Quantum Monte Carlo investigation of the magnetic properties of weakly interacting antiferromagnetic chains with an alternating exchange interaction with spin S=1/2

An approximation dependence of the spontaneous magnetic moment at a site, s igma/sigma(0) - 1 = 0.71(6)delta(2.5(2)), and the antiferromagnet-singlet s tate phase boundary, J(2)/J(1) = 0.52(3)delta, are determined by the quantu m Monte Carlo method in the self-consistent sublattice molecular field appr oximation for weakly interacting (J(2)) antiferromagnetic chains with spin S = 1/2 and alternating exchange interaction (J(1) +/- delta). The Neel tem perature and a number of critical temperatures which could be related with the filling energy of two singlets (Delta S-z = 0) and one triplet (Delta S -z= 1) spin bands, each of which is split by the sublattice field (h(x,y)no t equal h(z)) into two subbands, are determined on the basis of the compute d correlation radii of the two- and four-spin correlation function, the squ ared total spin <(S-z)(2)> with respect to the longitudinal components, the dimerization parameter, and the correlation functions between the nearest neighbors with respect to longitudinal and transverse spin components. On t he basis of the Monte Carlo calculations, the critical temperatures and pos sible energy gaps at the band center are determined for the antiferromagnet s CuWO4 and Bi2CuO4 and for the singlet compounds (VO)(2)P2O7 and CuGeO3 , agreeing satisfactorily with existing results, and new effects are also pre dicted. (C) 2000 MAIK "Nauka / Interperiodica".