Spin and phonon response in uranium compounds

Recent progress in understanding the spin localization and moment formation in metallic uranium compounds is summarized. Measurements have been made of the neutron scattering from UN, US, UTe and UPd3. The phonon spectra are unusual e.g. the (111) TA branch of UTe lies below the (111) TA branch from ζ = 0.2 to the zone boundary. Force-constant analysis of the UX series shows that when the U-U forces are large the moment is small. In non-ordering UPd3 the high-frequency band of crystal-field-like excitations is well described by a localized spin model. The low-frequency band (ν∼0.4 THz) is broadened, however, suggesting a delicate balance in the spin localization. In antiferromagnetic UN, where the U-U overlap is greater, the spin response is entirely broad with a large anisotropy gap. Ferromagnetic UTe has a quadratically rising spin-wave branch as well as a higher flat branch. Thus UPd3 and UTe provide clear evidence for the existence of crystalline electric fields in metallic actinide compounds.