  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.
