Superconductivity in Sn(1-x-d)In(x)Te and Pb(1-y)Tl(y)Te

Recent evidence for a charge-Kondo effect in superconducting samples of Pb$_{1-y}$Tl$_y$Te [PRL \textbf{94}, 157002 (2005)] raises the possibility that systems of degenerate semiconductors doped with valence skipping elements may be an ideal realm in which to study suggested negative U electronic pairing mechanisms in superconductors [PRL \textbf{61}, 2713 (1988)]. However, questions of exactly how the proposed charge-Kondo behavior relates to superconductivity in Pb$_{1-y}$Tl$_y$Te remain. In this work, we study the relationship between the DOS at the Fermi level and Tc in the related superconductor Sn$_{1-x-d}$In$_x$Te, where enhancement of Tc is found above a critical concentration $x_c > d/2$, where the Fermi level is pinned in the impurity band and the DOS is enhanced [Sov. Phys. Solid State \textbf{28}, 612 (1986)]. We find that the elevated DOS is insufficient to explain the enhanced Tc for these samples, suggesting an additional pairing mechanism is involved, such as the negative U mechanism mentioned above.