《科学》（20200807出版）一周论文导读

编译 | 李言

Science, 07 AUGUST 2020, VOL 369, ISSUE 6504

《科学》2020年8月7日，第369卷，6504期

天文学Astronomy

Global maps of the magnetic field in the solar corona

日冕磁场的全球性分布图

▲ 作者：Zihao Yang, Christian Bethge, Hui Tian, Steven Tomczyk, Richard Morton, Giulio Del Zanna, et al.

▲ 链接：

https://science.sciencemag.org/content/369/6504/694

▲ 摘要

要了解太阳大气中的许多物理过程，需要确定每个大气层的磁场。然而，直接测量太阳日冕中的磁场是很困难的。

利用日冕多通道偏振计的观测，我们确定了日冕中等离子体密度的空间分布和等离子体中盛行的横向磁铃动力波的相位速度。我们结合这些测量数据绘制了全球日冕磁场的平面成分。

日冕中导出的场强，从1.05到1.35太阳半径，大部分为1到4G。我们的结果证明了光谱在日冕磁场测量中成像的能力。

▲ Abstract

Understanding many physical processes in the solar atmosphere requires determination of the magnetic field in each atmospheric layer. However, direct measurements of the magnetic field in the Sun’s corona are difficult to obtain. Using observations with the Coronal Multi-channel Polarimeter, we have determined the spatial distribution of the plasma density in the corona and the phase speed of the prevailing transverse magnetohydrodynamic waves within the plasma. We combined these measurements to map the plane-of-sky component of the global coronal magnetic field. The derived field strengths in the corona, from 1.05 to 1.35 solar radii, are mostly 1 to 4 gauss. Our results demonstrate the capability of imaging spectroscopy in coronal magnetic field diagnostics.

环境科学Environmental Science

Unexpected air pollution with marked emission reductions during the COVID-19 outbreak in China

中国新冠疫情期间，排放量减少对空气污染的意外影响

▲ 作者：Tianhao Le, Yuan Wang, Lang Liu, Jiani Yang, Yuk L. Yung, Guohui Li, John H. Seinfeld

▲ 链接：

https://science.sciencemag.org/content/369/6504/702

▲ 摘要

新冠病毒流行期间，中国机动车停运和生产暂停，使空气污染缓解效率评估成为可能。通过卫星和地面观测，可以确定在城市封锁期间某些排放最多减少达90%。

我们的协同观察分析和模型模拟表明，异常高的湿度促进了气溶胶的非均相化学，以及停滞的气流和电厂和石化设施不间断的排放，导致严重雾霾的形成。

此外，由于冬季臭氧的非线性生成化学和滴定，氮氧化物的减少导致城市地区臭氧增强，进一步增加大气氧化能力，促进二次气溶胶的形成。

▲ Abstract

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 particulate matter levels simultaneously occurred in northern China. Our synergistic observation analyses and model simulations show that anomalously 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 atmospheric oxidizing capacity and facilitating secondary aerosol formation.

材料科学Materials Science

Chemical vapor deposition of layered two-dimensional MoSi2N4 materials

二维层状MoSi2N4 材料的化学气相沉积

▲ 作者：Yi-Lun Hong, Zhibo Liu, Lei Wang, Tianya Zhou, Wei Ma, Chuan Xu, Shun Feng, et al.

▲ 链接：

https://science.sciencemag.org/content/369/6504/670

▲ 摘要

在单层极限下识别二维层状材料已使得许多新现象和不寻常性质的发现。我们在非层状氮化钼的化学气相沉积生长过程中引入硅元素，可以钝化其表面悬键，从而制备出MoSi2N4的厘米级单层薄膜。

该单层由N-Si-N-Mo-N-Si-N的七个原子层构成，可以看作是一个MoN2层夹在两个Si-N双膜层之间。这种材料表现出半导体性质（带隙约1.94 eV），高强度（约66 GPa）和极佳的环境稳定性。

密度泛函理论计算预测了一个这样的单层二维层状材料家族，包括半导体、金属和磁性半金属。

▲ Abstract

Identifying two-dimensional layered materials in the monolayer limit has led to discoveries of numerous new phenomena 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 centimeter-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

多变量金属-有机框架中金属的排序

▲ 作者：Zhe Ji, Tong Li, Omar M. Yagh

▲ 链接：

https://science.sciencemag.org/content/369/6504/674

▲ 摘要

我们在含有钴（Co）、镉（Cd）、铅（Pb）和锰（Mn）的多元金属有机框架74中绘制了金属氧化物棒晶体中的金属序列。

这些晶体的原子探针断层扫描显示了我们所描述的金属离子的异质性空间序列，这些序列取决于所使用的金属和合成温度，例如随机（Co, Cd, 120℃）、短重复（Co, Cd, 85℃）、长重复（Co, Pb, 85℃）和插入（Co, Mn, 85℃）。

在不改变序列类型的情况下，我们检测了三种晶体，12个样品中Co的摩尔分数在0.4 ~ 0.9之间变化。与金属氧化物相比，金属有机骨架在杆状体中具有较高的耐受性，因而具有不同的金属序列。

▲ Abstract

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

电偶极子的突现螺旋结构

▲ 作者：Dmitry D. Khalyavin, Roger D. Johnson, Fabio Orlandi, Paolo G. Radaelli, Pascal Manuel, et al.

▲ 链接：

https://science.sciencemag.org/content/369/6504/680

▲ 摘要

块体材料中磁偶极子的长程排序产生了广泛的磁性结构，从简单的共线铁磁体和反铁磁体，到通过相互竞争交换作用稳定的复杂螺旋状磁结构。相比之下，介电晶体中的偶极序通常限于电偶极子的平行（铁电）和反平行（反铁电）的共线排列。

在此，我们报告通过光孔掺杂四倍钙钛矿BiMn7O12观察到的电偶极子的不相称螺旋序。与磁学相似，电偶极螺旋结构是通过竞争的不稳定性来稳定的。

具体来说，轨道的顺序和孤电子对的立体化学活性相互竞争，导致通过中间密度波从非手性立方结构到不相称的电偶极子和轨道螺旋的相变。

▲ Abstract

Long-range ordering of magnetic dipoles in bulk materials gives 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 analogy 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

头发如何使钢变形

▲ 作者：Gianluca Roscioli, Seyedeh Mohadeseh Taheri-Mousavi, Cemal Cem Tasan

▲ 链接：

https://science.sciencemag.org/content/369/6504/689

▲ 摘要

用于锋利边缘或工具的钢通常具有马氏体组织、高碳化物含量和各种涂层，以显示高硬度和耐磨性。然而，在切割头发、奶酪或土豆等较软的材料时，它们就几乎无法使用了。

尽管这是一种日常观察，但由于相互作用材料的结构复杂性和它们共同变形的复杂边界条件，其中的物理微观机制尚不清楚。

为了解释这个复杂性，我们进行了中断测试和设置了两个微观机械测试的原位电子显微镜切割实验。

我们调查结果分析和数值模拟，揭示板条马氏体的空间变异结构在明显磨损之前产生的混合模式II-III开裂现象中起到关键作用。

▲ Abstract

Steels for sharp edges or tools typically have martensitic microstructures, high carbide contents, and various coatings to exhibit high hardness and wear resistance. Yet they become practically unusable upon cutting much softer materials such as human hair, cheese, or potatoes. Despite this being an everyday observation, the underlying physical micromechanisms are poorly understood 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 appreciable wear.

物理学Physics

A new wrinkle on liquid sheets: Turning the mechanism of viscous bubble collapse upside down

液体薄片上一种新的起皱现象：颠覆粘性气泡坍塌机制

▲ 作者：Alexandros T. Oratis, John W. M. Bush, Howard A. Stone, James C. Bird

▲ 链接：

https://science.sciencemag.org/content/369/6504/685

▲ 摘要

粘性气泡在自然和工业环境中都很普遍。它们的破裂和塌陷可能伴有典型的与弹性板相关的特征，包括放射状褶皱的发展。此前，研究人员认为，薄膜的重量是导致薄膜塌陷和起皱不稳定的原因。

相反，我们在这里的实验表明，重力的作用微不足道：同样的坍缩和褶皱的产生与气泡的方向无关。

我们发现，表面张力驱动气泡崩溃和引发动态屈曲不稳定。由于薄膜的重量无关紧要。我们的研究结果表明，褶皱可能同样伴随着相对较小的弯曲粘性和粘弹性薄膜的破裂，包括那些在呼吸系统中负责由呼气事件产生气溶胶的薄膜。

▲ 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 respiratory tract responsible for aerosol production from exhalation events.