Strongly enhancend anisotropic upper critical field in carbon substituted MgB2: impact of Fermi surface changes
The upper critical field Hc2||c(0) of 85 kOe in Mg(B0.94C0.06)2 single crystals, determined from torque measurements, is more than twice as large as that one of 31 kOe in unsubstituted MgB2. Anisotropy of Hc2 increases from about 3.4 near Tc, the value close to that in MgB2, to about 4 at low temperature, the value considerably lower than that in MgB2. The corresponding Hc2||ab(0) of about 330-350 kOe is likely close to the maximum enhancement due to C substitution. The position of a sharp peak in the irreversible torque Hmax was found to be shifted to lower reduced fields H/Hc2 in comparison to its position in unsubstituted MgB2, indicating increasing disorder. The enhancement of Hc2 can be explained as a disorder effect only if the main result of disorder is to make the p bands more dirty while not affecting the s bands as much. In addition to disorder and weakened electron-phonon coupling, the impact of the Fermi level shifting into a region with lower s Fermi velocities has to be taken into account in the analysis of Hc2 data as well.