Wyniki wyszukiwania - Biblioteka Nauki

1

Influence of Many-Body Effects in Real Metals on Electron-Positron Momentum Distributions

100%

Boroński E.

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tom 125

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nr 3

706-709

EN

It is shown that if one takes into account the effective mass of the electron, which in real structures is actually different than the free electron mass, the electron and positron self-energy effects may result in flatter and smaller enhancement of the electron-positron momentum distribution. Thus, the many-body effects mentioned above, among other reasons like e.g. influence of lattice potential on electron and positron wave functions, can be responsible for decreasing of the discontinuity on the Fermi momentum and a greater smearing of the Fermi surface seen in several angular correlation of positron annihilation radiation experiments.

2

Investigation of Electron-Positron Correlations by Monte Carlo Simulation

100%

Boroński E.

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nr 4

576-585

EN

Earlier theoretical studies, as well as recent calculations of positron annihilation rates in metals, show that some basic problems concerning electron-positron (e-p) interaction have not been solved satisfactorily, even for homogeneous media (the electron gas). In turn, the "computer experiments" e.g. quantum Monte Carlo simulations applied to these problems by several authors yielded only fragmentary, inaccurate and even incorrect results for smaller electron densities. It is shown in the present paper that the quantum Monte Carlo method may be useful in investigations of positron interactions with electrons. Reasonable annihilation rates have been obtained owing to appropriate construction of the trial function and taking into account the 3-particle correlations (i.e. dependence of the electron-electron (e-e) interaction on the distance from the positron). Moreover, the method of "exact determination" (without any fitting) of positron annihilation rates on the basis of the variational trial function was proposed. One also found the way of calculating the momentum dependent enhancement factors, the quantities not achievable within the Monte Carlo method until now.

3

Electron-Positron Interaction in Jellium: Optimization of the Perturbed Hypernetted-Chain Approach

63%

Stachowiak H. , Boroński E.

Acta Physica Polonica A

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2006

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tom 110

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nr 5

709-720

EN

The perturbed hypernetted-chain approach to electron-positron interaction in jellium consists in assuming the wave function of the system in the form of a Slater determinant built of single-electron functions presented asψi k(s)=w(s)φ_i k(s). s is the distance between the electron and the positron. The function w(s) is obtained by solving the equation of Gondzik and Stachowiak. The functionφ_{i k}(s) is computed by applying a self-consistent Born approximation. The idea of the paper is to modify the function w(s) in such a way as to obtain from the Born approximation as precise results as possible. It is shown that the influence of such modifications on physical predictions is small. This shows how good is the perturbed hypernetted-chain approach. The annihilation rates obtained in such a way decrease somewhat, becoming closer to experimental expectations.

4

Electron-Positron Interaction in Metals. Theory and Experiment

63%

Stachowiak H. , Boroński E.

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nr 4

541-553

EN

The electron-positron interaction greatly complicates the interpretation of positron annihilation data. The two-detector Doppler measurements of Mijnarends et al. as well as our theoretical calculations point at the conclusion that the local density approximation to e^+-e^- interaction is a good way of treating this problem in real metals, at least the simplest ones. This shows that e^+-e^- interaction in an electron gas is the key to understanding this phenomenon also in inhomogeneous systems. On the basis of dozens of experiments one comes to the conclusion that the well known formula of Boroński and Nieminen for the electron accumulation on the positron in jellium describes the best the positron lifetimes in metals. However, it is based on the calculations of Lantto which start from a physically oversimplified trial function. The results of Arponen and Pajanne, of Rubaszek and Stachowiak, and of Stachowiak and Lach lead to too short positron lifetimes in spite of using less controversial assumptions. The discrepancy is of the order of 8 to 15% for r_s=2. This shows that we still do not fully understand e^+-e^- interaction even in an electron gas.

5

Electron-Positron Interaction in Jellium: Modification of the Perturbed Hypernetted-Chain Approach

63%

Stachowiak H. , Boroński E.

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nr 5

1523-1532

EN

The approach of Kahana to the electron-positron interaction in an electron gas contains the superfluous assumption that electron scattering on the positron can occur only to momentum states lying outside the Fermi sphere. The perturbed hypernetted-chain approach avoids that assumption, but self-consistency was achieved only in the Born approximation. In the present work a modification of perturbed hypernetted-chain approach allowed to reach self-consistency also at the Kohn-Sham level, at least for 1 ≤ r_s ≤ 3.5. The electron-positron correlation functions obtained in this way are compared to figures resulting from other approaches.

6

Electroni-Positron Scattering in Real Metals

51%

Boroński E. , Daniuk S. , Stachowiak H.

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nr 3

255-260

EN

The purpose of the paper is to point at the importance of some detailed studies of electron-positron scattering in real metals using wave functions obtained from band structure calculations. In this work some preliminary investigations of the matrix elements of electron-positron scattering, e.g. the transitions from occupied to non-occupied states close to the Fermi surface are presented. One can observe that for s → s transitions of the positron (s, p are orbital quantum numbers) electrons do not change their orbital quantum numbers but for s → p transitions of the positron, electron quantum numbers change by unity.

7

The Effective Electron-Positron Potential in Jellium

51%

Stachowiak H. , Boroński E. , Lach J.

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nr 3

391-394

EN

One of the methods of determining theoretical annihilation characteristics in real metals is the approximation called Bloch modified ladder approach. In this approach a Bethe-Goldstone type equation is solved with an effective electron-positron potential obtained previously for jellium of the corresponding electron density. If one wishes to include the dependence on the local electron density of the e^{+}-e^{-} effective potential in this formalism, it is necessary to know this potential for jellium, for metallic and above metallic densities. A review of different proposed e^{+}-e^{-} potentials is presented and their correctness is evaluated from the point of view of their application in a Bethe-Goldstone type formalism which is the jellium analogue of Bloch modified ladder approach.