NMR-STUDY OF NATURALLY-OCCURRING C-13 IN THE HALDANE-GAP MATERIAL NI(C2H8N2)(2)NO2CLO4

The spin-lattice relaxation time T-1 of naturally occurring C-13 in Ni (C2H8N2)(2)NO2ClO4, a Haldane-gap system commonly known as NENP, has b een measured as a function of temperature (T>4 K), magnetic field (H=3 .46 and 7.56 T), and crystal orientation [a, b (the Ni chains), and c axes]. Using the established magnetic-field dependence of the gap and the materials parameters known for NENP, the general trends of the T-1 data can be reproduced by standard analysis with the assumption that the relaxation is governed by the processes at q=pi. Most importantly, an unusual feature in the H(parallel to b)=7.56 T data can only be ex plained by using a temperature-dependent gap.