THz Magnetoelectric Atomic Rotations in the Chiral Compound ${\mathrm{Ba}}_{3}{\mathrm{NbFe}}_{3}{\mathrm{Si}}_{2}{\mathbf{O}}_{14}$

THz Magnetoelectric Atomic Rotations in the Chiral Compound ${Ba}_{3} {NbFe}_{3} {Si}_{2} O_{14}$

L. Chaix, S. de Brion, F. Lévy-Bertrand, V. Simonet, R. Ballou, B. Canals, P. Lejay, J. B. Brubach, G. Creff, F. Willaert, P. Roy, and A. Cano

Phys. Rev. Lett. 110, 157208 – Published 12 April 2013

Abstract

We have determined the terahertz spectrum of the chiral langasite ${Ba}_{3} {NbFe}_{3} {Si}_{2} O_{14}$ 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.