NMR study of naturally occurring ¹³C in the Haldane-gap material Ni(C₂H₈N₂)₂NO₂ClO₄

The spin-lattice relaxation time T₁ of naturally occurring ¹³C in Ni(C₂H₈N₂)₂NO₂ClO₄, 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 T₁ 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.