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 (J(2)/J(1))(14)] permit one to fix the parameters of SrCu2(BO3)(2) precisely: J(1) = 6.16(10) meV, x := J(2)/J(1) = 0.603(3), J(perpendicular to) = 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.