Compilation | Li Yan
Science, 07 AUGUST 2020, VOL 369, ISSUE 6504
Science《Science》August 7, 2020, Volume 369, Issue 6504
Astronomy
Global maps of the magnetic field in the solar corona
▲ Author: Zihao Yang, Christian Bethge, Hui Tian, Steven Tomczyk, Richard Morton, Giulio Del Zanna, et al.
▲ Link:
https://science.sciencemag.org/content/369/6504/694
▲ Abstract
To understand many physical processes in the solar atmosphere, it is necessary to determine the magnetic field of each atmosphere. However, it is difficult to directly measure the magnetic field in the solar corona.
Using the observation of the coronal multi-channel polarizer, we determined the spatial distribution of the plasma density in the coronal and the phase velocity of the lateral magnetic ring dynamic wave prevailing in the plasma. We combined these measurement data to plot the plane composition of the global coronal magnetic field. The field strength derived from the corona of
is from 1.05 to 1.35 solar radius, most of which are 1 to 4G. Our results demonstrate the ability of spectroscopy to image in coronal magnetic field measurements.
▲ Abstract
Understanding many physical processes in the solar atmosphere requires determination of the magnetic field in each atmosphere layer. However, direct measurements of the magnetic field in the Sun’s coronavirus are difficult to obtain. Using observations with the Coronal Multi-channel Polarimeter, we have determined the spatial distribution of the plasma density in the coronavirus and the phase speed of the prevailing transverse magneticohydrodynamic waves within the plasma. We combined these measures to map the plane-of-sky component of the global cornal magnetic field. The derived field strengths in the coronavirus, from 1.05 to 1.35 solar radii, are mostly 1 to 4 gauss. Our results demonstrate the capability of imaging spectroscopy in cornal magnetic field diagnostics.
Environmental Science
Unexpected air pollution with marked emission reductions during the COVID-19 outbreak in China
According to Chinatown, the reduction of emissions in China during the COVID-19 outbreak in China2
▲ Author: Tianhao Le, Yuan Wang, Lang Liu, Jiani Yang, Yuk L. Yung, Guohui Li, John H. Seinfeld
▲ Link:
https://science.sciencemag.org/content/369/6504/702
▲ Abstract
During the epidemic of the new coronavirus, China's motor vehicles were suspended and production was suspended, making it possible to evaluate the efficiency of air pollution mitigation. Through satellite and ground observations, it can be determined that some emissions during urban lockdowns will be reduced by up to 90%.
Our collaborative observational analysis and model simulations show that abnormally high humidity promotes heterogeneous chemistry of aerosol , as well as stagnant airflow and uninterrupted emissions from power plants and petrochemical facilities, leading to the formation of severe haze.
In addition, due to the nonlinear generation chemistry and titration of ozone in winter, the reduction of nitrogen oxides leads to the enhancement of ozone in urban areas, further increasing atmospheric oxidation capacity and promoting the formation of secondary aerosols.
▲ Abstracttml2
The absence of motor vehicle traffic and suspended manufacturing during the coronavirus disease 2019 (COVID-19) pandemic in China enabled assessment of the efficiency of air pollution mitigation. Up to 90% reduction of certain emissions during the city-lockdown period can be identified from satellite and ground-based observations. Unexpectedly, extreme particular matter levels Simultaneously occurred in northern China. Our synergistic observation analyses and model simulations show that anomaly high humidity promoted aerosol heterogeneous chemistry, along with stagnant airflow and uninterrupted emissions from power plants and petrochemical facilities, contributing to severe haze formation. Also, because of nonlinear production chemistry and titration of ozone in winter, reduced nitrogen oxides resulted in ozone enhancement in urban areas, further increasing the atmosphere oxidizing capacity and facilitating secondary aerosol formation.
Materials Science
Chemical vapor deposition of layered two-dimensional MoSi2N4 materials
2D layered MoSi2N4 material chemical vapor deposition of
▲ Author: Yi-Lun Hong, Zhibo Liu, Lei Wang, Tianya Zhou, Wei Ma, Chuan Xu, Shun Feng, et al.
▲ Link:
https://science.sciencemag.org/content/369/6504/670
▲ Abstract
Identification of two-dimensional layered materials under the limit of a single layer has led to the discovery of many new phenomena and unusual properties. We introduced silicon elements during the chemical vapor deposition growth of non-layered molybdenum nitride, which can passivate its surface hanging bonds, thereby preparing a centimeter-scale single-layer film of MoSi2N4.
This single layer consists of seven atomic layers of N-Si-N-Mo-N-Si-N, which can be regarded as a MoN2 layer sandwiched between two Si-N double-membrane layers. This material exhibits semiconductor properties (bandgap about 1.94 eV), high strength (about 66 GPa) and excellent environmental stability.
density functional theory calculation predicts such a family of single-layer two-dimensional layered materials, including semiconductors, metals and magnetic semi-metals.
▲ Abstracttml2
Identifying two-dimensional layered materials in the monolayer limit has led to discoveries of numerous new phenomenona and unusual properties. We introduced elemental silicon during chemical vapor deposition growth of nonlayered molybdenum nitride to passivate its surface, which enabled the growth of center-scale monolayer films of MoSi2N4. This monolayer was built up by septuple atomic layers of N-Si-N-Mo-N-Si-N, which can be viewed as a MoN2 layer sandwiched between two Si-N bilayers. This material exhibited semiconducting behavior (bandgap ~1.94 electron volts), high strength (~66 gigapascals), and excellent ambient stability. Density functional theory calculations predict a large family of such monolayer structured two-dimensional layered materials, including semiconductors, metals, and magnetic half-metals.
Sequencing of metals in multivariate metal-organic frameworks
Multivariate metal-sorting of metals in organic framework
▲ Author: Zhe Ji, Tong Li, Omar M. Yagh
▲ Link:
https://science.sciencemag.org/content/369/6504/674
▲ Abstract
We plot the metal sequences in metal oxide rod crystal in a multivariate metal organic framework 74 containing cobalt (Co), cadmium (Cd), lead (Pb) and manganese (Mn).
Atomic probe tomography of these crystals shows the heterogeneous spatial sequences of metal ions we describe, depending on the metal and synthesis temperature used, such as random (Co, Cd, 120°C), short repeats (Co, Cd, 85°C), long repeats (Co, Pb, 85°C), and insertions (Co, Mn, 85°C).
Without changing the sequence type, we detected three crystals, and the mole fraction of Co in 12 samples varies between 0.4 and 0.9.Compared with metal oxides, metal organic frameworks have higher tolerance in rod-shaped bodies and thus have different metal sequences.
▲ Abstract tml2
We mapped the metal sequences within crystals of metal-oxide rods in multivariate metal-organic framework–74 containing mixed combinations of cobalt (Co), cadmium (Cd), lead (Pb), and manganese (Mn). Atom probe tomography of these crystals revealed the presence of heterogeneous spatial sequences of metal ions that we describe, depending on the metal and synthesis temperature used, as random (Co, Cd, 120°C), short duplicates (Co, Cd, 85°C), long duplicates (Co, Pb, 85°C), and insertions (Co, Mn, 85°C). Three crystals were examined for each sequence type, and the molar fraction of Co among all 12 samples was observed to vary from 0.4 to 0.9, without changing the sequence type. Compared with metal oxides, metal-organic frameworks have high tolerance for coexistence of different metal sizes in their rods and therefore assume various metal sequences.
Emergent helical texture of electric dipoles
High-profile helical structure of electric dipoles
▲ Author: Dmitry D. Khalyavin, Roger D. Johnson, Fabio Orlandi, Paolo G. Radaelli, Pascal Manuel, et al.
▲ Link:
https://science.sciencemag.org/content/369/6504/680
▲ Abstract The long-range sorting of magnetic dipoles in bulk materials has produced a wide range of magnetic structures, from simple collinear ferromagnets and antiferromagnets to complex spiral magnetic structures that are stable through competition and exchange. In contrast, the dipole sequence in a dielectric crystal is generally limited to the collinear arrangement of parallel (ferroelectric) and antiparallel (antiferoelectric) of the electric dipoles.
Here, we report the asymmetric helical sequence of electrical dipoles observed by the light-pole doping of quadruple perovskite BiMn7O12. Similar to magnetism, the electric dipole helical structure is stabilized by competitive instability.
Specifically, the order of orbits and the stereochemical activity of the solitary electron pair compete with each other, resulting in a phase transition from an achiral cubic structure to an inconsistent electrical dipole and orbital helix through intermediate density waves.
▲ Abstracttml2
Long-range ordering of magnetic dipoles in bulk materials give rise to a broad range of magnetic structures, from simple collinear ferromagnets and antiferromagnets, to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, dipolar order in dielectric crystals is typically limited to parallel (ferroelectric) and antiparallel (antiferroelectric) collinear alignments of electric dipoles. Here, we report an observation of incommensurate helical ordering of electric dipoles by light hole doping of the quadruple perovskite BiMn7O12. In analysis with magnetism, the electric dipole helicoidal texture is stabilized by competing instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a nonchiral cubic structure to an incommensurate electric dipole and orbital helix via an intermediate density wave.
How hair deforms steel
How hair deforms steel
▲ Author: Gianluca Roscioli, Seyedeh Mohadeseh Taheri-Mousavi, Cemal Cem Tasan
▲ Link:
https://science.sciencemag.org/content/369/6504/689
▲ Abstract
Steels used in sharp edges or tools usually have martensite structure, high carbide content and various coatings to show high hardness and wear resistance. However, they are almost out of use when cutting softer materials such as hair, cheese or potatoes.
Although this is a daily observation, the physical microscopic mechanisms in it are not clear due to the structural complexity of the interacting materials and the complex boundary conditions in which they co-deform.
To explain this complexity, we conducted an in-situ electron microscope cutting experiment with interrupt tests and set up two micromechanical tests.
Our findings analysis and numerical simulation revealed that the spatial variant structure of slat martensite plays a key role in the hybrid mode II-III cracking phenomenon generated before significant wear.
▲ Abstracttml2
Steels for sharp edges or tools typically have martensitic microstructures, high carbide contents, and various coats to exhibit high hardness and wear resistance. Yet they become practically unusable upon cutting much soft materials such as human hair, cheese, or potatoes. Although this being an everyday observation, the underlying physical micromechanisms are poorly Understoo because of the structural complexity of the interacting materials and the complex boundary conditions of their co-deformation. To unravel this complexity, we carried out interrupted tests and in situ electron microscopy cutting experiments with two micromechanical testing setups. We investigated the findings analytically and numerically, revealing that the spatial variation of lath martensite structure plays the key role leading to a mixed-mode II-III cracking phenomenon before appreciate wear.
Physics
A new wrinkle on liquid sheets: Turning the mechanism of viscous bubble collapse upside down
1A new wrinkle on liquid sheets: turning the mechanism of viscous bubble collapse upside down
▲Author: Alexandros T. Oratis, John W. M. Bush, Howard A. Stone, James C. Bird
▲ Link:
https://science.sciencemag.org/content/369/6504/685
▲ Abstract
Sticky bubbles are common in both natural and industrial environments. Their rupture and collapse may be accompanied by typical features associated with elastic plates, including the development of radial folds. Previously, researchers believed that the weight of the film was the cause of film collapse and wrinkle instability.
In contrast, our experiments here show that the effect of gravity is trivial: the same collapse and wrinkle generation have nothing to do with the direction of the bubbles.
We found that surface tension drives bubble collapse and triggers dynamic buckling instability. Because the weight of the film is irrelevant. Our results suggest that folds may also be accompanied by relatively small bending viscous and viscoelastic film rupture, including those in the respiratory system that are responsible for the generation of aerosols by the expiration event.
▲ Abstract
Viscous bubbles are prevalent in both natural and industrial settings. Their rupture and collapse may be accompanied by features typically associated with elastic sheets, including the development of radial wrinkles. Previous investigators concluded that the film weight is responsible for both the film collapse and wrinkling instability. Conversely, we show here experimentally that gravity plays a negligible role: The same collapse and wrinkling arise independently of the bubble’s orientation. We found that surface tension drives the collapse and initiates a dynamic buckling instability. Because the film weight is irrelevant, our results suggest that wrinkling may likewise accompany the breakup of relatively small-scale, curved viscous and viscoelastic films, including those in the reconstruction tract responsible for aerosol production from exhalation events.
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