Observation ofS=1/2 degrees of freedom in anS=1 linear-chain Heisenberg antiferromagnet

THe ground state of the typical spin-1 linear-chain Heisenberg antiferromagnet Ni(${\mathrm{C}}_2$ ${\mathrm{H}}_8$ ${\mathrm{N}}_2$ ${)}_2$ ${\mathrm{NO}}_2$(${\mathrm{ClO}}_4$) containing a small amount of ${\mathrm{Cu}}^{2+}$ 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 ${\mathrm{Cu}}^{2+}$ sites resulting in spin-1/2 states at the ${\mathrm{Ni}}^{2+}$ sites neighboring the ${\mathrm{Cu}}^{2+}$. Thus the present study gives experimental evidence for the existence of the valence-bond-solid ground state in S=1 linear-chain Heisenberg antiferromagnets.