Light, small bipolarons: A new route to high transition temperature superconductivity*

A model for phonon-mediated high-T_c superconductivity based on superfluidity of light bipolarons is presented. I present numerically exact results obtained using a sign-problem-free quantum Monte Carlo approach for bipolaron binding energies, masses and radii for both Holstein (density-coupled) and Peierls/Su-Schrieffer-Heeger (bond-modulated) models of electron-phonon coupling, with and without both short- and long-range Coulomb interactions. The bond-modulated mechanism is shown to give rise to small-size, yet light-mass bipolarons, which condense at temperatures that generically and significantly exceed typical upper bounds on T_c of phonon-mediated superconductivity based on Migdal-Eliashberg theory.