FROM ONE TO 2 DIMENSIONS IN QUANTUM SPIN SYSTEMS

We study the first derivative of the staggered magnetization squared d m(tau)(theta)(2)/d theta and the second derivative d(2)e(0)(theta)/d t heta(2) of the ground-state energy per site. The parameter theta contr ols the anisotropy between horizontal and vertical couplings in a two- dimensional (2D) spin-1/2 antiferromagnetic Heisenberg model. It is sh own that both derivatives diverge at theta=1, where the anisotropic 2D model reduces to the 1D model. [S0163-1829(98)04402-6].