One-dimensional Hubbard-Holstein model with finite-range electron-phonon coupling

The Hubbard-Holstein model describes fermions on a discrete lattice, with on-site repulsion between fermions and a coupling to phonons that are localized on sites. Generally, at half-filling, increasing the coupling g to the phonons drives the system towards a Peierls charge density wave state, whereas increasing the electron-electron interaction U drives the fermions into a Mott antiferromagnet. At low g and U, or when doped, the system is metallic. In one dimension, using quantum Monte Carlo simulations, we study the case where fermions have a long-range coupling to phonons, with characteristic range xi, interpolating between the Holstein and Frohlich limits. Without electron-electron interaction, the fermions adopt a Peierls state when the coupling to the phonons is strong enough. This state is destabilized by a small coupling range xi and leads to a collapse of the fermions, and, consequently, phase separation. Increasing interaction U will drive any of these three phases (metallic, Peierls, phase separation) into a Mott insulator phase. The phase separation region is once again present in the U not equal 0 case, even for small values of the coupling range.