HIGH-FIELD ESR IN THE S=1 HEISENBERG ANTIFERROMAGNETIC CHAINS OF NENP

ESR studies of the excitation spectrum of Haldane spin systems are presented. We performed temperature-dependent high-held and high-frequency ESR measurements up to 21T and 1042GHz on the compound NENP [Ni(C2H8N2)(2)NO2ClO4]. Based on extensive experimental investigations we propose to distinguish between different classes of resonances: first, the transitions between excited states at the boundary of the Brillouin zone (q = pi) below the critical field; second, the high-frequency transitions from the ground state (q = 0) above the critical field and third, the low-frequency resonances at q = 0 in the whole field range. New results are presented in the high-held phase, which exhibit an appreciable temperature dependence of the resonance field and at low temperatures show the existence of multiple high-frequency excitation branches. In the low-frequency regime we also observe an additional ground state excitation.