Raman scattering in orthorhombic multiferroic RMnO3: The role of mixing of lowest harmonics among the components of magnetization

The local inversion symmetry breaking on the center of the Mn–O–Mn bond in the orthorhombically distorted MnO6 octahedra in RMnO3 gives rise to a distortion which produces local polarization and it averages out to 0 due to the symmetry of the ground state. This can be understood through a magnetoelectric coupling term in the free energy expansion which goes as ∼PM2 according to the Pbnm symmetry. The dynamical fluctuation of P→, which is nonzero, couples to the lowest harmonics of the components of M→ yielding a double peak structure in the electromagnon spectrum. The recently observed Raman data in this material can successfully be explained through this scenario.