Abstract
In this paper we have generalized the variational approach proposed by Zheng [J. Phys. Condens. Matter 1 (1989) 1641] and applied it to a model system with strong electron-phonon interaction. We have introduced a new variational ground state in which the phonon subsystem is in a correlated squeezed state and the electron subsystem is in a superconducting pairing state. The correlated squeezed state has built in the correlation between different phonon modes and thereby goes beyond earlier variational treatments. Also, with optimal values of the variational parameters, this trial state will by construction yield an energy lower than that obtained by Zheng. This means that our variational ansatz is more stable as the ground state of the system. Furthermore, in this new approximate ground state the reduction effect of phonons is much more alleviated, and thus the mass enhancement inherent to the polaron effect is considerably weakened. This, in turn, seems to suggest the possibility of higher critical temperatures for superconductivity.
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