BPS solitons with internal structures in a restricted baby Skyrme-Maxwell theory in a magnetic medium

We consider a restricted baby Skyrme-Maxwell scenario enlarged via the inclusion of a nontrivial magnetic permeability. We then proceed with the minimization of its total energy by means of the Bogomol'nyi-Prasad-Sommerfield (BPS) prescription, from which we get that the self-dual potential now depends on the magnetic permeability itself. As a result, we obtain not only the lower bound for the energy, but also the self-dual equations whose solutions saturate that bound. In such a context, we focus our attention on those time-independent gauged skyrmions with radial symmetry and no electric charge. We solve the effective self-dual equations numerically for different choices of the magnetic permeability, from which we obtain BPS magnetic fields whose internal structures form concentric rings. We also explain analytically the formation of these structures based on the values of a single real parameter which characterizes the respective magnetic permeabilities.
Comment: 17 pages, 12 figures, 6 tables. Comments are welcome