Spinon confinement and field-induced transition in a quasi-one-dimensional Ising-Heisenberg antiferromagnet*

We study a quasi-one-dimensional antiferromagnet with Ising anisotropy. Low-energy excitations in this system are spinons, which are confined by the interchain interaction and have a discrete excitation spectrum. We calculate the dynamical susceptibility considering interchain effects by a mean field theory. The result shows a discrete dispersion relation and agrees well with inelastic neutron scattering (INS) experiments on the compound BaCo₂V₂O₈.
We also discuss an effect of external magnetic field in such an anisotropic magnet. The field along the anisotropy axis causes Zeeman splitting in transverse component of the dynamical susceptibility. If the field is perpendicular to the anisotropy axis, the results depend drastically on the field direction in the xy plane. Since BaCo₂V₂O₈ the nondiagonal g-tensor, the application of uniform field provokes an effective staggered field. This leads to a quantum phase transition described through a dual-field double sine-Gordon model in terms of a bosonized effective field theory. We study such a transition using both field theory and quantitative analysis based on iTEBD. We compare the results with polarized INS experiments on BaCo₂V₂O₈.