An efficient valence-bond technique is introduced for the numerical study of singlet states of spin-1 antiferromagnetic chains and is applied to the general bilinear-biquadratic Hamiltonian. The results support the hypothesis that the gap first vanishes, with increasing biquadratic exchange, at the Bethe ansatz integrable point, which separates regular and dimerised regimes. The authors distinguish between real gaps and the mixing of degenerate ground states due to quantum tunnelling in small systems.
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Chang et al. (1989) studied this question.
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