s+is state with broken time-reversal symmetry in Fe-based superconductors

Abstract

We analyze the evolution of the superconducting gap structure in strongly hole-doped Ba1−xKxFe2As2 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 Γ-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 ϕ, which gradually involves from ϕ=π (the +− state) to ϕ=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.

Source-reported materials — not catalogue approval

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