s plus i s state with broken time-reversal symmetry in Fe-based superconductors

We analyze the evolution of the superconducting gap structure in strongly hole-doped Ba1-xKxFe2As2 between x = 1 and x similar to 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 KFe2As2 (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 a ++ 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 f = p (the + - state) to f = 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 the time-reversal-symmetry-breaking state. DOI: 10.1103/PhysRevB.87.144511