Quantitative Determination of Quantum Fluctuations in the Half- Spin Planar Antiferromagnet.

Frequency moments of the light scattering spectrum from spin- pair excitation are calculated for the spin 1/2 planar Heisenburg antiferromagnet and compared with those observed experimentally for La sub 2 CuO sub 4. The quantitative agreement between the theory and the experiments confirms that the observed linewidth is determined by quantum fluctuations in this system. The theory provides a quantitative means for extracting the microscopic exchange parameter J from the experimental spectrum. The result for La sub 2 CuO sub 4 is J = 930 +- 30 cm (-1).