Continuous-Time Quantum Monte Carlo Study of Strong Coupling Superconductivity in Holstein–Hubbard Model

Superconducting transition temperature is studied in the strong-coupling region of the HolsteinHubbard model that includes both electron-phonon and Coulomb interactions. By combining the dynamical mean field theory with the continuous-time quantumMonte Carlo method, transition temperature is derived numerically. Without Coulomb interaction, our calculation reproduces analytic expressions of the transition temperature for weak and strong coupling limits of electron-phonon interaction. When electron-phonon coupling is strong enough, Coulomb repulsion enhances superconductivity, in contrast to the weak coupling case. The enhancement is due to coherent motion of superconducting pairs that reduce the Coulomb repulsive energy of almost localized electrons.