Bound states of spatial optical dark/gray solitons in nonlocal media
  It is shown that three or more dark/gray solitons can form bound states in nonlocal media. More over dark/gray solitons can form bound states in several balance distances. Numerical simulations indicate that some of such bound states are unstable and will decay into a group of fundamental solitons, while others may be stable. There exist degenerate bound states with the same velocity, Hamiltonian, particle numbers and momentum but decaying in different ways and having different lifetimes. 
Shigen Ouyang, Wei Hu, Qi Guo
physics.optics