Observation of chiral spin fluctuations in insulating planar cuprates

A recent theory of Raman scattering in the Hubbard model predicts scatteirng of A₂ symmetry arising from chiral spin fluctuations. In the doped planar cuprates, chiral spin fluctuations have been speculated to play a central role in establishing the normal-state properties. In addition to spin fluctuation scattering in the B₁, A₁, and B₂ symmetries, we report here the direct observation of dynamic chiral spin fluctuations, appearing in the inelastically scattered light of A₂ symmetry. In Gd₂{CuO}₄$, the ${A}₂$ scattering broadly peaks near 4700 ${cm}^{{{-}}1}(~5J) and extends beyond 8000{cm}^{{{-}}1}$ (1 eV).