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

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.