Effect of transport scattering on the slope of the upper critical field in superconductors with different order parameters

The slope of the upper critical magnetic field at the transition temperature, S=(dH_c2(T)/dT)_T->Tc, is an easily accessible experimental quantity. It is shown that, in the case of transport (nonmagnetic) scattering, S increases in nodeless superconductors and decreases in superconductors with line nodes. In two-gap superconductors, the slope S always increases for s₊₊ pairing, but it decreases in the s_+- superconductors, similar to the line-nodal single-gap cases. Electron irradiation experiments show that in the Ba_1−xK_xFe₂As₂ family of iron-based superconductors of different stoichiometries, the slope S increases with the introduction of a nonmagnetic point-like disorder. This result implies that, contrary to common belief, these materials might fall into the s₊₊ category.