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Abstract:
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.
