Research: Valence skipping elements

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, potentially providing a realization of negative-U superconductivity. We initially addressed this question by considering two specific superconducting materials, Tl-doped PbTe and In-doped SnTe. 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. The focus of our ongoing experiments is on extending these ideas in the context of a dense lattice of mixed valence “impurities”, as found for example in the charge density wave insulators Ba2BiIIIBiVO6 (left figure) and Cs2AuIAuIIIX6 (right figure; X = halogen).
  Tl-doped PbTe and In-doped SnTe:
  PhD Theses
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Fisher Research Group
Geballe Laboratory for Advanced Materials
Dept. of Applied Physics
Stanford University
CA 94305-4045
Last Updated: Dec 30th 2017