Defect and Diffusion Forum Vol. 373

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https://doi.org/10.4028/www.scientific.net/DDF.373

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Preface, Committees, Sponsors

The Science Enjoyed by Professor Alec T. Stewart

Authors: Toshio Hyodo

Abstract: 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.

Positron Manipulation and Positronium Laser Excitation in AEgIS

Authors: Sebastiano Mariazzi, Ruggero Caravita, Stefano Aghion, Claude Amsler, Akitaka Ariga, Tomoko Ariga, Germano Bonomi, Philippe Bräunig, Johan Bremer, Roberto S. Brusa, Luis Cabaret, Massimo Caccia, Fabrizio Castelli, G. Cerchiari, Karl Chlouba, Simone Cialdi, Daniel Comparat, Giovanni Consolati, Andrea Demetrio, Lea di Noto, Michael Doser, Alexey Dudarev, Antonio Ereditato, Craig Evans, Rafael Ferragut, Julian Fesel, Andrea Fontana, Ola K. Forslund, Sebastian Gerber, Marco Giammarchi, Angela Gligorova, Sergei Gninenko, Francesco Guatieri, Stefan Haider, Helga Holmestad, Torkjell Huse, Ine L. Jernelv, Elena Jordan, Thomas Kaltenbacher, Alban Kellerbauer, Mitsuhiro Kimura, Torsten Koetting, Daniel Krasnicky, Vittorio Lagomarsino, Pierre Lansonneur, Patrice Lebrun, Sebastian Lehner, Joanna Liberadzka, Chloe Malbrunot, Lisa Marx, Viktor Matveev, Zeudi Mazzotta, Giancarlo Nebbia, Patrick Nedelec, Markus Oberthaler, Nicola Pacifico, Davide Pagano, Luca Penasa, Vojtech Petracek, Ciro Pistillo, Francesco Prelz, Marco Prevedelli, Luca Ravelli, Laura Resch, Benjamin Rienäcker, Ole M. Røhne, Stefan Rosenberger, Alberto Rotondi, Michele Sacerdoti, Heidi Sandaker, Romualdo Santoro, Paola Scampoli, Fiodor Sorrentino, Michal Spacek, James Storey, Izabela M. Strojek, Gemma Testera, Ingmari Tietje, Stefan Vamosi, Eberhard Widmann, Pauline Yzombard, Sandra Zavatarelli, Johann Zmeskal

Abstract: 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 10⁸ particles and the laser excitation of positronium to Rydberg states, using n=3 as intermediate level, are presented.

Enegretics of the Ps Formation: Role of the Solvated Electron

Authors: Sergey V. Stepanov, Vsevolod Byakov, Dmitry Zvezhinskiy, Gilles Duplâtre

Abstract: 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, V₀⁺, 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⁺_aq 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.

Low-Energy Ps-Xe Scattering Analyzed Using a Modified Effective Range Theory

Authors: Kengo Shibuya, Haruo Saito

Abstract: 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.

Positron Lifetime Calculation for Plastic Deformed Nanocrystalline Copper

Authors: Kai Zhou, Ting Zhang

Abstract: 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.

Parameter-Free Gradient Correction for Positron States in Oxides

Authors: Jan Kuriplach, Bernardo Barbiellini

Abstract: 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.

First-Principles Calculation of Defect Formation and Positron Annihilation States in Bi₂Te₃

Authors: Bin Zhao, Bo Zhou, Chong Yang Li, Ning Qi, Zhi Quan Chen

Abstract: Defect formation energy in Bi₂Te₃ 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 Bi₂Te₃ were also calculated using the atomic superposition (ATSUP) method. The positron bulk lifetime in Bi₂Te₃ is about 231 ps, and for the neutral vacancy-type defects without relaxation, the positron lifetimes of V_Bi, V_Te1 and V_Te2are 275 ps, 295 ps and 269 ps, respectively.

First-Principles Study on Positron States at Li-Adsorbed Al(100) Surfaces: Effect of Electron-Positron Correlation

Authors: Satoshi Hagiwara, Yasumitsu Suzuki, Kazuyuki Watanabe

Abstract: 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.

Simulation Study of Positron Beam Generation via Irradiating an Ultra-Intense Laser on a Hollow Target

Authors: Jian Xun Liu, Yan Yun Ma, Jun Zhao, Tong Pu Yu, Xiao Hu Yang, Wei Quan Wang, Jing Jiang

Abstract: 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 (4×10²³W/cm²) on a hollow target, 8.74×10¹⁵ 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 4.77×10²¹cm^-3. When the laser intensity is doubled, both the yield (5.31 ×10¹⁶) and the maximum density (1.57 ×10²²cm^-3) are greatly increased while the beam divergence gets even smaller (15.7 °).