Raman Scattering in the Mott InsulatorsLaTiO3andYTiO3: Evidence for Orbital Excitations

Raman scattering is used to observe pronounced electronic excitations around 230 meV—well above the two-phonon range—in the Mott insulators ${\mathrm{LaTiO}}_3$ and ${\mathrm{YTiO}}_3$. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of the superconducting cuprates, indicating an unanticipated universality of the electronic excitations in transition metal oxides.