A study of anisotropic 2D Heisenberg model with alternating bond and S=1/2 Monte-Carlo method

The two-dimensional Heisenberg model with S = 1/2 and the antiferromagnetic anisotropic exchange interactions between nearest neighbors (1 - J(x,y)/J(z)) and alternating bond (delta) on one of the translation vectors is considered. The thermodynamic characteristics, spin correlation functions are calculated by the quantum Monte-Carlo method. The border of stability of the antiferromagnet ground state delta = (1 - J(x,y)/J(z))(0.31) and the gapless disorder quantum state in the range of (1 - J(x,y)/J(z))(0.31) < delta < (0.3-0.35) is determined. The gap between the ground state and the excited triplet state H-c(delta) = 1.8(1) (delta - 0.35(3))(0.67(2)) is estimated for the isotropic case. (C) 1998 Elsevier Science B.V. All rights reserved.