Carbon doping of superconducting magnesium diboride

Abstract

We present details of synthesis optimization and physical properties of nearly single phase carbon doped MgB 2 with a nominal stoichiometry of Mg(B 0.8C 0.2) 2 synthesized from magnesium and boron carbide (B 4C) as starting materials. The superconducting transition temperature is ≈22 K (≈17 K lower than in pure MgB 2). The temperature dependence of the upper critical field is steeper than in pure MgB 2 with H c2 (10 K)≈9 T. Temperature dependent specific heat data taken in different applied magnetic fields suggest that the two-gap nature of superconductivity is still preserved for carbon doped MgB 2 even with such a heavily suppressed transition temperature. In addition, the anisotropy ratio of the upper critical field for T/ T c≈2/3 is γ≈2. This value is distinct from 1 (isotropic) and also distinct from 6 (the value found for pure MgB 2).