Comparison of ground state properties for odd half-integer and integer spin antiferromagnetic Heisenberg chains

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Abstract
Ground state properties of finite anisotropic antiferromagnetic Heisenberg chains are studied for odd half-integer spins S=1/2, 3/2 ... as well as integer spins S=1, 2 .... Finite size scaling analysis of the results clearly distinguishes the half-integer (S=1/2, 3/2) from the integer (S=1) spin situation. It gives strong support to a recent conjecture which postulates that the T=0 phase structure is very different in the two cases. According to this idea there exists a new phase between the planar and the antiferromagnetic region, for integer spins only. This phase, which includes the isotropic point, has a finite energy gap and no long range order.