Correlation Of Positron Annihilation Radiation Research Articles

The electron - positron momentum density and Fermi surface in

We have determined the three-dimensional electron - positron momentum density for the ordered binary alloy using two-dimensional angular correlation of positron annihilation radiation (2D-ACAR). The Fermi surface obtained from the momentum density is compared with the results from de Haas - van Alphen measurements and theoretical calculations. We have also observed the high-momentum components, and the enhancement effect on the momentum density.

Pressure dependence of positron anhihilation in germanium

Electron-positron momentum densities are calculated for the (001–110) plane in Ge at normal and increased pressure. The calculations are based on the independent particule approximation coupled with the pseudopotential method. The shapes of the profiles indicate that the angular correlation of positron annihilation radiation (ACPAR) along different crystallographic directions in Ge are highly sensitive to pressure.

Study of Defects in GaAs by 2D-ACAR Positron Annihilation

We describe the two dimensional angular correlation of positron annihilation radiation (2D-ACAR) method and show that it can be advantageously used to study the electronic structure of defects, in addition to standard positron lifetime and Doppler broadening measurements. Using annihilation fractions determined by lifetime measurements, we separate 2D-ACAR distributions for negatively charged and neutral arsenic vacancies in n-type GaAs. In electron-irradiated semi-insulating GaAs, we present 2D-ACAR results for the negatively charged gallium vacancy and for positron Rydberg states induced by gallium antisites. The 2D-ACAR for delocalized positrons, needed in the separation process, has been obtained from measurements on as-grown semi-insulating GaAs. Our results outline the capabilities of the 2D-ACAR technique when it is applied to the study of defects: the electronic structure in the vacancies are found to be well described by recent molecular dynamics calculations. The method also provides information about atomic relaxations around the vacancy sites, they are in agreement with the calculated relaxations. We conclude that the 2D-ACAR method is a promising tool to investigate defects in semiconductors.

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.

Localized positronium atoms in porous structures studied by 2D-ACAR

The localization of positronium (Ps) atoms was studied by measurements of two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) for porous glasses. Annihilations of positron-electron pairs with a large anisotropy were found in 2D-ACAR spectra. This fact can be attributed to the momentum uncertainty due to the localization of Ps in a finite dimension of pores. The present investigation showed the possibility of the detection of microstructures in amorphous materials by the 2D-ACAR technique.

Angular-correlation distribution of positron-electron annihilation in GaAs.

Angular correlation of positron annihilation radiations has been measured along the [100], [110], [111], [211], and [221] crystallographic axes in GaAs using the long-slit geometry. A pseudopotential calculation is carried out to understand the experimental data. There is good agreement between experimental and theoretical data. The structures seen in the low-momentum region of the data are interpreted in terms of the \ensuremath{\sigma} and \ensuremath{\pi} bonds of the ${\mathit{sp}}^{3}$ hybridized bonding orbitals. The experimental data when compared with the calculated data reveal the presence of electron-positron many-body correlation effects.

Positron Annihilation in Vitreous Silica Glasses

The annihilation characteristics of positrons in vitreous silica glasses (v-SiO2) were studied by measurements of two-dimensional angular correlation of positron annihilation radiations and positron lifetime spectra. From the measurements, it was found that positrons and positronium (Ps) atoms mainly annihilate from trapped states by vacancy-type defects in v-SiO2. For v-SiO2 specimens with cylindrical porous structures, annihilations of Ps with anisotropic momentum distributions were observed. This fact was attributed to the momentum uncertainty due to localization of Ps in a finite dimension of pores. This investigation showed possibilities for the detection of microstructures in v-SiO2 by the positron annihilation technique.

Study on the Three‐Dimensional Electron Momentum Density and the Fermi Surface in Cu by Positron Annihilation

AbstractThe electron momentum density and the Fermi surface in Cu are studied by a two‐dimensional angular correlation of positron annihilation radiation (2D‐ACAR). The three‐dimensional electron momentum density is reconstructed using the image reconstruction technique. The electron momentum density obtained in the extended zone scheme is reduced to that in the reduced zone scheme by using the Lock‐Crisp‐West (LCW) folding procedure. The Fermi surface in Cu is determined in the reduced zone scheme. The enhancement parameters and the autocorrelation function are compared with the theoretical calculations.

Theoretical studies of the angular correlation of positron annihilation in germanium

A pseudopotential formalism is applied to calculate the angular correlation of positron annihilation radiations (ACPAR) along the (100), (110) and (111) directions in Ge. The results are found to agree very well with the measured data. Calculations show that the ACPAR line shapes, particularly in the low-momentum region, are very much influenced by the detailed nature of bonding. Two-photon autocorrelation functions corresponding to the above symmetry directions are also presented and some interesting conclusions are drawn regarding positron-electron many-body correlation effects.

2D-angular correlation of positron annihilation on CeSb single crystals

The two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) was measured on a CeSb crystal as a function of temperature. The topology of the Fermi surface of CeSb is discussed in both the normal state and heavy Fermion state.

Angular correlation of positron annihilation radiation in perfluorohexane mixtures with nonpolar liquids

Positron-annihilation linear-slit angular-correlation (AC) spectra in perfluorohexane (C 6 F 14 ) mixtures with hexane and isooctane in the entire concentration interval are reported. Annihilation with electrons in fluorine results in an unusually broad AC-component. The data show that the free positron reacts to form a bound state with C 6 F 14 at low C 6 F 14 concentration. At high C 6 F 14 concentration the positron in Ps annihilates preferentially with electrons in the hydrocarbons, and not with the electrons in C 6 F 14 before this cannot be avoided. Hence, the energy of the Ps bubble state is lower if the surface consists of hydrocarbon molecules

Defects in vitreous SiO2 probed by positron annihilation

Annihilation characteristics of positrons in a single quartz crystal and vitreous silica glasses (v-SiO 2 ) were studied by measurements of two-dimensional angular correlation of positron annihilation radiations. From the measurements, it was found that positrons and positronium (Ps) atoms mainly annihilate from trapped states by vacancy-type defects in the v-SiO 2 specimen. For the v-SiO 2 specimen with a cylindrical porous structure, the annihilation of Ps with anisotropic momentum distributions was observed. This was attributed to the momentum uncertainty due to a localization of Ps in a finite dimension of pores. The present investigation showed possibilities for the detection of microstructures in v-SiO 2 by the positron annihilation technique

Defects in semiconductors observed by positron annihilation

The origin of the effect of defects on positron annihilation in semiconductors has been studied. The electron-positron momentum densities in elemental semiconductors (Si and Ge), III-V compound semiconductors (GaAs, InP and GaSb), diamond and the proton irradiated Si were investigated by a full-scale use of the two-dimensional angular correlation of positron annihilation radiations (2D-ACAR). The obtained results showed, as a whole, good agreement with the electron momentum distribution of the fully occupied Jones zone with a small exception for the fact that the low density channels are running along the three principal axes. This anisotropy was strong in elemental semiconductors, while it was weakened in compound semiconductors. This anisotropy and its dependence on the material were found to be generally understood by the incorporation of crystal symmetry. The anisotropy will be discussed by group theory in conjunction to the effect of defects on positron annihilation.

Calculation of momentum distributions in paramagnetic chromium: Compton scattering and positron annihilation

A detailed calculation of the momentum distributions in paramagnetic (BCC) chromium is carried out using the linear combination of Gaussian orbitals (LCGO) method. Using the same electron wavefunctions, theoretical results are obtained for Compton scattering and positron annihilation, and are presented in the form of electron momentum distributions rho (p), reciprocal form factors B(z), two-photon momentum distributions rho 2 gamma (p), one-dimensional angular correlation of positron annihilation radiation, Lock-Crisp-West distributions, etc. These results are analysed in terms of the Fermi surface topology of paramagnetic Cr. In particular, the present results for rho 2 gamma (p) are compared with those reconstructed by Kubota et al (1991) from their two-dimensional angular correlation of positron annihilation radiation measurements. Observed differences are ascribed to the effects of e+-e- many-body correlations and the reconstruction procedure.

Momentum distribution in vanadium: Compton scattering and positron annihilation.

Self-consistent, linear-combination-of-Gaussian-orbitals band-structure method is used within the independent particle model, to calculate the electron momentum distributions, \ensuremath{\rho}(p), and two-photon momentum distributions, ${\mathrm{\ensuremath{\rho}}}^{2\ensuremath{\gamma}}$(p), in metallic vanadium. We present results for \ensuremath{\rho}(p), Compton profiles, ${\mathrm{\ensuremath{\rho}}}^{2\ensuremath{\gamma}}$(p), one- and two-dimensional angular correlation of positron annihilation radiation, etc. Results are compared with other calculations and with experiments wherever available. In particular, the present results for ${\mathrm{\ensuremath{\rho}}}^{2\ensuremath{\gamma}}$(p) are analyzed in terms of contributions from different sheets of Fermi surface of V, and are compared with ${\mathrm{\ensuremath{\rho}}}^{2\ensuremath{\gamma}}$(p) reconstructed from experimental two-dimensional angular correlation of positron annihilation radiation data sets by Pecora et al.

The three-dimensional positron-electron momentum density study in Ge by means of a full-scale use of two-dimensional angular correlation of positron annihilation radiations

The positron-electron momentum density in germanium was studied by a full-scale use of the two-dimensional angular correlation of positron annihilation radiations (2D-ACAR). The term, a full-scale use of 2D-ACAR, means that the three-dimensional momentum-space density rho (p) of the annihilation positron-electron item is directly determined from a set of experimental 2D-ACAR spectra, followed by the reconstruction technique. The obtained electron momentum density, rho (p), showed, as a whole, good agreement with the electron momentum distribution of the fully filled Jones zone with the exception that a deep dip in the origin appears, which has been observed in the 1D-ACAR or semi-2D-ACAR integrated spectra in germanium. In the present work, the electron momentum density distribution shows a ridge along the (110) line and a vary deep valley exists along the (100) line, which is also the deviation from the Jones zone. In germanium, the valence 4p electrons are mainly participating in bonding sigma bonds, and the distribution in space is prominently anisotropic. This anisotropy is reflected on the electron momentum density distribution in the present work. The analysis based on group theory was performed by Saito et al. in order to investigate the origin of the anisotropy in the present work.

Electronic Structure in Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8</sub> Studied by Two Dimensional Angular Correlation of Positron Annihilation Radiations

Electronic Structure in Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8</sub> Studied by Two Dimensional Angular Correlation of Positron Annihilation Radiations

Electronic Structure in Semiconductors Studied by Two Dimensional Angular Correlation of Positron Annihilation Radiations

Electronic Structure in Semiconductors Studied by Two Dimensional Angular Correlation of Positron Annihilation Radiations

Fermi surface of 2HNbSe2 studied by de Haas-van Alphen oscillation and positron annihilation

Abstract Fermi surface (FS) studies of 2HNbSe 2 have been performed by adopting two different and complementary methods, i.e. by the measurements of de Haas-van Alphen (dHvA) oscillation and positron annihilation (PA). The dHvA effect in the superconducting mixed state has been confirmed by a standard field modulation ac susceptibility method at 0.5 K, showing the existence of a small pancake shape FS. Two dimensional angular correlation of PA radiations has given a cylindrical hole surface with its axis along ΓA. These findings are discussed with relation to APW band calculation and also to an observation of dHvA effect in the mixed state of high-T c superconductor.

Study on Momentum Density and Fermi Surface in Paramagnetic Cr and W by Positron Annihilation

AbstractThe three‐dimensional electron‐positron momentum densities in paramagnetic Cr and W are reconstructed from measurements of two‐dimensional angular correlation of positron annihilation radiations (2D‐ACAR) by the image reconstruction technique based on a direct Fourier transformation. The experimental momentum density for paramagnetic Cr and W shows the contribution of the Fermi surface sheets; the electron surface at Γ, the hole surface at H, the hole surface at N. The Fermi surface topology obtained from the present results agrees with the theory and other experiments.