EXPERIMENTS ON HALDANE GAP IN QUASI-ONE-DIMENSIONAL ANTIFERROMAGNETS

We review recent experimental works on several linear chain compounds which approximate the ideal one-dimensional Heisenberg antiferromagnet with S = 1. They include Ni(C2H8N2)2NO2ClO4 (NENP) and two other Ni (II) compounds with similar structure, AgVP2S6 and a new promising Ni (II) chain Ni(NO2)3 N(CH3)4 (TMNIN). All these compounds exhibit the low temperature susceptibility decrease and the absence of 3D long range magnetic order which are characteristic of the Haldane gap. We especially discuss the break of the gap induced by large applied fields in NENP and TMNIN and the effect of copper impurities introduced in the chains of NENP.