Electronic properties of the yttriumdicarbide superconductorsYC2,Y1−xThxC2,Y1−xCaxC2(0<x<~0.3)

Abstract

We characterize the superconducting state of the carbides YC${}_{2},$ ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}{\mathrm{C}}_{2},$ and ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{C}}_{2}$ $(0lxl~0.3)$ by means of magnetization and specific-heat measurements. ${\mathrm{YC}}_{2}$ 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 ${\mathrm{YC}}_{2}$ indicate a superconducting behavior close to the type-I limit with ${B}_{c2}(0)=59(2)$ mT. Specific-heat data of ${\mathrm{YC}}_{2},$ ${\mathrm{Y}}_{0.8}{\mathrm{Th}}_{0.2}{\mathrm{C}}_{2}$, and ${\mathrm{Y}}_{0.9}{\mathrm{Ca}}_{0.1}{\mathrm{C}}_{2}$ are analyzed in terms of weak-coupling BCS theory and the $\ensuremath{\alpha}$ model. The comparison with the model predictions as well as the ${}^{12}\mathrm{C}{/}^{13}$C-isotope effect on ${T}_{c}$ indicate excellent agreement with weak-coupling BCS theory for YC${}_{2}$. 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.