Clean to dirty limit and T-c suppression in NdFeAsO0.7F0.3 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 NdFeAsO0.7F0.3 (NdFeAs(O,F)) single crystal that has been progressively irradiated up to a 5.25 x 10(16) cm(-2) total alpha-particle dose. For the H parallel to ab-plane, dH(c2)/dT does not vary remarkably with irradiation, while for the H parallel to c-axis it increases sharply after the first irradiation of 3.60 x 10(15) cm(-2) and then more gradually with further irradiation doses. Focusing on the H parallel to 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 xi(BCS) to the mean free path l and we find that when T-c is reduced by a factor of four from its pristine value, xi(BCS)/l becomes as large as similar to 7 and l reaches values of similar to 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 YBa2Cu3O7-delta. 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.