Generalization of the shell model and correlation effects in strongly correlated oxides

Abstract

Direct analytical and numerical calculation show that the two-electron atomic configuration can be unstable with respect to a static or dynamic displacement of the electron shells. This enables us to develop a so-called nonrigid shell model for a partial account of the intra-atomic electron correlations. In frames of the model a correlated state of two-electron molecular configuration is described by a set of symmetrized shell displacements similarly to the well-known shell model developed for a description of the lattice dynamics. The former relate to the latter as the optical phonon modes relate to the acoustical one. The effect under consideration is expected to be of particular importance for anionic ${2p}^{6}$ background in oxides. The nonrigid anionic background forms an unconventional magnetoelectric system characterized by a set of independent orbital order parameters and competition of paramagnetic and diamagnetic responses. The model allows an introduction of the pseudospin formalism and effective ``spin Hamiltonian'' for a description of the short- and long-range ordering of nonrigid atomic backgrounds in crystals. We predict a number of unconventional correlation effects including: (i) a formation of polaronlike holes and charge-transfer excitons; (ii) the mechanism of the self-trapping for holes and/or charge-transfer excitons; (iii) a purely electronic (pseudo)Jahn-Teller effect due to a coupling between valent hole and a nonrigid anionic background; (iv) a combined (pseudo)Jahn-Teller effect provided by a joint account both of the electron shell displacements and conventional nuclear ones; (v) an appearance of the soliton chiral correlation states; (vi) unconventional magnetic response; (vii) unconventional hyperfine coupling.