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 s(+/-) and s(++) order parameters. We show that disorder causes the transition between s(+/-) and s(++) states and examine what observable effects this transition can produce.