Theory of magnetoelectric resonance in two-dimensional $S=3/2$ antiferromagnet ${\rm Ba_2CoGe_2O_7}$ via spin-dependent metal-ligand hybridization mechanism

TL;DR

This paper models magnetic excitations in a 2D $S=3/2$ antiferromagnet, explaining experimental terahertz absorption features and revealing electric-active modes due to spin-dependent hybridization, including cross-correlation effects.

Contribution

It introduces a theoretical framework explaining magnetic and electric excitations in ${ m Ba_2CoGe_2O_7}$, highlighting the role of spin-dependent hybridization and anisotropy.

Findings

Identification of magnetic resonance modes at 2 meV and 4 meV.

Explanation of electric-active mode via spin-dependent hybridization.

Observation of non-reciprocal linear dichroism due to cross-correlation effects.

Abstract

We investigate magnetic excitations in an $S=3/2$ Heisenberg model representing two-dimensional antiferromagnet ${\rm Ba_2CoGe_2O_7}$. In terahertz absorption experiment of the compound, Goldstone mode as well as novel magnetic excitations, conventional magnetic resonance at 2 meV and both electric- and magnetic-active excitation at 4 meV, have been observed. By introducing a hard uniaxial anisotropy term $\Lambda (S^z)^2$, three modes can be explained naturally. We also indicate that, via the spin-dependent metal-ligand hybridization mechanism, the 4 meV excitation is an electric-active mode through the coupling between spin and electric-dipole. Moreover, at 4 meV excitation, an interference between magnetic and electric responses emerges as a cross correlated effect. Such cross correlation effects explain the non-reciprocal linear directional dichroism observed in ${\rm Ba_2CoGe_2O_7}$.