Explanation of the exotic superconductors by a valence-fluctuation pairing mechanism

Abstract

Abstract The exotic superconductors considered here are those defined by Uemura and coworkers, the materials which approximately satisfy the empirical relation T c α λL −2, where λL is the London penetration depth. This category includes all of the presently known high-temperature superconductors, plus many others which are or have been prominent in basie superconductivity research. In a survey these materials were shown to share many characteristic features, electronic properties which are anomalous from the standpoint of BCS and strong-coupling (Eliashberg) systematics, and also distinctive features in the crystal chemistry. It is now argued that these characteristic features are consistent with and indeed suggestive of a valence-fluctuation pairing mechanism developed for the cuprate superconductors. The explanation is reasonably straightforward, and the match between the expected and the observed properties is remarkably extensive. The crystal chemistry aspects are seen here as enabling features, that is, features which are necessary in order to justify adeauately an Anderson-lattice form of model Hamiltonian, and to provide a suitable parameter regime.