Spin Dynamics and Orbital State in LaTiO_3

A neutron scattering study of the Mott-Hubbard insulator LaTiO${}_3$ (T${}_{\mathrm{N}}=132$ K) reveals a spin wave spectrum that is well described by a nearest-neighbor superexchange constant $J=15.5$ meV and a small Dzyaloshinskii-Moriya interaction ($D=1.1$ meV). The nearly isotropic spin wave spectrum is surprising in view of the absence of a static Jahn-Teller distortion that could quench the orbital angular momentum, and it may indicate strong orbital fluctuations. A resonant x-ray scattering study has uncovered no evidence of orbital order in LaTiO${}_3$.