NMR study of naturally occurringC13in the Haldane-gap material Ni(C2H8N2)2NO2ClO4

The spin-lattice relaxation time ${\mathrm{T}}_1$ of naturally occurring ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ in Ni(${\mathrm{C}}_2$ ${\mathrm{H}}_8$ ${\mathrm{N}}_2$ ${)}_2$ ${\mathrm{NO}}_2$ ${\mathrm{ClO}}_4$, a Haldane-gap system commonly known as NENP, has been 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 ${\mathrm{T}}_1$ data can be reproduced by standard analysis with the assumption that the relaxation is governed by the processes at q=π. Most importantly, an unusual feature in the H(∥b)=7.56 T data can only be explained by using a temperature-dependent gap.