Orbital wave and its observation in orbital-ordered titanates and vanadates

We present a theory of the collective orbital excitation termed orbital wave in perovskite titanates and vanadates with the triply degenerate t_2g orbitals. The dispersion relations of the orbital waves for the orbital-ordered LaVO₃, YVO₃ and YTiO₃ are examined in the effective spin-orbital coupled Hamiltonians associated with the Jahn-Teller type couplings. We propose possible scattering processes for the Raman and inelastic neutron scatterings from the orbital wave and calculate the scattering spectra for titanates and vanadates. It is found that both the excitation spectra and the observation methods of the orbital wave are distinct qualitatively from those for the e_g orbital-ordered systems.