Superconductivity in a narrow-band system with intersite electron pairing in two dimensions. II. Effects of nearest-neighbor exchange and correlated hopping

Abstract

Our previous mean-field study concerning superconducting pairings in the extended Hubbard model with on-site repulsive and intersite attractive interaction and arbitrary electron density is extended to include the effects of nearest-neighbor exchange (J) and correlated hopping (K) interaction. Detailed numerical analysis of s-, p-, and d-wave pairing solutions is performed for the two-dimensional square lattice with nearest- (t) and next-nearest- (t2) neighbor hopping, and the resulting mutual stability phase diagrams are given as a function of band filling and the interaction parameters. Antiferromagnetic exchange can enhance both the d-wave and the extended s-wave pairing. If it prevails over the repulsive intersite density-density interaction, such a term can be a leading mechanism for superconductivity. However, its effects strongly depend on the band filling and the t2t ratio, giving in particular for t2=0 s-wave (d-wave) pairing as most favorable in the low (high) density limit, but increasing t2 can even reverse this tendency. The correlated hopping term which breaks the electron-hole symmetry affects s-wave pairing mostly. A condensation transition (phase separation), possible in the presence of the attractive density-density interaction, is also analyzed within the random-phase approximation, and stability conditions of such an electron droplet phase with respect to other types of ordering are determined as a function of the band filling for the two-dimensional square lattice.

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