The study of interacting spin systems is of fundamental importance for modern condensed-matter physics. On frustrated lattices, magnetic exchange interactions cannot be simultaneously satisfied, and often give rise to competing exotic ground states¹. The frustrated two-dimensional Shastry–Sutherland lattice² realized by SrCu₂(BO₃)₂ (refs 3,4) is an important test case for our understanding of quantum magnetism. It was constructed to have an exactly solvable 2-spin dimer singlet ground state within a certain range of exchange parameters and frustration. While the exact dimer state and the antiferromagnetic order at both ends of the phase diagram are well known, the ground state and spin correlations in the intermediate frustration range have been widely debated^{2,4,5,6,7,8,9,10,11,12,13,14}. We report here the first experimental identification of the conjectured plaquette singlet intermediate phase in SrCu₂(BO₃)₂. It is observed by inelastic neutron scattering after pressure tuning to 21.5 kbar. This gapped singlet state leads to a transition to long-range antiferromagnetic order above 40 kbar, consistent with the existence of a deconfined quantum critical point.

对相互作用的自旋系统的研究对于现代凝聚态物理至关重要。在失意的晶格上，不能同时满足磁交换相互作用，并且常常会引起竞争性的奇异基态¹。沮丧的二维Shastry-萨瑟兰晶格²由SrCu实现₂（BO ₃）₂ （参考文献3,4）是我们了解量子磁性的重要测试案例。它被构造为在一定的交换参数和无奈范围内具有完全可溶的2旋二聚体单线态基态。虽然相图两端的确切二聚体状态和反铁磁顺序是众所周知的，但在中间受挫范围内的基态和自旋相关性已被广泛争论^{2,4,5,6,7,8,9,10 ，11,12,13,14}。我们在这里报告SrCu ₂（BO ₃）_2中推测的褶皱单线态中间相的首次实验鉴定。压力调整到21.5 kbar后，通过非弹性中子散射观察到它。这种空位单重态导致超过40 kbar的长距离反铁磁阶跃，这与存在限定的量子临界点相一致。