On surface ACAR spectra calculated within the mixed density approximation

Abstract

Various approaches to the surface problem that enables one to calculate angular correlation of annihilation radiation (ACAR) spectra are discussed. The validity of the mixed-density approximation (MDA) in the neighborhood of metal surface is investigated. For positrons trapped at the Al surface the MDA gives results that strongly disagree with those obtained using the exact ACAR formula. A new formula for use in calculating surface ACAR spectra, allowing avoidance of this deficiency of the MDA, is proposed.

Densities of Valence Electrons in Simple Metals Effective with Respect to Electron-Positron Correlations

“…For re = rp the twoparticle function Ψepkj( re, rp) may be written in the form [2][3][4] (this notation has been also repeated in Ref. while From formula (10b) it clearly follows that the momentum dependence of the electron-positron enhancement faction ε(p) inside the FS is due to the state selectivity of twoparticle correlation functions fkj (r) only.…”

Isotropic theoretical angular correlation of annihilation radiation spectra for positrons trapped at an Al surface?

“…It should be mentioned here that the MDA (introduced by Arponen et a1 1973), which turned out to be very efficient for investigating positron annihilation at lattice defects, seems to be rather controversial in the case of positron interaction with a metal surface because of the infinite size of vacuum, treated as a large void. This fact will be discussed in detail in Q 2.3 (see also Rubaszek 1989). Moreover, in contradiction to the positronium model, almost all the ACAR spectra obtained within this second formalism for the correlated systemwere anisotropic for the A1 surface, with the distributionof momentum perpendicular to the surface, N ( p , ) , broader than the parallel one, N(p1) (this anisotropy varies from 10% to 50%).…”

“…component. The only exceptions are the results of Brown et a1 (1988a, b) and Lou Yongming (1988), both obtained within independent-particles model (IPM) (the possibilities of this isotropy are discussed by Rubaszek (1989)).…”

“…The only exceptions are the isotropic IPM spectra obtained by Lou Yongming (1988) for a very simple electron model potential and those by Brown et a1 (1988a, b), contradicting their previous strong anisotropic IPM result (Brown et a1 1987). We would like to mention here that the possibilities of full isotropy of IPM surface ACAR spectra are discussed by Rubaszek (1989) using the autocorrelation function method. This isotropy appeared to be strongly dependent on the translational properties of the electron and positron wavefunctions perpendicular to the surface.…”

Electronic structure and electron-positron correlation effects in Mg

“…(vi) The idea of introducing into the integral contained in the definition of p ( p ) (equation ( 5 ) ) the EF depending on the local density and the local kinetic energy is due to Daniuk and co-workers [9]. Since then there were various attempts at modifying the position dependence of the EF by neglecting the dependence on the kinetic energy [18,24,52]. These attempts can lead, of course, to a limited success, especially, if the effective EF is not well described by the jellium results [53,54].…”