Nail G. Fazleev

Annihilation of positrons trapped at the (100) and (111) surfaces of Si

Abstract We present results of theoretical studies of positron surface states and positron annihilation characteristics at the clean non reconstructed (100) and (111) surfaces of Si performed within the modified atomistic, superposition method. It is found that in the case of non reconstructed semiconductor surfaces, the positron surface state is localized mainly on the vacuum side of the topmost layer. The computed positron surface state energies E b at the (100) and (111) surfaces of Si are −2.81 and −2.69 eV. In addition, calculations of the positron work functions with respect to the vacuum for bulk Si(100) and Si(111) yielded 2.34 and 2.23 eV, respectively demonstrating the stability of positron surface state on these surfaces. The positron surface state lifetime as well as probabilities for a positron trapped in a surface state to annihilate with relevant core-level electrons are computed for both surfaces, and compared with available experimental data.

Application of the Feynman–Kac path integral method in finding excited states of quantum systems

Abstract Numerical methods of applying the Feynman-Kac path integral approach to quantum mechanics are presented. Themethods are demonstrated on simple quantum mechanical systems, including the hydrogen atom, the simple harmonic oscillator and infinite square wells. New analytic results for the Wiener integrals are obtained and compared with numerical results. A measure of the statistical uncertainty is introduced and rates of convergence are investigated. Implementation of the method on both serial and parallel computers is discussed

Study of positron surface states on the alkali-metal-covered transition-metal surface

Abstract We present a first-principles study of annihilation probabilities of surface trapped positrons with core electrons at the Cu(100) surface with sub-monolayers of Cs adsorbed. Image-potential-induced positron surface states are calculated using a corrugated-mirror model in a full three-dimensional geometry. These states are studied for various arrangements of Cs atoms below and above the critical alkali-metal coverage of approximately 0.7 physical monolayer. Computer simulations of the deposition of Cs atoms are used to show that hexagonal close-packed Cs islands are formed at about 0.7 physical monolayer coverage of Cs. It is found that the abrupt decrease in the positron annihilation rate with Cu 3p electrons observed experimentally results from a metallization of the Cs islands. This causes a shift in the localization of the positron bound state from the Cs Cu interface to the vacuum side of the Cs overlayer, with the corresponding abrupt decrease in the positron annihilation probability with Cu electrons. Annihilation probabilities for positrons with Cs electrons are also computed and compared with experiment.

Study of ultra-thin Al films deposited on GaAs(100) using positron annihilation induced auger electron spectroscopy and electron induced auger electron spectroscopy

Abstract We report study of the stability of ultra-thin Al films deposited on GaAs(100) using positron annihilation induced Auger electron spectroscopy (PAES). After the sample was kept for 7 days at 300 K under UHV conditions, the normalized Al PAES intensity decreased by 33.7±4.6%. Over the same time period, the normalized Ga PAES intensity increased by 55.8±4.8%. PAES spectra provide a direct method of confirming the substitution of Ga for Al in the top layer and Ga diffuse into the Al overlayer faster than As.

Proximity effects in asymmetric layered ferromagnet/superconductor nanostructures

The asymmetric four-layered F′/S′/F″/S″ nanostructure consisting of rather dirty superconducting (S) and ferromagnetic (F) metals is studied within the theory of the proximity effect taking detailed account of the boundary conditions. The new π phase superconducting states are obtained for the F′/S′/F″/S″ structure in addition to the known “superlattice” states. The dependence of critical temperatures versus the F layer thicknesses is explored for a wide range of parameters. Decoupled superconductivity is predicted for this nanostructure and an optimal set of parameters is determined for which the difference between the critical temperatures for different states becomes significant. The complicated phase diagram of the asymmetric four-layered F′/S′/F″/S″ nanostructure is constructed, explored, and compared with the ones for F/S bilayers, symmetric F/S/F and S/F/S trilayers, and F/S superlattices.

Positrons as probes of Si(100) surface with adsorbed hydrogen and oxygen

Abstract Positron annihilation induced Auger spectra from the Si(100) surface exposed to hydrogen and oxygen are analyzed by performing calculations of positron surface states and annihilation characteristics of surface trapped positrons. Positron binding energies and work functions are also computed. It is found that the adsorption of hydrogen and oxygen on the Si(100) surface leads to a displacement of the positron surface state wave function away from the substrate Si atoms. As a result of this displacement, the overlap of the positron wave function with Si core electrons and, consequently, the annihilation probability of Si core electrons reduce, in agreement with experimental data.

Measurement of the Positron Annihilation Induced Auger Electron Spectrum from Ag(100)

Research has demonstrated that Positron Annihilation Induced Auger Spectroscopy (PAES) can be used to probe the top-most atomic layer of surfaces and to obtain Auger spectra that are completely free of beam-impact induced secondary background. The high degree of surface selectivity in PAES is a result of the fact that positrons implanted at low energies are trapped with high efficiency at an image-correlation potential well at the surface resulting in almost all of the positrons annihilating with atoms in the top-most layer. Secondary electrons associated with the impact of the incident positrons can be eliminated by a suitable choice of an incident beam energy. In this paper we present the results of measurements of the energy spectrum of electrons emitted as a result of positron annihilation induced Auger electron emission from a clean Ag(100) surface using a series of incident beam energies ranging from 20 eV down to 2 eV. A peak in the spectrum was observed at ~40 eV corresponding to the N2,3VV Auger transition in agreement with previous PAES studies. This peak was accompanied by an even larger low energy tail which persisted even at the lowest beam energies. Our results for Ag(100) are consistent with previous studies of Cu and Au and indicate that a significant fraction of electrons leaving the sample are emitted in the low energy tail and suggest a strong mechanism for energy sharing in the Auger process.

Surface states and annihilation characteristics of positrons trapped at the oxidized Cu(100) surface

In this work we present the results of theoretical studies of positron surface and bulk states and annihilation probabilities of surface-trapped positrons with relevant core electrons at the oxidized Cu(100) surface under conditions of high oxygen coverage. Oxidation of the Cu(100) surface has been studied by performing an ab-initio investigation of the stability and electronic structure of the Cu(100) missing row reconstructed surface at various on-surface and subsurface oxygen coverages ranging from 0.5 to 1.5 monolayers using density functional theory (DFT). All studied structures have been found to be energetically more favorable as compared to structures formed by purely on-surface oxygen adsorption. The observed decrease in the positron work function when oxygen atoms occupy on-surface and subsurface sites has been attributed to a significant charge redistribution within the first two layers, buckling effects within each layer and an interlayer expansion. The computed positron binding energy, positron surface state wave function, and annihilation probabilities of the surface trapped positrons with relevant core electrons demonstrate their sensitivity to oxygen coverage, atomic structure of the topmost layers of surfaces, and charge transfer effects. Theoretical results are compared with experimental data obtained from studies of oxidation of the Cu(100) surface using positron annihilation induced Auger electron spectroscopy (PAES). The results presented provide an explanation for the changes observed in the probability of annihilation of surface trapped positrons with Cu 3p core-level electrons as a function of annealing temperature.

Theoretical aspects of studies of oxide and semiconductor surfaces using low energy positrons

This paper presents the results of a theoretical study of positron surface and bulk states and annihilation characteristics of surface trapped positrons at the oxidized Cu(100) single crystal and at both As- and Ga-rich reconstructed GaAs(100) surfaces. The variations in atomic structure and chemical composition of the topmost layers of the surfaces associated with oxidation and reconstructions and the charge redistribution at the surfaces are found to affect localization and spatial extent of the positron surface-state wave functions. The computed positron binding energy, work function, and annihilation characteristics reveal their sensitivity to charge transfer effects, atomic structure and chemical composition of the topmost layers of the surfaces. Theoretical positron annihilation probabilities with relevant core electrons computed for the oxidized Cu(100) surface and the As- and Ga-rich reconstructed GaAs(100) surfaces are compared with experimental ones estimated from the positron annihilation induced Auger peak intensities measured from these surfaces.

Positron Annihilation Induced Auger Electron Spectroscopic Studies Of Reconstructed Semiconductor Surfaces

The positron annihilation induced Auger spectrum from GaAs(100) displays six As and three Ga Auger peaks below 110 eV corresponding to M4,5VV, M2M4V, M2,3M4,5M4,5 Auger transitions for As and M2,3M4,5M4,5 Auger transitions for Ga. The integrated Auger peak intensities have been used to obtain experimental annihilation probabilities of surface trapped positrons with As 3p and 3d and Ga 3p core level electrons. PAES data is analyzed by performing calculations of positron surface and bulk states and annihilation characteristics of surface trapped positrons with relevant Ga and As core level electrons for both Ga‐ and As‐rich (100) surfaces of GaAs, ideally terminated, non‐reconstructed and with (2×8), (2×4), and (4×4) reconstructions. The orientation‐dependent variations of the atomic and electron densities associated with reconstructions are found to affect localization of the positron wave function at the surface. Computed positron binding energy, work function, and annihilation characteristics demonstrate...