Enhanced superconducting pairing interaction in indium-doped tin telluride

The ferroelectric degenerate semiconductor Sn_1-δTe exhibits superconductivity with critical temperatures, Tc, of up to 0.3 K for hole densities of order 10²¹ cm^-3. When doped on the tin site with greater than xc=1.7(3)% indium atoms, however, superconductivity is observed up to 2 K, though the carrier density does not change significantly. We present specific-heat data showing that a stronger pairing interaction is present for x>xc than for x<xc. By examining the effect of In dopant atoms on both Tc and the temperature of the ferroelectric structural phase transition, TSPT, we show that phonon modes related to this transition are not responsible for this Tc enhancement, and discuss a plausible candidate based on negative U pairing associated with mixed valency on the indium impurity sites.