Broken time-reversal symmetry in s+is and s+id states of multi-band superconductors: vortices, skyrmions, domain walls and spontaneous magnetic fields.

The recent experiments on Iron-based superconductors reported two interesting sitiuations: the formation of s-wave superconducting state in Ba1−xKxFe2As2 that breaks time-reversal symmetry (BTRS) (i.e. the so-called s+is state) and disorder-driven crossover from s+- to s++ state. Both of these situations should be accompanied by unconventional physics that will be discussed in this talk. I will discuss the origin of the spontaneous magnetic fields in the s+is state [1], unconventional topological excitations arising due to BTRS such as domains walls and Skyrmions [2], the breakdown of type-I/type-II dichotomy due to a divergent coherence lengths at the s+- to s+is transition causing the magnetic field penetration length to be intermediate length scale: ξ1<λ< ξ2 [3,4] resulting in vortex clustering, and a rather generic coexistence of the s+- and s++ states near the s+- to s++ crossover [5].
[1] J. Garaud et al Physical Review Letters 112 (1), 017003,
[2] J. Garaud et al Physical Review B 87 (1), 014507
[3] J. Carlstrom Physical Review B 84 (13), 134518
[4] J. Garaud Physical Review B 98 (1), 014520
[5] J. Garaud Physical Review B 97 (5), 054520