LOW-FREQUENCY FLUCTUATIONS IN S = 1 HEISENBERG ANTIFERROMAGNETIC CHAINS - NUCLEAR-SPIN RELAXATION IN HIGH-FIELD IN NI(C2H8N2)2NO2(CLO4) (NENP)

The proton relaxation rate l/Tl is measured in NENP as a function of both temperature (2 ≤ T ≤ 40 K) and field (1 ≤ H ≤ 8 T). A quantitative analysis of l/T1 is given, which confirms a recent theory by Affleck describing the effects of a field on the excitation spectrum of a Haldane spin system. It is also shown that the application of a field results in a slowing-down of the fluctuations and that an uanomalym occurs when the nonmagnetic-magnetic transition is approached. This anomaly is well interpreted by assuming a softening of the quantum energy gap. © 1990 IOP Publishing Ltd.