[en] The coexistence of superconductivity with magnetism has recently re-emerged as a central topic in condensed matter physics. I will discuss two U-based ferromagnetic superconductors, UGe₂ and URhGe. The 'it ab initio' relativistic electronic structure calculations reveal very unusual microscopic properties of these materials, in which the coexisting superconducting and ferromagnetic phases are formed by the same electrons. The consistent picture of the magnetic properties and electronic structure of the UGe₂ will be shown to require inclusion of correlation effects beyond the local spin density approximation (LSDA). I will show that the 'LSDA + Hubbard U' (LSDA+U) approach reproduces magnetic properties and yields the largest Fermi surface sheet of the Fermi surface, which is comprised primarily of spin majority states. This occurrence and the quasi-two-dimensional geometry of the Fermi surface, support the likelihood of magnetically mediated p-wave triplet pairing in UGe₂. The URhGe is very different. I will show that LSDA provides better description for magnetic ground state properties than LSDA+U. It will be also shown that the canted magnetic structure of URhGe can originate from non-collinear arrangement of U atom orbital magnetic moments, while the spin magnetic moments are ferromagnetically ordered. The Fermi surface analysis will be presented and the possibility for phonon-mediated s-wave superconductivity in URhGe will be discussed. (author)

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[en] We have performed full-potential linearized augmented plane wave calculations of the Gd(0001) surface using the local density approximation (LDA) together with the Hubbard U (LDA+U) total energy functional. The use of LDA+U instead of LDA total energy calculations leads to a ferromagnetic ground state for both bulk Gd and the Gd surface, in agreement with experimental observation. The calculated downward shift of 4f eigenvalues for the Gd surface is in agreement with experimentally observed binding energies. Surface strain relaxation leads to a 90% enhancement of the interlayer surface-to-bulk effective exchange coupling. Application of a Landau-Ginzburg-type theory yields a 30% enhancement of the Curie temperature at the surface, in very good agreement with the experiment. (c) 2000 American Institute of Physics

[en] Effects of electron correlation on the electronic structure and magnetic properties of the Gd(0001) surface are investigated using the full-potential linearized augmented plane-wave implementation of correlated band theory (''LDA+U''). The use of LDA+U instead of LDA (local-density approximation) total-energy calculations produces the correct ferromagnetic ground state for both bulk Gd and the Gd surface. Surface strain relaxation leads to a 90% enhancement of the interlayer surface-to-bulk effective exchange coupling. Application of a Landau-Ginzburg-type theory yields a 30% enhancement of the Curie temperature at the surface, in very good agreement with the experiment. (c) 2000 The American Physical Society

[en] We propose a straightforward and efficient procedure to perform dynamical mean-field (DMFT) calculations on the top of the static mean-field LDA+U (local density approximation) approximation. Starting from self-consistent LDA+U ground state we included multiplet transitions using the Hubbard-I approximation, which yields a very good agreement with experimental photoelectron spectra of δ-Pu, Am, and their selected compounds. (authors)

[en] The physical properties of binary actinides and transition metal alloys are of great importance for the safety assessment of nuclear fuels. Since transition metals are major components of the cladding material of fuel rods (stainless steel, HT-9 etc.), the physical properties of those compounds formed by accidental fuelcladding interactions could have limiting factors on the fabrication, life time operation and disposal of nuclear fuels. Binary compound of the form ReT5 (Re = rare earth, T = Transition metal) has been in the focus of interest mainly because of their magnetic properties (small Re to T stoichiometric ratio, large spontaneous magnetization and high Curie temperature) and their ability to store large amount of hydrogen per formula unit (f. u.)

[en] Magnetic properties of the intermetallic compound U₂Fe₃Ge were studied on a single crystal. The compound crystallizes in the hexagonal Mg₂Cu₃Si structure, an ordered variant of the MgZn₂ Laves structure (C14). U₂Fe₃Ge displays ferromagnetic order below the Curie temperature T_C = 55 K and presents an exception to the Hill rule, as the nearest inter-uranium distances do not exceed 3.2 Å. Magnetic moments lie in the basal plane of the hexagonal lattice, with the spontaneous magnetic moment M_s = 1.0 μ_B/f.u. at T = 2 K. No anisotropy within the basal plane is detected. In contrast to typical U-based intermetallics, U₂Fe₃Ge exhibits very low magnetic anisotropy, whose field does not exceed 10 T. The dominance of U in the magnetism of U₂Fe₃Ge is suggested by the ⁵⁷Fe Mössbauer spectroscopy study, which indicates very low or even zero Fe moments. Electronic structure calculations are in agreement with the observed easy-plane anisotropy but fail to explain the lack of an Fe contribution to the magnetism of U₂Fe₃Ge.

[en] Thin layers of PuCoGa_x (x = 4 to 18) have been prepared by dc sputtering from a PuCoGa₅ single crystal target, and investigated in situ by X-ray and ultraviolet photoelectron spectroscopy. We could achieve broad composition variability (monitored by the Pu-4f, Co-2p and Ga-2p core-level spectra). The results are compared to the valence band spectra reported previously for PuCoGa₅. Our experiments reveal that some Ga excess (PuCoGa_∼7) was likely for those original data. We demonstrate that there is a tendency to the segregation of Ga at the surface, which has an important effect on the valence band spectra