Dynamics of spin fluctuations in lanthanum cuprate

Raman scattering studies of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ have revealed a high-frequency component (\ensuremath{\sim}3000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) which we interpret as due to scattering by magnon pairs. Comparison of the spectra with those of ${\mathrm{K}}_{2}$${\mathrm{NiF}}_{4}$ leads to a value of J\ensuremath{\sim}1100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the exchange interaction. The two-dimensional nature of the fluctuations suggested by earlier neutron scattering work is indicated here by the insensitivity of the spectra to temperature as well as to departures from stoichiometry which strongly influence the three-dimensional ordering temperature ${T}_{N}$.