Evidence for Charge Kondo Effect in Superconducting Tl-Doped PbTe

We report results of low-temperature thermodynamic and transport measurements of ${\mathrm{Pb}}_{1-x}{\mathrm{Tl}}_x\mathrm{Te}$ single crystals for Tl concentrations up to the solubility limit of approximately $x=1.5\%$. For all doped samples, we observe a low-temperature resistivity upturn that scales in magnitude with the Tl concentration. The temperature and field dependence of this upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature $T_K\sim 6\mathrm{K}$. The observation of such an effect supports an electronic pairing mechanism for superconductivity in this material and may account for the anomalously high $T_c$ values.