Neutron-inelastic-scattering measurements on uranium antimonide

We report neutron-inelastic-scattering measurements on the fcc compound USb, which orders antiferromagnetically at 240 K with the simple type I structure. The phonon spectra in the [ξξ0] and [00ξ] directions are similar to those measured previously in uranium rocksalt systems. The magnetic response consists of three parts: (i) A collective excitation which is longitudinal in nature; i.e., the fluctuations correspond to variations in magnitude of the moment along the quantization axis rather than deviations from this direction. At the magnetic zone center [110] the anisotropy gap is 1.53±{}0.05 THz and is degenerate with the acoustic-phonon energy at this point. With increasing temperature the excitation rapidly decreases in intensity and cannot be observed for T>TN2. (ii) A broad, almost temperature-independent excitation centered at {~} 6 THz, which is probably a crystal-field excitation. (iii) A magnetic-response function centered at $E=0$ but with a half-width that apparently increases with increasing temperature. We are unable to understand these results in a quantitative manner. However, the implications for our understanding of actinide systems, in particular the shortcomings of present models, are discussed.