Inhomogeneous States of Nonequilibrium Superconductors: Quasiparticle   Bags and Antiphase Domain Walls
  Nonequilibrium properties of short-coherence-length s-wave superconductors are analyzed in the presence of extrinsic and intrinsic inhomogeneities. In general, the lowest-energy configurations of quasiparticle excitations are topological textures where quasiparticles segregate into antiphase domain walls between superconducting regions whose order-parameter phases differ by $\pi$. Antiphase domain walls can be probed by various experimental techniques, for example, by optical absorption and NMR. At zero temperature, quasiparticles seldom appear as self-trapped bag states. However, for low concentrations of quasiparticles, they may be stabilized in superconductors by extrinsic defects. 
M.I. Salkola and J.R. Schrieffer