CORRELATED SQUEEZED-STATE APPROACH FOR THE GROUND-STATE OF A SYSTEM WITH STRONG ELECTRON-PHONON INTERACTION

We have investigated the ground state of an electron-phonon system wit h strong electron-phonon interaction and a weak on-site Coulomb repuls ion using the correlated squeezed-state approach. We have introduced a variational ground state in which the phonon subsystem is in a correl ated squeezed state and the electron subsystem is in a superconducting pairing state. In addition to the anharmonicity of each phonon mode i nduced by the linear coupling with the electrons, the correlated squee zed state is able to take into account the nonlinearity of the induced interaction between different phonon modes beyond the Hartree approxi mation. Also, with optimal values of the variational parameters this t rial state will by construction yield an energy lower than those obtai ned in previous studies, and thus our variational ansatz is a more sta ble ground state for this system. Furthermore, in this approximate gro und state the reduction effect of phonons is much more alleviated, and thus the mass enhancement inherent to the polaron effect is considera bly weakened. This weakening of the reduction effect should, in turn, affect other physical properties of the system; for instance, it does suggest the possibility of higher critical temperatures for supercondu ctivity in the strong-coupling limit where the superfluidlike bipolaro nic superconductivity is supposed to occur.