TRANSVERSE-SPIN CORRELATIONS AND SINGLE-MODE APPROXIMATION FOR THE SQUARE-LATTICE S = 1/2 HEISENBERG-MODEL

We calculate the equal-time transverse-spin correlations at zero temperature for the nearest-neighbor square-lattice spin-1/2 Heisenberg antiferromagnet by expansions around the Ising limit. To a good approximation, these correlations are proportional over the entire Brillouin zone, to the spin-wave result. If we use the single-mode approximation to estimate the spin-wave dispersion, we find linear gapless modes near both q=(0,0) and q=(pi,pi), with spin-wave velocity approximately 1.4 times the spin-wave answer of square-root 2J. This is larger than previous estimates of Z(c)=1.18+/-0.02, reflecting the mixing of higher magnons.