| 1. |
- Borg, Mikael, et al.
(författare)
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Experimental and theoretical surface core-level shifts of aluminum (100) and (111)
- 2004
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Ingår i: Physical Review B. - 1550-235X. ; 69
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Tidskriftsartikel (refereegranskat)abstract
- The surface core-level shifts of Al(111) and Al(100) have been measured using high-resolution core-level photoemission spectroscopy and calculated using density functional theory (DFT). For Al(100), the 2p core-level shift of the first (second) layer was determined to be –75 meV (+20 meV) from experiment and –71 meV (+20 meV) from the DFT calculations. For Al(111), the corresponding values are –27 meV (0 meV) from experiment and –14 meV (–) from the DFT calculations. Core-level splittings caused by the low-symmetry crystal fields at the (111) and (100) surfaces have also been studied. These splittings turn out to be much smaller than previously reported provided proper care is taken of the influence of the core hole screening and of core–valence exchange beyond the DFT level. Finally, the experimental Al 2p line shape was found to contain structure caused by a sharp no-phonon line and a broad and weak phonon replica.
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| 2. |
- Kurth, S, et al.
(författare)
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Time-dependent quantum transport: A practical scheme using density functional theory
- 2005
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 72:3
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Tidskriftsartikel (refereegranskat)abstract
- We present a computationally tractable scheme of time-dependent transport phenomena within open-boundary time-dependent density functional theory. Within this approach all the response properties of a system are determined from the time propagation of the set of "occupied" Kohn-Sham orbitals under the influence of the external bias. This central idea is combined with an open-boundary description of the geometry of the system that is divided into three regions: left/right leads and the device region ("real simulation region"). We have derived a general scheme to extract the set of initial states in the device region that will be propagated in time with proper transparent boundary-condition at the device/lead interface. This is possible due to a new modified Crank-Nicholson algorithm that allows an efficient time-propagation of open quantum systems. We illustrate the method in one-dimensional model systems as a first step towards a full first-principles implementation. In particular we show how a stationary current develops in the system independent of the transient-current history upon application of the bias. The present work is ideally suited to study ac transport and photon-induced charge-injection. Although the implementation has been done assuming clamped ions, we discuss how it can be extended to include dissipation due to electron-phonon coupling through the combined simulation of the electron-ion dynamics as well as electron-electron correlations.
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| 3. |
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| 4. |
- Birgersson, Martin, et al.
(författare)
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Density-functional theory applied to Rh(111) and CO/Rh(111) systems: Geometries, energies, and chemical shifts
- 2003
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 67:4, s. 1-04502
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Tidskriftsartikel (refereegranskat)abstract
- We present extensive density-functional theory (DFT) based calculations of the clean Rh(111) surface and of CO/Rh(111) overlayer systems. We study both ground-state structural properties and core-level shifts from differences in total energies at different coverages and adsorption sites. Most results are obtained using using norm-conserving or ultrasoft pseudopotentials. The overall reliability of the pseudopotential method is analyzed theoretically, and computationally by way of all-electron calculations. In general, core corrections are required in order to correctly simulate all-electron total energies, although the corrections are rather small for the systems considered here. Overall there is a very good agreement both between the pseudopotential and all-electron results as well as with high-resolution experimental spectra. The obtained agreement between theoretical and experimental core-level energies, however, requires that the correct geometrical parameters are used. For instance, inclusion of bucklings of the first Rh layer in the (2x2)-1CO and (root3x3)R30degrees-1CO overlayers is essential. For the overlayers studied here, different competing adsorption sites give almost the same frozen-lattice adsorption energies. However, the C 1s binding energy shows large differences between CO adsorbed in different sites. Thus calculations of the C 1s shifts allow us to predict the adsorption sites despite the small differences in ground-state energies. We also analyze sources of the shifts in terms of differences in Hartree potential and relaxation at different sites. As the DFT core eigenvalue lies above rather than below the core excitation energy some care is required in order to properly identify a relaxation energy in a DFT framework. In order to clarify the question we relate the DFT approach for core energies to approaches based on self-energies or the Hartree-Fock approximation.
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| 5. |
- Borg, Mikael, et al.
(författare)
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Structure and formation of the Al(100)-(root 5 x root 5) R27 degrees-Na phase: a LEED, DFT and HRCLS study
- 2002
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Ingår i: Surface Science. - 0039-6028. ; 515:2-3, s. 267-280
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Tidskriftsartikel (refereegranskat)abstract
- Adsorption of 0.2 ML Na on Al(1 0 0) at room temperature yields a disordered Al(1 0 0)-(1 x 1)-Na phase, which transforms reversibly to a well-ordered Al(1 0 0)-(root5 x root5)R27degrees-Na phase on cooling below 250 K. Based on low energy electron diffraction (LEED) and high resolution core-level spectroscopy (HRCLS) measurements, and on ab initio calculations, it is concluded that the structure of the Al(1 0 0)-(root5 x root5-)R27degrees-Na phase consists of Na atoms occupying substitutional sites. The structural parameters obtained from LEED and density functional theory analyses are in quantitative agreement. Adsorption of 0.2 ML Na at 100 K yields an Al(1 0 0)-c(2 x 2)-Na island structure, which transforms irreversibly into the (root5 x root5)R27degrees-Na structure by annealing above 190 K. The nature of the reversible and irreversible phase transformations to the (root5 x root5)R27degrees structure is investigated by HRCLS and LEED. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 6. |
- Almbladh, Carl-Olof, et al.
(författare)
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BANDSTRUCTURE AND LOCALIZED DESCRIPTIONS OF NARROW-BAND SYSTEMS
- 1988
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Ingår i: Narrow-Band Phenomena—Influence of Electrons with Both Band and Localized Character. ; 184, s. 81-89
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Bokkapitel (refereegranskat)abstract
- In this note I would like to give some personal views on the relevance of bandstructure treatments of narrow-band systems. At first sight one might think that bandstructure theory is not a very appropriate framework for quasi-localized electrons. Nevertheless, it has been found that density-functional (DF) theory in its modern form gives a rather impressive description of ground-state properties for the majority of systems, not only " in principle " but also in practice via calculations based on the local-density approximation (LDA). I will here discuss some key concepts in DF theory with emphasis on physical significance of the eigenvalues which appear in the DF one-electron-like equations. I will also to some extent discuss the dynamical selfenergy Σ, which describes the " one-electron " excitations reached by adding or removing an electron, although there is very little work done for narrow-band electrons.
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| 7. |
- Almbladh, Carl-Olof, et al.
(författare)
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Comments on core-hole lifetime effects in deep-level spectroscopies
- 1978
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Ingår i: Physical Review B. - 1550-235X. ; 17:2, s. 929-939
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Tidskriftsartikel (refereegranskat)abstract
- he effects of a finite core-hole lifetime on deep-level spectra in solids have recently attracted much interest. We here present a solvable model which enables a detailed analysis of lifetime effects. For x-ray photoemission and x-ray absorption, our analysis gives support for the usual convolution approach where the infinite lifetime spectrum is convoluted with a Lorentzian lifetime broadening. In particular, we demonstrate the inadequacy of a recent intuitive approach which predicts a suppression of the phonon broadening for short core-hole lifetimes. This is done by testing the different theories on our model. We also show that a straightforward diagrammatic analysis leads to the same conclusion. Finally, we discuss the phonon response to the total process of core-hole creation and subsequent annihilation in an x-ray or Auger emission event. It is explained in some detail that the lifetime independence of the shake-up connected with the first step in this process is in no contradiction with the fact that the phonon system is uneffected by the total deep-level process in the limit of short core-hole lifetimes. We also analyze the process of 1s hole creation and subsequent Auger K-LL emission and give explicit expressions for this process.
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| 8. |
- Almbladh, Carl-Olof, et al.
(författare)
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Density-Functional Theory and Excitation Energies
- 1985
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Ingår i: Density Functional Methods in Physics. - 9781475708189 - 9781475708202 ; , s. 209-231
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Bokkapitel (refereegranskat)abstract
- During the past decade, the local-density (LD) approximation within density-functional (DF) theory has been the most important method for obtaining the electronic properties of realistic systems. The limitations of the method have to a large extent been computational in nature rather than theoretical. The conceptually simple one-particle equations arising in the method have often been too difficult to solve in systems with low symmetry such as amorphous systems or surfaces. However, due to our increased understanding of these systems the computational techniques are quickly developing, and we forsee an even greater importance of the method in the near future. The theoretical limitations of the method will then become more evident and result in an urgent need for improvements beyond the LD approximation. Such a need exists already today in many systems such as, e.g., atoms and molecules. The description of correlation effects in these systems is necessary and important and the answers provided by the LD approximations are often too poor to be of practical use. Unfortunately, for a long time, only minor theoretical advances were made since the modern version of DF theory was laid down by Hohenberg, Kohn, and Sham1,2 in the mid sixties.
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| 9. |
- Almbladh, Carl-Olof, et al.
(författare)
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Effects of core-level degeneracies on core-level spectra
- 2004
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Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048. ; 137-40, s. 393-399
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Tidskriftsartikel (refereegranskat)abstract
- Effects of core-level degeneracies on X-ray photoemission spectra are considered theoretically and computationally. Processes within each degenerate sub-level manifold are treated by a leading-order cumulant approximation, and processes which connect spin-orbit or crystal-field split levels to leading order in the core-electron self-energy. The cumulant approximation is consistent with earlier asymptotic results and correctly describe the leading moments of the core-electron spectrum. The central quantity in both cases is an exponent function which may be expressed in terms of projected state densities and core-valence Coulomb matrix elements. Numerical results are presented for the Al L-2.3 and the Rh and Pd M-4.5 levels and compared with recent experiments. In the case of Rh M-4.5, the level-dependent effects are strong leading to an increased asymmetry, a much increased lifetime width of the M-4 level, and deviation from Lorentzian lifetime broadening. In Pd, the effects are much weaker owing to a much smaller Fermi-surface d density of states at the core-hole atom, and it is almost negligible in Al, the most noticeable effect being an increased width of the lower L-2 component. (C) 2004 Elsevier B.V. All rights reserved.
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| 10. |
- Almbladh, Carl-Olof
(författare)
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Energies of three-hole states in X-ray emission processes in metals
- 1985
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Ingår i: Journal of Physics C: Solid State Physics. - 0022-3719. ; 18:18, s. 3591-3594
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Tidskriftsartikel (refereegranskat)abstract
- Spherical solid model calculations reported earlier are used to estimate three-hole-state energies in Na, Mg and Al. The authors' estimate agrees well with recent experimental values obtained by Keski-Rahkonen and co-workers.
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