Superconductivity in Charge Kondo Systems

We present a theory for superconductivity and charge Kondo fluctuations, i.e., resonant quantum valence fluctuations by two charge units, for Tl-doped PbTe. We show that Tl is very special as it first supplies a certain amount of charge carriers to the PbTe-valence band and then puts itself into a self-tuned resonant state to yield a new, robust pairing mechanism for these carriers.

Figure 1

(a) Valence band hole concentration as a function of Tl content in ${\mathrm{Pb}}_{1-x}{\mathrm{Tl}}_x\mathrm{Te}$ in comparison with experiments [8]; (b) valence band density of states for $x<x^{*}$ (upper panel) and $x>x^{*}$ (lower panel). A pinning of the chemical potential at $\mu ={\mu }^{*}$ for $x>x^{*}$ gives rise to a degeneracy between the ${\mathrm{Tl}}^{1+}$ and ${\mathrm{Tl}}^{3+}$ states and to $n_0(x)=\mathrm{const}$.

Figure 2

$T_c$ as a function of concentration for various values of the dimensionless exchange coupling constant $\gamma ={\rho }_{F}J$. Experimental points [5] are plotted for comparison. Inset shows low-$T$ part of concentration dependence of $T_c$ where reentrance behavior appears.

Figure 3

Normalized superfluid density as a function of $T_K/T_c$ shown on a log scale.