  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.
