Elementary excitations in uranium dioxide: Unravelling the tangle

In solids with d- and f-electrons, under certain conditions the orbitals align to form an ordered structure. Collective excitations breaking this arrangement can take the form of oscillations of electric quadrupoles, the so-called quadrupolar waves. These represent a propagating pattern of charge densities implying a modulation of quadrupolar moments. Quadrupolar waves constitute a major component of the dynamics of uranium dioxide in its magneto-quadrupolar ordered phase. Together with spin-waves and phonons, these produce a very complex spectrum of elementary excitations having hybrid character over most of the Brillouin zone. Although neutron scattering spectra only reveal the tip of the iceberg of this spectrum, its complete and detailed modelling is needed to understand such experiments. Distinct roles of Jahn–Teller and superexchange mechanisms as sources of quadrupolar interactions can be clearly identified by such modelling.