Electromagnon in TbMnO$_3$ under magnetic field by Raman scattering

Magnetoelectric excitations in the multiferroic TbMnO$_3$ have been investigated by Raman spectroscopy. Our observations reveal electromagnons excitations at 30 cm$^{-1}$ and at 60 cm$^{-1}$ with electric polarization of light parallel to the a axis [1]. When a magnetic field is applied along the c axis, no flop of the spiral plane or polarization is observed but TbMnO$_3$ becomes paraelectric and a simple antiferromagnetic phase is developed. We show that the dipole character of the electromagnons disappears whereas their magnon compound appears immediately when the spins spiral is destabilized with a magnetic field along the c axis. The magnon dispersion curve associated with the electromagnons is preserved before the construction of the magnon dispersion of the simple antiferromagnetic phase at higher magnetic field. The effect of the phase transition on the phonon modes shows that the Mn-O distance is the key that controls the polar character of the electromagnons.\\[4pt] [1] P. Rovillain et al., PRB 81, 054428 (2010)