Superconducting properties of Y x Lu 1 − x Ni 2 B 2 C and La 3 Ni 2 B 2 N 3 − δ : A comparison between experiment and Eliashberg theory

Specific heat and resistivity measurements were performed on polycrystalline samples of the solid-solution ${\mathrm{Y}}_{x}{\mathrm{Lu}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ in order to determine thermodynamic properties such as the specific-heat difference $\ensuremath{\Delta}C,$ the thermodynamic critical field ${H}_{c}(T),$ as well as the upper critical field ${H}_{c2}(T).$ These properties were analyzed within the Eliashberg theory including anisotropy effects, yielding electron-phonon coupling anisotropy parameters $〈{a}_{\mathbf{k}}^{2}〉$ ranging between $0.02$ and $0.03$ for the whole series, and Fermi velocity anisotropy parameters of $〈{b}_{\mathbf{k}}^{2}〉=0.245\char21{}0.3.$ Excellent agreement between theory and experiment was achieved for these parameters, the Sommerfeld constant $\ensuremath{\gamma}$ and model phonon spectra determined from specific heat measurements. An analysis of the previously investigated boronitride ${\mathrm{La}}_{3}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}{\mathrm{N}}_{3\ensuremath{-}\ensuremath{\delta}}$ for comparison revealed the electron-phonon anisotropy to be of great significance in describing its thermodynamic properties and the calculations yielded $〈{a}_{\mathbf{k}}^{2}〉\ensuremath{\simeq}0.08$ and $〈{b}_{\mathbf{k}}^{2}〉\ensuremath{\simeq}0.245.$ The ${T}_{c}$ behavior within the series ${\mathrm{Y}}_{x}{\mathrm{Lu}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ is discussed in terms of the density of states at the Fermi level $N(0).$