CRITICAL-BEHAVIOR OF ANISOTROPIC SPIN-S HEISENBERG-CHAINS

Using a Lanczos method, we study the spectra of Heisenberg chains with spin S = 1, 3/2, 2, 5/2, and 3, as a function of the anisotropy parameter-lambda and the number of sites of the chain. We use periodic, twisted, and open boundary conditions. We found that for all values of the spin when -1 less-than-or-equal-to lambda less-than-or-equal-to 0, these mod-els are massless with critical behavior described by a conformal theory with central charge c = 1. We discuss the whole operator content. In particular, we found that the critical exponent eta(x) follows the behavior eta(x) = (pi-gamma)/2-pi-S and eta(z) = 2 where gamma = cos-1(lambda). For lambda > 0, our results indicate that the S = 1 model is massive, while for S > 1, the massless behavior characterized by the critical exponents described above continues up to lambda = lambda* (S). For lambda* < lambda less-than-or-equal-to 1, our results are consistent with a massive phase for the integer-spin models and a massless phase with a rapid variation of the critical exponents for the half-integer-spin models.