Orbiton features in YTiO$_3$ and LaTiO$_3$ probed by Raman light scattering

The existence of collective orbital excitations, termed orbitons, in YTiO$_3$ has been proposed by G. Khaliullin et al. [1] and S. Ishihara [2]. We have performed Raman and inelastic neutron scattering experiments on LaTiO$_3$ and YTiO$_3$ single crystals in order to determine the excitation spectrum for energies above the range of optical phonons. There are two unexpected features in the high energy Raman spectrum of both materials. A sharp structure at 165 meV and a broad continuum like feature around 230 meV. Our theoretical calculations of the phonon dispersion relation have revealed, that the peak at 165 meV is right at the high energy cutoff of the two-phonon density of states, but it's unexpected large intensity cannot be explained with two-phonon processes alone. The temperature dependence, polarization dependence and the resonance behavior probed with different laser lines for excitation, yields that the feature at 230 meV can possibly be assigned to orbital excitations. [1] G. Khaliullin and S. Okamoto, PRB {\bf 68}, 205109 (2003). [2] S. Ishihara. PRB {\bf 69}, 075118 (2004).