DIMER STATE IN THE 2-DIMENSIONAL ANISOTROPIC ALTERNATED-EXCHANGE HEISENBERG-MODEL

An analysis is made of the two-dimensional Heisenberg model with S = 1 /2, anisotropic exchange interaction between nearest neighbors, and al ternating exchange in two directions, [100] and [010] (corresponding t o condensation of the (pi,pi) mode) and in one direction [100] (corres ponding to condensation of the (pi,0) mode), The quantum Monte Carlo m ethod is used to calculate the thermodynamic characteristics and the s pin correlation functions which are used as the basis to determine the boundary of stability of an anisotropic antiferromagnetic with respec t to alternation of exchange delta = (1 - J(x,y) / J(z))(0.4) in the ( pi,pi) model and delta = (1 - J(x,y) / J(z))(0.31) in the (pi,0) model . In the (pi,0) model a disordered quantum state exists in the range ( 1 - J(x,y) / J(z))(0.31)< delta < (0.3-0.35). The energy (E - 0.68) = 0.36 delta(1.80(6)) and 0.21 delta(2.0(5)), the energy gap between the ground and excited states H-c(delta)= 1.96 delta(2.(1)), 1.8(1) (delt a - 0.35(3))(0.67(2)) were determined as a function of the alternation of exchange in the (pi,pi)- and (pi,0) models, respectively. (C) 1998 American Institute of Physics.