Two-triplet-dimer excitation spectra in the Shastry-Sutherland model for SrCu2(BO3)2

By using the perturbation expansion up to the fifth order, we study the two-triplet-dimer (TD) excitation spectra in the Shastry-Sutherland model, where the localized nature of a TD, the propagation of a TD pair by the correlated hopping and the long-range interactions between TD's play an essential role. It is found that the dispersion relations for first-neighbor TD pair excitations with S = 1 and p-type symmetry qualitatively explain the second-lowest branch observed in the neutron inelastic scattering experiment. It is also predicted that the second-lowest branch consists of two components, p(x)- and p(y)-states, with slightly different excitation energies. The origin of the singlet mode at 3.7 meV observed in the Raman scattering experiment is also discussed.