OBSERVATION OF FRACTIONAL SPIN DEGREES OF FREEDOM IN AN S=1 LINEAR CHAIN HEISENBERG ANTIFERROMAGNET

We have studied the typical spin one linear chain Heisenberg antiferromagnet, Ni ( C 2 H 8 N 2)2 NO 2( ClO 4) containing a small amount of Cu2+ by electron spin resonance (ESR) technique. The ESR results are successfully explained by the model that the valence bonds in the valence bond solid state are broken at the Cu 2+ sites resulting in fractional spin degrees of freedom at the Ni 2+ sites neighboring the Cu 2+.