Enhancement of irreversibility field in carbon-substituted MgB2 single crystals

We report on a detailed study of the irreversibility field H-irr of single crystals of Mg(B1-xCx)(2) (x = 0, 2, 3.5, 5 and 7.5%) in strong magnetic fields of up to 40 T. Highly sensitive torque measurements revealed that H-irr, is greatly enhanced by a factor of two (mu(0)H(irr)(0) similar or equal to 33 T with x = 5%) by carbon substitution compared with that of the pristine MgB2, owing to a reduction in in-plane coherence length. We also observed the temperature-dependent H-irr anisotropy for all carbon contents. This strongly suggests that the two-gap superconductivity of MgB2 is less influenced by a small amount of carbon substitution. We discuss these results by focusing on the impact of carrier doping and impurity scattering on a two-gap superconductor.