Small polaron formation in strongly correlated electronic systems

We study the crossover to small polarons in the Holstein model for finite d ensities in presence of strong electronic correlations. The small polaron f ormation condition lambda>1, which holds for a single particle in the adiab atic 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 cri terion can be formulated for polaron formation, provided that incoherent 1/ lambda corrections are taken into account.