Abstract
The magnetic excitations in La2CuO4 and La1.9Sr0.1CuO4 were studied by inelastic neutron scattering up to energies of 76 meV. For the pure sample, the results forE≧15 meV can be well described by conventional spin wave theory with a spin wave velocity ħc=0.89±0.07 eVA. For lower energies, the observed intensities were somewhat higher than expected from spin wave theory and did not follow the Bose-Einstein factor. For the doped sample, the linewidth in constant-E scans at smallE shows a very short correlation length of ξ=7.5 A only, which is considerably below the value expected from the concentration dependence reported by other authors. An increase of the linewidth for largeE indicates a reduced spin-wave stiffness when compared to the undoped material. However, the use of a spin-wave picture may not be appropriate as the standard expression for damped spin waves in a paramagnetic material is in serious conflict with experiment.
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