Off-site Cooper pairing in strong electron-phonon interacting systems

We have developed a new variational treatment to deal with the superconductivity properties of systems with strong electron-phonon interactions by introducing a "squeeze" transformation for phonon operators. It is shown that as long as the renormalized intersite electron correlation is attractive, a new type of Cooper pairing occurs whether the renormalized on-site electron correlation is repulsive or atttractive. The transformed electron-phonon interaction gives rise to the squeezed polarons which are, in general, different from the ordinary polarons. We have found from this theory that: (1) the squeezed polaronic state corresponds to a stable minimum of the total energy; (2) the effective mass of the squeezed polaron is much smaller than that of ordinary polarons; (3) both the gap equation and the expression for the critical temperature T_c in squeezed polaronic states are different from those in ordinary polaronic states as well as from those in BCS theory; and (4) high T_c is obtained for simple cubic and body centered cubic lattice structures with the electron concentration n=1.