Conformal anomaly and critical exponents of Heisenberg spin models with half-integer spin

Using a modified Lanczos method we have evaluated the conformal anomaly c of anisotropic Heisenberg models with S=(1/2 and (3/2 in the critical region (0≤λ≤1). For the theory with S=(1/2, we obtain c=1 in the whole range of the parameter λ and we check the expression ${\eta }_z$=1/[1-arccos(λ)/π] for the critical exponent of the $S^z$ correlation function. Studying the model with S=(3/2, the conformal anomaly seems to converge to 1 for 0≤λ≤1 but ${\eta }_z$ does not appear to follow the same qualitative behavior as for S=(1/2. .AE