Damped orbital excitations in the titanates

TL;DR

This paper investigates how orbital degeneracy in titanates is lifted through damped orbital excitations, using a Kugel-Khomskii model with perturbation theory, revealing temperature-dependent dispersions and damping effects.

Contribution

It introduces a model showing damped orbital excitations in RTiO3, explaining the removal of orbital degeneracy via orbital-phonon interactions and damping effects.

Findings

Orbital degeneracy is lifted by temperature-dependent dispersion.

Orbital excitations are heavily damped and do not form a separate thermodynamic branch.

The model explains the absence of distinct orbital excitation signatures at low temperatures.

Abstract

A possible mechanism for the removal of the orbital degeneracy in RTiO3 (where R=La, Y, ...) is considered. The calculation is based on the Kugel-Khomskii Hamiltonian for electrons residing in the t2g orbitals of the Ti ions, and uses a self-consistent pe rturbation expansion in the interaction between the orbital and the spin degrees of freedom. The latter are assumed to be ordered in a Neel state, brought about by delicate interactions that are not included in the Kugel-Khomskii Hamiltonian. Within our model calculations, each of the t2g bands is found to acquire a finite, temperature-dependent dispersion, that lifts the orbital degeneracy. The orbital excitations are found to be heavily damped over a rather wide band. Consequently, they do not participate as a separate branch of excitations in the low-temperature thermodynamics.e