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- Keshavarz, Samara, et al.
(författare)
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Layer-resolved magnetic exchange interactions of surfaces of late 3d elements : Effects of electronic correlations
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 92:16
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of an ab initio study of the magnetic properties of Fe, Co, and Ni surfaces. In particular, we discuss their electronic structure and magnetic exchange interactions (J(ij)), as obtained bymeans of a combination of density functional theory and dynamical mean-field theory. All studied systems have a pronounced tendency to ferromagnetism both for bulk and surface atoms. The presence of narrowband surface states is shown to enhance the magnetic moment as well as the exchange couplings. The most interesting results were obtained for the Fe surface where the atoms have a tendency to couple antiferromagnetically with each other. This interaction is relatively small when compared to interlayer ferromagnetic interaction, and it depends strongly on the lattice parameter. Local correlation effects are shown to lead to strong changes of the overall shape of the spectral functions. However, they seem not to play a decisive role in the overall picture of magnetic couplings studied here. We have also investigated the influence of correlations on the spin and orbital moments of bulklike and surface atoms. We found that dynamical correlations in general lead to enhanced values of the orbital moment.
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- 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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- Bood, JR, et al.
(författare)
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Urinary excretion of lipid mediators in response to repeated eucapnic voluntary hyperpnea in asthmatic subjects
- 2015
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Ingår i: Journal of applied physiology (Bethesda, Md. : 1985). - : American Physiological Society. - 1522-1601 .- 8750-7587. ; 119:3, s. 272-279
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Tidskriftsartikel (refereegranskat)abstract
- Exercise-induced bronchoconstriction displays refractoriness manifested as a decreased response to repeated exercise challenge within hours. The refractoriness may be attenuated by inhibition of the biosynthesis of prostaglandins (PG). The aim of the study was to determine which PGs and other lipid mediators are excreted during the refractory period. First, 16 subjects with mild stable asthma performed two repeated 4-min challenges with eucapnic voluntary hyperpnea (EVH) 1 and 3 h apart. There was a similar degree of refractoriness in both protocols (∼15% protection). The 1-h interval was too short to study mediator excretion because the urinary levels did not return to baseline before the second challenge. With the 3-h protocol, there was increased urinary excretion of cysteinyl-leukotrienes and metabolites of the mast cell product PGD2after both challenges. Next, another eight subjects performed two 6-min challenges with EVH 3 h apart, which produced a greater bronchoconstrictor response than the 4-min protocol (30.0 ± 5.4 vs. 17.7 ± 1.5%; P = 0.0029) and a greater degree of refractoriness (∼30%). Analysis by ultra-performance liquid chromatography triple quadrupole mass spectrometry confirmed excretion of the bronchoconstrictor cysteinyl-leukotrienes and PGD2during both challenges. In addition, there was increased excretion of the bronchoprotective PGE2, and also of the main metabolite of PGI2. This is the first report of excretion of PGE2and PGI2during the refractory period to EVH challenge, suggesting that they may mediate the refractoriness. Maintained excretion of PGD2and leukotriene E4following the repeat challenge argues against mast cell mediator depletion as the mechanism of refractoriness.
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- 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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| 9. |
- 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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