Line-shape predictions via Bethe ansatz for the one-dimensional spin-1/2 Heisenberg antiferromagnet in a magnetic field

The spin fluctuations parallel to the external magnetic field in the ground state of the one-dimensional (1D) s=1/2 Heisenberg antiferromngnet are dominated by a two-parameter set of collective excitations. In a cyclic chain of N sites and magnetization 0 < M-z < N/2, the ground state, which contains 2M(z) spinons, is reconfigured as the physical vacuum for a different species of quasiparticles, identifiable in the framework of the coordinate Bethe ansatz by characteristic configurations of Bethe quantum numbers. The dynamically dominant excitations are found to be scattering states of two such quasiparticles. For N->infinity these collective excitations form a continuum in (q,omega) space with an incommensurate soft mode. Their matrix elements in the dynamic spin structure factor S-zz(q,omega) are calculated directly from the Bethe wave functions for finite N. The resulting lineshape predictions N->infinity complement the exact results previously derived via algebraic analysis for the exact two-spinon part of S-zz(q,omega) in the zero-field limit. They are relevant for neutron-scattering experiments on quasi-1D antiferromagnetic compounds in a strong magnetic field.