Instantaneous spin correlations in La2CuO4

We have carried out a neutron-scattering study of the instantaneous spin-sp in correlations in La2CuO4 (T-N = 325 K) over the temperature range 337-824 K. Incident neutron energies varying from 14.7-115 meV have been employed in order to guarantee that the energy integration is carried out properly. The results so obtained for the spin-correlation length as a function of te mperature when expressed in reduced units agree quantitatively both with pr evious results for the two-dimensional (2D) tetragonal material Sr(2)CnO(2) Cl(2), and with quantum Monte Carlo results for the nearest-neighbor square lattice S = 1/2 Heisenberg model. All of the experimental tend numerical r esults for the correlation length are well described without any adjustable parameters by the behavior predicted for the quantum nonlinear sigma model in the low-temperature renormalized classical regime. The amplitude, on th e other hand, deviates subtly from the predicted low-temperature behavior. These results are discussed in the context of recent theory for the 2D quan tum Heisenberg model. [S0163-1829(99)08921-3].