Multilayer S=1/2 Heisenberg antiferromagnet

The multilayer S = 1/2 square lattice Heisenberg antiferromagnet with up to six layers is studied using various series expansions. For systems with an odd number of coupled planes, the ground-state energy, staggered magnetiza tion, and triplet excitation spectra are calculated using two different Isi ng expansions. The systems are found to have long-range Neel order and gapl ess excitations for all ratios of interlayer to intralayer couplings, as in the case of the single-layer system. For the systems with an even number o f coupled planes there is a second-order transition point, as the coupling ratio is varied, separating the gapless Neel phase and gapped quantum disor dered spin-liquid phase, and the critical points an located via expansions in the interlayer exchange coupling. This transition point is found to vary approximately as the inverse of the number of layers. The triplet excitati on spectra are also computed, and at the critical point the normalized spec tra appear to follow a universal function, independent of the number of lay ers. [S0163-1829(99)00801-2].