Two-particle bound states and one-particle structure factor in a   Heisenberg bilayer system
  The S=1/2 Heisenberg bilayer spin model at zero temperature is studied in the dimerized phase using analytic triplet-wave expansions and dimer series expansions. The occurrence of two-triplon bound states in the S=0 and S=1 channels, and antibound states in the S=2 channel, is predicted by the triplet-wave theory, and confirmed by series expansions. All bound states are found to vanish at or before the critical coupling separating the dimerized phase from the Neel phase. The critical behaviour of the total and single-particle static transverse structure factors is also studied by series, and found to conform with theoretical expectations. The single-particle state dominates the structure factor at all couplings. 
A. Collins, C.J. Hamer