  Ground-state and dynamical properties of the 2D Holstein t-J model are
examined by means of direct Lanczos diagonalization, using a truncation method
of the phononic Hilbert space. The single-hole spectral function shows the
formation of a narrow hole-polaron band as the electron-phonon coupling
increases, where the polaronic band collapse is favoured by strong Coulomb
correlations. In the two-hole sector, the hole-hole correlations unambiguously
indicate the existence of inter-site bipolaronic states. At quarter-filling, a
polaronic superlattice is formed in the adiabatic strong-coupling regime.
