Detecting pairing symmetry in Fe-based superconductors: Solitons and proximity patch

We suggest a mechanism which promotes the existence of a phase soliton -- topological defect formed in the relative phase of superconducting gaps of a two-band superconductor with $s_{+-}$ type of pairing. This mechanism exploits the proximity effect with a conventional $s$-wave superconductor which favors the alignment of the phases of the two-band superconductor which, in the case of $s_{+-}$ pairing, are $\pi$-shifted in the absence of proximity. In the case of a strong proximity such effect can be used to reduce soliton's energy below the energy of a soliton-free state thus making the soliton thermodynamically stable. Based on this observation we consider an experimental setup, applicable both for stable and metastable solitons, which can be used to distinguish between $s_{+-}$ and $s_{++}$ types of pairing in the iron-based multiband superconductors.