Impurities in multiband superconductors

Disorder — impurities and defects violating the ideal long-range order — is always present in solids. It can result in interesting and sometimes unexpected effects in multiband superconductors, especially if the superconductivity is unconventional, thus having symmetry other than the usual s-wave. This paper uses the examples of iron-based pnictides and chalcogenides to examine how both nonmagnetic and magnetic impurities affect superconducting states with $\rm{s}_\pm$ and $\rm{s}_{++}$ order parameters. We show that disorder causes the transition between $\rm{s}_\pm$ and $\rm{s}_{++}$ states and examine what observable effects this transition can produce.