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Recent Developments of the Bloch-Modified Ladder Theory

100%
Sormann H.
Acta Physica Polonica A
|
1993
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vol. 83
|
issue 3
381-390
EN
One of the possible theoretical approaches for desribing the physics of electron-positron pairs in the inhomogeneous electron gas is the so-called "quasi-free" Bloch-modified ladder theory. Despite the success of this approach, it contains two very serious deficiencies, namely the complete neglect of the Bloch character of the electron and positron scattering states and of the electron-positron interaction potential. In this contribution, the importance of these Bloch effects for the Bloch-modified ladder theory results, especially for the momentum dependence of the positron enhancement in d-band metals, is demonstrated for the first time.
2
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Many-Body Effects Observed in the Positron Annihilation Experiment

63%
Kontrym-Sznajd G., Sormann H.
Acta Physica Polonica A
|
2005
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vol. 107
|
issue 4
562-569
EN
This paper is devoted to study many-body effects in the positron annihilation experiment, both electron-positron (e-p) and electron-electron (e-e) correlations. Various theories of the e-p interaction in real solids were used to verify them by comparing theoretical and experimental e-p momentum densities in Cu and Y. We show that the lattice potential has an essential influence on the e-p correlation effects, i.e. their proper description must be done via periodic lattice potential as e.g. in the Bloch modified ladder theory. Moreover, it is not true that the dynamic parts of the direct e-p and e-e interactions cancel each other because e-e correlations are observed not only in the Compton scattering but also in the positron annihilation experiments.
3
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Umklapp Components of the Positron Momentum Density Depending on Different Models for the Positron Wave Function

63%
Kontrym-Sznajd G., Sormann H.
Acta Physica Polonica A
|
1995
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vol. 88
|
issue 1
185-192
EN
In this paper, we present a numerical investigation about the question how sensitively Fourier coefficients of the positron wave function ψ_{+} react to different (and not too strong) changes of ψ_{+}. In order to obtain general information about this problem, we studied this sensitivity for several bcc and fcc metals and for different models of the positron wave function. Summarizing our results, we can say that this sensitivity is generally small (or at least moderate) for Fourier coefficients belonging to reciprocal lattice vectors G which lie nearest to the centre of the momentum space. For the outer vectors G, the amount of this sensitivity is strongly dependent on the crystal structure of the metal and on the special like of the change of the positron wave function.
4
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Electronic Structure Seen by Positrons in Extended and Reduced Zone Schemes

63%
Kontrym-Sznajd G., Sormann H.
Acta Physica Polonica A
|
1995
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vol. 88
|
issue 1
177-184
EN
The influence of the positron on the momentum distribution of annihilation quanta is investigated. Basing on general considerations, we show that a noninteracting positron, which generally reduces electronic densities, may enlarge some particular electronic umklapp components. Numerical tests were performed for alkalis, Al, Cu and Pd by applying augmented plane wave band structure calculations. In the paper we discuss also the influence of this effect on the electron-positron densities after including the electron-positron correlation effects.
5
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Electron-Positron Enhancement in d-Metals Described by the Bloch-Modified Ladder Theory and the Local Density Approximation: a Comparative Study

63%
Sormann H., Wallner D.
Acta Physica Polonica A
|
1995
|
vol. 88
|
issue 1
225-235
EN
In this paper, we present a preliminary summary of our recent results on the enhancement of the electron-positron annihilation rate in d-band metals based on our recently published optimized quasi-free Bloch-modified ladder [QF-BML(opt.)] theory. This approach enables us to investigate the influence of the periodical lattice potential on the electron-positron annihilation in an approximative but nevertheless physically reasonable way. We used our theory for calculations of momentum-dependent enhancement factors belonging to electron states of different (s-, p-, d-) character in simple, transition and noble metals (Na, Cu, Pd, V). It is interesting to compare these new BML results with corresponding results obtained by the local density approximation (LDA) according to the work of Daniuk et al. We observe relatively strong differences between the BML and LDA enhancement factors for metals whose polarization process is dominated by s or p electrons. In such cases, we presume that the LDA approach has the tendency to overestimate the role of the more-localized d electrons in the polarization of the inhomogeneous electron gas. For transition metals whose physics is mainly determined by such d electrons, the discrepancies between BML and LDA enhancement results are significantly smaller.
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