High energy-resolution inelastic neutron scattering experiments on triplet bound state excitations in SrCu2(BO3)2

In this letter we report on the high energy-re solution neutron scattering Studies on the dynamic structure factor of the two-triplet bound states in the two-dimensional S= 1/2 Shastry-Sutherland system, SrCu2(BO3)(2), in the energy transfer range of 4 to 7 meV. In contrast to the earlier interpretation of the low energy-resolution results, these bound state excitations show hardly any dispersion, but do show a strong wave-vector-dependent structure factor. Although these findings are in overall agreement with the recent theoretical calculation of the 2-triplon dynamic structure factor by Knetter and Uhrig [Phys. Rev. Lett. 92 (2004) 027204], they also show distinct differences, which may be due to some symmetry-breaking terms in the Hamiltonian that are not included in the theory, such as the interdimer Dzyaloshinski-Moriya interactions recently discovered in this compound.