Observation of chiral spin fluctuations in insulating planar cuprates.

Abstract

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 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (\ensuremath{\sim}5J) and extends beyond 8000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (1 eV).