Superlight small bipolarons from realistic long-range Coulomb and Frohlich interactions

We report analytical and numerical results on the two-particle states of the polaronic t-Jp model derived recently with realistic Coulomb and electron-phonon (Fr¨ohlich) interactions in doped polar insulators. Eigenstates and eigenvalues are calculated for two different geometries. Our results show that the ground state is a bipolaronic singlet, made up of two polarons. The bipolaron size increases with increasing ratio of the polaron hopping integral t to the exchange interaction Jp but remains small in the whole range 0 t/Jp 1. Furthermore, the model exhibits a phase transition to a superconducting state with a critical temperature well in excess of 100K since the small bipolarons are perfectly mobile. In the range t/Jp 1, there are distinct charge and spin gaps opening in the density of states, specific heat, and magnetic susceptibility well above Tc.