Observation of the Haldane Gap in Y₂BaNiO₅Single Crystal

Magnetization measurement and inelastic neutron scattering have been performed on a single crystal of a new Haldane system Y₂ BaNiO₅. Magnetic susceptibility shows a weak anisotropy and decreases as temperature decreases, reflecting the singlet ground state at low temperature. The energy gaps were found to be Δ_///k_B = 118 K and Δ_⊥/k_B=104 K, parallel and perpendicular to the chain, respectively. Using inelastic neutron scattering magnetic excitations along the chain (a*-) and perpendicular (c*-) directions up to 30 meV energy transfer are studied. Energy gap of about 10 meV (116 K) is observed along the one-dimensional, antiferromagnetic zone center, in good agreement with the magnetization measurement. The observed dispersion relation in the chain direction with the gap can be well described by the exact numerical results of S= 1, Heisenberg antiferromagnetic chain yielding an intrachain exchange of J/k_B=280±10K.