Selective probing of chiral magnon modes in K₂Co(SeO₃)₂ using circularly polarized terahertz time-domain spectroscopy

Geometrical frustration in two-dimensional antiferromagnets is a key ingredient for realizing exotic quantum phases. K₂Co(SeO₃)₂ exhibits a unique magnetic state due to its strong easy-axis anisotropy in the triangular structure. Since this material was first reported in 2020^[1], several studies have suggested it as a candidate for a spin supersolid^[2]. Previous studies have reported that this material enters the up-up-down (UUD) phase under an applied magnetic field close to 0.75 T parallel to the easy axis (the c axis). In this study, we investigated its magnetic excitations using circularly-polarized terahertz time-domain spectroscopy (THz-TDS) at 1.5 K under magnetic fields up to 7 T. Beyond 1 T, we observed two distinct magnon modes, one shifting to higher frequencies (blue shift) and the other to lower frequencies (red shift). Circular polarization measurements with a quarter-wave plate confirmed that these modes possess opposite handedness. This disparate response to circularly polarization provides evidence of their intrinsic chirality. Our observation establishes an effective means to selectively probe chiral magnon modes.

[1] Ruidan Zhong, et al., Phys. Rev. Materials 4, 084406 (2020)

[2] Tong Chen, et al., arXiv:2402.15869 (2024)