Direct Evidence for the Localized Single-Triplet Excitations and the   Dispersive Multi-Triplets Excitations in SrCu2(BO3)2
  We performed inelastic neutron scattering on the 2D Shastry-Sutherland system SrCu2(11BO3)2 with an exact dimer ground state. Three energy levels at around 3, 5 and 9 meV were observed at 1.7 K. The lowest excitation at 3.0 meV is almost dispersionless with a bandwidth of 0.2 meV at most, showing a significant constraint on a single-triplet hopping owing to the orthogonality of the neighboring dimers. In contrast, the correlated two-triplets excitations at 5 meV exhibit a more dispersive behavior. 
H. Kageyama, M. Nishi, N. Aso, K. Onizuka, T. Yosihama, K. Nukui, K.   Kodama, K. Kakurai, Y. Ueda