Unraveling Emergent Magnetic Order in Topological Kagome Metals Using RIXS

Hsiao-Yu Huang^1*

¹Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu, Taiwan

* Presenter:Hsiao-Yu Huang, email:huang.hy@nsrrc.org.tw

Kagome lattice materials are central to the search for topology- and correlation-driven quantum states, featuring Dirac fermions, van Hove singularities, and topological flat bands. Understanding the interplay among strong electron correlations, magnetism, and these unique electronic features is key to revealing emergent orders. Resonant Inelastic X-ray Scattering (RIXS) has emerged as an exceptionally powerful tool for characterizing novel magnetic states. Using high-resolution RIXS at the TPS 41A beamline, we investigated the spin and electronic excitations in two representative kagome magnets: the Cr-based CsCr₃Sb₅ and the Mn-based YMn₆Sn₆. 

In CsCr₃Sb₅, a kagome metal that becomes superconducting under pressure, Cr L₃-edge RIXS in a cross-polarization geometry reveals clear non-dispersive magnetic excitations near the Brillouin-zone center. These excitations are coupled to quasiparticle excitations of flat bands located near the Fermi level. As the temperature decreases below the charge-density-wave transition (T_CDW=54), the flat bands shift by about 20 meV away from Fermi level, accompanied by a corresponding broadening and energy shift of the magnetic excitations—evidence that the low-temperature order involves both structural and magnetic degrees of freedom.

In YMn₆Sn₆, RIXS and DFT+DMFT calculations reveal dual localized and itinerant dd excitations characteristic of a Hund’s metal in an orbital-selective regime. The strong Hund’s exchange stabilizes orbitals directed along Mn–Mn bonds, which remain itinerant, while other Mn 3d orbitals become more localized or exhibit non–Fermi-liquid behavior, naturally explaining the ferromagnetic coupling in Mn bilayers via the double-exchange mechanism.

Together, these studies illustrate how RIXS directly probes the intertwined spin, orbital, and topological dynamics. The results reveal two distinct microscopic pathways that drive emergent magnetic order in kagome metals: one driven by the collective dynamics of topological flat bands (CsCr 3 Sb 5 ), and the other by local, Hund's-coupling-induced orbital differentiation (YMn 6 Sn 6 ).

References:
A. Singh, H.-Y. Huang, Y. Y. Chu et al., Development of the soft X-ray AGM–AGS RIXS beamline
at the Taiwan Photon Source. J. Synchrotron Radiat. 28, 977 (2021).
Z. Wang, Y. Guo, H.-Y. Huang et al., Spin excitations and flat electronic bands in a Cr-based
kagome superconductor. Nat Commun 16, 7573 (2025).
S. V. Streltsov, H.-Y. Huang, A. Ushakov et al., Orbital selectivity stabilized by Hund’s exchange in
kagome magnet YMn 6 Sn 6 . (in preparation)

Keywords: Kagome metal, RIXS, emergent orders, spin excitations , strongly electron correlation