Critical fluctuations in the carbon-doped magnesium diboride

Carbon doping of magnesium diboride resulting in the Mg(B0.9C0.1)(2) stoichiometry leads to the superconducting transition temperature T-c = 22 K which is about 17 K less than in the pure MgB2. We present the magnetotransport measurements on the doped polycrystalline samples up to 23 T in the temperature range from 1.5 K. Despite the significantly lowered T-c the upper critical magnetic field H-c2 is much higher here than in the pure MgB2 and achieves values of about 25-30 Tesla at low temperatures comparable to the MgB2 samples with the highest level of impurity scattering. The anomalous temperature dependence H-c2(T) revealing a positive curvature in the overall temperature range is discussed in the framework of the two-gap superconductivity, which is still present here. As another possibility an importance of the critical fluctuations is considered within the Ullah-Dorsey scaling model. (C) 2004 Elsevier B.V. All rights reserved