Electronic and magnetic properties of single-crystal Y${\mathrm{Ni}}_2$${\mathrm{B}}_2$C from ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR and magnetic-susceptibility measurements

The quaternary intermetallic compound superconductor Y${\mathrm{Ni}}_2$${\mathrm{B}}_2$C with transition temperature $T_c=15.5\mathrm{}$ K has been investigated by ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ nuclear magnetic resonance (NMR) and by magnetic susceptibility $\chi$ measurements both in the normal and the superconducting states. The NMR and relaxation measurements have been performed in a powder sample and single crystals. ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ ($I=\frac{3}{2}$) NMR spectra display patterns typical for an axially symmetric field gradient with quadrupole coupling frequency $v_Q=698\mathrm{}\pm 1$ kHz and ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ ($I=\frac{1}{2}$) data show spectra typical for a large anisotropic Knight shift, $K$, with axial symmetry ($3K_{\mathrm{ax}}=0.042\%$). In the normal state, the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ $K$ increases with decreasing temperature while ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ $K$ decreases. The temperature dependences of both the isotropic ($K_{\mathrm{iso}}$) and anisotropic ($3K_{\mathrm{ax}}$) components of the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ Knight shifts are presented together with dc magnetic susceptibility ($\chi$) measurements obtained from magnetization measurements and are explained by the sharp features of the density of states near the Fermi level in the system. The analysis of the NMR and $\chi \mathrm{}\left(T\right)\mathrm{}$ data when combined with the theoretical calculation of the Van Vleck contribution to $\chi \mathrm{}\left(T\right)\mathrm{}$ allows the determination of the hyperfine coupling constants for both nuclei investigated and permits the separation of the different contributions to the total measured $\chi \mathrm{}\left(T\right)\mathrm{}$. The nuclear spin-lattice relaxation rate (NSLR) ($T_1^{-1\mathrm{}}$) results for ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ show an enhancement of ${\left(T_{1}T\right)}^{-1\mathrm{}}$ when lowering the temperature, consistent with previous results. It is shown that the enhancement of the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ NSLR is not due to the effects of antiferromagnetic fluctuations of Ni magnetic moments but simply due to the increase of the $s$-band spin susceptibility with decreasing temperature as reflected in the temperature dependence of the Knight shift. Contrary to the case of ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$, the ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NSLR displays a ${\left(T_{1}T\right)}^{-1\mathrm{}}$ which is independent of temperature, indicating that the dominant contribution is from a large temperature-independent orbital Knight shift. In the superconducting state, the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ NSLR drops rapidly without a coherence peak and is found to fit BCS behavior with a superconducting gap parameter at $T=0\mathrm{}$ given by $2{\Delta }_0=(3.4\mathrm{}\pm 0.5)k_B{T}_c$.