POLARONS AND BIPOLARONS IN STRONGLY INTERACTING ELECTRON-PHONON SYSTEMS

The Holstein Hubbard and Holstein t-J models are studied for a wide ra nge of phonon frequencies, electron-electron, and electron-phonon inte raction strengths on finite lattices with up to ten sites by means of direct Lanczos diagonalization. Previously the necessary truncation of the phononic Hilbert space caused serious limitations to either very small systems (four or even two sites) or to weak electron-phonon coup ling, in particular in the adiabatic regime. Using parallel computers we were able to investigate the transition from ''large'' to ''small'' polarons in detail. By resolving the low-lying eigenstates of the Ham iltonian and by calculating the spectral function, we can identify a p olaron band in the strong-coupling case, whose dispersion deviates fro m the free-particle dispersion at low and intermediate phonon frequenc ies. For two electrons (holes) we establish the existence of bipolaron ic states and discuss the formation of a bipolaron band. For the two-d imensional Holstein t-J model, we demonstrate that the formation of ho le polarons is favored by strong Coulomb correlations. Analyzing hole- hole correlation functions, we find that hole binding is enhanced as a dynamical effect of the electron-phonon interaction.