This Letter presents the fine structure of energy levels for the edge states of a Haldane chain. In order to investigate the edge states, we have performed high field and multifrequency electron spin resonance (ESR) measurements of finite length S=1 antiferromagnetic chains in Y2BaNi0.96Mg0.04O5. Owing to the high spectral resolution by high fields and high frequencies, observed ESR signals can be separated into the contributions of the finite chains with various chain lengths. Our results clearly show that the edge spins actually interact with each other through the quantum spin chain and the interaction depends on the chain length N. This N dependence has been obtained experimentally for the first time, and shows that the correlation length xi in the real system is somewhat larger than that calculated by a simple Heisenberg model.

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High field ESR measurements of powder and aligned samples of MCuP2O7 (M = Sr, Pb) have been performed in the temperature range from 1.7 K to 265 K in the frequency range from 40 GHz to 370 GHz. A typical powder absorption line of Cu 2+ is observed in the powder sample. In the higher temperature region, g and g ⊥ are estimated to be 2.40 and 2.10 for SrCuP2O7 and 2.41 and 2.09 for PbCuP2O7, respectively. Both g and g ⊥ increase with decreasing temperature below 7 K. We have found that the shift direction is inconsistent with the ESR theory of Nagata-Tazuke for the one dimensional antiferromagnet. We have also found that the shifts have frequency dependence.