Coulomb-enhanced superconducting pair correlations and paired-electron liquid in the frustrated quarter-filled band

A necessary condition for superconductivity (SC) driven by electron correlations is that electron-electron (e-e) interactions enhance superconducting pair-pair correlations, relative to the noninteracting limit. We report high-precision numerical calculations of the ground state on four different finite lattices of up to 100 sites within the frustrated two-dimensional (2D) Hubbard Hamiltonian for a wide range of carrier concentration ρ (0<ρ<1). The average long-range pair-pair correlation for each cluster is enhanced by Hubbard U only for ρ≈0.5. At all other fillings e-e interactions mostly suppress pair correlations. Our work provides a key ingredient to the mechanism of SC in the 2D organic charge-transfer solids and many other unconventional superconductors with frustrated crystal lattices and ρ≃0.5.