| 1. |
- Wilhelm, F., et al.
(författare)
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X-ray magnetic circular dichroism experiments and theory of transuranium Laves phase compounds
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:2, s. 024424-
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Tidskriftsartikel (refereegranskat)abstract
- The actinide cubic Laves compounds NpAl2, NpOs2, NpFe2, and PuFe2 have been examined by x-ray magnetic circular dichroism (XMCD) at the actinide M-4,M-5 absorption edges and Os L-2,L-3 absorption edges. They have the interesting feature that the An-An spacing is close to the so-called Hill limit so that substantial hybridization between the 5f states on neighboring atoms is expected to occur. The XMCD experiments performed at the M-4,M-5 absorption edges of Np and Pu allow us to determine the spectroscopic branching ratio, which gives information on the coupling scheme in these materials. In all materials, the intermediate coupling scheme is found appropriate. Comparison with the magnetization data for NpOs2 and neutron results for PuFe2 allows a determination of the individual orbital and spin magnetic moments and the magnetic-dipole contribution m(md). The resulting orbital and spin magnetic moments are in good agreement with earlier values determined by neutron diffraction, and the values of m(md) are non-negligible, and close to those predicted for intermediate coupling. There is a comparatively large induced moment on the Os atom in NpOs2 such that the Os contribution to the total moment per formula unit is similar to 30% of the total. The spin and orbital moments at the Os site are parallel, in contrast to the antiparallel configuration that we find for Os impurities in a 3d ferromagnetic transition-metal alloy. Calculations using the LDA + U technique are reported. The ab initio computed XMCD spectra show good agreement with experimental spectra for small values (0-1 eV) of the Hubbard U parameter, which demonstrates that the 5f electrons in these compounds are relatively delocalized. The calculations confirm the sign and magnitude of the experimentally determined induced magnetic moments on the Os site in NpOs2. A posteriori, by comparison of the theoretical and measured XMCD spectra for a given material, we can determine the most appropriate LSDA + U variant and, more importantly, the applicable value of the Hubbard U parameter.
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| 2. |
- Hen, A., et al.
(författare)
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Structural investigation of Np2Co17 and analogue compounds under pressure
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:5, s. 054107-
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Tidskriftsartikel (refereegranskat)abstract
- The structural behavior of Np2Co17 is investigated by means of high-pressure diamond-anvil compression measurements and is compared with that of the isostructural compounds Lu2Co17 and Lu2Ni17. The Th2Ni17-type hexagonal crystal structure is preserved with no measurable discontinuous volume collapses up to the highest achieved pressure, p = 43 GPa. For Np2Co17, fits to the Birch-Murnaghan and Vinet equations of state give values of the isothermal bulk modulus and its pressure derivative of B-0 = 286 GPa and B-0' = 3, revealing that this Np compound is a highly incompressible solid with stiffness comparable to that of superhard covalently bonded materials. For the Lu2T17 (T = Co, Ni) compounds, the measured bulk modulus changes from B-0 = 137 GPa for T = Co to B-0 = 257 GPa for T = Ni. The isothermal equation of state for the studied compounds are in excellent agreement with the results of ab initio fully relativistic, full-potential local spin-density functional calculations. Theoretical estimates of the bulk modulus are given also for Np2Ni17, for which B-0 is predicted to assume values intermediate between those measured for Lu2Ni17 and Np2Co17.
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| 3. |
- Magnani, N., et al.
(författare)
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Magnetic Polarization of the Americium J=0 Ground State in AmFe2
- 2015
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 114:9
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Tidskriftsartikel (refereegranskat)abstract
- Trivalent americium has a nonmagnetic (J = 0) ground state arising from the cancellation of the orbital and spin moments. However, magnetism can be induced by a large molecular field if Am3+ is embedded in a ferromagnetic matrix. Using the technique of x-ray magnetic circular dichroism, we show that this is the case in AmFe2. Since < J(z)> = 0, the spin component is exactly twice as large as the orbital one, the total Am moment is opposite to that of Fe, and the magnetic dipole operator < T-z > can be determined directly; we discuss the progression of the latter across the actinide series.
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| 4. |
- Eckle, M, et al.
(författare)
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Electronic structure of UCx films prepared by sputter co-deposition
- 2004
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 334:1, s. 1-8
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Tidskriftsartikel (refereegranskat)abstract
- Thin layers of UCx (x = 0-12) have been prepared by sputter co-deposition of uranium and carbon in an Ar atmosphere. The films were investigated in-situ by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS, respectively). Special interest was put on the evolution of the electronic structure with the composition of the films, as deduced from the U-4f, C-1s and valence region spectra. With increasing carbon content, three types of carbon species were detected according to C-1s core level line, at 282, 282.6 and 284.5 eV binding energy (BE). They are attributed to the UC, UC2 and graphite phases, respectively. The U-4f core levels do not change strongly with increasing carbon content, showing well-itinerant U-5f electrons. Similarly, valence region spectra show three types of carbon species for different UCx films, which are differentiated by their C-2p signals. A strong hybridisation between C-2p and U-5f states is detected in UC, while the C-2p signal in UC2 appears only weakly hybridised, and for higher carbon contents a pi band characteristic of graphite appears.
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| 5. |
- El Jamal, Ghada, et al.
(författare)
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Mixed H2O/H2plasma-induced redox reactions of thin uranium oxide films under UHV conditions
- 2021
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Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 50:36, s. 12583-12591
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Tidskriftsartikel (refereegranskat)abstract
- X-ray photoelectron spectroscopy (XPS) has been used to study the effect of mixed H2O/H2gas plasma on the surfaces of UO2, U2O5and UO3thin films at 400 °C. The experiments were performedin situunder ultra-high vacuum conditions. Deconvolution of the U4f7/2peaks into U(iv), U(v) and U(vi) components revealed the surface composition of the films after 10 min plasma exposure as a function of H2concentration in the feed gas of the plasma. Some selected films (unexposed and exposed) were also analysed using grazing-incidence X-ray diffraction (GIXRD). The XPS results show that U(v) is formed as a major product upon 10 minutes exposure of UO3by a mixed H2O/H2plasma in a fairly wide H2concentration range. When starting with U(v) (U2O5), rather high H2concentrations are needed to reduce U(v) to U(iv) in 10 minutes. In the plasma induced oxidation of UO2, U(v) is never observed as a major product after 10 minutes and it would seem that once U(v) is formed in the oxidation of UO2it is rapidly oxidized further to U(vi). The grazing incidence X-ray diffraction analysis shows that there is a considerable impact of the plasma and heating conditions on the crystal structure of the films in line with the change of the oxidation state. This structural difference is proposed to be the main kinetic barrier for plasma induced transfer between U(iv) and U(v) in both directions.
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| 6. |
- Eloirdi, R., et al.
(författare)
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Dilution effect on the U-5f states : U in an Ag matrix
- 2005
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 386:1-2, s. 70-74
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of U-diluted in an Ag matrix has been studied in situ by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS, respectively). UxAg100-x thin films (x = 0-100 at.%) were produced by sputter co-deposition in an Ar atmosphere. UPS spectra of the Ag-4d line indicate formation of a homogeneous mixture despite the fact that U and Ag do not form stable alloys. A major goal of this work was to find out whether the dilution of U atoms in an inert matrix with no bonding states induces the localisation of the U-5f states. Both U-4f core level spectra and the U-5f spectra indicate U-5f delocalisation, down to 5 at.% of uranium in UxAg100-x films.
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| 7. |
- Walker, H. C., et al.
(författare)
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Magnetic, electrical, and thermodynamic properties of NpIr : Ambient and high-pressure measurements, and electronic structure calculations
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:19
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Tidskriftsartikel (refereegranskat)abstract
- We present bulk property measurements of NpIr, a newly synthesized member of the Np-Ir binary phase diagram, which is isostructural to the noncentrosymmetric pressure-induced ferromagnetic superconductor UIr. Magnetic susceptibility, electronic transport properties at ambient and high pressure, and heat capacity measurements have been performed for temperatures T = 0.55-300 K in a range of magnetic fields up to 14 T and under pressure up to 17.3 GPa. These reveal that NpIr is a moderately heavy fermion Kondo system with strong antiferromagnetic interactions, but there is no evidence of any phase transition down to 0.55 K or at the highest pressure achieved. Experimental results are compared with ab initio calculations of the electronic band structure and lattice heat capacity. An extremely low lattice thermal conductivity is predicted for NpIr at temperatures above 300 K.
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