Magnetic state of Yb in Kondo-lattice ${\mathrm{YbNi}}_2{\mathrm{B}}_2\mathrm{C}$

We report neutron-scattering experiments performed to investigate the dynamic magnetic properties of the Kondo-lattice compound ${\mathrm{YbNi}}_2{\mathrm{B}}_2\mathrm{C}.$ The spectrum of magnetic excitations is found to be broad, extending up to at least 150 meV, and contains inelastic peaks centered near 18 meV and 43 meV. At low energies, we observe quasielastic scattering with a width Γ=2.1 meV. The results suggest a ${\mathrm{Yb}}^{3+}$ ground state with predominantly localized $4f$ electrons subject to (i) a crystalline electric-field (CEF) potential, and (ii) a Kondo interaction, which at low temperatures is about an order of magnitude smaller than the CEF interaction. From an analysis of the dynamic magnetic response, we conclude that the crystalline electric field acting on the Yb ions has a similar anisotropy to that in other $R{\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$ compounds, but is uniformly enhanced by almost a factor of 2. The static and dynamic magnetic properties of ${\mathrm{YbNi}}_2{\mathrm{B}}_2\mathrm{C}$ are found to be reconciled quite well by means of an approximation scheme to the Anderson impurity model, and this procedure also indicates that the effective Kondo interaction varies with temperature due to the crystal-field splitting. We discuss the nature of the correlated-electron ground state of ${\mathrm{YbNi}}_2{\mathrm{B}}_2\mathrm{C}$ based on these and other experimental results, and suggest that this compound might be close to a quantum critical point on the nonmagnetic side.