Origin of the mid-infrared peaks in the optical conductivity of LaMnO$_{3}$

TL;DR

This paper models the infrared conductivity of LaMnO3, revealing that overtone modes of orbitons are responsible for mid-infrared peaks, aligning with experimental observations.

Contribution

It introduces a theoretical calculation considering orbital exchange and electron-phonon interactions to explain mid-infrared peaks in LaMnO3's optical spectra.

Findings

Overtone modes of orbitons are both Raman and infrared active.

Calculated spectra match experimental peaks near 1300 cm$^{-1}$.

Fundamental orbiton excitations are Raman active but not infrared active.

Abstract

We calculated the infrared conductivity spectrum of orbitally ordered LaMnO$_3$ in phonon frequency and overtone frequency ranges. We considered orbital exchange, Jahn-Teller electron-phonon coupling, and phonon--phonon coupling. The fundamental excitation of the phonon-coupled orbiton was only Raman active, not infrared active, while its overtone modes were both Raman and infrared active. Our calculations reproduced the small peaks near 1300 cm$^{-1}$ observed both in Raman scattering and infrared conductivity spectra, as consistent with previous experimental results.