Limits of the upper critical field in dirty two-gap superconductors

An overview of the theory of the upper critical field in dirty two-gap superconductors, with a particular emphasis on MgB2 is given. We focus here on the maximum H-c2 which may be achieved by increasing intraband scattering, and on the limitations imposed by weak interband scattering and paramagnetic effects. In particular, we discuss recent experiments which have demonstrated tenfold increase of H-c2 in dirty carbon-doped films as compared to single crystals, so that H-c2(0) parallel to the ab planes may approach the BCS paramagnetic limit, H-p[T] = 1.84T(c) (K] similar or equal to 60-70 T. New effects produced by weak interband scattering in the two-gap Ginzburg-Landau equations and features of H-c2(T) in ultrathin MgB2 films are addressed. (c) 2007 Elsevier B.V. All rights reserved.