Charge-fluctuation-induced superconducting state in two-dimensional quarter-filled electron systems

We investigate superconducting (SC) states in a two-dimensional extended Hubbard model at quarter-filling with on-site repulsive interaction (U) and nearest-neighbor repulsive interaction (V), which lead to spin fluctuation and charge fluctuation, respectively. The effect of charge fluctuation on the onset of the SC state is estimated by calculating pairing interaction within the random phase approximation. Solving a linearized Eliashberg equation with an anomalous self-energy as a function of both momentum and frequency, we examine the stability of a singlet SC state in a phase diagram of U and V with fixed temperatures. We obtain a novel result that the obtained SC state has d(xy) symmetry in the vicinity of both spin density wave (SDW) phase (U greater than or similar to V) and charge density wave (CDW) phase (V greater than or similar to U). The CDW-SC and SC-metal phase boundaries exhibit reentrant structures on the V-T (temperature) plane.