Continuous and Discontinuous Quantum Phase Transitions in a Model Two-Dimensional Magnet

The Shasty-Sutherland model consists of a set of spin 1/2 dimers on a 2-dimensional square lattice which are predicted to change from isolated, gapped excitations to a collective, ordered ground state by tuning the ratio of the intra to inter-dimer coupling. We compress the model Shastry-Sutherland material, SrCu2(BO3)2, in a diamond anvil cell at cryogenic temperatures to continuously tune the coupling energies and induce changes in state. High-resolution x-ray measurements exploit a remarkably strong spin-lattice coupling to ascertain the physics of the magnetic transition. The singlet-triplet gap energy is suppressed continuously with increasing pressure, vanishing completely by 2 GPa. This continuous quantum phase transition is followed by a structural distortion at higher pressure corresponding to the onset of long-range order.