The fifty years it has taken to understand the dynamics of UO₂ in 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.

Charge Localization and Magnetic Correlations in the Refined Structure of U3O7

“…Uranium oxides are recognized as a small-gap semiconductor (with either Mott or charge transfer insulator characteristics) where strong interactions exist at low temperature between the electronic and lattice degrees of freedom. 16 The magnetic structure of UO2 has been studied extensively during the past 60 years, 17,18 though certain details are not fully understood yet. The situation is even less established for the higher oxides retaining a fluoritetype structure such as U4O9 and U3O7 where some of the tetravalent U(IV) uranium ions are replaced by pentavalent U(V) or hexavalent U(VI) ions.…”

Generation of modulated magnetic structure based on cluster multipole: Application to $α$-Mn and Co$M_3$S$_6$

“…6(b) is induced from the magnetic structure with E − g -symmetry of k-group at X-point. This type of magnetic ordering has been observed in UO 2 [37,38]. This magnetic ordering with magnetic point group m 3 is absence of antiunitary symmetry operations, which belongs to type-I Shubnikov group [11].…”

The First Two Decades of Neutron Scattering at the Chalk River Laboratories

“…While the theory of the magnetic excitations included all the interactions presumed to exist at the time, it was recognized as not fully satisfactory. A more complete story has taken over half a century to resolve [121], and a very recent paper in a volume dedicated to the contributions of Roger Cowley [122] summarizes the now-known facts. There are quadrupole moments on the U 4+ ions, and there are also quadrupolar transitions between the three states in the molecular field.…”