In the charge-ordered phase of strongly doped manganites La1−xCaxMnO3 (x≥0.5) absorption lines appear in the terahertz spectral range for commensurate x values right below the charge-ordering temperature. They are connected to acoustic phonons that become optically active by folding of the Brillouin zone. At lower temperatures a strongly asymmetric extra absorption band develops at frequencies corresponding to the position of the lowest-energy van Hove singularity in the reduced Brillouin zone. The band is assigned to the boson peak, i.e. to the excess of lattice vibrational states over the standard Debye contribution. The folded phonons and the boson peak do not show up for incommensurate calcium contents when no distinct Brillouin zone folding exists.
@article{arxiv.1303.1992,
title = {Boson peak in overdoped manganites La_{1-x}Ca_{x}MnO_{3}},
author = {B. Gorshunov and E. Zhukova and V. I. Torgashev and L. S. Kadyrov and L. Motovilova and F. Fischgrabe and V. Moshnyaga T. Zhang and R. Kremer and U. Pracht and S. Zapf and M. Dressel},
journal= {arXiv preprint arXiv:1303.1992},
year = {2015}
}