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- Elgazzar, Saad, et al.
(författare)
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Ab initio computational and experimental investigation of the electronic structure of actinide 218 materials
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:23, s. 235117-
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Tidskriftsartikel (refereegranskat)abstract
- We report a comprehensive investigation of the electronic structure and magnetic properties of actinide 218 compounds, which crystallize in the tetragonal Ho2CoGa8 crystal structure. Specifically, we study experimentally the group of plutonium-based compounds Pu2MGa8 (with M=Rh, Co, and Fe), which are structurally related to the unconventional superconductors PuCoGa5 and PuRhGa5 and are measured to be nonmagnetic and nonsuperconducting down to 2 K, yet displaying relatively high linear specific-heat coefficients of 61 to 133 mJ/mol K-2. We perform density-functional theory based calculations, in which we apply three different approaches to access the tendency of 5f electron localization, the local spin-density approximation (LSDA) LSDA+U, and the 5f open-core approach. For comparison to the above-mentioned compounds we also investigate computationally the plutonium compounds with M=Ir and Pd, the uranium-based compounds U2MGa8 (with M=Co, Fe, Rh, and Ru), as well as Np2CoGa8, and Am2CoGa8. On the basis of ab initio LSDA calculations we optimize the equilibrium lattice parameters and the internal fractional coordinates within the Ho2CoGa8 crystal structure. The obtained lattice parameters are in relatively good agreement with experimental values, when we assume delocalized 5f states for all compounds except Am2CoGa8. We discuss the computed electronic structures and the theoretical Fermi surfaces. For the Pu-218 compounds we find that LSDA calculations, in which the 5f's are treated as delocalized, predict a magnetically ordered ground state, whereas LSDA+U calculations predict a nonmagnetic ground state in accordance with experiment. For the U-218 compounds the LSDA itinerant 5f approach predicts a nonmagnetic ground state, in accordance with available experimental data. For Am2CoGa8 our calculations are consistent with the scenario of localized 5f electrons. We find that, on account of the elongated tetragonal structure, most of the theoretical Fermi surfaces are quasi-two-dimensional.
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| 3. |
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| 4. |
- Heathman, S, et al.
(författare)
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Experimental and ab initio volume compressibility curves of NpCoGa5
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:2, s. 024106-
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Tidskriftsartikel (refereegranskat)abstract
- We report the results of high-pressure, angular-dispersive, x-ray diffraction measurements performed on NpCoGa5, a magnetic isostructural analog of the PuCoGa5 superconductor. No crystallographic transitions or discontinuous volume collapses have been observed by increasing pressure p up to 39 GPa. A fit to the Birch-Murnaghan equation of state gives values of the isothermal bulk modulus and its pressure derivative B-0=130 GPa and B-0'=4.8, respectively. The volume compressibility curve is in excellent agreement with the results of ab initio fully relativistic, full potential local spin-density-functional calculations. A comparison to experimental and ab initio calculated compressibility data of PuCoGa5 and PuRhGa5 is also made.
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| 6. |
- Oppeneer, Peter M., et al.
(författare)
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Electronic structure theory of the hidden-order material URu2Si2
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:20, s. 205103-
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Tidskriftsartikel (refereegranskat)abstract
- We report a comprehensive electronic structure investigation of the paramagnetic (PM), the large moment antiferromagnetic (LMAF), and the hidden order (HO) phases of URu2Si2. We have performed relativistic full-potential calculations on the basis of the density-functional theory, employing different exchange-correlation functionals to treat electron correlations within the open 5f shell of uranium. Specifically, we investigate-through a comparison between calculated and low-temperature experimental properties-whether the 5f electrons are localized or delocalized in URu2Si2. The local spin-density approximation (LSDA) and generalized gradient approximation (GGA) are adopted to explore itinerant 5f behavior, the GGA plus additional strong Coulomb interaction (GGA+U approach) is used to approximate moderately localized 5f states, and the 5f-core approximation is applied to probe potential properties of completely localized uranium 5f states. We also performed local-density approximation plus dynamical mean-field theory calculations (DMFT) to investigate the temperature evolution of the quasiparticle states at 100 K and above, unveiling a progressive opening of a quasiparticle gap at the chemical potential when temperature is reduced. A detailed comparison of calculated properties with known experimental data demonstrates that the LSDA and GGA approaches, in which the uranium 5f electrons are treated as itinerant, provide an excellent explanation of the available low-temperature experimental data of the PM and LMAF phases. We show furthermore that due to a material-specific Fermi-surface instability a large, but partial, Fermi-surface gapping of up to 750 K occurs upon antiferromagnetic symmetry breaking. The occurrence of the HO phase is explained through dynamical symmetry breaking induced by a mode of long-lived antiferromagnetic spin fluctuations. This dynamical symmetry breaking model explains why the Fermi-surface gapping in the HO phase is similar but smaller than that in the LMAF phase and it also explains why the HO and LMAF phases have the same Fermi surfaces yet different order parameters. A suitable order parameter for the HO is proposed to be the Fermi-surface gap, and the dynamic spin-spin correlation function is further suggested as a secondary order parameter.
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