Theory of Magnetoelectric Resonance in Two-Dimensional S=3/2 Antiferromagnet Ba2CoGe2O7 via Spin-Dependent Metal-Ligand Hybridization Mechanism

We investigate magnetic excitations in Ba2CoGe2O7. In terahertz absorption experiments of the compound, novel magnetic excitations, i.e., conventional magnetic resonance at 2 meV as well as both electric-and magnetic-active excitation at 4 meV, have been observed. These magnetic excitations can be explained naturally in an S = 3/2 Heisenberg model on a square lattice with uniaxial anisotropy and Dzyaloshinsky-Moriya terms. We also indicate that, via the spin-dependent metal-ligand hybridization mechanism, the 4 meV excitation is electric-active because of the coupling between the spin and electric dipole. Moreover, at the 4 meV excitation, an interference between magnetic and electric responses emerges as a cross-correlated effect. Such cross-correlation effects explain the nonreciprocal linear directional dichroism observed in Ba2CoGe2O7.