Symmetries and triplet dispersion in a modified Shastry-Sutherland model for SrCu2(BO3)(2)

We investigate the one-triplet dispersion in a modified Shastry-Sutherland model for SrCu2(BO3)(2) by means of a series expansion about the limit of s trong dimerization. Our perturbative method is based on a continuous unitar y transformation that maps the original Hamiltonian to an effective, energy -quanta-conserving block diagonal Hamiltonian H-eff. The dispersion splits into two branches which are nearly degenerate. We analyse thr symmetries of the model and show that space group operations are necessary to explain th e degeneracy of the dispersion at k = 0 and at the border of the magnetic B rillouin zone. Moreover, we investigate the behaviour of the dispersion for small /k/ and compare our results to inelastic neutron scattering data.