Soft-phonon-driven orbital order inCaMn7O12

Abstract

We use variable-temperature x-ray thermal diffuse scattering and inelastic scattering to investigate the lattice dynamics in single crystals of multiferroic CaMn7O12 which undergo a series of orbital and magnetic transitions at low temperatures. Upon approaching the charge and orbital ordering temperature To=250 K from above, we observe intense diffuse scattering features and a pronounced optical phonon softening centered around the superstructure reflections of the incommensurately modulated structure. The phonon anomaly appears well above To and continuously increases upon cooling, following a canonical power-law temperature dependence that confirms the transition at To to be of second order and related to a soft-phonon lattice instability. Microscopic mechanisms for the incommensurate charge and orbital ordering based on competing interactions and on momentumdependent electron-phonon coupling could both account for the observed extended momentum width of the phonon softening. Our results highlight the importance of the lattice interactions in the physics of this magnetically induced ferroelectric system.

Soft-phonon driven hexagonal-orthorhombic phase transition in BaVS3

“…Upon further cooling below T S , the intrinsic width increases back. This can be explained by the soft-phonon hardening after the settling of the orthorhombic transition, as observed in other compounds 25,26 . Some electronphonon coupling mechanism could be involved, the study of which lies outside the scope of this work.…”

Effect of chemical pressure on the electronic phase transition in Ca1−xSrxMn7O12 films

“…13,16 An orbital-ordering transition is observed at ∼250 K (T OO ) through scattering and Raman spectroscopy measurements. 17,18 Below 90 K (T N1 ), non-collinear helical magnetic and ferroelectric ordering is observed, 12,14,19 with a second helical magnetic transition at ∼45 K (T N2 ). 38 However, the effect of Sr substitution on the electronic properties remains largely unexplored, specifically at larger Sr concentrations (x > 0.2) and in thin films.…”

Magnetic transitions in CaMn7O12 : Raman observation of spin-phonon couplings

“…On the contrary, these modes don't remain active in the paramagnetic phase and we have not observed paraelectromagnons. A soft mode has also been reported at around 80cm-1 at low temperatures (10 K) 18,19 . Although we didnt report this mode in our study, the signal over noise ratio of our low energy Raman data is not allowing us to exclude with certainty the observation of this mode.…”