Many-body effects observed in the positron annihilation experiment

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 (BML) theory. Moreover, it is not true that 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. Keywords: positron annihilation, Compton scattering, momentum densities, many-body effectsComment: 9 pages, 3 figure

Many-body aspects of positron annihilation in the electron gas

We investigate positron annihilation in electron liquid as a case study for many-body theory, in particular the optimized Fermi Hypernetted Chain (FHNC-EL) method. We examine several approximation schemes and show that one has to go up to the most sophisticated implementation of the theory available at the moment in order to get annihilation rates that agree reasonably well with experimental data. Even though there is basically just one number to look at, the electron-positron pair distribution function at zero distance, it is exactly this number that dictates how the full pair distribution behaves: In most cases, it falls off monotonously towards unity as the distance increases. Cases where the electron-positron pair distribution exhibits a dip are precursors to the formation of bound electron--positron pairs. The formation of electron-positron pairs is indicated by a divergence of the FHNC-EL equations, from this we can estimate the density regime where positrons must be localized. This occurs in our calculations in the range 9.4 <= r_s <=10, where r_s is the dimensionless density parameter of the electron liquid.Comment: To appear in Phys. Rev. B (2003

Recent Developments of the Bloch-Modified Ladder Theory

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

Electron-Positron Enhancement in d-Metals Described by the Bloch-Modified Ladder Theory and the Local Density Approximation: a Comparative Study

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

Electronic Structure Seen by Positrons in Extended and Reduced Zone Schemes

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

Umklapp Components of the Positron Momentum Density Depending on Different Models for the Positron Wave Function

In this paper, we present a numerical investigation about the question how sensitively Fourier coefficients of the positron wave function ψ$\text{}_{+}$ react to different (and not too strong) changes of ψ$\text{}_{+}$. 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

Modeling the absorption behavior of solar thermal collector coatings utilizing graded a-C:H/TiC layers

Wavelength selective coatings are of common use in order to enhance the efficiency of devices heated by radiation such as solar thermal collectors. The use of suitable materials and the optimization of coating layer thicknesses are advisable ways to maximize the absorption. Further improvement is achievable by embedding particles in certain layers in order to modify material properties. We focus on optimizing the absorption behavior of a solar collector setup using copper as substrate, a layer of amorphous hydrogenated carbon with embedded titanium carbide particles (a-C:H=TiC), and an antireflection coating of amorphous silicon dioxide (aSiO2). For the setup utilizing homogeneous particle distribution, a relative absorption of 90.98% was found, while inhomogeneous particle embedding yielded 98.29%. These results are particularly interesting since until now, absorption of more than 95% was found only by using embedded Cr but not by using the more biocompatible Ti