Fig. 3: RIXS measurements.

A Illustration of RIXS scattering geometry. The scattering plane was perpendicular to the ab plane of CsCr₃Sb₅. The incident and scattered wave vectors of X-rays, i.e., k_in and k_out, are orthogonal. The polarization of incident X-rays was in the scattering plane, i.e., π-polarized, resulting in RIXS with a cross-polarization geometry. The polarization of scattered X-rays was unresolved. The projection of wavevector change q onto the ab plane is denoted as q_∥. B Cr L₃-edge X-ray absorption spectrum (XAS) of CsCr₃Sb₅ recorded at 300 K. Colored vertical bars indicate the X-ray energies used in RIXS measurements. C Incident-energy-dependent RIXS with q_∥ along the ΓM direction at 25 K. Spectra in color were recorded with π-polarized X-rays at selected energies. The RIXS spectrum with σ polarization, i.e., X-ray polarization perpendicular to the scattering plane, shows the instrumental energy resolution of RIXS. D Demonstration of curve fitting for RIXS data analysis. In addition to a linear background, a measured RIXS spectrum was fitted to a spectral profile consisting of three components: one elastic and two electronic excitations. See SM for fitting details. E First Brillouin zone in the a*b* plane of reciprocal space. The red arrow indicates q_∥ of momentum-dependent RIXS measurements. F Momentum-dependent RIXS with q_∥ along ΓK at 25 K. The energy of incident photons was set to 575.2 eV to optimize the shoulder feature. The dashed lines plot the elastic components; the colored shades indicate spectral profiles arising from spin excitations. G Dispersion of fitted E₀ of two spin excitations as a function of in-plane momentum q_∥.