Clean to dirty limit and T _c suppression in NdFeAsO_0.7F_0.3 studied by H _c2 analysis

In this work, we investigate the temperature dependence of the upper critical field, dH _c2/dT, in an increasingly disordered NdFeAsO_0.7F_0.3 (NdFeAs(O,F)) single crystal that has been progressively irradiated up to a 5.25 ×10¹⁶ cm^{- 2} total particle dose. For the H||ab-plane, dH _c2/dT does not vary remarkably with irradiation, while for the H||c-axis it increases sharply after the first irradiation of 3.60 ×10¹⁵ cm^-2 and then more gradually with further irradiation doses. Focusing on the H||c-axis, we develop a phenomenological analysis of the H _c2 slope which allows us to inspect the crossover from the clean to the dirty regime. From the H _c2 slope normalized to the critical temperature and to its clean limit value, we extract the ratio of the coherence length ξ _BCS to the mean free path and we find that when T _c is reduced by a factor of four from its pristine value, ξ _BCS/ becomes as large as ∼7 and reaches values of ∼1.8 nm, indicating that NdFeAs(O,F) is well into the dirty regime. Our analysis of the H _c2 slope also allows us to compare the pair-breaking effectiveness of scattering in different superconductors, showing similarity between unconventional NdFeAs(O,F) and moderate-T _c phonon-mediated devices, such as MgB₂ and A15 compounds, but much a stronger difference with YBa₂Cu₃O_7-δ. This work thus shows that dH _c2/dT is a reliable parameter, providing an alternative to residual resistivity, for investigating the pair-breaking mechanism induced by impurity scattering in superconductors.