《科學》（20200807出版）一周論文導讀

編譯 | 李言

Science, 07 AUGUST 2020, VOL 369, ISSUE 6504

《科學》2020年8月7日，第369卷，6504期

天文學Astronomy

Global maps of the magnetic field in the solar corona

日冕磁場的全球性分布圖

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

▲ 鏈接：

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

▲ 摘要

要了解太陽大氣中的許多物理過程，需要確定每個大氣層的磁場。然而，直接測量太陽日冕中的磁場是很困難的。

利用日冕多通道偏振計的觀測，我們確定了日冕中等離子體密度的空間分布和等離子體中盛行的橫向磁鈴動力波的相位速度。我們結合這些測量數據繪製了全球日冕磁場的平面成分。

日冕中導出的場強，從1.05到1.35太陽半徑，大部分為1到4G。我們的結果證明了光譜在日冕磁場測量中成像的能力。

▲ 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.

環境科學Environmental Science

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

中國新冠疫情期間，排放量減少對空氣污染的意外影響

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

▲ 鏈接：

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

▲ 摘要

新冠病毒流行期間，中國機動車停運和生產暫停，使空氣污染緩解效率評估成為可能。通過衛星和地面觀測，可以確定在城市封鎖期間某些排放最多減少達90%。

我們的協同觀察分析和模型模擬表明，異常高的濕度促進了氣溶膠的非均相化學，以及停滯的氣流和電廠和石化設施不間斷的排放，導致嚴重霧霾的形成。

此外，由於冬季臭氧的非線性生成化學和滴定，氮氧化物的減少導致城市地區臭氧增強，進一步增加大氣氧化能力，促進二次氣溶膠的形成。

▲ 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.

材料科學Materials Science

Chemical vapor deposition of layered two-dimensional MoSi2N4 materials

二維層狀MoSi2N4 材料的化學氣相沉積

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

▲ 鏈接：

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

▲ 摘要

在單層極限下識別二維層狀材料已使得許多新現象和不尋常性質的發現。我們在非層狀氮化鉬的化學氣相沉積生長過程中引入硅元素，可以鈍化其表面懸鍵，從而製備出MoSi2N4的厘米級單層薄膜。

該單層由N-Si-N-Mo-N-Si-N的七個原子層構成，可以看作是一個MoN2層夾在兩個Si-N雙膜層之間。這種材料表現出半導體性質（帶隙約1.94 eV），高強度（約66 GPa）和極佳的環境穩定性。

密度泛函理論計算預測了一個這樣的單層二維層狀材料家族，包括半導體、金屬和磁性半金屬。

▲ 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

多變數金屬-有機框架中金屬的排序

▲ 作者：Zhe Ji, Tong Li, Omar M. Yagh

▲ 鏈接：

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

▲ 摘要

我們在含有鈷（Co）、鎘（Cd）、鉛（Pb）和錳（Mn）的多元金屬有機框架74中繪製了金屬氧化物棒晶體中的金屬序列。

這些晶體的原子探針斷層掃描顯示了我們所描述的金屬離子的異質性空間序列，這些序列取決於所使用的金屬和合成溫度，例如隨機（Co, Cd, 120℃）、短重複（Co, Cd, 85℃）、長重複（Co, Pb, 85℃）和插入（Co, Mn, 85℃）。

在不改變序列類型的情況下，我們檢測了三種晶體，12個樣品中Co的摩爾分數在0.4 ~ 0.9之間變化。與金屬氧化物相比，金屬有機骨架在桿狀體中具有較高的耐受性，因而具有不同的金屬序列。

▲ 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

電偶極子的突現螺旋結構

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

▲ 鏈接：

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

▲ 摘要

塊體材料中磁偶極子的長程排序產生了廣泛的磁性結構，從簡單的共線鐵磁體和反鐵磁體，到通過相互競爭交換作用穩定的複雜螺旋狀磁結構。相比之下，介電晶體中的偶極序通常限於電偶極子的平行（鐵電）和反平行（反鐵電）的共線排列。

在此，我們報告通過光孔摻雜四倍鈣鈦礦BiMn7O12觀察到的電偶極子的不相稱螺旋序。與磁學相似，電偶極螺旋結構是通過競爭的不穩定性來穩定的。

具體來說，軌道的順序和孤電子對的立體化學活性相互競爭，導致通過中間密度波從非手性立方結構到不相稱的電偶極子和軌道螺旋的相變。

▲ 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

頭髮如何使鋼變形

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

▲ 鏈接：

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

▲ 摘要

用於鋒利邊緣或工具的鋼通常具有馬氏體組織、高碳化物含量和各種塗層，以顯示高硬度和耐磨性。然而，在切割頭髮、乳酪或土豆等較軟的材料時，它們就幾乎無法使用了。

儘管這是一種日常觀察，但由於相互作用材料的結構複雜性和它們共同變形的複雜邊界條件，其中的物理微觀機制尚不清楚。

為了解釋這個複雜性，我們進行了中斷測試和設置了兩個微觀機械測試的原位電子顯微鏡切割實驗。

我們調查結果分析和數值模擬，揭示板條馬氏體的空間變異結構在明顯磨損之前產生的混合模式II-III開裂現象中起到關鍵作用。

▲ 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.

物理學Physics

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

液體薄片上一種新的起皺現象：顛覆粘性氣泡坍塌機制

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

▲ 鏈接：

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

▲ 摘要

粘性氣泡在自然和工業環境中都很普遍。它們的破裂和塌陷可能伴有典型的與彈性板相關的特徵，包括放射狀褶皺的發展。此前，研究人員認為，薄膜的重量是導致薄膜塌陷和起皺不穩定的原因。

相反，我們在這裡的實驗表明，重力的作用微不足道：同樣的坍縮和褶皺的產生與氣泡的方向無關。

我們發現，表面張力驅動氣泡崩潰和引發動態屈曲不穩定。由於薄膜的重量無關緊要。我們的研究結果表明，褶皺可能同樣伴隨著相對較小的彎曲粘性和粘彈性薄膜的破裂，包括那些在呼吸系統中負責由呼氣事件產生氣溶膠的薄膜。

▲ 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.