Anomalous behavior of nonequilibrium excitations in UO2
Abstract: Ultrafast optical pump-probe studies of uranium dioxide (UO2) under pressure were performed in order to better understand the material's response to ionizing radiation. Photoexcitation generates oscillations in the time-resolved reflectivity at two distinct GHz-scale frequencies. The higher-frequency mode is attributed to a coherent longitudinal acoustic mode. The lower-frequency mode does not correspond to any known excitation under equilibrium conditions. The frequency and lifetime of the low-frequency mode are studied as a function of pressure. Abrupt changes in the pressure-dependent slopes of these attributes are observed at ∼10 GPa, which correlates with an electronic transition in UO2. Variation of probe wavelength reveals that the low-k dispersion of the low-frequency mode does not fit into either an optical or acoustic framework. Rather, we propose that this mode is related to the dynamical magnetic structure of UO2. The implications of these results help account for the anomalously small volume of damage known to be caused by ionizing radiation in UO2; we propose that the existence of the low-frequency mode enhances the material's transient thermal conductivity, while its long lifetime lengthens the timescale over which energy is dissipated. Both mechanisms enhance damage recovery.