Perspectives of the disproportionation driven superconductivity in strongly correlated 3d compounds

Disproportionation in 3d compounds can give rise to an unconventional electron-hole Bose liquid with a very rich phase diagram from a Bose metal, charge ordering insulator to an inhomogeneous Bose superfluid. Optimal conditions for the disproportionation driven high-T_c superconductivity are shown to realize only for several Jahn-Teller d^n configurations that permit the formation of well defined local composite bosons. These are the high-spin d^4, low-spin d^7, and d^9 configurations given the octahedral crystal field, and the d^1, high-spin d^6 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 d^6 systems at variance with d^1, d^7, and d^9 systems implies an unavoidable coexistence of the spin-triplet composite bosons and a magnetic lattice. We argue that unconventional high-T_c superconductivity observed in quasi-2D cuprates with tetragonally distorted CuO_6 octahedra and iron-based layered pnictides/chalcogenides with tetrahedrally coordinated Fe^2+ ions can be a key argument supporting the disproportionation scenario is at work in these compounds.

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