Polaronic effects in strongly coupled electron-phonon systems: Exact   diagonalization results for the 2D Holstein t-J model
  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. 
H. Fehske, G. Wellein, B. B\"auml, and H. B\"uttner (Physikalisches   Institut, Universt\"at Bayreuth, Germany)