Orbital dynamics in ferromagnetic transition metal oxides

Abstract

We consider a model of strongly correlated $e_g$ electrons interacting by superexchange orbital interactions in the ferromagnetic phase of LaMnO${}_3$. It is found that the classical orbital order with alternating occupied $e_g$ orbitals has a full rotational symmetry at orbital degeneracy, and the excitation spectrum derived using the linear spin-wave theory is gapless. The quantum (fluctuation) corrections to the order parameter and to the ground state energy restore the cubic symmetry of the model. By applying a uniaxial pressure orbital degeneracy is lifted in a tetragonal field and one finds an orbital-flop phase with a gap in the excitation spectrum. In two dimensions the classical order is more robust near the orbital degeneracy point and quantum effects are suppressed. The orbital excitation spectra obtained using finite temperature diagonalization of two-dimensional clusters consist of a quasiparticle accompanied by satellite structures. The orbital waves found within the linear spin-wave theory provide an excellent description of the dominant pole of these spectra.

13 pages, 12 figures, to appear in Phys. Rev. B

References in corpus (5)

• Electronic origin of magnetic and orbital ordering in insulating LaMnO_3
• Theory of Anomalous X-ray Scattering in Orbital Ordered Manganites
• Optical conductivity of colossal magnetorestistance compounds: Role of orbital degeneracy in the ferromagnetic phase
• Impurity-induced moments in underdoped cuprates
• Orbital dynamics: The origin of the anomalous optical spectra in ferromagnetic manganites

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