Two-gap nature of the bulk superconductor (Mg1-xAlx)B-2 with low (x <= 0.07) Al doping

Abstract

We have investigated the two-gap nature of bulk (Mg1-xAlx)B-2 for 0 <= x <= 0.07 by studying the upper critical field H-c2(T). The H-c2(T) is analyzed based on the dirty-limit two-gap model. While the effective diffusivities of the pi-band (D-sigma(e) (f) (f)) decreased gradually, those of the sigma-band (D-pi(e f f)) were relatively unchanged, which indicated that unlike the 2 dimensional a-band, the isotropic pi-band is seriously affected by the Al doping. The upward curvature of H-c2(T) near T-c is also explained by the dirty-limit two-gap model.