Small polaron formation in strongly correlated electronic systems

We study the crossover to small polarons in the Holstein model for finite densities in presence of strong electronic correlations. The small polaron formation condition lambda>1, which holds for a single particle in the adiabatic regime omega(0)/t much less than 1 is generalized to the many particle case by means of simple physical arguments. It is shown that a general criterion can be formulated for polaron formation, provided that incoherent 1/lambda corrections are taken into account.