Raman spectra reveal spin fluctuation scattering over the energy range 08000 cm-1 (01 eV) in single crystal La2CuO4 (T phase), Nd2CuO4, and Sm2CuO4 (T' phase). From the B1g symmetry spectra, values of the exchange parameter J for these crystals are determined to be 1030 50, 871 50, and 888 50 cm-1, respectively. All three materials also exhibit broad spectral features of A1g and B2g symmetry, which are forbidden within the classical spin-pair excitation model, but allowed by quantum fluctuations. The line shapes for spectra of all three symmetries agree with recent model calculations for the planar quantum antiferromagnet. While the spectra for crystals with the T' structure are nearly identical, subtle differences from the spectra of La2CuO4 (T structure) indicate a sensitivity to the structure outside of the CuO2 planes. © 1990 The American Physical Society.

Inelastic light scattering studies of La2CuO4, both pure and doped with lead, over the energy range 01 have revealed the presence of high-frequency A1g scattering components, in the spectral region 35006000 cm-1, while also providing the complete line shape for the previously reported spin-pair mode of B1g symmetry. Two main bands, broad and overlapping, are observed in this region with maxima near 3500 and 5200 cm-1. The various spectral features show different resonance behavior. We suggest that a model based upon coupled spin and charge excitations will be required to describe these spectra. © 1989 The American Physical Society.