We report a Cu $K$-edge resonant inelastic x-ray scattering (RIXS) study of spin excitations in the $S=1/2$ antiferromagnetic chain system ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$. The spectral weight observed below the charge-transfer gap appears in two-spinon continuum, indicating the fractionalization of a spin-conserving ($\mathrm{\Delta }S=0$) 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 ($\mathrm{\Delta }S=1$) excitations typically observed via inelastic neutron scattering or Cu $L_3$-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.

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• Spin dynamics
• 1-dimensional spin chains
• Oxides
• Exact diagonalization
• Resonant inelastic x-ray scattering