Cu nuclear quadrupole resonance and relaxation on the Yb-based heavy-fermion compound ${\mathrm{YbCu}}_5$

Cu nuclear quadrupole resonance (NQR) study has been performed on a heavy-fermion compound cubic ${\mathrm{YbCu}}_5$ which has a large electronic specific heat coefficient of $\gamma =550\mathrm{}{\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{}\mathrm{K}}^2.$ ${}^{63}\mathrm{Cu}$ and ${}^{65}\mathrm{Cu}$ NQR spectra confirmed that magnetic ordering is absent in cubic ${\mathrm{YbCu}}_5$ at least down to 1.6 K. ${}^{63}\mathrm{Cu}$ nuclear magnetic relaxation rate ${(1/T}_1)$ of cubic ${\mathrm{YbCu}}_5$ shows Korringa relation where ${1/T}_{1}T=\mathrm{const}$ below 4.2 K, indicating the evolution of Fermi-liquid state. With increasing temperature above 50 K, the $4f$-electron contribution to ${1/T}_1,$ ${(1/T}_1{)}_{4f},$ tends to be saturated to a constant value, implying that $4f$ electrons are localized with magnetic moments. The temperature dependence of ${(1/T}_1{)}_{4f}$ can be qualitatively explained by the temperature dependence of the $4f$-electron correlation rates $({\tau }_{4f}{)}^{-1},$ calculated within the impurity Kondo model.