Spontaneous currents in a superconductor with s+is symmetry

We analyze $s + is$ state proposed as a candidate superconducting state for strongly hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$. Such a state breaks time-reversal symmetry (TRS) but does not break any other discrete symmetry. We address the issue whether TRS breaking alone can generate spontaneous currents near impurity sites, which could be detected in, e.g., muSR experiments. We argue that there are no spontaneous currents if only TRS is broken. However, supercurrents do emerge if the system is put under external strain and C4 lattice rotation symmetry is externally broken.

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