Observation of S=1/2 degrees of freedom in an S=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.