Neutron scattering studies in the linear-chain Haldane materials, NDMAZ and NDMAP

Neutron scattering experiments were carried out in order to investigate the field-induced. antiferromagnetic long-range order and spin dynamics for th e new Haldane compounds Ni(C5H14N2)(2)N-3 (ClO4) (NDMAZ) and Ni(C5H14N2)(2) N-3 (PF6) (NDMAP). We observed the energy gaps corresponding to the excited triplet, Delta (chi) = 1.61 meV, Delta (gamma) = 1.76 meV, Delta (z) = 3.2 meV for NDMAZ and Delta (chi), = 0.48 meV, Delta (gamma), = 0.64 meV for N DMAP. The dispersion in NDMAZ was well explained by the theoretical curve w ith the intra-chain interaction J = 50 K. The field dependence of the gap e nergy below critical field (H-c), was observed for NDMAZ and NDMAP as apply ing the field perpendicular to the chain direction. The field dependence of the excitation levels in NDMAZ was well explained by the fermionic field-t heory treatment of the one-dimensional Heisenberg antiferromagnetic Hamilto nian with these gap energies and g-value, 2.3. In NDMAP, we found that the excitation peaks at Q = (0 0 0.5) became unobservably weak above H-c. This result would indicate a level crossing between the ground and the excited l evel at H-c and the change of the ground state above H-c. No trace of the m agnetic Bragg peak due to the field induced ordering could be observed in t he present study with our non-deuterated single crystal sample.