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| 2. |
- Bovin, JO, et al.
(författare)
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Imaging of atomic clouds outside the surfaces of gold crystals by electron microscopy
- 1985
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 317, s. 47-49
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Tidskriftsartikel (refereegranskat)abstract
- The structure of small metal crystals has been intensively examined by high-resolution electron microscopy (HREM). In particular, multiply-twinned gold and silver crystals have been characterized using the profile-imaging method1,2 at atomic resolution, and reconstructed metal surfaces observed2. Crystal structure images of large gold clusters consisting of 55 gold atoms arranged in a cubeoctahedron have been recorded3 using 2.5 Å resolution, and crystal growth, row by row, on a {111} surface has been documented4 using a low-light-level silicon-intensified target television (TV) camera and video system with an on-line image processor. Direct imaging of rearrangements of atomic columns on extended gold surface5 established that profile imaging can provide information about surface self-diffusion. The motion of surface atoms, recorded with a real-time video tape-recorder (VTR) system, and the formation of surface atom steps on {100} surfaces, although not {111}, has also been reported recently6. Dynamic HREM observations at TV rate showing defect motion in gold7 and CdTe (refs. 8, 9) has given information on ‘in-lattice’ rearrangements of columns of atoms. We report here surface profile images recorded with the electron beam along the 〈110〉 direction with spatial resolution of ∼2.0 Å which reveal changes in occupancy of the atom columns often within periods of less than 0.1 s. Surfaces with several adjacent atom columns involved in rapid structural changes frequently interacted with a cloud of atoms extending out to 9 Å off the crystal and changes in shape and density of the clouds were recorded. Although these clouds have already been described6,10, the present work is the first to analyse these events properly and to describe them in detail. The motion of atomic columns and the existence of atom clouds revealed here may have important consequences for crystal growth, surface science and catalysis studies.
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| 3. |
- Wallenberg, LR, et al.
(författare)
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On the crystal structure of small gold crystals and large gold clusters
- 1985
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 156:1, s. 256-264
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Tidskriftsartikel (refereegranskat)abstract
- A large gold cluster with the formula Au55[(C6H5)3P]12Cl6, was imaged with a high-resolution transmission electron microscope (HRTEM) and a proposed model was confirmed. Growth of gold crystals of approximately 4.0 nm size could be followed, row by row of atoms, with the use of a low light level TV camera and on-line image processing.
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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
- Almbladh, Carl-Olof, et al.
(författare)
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Exact results for the charge and spin densities, exchange-correlation potentials, and density-functional eigenvalues
- 1985
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Ingår i: Physical Review B. - 1550-235X. ; 31:6, s. 3231-3244
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Tidskriftsartikel (refereegranskat)abstract
- We derive asymptotically exact results for the charge and spin densities far away from finite systems (atoms and molecules) and far outside solid surfaces. These results are then used to obtain the correct asymptotic form of the exchange-correlation potential of density-functional (DF) theory and to prove that, for all systems, the eigenvalue of the uppermost occupied DF orbital equals the exact ionization potential. For spin-polarized finite systems we show that the uppermost DF eigenvalue in each spin channel is also given by exact excitation energies.
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| 8. |
- Almbladh, Carl-Olof, et al.
(författare)
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IMPORTANCE OF SURFACE EFFECTS AND DYNAMICAL CORE-HOLE EFFECTS IN AUGER SPECTRA OF METALS AND SEMICONDUCTORS
- 1987
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Ingår i: Journal de Physique. Colloques. ; 48:c-9, s. 879-890
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Tidskriftsartikel (refereegranskat)abstract
- Auger CVV spectra of sp-bonded materials are generally believed to be well described by one-electron theory in the bulk. We here perform detailed calculations on several simple metals and on silicon and obtain results which differ markedly from experiments. To explain these discrepancies we investigate effects of the surface and dynamical effects of the sudden disappearance of the core hole in the final state. To study core-hole effects we solve Mahan-Nozières-DeDominicis (MND) model numerically over the entire band. The core-hole potential and other parameters in the MND model are determined by self-consistent calculations on systems with a core-hole impurity. To study surface and mean free path effects we perform slab calculations in the case of silicon, whereas we for the metals use a simpler infinite barrier model in which the valence states near the surface are constructed from Bloch states in a simple way. In all cases investigated either the effects of the surface or the effects of the core hole give important modifications and a much improved agreement with experiments.
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| 9. |
- Almbladh, Carl-Olof
(författare)
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Importance of vertex corrections for obtaining correct selection rules in the theory of photoemission
- 1986
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Ingår i: Physical Review B. - 1550-235X. ; 34:6, s. 3798-3806
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Tidskriftsartikel (refereegranskat)abstract
- It is shown that the commonly used approximation for the no-loss angle-resolved photocurrent from solids in general violates the optical selection rules if the final-state inverse low-energy electron diffraction orbital is taken to be damped. The violations occur in the velocity formula, which is inconsistent with the corresponding and commonly used acceleration formula in which the matrix elements are evaluated with respect to the gradient of the one-electron potential. It has recently been shown that, in a correct description of the no-loss photocurrent, the bare matrix elements should be replaced by the appropriate vector-coupling vertex function. We demonstrate that this modification is needed in order to have correct selection rules and consistency between the velocity and acceleration formulas for photoemission and photoabsorption. Our formalism further allows us to interpret the commonly used acceleration formula in terms of a well-defined approximation which leaves out local-field effects, and to give approximations which obey the selection rules.
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| 10. |
- Almbladh, Carl-Olof, et al.
(författare)
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Many-Body Theory of Auger Processes in Metals and Semiconductors
- 1988
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Ingår i: Recent Progress in Many-Body Theories. - 9781461309734 ; 1, s. 317-328
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Bokkapitel (refereegranskat)abstract
- Auger spectra of solids arise from radiationless decay of innershell vacancies created by x-ray or electron bombardment. In a corevalence-valence (CVV) Auger process, for instance, one valence electron is filling the core hole while another valence electron is ejected and carries away the excess energy. The core hole is essentially dispersionless, and thus the spectrum of emitted electrons would, in the simplest one-electron picture, reflect the self-folded valence-electron state density. Such a simple model is, of course, grossly oversimplified, and the importance of matrix element effects has been demonstrated by several authors.1 However, no detailed investigation of many-body effects has been presented thus far for sp-bonded materials, and the current view is that one electron theory and bulk wavefunctions suffice to explain the experimental results.
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