摘要：The Holstein
Molecular Crystal Model is investigated by a
strong coupling perturbative method which,
unlike the standard Lang-Firsov approach,
accounts for retardation effects due to the
spreading of the polaron size. The effective
mass is calculated to the second perturbative
order in any lattice dimensionality for a broad
range of (anti)adiabatic regimes and
electron-phonon couplings. The crossover from a
large to a small polaron state is found in all
dimensionalities for adiabatic and intermediate
adiabatic regimes. The phonon dispersion largely
smoothes such crossover which
is signalled by polaron mass enhancement and on-site localization
of the correlation function. The notion of self-trapping together
with the conditions for the existence of light polarons, mainly in
two- and three-dimensions, is discussed. By the imaginary time
path integral formalism I show how nonlocal electron-phonon
correlations, due to dispersive phonons, renormalize downwards the
e-ph coupling justifying the possibility for
light and essentially small 2D Holstein polarons.
