Exchange mixing and soliton dynamics in the quantum spin chain CsCoCl3

Abstract

The lowest-lying excited states in the S=1/2 one-dimensional Ising-like antiferromagnet ${\mathrm{CsCoCl}}_{3}$ consist of domain-wall (soliton) pair states. Although the dynamical response function S(Q,\ensuremath{\omega}) has been calculated for these states, it has not proved possible to explain the results of neutron scattering and Raman experiments without recourse to the introduction of extra terms in the spin Hamiltonian. We argue against the two modifications to the Hamiltonian of ${\mathrm{CsCoCl}}_{3}$ proposed in previous papers, a staggered field term arising from a mean field approach to exchange mixing, and next-nearest-neighbor intrachain coupling, as being unphysical. Instead we derive a nearest-neighbor effective Hamiltonian, which takes account of the mixing of higher ${\mathrm{Co}}^{2+}$ ion crystal-field levels in a self-consistent manner. We also present a high-resolution inelastic neutron scattering experiment on ${\mathrm{CsCoCl}}_{3}$, which has allowed the dispersion of the excited states across the Brillouin zone to be studied more carefully than before. These results give a direct measure of S(Q,\ensuremath{\omega}) in the one-dimensional phase where there is a continuum of excited states, and in the three-dimensionally ordered phases, where weak interchain interactions split the continuum into a ``Zeeman ladder'' of discrete states. The predictions of our theory are found to be in quantitative agreement with experiment.