Details: 
Spin dimer compounds are based on strongly coupled pairs of spins (dimers), which by virtue of the crystal structure only weakly interact with each other. For antiferromagnetic intradimer exchange, the ground state is a product of singlets and there is a gap to excited triplet states. A fieldtuned quantum critical point (QCP) separates the lowfield quantum paramagnetic state from a state characterized by longrange magnetic order. In the absence of terms in the spin Hamiltonian that explicitly break U(1) symmetry, this phase transition can be interpreted as a BoseEinstein condensation (BEC) of delocalized triplets. Despite considerable theoretical interest in this class of material, there are very few examples available for experimental study. In this talk I will describe the high field behavior of two spin dimer compounds that we have recently been able to synthesize; BaCuSi2O6 (for which the dimers are arranged on a bodycentered tetragonal lattice), and Ba3Mn2O8 (vertical dimers on a triangular lattice). For both of these materials, geometric frustration plays a significant role in determining the magnetic properties and the resulting phase diagram.
