THz magneto-active vibrations in the chiral compound Ba3NbFe3Si2O14

Orateur : Sophie DE BRION (Institut Néel)

The langasite Ba3NbFe3Si2O14 displays a chiral structure and orders magnetically with a Néel temperature TN = 27 K. We have determined its terahertz (THz) spectrum by means of synchrotron radiation measurements in the energy range 8-60 cm-1= 0.3 -2.0 THz. Besides the expected spin wave excitations at 13 cm-1 disappearing at TN, we observe two aditionnal absorption bands centered at 23 cm-1 and 29 cm-1 that persist up to almost 4 times TN . According to their selection rules, we interpret them as twisting modes of the lattice whose magneto-electric activity reveals a structural transition into a polar helical state.

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