Carbon substitution in MgB₂ single crystals: Structural and superconducting properties

The growth of carbon-substituted magnesium diboride Mg(B_1−xC_x)₂ single crystals with 0⩽x⩽0.15 is reported, and the structural, transport, and magnetization data are presented. The superconducting transition temperature decreases monotonically with increasing carbon content in the full investigated range of substitution. By adjusting the nominal composition, T_c of substituted crystals can be tuned in a wide temperature range between 10 and 39 K. Simultaneous introduction of disorder by carbon substitution and significant increase of the upper critical field H_c2 is observed. Comparing with the nonsubstituted compound, H_c2 at 15 K for x=0.05 is enhanced by more than a factor of 2 for H oriented both perpendicular and parallel to the ab plane. This enhancement is accompanied by a reduction of the H_c2-anisotropy coefficient γ from 4.5 (for the nonsubstituted compound) to 3.4 and 2.8 for the crystals with x=0.05 and 0.095, respectively. At temperatures below 10 K, the single crystal with larger carbon content shows H_c2 (defined at zero resistance) higher than 7 and 24 T for H oriented perpendicular and parallel to the ab plane, respectively. Observed increase of H_c2 cannot be explained by the change in the coherence length due to the disorder-induced decrease of the mean free path only.