Theory of Raman scattering from orbital excitations in manganese oxides

Abstract

We present a theory of the Raman scattering from the orbital wave excitations in manganese oxides. Two excitation processes of the Raman scattering are proposed. The Raman scattering cross section is formulated by using the pseudospin operator for orbital degree of freedom in a Mn ion. The Raman spectra from the orbital wave excitations are calculated and their implications in the recent experimental results reported in LaMnO3 are discussed.

Inelastic light scattering from correlated electrons

“…Scattering from channels other than B 1g , and describing the anisotropic lineshape of the response, have been addressed via longer range spin exchange interactions and by exact diagonalizations of magnons coupled to phonons (Freitas and Singh, 2000), using an earlier approach of Lorenzana and Sawatzky (1995). Lastly, recent developments concern scattering from orbiton degrees of freedom (Okamoto et al, 2002) valence bond picture (Ho et al, 2001). 12 These approaches fail when the laser frequency is tuned near an optical transition.…”

Anomalous multi-order Raman scattering in LaMnO3: a signature of quantum lattice effects in a Jahn–Teller crystal

“…Unfortunately, an uncertainty in parameters of superexchange interaction, particularly in the Mott-Hubbard value U from one side, and in the JT energy ∆ from the other side, is still a challenge for theoreticians and it is not yet clear where the orbitons should be looked for in measurements. In a limit of weak JT coupling, the orbital excitations in LaMnO 3 are predicted to appear around 1600 cm −1 (0.2 eV), as a result of the superexchange interaction [14,15], which indicates on the feature, observed in the second-order Raman scattering experiment in the 900-1400 cm −1 (0.11-0.17 eV) range [8]. If the contributions from SE and JT effects are considered on equal footing, the elementary excitations of the system, according to the theory predictions, are mixed modes with both phonon and orbital character, and the phonon-orbital excitations appear as Franck-Condon sidebands at energy intervals of the Jahn-Teller vibrational energy [16].…”

Orbiton-mediated multiphonon scattering inLa1−xSrxMnO3

“…Within this mechanism orbitons are collective excitations and their energy is expected at much lower energies of 160 meV because of strong on-site Coulomb repulsion. 7,8 Thus, the determination of the respective contribution to orbital ordering and its energy scale are closely related to the nature of orbital excitations.…”