Magnetic excitations and dynamical Jahn-Teller distortions inUO2

Abstract

Inelastic neutron scattering with polarization analysis has been used to study the evolution of the magnetic response across the antiferromagnetic to paramagnetic transition in a single crystal of uranium dioxide ${(T}_{N}=30.8 \mathrm{K})$. The spin-wave dispersion curves have been determined at 16 K by measuring the spin-flip channel of the neutron cross section along the principal crystallographic directions. Evidence of magnon-phonon interactions along the $(0,0,\ensuremath{\xi})$ direction is given by the splitting of the lowest spin-wave branch, and by the Q dependence of the energy-integrated dynamic susceptibility. Above ${T}_{N},$ a magnetic inelastic response consisting of two dispersive peaks was observed between 3 and 10 meV. This signal was easily measurable even at 200 K, more than six times the N\'eel temperature, where spatial correlations between the uranium spins are essentially zero. We assume this result as evidence that in the time scale of our experiment the uranium triplet ground state is split into three singlets, due to a dynamical Jahn-Teller (JT) distortion of the oxygen cage which reduces the point symmetry at the uranium site. Since the position of the peaks and their dispersion are compatible to a $1\ensuremath{-}\mathbf{k}$ distortion along the $〈100〉$ direction, a picture emerges in which local, uncorrelated $1\ensuremath{-}\mathbf{k}$ dynamical JT distortions occur above ${T}_{N}$ along the three directions of the $〈100〉$ star; as ${T}_{N}$ is approached, a correlation builds up between the phases of the corresponding vibrations until, eventually, a static $3\ensuremath{-}\mathbf{k}$ structure is obtained below ${T}_{N}.$