Anomalous multi-order Raman scattering in LaMnO_3: a signature of a   quantum lattice effect in a Jahn-Teller crystal
  The multi-order Raman scattering is studied up to a fourth order for a detwinned LaMnO_3 crystal. Based on a comprehensive data analysis of the polarisation-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 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. 
N. N. Kovaleva, O. E. Kusmartseva, K. I. Kugel, A. A. Maksimov, D.   Nuzhnyy, A. M. Balbashov, E. I. Demikhov, A. Dejneka, V...
cond-mat.str-el