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

Abstract

We report inelastic light scattering measurements of lattice dynamics related to the incommensurate orbital order in $\mathrm{CaM}{\mathrm{n}}_7{\mathrm{O}}_{12}$. 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.

7 pages, 8 figures

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Cited by in corpus (3)

• Charge density waves and phonon-electron coupling in ZrTe${}_3$ investigated by Raman spectroscopy and first-principles calculations
• Soft phonon driven orbital order in CaMn7O12
• Identification and mechanical control of ferroelastic domain structure in rhombohedral CaMn${}_7$O${}_{12}$