Gulden T., Henn R.W., Jepsen O., Kremer R.K., Schnelle W., Simon A., Felser C.
Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany We characterize the superconducting state of the carbides YC_{2}, Y_{1-x}Th_{x}C_{2}, and^{ }Y_{1-x}Ca_{x}C_{2} (0 < x(less-than-or-equal-to)0.3) by means of magnetization and specific-heat measurements. YC_{2} is a superconductor with T_{c} = 4.02(5) K. Partial substitution with Ca and Th, as well as doping with the strongly pair breaking Gd, reduces the critical temperature. Isothermal magnetization measurements on YC_{2} indicate a superconducting behavior close to the type-I limit with B_{c2}(0) = 59(2) mT. Specific-heat data of YC_{2}, Y_{0.8}Th_{0.2}C_{2}, and Y_{0.9}Ca_{0.1}C_{2} are analyzed in terms of weak-coupling BCS theory and the alpha model. The comparison with the model predictions as well as the ^{12}C/^{13}C-isotope effect on T_{c} indicate excellent agreement with weak-coupling BCS theory for YC_{2}. A strong dependence of the superconducting properties on the carbon deficiency is observed. We describe high-temperature annealing procedures to optimize the superconducting properties of the samples. Ab initio calculations of the electronic band structure using the tight-binding linear muffin-tin orbital atomic-sphere approximation method are presented and the density of states at the Fermi energy is discussed in view of the experimental Pauli susceptibilities and heat-capacity results. A full reprint of this paper is available as postscript or PDF file from the APS WWW-Server.
Physical Review B, 56 9021-9, 1997.
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