The fifty years it has taken to understand the dynamics of UO2in its ordered state

Abstract

In 1966 Roger Cowley (together with Gerald Dolling) reported the first neutron inelastic scattering from the magnetic excitations from UO2 below its antiferromagnetic ordering temperature of 30 K. They showed the strong magnon-phonon coupling in this material and that the excitations appeared to contain an additional mode that was not anticipated. Cowley never returned to UO2, but showed a keen interest in the developments. Forty years after this pioneering work, unambiguous evidence was found (using resonance x-ray techniques) for the ordering below T N of the electric quadrupoles involving the anisotropy of the 5f charge distribution around the uranium nuclei. A further 10 years later, now armed with a full theory for the excitation spectrum expected for phonons, magnons, and quadrupoles, we can identify the latter as the source of the 'extra' mode reported first in 1966. The story is a long winding one, with the expected serendipity and dead ends, but is now (almost) completed.