Neutron scattering and scaling behavior in URu2Zn20 and YbFe2Zn20

The dynamic susceptibility χ^″(ΔE) , measured by inelastic neutron-scattering measurements, shows a broad peak centered at E_max=15meV for the cubic actinide compound URu₂Zn₂₀ and 7 meV at zone center and at the (1/2, 1/2, 1/2) zone boundary for the rare-earth counterpart compound YbFe₂Zn₂₀ . For URu₂Zn₂₀ , the low-temperature susceptibility and magnetic specific-heat coefficient γ=C_mag/T take the values χ=0.011emu/mole and γ=190mJ/moleK² at T=2K . These values are roughly three times smaller, and E_max is three times larger, than recently reported for the related compound UCo₂Zn₂₀ , so that χ and γ scale inversely with the characteristic energy for spin fluctuations, T_sf=E_max/k_B . While χ(T) , C_mag(T) , and E_max of the 4f compound YbFe₂Zn₂₀ are very well described by the Kondo impurity model, we show that the model works poorly for URu₂Zn₂₀ and UCo₂Zn₂₀ , suggesting that the scaling behavior of the actinide compounds arises from spin fluctuations of itinerant 5f electrons.