Soft vibrational mode associated with incommensurate orbital order in multiferroic CaMn₇O₁₂

Abstract

We report inelastic light scattering measurements of lattice dynamics related to the incommensurate orbital order in CaMn₇O₁₂. Below the ordering temperature T_o ≈ 250 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, Ω = Ω₀(1−T/T_o)^1/2. This temperature dependence demonstrates the second-order nature of the transition at T_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.

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