Origin of the mid-infrared peaks in the optical conductivity of LaMnO₃

We calculated the infrared conductivity spectrum of orbitally ordered LaMnO₃ 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.