New collective modes in multiferroic TbMnO3: The electromagnons

Using a phenomenological Landau theory as well as equations of motion, we look for new excitations in the multiferroic TbMnO3. Coupling these excitations to an electromagnetic field, we derived the magneto–polariton dispersion relation. Further, the frequency as well as temperature dependent dielectric susceptibility, and hence the refractive index, are obtained in this formulation. The imaginary part of the latter is computed and compared with the existing experimental data.