MOBILE INTERSITE BIPOLARONS IN THE DISCRETE HOLSTEIN-HUBBARD MODEL

We have explored the properties of a two-fermion system interacting wi th the phonon field in the framework of the one-dimensional discrete H olstein-Hubbard model. The variational method employed here introduces the correlations between the phonons and the electron wave functions. therefore the composite trial state is not factorized. As a consequen ce our approach gives reliable results in the whole space of the param eters. We will diner our attention to a new class of solutions found i n the intermediate range of values of the nonadiabaticity parameter ga mma. These solutions are characterized by anomalous (non-Gaussian) flu ctuations of the position of the oscillators. An intersite bipolaron w ith a relatively small effective mass is stable in a wide region of th e parameters due to both exchange and nonadiabaticity effects.