Electronic polarizability in a correlated electron system with strong covalency

Electronic polarizabilities in transition-metal oxides are studied in the tight-binding electron-lattice coupled model in which the electron covalency and the electron-electron interactions are included. As the interaction between the electronic charge and the electric field, we introduce the site diagonal interaction. The linear and second- and third-order nonlinear electronic polarizabilities are calculated by the exact diagonalization method for a finite cluster system. Signs and magnitudes of the nonlinear electronic polarizabilities are very sensitive to the changes in the electron covalency and the electron correlation in the system, and the magnitude diverges near the crossover point of many-body states. When the charge excitation energy is kept constant, however, the electron repulsion energy tends to suppress polarizabilities by increasing energies of higher excited states. Calculated results are compared with the experimental results for transition-metal oxides and material dependence of the polarizabilities is discussed.