DYNAMICAL PROPERTIES OF SMALL POLARONS

On the basis of the two-site polaron problem, which we solve by exact diagonalization, we analyze the spectral properties of polaronic syste ms in view of discerning localized from itinerant polarons and bound p olaron pairs from an ensemble of single polarons. The corresponding ex perimental techniques for that concern photoemission and inverse photo emission spectroscopy. The evolution of the density of states as a fun ction of concentration of charge carriers and strength of the electron -phonon interaction clearly shows the opening up of a gap between sing le-polaronic and bipolaronic states, in analogy to the Hubbard problem for strongly correlated electron systems. In studying the details of the intricately linked dynamics of the charge carriers and of the mole cular deformations which surround them, we find that in general the dy namical delocalization of the charge carriers helps to strengthen the phase coherence for itinerant polaronic states, except for the crossov er regime between adiabatic and antiadiabatic small polarons. The cros sover between these two regimes is triggered by two characteristic tim e scales: the renormalized electron hopping rate and the renormalized vibrational frequency becoming equal. This crossover regime is then ch aracterized by temporarily alternating self-localization and delocaliz ation of the charge carriers which is accompanied by phase slips in th e charge and molecular deformation oscillations and ultimately leads t o a dephasing between these two dynamical components of the polaron pr oblem. We visualize these features by a study of the temporal evolutio n of the charge redistribution and the change in molecular deformation s. The spectral and dynamical properties of polarons discussed here ar e beyond the applicability of the standard Lang-Firsov approach to the polaron problem.