Hidden order in hexagonal RMnO$_3$ multiferroics (R = Dy–Lu, In, Y, and Sc)
Résumé
Hexagonal RMnO$_3$ manganites are improper ferroelectrics in which the electric polarization is a by-product of the tripling of the unit cell. In YMnO$_3$, there is a second transition at ~ 920K whose nature remains unexplained. We argue that this transition can be seen as a sort of hidden order in which a residual symmetry displayed by the trimerization order parameter is spontaneously broken. This additional order gives rise to twelve structural domains instead of six, and structural domain boundaries that can be either ferroelectric or non-ferroelectric domain walls. We also suggest a generic P6$_3$cm ↔ P3c1 ↔ P$\bar3$c1 phase diagram, fully realized in InMnO$_3$
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