Th upper critical field and two-gap nature in Mg1-xAlxB2

The temperature dependence of the upper critical fields (H-c2 (T)) of the two-gap superconductors Mg1-xAlxB2 (x = 0, 0.1, 0.2, and 0.3) was obtained by resistivity measurements. The H-c2(T) values were analysed, within the dirty-limit two-gap model, using the calculated electron-phonon coupling constants and Coulomb pseudopotentials. As the Al contents increased, both T-c and H-c2(0) decreased, which was the direct manifestation of the increasing number of electrons. At the same time, the three-dimensional pi bands became much dirtier when Al was doped. In contrast, the intraband scattering of the two-dimensional sigma bands was relatively unaffected by the Al doping. These behaviours could be understood when the characters of both bands were considered. In all the samples that we investigated, the or bands were dirtier than the pi bands.