Momentum Distributions Of Electron-positron Pairs Research Articles

Threshold effects in electron-positron pair creation from the vacuum: Stabilization and longitudinal versus transverse momentum sharing

Momentum distributions of electron-positron pairs created from the vacuum by an oscillating in time electric field are calculated in the framework of quantum field theory. A pronounced enhancement of those distributions is observed as the frequency of the electric field passes across the one-photon threshold. Below that threshold the pairs preferentially carry a longitudinal momentum, while above the threshold they tend to carry a transverse momentum. Such momentum sharing has an impact on the number of produced pairs: It grows fast with increasing the field frequency below the threshold but it saturates at a roughly constant value above it. On the other hand, at the fixed frequency above the one-photon threshold, the number of pairs scales quadratically with the field strength. This typically perturbative scaling holds even for large electric fields. Thus, the validity of the perturbation theory is extended here to processes which result in creation of particles with substantial transverse momenta.

Modeling the momentum distributions of annihilating electron-positron pairs in solids

Measuring the Doppler broadening of the positron annihilation radiation or the angular correlation between the two annihilation gamma quanta reflects the momentum distribution of electrons seen by positrons in the material.Vacancy-type defects in solids localize positrons and the measured spectra are sensitive to the detailed chemical and geometric environments of the defects. However, the measured information is indirect and when using it in defect identification comparisons with theoretically predicted spectra is indispensable. In this article we present a computational scheme for calculating momentum distributions of electron-positron pairs annihilating in solids. Valence electron states and their interaction with ion cores are described using the all-electron projector augmented-wave method, and atomic orbitals are used to describe the core states. We apply our numerical scheme to selected systems and compare three different enhancement (electron-positron correlation) schemes previously used in the calculation of momentum distributions of annihilating electron-positron pairs within the density-functional theory. We show that the use of a state-dependent enhancement scheme leads to better results than a position-dependent enhancement factor in the case of ratios of Doppler spectra between different systems. Further, we demonstrate the applicability of our scheme for studying vacancy-type defects in metals and semiconductors. Especially we study the effect of forces due to a positron localized at a vacancy-type defect on the ionic relaxations.

Investigation of vacancy-type complexes in GaN and AlN using positron annihilation

The momentum distribution of electron-positron pairs in GaN and AlN has been investigated for the first time by measuring the one-dimensional angular correlation of the annihilation radiation (1D-ACAR). The characteristic parameter of the electron density r s ′ (the radius of the sphere occupied by an electron) estimated from the experimental data differs noticeably from r s calculated in terms of a standard model of noninteracting particles: r s ′ (AlN)≈1.28r s, and r s ′ (GaN)≈1.66r s, where r s(AlN)≈1.6091 au, and r s(GaN)≈1.6568 au. The chemical nature of atoms in the environment of the annihilating positron in AlN and GaN was identified from the electron-positron ionic radius of Al3+, Ga3+, and N5+ cores, which were determined from the characteristics of the high-momentum component of 1D-ACAR curves. The analysis was based on a comparison of the electron-positron ionic radii with those considered standard for Al and Ga metals and the related compounds GaP, GaAs, and GaSb. A conclusion is made that positron annihilation dominates in the regions of vacancy-type “nitrogen antisite”-“vacancy” complexes, [N Ga + V Ga] and [N Al + V Al], in GaN and AlN, respectively.

MOMENTUM DISTRIBUTION OF ANNIHILATING ELECTRON–POSITRON PAIRS IN LiF

The momentum distribution of electron-positron pairs annihilating in LiF was determined. The results were in good agreement with those of Long and DeBenedetti.