Multipolar, magnetic and vibrational lattice dynamics in the low temperature phase of uranium dioxide

We report the results of inelastic neutron scattering experiments performed with triple-axis spectrometers to investigate the low-temperature collective dynamics in the ordered phase of uranium dioxide. The results are in excellent agreement with the predictions of mean-field RPA calculations emphasizing the importance of multipolar superexchange interactions. By comparing neutron scattering intensities in different polarization channels and at equivalent points in different Brillouin zones, we show the mixed magneto-vibrational-quadrupolar character of the observed excitations. The high energy resolution afforded by the cold triple-axis spectrometer allowed us to study in detail the magnon-phonon interaction giving rise to avoided crossings along the $[00xi]$ reciprocal space direction.