Antiferromagnetism and a Dimensional Crossover in the Model Quantum Magnet SrCu$_{2}$(BO$_{3}$)$_{2}$

SrCu$_{2}$(BO$_{3}$)$_{2}$ has corner-sharing Cu$^{2+}$ S=1/2 dimers lying on a square lattice, corresponding to the two-dimensional Shastry-Sutherland model. We conduct high resolution x-ray and neutron diffraction measurements as a function of pressure, tuning the ground state from a singlet ground state to long-range order. We report a change in crystal symmetry as a function of temperature for pressures above 4 GPa, linked to antiferromagnetism and the tilting of the dimers out of the plane. The inclusion of Dzyaloshinskii-Moriya interactions in the Shastry-Sutherland Hamiltonian helps explain the experimental results.