MODELING A DIMER STATE IN CUGEO3 IN THE 2-DIMENSIONAL ANISOTROPIC HEISENBERG-MODEL WITH ALTERNATED EXCHANGE INTERACTION

The two-dimensional Heisenberg spin-1/2 model with alternated exchange interaction along the c axis and an anisotropic distribution of the e xchange interaction in the lattice, J(b)/J(c)=0.1, is examined. A quan tum Monte Carlo method is used to calculate the phase diagrams of the antiferromagnet, the dimer state in a plane, the value of the alternat ion delta of the exchange interaction, and the anisotropy Delta=1-J(xy )/J(z) of the exchange interaction, Delta similar to delta(0.58(6)). T he following characteristics are calculated for Delta=0.25: the depend ence of the temperature of the dimer-state-paramagnet transition on th e alternation of the exchange interaction, T-c(delta)=0.55(4>(delta-0. 082(6))(0.50(3)), the singlet-triplet energy gap, and the dependence o f the magnetization on the external field for some values of delta. Th e value of the exchange interaction, J(c)=127 K, the alternation of th e exchange interaction, delta=0.11J(c), and the correlation radius alo ng the c axis, xi(c) approximate to 28c, are determined. Finally, it i s found that the temperature dependence of the susceptibility and the specific heat are in good agreement with the experimental data. (C) 19 97 American Institute of Physics. [S1063-7761(97)01912-4].