Boson peak in overdoped manganites La_{1-x}Ca_{x}MnO_{3}

TL;DR

This paper investigates the boson peak phenomenon in overdoped manganites La_{1-x}Ca_{x}MnO_{3}, linking it to zone folding and vibrational state excess in the charge-ordered phase.

Contribution

It identifies the boson peak in manganites and connects it to zone folding and vibrational state excess, providing new insights into lattice dynamics in these materials.

Findings

Boson peak appears in charge-ordered manganites with commensurate doping.

Zone folding correlates with the emergence of the boson peak.

Incommensurate doping lacks zone folding and boson peak features.

Abstract

In the charge-ordered phase of strongly doped manganites La$_{1-x}$Ca$_x$MnO$_3$ ($x \geq 0.5$) 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.