晶体学驱动的二维自旋$3/2$磁体的分子化
Crystallography-driven molecularization of a two-dimensional spin-$3/2$ magnet
AI总结：
通过晶体学不等价性将磁性自由度分子化，解释二维自旋$3/2$材料Na$_2$Mn$_3$O$_7$中无磁有序但存在两个热力学交叉标度的反常行为。
摘要：
大自旋二维磁体通常预期在主导交换作用变得显著时发展出常规长程序。层状自旋$3/2$枫叶型化合物Na$_2$Mn$_3$O$_7$违背了这一预期：尽管存在可观的反铁磁相互作用且无明显无序，它不显示磁有序，而是呈现两个分离良好的热力学交叉标度。我们证明这种行为源于晶体学驱动的磁性自由度分子化。低对称性结构将Mn子晶格划分为不等价的交换路径，产生显著的层级，几乎孤立出反铁磁六边形。因此，磁关联分两个阶段发展：首先在单个六边形内以主导交换作用设定的标度发展，只有在更低温度下，受挫的六边形间耦合才试图在整个晶格上建立相干性。虽然孤立六边形再现了两步热力学结构，但实验相关的温度标度只有在六边形嵌入受挫二维网络后才出现。最终的量子基态是磁无序的，其特征是强六边形内关联和快速衰减的六边形间关联。这些结果将晶体学不等价性识别为一种材料层面的机制，用于稳定大自旋二维磁体中的分子化和量子无序态。
英文摘要：
Large-spin two-dimensional magnets are generally expected to develop conventional long-range order once the dominant exchange scale becomes appreciable. The layered spin-$3/2$ maple-leaf compound Na$_2$Mn$_3$O$_7$ defies this expectation: despite sizable antiferromagnetic interactions and no evident disorder, it exhibits no magnetic ordering and displays two well-separated thermodynamic crossover scales. We show that this behavior originates from a crystallography-driven molecularization of the magnetic degrees of freedom. The low-symmetry structure partitions the Mn sublattice into inequivalent exchange pathways, generating a pronounced hierarchy that nearly isolates antiferromagnetic hexagons. Magnetic correlations therefore develop in two stages: first within individual hexagons at a scale set by the dominant exchange, and only at much lower temperatures do frustrated inter-hexagon couplings attempt to establish coherence across the lattice. While isolated hexagons reproduce the two-step thermodynamic structure, the experimentally relevant temperature scales emerge only once the hexagons are embedded in the frustrated two-dimensional network. The resulting quantum ground state is magnetically disordered, characterized by strong intra-hexagon correlations and rapidly decaying inter-hexagon correlations. These results identify crystallographic inequivalence as a materials-level mechanism for stabilizing molecularized and quantum-disordered states even in large-spin two-dimensional magnets.
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