Domain walls of relative phase in two-component Bose-Einstein condensates

We consider a system of two interpenetrating Bose-Einstein condensates of atoms in two different hyperfine spin states. We show that in the presence of a small coupling drive between the two spin levels, there exist domain walls across which the relative phase of the two condensates changes by $2\pi .$ We give the physical interpretation of such walls. We show that the wall tension determines the force between certain pairs of vortices at large distances. We also show that the probability of the spontaneous decay of the domain wall is exponentially suppressed, both at finite and at zero temperature, and determine the exponents in the regime of small Rabi frequency. We briefly discuss how such domain walls could be created in future experiments.