Pourovskii Leonid V, Khmelevskyi Sergii
Centre de Physique Théorique, Ecole Polytechnique, CNRS, Institut Polytechnique de Paris, 91128 Palaiseau Cedex, France;
Collège de France, 75005 Paris, France.
Proc Natl Acad Sci U S A. 2021 Apr 6;118(14). doi: 10.1073/pnas.2025317118.
The nature of order in low-temperature phases of some materials is not directly seen by experiment. Such "hidden orders" (HOs) may inspire decades of research to identify the mechanism underlying those exotic states of matter. In insulators, HO phases originate in degenerate many-electron states on localized f or d shells that may harbor high-rank multipole moments. Coupled by intersite exchange, those moments form a vast space of competing order parameters. Here, we show how the ground-state order and magnetic excitations of a prototypical HO system, neptunium dioxide NpO, can be fully described by a low-energy Hamiltonian derived by a many-body ab initio force theorem method. Superexchange interactions between the lowest crystal-field quadruplet of Np ions induce a primary noncollinear order of time-odd rank 5 (triakontadipolar) moments with a secondary quadrupole order preserving the cubic symmetry of NpO Our study also reveals an unconventional multipolar exchange striction mechanism behind the anomalous volume contraction of the NpO HO phase.
某些材料低温相中的有序性质无法通过实验直接观察到。这种“隐藏序”（HOs）可能会激发数十年的研究，以确定这些奇异物质状态背后的机制。在绝缘体中，HO相起源于局域f或d壳层上的简并多电子态，这些态可能包含高阶多极矩。通过格点间交换相互作用，这些矩形成了一个由竞争序参量构成的广阔空间。在此，我们展示了如何通过多体从头算力定理方法导出的低能哈密顿量，来完整描述典型HO系统二氧化镎（NpO₂）的基态序和磁激发。Np离子最低晶体场四重态之间的超交换相互作用诱导出一个主要的非共线的5阶时间奇宇称（三十极偶极）矩序，以及一个保持NpO₂立方对称性的次要四极矩序。我们的研究还揭示了NpO₂ HO相异常体积收缩背后的一种非常规多极交换约束机制。
