Charge states of strongly correlated 3d oxides: from typical insulator to unconventional electron-hole Bose liquid

Abstract

We develop a model approach to describe charge fluctuations and different charge phases in strongly correlated 3d oxides. As a generic model system one considers that of centers each with three possible valence states described in frames of pseudo-spin (isospin) formalism by an effective anisotropic non-Heisenberg Hamiltonian. Simple uniform mean-field phases include an insulating monovalent phase, mixed-valence binary disproportionated phase, and mixed-valence ternary under-disproportionated phase. We consider two first phases in more details focusing on the problem of electron/hole states and different types of excitons in monovalent phase and formation of electron-hole Bose liquid in disproportionated phase. Pseudo-spin formalism provides a useful framework for revealing and describing different topological charge fluctuations, in particular, like domain walls or bubble domains in antiferromagnets. All the insulating systems such as monovalent phase may be subdivided to two classes: stable and unstable ones with regard to the formation of self-trapped charge transfer (CT) excitons. The latter systems appear to be unstable with regard to the formation of CT exciton clusters, or droplets of the electron-hole Bose liquid. The model approach suggested is believed to be applied to describe a physics of strongly correlated oxides with charge transfer excitonic instability and a mixed valence. We shortly discuss an unconventional scenario of the essential physics of cuprates and manganites that implies their instability with regard to the self-trapping of charge transfer excitons and the formation of electron-hole Bose liquid.