Charge Disproportionation and Superconductivity in Tl- and In-doped Lead Telluride
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- Larsson, S. J Supercond Nov Magn (2012) 25: 319. doi:10.1007/s10948-011-1309-z
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The “negative-U model” is analyzed and applied to Tl- and In-doped lead telluride, Pb1−xTlxTe, and Pb1−xInxTe, respectively. The former, but not the latter, is superconducting (Tc<1.5 K) in a certain concentration range (xc>0.003). At very low concentrations, carriers are created in the valence band and the conductivity is metallic, of the ordinary kind. Matsushita et al. and Erickson et al. explain this as due to disproportionation 2Tl2+→Tl+ and Tl3+. The latter authors are of the opinion that indium also disproportionates in the same way as thallium. If this would be the case, it would be hard to explain the absence of superconductivity at indium doping. In this paper it is shown that In2+ does not disproportionate under the given circumstances. The Hubbard model, extended and generalized, is therefore consistent with all experimental results.