Anomalous multi-order Raman scattering in LaMnO₃: A signature of quantum lattice effects in a Jahn-Teller crystal

The multi-order Raman scattering is studied up to fourth order for a detwinned LaMnO₃ crystal. Based on a comprehensive data analysis of the polarization-dependent Raman spectra, we show that the anomalous features in the multi-order scattering could be the sidebands on the low-energy mode at about 25 cm^-1. We suggest that this low-energy mode stems from the tunneling transition between the potential energy minima arising near the Jahn-Teller Mn³⁺ ion due to the lattice anharmonicity and that the multi-order scattering is activated by this low-energy electronic motion. The sidebands are dominated by the oxygen contribution to the phonon density-of-states, however, there is an admixture of an additional component, which may arise from coupling between the low-energy electronic motion and the vibrational modes.