Positron Annihilation Angular Correlation Research Articles

Topological Quasi-Positroniums in Graphite-Alkali Metal Intercalation Compounds

We have analyzed the positron annihilation Angular Correlation data in the second stage graphite-potassium intercalation C24K with the theoretical formula extended from "topological quasi-positronium model" and have proposed one of the possible origins of the anisotropic narrow component in alkali-metal graphite intercalations compounds (AGICs).

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)

The Study of the Electron–Positron Pair in CS2

Abstract Ole E. Mogensen reported that CS2 was the only liquid which showed large decrease of the Ps intensity with lowering the temperature, even though electron and positron could encounter with each other at low temperatures such as −100 °C. The intensity change by the positron annihilation lifetime was observed at a little lower temperature by the positron annihilation angular correlation. Here I measured the change of intensity and lifetime of the longest lifetime component caused by varying the surface tension, the dielectric constant, and the temperature, to clarify the positron state in CS2. These experimental results confirm the ideas of the “fourth positron state” and the “squeezed positronium state” which were proposed by Ole E. Mogensen.

Study of various types of diamonds by measurements of double crystal x-ray diffraction and positron annihilation

Annihilation characteristics of positrons in various types of diamonds were studied by measurements of two-dimensional angular correlation of positron annihilation, those of Doppler Broadening profiles and those of lifetime spectra. From the measurements, it was found that there is a small amount of defects in type Ib synthesized diamond and the positron lifetime is 115 ps. On the other hand, it was found that positronium is formed in natural diamonds. Results of the double x-ray measurements suggested that IIa, IIb, and Ia type natural diamonds also may have empty space between crystallites or grain boundaries.

Evolution of the electronic structure of La2-xSrxCuO4 with doping determined by positron-annihilation spectroscopy.

We have measured and calculated the electron-positron momentum distribution of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ samples for Sr concentrations of 0, 0.1, 0.13, and 0.2. Measured distributions were obtained at room temperature with high statistical precision, greater than 4\ifmmode\times\else\texttimes\fi{}${10}^{8}$ events, in the Lawrence Livermore National Laboratory positron-annihilation angular correlation spectrometer on single-crystal samples fabricated using the traveling solvent floating zone technique. Corresponding theoretical momentum-density calculations were performed using the linear muffin-tin-orbital method. The momentum distribution of all samples contained features derived from the overlap of the positron distribution with the valence electrons. In addition, discontinuities typical of a Fermi surface are seen in the doped samples. The form and position of these features are in general agreement with the Fermi surface and overall momentum distributions as predicted by band theory. However, the evolution of the Fermi surface with doping differed significantly from expectations based on single electron band theories.

Electronic Structure and Fermiology of High-TcSuperconductors

We discuss several problems of current interest concerning the electronic structure and Fermiology of complex materials where computational approaches have been very useful. Specifically, we consider the first-principles band theory predictions of spectral intensities for angle-resolved photoemission (ARPES) and positron annihilation angular correlation (2D-ACAR) spectra from YB a2 Cu 3 O 7 , and the effects of substitutions on the elctronic structures of simple cubic perovskites and the La214 high-T c compound.

Theoretical studies of the angular correlation of positron annihilation in germanium

A pseudopotential formalism is applied to calculate the angular correlation of positron annihilation radiations (ACPAR) along the (100), (110) and (111) directions in Ge. The results are found to agree very well with the measured data. Calculations show that the ACPAR line shapes, particularly in the low-momentum region, are very much influenced by the detailed nature of bonding. Two-photon autocorrelation functions corresponding to the above symmetry directions are also presented and some interesting conclusions are drawn regarding positron-electron many-body correlation effects.

Positron Annihilation Studies of the Electronic Structure and Fermiology of High-Tc Superconductors

Abstract We discuss the application of the positron annihilation angular correlation (ACAR) spectroscopy for investigating the electronic structure and Fermiology of high-Tc superconductors, with focus on the YBa2Cu3O7 system where most of the experimental and theoretical work has to date been concentrated. Detailed comparisons between the measured 2D-ACAR positron spectra and the corresponding band theory predictions show a remarkable agreement (for the normal state), indicat-ing that the electronic structure and Fermi surface of this material is described reasonably by the conventional picture.

Positron annihilation study of irradiation effects on graphites

Positron annihilation lifetime measurement has been performed for various kinds of unirradiated and irradiated graphites. Of the unirradiated speciemens, HOPG-type graphites showed the shortest lifetime component ( ~ 220 ps) and isotropic graphites showed a longer one ( ~ 350 ps). The longest lifetime was observed in glassy carbons ( ~ 400 ps). The electron or neutron irradiation-induced change of the positron annihilation lifetime depended on the type of graphite namely, in HOPG an increase of the positron lifetime was observed, but in isotropic graphites a decrease of the lifetime was observed. Positron annihilation angular correlation measurements have also been performed. In HOPG, for the case of P z ∥ c-axis, an angular correlation curve with a minimum at θ = 0 was observed, but the irradiation made this minimum obscure.

Fermi surface, ground-state electronic structure, and positron experiments in YBa2Cu3O7.

We present calculations of electron-positron momentum density in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ based on band theory in the local-density approximation. Theoretical predictions are in semiquantitative accord with the corresponding two-dimensional positron-annihilation angular-correlation measurements. These results indicate that the conventional local-density-approximation band-theory picture provides a reasonable description of the ground-state electronic structure and the Fermi surface of Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. Several Fermi-surface-related features are suggested in the positron spectra for the first time.

Calculations of the angular correlation of positron annihilation at ideal and defected aluminium surfaces

Previous calculations of the angular correlation of annihilation radiation (ACAR) spectra for positron surface states on aluminium have resulted in varying degrees of anisotropy with respect to momentum components parallel and perpendicular to the surface. However, all experiments to date have yielded almost perfect isotropy. The present calculations show that the ACAR spectrum of a positron localised in a surface monovacancy is nearly isotropic, providing some evidence that this defect-trapping state may be responsible for the observations.

Positron annihilation in synthetic zeolites

Both lifetime and angular correlation of positron annihilation have been measured for a series of synthetic zeolites for which the void structures are known fairly well. All of the zeolites had long lifetime components and a narrow momentum component which are ascribable to o-Ps and p-Ps annihilations in the voids, respectively. The correlation between the width of the p-Ps narrow component and the size of the largest voids showed a remarkable agreement with a theoretical estimate based on the spherical potential well model. The measurement of p-Ps momentum thus appears to be prospective as a tool to determine the size of voids of materials. The lifetime of the long lifetime component, on the other hand, showed a poor correlation with the void size even from a qualitative viewpoint, suggesting that factors other than the simple void size effect are dominant in determining the o-Ps lifetimes. Discussion is made on the cause of the different dependences of o-Ps and p-Ps annihilation parameters on the void size. It has also been found that p-Ps fraction is always larger than one third of the o-Ps fraction in all the zeolites studied. A discussion is presented on this point, too.

Two-dimensional electron momentum distribution in graphite revealed by means of angular correlation of positron annihilation

Two-dimensional angular correlation measurements with positron annihilation radiation (2D-ACAR) on a single crystal of Kish graphite reveal that the motion of the annihilating electrons is nearly free along the graphite layers and restricted within some distance shorter than the lattice constant in the direction perpendicular to the layers. The hollows appearing on the 2D-ACAR surfaces are explained in terms of preferential annihilation of positrons with π-electrons in graphite.

Structure Anisotropy of Mica Crystals by Positron Annihilation Method

Positron annihilation angular correlation measurements were carried out for two mica monocrystals (muscovite and phlogopite) in two orientations each (001 and 010).The experimental angular correlation curves for the first orientation were nearly the same. A substantial difference was found for the 010 orientation. The effect was ascribed to the structure peculiarities of the crystals and was found to correlate with their rigidity.

Electronic structure studies with positrons: a new approach to wavefunction effects

A new approach to the analysis of positron annihilation angular correlation data based on the definition of the LCW folded k-space density in terms of real-space matrix elements and an orbital angular momentum l-based wavefunction expansion is presented and discussed. The computational convenience and interpretation utility of this approach is illustrated with an example calculation for niobium.

Positron annihilation in pivalic acid. Temperature dependence of angular correlation curves

Positron annihilation angular correlation curves have been measured as a function of temperature for trimethylacetic (pivalic) acid in both the brittle and plastic phases. A simple fitting of the data to a sum of three gaussians shows the presence of a narrow component due to para-positronium ( p-Ps) annihilation. In the brittle phase the intensity of the narrow component is inconsistent with previous positron lifetime data. A more detailed analysis, requiring consistency with the lifetime data, results in the determination of the shapes of the angular correlation components for free positron-, pick-off-, and p-Ps intrinsic annihilation. The p-Ps component has a width (fwhm) of 3.75 mrad in the brittle phase, probably due to Ps self-trapping or trapping in defects smaller than molecular vacancies. In the plastic phase the width (fwhm) is 3.25 mrad which is ascribed to Ps localization in vacancies and divacancies in accordance with positron lifetime data.

The temperature dependence of the positronium bubble state. An angular correlation and doppler broadening study of a phenyl ether

Abstract Positron annihilation angular correlation and Doppler broadening measuremennts are reported as a function of temperature for the supercooling liquid bis[m-(m-phenoxyphenoxy)phenyl]ether (6PO). A comparison of these observations with predictions based on lifetime measurements is presented and indicates limitations of particular experiments in characterising the annihilation processes. The ratio I′3/I1, where I′3 is the ortho-Ps yield and I1 the narrow-peak intensity, was observed to be 4–6 instead of its “ideal” value of 3 in the glassy state. This is interpreted as evidence for a Ps center-of-mass wavefunction consisting of a component strongly localized on the Ps trap and a broader, “spilled over” component. Only the latter gives rise to a narrow peak of intensity I1. The variation of I′3/I1 with temperature depends on the Ps state and hence, provides evidence for the effect of the initial size of the Ps trap and the molecular motion within the Ps lifetime.

Electronic structure and positron annihilation in B2-ordered CuZn

The momentum density of annihilation pairs (MDAP) in B2-ordered stoichimetric CuZn alloy is determined ab initio on the basis of an APW band-structure calculation. The effect of the electron-positron interaction is considered by means of an energy-dependent Kahana-like enhancement factor. A direct comparison of calculated and experimental positron annihilation angular correlation curves is performed, taking into account the angular resolution of the measuring device and the thermal motion of the positron. As in iron alloys, it is found that the enhancement of the contribution of d electrons to the MDAP is appreciably lower than that of the contribution of sp electrons belonging to the conduction band. A simple model explaining semiquantitatively the different enhancement factors for both groups of electrons is proposed.

Correlation energy from compton scattering and positron annihilation data

AbstractThe second moments of Compton profiles are used to obtain electronic energies. It is proposed that the second moment of a line‐slit positron annihilation angular correlation curve, in comparison with the second moment of Compton profiles, can yield the correlation energy contribution to the cohesive energy of a material. The need for correcting the PAAC data for the positron enhancement factor and thermalization effects is examined. Numerical estimates of the errors involved are given for sodium and lithium and the possible applicability of the proposal is shown.

Positron annihilation in electron-irradiated n-type gap crystals

Abstract The defects in n-GaP crystals irradiated by 2.3 MeV electrons up to 1 × 1019 cm−2 at RT were studied by means of positron annihilation (angular correlation) and electrical property measurements. It was found that positrons are trapped in some radiation-induced vacancy-type defects (acceptors) but that the effect saturates at high electron fluences (D1 × 1018 cm−2). The trapping rate in irradiated samples increases with temperature in the range 77–300 K. Post-irradiation isochronal annealing reveals the positron traps clustering at about 200–280°C. All positron sensitive radiation-induced defects disappear upon annealing up to 500°C.