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 persits 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.
@article{arxiv.1303.1706,
title = {THz Magneto-electric atomic rotations in the chiral compound Ba$_3$NbFe$_3$Si$_2$O$_{14}$},
author = {Laura Chaix and Sophie De Brion and Florence Levy-Bertrand and Virginie Simonet and Rafik Ballou and Benjamin Canals and Pascal Lejay and Jean-Blaise Brubach and Gaëlle Greff and Fabrice Willaert and Pascale Roy and Andres Cano},
journal= {arXiv preprint arXiv:1303.1706},
year = {2013}
}