Free-atom—metal shifts in theM4,5N4,5N4,5Auger spectra of Ag, Cd, In, Sn, Sb, and Te

Period-multiplying bifurcations and multifurcations in conservative mappings

The authors have investigated numerically and analytically the period-doubling bifurcations and multifurcations of the periodic orbits of the conservative sine-Gordon mappings. They have derived a general equation for the appearance of multifurcations in conservative mappings. In agreement with many recent studies, they also find evidence that such mappings possess universality properties. They also discuss the role of multifurcations in conservative mappings exhibiting chaotic behaviour.

Defect recovery in aluminum irradiated with protons at 20 K.

Aluminum single crystals have been irradiated with 7.0-MeV protons at 20 K. The irradiation damage and its recovery are studied with positron-lifetime spectroscopy between 20 and 500 K. Stage-I recovery is observed at 40 K. At 240 K, loss of freely migrating vacancies is observed. Hydrogen in vacancies is found to stabilize the vacancies and prolong stage III to above 280 K, where the hydrogen bound to vacancies is released. Single and multiple occupancy of hydrogen atoms at monovacancies is put forward as the reason for the two recovery stages between 280 and 400 K. A binding energy of 0.53 +- 0.03 eV is found for a hydrogen atom trapped at a monovacancy. The results are in excellent agree…

Kohn-Sham Decomposition in Real-Time Time-Dependent Density-Functional Theory An Efficient Tool for Analyzing Plasmonic Excitations

The real-time-propagation formulation of time-dependent density-functional theory (RT-TDDFT) is an efficient method for modeling the optical response of molecules and nanoparticles. Compared to the widely adopted linear-response TDDFT approaches based on, e.g., the Casida equations, RT-TDDFT appears, however, lacking efficient analysis methods. This applies in particular to a decomposition of the response in the basis of the underlying single-electron states. In this work, we overcome this limitation by developing an analysis method for obtaining the Kohn-Sham electron-hole decomposition in RT-TDDFT. We demonstrate the equivalence between the developed method and the Casida approach by a be…

Electronic polarizability of small sodium clusters.

Abstract : Small sodium clusters consisting of 1 to 40 atoms are described as spheres of interacting homogeneous electron gas (jellium model). The static electronic polarizability is calculated using self consistent density functional methods. An excellent agreement with recent experimental results is observed.

Quantum chemical simulations of bound hold polarons (V Mg centers) in corundum crystals

The semi-empirical INDO method has been applied to the calculations of the bound hole small-radius polarons in corundum. Results for optimized atomic and electronic structure using two different approaches (molecular cluster and periodic, supercell model) are critically compared. Both models find that two-site configurations of bound hole polarons have the lowest energy (which does not exclude existence of one-site polarons also characterized by essential relaxation energies). Experimental ENDOR data on V Mg defects are discussed in the light of the calculations.

Surface Physics With Slow Positrons

Recent progress in slow beam studies of positron-surface interactions is reviewed. The key physical phenomena are introduced, and the present knowledge of the parameters involved is reviewed. The potential of the slow positron technique for surface science is discussed.

Theory of hydrogen and helium impurities in metals

A powerful computational scheme is presented for calculating the static properties of light interstitials in metallic hosts. The method entails (i) the construction of the potential-energy field using the quasiatom concept, (ii) the wave-mechanical solution of the impurity distribution ("zero-point motion"), (iii) calculation of the forces exerted on the adjacent host atoms and their displacements, and (iv) iteration to self-consistency. We investigate self-trapping phenomena in bcc and fcc metals in detail, and calculate both the ground and low-lying excited states. Implications of the wave-mechanical or band picture to diffusion mechanisms and inelastic scattering experiments are discusse…

Muon states in uniaxially strained iron

Effects of lattice relaxation, quantum motion, and uniaxial strain on the internal field at a positive-muon site in iron have been calculated. The uniaxial strain gives rise to a statistical shift of the muon population at interstitial sites. The effect of the population shift is found to be primarily responsible for the observed changes in the muon-precession frequency. The theory also predicts a 1T temperature dependence of the frequency shifts. Peer reviewed

Effect of zero-point motion on the superconducting transition temperature of PdH(D)

Using self-consistent density functional formalism we show that the electronic structure of PdH(D) is influenced by the zero-point vibration of hydrogen and deuterium. This quantum effect makes a small but significant contribution to the superconducting transition temperature ${T}_{c}$ of PdH(D). The reverse isotope effect on ${T}_{c}$ is found to be dominated by the changes in the force constants between PdH and PdD.

Helium bubbles in alpha-irradiated aluminium: positron lifetime studies

The formation of He-stabilised voids in aluminium single crystals injected with helium is monitored by positron lifetime measurements. The bubbles are observed to grow during annealing from 300 to 930K. The bubbles are extremely stable and survive annealing up to the melting point. Positron lifetime data are used to discuss the He density inside the bubbles and their growth mechanism.

Monte-Carlo calculations of keV electron and positron slowing down in solids

A Monte-Carlo simulation technique based on the screened Rutherford differential cross section for the elastic scattering and Gryzinski's semiempirical expression for the inelastic core and valence electron excitation is used to describe electrons and positrons slowing down in solids. The theoretical results are compared with the experimental backscattering, absorption and transmission results for aluminum, silicon, copper, and gold thin film and semi-infinite targets and good agreement is observed. The simulated stopping profiles are fitted with a simple analytic expression. The profiles are Laplace-transformed to give a useful data base for analyzing phenomena associated with slow positro…

Theory of positronium momentum spectra at metallic surfaces

Theoretical calculations of momentum distributions of positronium (Ps) atoms ejected from clean metal surfaces are presented and compared with recent experimental results. The authors find that the momentum dependence of the Ps-forming interaction significantly affects the shape of the spectra. They also show that, within the model, the energy distribution of Ps atoms does not vary directly with the density of electronic states just outside the surface.

Computed positron lifetimes in vacancies and vacancy-iron clusters in gold

Abstract Annihilation characteristics are calculated for positrons trapped in clean and impurity decorated vacancy clusters in Au. The positron lifetime depends strongly on the structure of the clusters. In a strongly relaxed vacancy cluster, the lifetime can become smaller than the lifetime in a single vacancy. The substitution of some neighbour atoms of a vacancy cluster by Fe atoms has only a minor effect on the positron lifetimes.

Positron trapping rate into vacancy clusters

The trapping rate of positrons into vacancy clusters in metals has been calculated. It increases with the trap size and binding energy and approximately scales with the number of vacancies in small clusters. The phonon-mediated contribution to the trapping rate is small. The temperature dependence of the trapping process is discussed.

Atoms embedded in an electron gas: Immersion energies

Energies of atoms, H through Ar, embedded in a homogeneous electron gas are calculated within the density-functional scheme as a function of the electron-gas density. The energy-versus-density curves and the induced densities of states are analyzed and discussed in terms of the interaction properties of an atom with its environment. The low-density limit of the immersion energy is related to the electron-atom scattering length. The results should prove useful in detailed investigations of the recently suggested "quasiatom" or "effective-medium" approaches to chemical binding. The lowest-order estimates of the binding energies of diatomic molecules and chemisorbed atoms are obtained. Peer re…

The resistivity and thermopower of amorphous Mg-Zn alloys

The resistivity and thermopower of amorphous Mg-Zn alloys have been computed as a function of temperature and composition. The diffraction model incorporating the dynamical partial structure factors is applied. The effect of the electron mean free path is investigated. The authors find that the resistivity is well described by the model, and that the inclusion of the mean free path does not change the results considerably. In the case of thermopower the diffraction model turns out to be inadequate: it gives a composition dependence which is against the experimental evidence. This suggests that there exists another scattering mechanism, which is not accounted for by the diffraction model. Th…

Size-dependent photoemission shifts in small metal clusters

Density-functional calculations of the change in self-consistent-field energy ( Delta SCF) type are reported for core-level photoemission shifts in small metal spheres. The results for the atom-in-jellium vacancy model show that the binding energies are increased from bulk-metal values, but the photoemission shifts show considerable oscillations as a function of cluster size.

Quantum-chemical simulations of free and bound hole polarons in corundum crystal

Abstract The semi-empirical method of the so-called intermediate neglect of differential overlap (INDO) has been applied to the calculations of the hole small-radius polarons in corundum crystals. Results for optimized atomic and electronic structure using two different approaches (the molecular cluster and periodic, supercell model) are critically compared. It is shown that the main results are similar in both cases.

Electronic structure calculations with GPAW: a real-space implementation of the projector augmented-wave method.

Electronic structure calculations have become an indispensable tool in many areas of materials science and quantum chemistry. Even though the Kohn-Sham formulation of the density-functional theory (DFT) simplifies the many-body problem significantly, one is still confronted with several numerical challenges. In this article we present the projector augmented-wave (PAW) method as implemented in the GPAW program package (https://wiki.fysik.dtu.dk/gpaw) using a uniform real-space grid representation of the electronic wavefunctions. Compared to more traditional plane wave or localized basis set approaches, real-space grids offer several advantages, most notably good computational scalability an…

Electronic polarizability of small metal spheres

We present the results of calculations for the ground-state electron structure, static polarizability, and dynamic response of small metal (jellium) spheres in vacuum or embedded in a dielectric. Fully self-consistent time-dependent density-functional methods are used. In particular, the static and dynamic responses to an incident electric field (dipolar polarizability and photoabsorption) are obtained. The results show substantial deviations from either classical or approximate quantum-mechanical solutions, and provide reference data for simplified treatments. Peer reviewed

Nonlinear calculations of the energy loss of slow ions in an electron gas

Abstract The stopping power of an electron gas for slow ons was calculated based on nonlinear, density-functional methods. These new theoretical results show substatnial increases in stopping powers for protons compared to calculations based on linear theory and provide a good qualitative description of the Z1-oscillations found in experimental data.

Challenges in Truncating the Hierarchy of Time-Dependent Reduced Density Matrices Equations: Open Problems

In this work, we analyze the Born, Bogoliubov, Green, Kirkwood and Yvon (BBGKY) hierarchy of equations for describing the full time-evolution of a many-body fermionic system in terms of its reduced density matrices (at all orders). We provide an exhaustive study of the challenges and open problems linked to the truncation of such hierarchy of equations to make them practically applicable. We restrict our analysis to the coupled evolution of the one- and two-body reduced density matrices, where higher order correlation effects are embodied into the approximation used to close the equations. We prove that within this approach, the number of electrons and total energy are conserved, regardless…

Electronic structure and positron states at vacancies in Si and GaAs

Noble-gas bubbles in metals: Molecular-dynamics simulations and positron states.

A theoretical treatment of atomic structure and positron states in noble-gas bubbles in metals is presented. The Al-He and Cu-Kr systems are considered as specific examples. For large bubbles (radii above a few tens of angstroms) a calculational scheme is developed combining molecular-dynamics results for the metal--noble-gas interface with positron calculations. It is demonstrated that a positron is trapped at the surface of a noble-gas bubble, i.e., at the metal-gas interface. The annihilation rate with metal electrons is similar to that at a clean surface, while simultaneously there is a significant annihilation rate with gas-atom electrons. This enables relationships between the gas den…

Spherical solid model for muon and hydrogen in metals

The spherical solid model and the spin density functional formalism have been applied to calculate the screening of a positive point charge at different lattice sites in Al, Na and Cu. Results are obtained for the Knight shift, the electric field gradient, the heat of solution and the diffusion barrier. It is found essential to use the spin-polarised form to evaluate the Knight shift, especially at low metallic densities and for impurities with a high nuclear charge. Both the Knight shift and the electric field gradient are found to be markedly different for substitutional and interstitial positions. The calculated heat of solution of hydrogen is lowest for the octahedral position in FCC Al…

Molecular dynamics simulation of epitaxial growth of the Si(001) surface

Abstract Molecular beam epitaxy on a Si(100) substrate has been studied using a molecular dynamics method with the Stillinger-Weber model potential. At high substrate temperature, 800 K, well ordered crystalline layers are found to grow underneath an amorphous overlayer of approximately 5 A thick. A limiting temperature for epitaxial growth is found to be 480 K, below which the growth does not produce ordered layers. When the sample deposited below 480 K is heated up to 800 K and the deposition is started again the original adatoms start to form ordered atomic layers. Thus the collisions of the deposited atoms in addition to the substrate temperature seem to play an essential role in the gr…

Properties of condensed spin-aligned atomic hydrogen from variational calculations

The optimal Jastrow-type ground-state wave function of spin-aligned atomic hydrogen is calculated using the pair potential of Kolos and Wolniewicz. The optimization is performed by solving the Euler equation in the hypernetted chain approximation. Accurate energies as well as pair-distribution functions are obtained. The Bose-Einstein condensate fraction is evaluated from the one-particle momentum distribution. The pair distribution function is also used to obtain stability criteria for the system and minimal values for the aligning magnetic field are calculated at low densities. The resulting values of the minimal aligning fields are considerably higher than those obtained previously.

Electronic Properties of Point Defects in Metals

Recent progress in electronic structure and related properties of point defects in metals is reviewed. Topics discussed include transition metal impurities in a non-magnetic host, construction of potential energy surfaces from effective-medium theories, trapping of light interstitials, and pair potential in metals.

Computational analysis of positron experiments

A number of applications of the calculational scheme developed by Puska and Nieminen (1982-3) are reported and the predictive power of the scheme is substantiated. Effects on positron parameters of relaxation and of N or H impurities in vacancies in Mo are calculated and employed to analyse recent experiments. Predictions pertaining to H decoration of vacancies in Al and Ni suggest the use of positron lifetime studies of these systems. Positron responses to submicroscopic vacancy clusters decorated with Kr and to large Kr bubbles in Cu are calculated and used to analyse recent experiments. To accomplish this the scheme is generalised to incorporate crystals of inert gas. In turn this makes …

Atomistic Calculations of Positron Surface States

We report on the results of an atomistic, discrete-lattice calculation of positron surface states on the three principal surfaces of Al and Cu. We are able to (i) accurately reproduce the observed values and anisotropy of the binding energies, and (ii) predict the surface state life times. Furthermore, we calculate (iii) the positron lateral diffusion constant, and find it considerably enhanced over the bulk value. We also investigate (iv) the positron trapping at surface vacancies, and (v) the effect of ordered chemisorbed monolayers of oxygen. We find that the oxidation lowers the binding energy and makes the surface state unstable with respect to positronium emission on Al (100) and Al (…

Atoms embedded in an electron gas: Phase shifts and cross sections

The Fermi-level scattering phase shifts and the transport cross sections are reported for atoms embedded in a homogeneous electron gas. The applications of the results are discussed, using the electronic stopping power for slow ions and impurity resistivity as examples. Peer reviewed

Positron Annihilation in Alkali Halides at Low Temperatures

We report on low-temperature positron studies for pure, single crystals of the alkali halides KCl, NaCl and NaF. Strong temperature dependences are observed in the lifetime and angular correlation measurements in the temperature range 10–300 K. Delocalized para-positronium is observed at low temperatures in the three crystals. The broadening, with rising temperature, of the narrow peak in the angular correlation curve can be accounted for by the self-trapping model of positronium. The results are discussed in the light of this model, in terms of positronium localization at imperfections and in terms of positron interactions with imperfections created in the spur of the positron. The increas…

Round Robin computer simulation of ejection probability in sputtering

Abstract We have studied the ejection of a copper atom through a planar copper surface as a function of recoil velocity and depth of origin. Results were obtained from six molecular dynamics codes, four binary collision lattice simulation codes, and eight Monte Carlo codes. Most results were found with a Born-Mayer interaction potential between the atoms with Gibson 2 parameters and a planar surface barrier, but variations on this standard were allowed for, as well as differences in the adopted cutoff radius for the interaction potential, electronic stopping, and target temperature. Large differences were found between the predictions of the various codes, but the cause of these differences…

Quantum Motion of Chemisorbed Hydrogen on Ni Surfaces

Quantum mechanical energy levels and wave functions have been calculated for the motion of chemisorbed hydrogen atoms on Ni surfaces. The results show considerable quantum effects for the adatom in both the ground and the excited states. The description of the adparticles as being delocalized along the surface offers a novel interpretation of several phenomena, in particular the vibrational excitations. Peer reviewed

Renormalisation group study of Anderson localisation in two dimensions: effect of second-order terms

The localisation of electrons moving in a random potential is studied in two dimensions using the real space renormalisation group method of Domany and Sarker. The effects of the cell size and of the second-order terms in the perturbation expansion are examined. While the method is not particularly sensitive to the cell size, its results depend crucially on the truncation of the perturbation series.

Muon states in metals: Recent progress

We report on our results in two interesting questions related to muon spin rotation studies in condensed matter: (i) energetics of muons in metals, including lattice relaxation and zero point motion in self-trapping phenomena, and (ii) systematics of Knight shifts and hyperfine fields.

3d impurities in Al: density functional results

Self-consistent spin density functional calculations have been carried out for 3d transition metal impurities in aluminium. The width of the virtual level decreases as it moves away from the Fermi energy with increasing occupancy. The results are compared with recent XPS measurements.

Nonlinear density dependence of the positron decay rate in helium

The decay rate of thermalized positrons in helium is calculated as a function of the fluid density, incorporating the multiple scattering of the positron off the atoms. The obtained nonlinearity agrees with experimental data outside the self-trapping region.

Molecular dynamics simulation of the damage production in Al (110) surface with slow argon ions

We have developed a molecular dynamics simulation program to gain more insight into the sputtering process, especially the damage produced by it. We have studied the sputtering of aluminium (110) surface with argon ions. The Morse pair potentail was used for Al−Al interaction, the Lennard-Jones potential for Ar−Ar interaction and both the Moliere potential and the universal potential of Ziegler et al. for Ar−Al interaction. An electronic friction term proportional to the particle velocities was also used. The studied incident argon ion energies and angles were 200 and 400 eV and 0° (normal), 25°, 45° and 75°, respectively. The calculated sputtering yield and the overall shape and the mean d…

Electronically induced trapping of hydrogen by impurities in niobium

The binding energies of hydrogen and its isotopes to substitutional impurities Ti, Cr, and V in niobium have been calculated. The hydrogen-metal interaction is based on the effective-medium theory. The wave mechanics of the hydrogenic interstitials are explicity dealt with, and the lattice distortion created by the hydrogen is incorporated through the method of lattice statics. The difference in the electronic structure between impurity and host atoms is shown to be largely responsible for the binding of hydrogen to the impurities. The results are in agreement with recent inelastic neutron scattering experiments. Peer reviewed

Screening of positrons in semiconductors and insulators

Theoretical models are presented for the enhancement of the electron density at a positron in a semiconductor or insulator host. The model better suited for typical semiconductors is based on the many-body theory for the screening of a positron in electron gas. The starting point of the model for insulators is the atomic polarizability. The common parameter in both models is the high-frequency dielectric constant. Moreover, the enhancement depends on the ambient electron density in the semiconductor model and on the unit-cell volume in the insulator model. With use of the models developed, positron lifetimes in perfect semiconductor and insulator crystals have been calculated. In the calcul…

Spontaneous fragmentation of multiply charged metal clusters.

Emission of thermal positrons from metal surfaces

Positron studies of hydrogen-defect interactions in proton irradiated molybdenum

Molybdenum single crystals are irradiated at 20 K with 6 MeV protons. The radiation damage and lattice defect annealing is studied by positron lifetime spectroscopy in the temperature range from 15 to 720 K. Loss of vacancies due to recombination with mobile interstitials is observed at 40 K (Stage I) in agreement with resistivity measurements. This is the first time Stage I is observed by positrons below 77 K. The implanted hydrogen decorates the vacancies around 100 K, which is consistent with a hydrogen migration energy in molybdenum:E H = 0.3–0.4 eV. Clustering of spatially correlated vacancies takes place in a wide temperature region below the usual vacancy clustering stage (Stage III)…

Hydrogen in metals: Quantum aspects

Hydrogen atoms are usually considered chemisorbed at well-defined sites on surfaces. We advocate a completelydifferent view, and demonstrate that chemisorbed hydrogen exhibits pronounced quantum effects. The hydrogen atom is to a large degree delocalized in both ground and excited-stated configurations: a proper description can only be given in terms of hydrogen energy bands. An analogous picture emerges for hydrogen isotopes (including muon) diffusing interstitially in bulk metals. The ground state there corresponds to a self-trapped situation: a localized impurity with an associated lattice distortion field. A powerful computational scheme is presented, which entails (i) the construction …

Molecular dynamics investigation of the premelting effects of Lennard-Jones (111) surfaces

Molecular dynamics simulations have been performed to study the premelting effects of noble-gas surfaces (argon) close to but below the bulk melting temperature. In particular, the increase of disorder as a function of temperature at (111) surface has been considered. The truncated Lennard-Jones (6-12) potential is used to describe the interactions between particles. Surface premelting has been analyzed by means of total energies, trajectory plots, mean sequare displacement functions, diffusion coefficients, vacancy concentrations and two-dimensional order parameters. The (111) surface starts to disorder by vacancy formation, which leads to the premelting of the surface layer far below the …

Density functional calculation of stopping power of an electron gas for slow ions

Abstract We describe the first calculation of the stopping power of an electron gas for slow ions using the density-functional formalism. We evaluate the nonlinear self-consistent potential around the ion and from scattering theory determine the energy loss directly. Comparison with the results of linear theory is made.

Time-dependent simulation of Czochralski silicon crystal growth

We have developed a detailed mathematical model and numerical simulation tools based on the streamline upwind/Petrov-Galerkin (SUPG) finite element formulation for the Czochralski silicon crystal growth. In this paper we consider the mathematical modeling and numerical simulation of the time-dependent melt flow and temperature field in a rotationally symmetric crystal growth environment. Heat inside the Czochralski furnace is transferred by conduction, convection and radiation, Radiating surfaces are assumed to be opaque, diffuse and gray. Hence the radiative heat exchange can be modeled with a non-local boundary condition on the radiating part of the surface. The position of the crystal-me…

Challenges in truncating the hierarchy of time-dependent reduced density matrices equations

In this work, we analyze the Born, Bogoliubov, Green, Kirkwood, and Yvon (BBGKY) hierarchy of equations for describing the full time evolution of a many-body fermionic system in terms of its reduced density matrices (at all orders). We provide an exhaustive study of the challenges and open problems linked to the truncation of such a hierarchy of equations to make them practically applicable. We restrict our analysis to the coupled evolution of the one- and two-body reduced density matrices, where higher-order correlation effects are embodied into the approximation used to close the equations. We prove that within this approach, the number of electrons and total energy are conserved, regardl…

Positron Surface States on Clean and Oxidized Al and in Surface Vacancies

This Letter reports on the first discrete-lattice calculation of positron surface states on the surfaces of Al. The authors reproduce the observed values and anisotropy of the binding energies on clean surfaces, and predict the surface-state lifetimes. The temperature-independent lateral diffusion constant is calculated. Monovacancies on surfaces are predicted not to trap positrons. The effect of ordered chemisorbed monolayers of oxygen is investigated: Oxidation makes the surface state unstable with respect to positronium emission. Peer reviewed

Role of elastic and electronic interactions in trapping of hydrogen by impurities in transition metals

The interplay between the lattice distortion and the electronic contributions to the trapping of migrating hydrogen isotopes by substitutional impurities is investigated. We use a comprehensive calculational scheme incorporating (i) the effective-medium theory for the electronic interaction, (ii) the lattice Green’s function for elastic coupling, and (iii) the hydrogen quantum motion. The calculations for Ti and Cr impurities in V host show that lattice strain effects dominate. Cr, which otherwise provides an electronic trap site, does not induce trapping when elastic effects are incorporated. The situation in the case of Ti is just the reverse. We find no isotope dependence of the binding …

Two-component density-functional theory: Application to positron states.

A quantitative approach to calculating properties of inhomogeneous two-component Coulomb-Fermi systems is presented. As an application, the ground-state electronic structure of a jellium vacancy containing a trapped positron is calculated self-consistently. While the resulting density profiles and energetics are quite different from those obtained neglecting cross correlations, the conventional estimates for the annihilation rates are shown to remain valid, due to canceling effects of the increase in the mean electron density and the decrease in short-range screening.

Semi-empirical supercell calculations for free- and bound-hole polarons in crystal

Two different parametrizations of the semi-empirical method of the intermediate neglect of the differential overlap (INDO) are applied to the calculations of the small-radius hole polarons in the corundum crystal. The 80-atom supercell has been used for the study of the atomic and electronic structure of a free small-radius hole polaron (the self-trapped hole, STH) and a hole polaron bound by a Mg impurity (the so-called centre), respectively. Both parametrizations indicate that the two-site (quasi-molecular) configurations of both kinds of polaron have the lowest energy (which does not exclude the existence of one-site polarons also characterized by considerable relaxation energies). For c…

Cluster calculations for H2dissociation on Cu and Ni

Self-consistent cluster calculations have been carried out for hydrogen dissociation on Cu and Ni clusters using local-density theory and the LCAO-DVM expansion. We find physisorption, chemisorption and dissociation minima in the resulting two-dimensional potential energy surfaces, and for the Ni cluster, also an indication of the associative molecular chemisorption state. For Cu we find a considerable barrier at the seam separating the molecular chemisorption and dissociative minima. The analysis of one-electron levels supports the picture of Harris and Andersson that the s to d conversion present on Ni surfaces does not similarly lower the barrier on Cu surface.

Nonlinear stopping power of an electron gas for slow ions

Theoretical calculations of the stopping power of the electron gas for slow ions, v${v}_{F}$, are reviewed. New results are presented for stopping power and effective charge based on nonlinear density-functional calculations. Extensive comparisons with available experimental data show that these new theoretical results are clearly superior to earlier calculations based on linear theory.

Near-surface defect profiling with slow positrons: Argon-sputtered Al(110).

We report on slow-positron measurements of atomic defect distribution near a solid surface. Defects are produced by argon-ion bombardment of an Al(110) surface in ultrahigh vacuum. Defect profiles have a typical width of 15–25 Å and contain a broader tail extending to 50–100 Å. The defect density at the outermost atomic layers saturates at high argon fluences to a few atomic percent, depending on sputtering conditions. Defect production rate at >1 keV Ar+ energies is typically 1–5 vacancy-interstitial pairs per incident ion. Molecular-dynamics simulations of the collision cascade predict similar defect distributions. Peer reviewed

Comment on the Positron Surface-State Lifetime

Quand on calcule de facon coherente de l'energie de correlation des positons et la vitesse d'annihilation, la theorie explique au moins de facon qualitative la duree de vie de l'etat de surface des positons

Positron detrapping from defects: A thermodynamic approach

The rate of positron detrapping in thermal equilibrium from lattice defects has been calculated by relating it to the specific trapping rate. The results for vacancies, dislocations and surfaces each show a different temperature dependence for the escape rate. For vacancies a measure of the importance of the detrapping can be obtained from the ratio of the vacancy formation energy to the positron binding energy in the defect. The positronium desorption rate from a surface is also calculated and agreement with experimental results is found.

Embedded-atom calculations of Auger and x-ray photoemission shifts for metallic elements

Change in self-consistent-field energy density-functional calculations are reported for Auger and core-level binding-energy shifts in sp-bonded metals. The basic model, atom in jellium vacancy, gives good agreement with experiment, especially in the Auger case. The chemical and relaxation contributions to the shifts are discussed, and the extra-atomic response is analyzed in detail, both in position and energy space. The adequacy of the "excited-atom" approach to the energy shifts is discussed. Peer reviewed

Electron-positron density-functional theory.

A two-component density-functional theory is presented for electron-positron systems. The phase diagram of a two-component Fermi-Coulomb system is discussed, and explicit expressions are derived for exchange-correlation functionals for use in the local-density approximation. The scheme is then applied in a fully self-consistent calculation of electron and positron densities in atomic vacancies in metals, treated in the jellium model. Comparison with conventional calculations, which do not meet true electron-positron self-consistency, reveals considerable changes in the density distributions. However, we demonstrate that there are cancellation effects which render the corresponding changes i…

Core Polarizabilities in Metals

Linear response formalism within the density-functional scheme is applied in a calculation of core polarizabilities in simple metals. While the core polarizability changes relatively little (around 10%) from its free-ion value in the alkalis, Mg and Al, large increases are found for metals like Ga, Cd, In, and Sn with full d-shells. Low-frequency values for the dynamic polarizability are also obtained.

Hydrogen and deuterium decoration of In-vacancy complexes in nickel.

The quantum-mechanical states of hydrogen and deuterium in pure and defected nickel have been calculated using the effective-medium theory. The defects considered include monovacancies, the substitutional In impurity, a complex of four vacancies, and a complex of an In impurity decorated with a tetrahedron of four vacancies. While the substitutional In impurity does not trap hydrogen, the vacancy and the vacancy complexes with and without In association do. The calculated binding energy to the four vacancy complex is nearly insensitive to the hydrogen isotopic mass and to the In decoration. These results, along with the dependence of the hydrogen binding energy on multiple hydrogen occupanc…

Density-Functional Calculations of Auger and X-Ray Photoemission Shifts for Metallic Elements

ΔSCF density-functional calculations are reported for Auger, and core level binding energy shifts in sp-bonded metals. The basic model, atom-in-jellium-vacancy, gives good agreement with experiment, especially in the Auger case. The chemical and relaxation contributions to the shifts are discussed. The shifts are calculated also by using the thermochemical model and the results obtained are in agreement with experimental data. The applicability of the "excited-atom" approach to the Auger energy shifts is found restricted.

Helium bubbles in metals: Molecular-dynamics simulations and positron states.

By combining molecular-dynamics results for the aluminum-helium interface with positron-state calculations it is demonstrated that a positron is trapped at the surface of a He bubble in Al. The annihilation rate with Al electrons is similar to that at a clean surface, while simultaneously there is a significant annihilation rate with He electrons. This enables one to obtain a useful relation between the positron lifetime and helium densities in bubbles.

Stability of carbon nanotubes under electron irradiation: Role of tube diameter and chirality

As recent experiments demonstrate, the inner shells of multiwalled carbon nanotubes are more sensitive to electron irradiation than the outer shells. To understand the origin of such counterintuitive behavior, we employ a density-functional-theory based tight-binding method and calculate the displacement threshold energies for carbon atoms in single-walled nanotubes with different diameters and chiralities. We show that the displacement energy and the defect production rate strongly depend on the diameter of the nanotube and its chirality, with the displacement energy being lower, but saturating towards the value for graphite when the tube diameter increases. This implies that the threshold…