Quantum Monte Carlo study of spin 1/2 ± Jz chains

Disorder of anisotropy is introduced into spin 1/2 quantum XXZ chains. This model, which is refered as the spin 1/2 ± Jz chain, is studied by the quantum Monte Carlo method. It displays the critical phenomena of the x−y phase, though the critical exponents γ, η and ν must be corrected. However, the antiferromagnetic alternating spin chains show no critical phenomena. It is found that the susceptibility along the z axis and averaged zz pair spin correlation functions of alternating spin chains show obviously different behavior from those of ±Jz chains.