A. Kristoffersen et al., CORRELATED SQUEEZED-STATE APPROACH FOR THE GROUND-STATE OF STRONGLY CORRELATED ELECTRONS COUPLED TO LOCAL HOLSTEIN PHONONS, Nuovo cimento della Societa italiana di fisica. D, Condensed matter,atomic, molecular and chemical physics, biophysics, 18(8), 1996, pp. 931-946
In the present work we have applied the correlated squeezed-state appr
oach to investigate the ground state of the extended Hubbard model whi
ch is coupled to local Holstein phonons. Our study begins with decoupl
ing the electron and phonon subsystems approximately by introducing a
variational correlated squeezed-state ansatz for the phonons. Then ass
uming the renormalized intersite electron correlation of the effective
electronic Hamiltonian to be attractive and the renormalized on-site
correlation repulsive, we have applied the generalized Hartree-Fock ap
proximation to obtain the ground state of the system, which is a super
conducting state with intersite pairing. With optimal values of the va
riational parameters the correlated squeezed-state approach will by co
nstruction yield a ground-state energy lower than those obtained in pr
evious studies. This means that our variational ansatz is more stable
as the ground state of the system. Furthermore, our variational study
shows that in the correlated squeezed state the polaronic reduction ef
fect of phonons is much more alleviated, and thus the mass enhancement
inherent to the polaron effect is noticeably weakened. This weakening
of the reduction effect should, in turn, significantly affect other p
hysical properties of the system.