Enhancement of the electron-lattice interaction due to strong electron correlation

Effects of strong electron correlation on the electron-lattice interaction are studied by means of the exact diagonalization method for a finite-size cluster system. A simple two-component tight-binding Hamiltonian with the intrasite and intersite Coulomb repulsions is adopted. Upon changing the correlation energy (U) and the energy-level separation (Δ), the system is found to undergo a transition between the ionic insulating phase and the Mott insulating phase. Near the phase boundary the system becomes unstable under a small perturbation and a dramatic enhancement of the electron-lattice interaction is observed. Spin and charge properties are also strongly affected by the lattice distortion in a crossover region. The implication of the calculated results to the origins of the displacive-type ferroelectricity and the superconductivity in transition-metal oxides is discussed.