THE PHASE-DIAGRAM OF THE 2D HOLSTEIN-T-J MODEL NEAR HALF FILLING

We have used an exact diagonalization technique to explore the ground- state phase diagram of the two-dimensional Holstein-t-J model near hal f filling. The nature of the ground state is analysed. calculating sel f consistently the local lattice distortions, on-site spin and charge densities, as well as the magnetic structure form factor. We demonstra te that doped holes tend to form hole-polaron or bipolaron states. Com pared to that in the uncorrelated model (spinless fermions), the criti cal electron-phonon coupling strength for the self-trapping transition of the doped charge carriers is considerably reduced by the antiferro magrietic exchange interaction. The relevance to the cuprate supercond uctors is discussed.