SPIN EXCITATIONS IN THE S = 1/2 HEISENBERG ANTIFERROMAGNETIC MODEL ON A SQUARE LATTICE

The authors investigate the properties of the two-dimensional quantum S=1/2 Heisenberg antiferromagnetic model on a square lattice by means of a block approach. The lattice is divided into four spin square blocks and the Hamiltonian is expressed in terms of low-energy block states. In the framework of the mean-field theory based on the 'auxiliary-boson' formalism; they show that the spin excitation spectrum contains one singlet mode with the temperature-dependent gap and three degenerate triplet modes. The asymptotic low-temperature spin-spin correlations are proportional to R-3/2 exp(-R(1/ ξ 1+1/ ξ 2)), where ξ 1=exp(Ā/T), ξ 2=constant and Ā =0.5J. The uniform susceptibility tends to zero in agreement with numerical results; the energy per site is close to -0.57J. Beyond the meanfield theory the authors find gapless collective excitations analogous to the gapless mode in the neutral Fermi gas with attraction. © 1990 IOP Publishing Ltd.