Spin excitations in an anisotropic bond-alternating quantum S=1 chain in a magnetic field:: Contrast to haldane spin chains -: art. no. 177202

Inelastic neutron scattering experiments on the S=1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9D24N4)(NO2)ClO4 have been performed under magnetic fields below and above a critical field H-c at which the energy gap closes. Normal field dependence of Zeeman splitting of the excited triplet modes below H-c has been observed, but the highest mode is unusually small and smears out with increasing field. This can be explained by an interaction with a low-lying two magnon continuum at q(parallel to)=pi that is present in dimerized chains but absent in uniform ones. Above H-c, we find only one excited mode, in stark contrast with three massive excitations previously observed in the structurally similar Haldane-gap material NDMAP [A. Zheludev , Phys. Rev. B 68, 134438 (2003)].