Polarons in the two-dimensional Holstein-t-J model

To describe the dynamics of holes in CuO2 planes of high-Tc superconductors we analyse the two-dimensional t-J model supplemented with the interaction of holes with nonpolar optical phonons. Elementary excitations of this model are treated non adiabatically within the spin-wave approximation on an infinite square lattice by means of an iterative Lanczos algorithm. For parameter values relevant for high-Tc superconductors we have found a sharp crossover in the ground state from a nearly free excitation for small hole-phonon coupling to a self-trapped excitation for large hole-phonon coupling. In the vicinity of this crossover coexistence of these two types of states and ferron-phonon complexes has been found. We also analyse the dependence of the critical hole-phonon coupling strength on the exchange constant J. (author). 3 figs., 36 refs