Two-Triplet-Dimer Excitation Spectra in the Shastry-Sutherland Model for SrCu_{<b> 2</b>}(BO_{<b> 3</b>})_{<b> 2</b>}

Abstract

By using the perturbation expansion up to the fifth order, we study the\ntwo-triplet-dimer excitation spectra in the Shastry-Sutherland model, where the\nlocalized nature of a triplet-dimer, the propagation of a triplet-dimer pair by\nthe correlated hopping and the long-range interactions between triplet-dimers\nplay an essential role. It is found that the dispersion relations for\nfirst-neighbor triplet-dimer pair excitations with S=1 and p-type symmetry\nqualitatively explain the second-lowest branch observed in the neutron\ninelastic scattering experiment. It is also predicted that the second-lowest\nbranch consists of two components, p_x- and p_y-states, with slightly different\nexcitation energies. The origin of the singlet mode at 3.7meV observed in the\nRaman scattering experiment is also discussed.\n