Difference between Al and C doping in anisotropic upper critical field development in MgB2 -: art. no. 144512

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.