CRITICAL-BEHAVIOR OF COUPLED SPIN CHAINS

We investigate, using numerical computation of the eigenvalues of short chains, the critical behaviour of two composite spin models, which interpolate smoothly between isotropic Heisenberg chains with different values of S. For the first model which interpolates between S = 1/2 and S = 3/2 we find that the model is critical over the whole range and the values of the central charge and critical exponents (scaling dimensions) appear to be constant in the thermodynamic limit. In the second model, which interpolates between S = 1/2 and S = 1 we find that, except at S = 1/2, the central charge is effectively zero, implying a non-critical behaviour.