Optical study of orbital excitations in transition-metal oxides

TL;DR

This study uses optical spectroscopy to analyze orbital excitations in various transition-metal oxides, comparing experimental results with theoretical models to understand the role of lattice coupling and orbital fluctuations.

Contribution

It provides a comprehensive experimental and theoretical analysis of orbital excitations across multiple transition-metal oxides, highlighting the importance of lattice coupling and identifying the need for advanced models.

Findings

Orbital excitations are observed in all compounds except LaMnO3.

Good agreement between experiments and configuration-interaction calculations emphasizes lattice coupling.

Further theoretical work is needed to clarify the role of collective orbital modes in optical spectra.

Abstract

The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10, ranging from early to late transition-metal ions, from t_2g to e_g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO3, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yield good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g., the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular in case of the orbital excitations at about 0.25 eV in RTiO3. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing.