Susceptibility and Dzyaloshinskii-Moriya interaction in the Haldane-gap compound Ni(C 2 H 8 N 2 ) 2 NO 2 (ClO 4 )

The Haldane gap material NENP exhibits anomalies in its Knight shift, far infrared absorption and field-dependent gaps, which have been explained using the staggered $g$-tensor that occurs due to the low crystal symmetry. We point out that the low-temperature susceptibility is also anomalous and that a consistent interpretation of all data may require consideration of the Dzyaloshinskii-Moriya interaction.