Terahertz spectroscopy evidence of an electromagnon in honeycomb Dirac magnet CoTiO₃

In CoTiO₃, the Co²⁺ ions form a honeycomb lattice in the ab plane. Scattering measurements indicate in-plane ferromagnetic interactions and interplane antiferromagnetic interactions. The measured magnon dispersion relation shows nodal lines around the K points, just like the electrons in graphene. This makes this material a potential platform for the existence of topological magnons.

We report on time-domain THz spectroscopy measurements on single crystals of CoTiO₃. By controlling the polarization of the THz wave with respect to the crystal axes, we can determine the nature of the magnetic excitations. At zero magnetic field, we find that the 1.3 THz (~5.4 meV) magnon is excited only by the in-plane electric field of the THz wave, making this magnon an electromagnon. In addition, we determine the zone-center gap via the measurement of the in-plane magnetic-dipole antiferromagnetic resonance at ~230 GHz (~0.9 meV).