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

TL;DR

This study uses Cu K-edge RIXS to observe a spin-conserving two-spinon continuum in Sr$_2$CuO$_3$, revealing fractionalized magnetic excitations with a distinct momentum-dependent intensity pattern.

Contribution

First direct observation of a spin-conserving two-spinon continuum in a 1D antiferromagnetic chain using Cu K-edge RIXS, highlighting fractionalization phenomena.

Findings

Two-spinon continuum observed below charge-transfer gap.

Intensity peaks at the midpoint of the Brillouin zone.

Behavior differs from spin-flip excitations seen in neutron scattering.

Abstract

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