FINITE-CHAIN EFFECTS ON THE SPIN EXCITATIONS IN THE S=1 HALDANE SYSTEM NINO WITH CU IMPURITIES

We measured the spin-excitation spectrum for the quasi-one-dimensional S = 1 antiferromagnet [Ni(C3H10N2)(2)] NO2(ClO4) (NINO) and NINO with nominally 5 at% Cu2+ impurities (NINO : CU2+) by inelastic neutron sc attering. We clearly observed a peak corresponding to an energy gap E( G)(perpendicular to) = 1.2 meV (approximate to 14 K) at q = pi, in bot h single crystals. The low-q spectra are also almost completely identi cal. However, the q dependence of the integrated intensity shows diffe rences between NINO and NINO :Cu2+. The correlation length zeta/d is e stimated to be 6.0 for NINO and 4.4 for NINO:Cu2+, where d is the spac ing between spins in a chain. These results are consistent with the na ture of the localized and disordered state of the Haldane system, whic h is expected to be robust against finite-chain effects.