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 NpO₂, PuO₂, and higher-oxides, U₃O₈, and Np₂O₅, the hidden order (HO) material URu₂Si₂, and correlated plutonium compounds.

NpO₂ 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 NpO₂. DFT+U calculations offer a precise microscopic description of the 3q-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 U₃O₈, PuO_2+x, and Np₂O₅, for which non-collinear magnetic ordering is predicted. The possible further oxidation of PuO₂ 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 URu₂Si₂ 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).

Actinide-based sputter deposited films were so far used in the context of surface-science studies (such as [1]) and for exploration of electronic structure by photoelectron spectroscopy (e.g. [2,3]). In addition, sputter deposition was used in attempts to synthesize amorphous uranium alloys for ex-situ studies of magnetic properties. Such early (late 1980's) attempts in U.S. [4] or Japan [5] were undertaken in simple setups and lack proper diagnostics of the deposited material. Considering strong electropositivity of U, oxidation has to be suspected for films prepared in HV conditions. More recently, U metal in multilayers with possibility of epitaxial growth were sputter deposited with the aim to induce uranium magnetic moments [6]. We have used sputter deposition to investigate structure and magnetic properties of various U-based compounds as a function of deposition conditions (deposition rate, substrate type and temperature). Employing diagnostics by XPS, Glancing Angle XRD, and RBS, it was established that UN films have a long-term stability, which allows comfortably to make ex-situ studies over months. The reason can be seen in pronounced compressive residual strains, imposed during the deposition, which prevent progressing the surface oxidation into the bulk of several hundred nm thick films. Departing more from a fully crystalline state, the antiferromagnetism of UN is masked by a weak ferromagnetism, as usual for nanograined AF structures, and finally both moments and their order disappears [7]. Similar suppression of magnetism was found for ferromagnetic US [8].

Recently we undertook sputter-deposition experiments on Fe-rich U-Fe alloys derived from the Laves phase UFe₂, which combines the 3d and 5f magnetism in a compound with a relatively high Curie temperature (T_C = 162 K). An Fe-excess is expected to increase the T_C value markedly. Nanocrystalline material obtained up to the stoichiometry UFe_2.3 by splat cooling, with the excessive Fe atoms entering the U sublattice has T_C enhanced up to 230-240 K. More Fe leads to the segregation of α-Fe. We succeeded to synthesize amorphous films by U and Fe co-sputtering, with stoichiometry up to UFe₃. T_C is enhanced up to 450 K in this case.

[1] T. Gouder et al., Surface Science 601 (2007) L77.

[2] T. Gouder et al., Phys.Rev.Letters 84 (2000) 3378.

[3] L. Havela et al., Phys.Rev.B 65 (2002) 235118.

[4] P.P. Freitas et al., J.Appl.Phys. 63 (1988) 3746.

[5] S. Itoh et al., Physica B 281-282 (2000) 230.

[6] R. Springell et al., Phys. Rev. B 77 (2008) 064423.

[7] D. Rafaja et al., J.Alloys Comp. 386 (2005) 87.

[8] L. Havela et al., J. Alloys Comp. 408-412 (2006) 1320.