Le Jeudi 18 Octobre à 14h
Salle Remy Lemaire, K223
"THz magneto-active vibrations in the chiral compound Ba3NbFe3Si2O14"
Sophie de Brion (Institut Néel)
The langasite Ba3NbFe3Si2O14 displays a chiral structure and orders magnetically with a N-eel 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.