Calculation of Positron Response from Embedded Nanoparticles

Abstract

Nanoparticles embedded in a matrix can trap positrons under certain conditions. In such cases nanoparticles can be effectively studied by means of positron annihilation because positron annihilation characteristics contain information related to nanoparticles' electronic and atomic structure. Of great importance is to calculate the positron response from such nanoparticles. Then, nanoparticles can, in principle, be identified by comparing the measured and calculated positron annihilation response. For this purpose we present an efficient computational method based on the well-known atomic superposition technique. This method is explained in detail, justified on the basis of first principles calculations, and applied to Cu nanoparticles embedded in the Fe matrix.

Effect of thermal treatment condition on the Ag precipitates in Al–Ag alloy studied by positron annihilation

“…(1), clusters with very small size can trap positrons, but this does not necessarily mean that the positron wavefunction will be totally confined inside the clusters. In Fe-Cu alloy, the estimated minimum diameter is 0.74 nm for Cu clusters to trap positrons, however theoretical calculation indicates that at least a diameter of 1.0 nm is needed to completely confine the positrons, so that the positron annihilation parameter will be close to that of pure Cu [31]. For smaller clusters, the positron wavefunction will be weakly localized.…”

Simulation of positron annihilation response to mechanical deformation of nanostructured Ni3Al

“…Positron annihilation lifetimes (PALs) were calculated by means of the ATSUP program [3,11] based on the atomic superposition method [12,13]. Both FREEVOL and ATSUP codes were updated in order to allow handling nanowires.…”

Simulation of Positron Annihilation Response to Mechanical Deformation of Nanostructured AgCo

“…Positron lifetimes (PLs) and high momentum parts (HMPs) of the momentum distribution of annihilation γ-quanta were calculated by means of the ATSUP programme [6,7] based on the atomic superposition method [8,9]. The enhancement factor and correlation part of the positron potential were determined using the parameterisation obtained by Boroński and Nieminen [10].…”

“…In the same way, E Co was used to calculate the PBE for defects C1 and C2 and vacancy clusters in Co. In order to account for the Ag-Co affinity difference, the positron response for AC1 defect was calculated applying a shift (E S ) of the positron potential around Co atomic sites in a sphere with the radius R = 2.75Å [7]. An appropriate E S value was estimated using the formula [14] are the positron affinities of Ag and Co, respectively.…”

“…An appropriate E S value was estimated using the formula [14] are the positron affinities of Ag and Co, respectively. The meaning of this formula is that the potential of Co atoms is shifted in order to get the correct positron energy differences between the Ag and the Co phases (see [7] for details). Then, the reference level E AgCo for calculating PBE at AC1 was estimated as…”