On the nonlinear dynamics of the easy-plane antiferromagnetic chain in an external magnetic field

We present a theoretical investigation of soliton excitations in an easy-plane 1d antiferromagnet in an external magnetic field beyond the Sine-Gordon approximation. We find that there exist two types of solitons. One of them depends strongly on the magnetic field, becoming unstable at a critical magnetic field B_c. The other one is stable for all magnetic fields and can be described by a Sine-Gordon model. We discuss the relevance of these excitations for neutron scattering experiments.