl valence fluctuating between Tl + and Tl3+ states.; We investigate the superconducting and normal state properties of single crystal Tl-doped PbTe samples for a range of Tl concentrations up to the solubility limit of approximately 1.5%. Estimates of superconducting parameters from heat capacity and upper critical field measurements show that the material is a Type-II, weak-coupled BCS superconductor in the dirty limit. Evidence for the mixed valence state appears as a carrier concentration saturation in the Hall effect measurements, indicating a self-compensating mixture of Tl+ (acceptors) and Tl3+ (donors) beyond a characteristic Tl concentration near xc. Furthermore, we show evidence that an anomalous low-temperature resistivity upturn may be associated with a charge Kondo effect arising; from the quantum valence fluctuations between the degenerate valence states of the Tl impurities. The resistivity anomaly was observed only in superconducting samples where x > xc These observations would support the notion of an alternate pairing mechanism for superconductivity in this material which is linked to the presence of the mixed valence state. |
