Interband and intraband effects in the upper critical field of disordered MgB2

In this work the superconducting properties of disordered MgB2 in applied magnetic field are studied within the lambda(theta theta) model, by taking into account the presence of both interband and intraband scattering with impurities. This approach allows to extract the suppression of the critical temperature T-c and the enhancement of the upper critical field H-c2, as a consequence of the introduction of impurities in the samples. We analyze the dependence of H-c2 on temperature, anisotropy of the electronic structure, and intraband sigma and pi band scattering rates. Comparing our numerical calculations with experimental data on irradiated samples, we find that irradiation defects mainly affect the mobility of sigma carriers. These results rationalize why the H-c2 anisotropy of irradiated samples is quickly reduced with increasing doses and full suppression of superconductivity occurs at rather low-resistivity values. Moreover, our calculations point out that disorder in the pi bands affects only weakly the coupling constants and thus it could yield a significant enhancement of H-c2 without severe suppression of T-c.