  The polaron formation is investigated in the intermediate regime of the
Holstein model by using an exact diagonalization technique for the
one-dimensional infinite lattice. The numerical results for the electron and
phonon propagators are compared with the nonadiabatic weak- and strong-coupling
perturbation theories, as well as with the harmonic adiabatic approximation. A
qualitative explanation of the crossover regime between the self-trapped and
free-particle-like behaviors, not well-understood previously, is proposed. It
is shown that a fine balance of nonadiabatic and adiabatic contributions
determines the motion of small polarons, making them light. A comprehensive
analysis of spatially and temporally resolved low-frequency lattice
correlations that characterize the translationally invariant polaron states is
derived. Various behaviors of the polaronic deformation field, ranging from
classical adiabatic for strong couplings to quantum nonadiabatic for weak
couplings, are discussed.
