Mobile bipolaron

We explore the properties of the bipolaron in a 1D Holstein-Hubbard model w ith dynamical quantum phonons. Using a recently developed variational metho d combined with analytical strong coupling calculations, we compute correla tion functions, effective mass, bipolaron isotope effect, and the phase dia gram. The two site bipolaron has a significantly reduced mass and isotope e ffect compared to the on-site bipolaron, and is bound in the strong couplin g regime up to twice the Hubbard U naively expected. The model can be descr ibed in this regime as an effective t-J-V model with nearest neighbor repul sion. These are the most accurate bipolaron calculations to date.