Carbon-substituted MgB₂ single crystals

Carbon-substituted MgB₂ single crystals, Mg(B _1-xC_x)₂ of 0.3-1.0 mm size were grown for x = 0.02-0.15 by a high-pressure technique. The doping dependence of lattice constants studied in a range of x = 0.0-0.2 shows a monotonic decrease in a, while the c parameter remains almost invariant. Using X-ray diffraction and Auger electron spectroscopy, the solubility limit of C in MgB₂ was estimated to be about 15±1%, which is substantially larger than that reported for the polycrystalline samples synthesized by encapsulation techniques. Measurements of the temperature dependence of magnetization and resistivity showed a dramatic decrease in T_c with C-substitution, followed by complete suppression of superconductivity for x > 0.125. Resistivity measurements in magnetic fields parallel and perpendicular to the basal plane of the crystals showed a nearly isotropic state in the heavily doped crystals (x > 0.1).