Abstract
The T=0 dynamics of the one-dimensional S= 1 2 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 S zz(q,ω). We provide quantitative results for the spectral-weight distribution in S zz(q,ω) at T=0 from bound states and continuum states, including a detailed analysis of the singularity in S zz(q,ω) at the lower band edge. Further evidence is found for the prediction that some T=0 critical properties of the planar S= 1 2 ferro- and antiferromagnet are governed by exponents which depend continuously on the planar anisotropy.
Highlights
The static and dynamic properties of one-dimensional (1D) ferromagnets (FM) have been studied extensively for some time, both theoretically and experimentally [1]
A neutron scattering study completed very recently [6] has confirmed that the dynamical properties of the quasi-1D FM CuCl2 · DMSO cannot be understood in terms of the excitations predicted by classical theories
At ∆ = 0 we observe that only the excitations of continuum C carry spectral weight, in agreement with exact analytic results for the XY model [11]
Summary
The static and dynamic properties of one-dimensional (1D) ferromagnets (FM) have been studied extensively for some time, both theoretically and experimentally [1]. At ∆ = 0 we observe that only the excitations of continuum C carry spectral weight (i.e. have Mλz = 0), in agreement with exact analytic results for the XY model [11]. The bound states in branches with even r, on the other hand, are of different origin and have zero spectral weight in Szz(q, ω) at any value of ∆ [12].
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