AVS 58th Annual International Symposium and Exhibition | |

Actinides and Rare Earths Focus Topic |
Wednesday Sessions |

Session AC+MI-WeA |

Electronic Structure Theory of Complex Ordered Actinide Materials

Session: |
Magnetic and Electron Correlation Effects in Actinides and Rare Earths |

Presenter: |
Peter M. Oppeneer, Uppsala University, Sweden |

Correspondent: |
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Actinide materials display many complex and correlated behaviors that originate from the special properties of the open f-shell atom embedded in a specific material’s environment. First-principles investigations provide a route to assess these anomalous phenomena in a materials specific way, providing direct, fundamental insight.

Here we consider recently obtained *ab initio* modeling results for actinide materials that are in the focus of current interest: actinide oxides, such as NpO_{2}, PuO_{2}, and higher-oxides, U_{3}O_{8}, and Np_{2}O_{5}, the hidden order (HO) material URu_{2}Si_{2}, and correlated plutonium compounds.

NpO_{2} is one of the very few materials in which complex multipolar order has been identified. Using the density-functional theory (DFT)-based LDA+*U* method we provide a first-principles theory of multipolar order and superexchange in NpO_{2}. DFT+*U* calculations offer a precise microscopic description of the 3** q**-antiferro ordered phase. We find that the usually neglected higher-order multipoles (electric hexadecapoles and magnetic triakontadipoles) are at least equally significant as the electric quadrupoles and magnetic octupoles [1].

We further investigate light actinide oxides in higher oxidation states, such as U_{3}O_{8}, PuO_{2+x}, and Np_{2}O_{5}, for which non-collinear magnetic ordering is predicted. The possible further oxidation of PuO_{2} to PuO_{2+x }is studied using DFT+*U* calculations in combination with x-ray absorption measurements [2].

The Pu monochalcogenides are intriguing materials, in which a correlated temperature gap develops, reminiscent of the behavior seen in Kondo insulators. Using dynamical mean field theory (DMFT) in comparison to LDA+*U* calculations, we show that dynamical self-energy fluctuations are important for the formation of an unusual gap. Static approximations to the self-energy as the LDA+*U* fail to provide a gap.

For URu_{2}Si_{2} we report extensive electronic structure investigations [3], using full-potential LSDA, LSDA+*U*, and DMFT approaches to assess the origin of the hidden order. Our investigation show that the itinerant f-electron picture provides an excellent description of the materials properties of this fascinating compound. The Fermi surface which is crucial for the HO transition and the occurrence of unconventional superconductivity is accurately given. Our study points to the formation of long-lived spin fluctuations that are the driving quasiparticles for the HO.

1. M.-T. Suzuki, N. Magnani, and P.M. Oppeneer, Phys. Rev. B **82**,** **241103(R) (2010).

2. A. Modin, Y. Yun, M.-T. Suzuki et al., Phys. Rev. B **83**,** 075113** (2011).

3. P.M. Oppeneer, J. Rusz, S. Elgazzar, M.-T. Suzuki, T. Durakiewicz, and J.A. Mydosh, Phys. Rev. B **82**,** **205103** **(2010).