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.

Figure 1

Variation of $T_c$ with Tl content $x$ for ${\mathrm{Pb}}_{1-x}{\mathrm{Tl}}_x\mathrm{Te}$. Line shows linear fit. Inset shows representative resistivity data for single crystals.

Figure 2

Low-temperature resistivity of ${\mathrm{Pb}}_{1-x}{\mathrm{Tl}}_x\mathrm{Te}$ for $x=0.3$, 0.4, 0.8, and 1.3%. Inset to panel (a) shows residual resistivity ${\rho }_0$ (left axis, solid symbols) and ${\rho }_0-{\rho }_{\min }$ (right axis, open symbols) as a function of $x$. Insets to (c) and (d) show $T^2$ behavior at low temperatures for the smallest Tl concentrations.

Figure 3

Representative magnetoresistance, $\Delta \rho =\rho (H)-\rho (H=0)$, for $x=0.4\%$ at 1.8 K. Upper inset shows temperature dependence of the resistivity in fields of 0 and 5 T. The data for 5 T have been shifted down by ${\rho }_0(5\mathrm{T})-{\rho }_0(0\mathrm{T})=4.8\mu \Omega \mathrm{cm}$. Lower inset shows temperature dependence of susceptibility for different Tl concentrations.