Impurities in multiband superconductors
  Disorder - impurities and defects violating an ideal 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 other than the usual s-wave symmetry. This paper uses the examples of iron-based pnictides and chalcogenides to examine how both nonmagnetic and magnetic impurities affect superconducting states with $s_\pm$ and $s_{++}$ order parameters. We show that disorder causes the transitions between $s_\pm$ and $s_{++}$ states and examine observable effects these transitions can produce. 
M.M. Korshunov, Yu.N. Togushova, O.V. Dolgov
cond-mat.supr-con cond-mat.str-el