Observation of spin-conserving two-spinon continuum in the S = 1 2 antiferromagnetic chain system Sr 2 CuO 3 using Cu K -edge resonant inelastic x-ray scattering

We report a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of spin excitations in the S=1/2 antiferromagnetic chain system Sr₂CuO₃. The spectral weight observed below the charge-transfer gap appears in two-spinon continuum, indicating the fractionalization of a spin-conserving ($ΔS = 0$) magnetic excitation into two-spinon states. The intensity of these excitations reaches a maximum near the midpoint between the center and the boundary of the Brillouin zone, and decreases toward the zone boundary; this behavior contrasts with that of the spin-flip ($ΔS = 1$) excitations typically observed via inelastic neutron scattering or Cu L₃-edge RIXS. The momentum dependence of the intensity is described by the spin-exchange dynamical structure factor. A phenomenological analysis of the symmetry between the polarization and the d orbital explains the resonance condition for the two-spinon excitations.