Symmetry of superconducting pairing state in a staggered field

In order to investigate the symmetry of a superconducting (SC) pairing state in a staggered field, we solve the linearized Iliashberg's equation based on the random phase approximation and the fluctuation exchange approximation in the Hubbard model under a staggered field. We show that an odd-frequency spin-triplet (equal-spin) s-wave pairing state can be realized in the system. It is caused by the fact that the staggered field suppresses the in-plane spin susceptibility and enhances the charge susceptibility. The latter is due to a spin-dependent modulation in the electron density. The result suggests that the antiferromagnetic (AF) background gives a contribution inherently different from the AF spin fluctuation to SC pairing state.