Polarized neutron scattering study of the magnetic response across TN in a single crystal of UO2

Abstract

We present the results of an inelastic neutron scattering experiment with polarization analysis performed on a single crystal of UO2. A magnetic inelastic response is observed above TN in the energy region between 3 and 10 meV, at least up to 6.5TN. This result provides evidence of the fact that in the time scale of our experiment the uranium triplet ground state is split into three singlets because of a dynamical Jahn–Teller distortion of the oxygen cage, reducing the point symmetry at the uranium site. The position of the peaks and their dispersion are compatible with monoclinic 1-k distortions along the three directions of the 〈100〉 star. This suggests a picture in which as TN is approached, a correlation builds up between the phases of the corresponding vibrations until, eventually, a static 3-k distortion is obtained below TN.