s+is State with Broken Time Reversal Symmetry in Fe-Based Superconductors

We analyze the evolution of the superconducting gap structure in strongly hole doped Ba_{1-x}K_xFe_2As_2 between x=1 and x ~ 0.4 (optimal doping). In the latter case, the pairing state is most likely s+-, with different gap signs on hole and electron pockets, but with the same signs of the gap on the two Gamma-centered hole pockets (a ++ state on hole pockets). In a pure KFe_2As_2 (x=1), which has only hole pockets, laser ARPES data suggested another s+- state, in which the gap changes sign between hole pockets (a +- state). We analyze how ++ gap transforms into a +- gap as x -> 1. We found that this transformation occurs via an intermediate s+is, state in which the gaps on the two hole pockets differ in phase by `phi', which gradually involves from `phi' = pi (the +- state) to phi =0 (the ++ state). This state breaks time-reversal symmetry and has huge potential for applications. We compute the dispersion of collective excitations and show that two different Leggett-type phase modes soften at the two end points of TRSB state.

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