Perspectives of disproportionation driven superconductivity in strongly correlated 3d compounds

Abstract

Disproportionation in 3d compounds can give rise to an unconventional electron-hole Bose liquid with a very rich phase diagram, from a Bose metal, to a charge ordering insulator and an inhomogeneous Bose-superfluid. Optimal conditions for disproportionation driven high-Tc superconductivity are shown to be realized only for several Jahn-Teller dn configurations that permit the formation of well defined local composite bosons. These are the high-spin d4, low-spin d7, and d 9 configurations given the octahedral crystal field, and the d 1, high-spin d6 configurations given the tetrahedral crystal field. The disproportionation reaction has a peculiar 'anti-Jahn-Teller' character lifting the bare orbital degeneracy. Superconductivity in the d 4 and d6 systems at variance with d1, d 7, and d9 systems implies unavoidable coexistence of the spin-triplet composite bosons and the magnetic lattice. We argue that unconventional high-Tc superconductivity, observed in quasi-2d cuprates with tetragonally distorted CuO6 octahedra and iron-based layered pnictides/chalcogenides with tetrahedrally coordinated Fe2+ ions presents a key argument to support the fact that the disproportionation scenario is at work in these compounds. © 2013 IOP Publishing Ltd.