Mobile intersite bipolarons in the discrete Holstein-Hubbard model

We have explored the properties of a two-fermion system interacting with the phonon field in the framework of the one-dimensional discrete Holstein-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 consequence our approach gives reliable results in the whole space of the parameters. We will direct our attention to a new class of solutions found in the intermediate range of values of the nonadiabaticity parameter ?. These solutions are characterized by anomalous (non-Gaussian) fluctuations of the position of the oscillators. An intersite bipolaron with a relatively small effective mass is stable in a wide region of the parameters due to both exchange and nonadiabaticity effects.