Orbital ordering in ${\mathrm{L}\mathrm{a}\mathrm{M}\mathrm{n}\mathrm{O}}_3$ Investigated by Resonance Raman Spectroscopy

Orbital ordering leads to an unconventional excitation spectrum that we investigate by resonance Raman scattering using incident photon energies between 1.7 and 5.0 eV. We use spectral ellipsometry to determine the corresponding dielectric function. Our results show resonant behavior of the phonon Raman cross section when the laser frequency is close to the orbiton-excitation energy of 2 eV in ${\mathrm{L}\mathrm{a}\mathrm{M}\mathrm{n}\mathrm{O}}_3$. We show an excellent agreement between theoretical calculations based on the Franck-Condon mechanism activating multiphonon Raman scattering in first order of the electron-phonon coupling and the experimental data of phonons with different symmetries.

Figure 1

Experimental spectra showing the oxygen vibrations of the distorted ${\mathrm{M}\mathrm{n}\mathrm{O}}_6$ octahedra in the one- (a) and two-phonon (b) regions.

Figure 2

Resonance profiles of the one-phonon excitations (a) and the comparison between the resonance profiles of the one- and two-phonon excitations (b). The inset in (a) shows ${ϵ}_1$ (solid line) and ${ϵ}_2$ (dashed line) of the dielectric function and in (b) the ratio of the one- to two-phonon intensities.

Figure 3

Solid lines show the resonance profiles at orbiton energy of the one- and two-phonon peaks according to the Franck-Condon mechanism. Markers represent experimental data for the $611{\mathrm{c}\mathrm{m}}^{-1}$ one-phonon (squares) and the $1300{\mathrm{c}\mathrm{m}}^{-1}$ two-phonon (circles) resonance.