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 (01). For the theory with S= (1/2, we obtain c=1 in the whole range of the parameter and we check the expression ₙ=1/1-arccos () / 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 01 but ₙ does not appear to follow the same qualitative behavior as for S= (1/2. . AE