《自然》（20200806出版）一周論文導讀

編譯 | 未玖

nature, 6 August 2020, VOL 584, ISSUE 7819

《自然》2020年8月6日，第584卷，7819期

天文學Astronomy

Small lightning flashes from shallow electrical storms on Jupiter

木星上弱電風暴產生的小閃電

▲ 作者：Heidi N. Becker, James W. Alexander, Sushil K. Atreya, Scott J. Bolton, Martin J. Brennan, Shannon T. Brown, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2532-1

▲ 摘要

在過去的幾十年裡，木星飛行任務多次觀測到了閃電。根據圖像得出的閃光率估計為每年每平方公裡4×10-3次閃光。

「航行者」閃電的空間範圍約為30公裡（半峰半寬度，HWHM），但由於相機對最亮的木星閃電發射光譜線（656.3納米氫原子Hα線）的反應較弱，不太可能探測到閃光的暗淡外緣。

一些相機的空間解析度使研究人員能夠確認22次HWHM大於42公裡的閃光，並估計一次HWHM為37至45公裡的閃光。

這些閃光的光能相當於地球上的「超級閃電」，即（0.02–1.6）×1010焦耳，解釋為木星大氣5巴水平附近產生的溼對流的示蹤劑。

先前對閃電的觀測受相機靈敏度、距木星的距離和長時間曝光（約680毫秒到85秒）的限制，這意味著一些觀測結果可能是疊加的閃電。

研究組報導了朱諾號太空船對閃電的光學觀測，其能量約為105–108焦耳，閃光持續時間短至5.4毫秒，閃電間隔為數十毫秒，具有典型的陸地能量。閃光率約為每年每平方公裡6.1×10−2次閃光，比迄今所觀測的要高一個數量級。

有幾次閃電的空間範圍如此之小，它們必須在沒有液態水的2巴水平以上生成。這意味著要全面了解木星的大氣對流和成分，就需要考慮木星上產生閃電的多種機制。

▲ Abstract

Lightning flashes have been observed by a number of missions that visited or flew by Jupiter over the past several decades. Imagery led to a flash rate estimate of about 4×10-3 flashes per square kilometre per year. The spatial extent of Voyager flashes was estimated to be about 30 kilometres (half-width at half-maximum intensity, HWHM), but the camera was unlikely to have detected the dim outer edges of the flashes, given its weak response to the brightest spectral line of Jovian lightning emission, the 656.3-nanometre Hα line of atomic hydrogen. The spatial resolution of some cameras allowed investigators to confirm 22 flashes with HWHM greater than 42 kilometres, and to estimate one with an HWHM of 37 to 45 kilometres. These flashes, with optical energies comparable to terrestrial 『superbolts』—of (0.02–1.6) × 1010 joules—have been interpreted as tracers of moist convection originating near the 5-bar level of Jupiter’s atmosphere. Previous observations of lightning have been limited by camera sensitivity, distance from Jupiter and long exposures (about 680 milliseconds to 85 seconds), meaning that some measurements were probably superimposed flashes reported as one. Here we report optical observations of lightning flashes by the Juno spacecraft with energies of approximately 105–108 joules, flash durations as short as 5.4 milliseconds and inter-flash separations of tens of milliseconds, with typical terrestrial energies. The flash rate is about 6.1 × 10−2 flashes per square kilometre per year, more than an order of magnitude greater than hitherto seen. Several flashes are of such small spatial extent that they must originate above the 2-bar level, where there is no liquid water. This implies that multiple mechanisms for generating lightning on Jupiter need to be considered for a full understanding of the planet’s atmospheric convection and composition.

地理學Geography

Thermal displacement by marine heatwaves

海洋熱浪引起的熱位移

▲ 作者：Michael G. Jacox, Michael A. Alexander, Steven J. Bograd & James D. Scott

▲ 連結：

https://www.nature.com/articles/s41586-020-2534-z

▲ 摘要

海洋熱浪（MHW），即離散但長時間異常溫暖的海洋溫度，會極大地改變海洋生態系統，對生態和社會經濟產生深遠影響。在了解全球MHW的模式、驅動因素和趨勢方面，人們已經付出了相當大的努力。

通常，MHW的特徵是基於它們在某特定位置的強度和持久性來確定的，這一方法特別適用於必須承受更高溫度的珊瑚和其他無柄生物。

然而，許多生態和商業上重要的海洋物種通過遷移到有利的棲息地來應對環境破壞，而遷移海洋物種的急劇變化是MHWs的顯著影響之一。

雖然空間溫度變化已在長期變暖趨勢的背景下得到了廣泛研究，但現有的全球MHW分析卻未對其進行解釋。

研究組引入熱位移作為一項指標，它通過表面溫度等值線的空間位移而非局部溫度反常來表徵MHWs，並使用基於觀測的全球海表溫度數據集來計算1982-2019年所有MHW的熱位移。

研究組表明，在MHW期間，全球海洋中的熱位移從幾十到數千公裡不等，且在空間上與MHW強度並無關聯。此外，在MHW期間的短期熱位移與從變暖趨勢推斷出的世紀尺度的變化具有可比性，儘管它們的全球空間模式有很大不同。

這些結果擴展了人們對MHW及其對海洋物種潛在影響的理解，揭示了哪些區域最容易受到熱位移的影響，以及在預期海洋變暖情況下熱位移是如何變化的。

研究結果還強調，海洋資源管理需要考慮到由MHW驅動的空間變化，這種變化的規模與與長期氣候變化相當，且業已發生。

▲ Abstract

Marine heatwaves (MHWs)—discrete but prolonged periods of anomalously warm ocean temperatures—can drastically alter ocean ecosystems, with profound ecological and socioeconomic impacts. Considerable effort has been directed at understanding the patterns, drivers and trends of MHWs globally. Typically, MHWs are characterized on the basis of their intensity and persistence at a given location—an approach that is particularly relevant for corals and other sessile organisms that must endure increased temperatures. However, many ecologically and commercially important marine species respond to environmental disruptions by relocating to favourable habitats, and dramatic range shifts of mobile marine species are among the conspicuous impacts of MHWs. Whereas spatial temperature shifts have been studied extensively in the context of long-term warming trends, they are unaccounted for in existing global MHW analyses. Here we introduce thermal displacement as a metric that characterizes MHWs by the spatial shifts of surface temperature contours, instead of by local temperature anomalies, and use an observation-based global sea surface temperature dataset to calculate thermal displacements for all MHWs from 1982 to 2019. We show that thermal displacements during MHWs vary from tens to thousands of kilometres across the world’s oceans and do not correlate spatially with MHW intensity. Furthermore, short-term thermal displacements during MHWs are of comparable magnitude to century-scale shifts inferred from warming trends, although their global spatial patterns are very different. These results expand our understanding of MHWs and their potential impacts on marine species, revealing which regions are most susceptible to thermal displacement, and how such shifts may change under projected ocean warming. The findings also highlight the need for marine resource management to account for MHW-driven spatial shifts, which are of comparable scale to those associated with long-term climate change and are already happening.

物理學Physics

Exponentially faster cooling in a colloidal system

膠體系統中的指數級快速冷卻

▲ 作者：Avinash Kumar & John Bechhoefer

▲ 連結：

https://www.nature.com/articles/s41586-020-2560-x

▲ 摘要

當冷卻物體的溫度隨著弛豫到熱平衡而降低時，人們直覺上認為熱物體比冷物體需要更長的時間來冷卻。然而，大約2300年前，亞里斯多德觀察到「要迅速冷卻熱水，首先要將其放在陽光下」。

在1960年代，這種違反直覺的現象被重新發現為「熱水比冷水能更快結冰」的說法，並被稱為姆潘巴效應。此後，它一直是許多實驗研究和一些爭議的主題。

儘管已經提出了許多特定機制，但人們對於根本原因還沒有普遍的共識。研究組展示了在一個可控環境中的姆潘巴效應，即浸入熱浴水中的膠體系統的熱猝滅。研究結果可重複，且在數量上與基於最近提出的理論框架的計算結果相一致。

通過仔細選擇參數，研究組觀察到冷卻速度比使用典型參數觀察到的要快，這與最近預測的強姆潘巴效應一致。

研究組的實驗概述了加速去熱和弛豫以達到熱平衡所需的一般條件，並支持這樣一種觀點：姆潘巴效應不僅僅是關於水如何凍結成冰的科學獵奇（這是水的許多反常特徵之一），還是一種具有廣泛技術重要性的一系列異常弛豫現象的模型。

▲ Abstract

As the temperature of a cooling object decreases as it relaxes to thermal equilibrium, it is intuitively assumed that a hot object should take longer to cool than a warm one. Yet, some 2,300 years ago, Aristotle observed that 「to cool hot water quickly, begin by putting it in the sun」. In the 1960s, this counterintuitive phenomenon was rediscovered as the statement that 「hot water can freeze faster than cold water」 and has become known as the Mpemba effect; it has since been the subject of much experimental investigation and some controversy. Although many specific mechanisms have been proposed, no general consensus exists as to the underlying cause. Here we demonstrate the Mpemba effect in a controlled setting—the thermal quench of a colloidal system immersed in water, which serves as a heat bath. Our results are reproducible and agree quantitatively with calculations based on a recently proposed theoretical framework. By carefully choosing parameters, we observe cooling that is exponentially faster than that observed using typical parameters, in accord with the recently predicted strong Mpemba effect. Our experiments outline the generic conditions needed to accelerate heat removal and relaxation to thermal equilibrium and support the idea that the Mpemba effect is not simply a scientific curiosity concerning how water freezes into ice—one of the many anomalous features of water—but rather the prototype for a wide range of anomalous relaxation phenomena of broad technological importance.

Quantum distance and anomalous Landau levels of flat bands

平帶的量子距離和反常朗道能級

▲ 作者：Jun-Won Rhim, Kyoo Kim & Bohm-Jung Yang

▲ 連結：

https://www.nature.com/articles/s41586-020-2540-1

▲ 摘要

昂薩格提出的磁場下電子態的半經典量子化不僅描述了朗道能級譜，還描述了金屬在磁場下的幾何響應。即使在具有相對論性能量色散的石墨烯中，昂薩格規則也正確地描述了 p Berry相，以及狄拉克粒子的反常朗道能級譜。

然而，目前尚不清楚這種半經典的觀點是否適用於無色散平帶系統，該系統允許有無窮數量的簡併半經典軌道。

研究組證明了一類稱為「奇異平帶」的無色散平帶的半經典量子化規則失效。奇異平帶與另一個色散帶的帶交叉，受能帶平坦度條件約束，表現出異常的磁響應。

奇異平帶的朗道能級擴散到能量空間的空白區，在沒有磁場的情況下不存在電子態，並且對朗道能級指數n表現出不尋常的1/n依賴關係，這導致了軌道磁化率的發散。奇異平帶朗道能級的總能量擴散由相關布洛赫態的量子幾何決定，其特徵是希爾伯特-施密特量子距離。

研究組證明，平帶的總朗道能級擴散與最大希爾伯特-施密特量子距離之間存在普遍和簡單的關係，可以在各種候選材料中進行驗證。

結果表明，平帶的反常朗道能級譜有望用於直接測量凝聚態波函數的量子幾何。

▲ Abstract

Semiclassical quantization of electronic states under a magnetic field, as proposed by Onsager, describes not only the Landau level spectrum but also the geometric responses of metals under a magnetic field. Even in graphene with relativistic energy dispersion, Onsager’s rule correctly describes the p Berry phase, as well as the unusual Landau level spectrum of Dirac particles. However, it is unclear whether this semiclassical idea is valid in dispersionless flat-band systems, in which an infinite number of degenerate semiclassical orbits are allowed. Here we show that the semiclassical quantization rule breaks down for a class of dispersionless flat bands called 『singular flat bands』. The singular flat band has a band crossing with another dispersive band that is enforced by the band-flatness condition, and shows anomalous magnetic responses. The Landau levels of a singular flat band develop in the empty region in which no electronic states exist in the absence of a magnetic field, and exhibit an unusual 1/n dependence on the Landau level index n, which results in diverging orbital magnetic susceptibility. The total energy spread of the Landau levels of a singular flat band is determined by the quantum geometry of the relevant Bloch states, which is characterized by their Hilbert–Schmidt quantum distance. We show that there is a universal and simple relationship between the total Landau level spread of a flat band and the maximum Hilbert–Schmidt quantum distance, which can be verified in various candidate materials. The results indicate that the anomalous Landau level spectrum of flat bands is promising for the direct measurement of the quantum geometry of wavefunctions in condensed matter.

化學Chemistry

Photoenzymatic enantioselective intermolecular radical hydroalkylation

光酶對映選擇性分子間自由基氫烷基化

▲ 作者：Xiaoqiang Huang, Binju Wang, Yajie Wang, Guangde Jiang, Jianqiang Feng & Huimin Zhao

▲ 連結：

https://www.nature.com/articles/s41586-020-2406-6

▲ 摘要

酶越來越廣泛地應用於非對稱合成，但通常受到天然酶的活性限制。最近，人們對光催化的興趣促使其從已知酶中發現新型活性。

然而，到目前為止，光誘導的酶催化尚未用於兩個分子的交叉偶聯。主要困難是酶固有的低光活性和難以實現對遠程前手性自由基中間體的立體化學控制。

研究組報導了一種可見光誘導的末端烯烴的分子間自由基氫烷基化反應，該反應非自然發生，使用現成的α-滷代羰基化合物作為反應物，由「烯」還原酶催化。

這為合成帶γ-立體中心的數種羰基化合物提供了一種有效方法，具有優異的產率（高達99%）和對映選擇性（對映體過量99%），否則很難通過化學催化來獲得。

機理研究表明，底物（α-滷代羰基化合物）與「烯」還原酶的複合物的形成觸發了對映選擇性光誘導的自由基反應。研究組的工作進一步擴展了光催化和酶催化相結合的生物催化、合成有用的非對稱轉化的反應譜。

▲ Abstract

Enzymes are increasingly explored for use in asymmetric synthesis, but their applications are generally limited by the reactions available to naturally occurring enzymes. Recently, interest in photocatalysis has spurred the discovery of novel reactivity from known enzymes. However, so far photoinduced enzymatic catalysis has not been used for the cross-coupling of two molecules. The major challenges are the inherent poor photoreactivity of enzymes and the difficulty in achieving stereochemical control of the remote prochiral radical intermediate. Here we report a visible-light-induced intermolecular radical hydroalkylation of terminal alkenes that does not occur naturally, catalysed by an 『ene』 reductase using readily available α-halo carbonyl compounds as reactants. This method provides an efficient approach to the synthesis of various carbonyl compounds bearing a γ-stereocentre with excellent yields and enantioselectivities (up to 99 per cent yield with 99 per cent enantiomeric excess), which otherwise are difficult to access using chemocatalysis. Mechanistic studies suggest that the formation of the complex of the substrates (α-halo carbonyl compounds) and the 『ene』 reductase triggers the enantioselective photoinduced radical reaction. Our work further expands the reactivity repertoire of biocatalytic, synthetically useful asymmetric transformations by the merger of photocatalysis and enzyme catalysis.

A photochemical dehydrogenative strategy for aniline synthesis

苯胺合成的光化學脫氫策略

▲ 作者：Shashikant U. Dighe, Fabio Juliá, Alberto Luridiana, James J. Douglas & Daniele Leonori

▲ 連結：

https://www.nature.com/articles/s41586-020-2539-7

▲ 摘要

將兩個分子片段可靠地結合在一起的化學反應（交叉偶聯）對於發現和製造藥物和農用化學品至關重要。

目前，只有在過渡金屬催化過程中，才能在特定和預先確定的位置（鄰位、間位和對位）將胺引入官能化芳香族化合物中，並且需要含滷素或含硼的底物。在芳族單元周圍引入這些基團是由該方法的固有反應性決定的（親電滷化或C–H硼化），因此通常不可能對所有位置進行選擇性靶向。

研究組報導了一種利用飽和環己酮作為芳基親電試劑來構建苯胺的非典型交叉偶聯方法。

胺和羰基之間的縮合反應是自然界中經常發生的一種過程，通常被（生物）有機化學家所使用，它能夠在預定和選擇性的位點形成碳氮（C–N）鍵，基於光氧化還原和鈷基的催化系統可使環己烯環逐步去飽和，形成苯胺。

鑑於通過完全區域控制利用成熟的羰基反應，很容易將環己酮官能化，因此該法繞過了芳香族化學中一些常見的選擇性問題。

研究組通過製備商業藥物以及對天然產物、類固醇和萜烯原料進行後期氨化-芳香化，證明了這種C–N偶聯方案的實用性。

▲ Abstract

Chemical reactions that reliably join two molecular fragments together (cross-couplings) are essential to the discovery and manufacture of pharmaceuticals and agrochemicals. The introduction of amines onto functionalized aromatics at specific and pre-determined positions (ortho versus meta versus para) is currently achievable only in transition-metal-catalysed processes and requires halogen- or boron-containing substrates. The introduction of these groups around the aromatic unit is dictated by the intrinsic reactivity profile of the method (electrophilic halogenation or C–H borylation) so selective targeting of all positions is often not possible. Here we report a non-canonical cross-coupling approach for the construction of anilines, exploiting saturated cyclohexanones as aryl electrophile surrogates. Condensation between amines and carbonyls, a process that frequently occurs in nature and is often used by (bio-)organic chemists, enables a predetermined and site-selective carbon–nitrogen (C–N) bond formation, while a photoredox- and cobalt-based catalytic system progressively desaturates the cyclohexene ring en route to the aniline. Given that functionalized cyclohexanones are readily accessible with complete regiocontrol using the well established carbonyl reactivity, this approach bypasses some of the frequent selectivity issues of aromatic chemistry. We demonstrate the utility of this C–N coupling protocol by preparing commercial medicines and by the late-stage amination–aromatization of natural products, steroids and terpene feedstocks.