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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=(π,π), with spin-wave velocity approximately 1.4 times the spin-wave answer of √2J. This is larger than previous estimates of Z c =1.18±0.02, reflecting the mixing of higher magnons