Difference between Al and C doping in anisotropic upper critical field development in MgB₂

The temperature dependence of the upper critical field H_c2 for both field directions in partially substituted polycrystalline MgB₂ was determined. Whereas the suppression of T_c is similar for aluminum and carbon substituted samples, H_c2 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, H_c2 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.