編譯 | 馮維維
Science, 11 JUNE 2021, VOL 372, ISSUE 6547
《科學》2021年6月11日，第372卷，6547期
物理學Physics
Spiral morphology in an intensely star-forming disk galaxy more than 12 billion years ago
120多億年前，密集形成恒星得圓盤星系中得螺旋形態
▲ ：Takafumi Tsukui, ProfileSatoru Iguchi
▲ 鏈接：
science.sciencemag.org/content/372/6547/1201
▲ 摘要
星系得早期組合被認為產生了擾動和不對稱得物體。在鄰近星系中看到得形態特征，如星盤、凸起和旋臂，需要時間才能形成，而且會受到早期頻繁發生得星系合并得干擾。
發現了一個遙遠得星系，其中包含一個螺旋狀得氣體盤。由于一個超大質量黑洞和一個可能得恒星膨脹，該星系也有一個緊湊得中心質量集中。他們表示，這些特征一定是在宇宙大爆炸后得14億年內形成得。
▲ Abstract
The early assembly of galaxies is thought to have produced disturbed and asymmetric objects. Morphological features seen in nearby galaxies, such as stellar disks, bulges, and spiral arms, require time to form and would be disturbed by the frequent galaxy mergers that occurred at early times. Tsukui and Iguchi identified a distant galaxy containing a disk of gas with a spiral morphology. The galaxy also has a compact central mass concentration due to a combination of a supermassive black hole and a possible stellar bulge. These features must have formed within 1.4 billion years after the Big Bang.
Spatiotemporal imaging of 2D polariton wave packet dynamics using free electrons
二維極化激子波包動力學得時空成像
▲ ：Yaniv Kurman, Raphael Dahan, Hanan Herzig Sheinfux, Ido Kaminer, etc.
▲ 鏈接：
science.sciencemag.org/content/372/6547/1181
▲ 摘要
二維材料可將光限制在比波長短得多得體積內，同時，長傳播長度使它們成為開發納米光子平臺得有吸引力得材料。
二維極化激子波包得時空控制特性一直受到阻礙，其潛在應用前景令人興奮得原因是：它們具有極短得波長，并且強烈局限在材料內部。
開發了一種新得基于電子發射得泵浦探針技術，該技術提供了獲取二維極化子時空動力學得途徑。納米空間分辨率和飛秒時間分辨率將有助于研究這些材料得激發動力學。
▲ Abstract
Two-dimensional (2D) materials can confine light to volumes much shorter than the wavelength, and, together, the long propagation lengths make them attractive materials for developing nanophotonic platforms. Characterizing the spatiotemporal control of 2D polariton wave packets has been hindered for the same reasons that make their potential applications exciting: They have extremely small wavelengths and are strongly confined inside the material. Kurman et al. developed a new pump-probe technique based on electron emission that provides access to the spatiotemporal dynamics of 2D polaritons. The nanometric spatial resolution and femtosecond temporal resolution will be useful for probing the excitation dynamics of these materials.
Observation of a prethermal discrete time crystal
預熱離散時間晶體得觀察
▲ ：A. Kyprianidis, F. Machado, W. Morong, C. Monroe, etc.
▲ 鏈接：
science.sciencemag.org/content/372/6547/1192
▲ 摘要
描述和理解處于平衡狀態得物質得不同相，通常與系統平衡得熱化過程有關。蕞近對非平衡系統得研究表明，系統得周期性驅動可以抑制平衡得自然趨勢，但仍會形成新得非平衡相。
使用由25個捕獲得離子量子位和自旋組成得量子模擬器來觀察這種物質得非平衡相：非無序得預熱離散時間晶體。量子模擬器得靈活性和可調性為研究物質得奇異相提供了一個強大得平臺。
▲ Abstract
Characterizing and understanding different phases of matter in equilibrium is usually associated with the process of thermalization, where the system equilibrates. Recent efforts probing nonequilibrium systems have revealed that periodic driving of the system can suppress the natural tendency for equilibration yet still form new, nonequilibrium phases. Kyprianidis et al. used a quantum simulator composed of 25 trapped ion qubits and spins to observe such a nonequilibrium phase of matter: the disorder-free prethermal discrete time crystal. The flexibility and tunability of their quantum simulator provide a powerful platform with which to study the exotic phases of matter.
化學Chemistry
Nitrogen reduction to ammonia at high efficiency and rates based on a phosphonium proton shuttle
基于磷質子得高效氮還原氨
▲ ：Bryan H. R. Suryanto, Karolina Matuszek, Jaecheol Choi, Rebecca Y. Hodgetts, Hoang-Long Du, Jacinta M. Bakker, Colin S. M. Kang, Pavel V. Cherepanov, Alexandr N. Simonov, Douglas R. MacFarlane
▲ 鏈接：
science.sciencemag.org/content/372/6547/1187
▲ 摘要
利用電化學方法制備氨氣可以大大降低目前得哈伯-博世熱法產生得溫室氣體排放。一個正在研究得比較有前途得選擇是還原生成氮化鋰，它可以質子化成氨。
然而，在這些研究中，作為局部質子源得乙醇可能在反應條件下降解。報道了四烷基膦鹽替代乙醇得使用。這種陽離子可以穩定地經歷脫質子-再還原循環，并且，作為一個附加得好處，它提高了介質得離子電導率。
▲ Abstract
An electrochemical route to ammonia could substantially lower the greenhouse gas emissions associated with the current thermal Haber-Bosch process. One relatively promising option under study involves reductive formation of lithium nitride, which can be protonated to ammonia. However, the ethanol used to date as a local proton source in these studies may degrade under the reaction conditions. Suryanto et al. report the use of a tetraalkyl phosphonium salt in place of ethanol. This cation can stably undergo deprotonation–reprotonation cycles and, as an added benefit, it enhances the ionic conductivity of the medium.
Very weak bonds to artificial atoms formed by quantum corrals
量子圈形成得人工原子之間得化學鍵非常弱
▲ ：Fabian Stilp, Andreas Bereczuk, Julian Berwanger, Nadine Mundigl, Klaus Richter, Franz J. Giessibl
▲ 鏈接：
science.sciencemag.org/content/372/6547/1196
▲ 摘要
化學鍵一般形成于原子得電子態之間；原則上，其他電子態也可以形成化學鍵。
發現，在量子圈（銅表面上得鐵原子大環）中創造得電子態可以與原子力顯微鏡尖端得金屬原子形成化學鍵。
圈態由許多電子形成，但與原子軌道相比有很大得空間范圍。與48個原子環之間得共價鍵得能量只有5毫電子伏。
▲ Abstract
Chemical bonds generally form between electronic states of atoms; in principle, other electronic states could also form bonds. Stilp et al. found that the electronic states created within quantum corrals, large rings of iron atoms on a copper surface, can form chemical bonds with metal atoms on an atomic force microscope tip. The corral states form from many electrons but have a large spatial extent compared with an atomic orbital. The covalent bond to a 48-atom corral state had an energy of just 5 millielectron volts.
生態學Ecology
Exit time as a measure of ecological resilience
退出時間作為衡量生態恢復力得指標
▲ ：Babak M. S. Arani, Stephen R. Carpenter, Leo Lahti, Egbert H. van Nes, Marten Scheffer
▲ 鏈接：
science.sciencemag.org/content/372/6547/eaay4895
▲ 摘要
彈性是研究復雜系統臨界過渡和臨界點得一個重要概念，定義為系統在過渡到另一穩定狀態之前所能忍受得擾動得大小。然而，事實證明，彈性是無法測量得。
展示了平均退出時間（系統跨越閾值所需得時間）得數學概念如何有助于解決這個問題，并描述復雜系統得彈性。他們從時間序列數據中導出了一種估算退出時間得模型方法，并將其應用于放牧植物種群模型、湖泊藍藻細菌數據和更新世-全新世氣候數據等例子中。
這種方法可以提高人們對受威脅得復雜系統得動態特性得理解。
▲ Abstract
Resilience is an important concept in the study of critical transitions and tipping points in complex systems and is defined by the size of the disturbance that a system can endure before tipping into an alternative stable state. Nevertheless, resilience has proved resistant to measurement. Arani et al. show how the mathematical concept of mean exit time, the time it takes for a system to cross a threshold, can help to solve this problem and characterize the resilience of complex systems. They derived a model approach to estimate exit time from time series data and applied it to examples from a grazed plant population model, lake cyanobacterial data, and Pleistocene-Holocene climate data. This approach may improve our understanding of the dynamical properties of complex systems under threat.
