Excitation Spectrum and T = 0 Dynamics of the One-Dimensional Planar Spin-1/2 Ferromagnet

The T=0 dynamics of the one-dimensional ferromagnet with planar exchange anisotropy is studied by finite-chain calculations and a Green function approach. We demonstrate that the excitation spectrum relevant for appropriate low-T inelastic neutron scattering experiments is much more complex than predicted by linear spin-wave theory. It includes two continua and a set of discrete branches. Some of the low-lying excitations predicted by rigorous calculations, on the other hand, are shown to contribute no spectral weight to the T=0 dynamic structure factor Szz(q,ω). We provide quantitative results for the spectral-weight distribution in Szz(q,ω) at T=0 from bound states and continuum states, including a detailed analysis of the singularity in Szz(q,ω) at the lower band edge. Further evidence is found for the prediction that some T=0 critical properties of the planar ferro- and antiferromagnet are governed by exponents which depend continuously on the planar anisotropy