Electron-positron Momentum Distribution Research Articles

Pulsar emission: Langmuir modes in a relativistic multicomponent plasma

Ions, protons and possibly a small flux of electrons and positrons are accelerated outward from the polar cap of a normal or millisecond pulsar whose rotational spin is antiparallel with its magnetic moment. The Langmuir modes of this relativistic plasma have several properties of significance for the origin of coherent radio emission. The characteristics of the mode are determined by the sequence of singularities in the dielectric tensor at real angular frequencies, which in turn is fixed by the electron-positron momentum distribution. We find that under a certain condition on its momentum distribution, an electron-positron flux two orders of magnitude smaller than the Goldreich-Julian flux stabilizes the plasma and extinguishes the mode. But more generally, both the growth rate and wavenumber of the multi-component Langmuir mode can be as much as an order of magnitude larger than those of the two-component ion-proton mode. It appears to be a further effective source for the plasma turbulence whose decay is probably responsible for the observed emission.

Three-dimensional electron-positron momentum distribution of O3+-irradiated 6H SiC using two positron spectroscopy techniques simultaneously

A three-dimensional (3D) positron annihilation spectroscopy system (3DPASS) capable of determining 3D electron-positron (e−-e+) momentum densities from measurements of deviations from co-linearity and energies of photons from e−-e+ annihilation events was employed to examine the effects of O-atom defects in 6H SiC. Three-dimensional momentum datasets were determined for 6H SiC irradiated with 24 MeV O3+ ions. Angular correlation of annihilation radiation (ACAR) and coincidence Doppler-broadening of annihilation radiation (CDBAR) analyses are presented. In addition, a novel technique is illustrated for analyzing 3D momentum datasets in which the parallel momentum component, p|| (obtained from the CDBAR measurement) is selected for annihilation events that possess a particular perpendicular momentum component, p- observed in the 2D ACAR spectrum.

Analysis of electron-positron momentum spectra of metallic alloys as supported by first-principles calculations

Electron-positron momentum distributions measured by the coincidence Doppler broadening method can be used in the chemical analysis of the annihilation environment, typically a vacancy-impurity complex in a solid. In the present work, we study possibilities for a quantitative analysis, i.e., for distinguishing the average numbers of different atomic species around the defect. First-principles electronic structure calculations self-consistently determining electron and positron densities and ion positions are performed for vacancy-solute complexes in $\mathrm{Al}\text{\ensuremath{-}}\mathrm{Cu}$, $\mathrm{Al}\text{\ensuremath{-}}\mathrm{Mg}\text{\ensuremath{-}}\mathrm{Cu}$, and $\mathrm{Al}\text{\ensuremath{-}}\mathrm{Mg}\text{\ensuremath{-}}\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Ag}$ alloys. The ensuing simulated coincidence Doppler broadening spectra are compared with measured ones for defect identification. A linear fitting procedure, which uses the spectra for positrons trapped at vacancies in pure constituent metals as components, has previously been employed to find the relative percentages of different atomic species around the vacancy [A. Somoza et al. Phys. Rev. B 65, 094107 (2002)]. We test the reliability of the procedure by the help of first-principles results for vacancy-solute complexes and vacancies in constituent metals.

Structure ofSiO2∕4H−SiCinterface probed by positron annihilation spectroscopy

The structure of the ${\mathrm{SiO}}_{2}∕4\mathrm{H}\text{\ensuremath{-}}\mathrm{SiC}$ interface produced by dry oxidation has been studied using positron annihilation spectroscopy using energy-variable slow positron beams. Based on the Doppler broadening shape and wing parameter ($S\text{\ensuremath{-}}W$) correlation, the interface layer was clearly distinguished from the ${\mathrm{SiO}}_{2}$ and $\mathrm{SiC}$ layers. A single positron lifetime of $451\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, which is sufficiently longer than that in the $\mathrm{SiC}$ substrate $(\ensuremath{\sim}140\phantom{\rule{0.3em}{0ex}}\mathrm{ps})$ and close to the second lifetime in the ${\mathrm{SiO}}_{2}$ layer, was obtained when the incident positron energy was adjusted at the interface layer. The electron-positron momentum distribution associated with the interface layer was well explained by a theoretical calculation that considered the annihilation of the positrons by the oxygen valence electrons in the ${\mathrm{SiO}}_{2}$ layer. The annealing process after the oxidation resulted in the modification of the electron-positron momentum distribution in a manner similar to that of the interface traps, thereby suggesting that the interface traps correlate with the positron annihilation site.

Angular correlation of annihilation radiation associated with vacancy defects in electron-irradiated6H-SiC

Electron-positron momentum distributions associated with vacancy defects in $6H$-SiC after irradiation with 2-MeV electrons and annealing at 1000 \ifmmode^\circ\else\textdegree\fi{}C have been studied using angular correlation of annihilation radiation measurements. It was confirmed that the above vacancy defects have dangling bonds along the $c$ axis and the rotational symmetry around it. The first-principles calculation suggested that the vacancy defects are attributable to either carbon-vacancy-carbon-antisite complexes or silicon-vacancy-nitrogen pairs, while isolated carbon vacancies, silicon vacancies, and nearest neighbor divacancies are ruled out.

Aging effects in polypyrrole probed by positron annihilation

Abstact Positron annihilation spectroscopy and conductivity measurements have been used to probe aging-induced changes in parahydroxybenzenesulfonate doped polypyrrole. Polypyrrole was synthesized by a chemical oxidation process and aged in air at 110 °C and 140 °C up to 250 days. Positron annihilation experiments reveal modifications of the electronic structure accompanied by a dramatic loss of conductivity as a function of the aging time and temperature. The electron-positron momentum distribution at the 511 keV annihilation peak exibits a strong shift towards higher momentum annihilation events upon aging. The experimental results are explained in terms of positron trapping in the vicinity of negative dopants whose concentration is reduced after aging.

Positron lifetime studies of decomposition in 2024 (Al-Cu-Mg) and 7010 (Al-Zn-Cu-Mg) alloys

In the current paper, the decomposition behavior of the engineering alloys 2024 (Al-Cu-Mg) and 7010 (Al-Zn-Cu-Mg) is studied using positron lifetime measurements. Positrons probe open volume defects such as vacancies and dislocations. However, they may also be used to investigate coherent zones and incoherent precipitates. In order to understand the rather complicated precipitation sequences and the response of positrons to different type of precipitates occurring in 2024 and 7010 alloys, binary and ternary laboratory alloys were also investigated under the same experimental conditions as the engineering alloys. The interpretations of the results are based on experiences of the group from extensive positron studies of laboratory alloys such as Al-Zn, Al-Zn-Mg, Al-Cu, and further Al alloys (see also the review (4)). Their collected results are shown as lifetimes and curve-shape parameters S of the electron-positron momentum distribution curves characteristic for different precipitates in Al alloys.

Electron-positron momentum distributions and positron lifetime in semiconductors in the generalized gradient approximation

The positron annihilation characteristics have been calculated taking the electron-positron correlation in the generalized gradient approximation (GGA). The calculated electron-positron momentum distributions in Si along the [110] direction in the GGA scheme agree very well with the experiment. The comparison of anisotropies of the momentum distributions along different crystal directions with the theory shows that only the GGA scheme gives the exact values. The enhancement factor for the valence electrons in the electron-positron momentum density is found to be weakly dependent on the momentum. The positron lifetimes in group IV, III-V, and II-VI semiconductors agree very well with the previous calculations and the experiment.

Study on distribution of electron momentum density and Fermi surface in lead by positron annihilation

The momentum density of electrons and the Fermi surface are measured in metal lead by the technique of positron annihilation. The three-dimensional electron-positron momentum distribution is reconstructed from measurements of the two-dimensional angular correlation of positron annihilation radiation (2D-ACAR), followed by the image reconstruction technique based on a direct Fourier transformation. On the basis of the obtained experimental spectrum, the Fermi surface topology, the high momentum components (HMC), the enhancement effect, and the autocorrelation function are discussed. The obtained experimental results are in good agreement with theoretical calculations.

Evolution of the electronic structure of La2-xSrxCuO4 with doping determined by positron-annihilation spectroscopy.

We have measured and calculated the electron-positron momentum distribution of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ samples for Sr concentrations of 0, 0.1, 0.13, and 0.2. Measured distributions were obtained at room temperature with high statistical precision, greater than 4\ifmmode\times\else\texttimes\fi{}${10}^{8}$ events, in the Lawrence Livermore National Laboratory positron-annihilation angular correlation spectrometer on single-crystal samples fabricated using the traveling solvent floating zone technique. Corresponding theoretical momentum-density calculations were performed using the linear muffin-tin-orbital method. The momentum distribution of all samples contained features derived from the overlap of the positron distribution with the valence electrons. In addition, discontinuities typical of a Fermi surface are seen in the doped samples. The form and position of these features are in general agreement with the Fermi surface and overall momentum distributions as predicted by band theory. However, the evolution of the Fermi surface with doping differed significantly from expectations based on single electron band theories.

Positron annihilation study of Sr doping in La 2− xSr xCuO 4

We present a combined experimental and theoretical study of the effects of Sr doping on the electronic structure of La 2− x Sr x CuO 4. Electron-positron momentum distributions have been measured to high statistical precision (> 4 × 10 8 counts) at room temperature for samples with Sr concentrations of x = 0.0, 0.1, 0.13 and 0.2. Analysis of all tour spectra reveal strong features due to electron-positron wavefunction overlap, in quantitative agreement with theoretical calculations. The Sr doped samples show discontinuities consistent with the presence of a Fermi surface. The form and position of these features are in general agreement with the predictions of band theory. Correspondence between theory and experiment, as well as some differences, are revealed by a detailed study of the changes in electron-positron momentum density with increasing Sr doping.

Positron-annihilation studies of a decagonal single quasicrystal of Al70Ni15Co15.

Measurements of the two-dimensional angular correlation of positron-annihilation radiation and positron lifetime have been carried out for a thermodynamically stable decagonal single quasicrystal of ${\mathrm{Al}}_{70}$${\mathrm{Ni}}_{15}$${\mathrm{Co}}_{15}$. The results show almost isotropic electron-positron momentum distributions in spite of the strong anisotropy in the atomic structure of the decagonal phase. Two positron lifetimes, which are much longer than the expected lifetime corresponding to the electron density of ${\mathrm{Al}}_{70}$${\mathrm{Ni}}_{15}$${\mathrm{Co}}_{15}$, were observed. These results suggest that the annihilating positrons are trapped by isotropic structural defects.

Positron annihilation in perovskite superconductors; Theory and experiment

Positron Annihilation Spectroscopy is shown to be of potential value for probing the electronic structure and the changes accompanying the superconducting transition of the new high T c materials. The experimental results of electron-positron momentum distribution for La 2CuO 4 agree with a ligand field approach, suggesting a strong electron localization and the importance of the covalency.

Electron and electron-positron momentum distributions in concentrated random alloys

Presents a formalism for calculating the two-photon momentum densities rho 2 gamma (p) for substitutional random alloys with the KKR CPA band-theory scheme and apply it to Cu60Ni40 and Cu60Pd40 random alloys. In particular they study rho 2 gamma (p) along the directions Gamma X, Gamma K and Gamma L in Cu60Ni40 and along Gamma X in Cu60Pd40 and compare it with the appropriate electron momentum distribution. They conclude that the extent to which the positron wavefunction influences the results of positron annihilation experiments is slight and it can now be studied in quantitative detail.

Electron-positron momentum distribution in cadmium: evidence for self-trapped positrons below the vacancy trapping region

Equilibrium measurements of the temperature dependence of the peak coincidence rate from positrons annihilating in Cd show that there are two regions of linear slope at temperatures below the vacancy trapping region. The electron-positron momentum distribution measured in the region of intermediate slope exhibits smearing consistent with positron localization in a self-trapped state. In this state a decrease of 8% in the core annihilation rate is also observed.