Critical fluctuations in the carbon-doped magnesium diboride

Abstract

Carbon doping of magnesium diboride resulting in the Mg(B 0.9C 0.1) 2 stoichiometry leads to the superconducting transition temperature T c=22 K which is about 17 K less than in the pure MgB 2. 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 MgB 2 and achieves values of about 25–30 Tesla at low temperatures comparable to the MgB 2 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.