superconductors*

In most superconductors non-magnetic point-like defects enhance pinning and upper critical field. However, in s_± superconductors, for example iron-based compounds, point-like disorder influences not only intra-band potential scattering, but also inter-band pair-breaking scattering rates. The latter leads to suppression of T_c, H_c2 and the condensation energy. The balance between these opposite effects leads to an increase of (dH_c2/dT)_T=Tc slope with the decrease of T_c, opposite to what is observed as a function of doping. Even more unusual, it may also lead to a suppression of strong pinning believed to be responsible for a sharp peak in magnetization near H=0. In our experiments controlled disorder was induced by 2.5 MeV electron irradiation.