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The ESR technique experimentally confirmed the Valence Bond Solid picture
of the Haldane ground state [89,90]. In a Cu
doped NENP, Hagiwara et al.  observed the ESR
signal from the `three S=1/2 spin cluster', which corresponds to one
Cu2+ atomic moment and the two `chain-end S=1/2 spins' (see
Fig.35d). A similar experiment confirmed the
chain-end S=1/2 spins with a non-magnetic ion (Zn, Cd and Hg) doping
to NENP .
Another ESR measurement observed a transition within the triplet
excited states [91,93]. Date and Kindo
 proposed, from the sign of the single
ion anisotropy parameter D, that the excited state of NENP is a
local excitation which runs quickly along the chain.