Effects of carbon doping on superconductivity in magnesium diboride

We introduce carbon onto boron sites in MgB2. The resulting changes in crystal lattice constants and superconducting transition temperature T-c are characterized by x-ray-diffraction, magnetic susceptibility, and electrical resistance measurements. The consequence of approximately 10% carbon doping of boron sites is a 1% contraction of the intraplane lattice dimension (with no appreciable change in the interplane dimension) and a lowering of T-c by approximately 7 K. The relative contributions to the shift in T-c from lattice contraction and charge transfer are evaluated.