Transport Problems with a Focus on Fluid and Heat Flow

Vol. 379

Vol. 379

Transfer Phenomena in Fluid and Heat Flows IV

Vol. 378

Vol. 378

New Development for Heat Transfer in Solids and Fluid Flow

Vol. 377

Vol. 377

Materials and Technologies in Dentistry: Defects, Processing and Characterization

Vol. 376

Vol. 376

Defects and Diffusion Phenomena in Materials for Nuclear Technologies

Vol. 375

Vol. 375

Transfer Phenomena in Fluid and Heat Flows III

Vol. 374

Vol. 374

Positron Annihilation - ICPA-17

Vol. 373

Vol. 373

Transfer Phenomena in Fluid and Heat Flows II

Vol. 372

Vol. 372

Recent Developments in Mass Transport and Related Phenomena in Materials

Vol. 371

Vol. 371

Transfer Phenomena in Fluid and Heat Flows

Vol. 370

Vol. 370

Diffusion in Solids and Liquids XI

Vol. 369

Vol. 369

Local Mechanical Properties

Vol. 368

Vol. 368

Diffusion Phenomena: Aspects of Characterization and Experiments

Vol. 367

Vol. 367

# Positron Annihilation - ICPA-17

文章题目 页数

摘要: Late Professor Stewart initiated and shaped the International Conference on Positron Annihilation (ICPA) series. As a first-generation experimental positron-annihilation scientist, he made full use of the angular correlation of annihilation radiation (ACAR) method. He applied this method to study Fermi surfaces of metals, positron wave-functions in crystals, positron-electron and -phonon many-body interactions, and the vacancy formation energy in solids. He also studied with this method positronium in liquids and solids (T. Hyodo, J. Phys. Conf. Series, 618 (2015) 012002). All these studies enjoyed by Professor Stewart will long be remembered by the positron study community.

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摘要: Production of antihydrogen by using the charge exchange reaction, as proposed by AEgIS (Antimatter Experiment: gravity, Interferometry, Spectroscopy), requires the formation of a dense cloud of positronium atoms excited to Rydberg states. In this work, the recent advances in AEgIS towards this result are described. Namely, the manipulation of positrons to produce bunches containing more than 10

^{8}particles and the laser excitation of positronium to Rydberg states, using n=3 as intermediate level, are presented.
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摘要: Taking liquid water as an example, we have considered energetics of the positronium formation. It is shown that quasi-free Ps as well as the Ps localized in a bubble cannot decay into hydrated electron and positron. The most probable value of the positron work function in water,

*V*_{0}^{+}, is 1.5…2 eV. Ps formation from the hydrated electron and positron is energetically possible. By the end of thermalization major fraction of positrons escapes the blob and hydrates outside. The low-mobile е^{+}*has no time to diffuse back and form Ps with intrablob e-. These е*_{aq}^{+}*mostly annihilate as “free positrons”. Positrons, which are thermalized within the blob, recombine with the quasi-free intrablob electrons and form Ps.*_{aq}
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摘要: We have investigated positronium‒xenon collisions at energies below 100 meV to find a strong temperature dependences of the annihilation rates. A partial wave analysis based on a modified effective range theory (MERT) is tested to explain the temperature dependences and to find significant contributions of the

*p*-wave scattering component. The fact that MERT works well for analyzing positronium‒xenon collisions indicates that positronium is polarized during the collisions as proposed by other theoretical and experimental researchers.
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摘要: Positron lifetime calculation has been performed on a computer-generated nanocrystalline copper with a mean grain size of 9.1 nm during its deformation. For the undeformed and deformed nanocrystalline copper, calculated positron lifetimes are around 157 ps which come from the positron annihilation in the free volume in grain boundaries. Due to the grain-boundary deformation mechanism, no vacancies or vacancy clusters will be induced in grains during the plastic deformation of the nanocrystalline copper, which is different to the deformation of the conventional polycrystal. From this point of view,

*in-situ*positron annihilation measurements can provide important experimental information on the deformation mechanism of nanocrystalline metals.
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摘要: Recently, the theory of gradient-corrected electron-positron correlations in solids was improved and extended in order to avoid the adjustable, empirical parameter α which is now part of theory and a smooth function of the electron density.This new, parameter-free approach is applied to selected oxides in order to discuss their interstitial space morphology reflected by the positron charge distribution. In addition, the positron lifetime and affinity are calculated using a highly precise computational method. An attempt is made to correlate these quantities with the volume of the reduced formula unit.The results for some oxides - such as Li-ion battery cathode materials and high temperature superconductors - are discussed in detail and prospects for future experiments are given.

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摘要: Defect formation energy in Bi

_{2}Te_{3}thermoelectric material was calculated using a first principles approach based on the Density Functional Theory (DFT). For vacancy-type defect, the Te1 vacancy (V_{Te1}) is the most stable defect with low formation energy in both Bi-rich and Te-rich conditions, which indicates that the Te1 vacancies have higher probability to be formed. For antisite defects, the formation energy of Bi_{Te1}is much lower than that of Bi_{Te2}in Bi-rich condition, while in Te-rich condition it is beneficial for Te_{Bi}with lower formation energy. Positron wave function distribution and positron lifetimes of different annihilation states in Bi_{2}Te_{3}were also calculated using the atomic superposition (ATSUP) method. The positron bulk lifetime in Bi_{2}Te_{3}is about 231 ps, and for the neutral vacancy-type defects without relaxation, the positron lifetimes of V_{Bi}, V_{Te1}and V_{Te2 }are 275 ps, 295 ps and 269 ps, respectively.
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摘要: We investigated positron states at Li-adsorbed Al(100) surfaces using \textit{ab-initio} two-component density functional theory. The results thus obtained by using semi-local electron-positron (e-p) correlation functionals with the generalized gradient approximation well reproduce the experimental results, showing that reliable e-p correlation functionals are needed to properly describe the various properties of positron surface state.

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摘要: By simulation with the relativistic PIC code EPOCH, it is found that a hollow target ismore beneficial than a plane one for the collimation and compression of the positrons. Irradiating anultra-intense laser (4×10

^{23 }W/cm^{2}) on a hollow target, 8.74×10^{15}positrons are generated. Due tothe focusing effect of the transverse electric field formed in the hollow wall, the divergence angle isaround 21° , while the maximum density of the beam is 4.77×10^{21 }cm^{-3}. When the laser intensity is doubled, both the yield (5.31 ×10^{16}) and the maximum density (1.57 ×10^{22 }cm^{-3}) are greatly increased while the beam divergence gets even smaller (15.7 °).
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