SPIN DYNAMICS IN THE HALDANE-GAP SYSTEM NENP

In this paper we review recent results on the spin dynamics in zero fi eld of the nearly ideal one-dimensional S = 1 antiferromagnet Ni(C2H8N 2)(2)NO2ClO4 (alias NENP). The experimental results obtained from susc eptibility, nuclear magnetic resonance and inelastic neutron scatterin g experiments are in good quantitative agreement with most theoretical predictions (both analytical and numerical) established for the spin- 1 Haldane-gap system in presence of finite orthorhombic anisotropy: no n-magnetic singlet ground state, three gaps in the excitation spectrum corresponding to the excited triplet (Delta(y)(0) approximate to 1.05 meV, Delta(x)(0) approximate to 1.23 meV, Delta(x)(0) approximate to 2.5 meV) and finite values of the correlation lengths at T = 0 (xi(xy) /d approximate to 8, xi(z)/d approximate to 4).