物理學Physics

▲ 作者：C. J. Baker, W. Bertsche

▲ 鏈接：

https://www.nature.com/articles/s41586-021-03289-6

▲ 摘要

光子——電磁場激發的量子——是無質量的，但攜帶動量。作者演示了反氫原子的激光冷卻，這個反物質原子由一個反質子和一個正電子組成。利用窄線寬脈沖Lyman-α激光激發反氫的1S-2P躍遷，對磁阱反氫樣品進行了多普勒冷卻。

雖然作者只在一個維度上應用激光冷卻，陷阱耦合反原子的縱向和橫向運動，導致在所有的三維冷卻。作者觀察到橫能量中值的減少超過一個數量級——有相當一部分反原子獲得亞微電子伏橫動能。

作者還報道了在激光冷卻的反氫原子樣品中激光驅動1S-2S躍遷的觀察。觀察到的光譜線比沒有激光冷卻時的光譜線窄約四倍。激光冷卻的演示及其立即應用對反物質研究具有深遠的意義。

一個更局部、密度更大、溫度更低的反氫樣品將極大地改善正在進行的實驗中對反氫的光譜和引力的研究。此外，通過激光操縱反物質原子運動的能力將可能為未來的實驗提供突破性的機會。

▲ Abstract

The photon—the quantum excitation of the electromagnetic field—is massless but carries momentum. Here we demonstrate laser cooling of antihydrogen, the antimatter atom consisting of an antiproton and a positron. By exciting the 1S–2P transition in antihydrogen with pulsed, narrow-linewidth, Lyman-α laser radiation, we Doppler-cool a sample of magnetically trapped antihydrogen. Although we apply laser cooling in only one dimension, the trap couples the longitudinal and transverse motions of the anti-atoms, leading to cooling in all three dimensions. We observe a reduction in the median transverse energy by more than an order of magnitude—with a substantial fraction of the anti-atoms attaining submicroelectronvolt transverse kinetic energies. We also report the observation of the laser-driven 1S–2S transition in samples of laser-cooled antihydrogen atoms. The observed spectral line is approximately four times narrower than that obtained without laser cooling. The demonstration of laser cooling and its immediate application has far-reaching implications for antimatter studies. A more localized, denser and colder sample of antihydrogen will drastically improve spectroscopic and gravitational studies of antihydrogen in ongoing experiments. Furthermore, the demonstrated ability to manipulate the motion of antimatter atoms by laser light will potentially provide ground-breaking opportunities for future experiments.

▲ 作者：Sanghoo Park, Wonho Choe, Hyungyu Lee, Joo Young Park, Jinwoo Kim, Se Youn Moon & Uro? Cvelbar

▲ 鏈接：

https://www.nature.com/articles/s41586-021-03359-9

▲ 摘要

撞擊氣體射流會導致液體表面凹陷，任何觀察過通過吸管吹氣而產生空腔的人都很熟悉這種現象。作者基于陰影圖實驗和計算兩相流體和等離子體模型，論證了在氣體射流撞擊水的情況下，弱電離氣體對這種不穩定性的穩定作用。

他們重點研究了與電流體動力（EHD）氣體流動相關的界面動力學，即所謂的電風，它是在電場下由加速帶電粒子向中性氣體的動量轉移所引起的。由周期性脈沖電離波組成的弱電離氣體射流被稱為等離子體，它通過電流體動力流對水面施加的力比單獨的中性氣體射流更大，導致空腔膨脹而不產生不穩定。

此外，“腔內”等離子體相互作用產生的雙向電流體動力氣體流和平行于氣-水界面的電場使表面更加穩定。

▲ Abstract

Impinging gas jets can induce depressions in liquid surfaces, a phenomenon familiar to anyone who has observed the cavity produced by blowing air through a straw directly above a cup of juice. Here we demonstrate the stabilization of such instabilities by weakly ionized gas for the case of a gas jet impinging on water, based on shadowgraph experiments and computational two-phase fluid and plasma modelling. We focus on the interfacial dynamics relevant to electrohydrodynamic (EHD) gas flow, so-called electric wind, which is induced by the momentum transfer from accelerated charged particles to neutral gas under an electric field. A weakly ionized gas jet consisting of periodic pulsed ionization waves5, called plasma bullets, exerts more force via electrohydrodynamic flow on the water surface than a neutral gas jet alone, resulting in cavity expansion without destabilization. Furthermore, both the bidirectional electrohydrodynamic gas flow and electric field parallel to the gas–water interface produced by plasma interacting ‘in the cavity’ render the surface more stable.

材料科學Material Science

▲ 作者：Wooik Jung, Yoon-Ho Jung, Peter V. Pikhitsa, Jicheng Feng, Younghwan Yang, Minkyung Kim, Hao-Yuan Tsai, Takuo Tanaka, Jooyeon Shin, Kwang-Yeong Kim, Hoseop Choi, Junsuk Rho & Mansoo Choi

▲ 鏈接：

https://www.nature.com/articles/s41586-021-03353-1

▲ 摘要

三維（3D）打印已經徹底改變了電子、光學、能源、機器人、生物工程和傳感的制造工藝。作者介紹了一種直接3D打印金屬納米結構陣列的技術，這種金屬納米結構具有靈活的幾何形狀和特征大小，可達數百納米，使用各種材料。印刷過程是在干燥條件下進行的，不需要聚合物或油墨。

相反，離子和帶電荷的氣溶膠粒子被引導到一個包含一組空穴的介電掩膜上，這些空穴漂浮在偏置硅襯底上。這些離子聚集在每個洞周圍，產生靜電透鏡，將帶電的氣溶膠粒子聚焦成納米級噴射。

這些射流由在含孔掩模下形成的聚合電場線引導，其作用類似于傳統3D打印機的噴嘴，使氣溶膠顆粒能夠3D打印到硅基板上。通過在打印過程中移動襯底，他們成功地打印出了各種3D結構，包括螺旋、懸垂的納米管、圓環和字母。

▲ Abstract

Three-dimensional (3D) printing has revolutionized manufacturing processes for electronics, optics, energy, robotics, bioengineering and sensing. Here we introduce a technique for direct 3D printing of arrays of metal nanostructures with flexible geometry and feature sizes down to hundreds of nanometres, using various materials. The printing process occurs in a dry atmosphere, without the need for polymers or inks. Instead, ions and charged aerosol particles are directed onto a dielectric mask containing an array of holes that floats over a biased silicon substrate. The ions accumulate around each hole, generating electrostatic lenses that focus the charged aerosol particles into nanoscale jets. These jets are guided by converged electric-field lines that form under the hole-containing mask, which acts similarly to the nozzle of a conventional 3D printer, enabling 3D printing of aerosol particles onto the silicon substrate. By moving the substrate during printing, we successfully print various 3D structures, including helices, overhanging nanopillars, rings and letters.

▲ 作者：Yao Yang, Jihan Zhou, Fan Zhu, Yakun Yuan, Dillan J. Chang, Dennis S. Kim, Minh Pham, Arjun Rana, Xuezeng Tian, Yonggang Yao, Stanley J. Osher, Andreas K. Schmid, Liangbing Hu, Peter Ercius & Jianwei Miao

▲ 鏈接：

https://www.nature.com/articles/s41586-021-03354-0

▲ 摘要

非晶態固體如玻璃、塑料和非晶態薄膜在日常生活中無處不在，在電信、電子、太陽能電池等領域有著廣泛的應用。作者發展了一種原子電子斷層掃描重建方法，以實驗確定非晶固體的三維原子位置。

他們用一種多組分非晶合金作為原理證明，定量表征了三維原子排列的中短程順序。他們觀察到，雖然近程序的三維原子排列是幾何無序的，但一些近程序的結構相互連接形成類晶體超團簇，并形成中程序。

作者們非晶樣品中發現了四種類型的類晶中程有序晶體：面心立方、六邊形密排、體心立方和簡單立方共存，它們表現為平動而非取向有序。這些觀察結果為支持金屬玻璃有效簇狀堆積模型的總體框架提供了直接的實驗證據。

作者期望這項研究能為確定廣泛的非晶固體的三維結構鋪平道路，這將改變人們對非晶材料和相關現象的基本認識。

▲ Abstract

Amorphous solids such as glass, plastics and amorphous thin films are ubiquitous in our daily life and have broad applications ranging from telecommunications to electronics and solar cells. Here we develop an atomic electron tomography reconstruction method to experimentally determine the 3D atomic positions of an amorphous solid. Using a multi-component glass-forming alloy as proof of principle, we quantitatively characterize the short- and medium-range order of the 3D atomic arrangement. We observe that, although the 3D atomic packing of the short-range order is geometrically disordered, some short-range-order structures connect with each other to form crystal-like superclusters and give rise to medium-range order. We identify four types of crystal-like medium-range order—face-centred cubic, hexagonal close-packed, body-centred cubic and simple cubic—coexisting in the amorphous sample, showing translational but not orientational order. These observations provide direct experimental evidence to support the general framework of the efficient cluster packing model for metallic glasses. We expect that this work will pave the way for the determination of the 3D structure of a wide range of amorphous solids, which could transform our fundamental understanding of non-crystalline materials and related phenomena.

地球科學Geosciences

▲ 作者：Vincent Humphrey, Alexis Berg, Philippe Ciais, Pierre Gentine, Martin Jung, Markus Reichstein, Sonia I. Seneviratne & Christian Frankenberg

▲ 鏈接：

https://www.nature.com/articles/s41586-021-03325-5

▲ 摘要

陸地生態系統碳吸收的年變化對大氣二氧化碳濃度有重要影響。作者使用因子氣候模型模擬，表明土壤水分的變化驅動了全球土地碳吸收的90%的年際變化，主要通過其對光合作用的影響。

他們發現，這種生態系統響應大多是間接發生的，因為土壤濕度-大氣響應放大了溫度和濕度異常，增強了土壤水分脅迫的直接影響。這種反饋機制的強度解釋了為什么耦合氣候模式表明土壤濕度具有主導作用，這在陸面模型模擬和觀測分析中并不明顯。

研究表明，模擬的土地碳吸收的全球變率大部分是由土壤濕度控制的溫度和水汽壓虧缺效應驅動的。

▲ Abstract

Year-to-year changes in carbon uptake by terrestrial ecosystems have an essential role in determining atmospheric carbon dioxide concentrations. Here we use factorial climate model simulations and show that variability in soil moisture drives 90 per cent of the inter-annual variability in global land carbon uptake, mainly through its impact on photosynthesis. We find that most of this ecosystem response occurs indirectly as soil moisture–atmosphere feedback amplifies temperature and humidity anomalies and enhances the direct effects of soil water stress. The strength of this feedback mechanism explains why coupled climate models indicate that soil moisture has a dominant role, which is not readily apparent from land surface model simulations and observational analyses. Our results show that most of the global variability in modelled land carbon uptake is driven by temperature and vapour pressure deficit effects that are controlled by soil moisture.

▲ 作者：Diego R. Barneche, Chris J. Hulatt, Matteo Dossena, Daniel Padfield, Guy Woodward, Mark Trimmer & Gabriel Yvon-Durocher

▲ 鏈接：

https://www.nature.com/articles/s41586-021-03352-2

▲ 摘要

在生態系統中，能源從資源向消費者轉移的效率決定了食物網的生物量結構。作者量化了暴露在7年實驗變暖環境中的淡水浮游生物的氮轉移效率（整體能量轉移的代理）。

直接實驗證據表明，相對于環境條件，4℃升溫可使營養轉移效率降低56%。此外，暖池浮游植物和浮游動物生物量均較低，表明能量的吸收、轉化和轉移發生了重大變化。

這些發現協調了在不同類群中觀察到的由變暖驅動的增長成本和碳利用效率的變化，在生態系統水平上，呼吸總量與初級生產總量之比增加。結果表明，隨著地球變暖，光合作用固定的碳越來越多地會消失到大氣中，破壞通過食物鏈的能量流動，這將對較大的消費者和整個生態系統的功能產生負面影響。

▲ Abstract

In ecosystems, the efficiency of energy transfer from resources to consumers determines the biomass structure of food webs. Here we quantify nitrogen transfer efficiency—a proxy for overall energy transfer—in freshwater plankton in artificial ponds that have been exposed to seven years of experimental warming. We provide direct experimental evidence that, relative to ambient conditions, 4?°C of warming can decrease trophic transfer efficiency by up to 56%. In addition, the biomass of both phytoplankton and zooplankton was lower in the warmed ponds, which indicates major shifts in energy uptake, transformation and transfer. These findings reconcile observed warming-driven changes in individual-level growth costs and in carbon-use efficiency across diverse taxa with increases in the ratio of total respiration to gross primary production at the ecosystem level. Our results imply that an increasing proportion of the carbon fixed by photosynthesis will be lost to the atmosphere as the planet warms, impairing energy flux through food chains, which will have negative implications for larger consumers and for the functioning of entire ecosystems.

▲ 作者：Robert H. Smithies, Yongjun Lu, Christopher L. Kirkland, Tim E. Johnson, David R. Mole, David C. Champion, Laure Martin, Heejin Jeon, Michael T. D. Wingate & Simon P. Johnson

▲ 鏈接：

https://www.nature.com/articles/s41586-021-03337-1

▲ 摘要

目前地球大陸地殼的大部分體積是在太古代末期（25億年前）形成的，通過在大約25~50公里深處水化玄武巖的熔融，形成英云長花崗巖-長長花崗巖-花崗閃長巖（TTG）套的鈉質花崗巖。

作者利用鋯石氧同位素組成的變化，結合全巖石地球化學，來識別兩組不同的TTG。強鈉TTG代表了最原始的巖漿，含有氧同位素組成的鋯石，反映了源巖已被原始地幔源水水化。

研究表明，足夠的原始水已經存在于地球早期的基性地殼中，以產生大陸的原始核，并通過早期地球特有的動態過程產生額外的水合源。

▲ Abstract

Much of the current volume of Earth’s continental crust had formed by the end of the Archaean eon (2.5 billion years ago), through melting of hydrated basaltic rocks at depths of approximately 25–50 kilometres, forming sodic granites of the tonalite–trondhjemite–granodiorite (TTG) suite. Here we use variations in the oxygen isotope composition of zircon, coupled with whole-rock geochemistry, to identify two distinct groups of TTG. Strongly sodic TTGs represent the most-primitive magmas and contain zircon with oxygen isotope compositions that reflect source rocks that had been hydrated by primordial mantle-derived water. These primitive TTGs do not require a source highly enriched in incompatible trace elements, as ‘average’ TTG does. These results remove many of the paradoxical impediments to understanding early continental crust formation. Our work suggests that sufficient primordial water was already present in Earth’s early mafic crust to produce the primitive nuclei of the continents, with additional hydrated sources created through dynamic processes that are unique to the early Earth.