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1. High-precision Transition Energy Measurements of Neon-like Fe xvii Ions

   https://doi.org/10.3847/1538-4357/ad454b  

   Shah, Chintan; Togawa, Moto; Botz, Marc; Danisch, Jonas; Goes, Joschka_J; Bernitt, Sonja; Maxton, Marleen; Köbnick, Kai; Buck, Jens; Seltmann, Jörn; et al (June 2024, The Astrophysical Journal)

   Abstract We improve by a factor of 4–20 the energy accuracy of the strongest soft X-ray transitions of Fexviiions by resonantly exciting them in an electron beam ion trap with a monochromatic beam at the P04 beamline of the PETRA III synchrotron facility. By simultaneously tracking instantaneous photon-energy fluctuations with a high-resolution photoelectron spectrometer, we minimize systematic uncertainties down to 10–15 meV, or velocity equivalent ±∼5 km s⁻¹in their rest energies, substantially improving our knowledge of this key astrophysical ion. Our large-scale configuration-interaction computations include more than 4 million relativistic configurations and agree with the experiment at a level without precedent for a 10-electron system. Thereby, theoretical uncertainties for interelectronic correlations become far smaller than those of quantum electrodynamics (QED) corrections. The present QED benchmark strengthens our trust in future calculations of many other complex atomic ions of interest to astrophysics, plasma physics, and the development of optical clocks with highly charged ions. 

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2. Sympathetic cooling of positrons to cryogenic temperatures for antihydrogen production

   https://doi.org/10.1038/s41467-021-26086-1  

   Baker, C. J.; Bertsche, W.; Capra, A.; Cesar, C. L.; Charlton, M.; Mathad, A. Cridland; Eriksson, S.; Evans, A.; Evetts, N.; Fabbri, S.; et al (December 2021, Nature Communications)

   Abstract The positron, the antiparticle of the electron, predicted by Dirac in 1931 and discovered by Anderson in 1933, plays a key role in many scientific and everyday endeavours. Notably, the positron is a constituent of antihydrogen, the only long-lived neutral antimatter bound state that can currently be synthesized at low energy, presenting a prominent system for testing fundamental symmetries with high precision. Here, we report on the use of laser cooled Be + ions to sympathetically cool a large and dense plasma of positrons to directly measured temperatures below 7 K in a Penning trap for antihydrogen synthesis. This will likely herald a significant increase in the amount of antihydrogen available for experimentation, thus facilitating further improvements in studies of fundamental symmetries. 

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3. Microphysics of Particle Reflection in Weibel-mediated Shocks

   https://doi.org/10.3847/1538-4357/ad527d  

   Parsons, Jasmine; Spitkovsky, Anatoly; Vanthieghem, Arno (August 2024, The Astrophysical Journal)

   Abstract Particle-in-cell (PIC) simulations have shown that relativistic collisionless shocks mediated by the Weibel instability accelerate ∼1% of incoming particles, while the majority are transmitted through the shock and become thermalized. The microphysical processes that determine whether an incoming particle will be transmitted or reflected are poorly understood. We study the microphysics of particle reflection in Weibel-mediated shocks by tracking a shell of test particles in a PIC simulation of a shock in pair plasma. We find that electrons in positron-dominated filaments and positrons in electron-dominated filaments efficiently reflect off of strong magnetic structures at the shock. To participate in diffusive shock acceleration, however, these reflected particles headed toward the upstream must avoid getting advected downstream. This is enabled by incoming filaments, which trap reflected particles carrying the same sign of current as the filaments. The final injection efficiency on the order of ∼1% thus results from the effectiveness of the initial reflection at the shock and the reflected particles’ probability of survival in the upstream postreflection. We develop a model that predicts the fraction of high-energy particles as a function of the properties of Weibel filamentation. 

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4. Imaging Beam‐Sensitive Materials by Electron Microscopy

   https://doi.org/10.1002/adma.201907619  

   Chen, Qiaoli; Dwyer, Christian; Sheng, Guan; Zhu, Chongzhi; Li, Xiaonian; Zheng, Changlin; Zhu, Yihan (February 2020, Advanced Materials)

   Abstract Electron microscopy allows the extraction of multidimensional spatiotemporally correlated structural information of diverse materials down to atomic resolution, which is essential for figuring out their structure–property relationships. Unfortunately, the high‐energy electrons that carry this important information can cause damage by modulating the structures of the materials. This has become a significant problem concerning the recent boost in materials science applications of a wide range of beam‐sensitive materials, including metal–organic frameworks, covalent–organic frameworks, organic–inorganic hybrid materials, 2D materials, and zeolites. To this end, developing electron microscopy techniques that minimize the electron beam damage for the extraction of intrinsic structural information turns out to be a compelling but challenging need. This article provides a comprehensive review on the revolutionary strategies toward the electron microscopic imaging of beam‐sensitive materials and associated materials science discoveries, based on the principles of electron–matter interaction and mechanisms of electron beam damage. Finally, perspectives and future trends in this field are put forward. 

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5. Positron pair interactions in a nearly free-electron metal

   https://doi.org/10.1140/epjd/s10053-022-00406-6  

   Mills, Jr., Allen Paine; Fuentes-Garcia, Melina (June 2022, The European Physical Journal D)

   AbstractThe electric field surrounding a single positron in a metal is screened by an increase in the local electron density which, in the case of nearly free-electron metals (like Al, Na, etc.), has a radial distribution similar to that of the electron in positronium (Ps). In such metals, a singlet pair of positrons would experience an attractive interaction and at low enough electron densities could possibly form a bound state that is held together by exchange and correlation energies, thus forming structures analogous to that of the positronium molecule (Ps$$_2$$ ${}_2$), with binding energies of a few tenths of an eV. Such di-positrons could be prevalent at positron densities of around 10$$^{18}$$ ${}^{18}$cm$$^{-3}$$ ${}^{-3}$and, if so, would be evident from an apparent broadening of the sharp step at the Fermi surface in measurements of the electron momentum distribution by the angular correlation of the 2$$\gamma $$ $\gamma$annihilation radiation. Even if di-positrons are not directly formed in a metal, optical spectroscopy of Ps$$_2$$ ${}_2$formed in vacuum via pairs of positrons simultaneously being emitted from the surface could be applied to the direct measurement of the momentum distribution of Cooper pairs. If they exist, di-positrons in metals would yield interesting information about electron and positron interactions and at very high densities might allow the study of a di-positron Bose–Einstein condensate immersed in an electron gas. Graphic Abstract 

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