In a classical antiferromagnet, the magnetic excitations, so called spin waves, correspond to a precession of the magnetic moments around their equilibrium position. As a result, a well-defined dispersion curve of transverse excitations is observed. Interestingly, the nature of the spin excitations is completely different in isotropic spin chains, where the exchange interaction is strong in one direction of space and negligible in the two other ones: The spin excitation spectrum consists of a continuum of transverse spinons, created or destroyed in pairs, similarly to domain walls in an Ising antiferromagnet. In presence of sizable inter-chain interaction, a Néel antiferromagnetic ordering can occur in such a system, and an appealing issue is to study how this ordering affects the spin dynamics. In this context, we have explored the magnetic excitations emerging from the Néel phase of the chain compound BaCo₂V₂O₈, showing a moderate Ising anisotropy along with sizable interchain interactions. 

An inelastic neutron scattering experiment has been conducted in the Néel phase of a BaCo₂V₂O₈ single crystal on the three axis spectrometer IN12 at Institut Laue Langevin ^[Reference]. Discrete excitations were found, instead of a continuum, composed of two interlaced series of long-lived modes polarized perpendicular (transverse) and parallel (longitudinal) to the direction of the static antiferromagnetic ordered magnetic moments (Figure). This discretization of the excitations, referred to as Zeeman ladders, is attributed to a confinement of spinons in an attractive linear potential, due to the sizable interchain interaction. The observation, for the first time, of such well-defined longitudinal excitations is ascribed to the presence of a moderate Ising anisotropy, in addition to the sizable interchain interaction. We now plan to study the evolution of the excitation spectrum of BaCo₂V₂O₈ in an applied magnetic field, where a new type of magnetic ordering is induced.