| 131. |
- 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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| 132. |
- Almbladh, Carl-Olof, et al.
(författare)
-
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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| 133. |
- Almbladh, Carl-Olof
(författare)
-
On the theory of photoemission
- 1985
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Ingår i: Physica Scripta. - 0031-8949. ; 32, s. 341-352
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Tidskriftsartikel (refereegranskat)abstract
- This paper is concerned with the a priori theory of the photoemission process in solids. Scattering-theoretical and response-theoretical approaches are discussed and a new demonstration of their equivalence is given. The elastic photoemission spectrum is analyzed in detail and an expression exact to all orders in interparticle interaction is obtained. This expression is similar to the renormalized three-current correlation formula used by Pendry and by others but includes also a screening and exchange-correlation correction to the optical field. The loss part of the spectrum is also discussed and a new quantum-mechanical approximation for the energy losses is obtained. For long mean free paths this correctly reduces to a three-step model in which the energy dissipation is given by a semi-classical transport equation.
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| 134. |
- Almbladh, Carl-Olof, et al.
(författare)
-
Phonon effects in deep-level spectra of metals: Beyond the linear-screening and linear-coupling models
- 1983
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Ingår i: Physical Review B. - 1550-235X. ; 28:12, s. 7362-7365
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Tidskriftsartikel (refereegranskat)abstract
- A priori calculations of phonon broadening of core levels in metals are presented. The effects of nonlinear screening and higher-order couplings are taken into account for the first time. The nonlinear screening effects are very important for Li and must be accounted for in order to obtain agreement with experiment.
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| 135. |
- Almbladh, Carl-Olof, et al.
(författare)
-
Phonon effects on deep-level excitations in metals: Beyond the linear-screening and linear-coupling models
- 1985
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Ingår i: Journal of Physics F: Metal Physics. - 0305-4608. ; 15:4, s. 991-1012
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Tidskriftsartikel (refereegranskat)abstract
- New theoretical developments in impurity lattice dynamics are presented, with the emphasis on phonon broadening of core levels in metals. Detailed calculations have been made for Li and Na. The effects of non-linear screening are evaluated for the first time and are shown to increase the phonon broadening by about 70% for Li compared with the linear-screening value. The present a priori estimate of the phonon broadening of the Li 1s level is the first which agrees with experiment. The effects of core-hole-lattice couplings of second order in ion displacements are also evaluated and shown to be unimportant for Li and Na. The usual linear-screening theory for lattice dynamics and for impurity-lattice couplings is analysed critically with regard to its sensitivity to the approximations for the pseudopotential and for the screening function. Both optimised model potentials and norm-conserving a priori potentials are considered. Finally, an indirect phonon broadening mechanism proposed previously is analysed and shown to give a negligible effect.
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| 136. |
- Almbladh, Carl-Olof, et al.
(författare)
-
Statistical mechanics of band states and impurity states in semiconductors
- 1981
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Ingår i: Journal of Physics C: Solid State Physics. - 0022-3719. ; 14:31, s. 4575-4601
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Tidskriftsartikel (refereegranskat)abstract
- Thermodynamic analogies have previously been invoked to interpret the temperature- and pressure-dependent electronic levels in a semiconductor as standard Gibbs free energies. In this paper it is shown by means of statistical mechanical arguments using the pressure ensemble that standard Gibbs free energies are the quantities determining equilibrium occupations of electronic levels and hence are the thermodynamic quantities that enter detailed balance relationships. Optical band gaps are identified with the 'nophonon line' gap for a transition in which the occupation numbers for the phonons and remaining electrons are unchanged. An exact expression for no phonon optical transition energies is given. It is shown rigorously that the optical no-phonon-line energy is identical to the corresponding free-energy change for any transition between band states and also for transitions involving localised states provided: (i) the effects of electronic degeneracy changes are excluded from the free energy definition; (ii) the localised state is not associated with a local lattice mode.
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| 137. |
- Almbladh, Carl-Olof, et al.
(författare)
-
Statistical mechanics of electronic energy levels in semiconductors
- 1982
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Ingår i: Solid State Communications. - 1879-2766. ; 41:2, s. 173-176
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Tidskriftsartikel (refereegranskat)abstract
- It is shown, using Guggenheim's pressure ensemble that partial Gibbs free energies are the quantities determining equilibrium occupations of electronic levels and hence are the quantities that enter detailed balance relationships. It is also shown that the energy of an optical no-phonon transition between band states equals the corresponding free energy change. This is also true for transitions involving localized impurity states provided: 1.(a) the effects of electronic degeneracy are excluded from the free energy definition, and2.(b) the localized state does not couple to a local phonon mode.
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| 138. |
- Almbladh, Carl-Olof, et al.
(författare)
-
Theory of Auger core-valence-valence processes in simple metals. I. Total yields and core-level lifetime widths
- 1989
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Ingår i: Physical Review B. - 1550-235X. ; 39:6, s. 3489-3502
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Tidskriftsartikel (refereegranskat)abstract
- There is a considerable disagreement in the literature on the description of lifetime effects arising from core-valence transitions in solids. We calculate here Auger and radiative widths of shallow core levels in Li, Be, Na, Mg, and Al with use of principles consistent with dynamical theories of secondary-emission processes developed earlier. The lifetime has no simple relation to the usual self-energy but is instead directly related to emission yields. The problem of choosing reliable approximations for Auger rates and matrix elements is analyzed theoretically and computationally. We also comment on some earlier approaches. Much of our discussion pertains also to calculations of Auger line shapes from first principles. For long hole lifetimes the total and partial level widths obey an initial-state rule and follow from wave functions perturbed by a static core hole. To obtain these impurity wave functions we perform self-consistent supercell calculations. The core-hole screening increases the Auger rates by factors of the order 2–4 compared with results from ground-state orbitals but has never been properly included before. The width of the 1s level in Li is rather accurately known because it monitors large effects of incomplete lattice relaxation. For Li we obtain here a width 17 meV in excellent agreement with the value 16 meV deduced earlier from measurements by Callcott et al.
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| 139. |
- Almbladh, Carl-Olof, et al.
(författare)
-
Theory of Auger core-valence-valence processes in simple metals. II. Dynamical and surface effects on Auger line shapes
- 1989
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Ingår i: Physical Review B. - 1550-235X. ; 39:6, s. 3503-3516
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Tidskriftsartikel (refereegranskat)abstract
- Auger CVV spectra of simple metals are generally believed to be well described by one-electron-like theories in the bulk which account for matrix elements and, in some cases, also static core-hole screening effects. We present here detailed calculations on Li, Be, Na, Mg, and Al using self-consistent bulk wave functions and proper matrix elements. The resulting spectra differ markedly from experiment and peak at too low energies. To explain this discrepancy 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-De Dominicis (MND) model numerically over the entire band. The core-hole potential and other parameters in the MND model are determined by self-consistent calculations of the core-hole impurity. The results are compared with simpler approximations based on the final-state rule due to von Barth and Grossmann. To study surface and mean-free-path effects we perform slab calculations for Al but use a simpler infinite-barrier model in the remaining cases. The model reproduces the slab spectra for Al with very good accuracy. 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 experiment.
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| 140. |
- Almbladh, Carl-Olof, et al.
(författare)
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Theory of Band-like Auger Spectra in sp-Bonded Materials
- 1988
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Ingår i: Springer Series in Solid-State Sciences: Core-Level Spectroscopy in Condensed Systems. - 9783642834370 ; , s. 263-280
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Konferensbidrag (refereegranskat)abstract
- Auger-electron spectroscopy (AES) has developed to a major tool for chemical identification and for characterization of solid surfaces. As first emphasized by Lander[1], it can also be used a spectroscopy for gaining information on occupied electron states and their interactions. However, AES gives a far less direct picture of the occupied states than does, for example, angle-resolved photoemission spectroscopy. Owing to the broad selection rules, Auger spectra sample large classes of one-electron states, and the interpretation is also complicated by possible effects of the interparticle interaction and of the surface. I will here try to convince the reader that Auger spectra from sp-bonded materials can nevertheless be fairly well understood using conceptually rather simple models. I will, however, also emphasize the need of realistic evaluations of such models in order to verify that they contain the correct physical mechanisms. A large part of this review will be focused on recent, yet unpublished work by the Lund group.
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