Electron-positron annihilation characteristics at a metal surface: simple metals

Abstract

Momentum densities of annihilating elecfroc-psimn pairs. posimn lifetimes, work funcuons and binding energies as well as e l e c t r o ~s l m n enhancement factors at the surfaces of simple metals and cadmium are studied. 'Ihe effect of ekcuowposimn correlations on the surfaceitate posimn a " o n chanctellstics is set out Institute of ExperimenLll Physics, Universily of Wndaw. 50-204 Wmcfaw, PI. Maxa Boma 1 . Introduction Studies of slow positron interactions with metal surfaces (for reviews, see e.g. I11-[41) opened a new channel in investigations of the electronic structure of solids by positron annihilation. In particular, the extended well known angular correlation of annihilation radiation (ACAR) technique (for reviews, see e.g. [5,6]) has been successfully applied to probing electron and positron surface states (ss) in AI ([7-lo]), Cu [ll], Ni [9,10,12], Pb [131, Si [9,10] and graphite [14]. Slow-positron experiments have conlirmed the presence of the posiwn Ss at the clean AI [I101 surface 1151; the ss component of the measured lifetime spectrum amounts to 580 ps, i.e. about 15% more than the spin-averaged free positronium value. This result

Theory of positrons in solids and on solid surfaces

“…39(b). Brown et al used Rubaszek et al (1993) show that the proper inclusion of positron-electron correlation effects in the calculation of the ACAR spectra is essential. They find that, in contrast to positron annihilation in bulk solids, the enhancement factor for the surface state is a decreasing function of the momentum.…”

“…The resulting ACAR curves are quite isotropic and in fair agreement with experiments. Rubaszek et al (1993) have also calculated the positron lifetimes for the surface states on several simple metals. In the case of Al, they obtain a lifetime of 580-590 ps, in good agreement with the experimental one of 580 ps .…”

Positron annihilation with core and valence electrons

“…These are often computed using the independent-particle approximation (IPA) [12,24], or in a modified IPA framework that includes phenomenological enhancement factors that attempt to account for the important effects of electronpositron correlations [15,16,20,23,[25][26][27][28][29]. In most cases, the materials of interest are condensed matter systems, and the enhancement factors are usually calculated using a variety of density functional theory methods, e.g., the local density approximation (LDA) [2,30], the generalized gradient approximation (GGA) [31][32][33][34], or the weighted density approximation (WDA) [35][36][37], all of which rely heavily on theoretical considerations of the electron gas. However, due to the strong variations in the density, the LDA is not expected to work well for the core electrons, and has been found to overestimate the annihilation rates [37] The enhancement factor approach within the one-component and Boroński-Nieminen twocomponent LDA [30], as well as within the GGA, have been tested and compared with bound positron-atom stochastic variational calculations in Ref.…”

Application of the weighted-density approximation to the accurate description of electron-positron correlation effects in materials

“…A full first-principles alternative is provided by the weighted density approximation (WDA) [26][27][28][29][30] . In this approach, which is inherently non-local as opposed to the LDA/GGA, the screening cloud is modelled using the electron-positron pair correlation function of the homogeneous electron gas.…”

“…The application of the WDA for modeling electronpositron correlations has been discussed previously in connection with jellium surfaces [27][28][29] as well as bulk materials 30 . Here, we revisit the problem and construct a new WDA scheme based on more recent quantum Monte Carlo (QMC) results 31 .…”