Bipolaronic superconductivity out of a Coulomb gas

Abstract: Employing unbiased sign-problem-free quantum Monte Carlo, we investigate the effects of long-range Coulomb forces on BEC of bipolarons using a model of bond phonon-modulated electron hopping. In absence of long-range repulsion, this model was recently shown to give rise to small-size, light-mass bipolarons that undergo a superfluid transition at high values of the critical transition temperature $T_\mathrm{c}$. We find that $T_\mathrm{c}$ in our model even with the long-range Coulomb repulsion remains much larger than that of Holstein bipolarons, and can be on the order of or greater than the typical upper bounds on phonon-mediated $T_\mathrm{c}$ based on the Migdal-Eliashberg and McMillan approximations. Our work points to a physically simple mechanism for superconductivity in the low-density regime that may be relevant to current experiments on dilute superconductors.