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THe ground state of the typical spin-1 linear-chain Heisenberg antiferromagnet Ni (C₂H₈N₂) ₂NO₂ (ClO₄) containing a small amount of Cu^2+ is studied by the electron-spin-resonance (ESR) technique. The ESR results are qualitatively explained by the model that the valence bonds are broken at the Cu^2+ sites resulting in spin-1/2 states at the Ni^2+ sites neighboring the Cu^2+. Thus the present study gives experimental evidence for the existence of the valence-bond-solid ground state in S=1 linear-chain Heisenberg antiferromagnets.
Hagiwara et al. (1990) studied this question.