The different development of the anisotropic upper critical field in MgB$_2$ by aluminum and carbon doping

The temperature dependence of the upper critical field, Hc2, for both field directions in partially substituted polycrystalline MgB2 was determined. Whereas the suppression of Tc is similar for aluminum and carbon substituted samples, Hc2 is affected by the substitution in profoundly different ways. In the case of Al substitution changes can tentatively be described by intrinsic effects (shift of the Fermi level). In the C substituted samples, Hc2 is increased drastically, and extrinsic effects (disorder) have to play a major role. The strong contrast between the two substitutions is discussed, taking into account three relevant scattering rates.