Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany
We characterize the superconducting state of the carbides YC2, Y1-xThxC2, and Y1-xCaxC2 (0 < x(less-than-or-equal-to)0.3) by means of magnetization and specific-heat measurements. YC2 is a superconductor with Tc = 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 YC2 indicate a superconducting behavior close to the type-I limit with Bc2(0) = 59(2) mT. Specific-heat data of YC2, Y0.8Th0.2C2, and Y0.9Ca0.1C2 are analyzed in terms of weak-coupling BCS theory and the alpha model. The comparison with the model predictions as well as the 12C/13C-isotope effect on Tc indicate excellent agreement with weak-coupling BCS theory for YC2. 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.
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Physical Review B, 56 9021-9, 1997.
Max-Planck Institut für Festkörperforschung;
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