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| 2. |
- Locht, Inka L. M., et al.
(författare)
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Standard model of the rare earths analyzed from the Hubbard I approximation
- 2016
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Ingår i: PHYSICAL REVIEW B. - : American Physical Society. - 2469-9950. ; 94:8
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Tidskriftsartikel (refereegranskat)abstract
- In this work we examine critically the electronic structure of the rare-earth elements by use of the so-called Hubbard I approximation. From the theoretical side all measured features of both occupied and unoccupied states are reproduced, without significant deviations between observations and theory. We also examine cohesive properties like the equilibrium volume and bulk modulus, where we find, in general, a good agreement between theory andmeasurements. In addition, we have reproduced the spin and orbital moments of these elements as they are reflected from measurements of the saturation moment. We have also employed the Hubbard I approximation to extract the interatomic exchange parameters of an effective spin Hamiltonian for the heavy rare earths. We show that the Hubbard I approximation gives results which are consistent with calculations where 4f electrons are treated as core states for Gd. The latter approach was also used to address the series of the heavy/late rare earths. Via Monte Carlo simulations we obtained ordering temperatures which reproduce measurements within about 20%. We have further illustrated the accuracy of these exchange parameters by comparing measured and calculated magnetic configurations for the heavy rare earths and the magnon dispersion for Gd. The Hubbard I approximation is compared to other theories of the electronic structure, and we argue that it is superior. We discuss the relevance of our results in general and how this makes it possible to treat the electronic structure of materials containing rare-earth elements, such as permanent magnets, magnetostrictive compounds, photovoltaics, optical fibers, topological insulators, and molecular magnets.
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| 3. |
- Odell, Anders, et al.
(författare)
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Investigation of the Conducting Properties of a Photoswitching Dithienylethene Molecule
- 2010
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Ingår i: ACS NANO. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 4:5, s. 2635-2642
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Tidskriftsartikel (refereegranskat)abstract
- Photoswitching molecules are attractive candidates as organic materials for optoelectronics applications because light impulses can switch them between states with different conducting characteristics. Here, we report a fully self-consistent density functional theory calculation of the electron transport properties of photoswitching dithienylethene attached to Au leads in both the open and closed conformations. The molecule is found to be a good conductor in both conformations, with the low-bias current for the closed one being about 20 times larger than that of the open. Importantly, the current voltage characteristics away from the linear response are largely determined by molecular orbital rehybridization in an electric field, in close analogy to what happens for Mn-12 molecules. However, in the case of dithienylethene attached to Au, such a mechanism is effective also in conditions of strong electronic coupling to the electrodes. This makes the dithienylethene family an intriguing materials platform for constructing highly conducting organic optoelectronics switches.
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| 4. |
- Peters, Lars, et al.
(författare)
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Magnetism and exchange interaction of small rare-earth clusters; Tb as a representative
- 2016
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Here we follow, both experimentally and theoretically, the development of magnetism in Tb clusters from the atomic limit, adding one atom at a time. The exchange interaction is, surprisingly, observed to drastically increase compared to that of bulk, and to exhibit irregular oscillations as a function of the interatomic distance. From electronic structure theory we find that the theoretical magnetic moments oscillate with cluster size in exact agreement with experimental data. Unlike the bulk, the oscillation is not caused by the RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a competition between wave-function overlap of the 5d shell and the on-site exchange interaction, which leads to a competition between ferromagnetic double-exchange and antiferromagnetic super-exchange. This understanding opens up new ways to tune the magnetic properties of rare-earth based magnets with nano-sized building blocks.
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| 5. |
- Peters, L., et al.
(författare)
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Valence and spectral properties of rare-earth clusters
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:3
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Tidskriftsartikel (refereegranskat)abstract
- The rare earths are known to have intriguing changes of the valence, depending on the chemical surrounding or geometry. Here, we aim at predicting the transition of valence when passing from the atomic divalent limit to the bulk trivalent limit. This transition is analyzed by addressing clusters of various size for selected rare-earth elements, i.e., Sm, Tb, and Tm, via a theoretical treatment that combines density functional theory with atomic multiplet theory. Our results show that Tm clusters change from pure divalent to pure trivalent at a size of six atoms, while Tb clusters are already divalent for two atoms and stay so until eight atoms and the bulk limit. Instead, Sm clusters are respectively purely divalent up to eight atoms. For larger Sm clusters, a transition to a trivalent configuration is expected and likely accompanied by a regime of mixed valence. The valence of all rare-earth clusters, as a function of size, is predicted from the interpolation of our calculated results. These predictions are argued to be best investigated by spectroscopic measurements. To ease experimental analysis, we provide theoretical spectra, based on dynamical mean-field theory in the Hubbard I approximation.
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| 6. |
- Vaitheeswaran, G., et al.
(författare)
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Elastic constants and high-pressure structural transitions in lanthanum monochalcogenides from experiment and theory
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:18
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Tidskriftsartikel (refereegranskat)abstract
- The high-pressure structural behavior of lanthanum monochalcogenides is investigated by theory and experiment. Theory comprises density-functional calculations of LaS, LaSe, and LaTe with the general gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. The experimental studies consist of high-pressure angle dispersive x-ray-diffraction investigations of LaS and LaSe up to a maximum pressure of 41 GPa. A structural phase transition from the NaCl-type to CsCl-type crystal structure is found to occur in all cases. The experimental transition pressures are 27-28 and 19 GPa for LaS and LaSe, respectively, while the calculated transition pressures are 29, 21, and 10 GPa for LaS, LaSe, and LaTe, respectively. The calculated ground-state properties such as equilibrium lattice constant, bulk modulus and its pressure derivative, and Debye temperatures are in good agreement with experimental results. Elastic constants are predicted from the calculations.
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| 7. |
- Vaitheeswaran, G., et al.
(författare)
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High-pressure structural, elastic, and electronic properties of the scintillator host material KMgF3
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:1
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Tidskriftsartikel (refereegranskat)abstract
- The high-pressure structural behavior of the fluoroperovskite KMgF3 is investigated by theory and experiment. Density functional calculations were performed within the local density approximation and the generalized gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. In situ high-pressure powder x-ray diffraction experiments were performed up to a maximum pressure of 40 GPa using synchrotron radiation. We find that the cubic Pm (3) over barm crystal symmetry persists throughout the pressure range studied. The calculated ground state properties-the equilibrium lattice constant, bulk modulus, and elastic constants-are in good agreement with experimental results. By analyzing the ratio between the bulk and shear moduli, we conclude that KMgF3 is brittle in nature. Under ambient conditions, KMgF3 is found to be an indirect gap insulator, with the gap increasing under pressure.
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| 8. |
- Hugosson, Håkan Wilhelm, 1972-, et al.
(författare)
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Theory of phase stabilities and bonding mechanisms in stoichiometric and substoichiometric molybdenum carbide
- 1999
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 86:7, s. 3758-3767
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Tidskriftsartikel (refereegranskat)abstract
- ABSTRACTFirst principles, total energy methods have been applied to predict the relative stabilities of the four experimentally verified MoC phases: the cubic δ(NaCl) phase and the three hexagonal γ(WC), η and γ′(TiAs) phases. The effect of vacancies on the relative stability of these four phases was investigated using a model structure with ordered vacancies within the carbon sublattice. For stoichiometric MoC, the γ phase was found to be the most stable followed by γ′, δ, and η, but for substoichiometric MoC0.75,MoC0.75, the order of relative stability was changed and the substoichiometric δ phase was found to have the lowest energy followed by γ′ and γ. A study of the electronic structure revealed vacancy induced peaks in the density of state and the electron density attached to these peaks was analyzed and found to emanate from unscreened Mo–Mo bonds through the carbon vacancy site. Finally, the oxygen stabilization of the γ′ MoC phase was studied.
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| 9. |
- Odell, Anders, 1979-, et al.
(författare)
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Comparison between s- and d-electron mediated transport in a photoswitching dithienylethene molecule using ab initio transport methods
- 2011
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 84:16, s. 165402-
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the electrode's Fermi surface on the transport properties of a photoswitching molecule is investigated with state-of-the-art ab initio transport methods. We report results for the conducting properties of the two forms of dithienylethene attached either to Ag or to nonmagnetic Ni leads. The I-V curves of the Ag/dithienylethene/Ag device are found to be very similar to those reported previously for Au. In contrast, when Ni is used as the electrode material the zero-bias transmission coefficient is profoundly different as a result of the role played by the Ni d bands in the bonding between the molecule and the electrodes. Intriguingly, despite these differences the overall conducting properties depend little on the electrode material. We thus conclude that electron transport in dithienylethene is, for the cases studied, mainly governed by the intrinsic electronic structure of the molecule.
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| 10. |
- Odell, Anders, et al.
(författare)
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Geometric integration in Born-Oppenheimer molecular dynamics
- 2011
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 135:22, s. 224105-
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Tidskriftsartikel (refereegranskat)abstract
- Geometric integration schemes for extended Lagrangian self-consistent Born-Oppenheimer moleculardynamics, including a weak dissipation to remove numerical noise, are developed and analyzed.The extended Lagrangian framework enables the geometric integration of both the nuclear and electronicdegrees of freedom. This provides highly efficient simulations that are stable and energy conservingeven under incomplete and approximate self-consistent field (SCF) convergence. We investigatethree different geometric integration schemes: (1) regular time reversible Verlet, (2) secondorder optimal symplectic, and (3) third order optimal symplectic. We look at energy conservation,accuracy, and stability as a function of dissipation, integration time step, and SCF convergence. Wefind that the inclusion of dissipation in the symplectic integration methods gives an efficient dampingof numerical noise or perturbations that otherwise may accumulate from finite arithmetics in aperfect reversible dynamics.
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