Explore the vast range of titles available.

The surprising discrepancy between results from different methods for measuring the proton charge radius is referred to as the proton radius puzzle. In particular, measurements using electrons seem to lead to a different radius compared with those using muons. Here, a direct measurement of the n = 2 Lamb shift of atomic hydrogen is presented. Our measurement determines the proton radius to be r p = 0.833 femtometers, with an uncertainty of ±0.010 femtometers. This electron-based measurement of r p agrees with that obtained from the analogous muon-based Lamb shift measurement but is not consistent with the larger radius that was obtained from the averaging of previous electron-based measurements.

As-cast irons and aluminum alloys are used in various industrial fields and their phase and microstructure properties are strongly affected by the undercooling degree. However, existing studies regarding the undercooling degree are mostly limited to qualitative analyses. In this paper, a quantitative analysis of the undercooling degree is performed by collecting experimental data and employing machine learning. Nine machining learning models including Random Forest (RF), eXtreme Gradient Boosting (XGBOOST), Ridge Regression (RIDGE) and Gradient Boosting Regressor (GBDT) methods are used to predict the undercooling degree via six features, which include the cooling rate (CR), mean atomic covalence radius (MAR) and mismatch (MM). Four additional effective models of machine learning algorithms are then selected for a further analysis and cross-validation. Finally, the optimal machine learning model is selected for the dataset and the best combination of features is found by comparing the prediction accuracy of all possible feature combinations. It is found that RF model with CR and MAR features has the optimal performance results for predicting the undercooling degree.

The structural stability of M/Ag(111)–3×3R30°  surface alloys is systematically investigated by using first-principles calculations, where M is a member of group III (B, Al, Ga, In, Tl), IV (C, Si, Ge, Sn, Pb), and V (N, P, As, Sb, Bi) elements. We focus on the corrugation parameter d which is determined by the height of the M atom from the Ag atom in the plane of the top-most atom, and the relation between atomic radii and corrugations in M/Ag(111) is obtained. The tendencies of the corrugation parameter d can be understood by using a simple hard spherical atomic model. We introduce a new type of atomic radii determined by the corrugation in surface alloys, surface alloy atomic radii, which can be useful for rapid predictions of the structures of surface alloys, not only for M/Ag (111)–3×3R30° systems but also for other surface alloys.

Like physical quantities such as nuclear mass and electric quadrupole moment, the nuclear charge radius is one of the important physical quantities that constitute the essential properties of the atomic nucleus. This paper studies the evolution process of the nuclear charge radius. Based on the local interaction of the atomic nucleus, the original semi-empirical formula for calculating the nuclear charge radius has been improved. The accuracy and reliability of the formula are verified by combining experimental data, with the expectation of providing a reference for the accurate measurement of the nuclear charge radius of nuclides in the future.

Mechanical interlocking of molecules (catenation) is a nontrivial challenge in modern synthetic chemistry and materials science 1 , 2 . One strategy to achieve catenation is the design of pre-annular molecules that are capable of both efficient cyclization and of pre-organizing another precursor to engage in subsequent interlocking 3 – 9 . This task is particularly difficult when the annular target is composed of a large ensemble of molecules, that is, when it is a supramolecular assembly. However, the construction of such unprecedented assemblies would enable the visualization of nontrivial nanotopologies through microscopy techniques, which would not only satisfy academic curiosity but also pave the way to the development of materials with nanotopology-derived properties. Here we report the synthesis of such a nanotopology using fibrous supramolecular assemblies with intrinsic curvature. Using a solvent-mixing strategy, we kinetically organized a molecule that can elongate into toroids with a radius of about 13 nanometres. Atomic force microscopy on the resulting nanoscale toroids revealed a high percentage of catenation, which is sufficient to yield ‘nanolympiadane’ 10 , a nanoscale catenane composed of five interlocked toroids. Spectroscopic and theoretical studies suggested that this unusually high degree of catenation stems from the secondary nucleation of the precursor molecules around the toroids. By modifying the self-assembly protocol to promote ring closure and secondary nucleation, a maximum catenation number of 22 was confirmed by atomic force microscopy. Nanoscale toroids with a high percentage of poly-catenation and radii of up to about 13 nm are kinetically organized using fibrous supramolecular assemblies with intrinsic curvature and a solvent-mixing strategy.

High-entropy alloy nanoparticles (HEA-NPs) show great potential as functional materials 1 – 3 . However, thus far, the realized high-entropy alloys have been restricted to palettes of similar elements, which greatly hinders the material design, property optimization and mechanistic exploration for different applications 4 , 5 . Herein, we discovered that liquid metal endowing negative mixing enthalpy with other elements could provide a stable thermodynamic condition and act as a desirable dynamic mixing reservoir, thus realizing the synthesis of HEA-NPs with a diverse range of metal elements in mild reaction conditions. The involved elements have a wide range of atomic radii (1.24–1.97 Å) and melting points (303–3,683 K). We also realized the precisely fabricated structures of nanoparticles via mixing enthalpy tuning. Moreover, the real-time conversion process (that is, from liquid metal to crystalline HEA-NPs) is captured in situ, which confirmed a dynamic fission–fusion behaviour during the alloying process. We discovered that liquid metal endowing negative mixing enthalpy with other elements could provide a stable thermodynamic condition and act as a desirable dynamic mixing reservoir, realizing the synthesis of high-entropy alloy nanoparticles.

Elastic electron–proton scattering (e–p) and the spectroscopy of hydrogen atoms are the two methods traditionally used to determine the proton charge radius, r p . In 2010, a new method using muonic hydrogen atoms 1 found a substantial discrepancy compared with previous results 2 , which became known as the ‘proton radius puzzle’. Despite experimental and theoretical efforts, the puzzle remains unresolved. In fact, there is a discrepancy between the two most recent spectroscopic measurements conducted on ordinary hydrogen 3 , 4 . Here we report on the proton charge radius experiment at Jefferson Laboratory (PRad), a high-precision e–p experiment that was established after the discrepancy was identified. We used a magnetic-spectrometer-free method along with a windowless hydrogen gas target, which overcame several limitations of previous e–p experiments and enabled measurements at very small forward-scattering angles. Our result, r p  = 0.831 ± 0.007 stat  ± 0.012 syst  femtometres, is smaller than the most recent high-precision e–p measurement 5 and 2.7 standard deviations smaller than the average of all e–p experimental results 6 . The smaller r p we have now measured supports the value found by two previous muonic hydrogen experiments 1 , 7 . In addition, our finding agrees with the revised value (announced in 2019) for the Rydberg constant 8 —one of the most accurately evaluated fundamental constants in physics. A magnetic-spectrometer-free method for electron–proton scattering data reveals a proton charge radius 2.7 standard deviations smaller than the currently accepted value from electron–proton scattering, yet consistent with other recent experiments.

The energy levels of hydrogen-like atomic systems can be calculated with great precision. Starting from their quantum mechanical solution, they have been refined over the years to include the electron spin, the relativistic and quantum field effects, and tiny energy shifts related to the complex structure of the nucleus. These energy shifts caused by the nuclear structure are vastly magnified in hydrogen-like systems formed by a negative muon and a nucleus, so spectroscopy of these muonic ions can be used to investigate the nuclear structure with high precision. Here we present the measurement of two 2S–2P transitions in the muonic helium-4 ion that yields a precise determination of the root-mean-square charge radius of the α particle of 1.67824(83) femtometres. This determination from atomic spectroscopy is in excellent agreement with the value from electron scattering 1 , but a factor of 4.8 more precise, providing a benchmark for few-nucleon theories, lattice quantum chromodynamics and electron scattering. This agreement also constrains several beyond-standard-model theories proposed to explain the proton-radius puzzle 2 – 5 , in line with recent determinations of the proton charge radius 6 – 9 , and establishes spectroscopy of light muonic atoms and ions as a precise tool for studies of nuclear properties. The 2S–2P transitions in muonic helium-4 ions are measured using laser spectroscopy and used to obtain an α-particle charge-radius value five times more precise than that from electron scattering.

Realization of stable and industrial-level H 2 O 2 electroproduction still faces great challenge due large partly to the easy decomposition of H 2 O 2 . Herein, a two-dimensional dithiine-linked phthalocyaninato cobalt (CoPc)-based covalent organic framework (COF), CoPc-S-COF, was afforded from the reaction of hexadecafluorophthalocyaninato cobalt (II) with 1,2,4,5-benzenetetrathiol. Introduction of the sulfur atoms with large atomic radius and two lone-pairs of electrons in the C-S-C linking unit leads to an undulated layered structure and an increased electron density of the Co center for CoPc-S-COF according to a series of experiments in combination with theoretical calculations. The former structural effect allows the exposition of more Co sites to enhance the COF catalytic performance, while the latter electronic effect activates the 2e − oxygen reduction reaction (2e − ORR) but deactivates the H 2 O 2 decomposition capability of the same Co center, as a total result enabling CoPc-S-COF to display good electrocatalytic H 2 O 2 production performance with a remarkable H 2 O 2 selectivity of >95% and a stable H 2 O 2 production with a concentration of 0.48 wt% under a high current density of 125 mA cm −2 at an applied potential of ca . 0.67 V versus RHE for 20 h in a flow cell, representing the thus far reported best H 2 O 2 synthesis COFs electrocatalysts. Realization of stable and industrial-level hydrogen peroxide electroproduction still faces great challenge due large partly to the easy decomposition of this product. Here the authors report a strategy to achieve superior performance by promoting an increased electron density of Co center due to the introduction of sulfur atoms in the linking units of 2D CoPc-S-COF

In this paper, correlation of thermal stability ( ΔTx ) with the bond parameters such as electronegativity ( Δx ), atomic radius mismatch ( δ ), and valence electron concentration ( Δn1/3 ) for Mg-based multicomponent bulk metallic glasses (BMGs) have been evaluated. A statistical approach of regression analysis has been adopted to investigate correlations among these parameters. Available experimental data have been used for the systematic investigation from ternary to multicomponent Mg-based BMGs. In addition, the applicability of the criteria has been assessed for the systems with and without rare earth (RE) elements. We have found that BMG systems containing RE group elements have significant effect on width of supercooled liquid region. Results obtained from our modified empirical equation have been compared with that of earlier models and have shown better correlations.