The spatially localized bound states of two electrons in the adiabatic two-dimensional Holstein-Hubbard model on a square lattice are investigated both numerically and analytically. The interplay between the electron-phonon coupling g, which tends to form bipolarons and the repulsive Hubbard...

Phase separation in the strongly correlated Falicov-Kimball model in infinite dimensions is examined. We show that the phase separation can occur for any values of the interaction constant J <Superscript>*</Superscript> when the site energy ε <Superscript>0</Superscript> of the localized electrons is equal to zero. Electron-poor regions always...</superscript></superscript>