High Field behavior of the Spin Gap compound Sr$_2$Cu(BO$_3$)$_2$

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.