The optical spectra in the family of multiferroic manganites $R$MnO${}_3$ is a great puzzle. Current models cannot explain the fact that two strong electromagnons are present in the noncollinear spin cycloidal phase, with only one electromagnon surviving the transition into the collinear spin sinusoidal phase. We show that this is a signature of the presence of anomalous magnetoelectric coupling that breaks rotational invariance in spin space and generates oscillatory polarization in the ground state.

• Received 25 October 2011

DOI:https://doi.org/10.1103/PhysRevB.85.104412