The soft mode behaviour and electron correlation effect in the pseudo Jahn-Teller system

Abstract

The relationship between electron correlation and lattice instability is discussed in this paper. A exciton Hamiltonian coupled with lattice motion is proposed to include the many body effect of the electron correlation. In strong coupling case we can limit our discussion to an effective Hamiltonian in which only the ground state and the low excited state is included. A pseudo spin representation is introduced, and the Green function technique is used to calculate the frequency of the soft mode. Our result implies that electron correlation will greatly enhance the lattice instability.