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

Abstract

We characterize the superconducting state of the carbides YC2, Y1−xThxC2, and Y1−xCaxC2 (0<x<~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 α 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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