Excitations of the orbital order in RMnO3 manganites: light scattering experiments

Raman spectroscopy was used to seek out the orbital excitations in orbitally ordered RMnO3. The high energy Raman peaks are found to correspond to second and third order phonons which couple in some degree to the orbital order. Franck-Condon-type Raman resonance of multiphonons is observed but the cross sections are smaller than expected. A surging emission band at 1.5 eV, detected at high temperatures, disappears when the orbital order is established. The crossing of the configurational coordinate diagrams of the localized 3d Mn lowest levels, in the orbitally ordered state, originates the nonradiative relaxation and the quenching of the luminescence.