Phase diagram of the two-dimensional extended Hubbard model: Pairing from charge and spin fluctuations

We study theoretically the symmetry of pairing when charge and spin fluctuations coexist to mediate the pairing for the extended Hubbard model on the square lattice, where the off-site repulsion V controls the charge fluctuation. Phase diagram is obtained, with the fluctuation exchange approximation, on the plane of V and the band filling n (quarter to half filling), which reveals: (i) a transition between dxy and dx2-y2 pairing symmetries for a weaker V, (ii) f-pairing in between the dx2-y2 and CDW phases for a stronger V, and (iii) for an anisotropic (quasi-1D) case the pairing symmetry changed, in agreement with a previously proposed "generic phase diagram" (Phys. Rev. B 63 (2001) 094509), as d→f→s with V. All these are consequences of the structure in the charge and spin susceptibilities which have peaks habitating at common or segregated positions, depending on V and n, in k space.