Hidden order and multipolar exchange striction in a correlated f-electron system

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 118; no. 14; pp. 1 - 8
• Main Authors: Pourovskii, Leonid V., Khmelevskyi, Sergii
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 06.04.2021
• Subjects: Condensed Matter; Physical Sciences; Physics; Strongly Correlated Electrons; ab initio; linear response; effective Hamiltonians; hidden order; strongly correlated electrons
• ISSN: 0027-8424; 1091-6490; 1091-6490
• 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 NpO₂, 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 Np4+ ions induce a primary noncollinear order of time-odd rank 5 (triakontadipolar) moments with a secondary quadrupole order preserving the cubic symmetry of NpO₂. Our study also reveals an unconventional multipolar exchange striction mechanism behind the anomalous volume contraction of the NpO₂ HO phase.