We demonstrate a symmetry-tied circular dichroism (CD) in resonant inelastic X-ray scattering (RIXS) at the magnon feature of the altermagnet CrSb. By comparing right- and left-circular polarization while rotating the crystal, we observe a robust fingerprint: the CD flips sign at opposite crystal momenta and follows a three-fold azimuthal modulation set by crystal symmetry; switching between magnetic domains reverses the signal consistently with the Néel-vector orientation. Theory shows the chiral imprint survives even when two magnon modes overlap, and circular polarization provides opposite dichroic contrast for the nearly degenerate branches—enabling detection even when the energy splitting cannot be resolved directly. The measurements run at room temperature and zero field (I21 RIXS, Diamond). The approach offers a practical, symmetry-aware readout to identify altermagnets and map where and how magnon chirality emerges across materials.

References:

• Nikolaos Biniskos, Manuel dos Santos Dias, Stefano Agrestini, David Sviták, Ke-Jin Zhou, Jiří Pospíšil, Petr Čermák, “Systematic Mapping of Altermagnetic Magnons by Resonant Inelastic X-ray Circular Dichroism,” Nature Communications (2025) accepted for publication. Link to Arxiv: https://doi.org/10.48550/arXiv.2503.02533