QUANTUM DISORDERED STATE WITHOUT FRUSTRATION IN THE DOUBLE-LAYER HEISENBERG-ANTIFERROMAGNET - DIMER EXPANSION AND PROJECTOR MONTE-CARLO STUDY

The quantum disordered state (QDOS) of the spin 1/2 double layer square lattice Heisenberg antiferromagnet is studied. Using the dimer expansion from the limit of the large interlayer coupling J', the staggered susceptibility-chi, the antiferromagnetic structure factor S(pi) and the antiferromagnetic correlation length-xi are calculated up to the 6-th order in the intralayer coupling J. The ratio analysis shows that the QDOS becomes unstable against the Neel ordering at J'/J congruent-to 2.56. The critical exponents are not inconsistent with the universality class of the 3-dimensional classical Heisenberg model, suggesting that our QDOS corresponds to that expected in the 2-dimensional square lattice Heisenberg antiferromagnet with unphysically small spin (< 0.276). The results of the projector Monte Carlo simulation also confirms the dimer expansion results.