Multiphonon resonant Raman scattering predicted in LaMnO3 from the Franck-Condon process via self-trapped excitons - art. no. 085118

Resonant behavior of the Raman process is predicted when the laser frequenc y is close to the orbital excitation energy of LaMnO3 at 2 eV. The incident photon creates a vibrationally excited self-trapped "orbiton" state from t he orbitally ordered Jahn-Teller (JT) ground state. Trapping occurs by loca l oxygen rearrangement. Then the Franck-Condon mechanism activates multipho non Raman scattering. The amplitude of the n-phonon process is first order in the electron-phonon coupling g. The resonance occurs via a dipole forbid den d to d transition. We previously suggested that this transition (also s een in optical reflectivity) becomes allowed because of asymmetric oxygen f luctuations. Here we calculate the magnitude of the corresponding matrix el ement using local spin-density functional theory, This calculation agrees t o better than a factor of two with our previous value extracted from experi ment. This allows us to calculate the absolute value of the Raman tensor fo r multiphonon scattering. Observation of this effect would be a direct conf irmation of the importance of the JT electron-phonon term and the presence of self-trapped orbital excitons, or "orbitons."