Motivated by recent terahertz (THz) spectroscopy measurements inα -RuCl3 , we develop a theory for magnetoelectric (ME) effects in Mott insulators ofd5 transition metal ions in an octahedral crystal field. For4d and5d compounds, the relatively wide-spread orbitals favor charge fluctuations of localized electrons to neighboring ions and a significant ME effect from electronic mechanisms is expected. From a three-orbital Hubbard model with strong spin-orbit coupling, we derive the mechanisms for the electric polarization originating from virtual hopping of the localized holes carrying the spins. We consider the electric polarization generated by pairs of spin operators on nearest-neighbor bonds with either an edge-sharing geometry (i.e., two ligands are shared) or a corner-sharing geometry (i.e., one ligand is shared). The allowed couplings are first derived using a symmetry approach. Then, we explicitly calculate the coupling constants and evaluate the effective polarization operator in the ground state manifold using perturbation theory. The results are relevant when considering the THz optical conductivity of magnetic systems such as some perovskite iridates or Kitaev materials. In particular, they help explain the recent THz optical measurements ofα -RuCl3 for which the electric-dipole-induced contribution has been shown to be strong.
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Pages- Terahertz Imaging & Detection
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