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

Leonid V Pourovskii; Sergii Khmelevskyi

doi:10.1073/pnas.2025317118

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

Proc Natl Acad Sci U S A. 2021 Apr 6;118(14):e2025317118. doi: 10.1073/pnas.2025317118.

Authors

Leonid V Pourovskii^{1

2}, Sergii Khmelevskyi³

Affiliations

¹ Centre de Physique Théorique, Ecole Polytechnique, CNRS, Institut Polytechnique de Paris, 91128 Palaiseau Cedex, France; leonid@cpht.polytechnique.fr.
² Collège de France, 75005 Paris, France.
³ Research Center for Computational Materials Science and Engineering, Vienna University of Technology, 1040 Vienna, Austria.

Abstract

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.

Keywords: ab initio; effective Hamiltonians; hidden order; linear response; strongly correlated electrons.