Light scattering from quantum spin fluctuations in R2CuO4 (R=La, Nd, Sm).

Abstract

Raman spectra reveal spin fluctuation scattering over the energy range 0--8000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (0--1 eV) in single crystal ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ (T phase), ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$, and ${\mathrm{Sm}}_{2}$${\mathrm{CuO}}_{4}$ (T' phase). From the ${B}_{1g}$ symmetry spectra, values of the exchange parameter J for these crystals are determined to be 1030\ifmmode\pm\else\textpm\fi{}50, 871\ifmmode\pm\else\textpm\fi{}50, and 888\ifmmode\pm\else\textpm\fi{}50 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. All three materials also exhibit broad spectral features of ${A}_{1g}$ and ${B}_{2g}$ symmetry, which are forbidden within the classical spin-pair excitation model, but allowed by quantum fluctuations. The line shapes for spectra of all three symmetries agree with recent model calculations for the planar quantum antiferromagnet. While the spectra for crystals with the T' structure are nearly identical, subtle differences from the spectra of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ (T structure) indicate a sensitivity to the structure outside of the ${\mathrm{CuO}}_{2}$ planes.