L. Chaix, S. de Brion, F. Lévy-Bertrand, V. Simonet, R. Ballou, B. Canals, and P. Lejay, J.-B. Brubach, G. Greff, F. Willaert, P. Roy, A. Cano
We have determined the terahertz spectrum of the chiral langasite Ba3NbFe3Si2O14 by means of synchrotron-radiation measurements. Two excitations are revealed that are shown to have a different nature. The first one, purely magnetic, is observed at low temperature in the magnetically ordered phase and is assigned to a magnon. The second one persists far into the paramagnetic phase and exhibits both an electric and a magnetic activity at slightly different energies. This magnetoelectric excitation is interpreted in terms of atomic rotations and requires a helical electric polarization.