  We report magnetization and heat capacity measurements of single crystal
samples of the spin gap compound Sr$_2$Cu(BO$_3$)$_2$. Low-field data show that
the material has a singlet ground state comprising dimers with intradimer
coupling J = 100 K. High field data reveal the role of weak interdimer
coupling. For fields that are large compared to the spin gap, triplet
excitations are observed for significantly smaller fields than predicted for
isolated dimers, indicating that weak inter-dimer coupling leads to triplet
delocalization. High field magnetization behavior at low temperatures suggests
additional cooperative effects.
