ZERO-TEMPERATURE SPIN-WAVE SPECTRUM OF THE CRITICAL 2-LAYER S=1 2 ANTIFERROMAGNET/

Series expansions about the interlayer dimer limit are calculated to e ighth order in the intralayer coupling for the T=0 antiferromagnetic s usceptibility and elementary excitation spectrum of the two-layer, S=1 /2, square lattice Heisenberg antiferromagnet. Extrapolations of the s usceptibility and the gap series yield an estimate of the critical int erlayer/intralayer coupling ratio J(2)/J(1)=2.54+/-0.02, in agreement with several previous numerical calculations. The spin-wave velocity a t criticality is determined by an analysis of the excitation spectrum series: the result, HBARc/J(1)a=1.89+/-0.02, is very precise and consi stent with other estimates based on numerical calculations at finite t emperature. The critical spin-wave dispersion at general wave vectors is shown to deviate significantly from a uniformly renormalized mean-f ield spectrum.