COMPETITION BETWEEN ELECTRON-PHONON ATTRACTION AND WEAK COULOMB REPULSION

The Holstein-Hubbard model is examined in the limit of infinite dimens ions. Conventional wisdom states that charge-density-wave (CDW) order is more strongly affected by Coulomb repulsion than superconducting or der (SC) because of the pseudopotential effect. We find that the CDW-S C phase boundary does move toward half filling as the Coulomb repulsio n increases, but, surprisingly, the CDW transition temperature is init ially more robust against Coulomb repulsion than the superconducting t ransition temperature. This puzzling feature is resolved by an analysi s of weak-coupling perturbation theory.