Chiral Magnon Dynamics in a Kitaev Magnet Revealed by Magneto-Optics

Magnetic excitations in quantum materials can acquire chirality when time-reversal or spatial-inversion symmetries are broken, giving rise to nonreciprocal spin dynamics and circular dichroism. Such chiral magnons play a central role in contemporary condensed-matter physics, exist in magnetic, topological, and multiferroic materials, and thereby promote chiral spintronics. The underlying phenomena range from nonreciprocal magnon transport and optical activity to chiral spin textures such as skyrmions. They also include recently identified unconventional magnetic states, such as $g$-wave altermagnets and odd-parity $p$-wave magnets. In this context, bond-dependent magnetic interactions, such as those in Kitaev materials, provide a natural microscopic mechanism that can generate unconventional magnetic textures and anisotropic spin dynamics. While signatures of Kitaev physics have been widely sought in honeycomb magnets, direct experimental evidence for chiral magnons arising from Kitaev exchange remains scarce. Here we show, both experimentally and theoretically, that the zero-field splitting and field-dependent optical handedness of the chiral electromagnons in the helical magnetic order of NiI$_2$, arise from the interplay of Kitaev-type exchange and single-ion anisotropy.