Anisotropy in cubic UO2 caused by electron-lattice interactions

Despite many years of research, the full complexity of the electron-lattice interactions in ${\mathrm{UO}}₂$ is not fully understood. We present x-ray inelastic scattering at low temperature showing that the interaction between electronic degrees of freedom and transverse acoustic phonons is strong only along the reciprocal space direction [100]. The anisotropy is reflected in the phonon-linewidth broadening, which persists also well above the N\'eel temperature. This intrinsic effect infers an anisotropy in the thermal conductivity, which has been observed, but which is formally forbidden in a cubic material. We have no model capable of connecting our experimental observations with the low thermal conductivity of ${\mathrm{UO}}₂$ below room temperature.