Pairing State with a Time-Reversal Symmetry Breaking in FeAs-Based Superconductors

We investigate the competition between the extended s_±-wave and d_x²-y²-wave pairing order parameters in the iron-based superconductors. Because of the frustrating pairing interactions among the electron and the hole Fermi pockets, a time-reversal symmetry breaking s+id pairing state could be favored. We analyze this pairing state within the Ginzburg-Landau theory and explore the experimental consequences. In such a state, spatial inhomogeneity induces a supercurrent near a nonmagnetic impurity and the corners of a square sample. The resonance mode between the s_±- and d_x²-y²-wave order parameters can be detected through the B_1g Raman spectroscopy.