Observation of chiral spin fluctuations in insulating planar cuprates

A recent theory of Raman scattering in the Hubbard model predicts scatteirng of ${\mathit{A}}_2$ 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 ${\mathit{B}}_1$, ${\mathit{A}}_1$, and ${\mathit{B}}_2$ symmetries, we report here the direct observation of dynamic chiral spin fluctuations, appearing in the inelastically scattered light of ${\mathit{A}}_2$ symmetry. In ${\mathrm{Gd}}_2$ ${\mathrm{CuO}}_4$, the ${\mathit{A}}_2$ scattering broadly peaks near 4700 ${\mathrm{cm}}^{\mathrm{-}1}$ (∼5J) and extends beyond 8000 ${\mathrm{cm}}^{\mathrm{-}1}$ (1 eV).