Superconducting Properties of Tl-Doped PbTe

Tl-doped PbTe (Pb$_{1-x}$Tl$_x$Te) has a remarkably high $T_c$ given its relatively low carrier concentration and also exhibits unusual electronic properties [1--2]. For Tl concentrations $x$ beyond a critical value $x_c \sim 0.3\%$, it is observed to superconduct with $T_c$ rapidly increasing with $x$ up to a maximum doping of 1.5$\%$ and a maximum $T_c$ of 1.5 K. Here we present results of heat capacity, transport, and microwave conductivity experiments for single crystal samples. We extract estimates of superconducting parameters including $\Delta C/ \gamma T_c$, $H_{c2}$, coherence length, and penetration depth. For the highest Tl concentrations, we find that the heat capacity anomaly and the temperature dependence of the superfluid density are consistent with BCS predictions and that the material is in the dirty limit. [1] Y. Matsushita, H. Bluhm, T.~H. Geballe, and I.~R. Fisher, Phys. Rev. Lett. \textbf {94}, 157002 (2005). [2] M. Dzero and J. Schmalian, Phys. Rev. Lett. \textbf{94}, 157003 (2005).