Quantum phases diagram of the the Shastry-Sutherland compound SrCu₂(BO₃)₂ at high magnetic field and high pressure

In the Shastry-Sutherland model, the ground state is expected to change from a spin dimer singlet state to a novel, intermediate 4-spin plaquette singlet state, then to an antiferromagnet state, when the relative strength of the nearest neighbor and next-nearest neighbor interactions (J/J’) is tuned. SrCu₂(BO₃)₂ realizes the Shastry-Sutherland model at ambient pressure and can be tuned to other phases by application of high pressure, thus attracting considerable interests. Recent experimental advances have revealed the presence of an intermediate ground state with signatures of a 4-spin plaquette singlet state at high pressure. However, this intermediate phase also feature signatures that clearly deviate from the prediction of the ideal Shastry-Sutherland model. Therefore, the nature of this plaquette phase and how it evolves into other phases remain unclear. Here, we report a comprehensive study of the quantum phase diagram of the plaquette state by tuning the pressure and magnetic field, using a highly sensitive magnetization measurement technique. Our results reveal the emergence of a sequence of ground states once the spin dimer singlet and the plaquette singlet phase are suppressed by the field, which are further corroborated by our theoretical simulations.