INELASTIC-NEUTRON-SCATTERING STUDY OF THE SPIN DYNAMICS IN THE HALDANE-GAP SYSTEM NI(C2H8N2)2NO2CLO4

We have investigated the spin dynamics of the nearly ideal one-dimensi onal S = 1 antiferromagnet Ni(C2H8N2)2NO2ClO4 (NENP) by inelastic neut ron scattering. The measurements have been performed as a function of both the wave vector (in the vicinity of the antiferromagnetic point q = pi) and the magnetic field, for field configurations parallel (up t o 5 T) or perpendicular to the chain axis (up to 10 T). Our experiment al results at low temperature are in good quantitative agreement with most theoretical predictions (both analytical and numerical) establish ed for the spin-1 Haldane-gap system in presence of finite orthorhombi c anisotropy: nonmagnetic singlet ground state, three gaps in the exci tation spectrum corresponding to the excited triplet (DELTA(y)0 almost -equal-to 1.05 meV, DELTA(x)0 almost-equal-to 1.23 meV, DELTA(z)0 almo st-equal-to 2.5 meV), finite correlation lengths at T = 0 (xi(xy)/d al most-equal-to 8, xi(z)/d almost-equal-to 4) and unconventional field d ependencies. The evolution with field of the various modes and their r espective polarization have been determined experimentally and are com pared to recent field-theory treatments of the S = 1 anisotropic antif erromagnetic chain together with numerical calculations on finite-size systems. We present results concerning the field dependencies of the correlation lengths, both in field parallel and perpendicular to the c hain axis, which are not fully understood from the existing theories.