Phase transitions in condensed matter are a source of exotic emergent properties. We study the fully frustrated bilayer Heisenberg antiferromagnet to demonstrate that an applied magnetic field creates a previously unknown emergent criticality. The quantum phase diagram contains four states with distinctly different symmetries, all but one pair separated by first-order transitions. We show by quantum Monte Carlo simulations that the thermal phase diagram is dominated by a wall of discontinuities extending between the dimer-triplet phases and the singlet-containing phases. This wall is terminated at finite temperatures by a critical line, which becomes multicritical where the Berezinskii-Kosterlitz-Thouless (BKT) transition of the dimer-triplet antiferromagnet and the thermal Ising transition of the singlet-triplet crystal phase also terminate. The combination of merging symmetries leads to a 4-state Potts universality not contained in the microscopic Hamiltonian, which we interpret within the Ashkin-Teller model. Our results represent a systematic step in understanding emergent phenomena in quantum magnetic materials, including the “Shastry-Sutherland compound” SrCu₂(BO₃)₂.

凝聚态物质中的相变是奇异的涌现特性的来源。我们研究了完全受挫双层海森堡反铁磁体，以证明施加的磁场会产生以前未知的紧急临界性。量子相图包含四种具有明显不同对称性的状态，除了一对之外，所有状态都由一阶跃迁分隔开。我们通过量子蒙特卡罗模拟表明，热相图主要由在二聚体三重态相和包含单重态相之间延伸的不连续壁主导。该壁在有限温度下由一条临界线终止，该临界线变成多临界线，其中二聚体-三重态反铁磁体的别列津斯基-科斯特利茨-托利斯（BKT）转变和单重态-三重态晶相的热伊辛转变也终止。合并对称性的组合导致了微观哈密顿量中不包含的 4 态 Potts 普适性，我们在 Ashkin-Teller 模型中对其进行了解释。我们的结果代表了理解量子磁性材料（包括“Shastry-Sutherland 化合物”SrCu ₂ (BO ₃ ) _{2 ）}中的涌现现象的系统步骤。