QUANTUM MONTE-CARLO STUDY OF SPIN 1 2+/-J(Z) CHAINS/

Disorder of anisotropy is introduced into spin 1/2 quantum XXZ chains, This model, which is refered as the spin 1/2 +/- J(z) chain, is studi ed by the quantum Monte Carlo method. It displays the critical phenome na of the x-y phase, though the critical exponents gamma, eta and nu m ust be corrected. However, the antiferromagnetic alternating spin chai ns show no critical phenomena. It is found that the susceptibility alo ng the z axis and averaged zz pair spin correlation functions of alter nating spin chains show obviously different behavior from those of +/- J(z) chains.