Dispersion and Symmetry of Bound States in the Shastry-Sutherland Model

Bound states made from two triplet excitations on the Shastry-Sutherland lattice are investigated. Based on the perturbative unitary transformation by flow equations quantitative properties like dispersions and qualitative properties like symmetries are determined. The high order results [up to (J₂/J₁)¹⁴] permit one to fix the parameters of SrCu₂(BO₃)₂ precisely: J₁ = 6.16(10)meV, x≔J₂/J₁ = 0.603(3), J_⊥ = 1.3(2)meV. At the border of the magnetic Brillouin zone a general double degeneracy is derived. An unexpected instability in the triplet channel at x = 0.63 indicates a transition towards another phase. The possible nature of this phase is discussed.