Magnetic excitations and dynamical Jahn-Teller distortions in UO2

Inelastic neutron scattering with polarization analysis has been used to st udy the evolution of the magnetic response across the antiferromagnetic to paramagnetic transition in a single crystal of uranium dioxide (T-N = 30.8 K). The spin-wave dispersion curves have been determined at 16 K by measuri ng the spin-flip channel of the neutron cross section along the principal c rystallographic directions. Evidence of magnon-phonon interactions along th e (0,0,xi) direction is given by the splitting of the lowest spin-wave bran ch, and by the Q dependence of the energy-integrated dynamic susceptibility . Above T-N, a magnetic inelastic response consisting of two dispersive pea ks was observed between 3 and 10 meV. This signal was easily measurable eve n at 200 K, more than six times the Neel temperature, where spatial correla tions between the uranium spins are essentially zero. We assume this result as evidence that in the time scale of our experiment the uranium triplet g round state is split into three singlets, due to a dynamical Jahn-Teller (J T) distortion of the oxygen cage which reduces the point symmetry at the ur anium site. Since the position of the peaks and their dispersion are compat ible to a 1-k distortion along the (100) direction, a picture emerges in wh ich local, uncorrelated 1-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, eventu ally, a static 3-k structure is obtained below T-N. [S0163-1829(99)09621-6] .