title: Dispersion and symmetry of bound states in the shastry-sutherland model
abstract: 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( perpendicular) = 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.
authors.affiliation: Institut fur Theoretische Physik, Universitat zu Koln, D-50937 Koln, Germany.
authors.initials: C
authors.lastname: Knetter
authors.initials: A
authors.lastname: Buhler
authors.initials: E
authors.lastname: Muller-Hartmann
authors.initials: GS
authors.lastname: Uhrig