We predict a universal mid-gap bound state in topological superfluids,
induced by either non-magnetic or magnetic impurities in the strong scattering
limit. This universal state is similar to the lowest-energy Caroli-de
Gennes-Martricon bound state in a vortex core, but is bound to localized
impurities. We argue that the observation of such a universal bound state can
be a clear signature for identifying topological superfluids. We theoretically
examine our argument for a spin-orbit coupled ultracold atomic Fermi gas
trapped in a two-dimensional harmonic potential, by performing extensive
self-consistent calculations within the mean-field Bogoliubov-de Gennes theory.
A realistic scenario for observing universal bound state in ultracold 40K
atoms is proposed.Comment: 5 pages + 4 figures; published in PRL; see the relevant study in 1D:
Phys. Rev. A 87, 013622 (2013); see also the accompanying Physics Synopsis:
http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.110.02040
