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Several elements (for instance Pb, Bi, Tl…) are often found to skip a valence in solids. A longstanding question in the field of superconductivity has been whether valence fluctuations associated with such elements (i.e. fluctuations involving pairs of electrons) can contribute to the pairing interaction, perhaps via a kind of correlation-enhanced electron-phonon interaction, or potentially even providing a realization of negative-U superconductivity.
Our work in this area has taken two directions. First, we have explored the behavior of dilute valence-skipping impurities. Here, our research has focussed on two specific superconducting materials, Tl-doped PbTe and In-doped SnTe. Among other things, we found evidence for a new correlated electron effect associated with valence fluctuations of the impurities in these materials - a charge Kondo effect - with implications for the pairing mechanism in this and related materials.
Secondly, we have extended these ideas in the context of a dense lattice of mixed valence “impurities”, as found for example in the charge disproportionated charge density wave insulators Ba2BiIIIBiVO6 (left figure) and Cs2AuIAuIIIX6 (right figure; X = halogen).
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