Carbon solubility and superconductivity in MgB2

Successful replacement of B by C in the series MgB2-xCx for values of x upto 0.3 is reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x = 0.30 into the MgB2 lattice. The superconducting transition temperature. T. measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x = 0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch-Gruneisen formula for all the carbon compositions studied. The Debye temperature, 130, extracted from the fit, is seen to decrease with carbon content from 900 to 525 K, whereas the electron-phonon interaction parameter, lambda, obtained from the McMillan equation using the measured T-c and 0(D), is seen to increase monotonically from 0.8 in MgB2 to 0.9 in the x = 0.50 sample. The ratio of the resistivities between 300 and 40 K versus T. is seen to follow the Testardi correlation for the C substituted samples. The decrease in T. is argued to mainly arise due to large decrease in theta(D) with C concentration and a decrease in the hole density of states at N(E-F). (C) 2002 Elsevier Science B.V. All rights reserved.