BI- POLARON FORMATION AND OPTICAL CONDUCTIVITY IN THE HOLSTEIN-HUBBARD MODEL/

The polaron formation at finite densities is studied in the Holstein-H ubbard model transformed, by a modified Lang-Firsov transformation, to a polaron Hamiltonian. In a variational approach, an effective polaro nic Hubbard model is derived, where squeezing effects ate taken into a ccount. The correlations are treated in the slave-boson saddle-point a pproximation. Including charge- and magnetically ordered states the ph ase diagram is calculated, and a self-trapping transition from light t o heavy polarons is found. The optical conductivity of heavy polarons reveals correlation-induced structures. By a Schrieffer-Wolff transfor mation up to second order in the residual polaron-multiphonon interact ion, an effective polaron model is derived, where for a polaronic attr action an on-site bipolaron model is obtained.