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Electron spin resonance (ESR) [#!HagiwaraPRL90!#,#!GlarumPRL91!#,#!DatePRL90!#,#!LuPRL91!#,#!BrunelPRL92!#]

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. [89] 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 [90].

Another ESR measurement observed a transition within the triplet excited states [91,93]. Date and Kindo [91] 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.