Orbital dynamics in ferromagnetic transition metal oxides

TL;DR

This paper investigates the orbital dynamics and excitations in ferromagnetic LaMnO₃, revealing how quantum fluctuations and external pressure influence orbital order, degeneracy, and excitation spectra.

Contribution

It introduces a detailed analysis of orbital excitations in LaMnO₃, highlighting the effects of quantum fluctuations and pressure on orbital degeneracy and order.

Findings

Classical orbital order has full rotational symmetry at degeneracy.

Quantum corrections restore cubic symmetry and open a gap under pressure.

Orbital excitation spectra are well described by linear spin-wave theory.

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.