Correlation Of Positron Annihilation Radiation Research Articles

Electronic studies of alloy Ga1-xAlxN alloys

Electronic band structures and charge densities of GaN, AlN, and their alloy Ga0.5Al0.5N in zinc-blende structure using empirical pseudopotential method are investigated. For the Ga1-xAlxN ternary alloy, the virtual crystal approximation is coupled with the pseudopotential method. The energies along f, X, and L of Ga1-xAlxN alloy as a function of the alloy concentration is calculated. Angular correlation of positron annihilation radiation along different crystallographic directions in GaN, AlN, and Ga0.5Al0.5N is calculated. Other quantities such as ionicity character and bulk modulus by means of recent models with respect to the alloy concentration are discussed.

On Diagnostics of Double Electric Layer in the Zone of Metal–Polymer Adhesion Contact Using the Positron-Annihilation-Probe Method

Application of the positron-annihilation-probe technique for nondestructive diagnostics of the double electric layer (DEL) at the polymer–metal interface and, hence, of the electric component of polymer adhesion to the metal is described. Separated curves of angular correlation of positron-annihilation radiation (ACPAR) for metal, polymer, and metal with a polymer layer allow detecting the presence of a strong electric field of a DEL at the nickel–epoxy-resin interface. The value of the DEL potential is determined to be 15 keV. The method of a positron-annihilation probe allows identifying the presence of a DEL on the internal interfaces of materials and estimating its energy parameters so as to correctly reflect the value of the contribution of the electrostatic component to the interphase interaction.

Fermi surface determination from momentum density projections

The problem of determining a metal’s Fermi surface from measured projections of the electron or electron/positron momentum densities, such as those obtained by Compton scattering or angular correlation of positron annihilation radiation, respectively, is examined in a Bayesian formulation. A consistent approach with an explicit treatment of the Fermi surface already at the reconstruction stage is presented, and its advantages compared to previous practice are discussed. A validation of the proposed method on simulated data shows its systematic accuracy to be very satisfactory and its statistical precision on modest experimental data to be surprisingly good.

Electron momentum distribution and singlet-singlet annihilation in the organic anthracene molecular crystals using positron 2D-ACAR and fluorescence spectroscopy.

We present the mapping of electron momentum distribution (EMD) in a single crystal of anthracene by two-dimensional angular correlation of positron annihilation radiation (2D-ACAR). The projected EMD is explained on the basis of the crystallographic features of the material. The EMD spectra provide information about the positron states and their behavior and also about the hindrance of the positronium (Ps) formation in this material. The EMD has exhibited evidence for the absence of free volume defects. The characteristic EMD features regarding the delocalized electronic states are explained. Further, scintillation characteristics such as fluorescence and time-correlated single photon counting have also been studied. The emission peaks are attributed to vibrational bands of fluorescence emission from the singlet excitons and lifetime components are observed to be due to singlet fission and the singlet-singlet excitons annihilation.

Recovering the Fermi surface with 2D-ACAR spectroscopy in samples with defects

When two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) experiments are performed in metals containing defects, conventional analysis in which the measured momentum distribution is folded back into the first Brillouin zone is rendered ineffective due to the contribution from positrons annihilating from the defect. However, by working with the radial anisotropy of the spectrum, it is shown that an image of the Fermi surface can be recovered since the defect contribution is essentially isotropic.

Reaction of positronium with doped ions in silica-based glasses in the size determination of subnanometer structural open spaces

Positron annihilation spectroscopy is employed to estimate the size of subnanometer-scale open spaces in insulating materials. In most cases, the size is estimated from the lifetime of long-lived ortho-positronium (o-Ps) by pickoff annihilation using a simplified model. However, reactions of Ps with surrounding electrons other than the pickoff reaction, such as spin conversion or chemical reaction, could give a substantially underestimated size using the simplified model. In the present paper, we report that the size of the open spaces can be evaluated correctly by the angular correlation of positron annihilation radiation (ACAR) with a magnetic field using the spin-polarization effect on Ps formation, even if such reactions of Ps occur in the material. This method is applied to the subnanometer-scale structural open spaces of silica-based glass doped with Fe. We demonstrate the influence of the Ps reaction on size-estimation of the open spaces from the o-Ps lifetime. Furthermore, the type of reaction, whether spin conversion or chemical, is distinguished from the magnetic field dependence of the Ps self-annihilation component intensity in the ACAR spectra. The Ps reaction in silica-based glass doped with Fe is a chemical reaction (most likely oxidation) rather than spin conversion, with Fe ions. The chemical quenching rate with Fe ions is determined from the dependence of the o-Ps lifetime on the Fe content.

Skyrmion-like Solitons and Topological Quasi-positronium in Graphite-potassium Intercalation Compounds

We have analyzed the narrow components of the angular correlation of positron annihilation radiation of C24K, by using the topological charge density of the skyrmion-like soliton, and have estimated experimentally the configuration of the skyrmion-like soliton.

Eliminating spatial distortions in Anger-type gamma cameras

A procedure to quantify and correct the spatial distortions inherent to Anger-type gamma cameras is presented. It consists in imaging a pattern of regularly spaced holes, assigning to each pair of lattice indices the actual position on the detector and generating a look-up matrix describing the inverse mapping. This allows one to correct the position of the distinct events either during or after the measurement with minimal computational effort. The corrected spectrum is indistinguishable from a spectrum taken with an ideal detector in a statistical sense. The effect of the increased resolution on measurements of angular correlation of positron annihilation radiation is demonstrated. The presented scheme is applicable for all types of area detectors.

Bulk Fermi surface and momentum density in heavily doped La2−xSrxCuO4using high-resolution Compton scattering and positron annihilation spectroscopies

We have observed the bulk Fermi surface (FS) in an overdoped ($x$=0.3) single crystal of La$_{2-x}$Sr$_x$CuO$_4$ by using Compton scattering. A two-dimensional (2D) momentum density reconstruction from measured Compton profiles yields a clear FS signature in the third Brillouin zone along [100]. The quantitative agreement between density functional theory (DFT) calculations and momentum density experiment suggests that Fermi-liquid physics is restored in the overdoped regime. In particular the predicted FS topology is found to be in good accord with the corresponding experimental data. We find similar quantitative agreement between the measured 2D angular correlation of positron annihilation radiation (2D-ACAR) spectra and the DFT based computations. However, 2D-ACAR does not give such a clear signature of the FS in the extended momentum space in either the theory or the experiment.

Study of PRIMAVERA steel samples by a positron annihilation spectroscopy technique

In the present article, a positron annihilation spectroscopy investigation of VVER-440/230 weld materials is discussed. Important characteristics of metals such as Fermi energy, concentration of electrons in the conduction band, size and concentration of defects were experimentally determined for three model materials with higher level of copper (0.16 wt.%) and phosphorus (0.027–0.038 wt.%). The impact of neutron irradiation and subsequent annealing on crystal lattice parameters was investigated. The experiments with the angular correlation of positron annihilation radiation (ACAR) complement the published positron annihilation spectroscopy (PAS) studies of the radiation treated VVER materials as well as previous experiments on PRIMAVERA materials. The availability of the experimental reactor to prepare strong 64Cu positron sources provided for unique experimental conditions, such as good resolution of spectra (0.4 mrad) and reasonable short time of measurement (36 h). The present paper aims to contribute to further understanding of RPV (reactor pressure vessel) steels behaviour under irradiation conditions as well as annealing recovery procedures, which have already been applied at several VVER NPP units in Europe.

Interactions between Fermi surfaces and Brillouin zone boundaries and phase stability of embedded metallic nanoparticles

A general approach is outlined to predict the chemical compositions of solute nanoclusters embedded in materials with nearly free electrons. Based on the experimental results of the two-dimensional angular correlation of positron annihilation radiation and the corresponding theoretical calculations, we show that the Fermi surface (FS)--Brillouin zone interaction is a key to understand the chemical composition realized in solute nanoclusters because the presence of FS necks at zone boundaries causes a reduction in electron energy due to the band-gap effect.

陽電子量子ドット閉じ込めを利用したＦｅ中Ｃｕナノ析出物の寸法評価法の開発

A new method for size estimation of Cu (sub)nano-precipitates in Fe based on a positron quantum-dot-state is developed. Utilizing a unique characteristic of the positron quantum-dot confinement in positron affinitive embedded nano-precipitates, the smearing of the momentum distribution of positron-electron pair around the Fermi momentum in the Cu nano-precipitates is observed by two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) measurements. Due to the uncertainly relation between position and momentum, the size of the Cu nano-precipitates is estimated from the width of the momentum smearing which is inversely proportional to the average size of the precipitates where positrons are confined. Fe–Cu alloys were aged at 475°C for 0.1 to 20 h to form the Cu nano-precipitates with various sizes. The sizes estimated from 2D-ACAR are compared with those by three-dimensional atom probe (3D-AP). Both the results agree with each other. Furthermore, the present method gives the sizes of the Cu subnano-precipitates which are too small to be detected even by the other technique, such as 3D-AP.

2D‐ACAR studies on microstructure characteristics of porous silicon

AbstractTwo dimensional electron momentum distributions (EMD) have been mapped by the two‐dimensional angular correlation of positron annihilation radiation (2D‐ACAR) to study the electronic structure and microstructural characteristics of porous silicon. The open volume defects associated with vacancies and their complexes present in the porous layer have been analysed using the EMD spectra. Also, the correlation between EMD and the pore formation mechanism was studied and the surface properties were analyzed along with the photoluminescence spectra. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase transition studies in trans ‐stilbene crystal using 2D‐ACAR

AbstractTwo‐dimensional electron momentum distributions (EMD) mapping in a (c‐a) plane of molecular trans‐stilbene crystal have been made, using the two‐dimensional angular correlation of positron annihilation radiation (2D‐ACAR) at room temperature (RT), 210 K and 110 K. Further, we discuss the changes in static and dynamic electron momentum distribution and the positron behavior in molecular crystals at different temperatures to clarify their static and dynamic origin. An attempt has been made to visualize the electronic structure of organic molecular crystal trans‐stilbene during phase transition. The mapped EMD is explained with the combination of disordered to ordered transition concept and molecular vibration freezing model. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Local structure reconstruction in hydrogenated amorphous silicon from angular correlation and synchrotron diffraction studies

Hydrogenated amorphous silicon (a-Si:H) is a widely used thin film semiconductor material which is still incompletely understood. It is generally assumed to form a continuous random network, with a high concentration of coordination defects (dangling bonds), which are hydrogen terminated. Neither the exact nature of these sites nor the degree of medium range order has been fully determined. In this paper, we present the first results for the local structure, from a combined study using angular correlation of positron annihilation radiation (ACAR) and synchrotron radiation diffraction. Reciprocal space information is obtained directly, for the mesoscale structure and the local defect structure, from the orientation dependent diffraction and 2D-ACAR patterns, respectively. Furthermore, inversion of both patterns yields a comparison of real space information through maps of the silicon–silicon pair correlation function and the electron–positron autocorrelation function B 2 γ ( r). From this information, it is possible to identify the dominant structural defect as a vacancy-size dangling bond cluster, around which the network strain is fully relaxed.

COMPOSITION DEPENDENCE OF THE POSITRON ANNIHILATION IN Zn1-xBexSe

In order to clarify the electronic and optical properties of wide-energy gap zinc-blende structure ZnSe , BeSe and their alloys, a simple pseudo-potential scheme (EPM), within an affective potential the virtual crystal approximation (VCA) which incorporates compositional disorder as an effective potential are presented. Angular correlation of positron annihilation radiation (ACPAR) along different crystallographic directions in Zn 1-x Be x Se are calculated. We observe that the electron-positron momentum density increases rapidly with increasing Beryllium content.

Electron momentum distribution mapping of trans-stilbene single crystal by positron annihilation

Two-dimensional electron momentum distributions mapping in a molecular trans-stilbene crystal has been done, first time using two-dimensional angular correlation of positron annihilation radiation. The measured coincident annihilation spectra for the plane corresponding to c– a, projected to crystallographic b direction have been elucidated.

Different ways of dealing with Compton scattering and positron annihilation experimental data

Different ways of dealing with one-dimensional (1D) spectra, measured e.g. in the Compton scattering or angular correlation of positron annihilation radiation (ACAR) experiments are presented. On the example of divalent hexagonal close packed metals it is shown what kind of information on the electronic structure one can get from 1D profiles, interpreted in terms of either 2D or 3D momentum densities. 2D and 3D densities are reconstructed from merely two and seven 1D profiles, respectively. Applied reconstruction techniques are particular solutions of the Radon transform in terms of orthogonal Gegenabauer polynomials. We propose their modification connected with so-called two-step reconstruction. The analysis is performed both in the extended p and reduced k zone schemes. It is demonstrated that if positron wave function or many-body effects are strongly momentum dependent, analysis of 2D densities folded into k space may lead to wrong conclusions concerning the Fermi surface. In the case of 2D ACAR data in Mg we found very strong many-body effects. PACS numbers: 71.18.+y, 13.60.Fz, 87.59.Fm

Empirical pseudopotential study of electronic, positron, and structural properties of Ga 1−xAl xN

Electronic and positron band structures and charge densities of Ga 0.5Al 0.5N using the empirical pseudopotential method (EPM) are investigated. For the ternary alloy Ga 1− x Al x N, the virtual crystal approximation (VCA) is coupled with the pseudopotential method. The energies along Γ, X and L of Ga 1− x Al x N alloy as a function of the mole fraction are calculated. Angular correlation of positron annihilation radiation (ACPAR) along different crystallographic directions in Ga 0.5Al 0.5N is calculated. Other quantities such as ionicity character and bulk modulus by means of recent models with respect to the mole fraction are discussed.

Theoretical studies of the angular correlation of positron annihilation in Al1− xInxN

AbstractAngular correlation of positron annihilation radiation (ACPAR) along the (001)–(110) directions in Al1−xInxN are calculated. We observe that the momentum density distribution for different concentrations increases rapidly with increasing indium content. The computational technique used here is based on the independent particle model (IPM) coupled with the use of the electron pseudo‐wavefunction and the virtual crystal approximation (VCA), which incorporates compositional disorder as an effective potential. The variation of the positron lifetime in these alloys is also presented. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)