Soft vibrational mode associated with incommensurate orbital order in multiferroic CaMn$_7$O$_{12}$

TL;DR

This study reveals a soft vibrational mode linked to incommensurate orbital order in CaMn₇O₁₂, showing the lattice's active role in the orbital-ordering transition through inelastic light scattering measurements.

Contribution

It provides the first observation of a soft vibrational mode associated with incommensurate orbital order in CaMn₇O₁₂, highlighting the lattice's involvement in the ordering mechanism.

Findings

Observation of extra phonon peaks due to zone folding

Identification of a soft mode with a specific temperature dependence

Confirmation of a second-order orbital-ordering transition

Abstract

We report inelastic light scattering measurements of lattice dynamics related to the incommensurate orbital order in $\mathrm{CaMn_7O_{12}}$. Below the ordering temperature $T_\mathrm{o} \approx 250 \,\mathrm{K}$, we observe extra phonon peaks as a result of Brillouin-zone folding, as well as a soft vibrational mode with a power-law $T$-dependent energy, $\Omega = \Omega_{0}(1 - T/T_{\mathrm{o}})^{1/2}$. This temperature dependence demonstrates the second-order nature of the transition at $T_\mathrm{o}$, and it indicates that the soft mode can be regarded as the amplitude excitation of the composite order parameter. Our result strongly suggests that the lattice degrees of freedom are actively involved in the orbital-ordering mechanism.