Neutron scattering from spins and phonons in uranium compounds

Recent single-crystal neutron experiments at Chalk River on the spin excitations in the rock salt structure compounds UN, US and UTe, have revealed the presence of a broad spectral response, instead of the sharp spin-wave peaks expected at low temperatures in magnetically-ordered crystals. The presence of broad features in chi(q,w) correlates with strong suppression of the ordered moment, with small ordering temperatures and with small lattice constants. It is suggested that the damping of the collective spin response in the UX series arises from intermediate valency. Evidence for this view is provided by the observation in photoemission of a narrow 5f-electron peak at the Fermi energy and by the fact that the actinide lattice constants are consistent with a valency change from 4+ to 3+ with increasing atomic number. By contrast, in UPd3, where sharp spin excitations are observed, the 5f-electron peak lies below the Fermi energy. (Auth.)