Multipolar interactions in rare-earth metals and alloys

The Hamiltonian of the indirect quadrupole-quadrupole interaction of rare-earth ions in metals and alloys is constructed self-consistently, taking into account the exchange interactions and correlations in a system of conduction electrons (CE) and the antishielding effects. The manifestations of the indirect multipolar interactions of paramagnetic ions in the parameters of ESR on localized moments in metals and alloys are studied. © 1992

Surface states and annihilation characteristics of positrons trapped at reconstructed semiconductor surfaces

Positron probes of the Si(1 0 0) surface that plays a fundamental role in modern science and technology are capable to non-destructively provide information that is both unique to the probe and complimentary to that extracted using other more standard techniques. This paper presents a theoretical study of positron "image-potential" surface states and annihilation characteristics of surface trapped positrons at the Si(1 0 0) surface. Calculations are performed for the reconstructed Si(1 0 0)-p(2 × 2) surface using the modified superimposed-atom method to account for discrete-lattice effects, and the results are compared with those obtained for the non-reconstructed and reconstructed Si(1 0 0)-(2 × 1) and Si(1 1 1)-(7 × 7) surfaces. The effect of orientation-dependent variations of the atomic and electron densities on localization and extent of the positron surface state wave function at the semiconductor surface is explored. The positron surface state wave function is found to extend into the Si lattice in the regions where atoms are displaced from their ideal terminated positions due to the p(2 × 2) reconstruction. Estimates of the positron binding energy and positron annihilation characteristics reveal their sensitivity to the specific atomic structure of the topmost layers of Si. The observed sensitivity of annihilation probabilities to crystal face indicates that positron spectroscopy techniques could serve as an important surface diagnostic tool capable of distinguishing different semiconductor surfaces and defining their state of reconstruction. © 2005 Elsevier B.V. All rights reserved

Theoretical studies of positron states and annihilation characteristics 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. An ab-initio study of the electronic properties of the Cu(100) missing row reconstructed surface at various on surface and sub-surface oxygen coverages has been performed on the basis of the density functional theory (DFT) using the Dmol3 code and the generalized gradient approximation (GGA). Surface structures in calculations have been constructed by adding oxygen atoms to various surface hollow and sub-surface octahedral sites of the 0.5 monolayer (ML) missing row reconstructed phase of the Cu(100) surface with oxygen coverages ranging from 0.5 to 1.5 ML. The charge redistribution at the surface and variations in atomic structure and chemical composition of the topmost layers associated with oxidation and surface reconstruction have been found to affect the spatial extent and localization of the positron surface state wave function and annihilation probabilities of surface trapped positrons with relevant core electrons. 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). It has been shown that positron annihilation probabilities with Cu 3s and 3p core electrons decrease when total (on-surface and sub-surface) oxygen coverage of the Cu(100) surface increases up to 1 ML. The calculations show that for high oxygen coverage when total oxygen coverage is 1. 5 ML the positron is not bound to the surface. © 2013 AIP Publishing LLC

Nuclear magnetic resonance in dilute magnetic alloys and superconductors

The longitudinal and transverse spin-lattice relaxation rates for paramagnetic ion-nuclei in dilute Kondo systems are calculated. It is shown that obsevation of the Kondo anomalies in the NMR parameters is facilitated by either high temperatures (kT >> ωs, the resonance frequency for localized moments) for low temperatures (kT < <ωs). The longitudinal spin-lattice relaxation of paramagnetic ion-nuclei in "dirty" type II superconductors is investigated. The influence of the order parameter fluctuations on relaxation of paramagnetic ion-nuclei in type II superconductors is studied at temperatures slightly above the transition temperature Tc. © 1991

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

Nuclear magnetic resonance in dilute magnetic alloys and superconductors

The longitudinal and transverse spin-lattice relaxation rates for paramagnetic ion-nuclei in dilute Kondo systems are calculated. It is shown that observation of the Kondo anomalies in the NMR parameters is facilitated by either high temperatures (kT ≫ ω-s$/, the resonance frequency for localized moments) or low temperatures (kT ≪ ω-s$/). The longitudinal spin-lattice relaxation of paramagnetic ion-nuclei in 'dirty' type II superconductors is investigated. The influence of the order parameter fluctuations on relaxation of paramagnetic ion-nuclei in type II superconductors is studied at temperatures slightly above the transition temperature Tc

Positron states and annihilation characteristics at the (100), (110), and (111) surfaces of alkali metals

In this paper we present results of theoretical studies of positron states and annihilation characteristics at the clean surfaces of alkali metals. Positron surface states and positron work functions have been computed for the (100), (110), and (111) surfaces of Li, Na, K, Rb, and Cs using the modified superimposed-atom method to account for discrete-lattice effects, and the results are compared with those obtained for the transition-metal surfaces. Stable positron surface states are found in all cases, with the Li states lying about 0.5 eV below the bulk positron band, and other alkali metals having positron surface states a few hundredths of an eV below the bulk bands. The results for the positronium activation energy and positronium work function for the clean surfaces of alkali metals are presented as well. Surface and bulk state lifetimes and probabilities for a positron trapped in a surface state to annihilate with relevant core-level electrons are also computed and compared with available experimental data

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