ELECTRONIC-PROPERTIES OF THE YTTRIUMDICARBIDE SUPERCONDUCTORS YC2, Y1-XTHXC2, Y1-XCAXC2 (0-LESS-THAN-X-LESS-THAN-OR-EQUAL-TO-0.3)

We characterize the superconducting state of the carbides YC2, Y1-xThx C2, and Y1-xCaxC2 (0<x less than or equal to 0.3) by means of magnetiz ation 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 wit h the strongly pair breaking Gd, reduces the critical temperature. Iso thermal magnetization measurements on YC2 indicate a superconducting b ehavior 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 we ak-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 e xcellent agreement with weak-coupling BCS theory for YC2. A strong dep endence of the superconducting properties on the carbon deficiency is observed. We describe high-temperature annealing procedures to optimiz e the superconducting properties of the samples. Ab initio calculation s of the electronic band structure using the tight-binding linear muff in-tin orbital atomic-sphere approximation method are presented and th e density of states at the Fermi energy is discussed in view of the ex perimental Pauli susceptibilities and heat-capacity results.