Spin correlations in the 2D Heisenberg antiferromagnet ${\mathrm{Sr}}_2$${\mathrm{CuO}}_2$${\mathrm{Cl}}_2$: Neutron scattering, Monte Carlo simulation, and theory

We report a neutron scattering study of the spin correlations in the model 2D, S=1/2, square-lattice Heisenberg antiferromagnetic ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_2$ ${\mathrm{Cl}}_2$. The spin correlation lengths obtained agree quantitatively with values deduced from Monte Carlo simulations over a wide range of temperature. The combined data, which cover the length scale from 1 to 200 lattice constants, are predicted accurately with no adjustable parameters by renormalized classical theory for the quantum nonlinear sigma model.