| 1. |
- Björkman, Henrik, 1977-
(författare)
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Alternative Methods for Assessment of Split Renal Function
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Living kidney donation is a clinical situation with unique features in the sense that healthy individuals voluntarily expose themselves to certain risks and inconveniences. Therefore, eliminating as much of the associated discomfort as possible is crucial. The primary aim of this study was to evaluate whether it is possible to use the examination with computed tomography (CT), which is essential to the investigation, also for determining the ratio of the two kidneys’ function – the split renal function. If possible, an examination with gamma camera renography could be excluded from the work-up. To investigate this possibility, 27 subjects who had underwent CT and renography as part of kidney donor investigation were studied retrospectively. The quantity of contrast material in each kidney was considered proportional to that kidney’s function, and measurement was made in each of the two available contrast phases. The results were compared to the results from renography. A similar analysis was conducted in 38 patients investigated for suspected renal artery stenosis with CT and renography, including a study of an automatized method for the acquisition of data from CT. For further scrutiny, a respiratory triggered dynamic contrast-enhanced magnetic resonance imaging (MRI) examination was investigated in 26 individuals. Results of split renal function were compared with renography and with CT in a subgroup. To study the possibility of facilitating the data analysis with CT, a formula for approximation of the contrast attenuation was studied in 64 subjects. An analysis of the significance of choice of contrast phase was also conducted in 43 subjects. Unsatisfactory agreement with renography resulted from the CT analysis of previous donors, partly due to technical shortcomings. However, the technique was recognized to have a potential value. In the subsequent material, the settings were improved, with beneficial effects on the agreement. Respiratory-triggered MRI generated high quality examinations of renal uptake and excretion, with results harmonizing well with renography and CT. The approximation formula applied to CT resulted in higher accuracy for renal volume assessment than with the automatic method, and an acceptable agreement of the split renal function estimate. From the presented results, a revision of the current donor investigation protocol is suggested. CT gives sufficient information to exclude renography as a routine examination. In cases of uncertainty, renography is recommended for secondary evaluation.
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| 2. |
- Björkman, Torbjörn, 1977-, et al.
(författare)
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Adaptive smearing for Brillouin zone integration
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Annan publikation (populärvet., debatt m.m.)abstract
- We suggest a simple scheme for automatically determining the witdth parameter of smearing methods of the Brillouin zone integration in electronic structure calculation. The scheme retains one free parameter that at any time can be eliminated by choosing a denser k-space mesh until the desired accuracy is obtained. Tests are carried out in the context of Methfessel-Paxton smearing. This adaptive Gaussian smearing (AGS) is easily implemented, variational with respect to partial occupancies and free from spurious occupancies that are negative or larger than one. Its convergence properties are similar to those obtained with the modified tetrahedron method for energy resolution of ≥ 0 .1 meV.
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| 3. |
- Björkman, Torbjörn, 1977-, et al.
(författare)
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Coupling between the 4f core binding energy and the 5f valence band occupation of elemental Pu and Pu-based compounds
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78:24, s. 245101-
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that there is a close coupling between 4f core binding energy and 5f valence band occu- pation for Pu-based materials. This enables accurate determination of the valence occupation of this class of materials by simply measuring the 4f core binding energy. Our study demonstrates that all allotropes of elemental Pu have a 5f occupation close to 5, a finding which is inconsistent with several of the current theoretical models attempting to describe this unique element. A few of the theoretical models proposed for this material are however consistent with our finding; a fact which is discussed in detail.
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| 4. |
- Björkman, Torbjörn, 1977-
(författare)
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Magnetic and Structural Properties of f-electron Systems from First Principles Theory
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A series of studies of f-electron systems based on density functional theory methods have been performed. The focus of the studies has been on magnetic and structural properties, as well as investigating ways to handle strong electron correlation in these systems. A version of the self-interaction correction (SIC) method has been developed for a full-potential linear muffin-tin orbital method. The method is demonstrated to have the strong capabilities of previous SIC implementations, to study energetics and phase stabilities of d- and f-electron systems with localisation-delocalisation transitions, but with no geometrical constraints from the underlying band structure method. The method is applied to the high-TC superconductor CeOFeAs, in which the f-shell of the Ce atoms is argued to undergo a Mott transition to a delocalised state under pressure. The non-collinear magnetic structures of two rare earth compounds, TbNi5 and CeRhIn5 have been studied, and in both cases the complex magnetic ordering can be attributed to the effects of Fermi surface nesting. The magnetic properties of the FeMnP0.75Si0.25 system has been studied and found to have an extreme sensitivity to the amount of disorder of the Fe-Mn sublattice. Elements with valence f electrons typically exhibit very complex phase diagrams, with the frequently occurring phenomenon that they melt from a bcc phase that is unstable in calculations based solely on the electronic structure. The high temperature bcc phase of the elements La and Th were studied by means of the self-consistent ab initio lattice dynamics method that accounts for phonon-phonon interaction. Delicate magnetic and structural properties are often sensible to details of how the Brillouin zone (BZ) integration is performed. An improved scheme is proposed that adapts to the BZ mesh and allows for better energy convergence of small energy differences in the smearing type methods. Correlation effects in the 5f-states of plutonium has in recent years been the focus of attention for many theoretical studies employing extensions to DFT schemes. These different schemes have often produced large variations in 5f occupation numbers, and therefore a survey was made of experimental occupation numbers and 4f core level shifts to establish a value for the 5f occupation without any computational bias.
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| 5. |
- Björkman, Torbjörn, 1977-, et al.
(författare)
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Self-interaction correction scheme for a full-potential linear muffin-tin orbital method
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Annan publikation (populärvet., debatt m.m.)abstract
- The self-interaction correction has been implemented in a full-potential linear muffin-tin orbital (FP-LMTO) scheme. The method is tested on several known materials where self-interaction is important; transition metal oxides and rare-earth elements. We find in general good agreement with previous implementations based on the atomic-sphere approximation. The implementation can be used for an crystal structure, without geometrical constraints. In addition, it is possible to consider non-spherical components to the electron density, which in self-interaction corrected electronic structure methods poses a particularly challenging problem, which is analyzed in detail.
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| 6. |
- Björkman, Torbjörn, 1977-, et al.
(författare)
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Theoretical investigation of the volume collapse of CeOFeAs under pressure
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We conduct a density functional theory study of the high TC superconductor CeOFeAs using the self-interaction corrected local density approximation (LDA-SIC). We find that under compression the compound undergoes a transition from localized to delocalized Ce 4f -states at a pressure close to the experimentally observed point of vanishing superconductivity. This suggests that the observed abruptly vanishing superconductivity under pressure may have its origin in a Mott transition of the 4f electrons of the CeO layer.
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| 7. |
- Björkman, Torbjörn, 1977-, et al.
(författare)
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Theoretical studies of the incommensurate magnetic structure of a heavy fermion system: CeRhIn5
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:9, s. 094433-
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the electronic structure and the incommensurate magnetic configuration of the pressure-induced superconductor CeRhIn5. Noncollinear first-principles calculations were performed in the local-density approximation plus U scheme. The observed magnetic configuration is described accurately in our calculations, especially considering the minute energy scale which is relevant (microelectron volt). The band structure and Fermi surfaces were investigated and nesting was found to be responsible for the complex noncollinear magnetic state of CeRhIn5.
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| 8. |
- Hudl, Matthias, et al.
(författare)
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Order–disorder induced magnetic structures of FeMnP0.75Si0.25
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:13, s. 134420-
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Tidskriftsartikel (refereegranskat)abstract
- We report on the synthesis and structural characterization of the magnetocaloric FeMnP0.75Si0.25 compound. Two types of samples (as quenched and annealed) were synthesized and characterized structurally and magnetically. We have found that minute changes in the degree of crystallographic order causes a large change in the magnetic properties. The annealed sample, with higher degree of order is antiferromagnetic with a zero net moment. The as-quenched sample has a net moment of 1.26 μB /f.u. and ferrimagnetic-like behavior. Theoretical calculations give rather large values for the Fe and Mn magnetic moments, both when occupied on the tetrahedral and pyramidal lattice site. The largest being the Mn moment for the pyramidal site reaches values as high as 2.8 μB /atom.
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| 9. |
- Souvatzis, Petros, et al.
(författare)
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Dynamical stabilization of the body centered cubic phase in lanthanum and thorium by phonon–phonon interaction
- 2009
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 21:17, s. 175402-
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Tidskriftsartikel (refereegranskat)abstract
- A recently developed self-consistent ab initio lattice dynamical method has been applied to the high temperature body centered cubic (bcc) phase of La and Th, which are dynamically unstable at low temperatures. The bcc phase of these metals is found to be stabilized by phonon–phonon interactions. The calculated high temperature phonon frequencies for La are found to be in good agreement with the corresponding experimental data.
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