Electronic properties of the yttriumdicarbide superconductors YC2, Y1-xThxC2, Y1-xCaxC2 (0<x<=0.3)

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 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 YC2 indicate a superconducting behavior close to the type-I limit with B-c2(0)=59(2) mT. Specific-heat data of YC2, Y0.8Th0.2C2, and Y0.9Ca0.1C2 are analysed in terms of weak-coupling BCS theory and the ct model. The comparison with the model predictions as well as the C-12/C-13-isotope effect on T-c 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.