Carbon-substituted MgB2 single crystals

Carbon-substituted MgB2 single crystals, Mg(B1-xCx)(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, 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). (C) 2003 Elsevier B.V. All rights reserved.