《自然》（20240905出版）一周論文導讀

編譯|未玖

Nature,  5 September 2024, Volume 633, Issue 8028

《自然》2024年9月5日，第633卷，8028期

天文學Astronomy

Gravitational instability in a planet-forming disk

行星形成盤的引力不穩定性

▲ 作者：Jessica Speedie, Ruobing Dong, Cassandra Hall, Cristiano Longarini, Benedetta Veronesi, Teresa Paneque-Carre?o, et al.

▲鏈接：

https://www.nature.com/articles/s41586-024-07877-0

▲摘要：

關于行星在星周盤中形成的正則理論提出，行星最初是由小得多的種子長成的。長期考慮的替代理論提出，巨大的原行星可由引力不穩定引起的巨大旋臂的坍塌碎片直接形成——如果星周盤引力不穩定的話。

要實現這一點，星周盤的質量必須比中心恒星大得多：星周盤與恒星的質量比為1:10被廣泛認為是觸發引力不穩定性、激發大量非開普勒動力學、產生明顯旋臂的粗略閾值。雖然估計星周盤質量歷來頗具挑戰性，但氣體的運動可通過其對盤速度結構的影響揭示引力不穩定性的存在。

研究組利用阿塔卡馬大型毫米波/亞毫米波陣列（ALMA）對13CO和C18O譜線發射的深入觀測，提出了AB Aurigae星周盤引力不穩定的運動學證據。觀測到的運動信號與模擬和分析建模的預測非常相似。通過定量比較，研究組推斷出在天空中1″到5″范圍內的星周盤質量高達恒星質量的三分之一。

▲ Abstract：

The canonical theory for planet formation in circumstellar disks proposes that planets are grown from initially much smaller seeds. The long-considered alternative theory proposes that giant protoplanets can be formed directly from collapsing fragments of vast spiral arms induced by gravitational instability—if the disk is gravitationally unstable. For this to be possible, the disk must be massive compared with the central star: a disk-to-star mass ratio of 1:10 is widely held as the rough threshold for triggering gravitational instability, inciting substantial non-Keplerian dynamics and generating prominent spiral arms. Although estimating disk masses has historically been challenging, the motion of the gas can reveal the presence of gravitational instability through its effect on the disk-velocity structure. Here we present kinematic evidence of gravitational instability in the disk around AB Aurigae, using deep observations of 13CO and C18O line emission with the Atacama Large Millimeter/submillimeter Array (ALMA). The observed kinematic signals strongly resemble predictions from simulations and analytic modelling. From quantitative comparisons, we infer a disk mass of up to a third of the stellar mass enclosed within 1″ to 5″ on the sky.

物理學Physics

Long-lived isospin excitations in magic-angle twisted bilayer graphene

魔角扭曲雙層石墨烯的長壽命同位旋激發

▲ 作者：Tian Xie, Siyuan Xu, Zhiyu Dong, Zhiyuan Cui, Yunbo Ou, Melike Erdi, et al.

▲鏈接：

https://www.nature.com/articles/s41586-024-07880-5

▲摘要：

人們已報道了魔角扭曲雙層石墨烯（MATBG）諸多相關的多體相，包括傳統相和奇異相。與這些相關態有關的動力學對于理解潛在的物理學至關重要，但尚未得到探索。

研究組結合激子傳感和光泵浦-探針光譜來研究具有WSe2基底的MATBG在整個平帶上的同位旋序動力學，達到了亞皮秒分辨率。結果觀察到，在ν=2附近和ν=-3和-2之間的寬填充范圍內，同位旋動力學非常緩慢，壽命高達300 ps，與電子溫度更快的冷卻（約10 ps）解耦。這種非熱行為表明在同位旋自由度中存在異常的長壽命模式。

這一出乎理論預測的觀察結果，表明了長程傳播集體模式、強同位旋漲落和記憶效應的存在，并可能與谷間相干或不匹配的凱庫勒螺旋基態有關。研究組進一步證明了先前在整數填充周圍發現的同位旋序的非平衡控制。具體來說，通過超快操作，可暫時偏離整數填充。該研究展示了MATBG中集體激發的獨特探索，并為主動控制莫爾系統中的非平衡現象鋪平了道路。

▲ Abstract：

Numerous correlated many-body phases, both conventional and exotic, have been reported in magic-angle twisted bilayer graphene (MATBG). However, the dynamics associated with these correlated states, crucial for understanding the underlying physics, remain unexplored. Here we combine exciton sensing and optical pump–probe spectroscopy to investigate the dynamics of isospin orders in MATBG with WSe2 substrate across the entire flat band, achieving sub-picosecond resolution. We observe remarkably slow isospin dynamics in a broad filling range around ν =2 and between ν =-3 and-2, with lifetimes of up to 300 ps that decouple from the much faster cooling of electronic temperature (about 10 ps). This non-thermal behaviour demonstrates the presence of abnormally long-lived modes in the isospin degrees of freedom. This observation, not anticipated by theory, implies the existence of long-range propagating collective modes, strong isospin fluctuations and memory effects and is probably associated with an intervalley coherent or incommensurate Kekulé spiral ground state. We further demonstrate non-equilibrium control of the isospin orders previously found around integer fillings. Specifically, through ultrafast manipulation, it can be transiently shifted away from integer fillings. Our study demonstrates a unique probe of collective excitations in MATBG and paves the way for actively controlling non-equilibrium phenomena in moiré systems.

材料科學Materials Science

Photoredox phase engineering of transition metal dichalcogenides

過渡金屬二硫族化物的光氧化還原相工程

▲ 作者：Juhwan Lim, Jung-In Lee, Ye Wang, Nicolas Gauriot, Ebin Sebastian, Manish Chhowalla, et al.

▲鏈接：

https://www.nature.com/articles/s41586-024-07872-5

▲摘要：

晶體相工程在精確控制材料的物理和電子特性方面起著重要作用。在二維過渡金屬二硫族化物（2D TMDs）中，利用有機金屬化劑正丁基鋰（n-BuLi）進行化學鋰化，將半導體2H（三角）相轉化為金屬1T（八面體）相的相工程已在晶體管、催化和電池等領域得到了廣泛探索應用。雖然這種化學相工程可在環境溫度和壓力下進行，但其潛在機制尚不清楚，并且n-BuLi的使用引發了明顯的安全問題。

研究組光學可視化了單層和雙層二維TMDs中從2H相到1T相的典型相變，并發現使用455 nm的低功率照明可將該反應加速多達6個數量級。結果發現，間隙上方的照明通過光氧化還原過程改善了限速電荷轉移動力學。研究組使用該方法來實現快速和高質量的相位工程，并證明其可利用衍射限制邊緣分辨率將任意相圖案刻入少層TMDs中。

最后，他們用更安全的多環芳香族有機鋰試劑取代了自燃性n-BuLi，并表明其作為相變劑的性能優于n-BuLi。該工作為探索電化學過程的原位表征提供了機會，并為通過光氧化還原相工程可持續地擴大材料和器件的規模鋪平了道路。

▲ Abstract：

Crystallographic phase engineering plays an important part in the precise control of the physical and electronic properties of materials. In two-dimensional transition metal dichalcogenides (2D TMDs), phase engineering using chemical lithiation with the organometallization agent n-butyllithium (n-BuLi), to convert the semiconducting 2H (trigonal) to the metallic 1T (octahedral) phase, has been widely explored for applications in areas such as transistors, catalysis and batteries. Although this chemical phase engineering can be performed at ambient temperatures and pressures, the underlying mechanisms are poorly understood, and the use of n-BuLi raises notable safety concerns. Here we optically visualize the archetypical phase transition from the 2H to the 1T phase in mono- and bilayer 2D TMDs and discover that this reaction can be accelerated by up to six orders of magnitude using low-power illumination at 455 nm. We identify that the above-gap illumination improves the rate-limiting charge-transfer kinetics through a photoredox process. We use this method to achieve rapid and high-quality phase engineering of TMDs and demonstrate that this methodology can be harnessed to inscribe arbitrary phase patterns with diffraction-limited edge resolution into few-layer TMDs. Finally, we replace pyrophoric n-BuLi with safer polycyclic aromatic organolithiation agents and show that their performance exceeds that of n-BuLi as a phase transition agent. Our work opens opportunities for exploring the in situ characterization of electrochemical processes and paves the way for sustainably scaling up materials and devices by photoredox phase engineering.

化學Chemistry

Catalytic asymmetric synthesis of meta benzene isosteres

間苯等排體的催化不對稱合成

▲ 作者：Mingkai Zhang, Matthew Chapman, Bhagyesh R. Sarode, Bingcong Xiong, Hao Liang, James K. Chen, et al.

▲鏈接：

https://www.nature.com/articles/s41586-024-07865-4

▲摘要：

雖然芳香環是藥物活性化合物中的常見元素，但這些基序的存在給藥物的可開發性帶來了一些不利因素。用非芳香族等排基序取代芳香環可改善藥物化合物的非最佳效價、代謝穩定性、溶解度和親脂性。

此外，芳香環是平面的，缺乏三維性，但大多數藥物靶點的結合口袋是手性的。因此，等排替代的立體化學構型可能為提高衍生配體對靶受體的親和力提供一個額外機會。這種方法的一個顯著障礙是缺乏從現成前體中簡單和可擴展地催化對映選擇性合成候選同位異構體。

研究組提出了一種以前未知的鈀催化反應，將烴類衍生的前體轉化為手性含硼的去甲三環烴，并表明這些去甲三環烴的形狀使其成為間二取代芳環的合理等排異構體。使用手性催化劑，鈀催化的反應能夠以對映選擇性的方式完成，后續的硼基轉化提供了獲得廣泛結構的途徑。

研究組還表明，將去甲三環類摻入藥物基序可改善生物物理性質以及立體化學依賴的活性。他們預計這些特征加上簡單、廉價的功能化去甲三環支架的合成，將使該平臺成為組裝新生物活性物質的堅實基礎。

▲ Abstract：

Although aromatic rings are common elements in pharmaceutically active compounds, the presence of these motifs brings several liabilities with respect to the developability of a drug. Nonoptimal potency, metabolic stability, solubility and lipophilicity in pharmaceutical compounds can be improved by replacing aromatic rings with non-aromatic isosteric motifs. Moreover, whereas aromatic rings are planar and lack three-dimensionality, the binding pockets of most pharmaceutical targets are chiral. Thus, the stereochemical configuration of the isosteric replacements may offer an added opportunity to improve the affinity of derived ligands for target receptors. A notable impediment to this approach is the lack of simple and scalable catalytic enantioselective syntheses of candidate isosteres from readily available precursors. Here we present a previously unknown palladium-catalysed reaction that converts hydrocarbon-derived precursors to chiral boron-containing nortricyclanes and we show that the shape of these nortricyclanes makes them plausible isosteres for meta disubstituted aromatic rings. With chiral catalysts, the Pd-catalysed reaction can be accomplished in an enantioselective fashion and subsequent transformation of the boron group provides access to a broad array of structures. We also show that the incorporation of nortricyclanes into pharmaceutical motifs can result in improved biophysical properties along with stereochemistry-dependent activity. We anticipate that these features, coupled with the simple, inexpensive synthesis of the functionalized nortricyclane scaffold, will render this platform a useful foundation for the assembly of new biologically active agents.

地球科學Earth Science

Effective realization of abatement measures can reduce HFC-23 emissions

有效實施減排措施可減少HFC-23的排放

▲ 作者：Dominique Rust, Martin K. Vollmer, Stephan Henne, Arnoud Frumau, Pim van den Bulk, Arjan Hensen, et al.

▲鏈接：

https://www.nature.com/articles/s41586-024-07833-y

▲摘要：

HFC-23是一種強效溫室氣體，主要在HCFC-22的合成和加工過程中作為有害副產品排放。此前，清潔發展機制和各國努力呼吁實施減排技術以減少HFC-23排放。然而，2015至2019年間，大氣觀測得出的全球排放量與報告減排的預期排放量之間存在差異。這從根本上表明減排戰略執行不足，亟需對單個化工設施層面的排放進行獨立核查。

研究組使用區域大氣觀測和一種新型、故意釋放的示蹤劑來量化一個配備了廢氣銷毀技術的HCFC-22和含氟聚合物生產設施的HFC-23排放量。結果發現推斷的HFC-23/HCFC-22排放系數為0.19%（0.13~0.24%），大致符合減排項目認為可行的排放系數。對全球HCFC-22產量的外推強調，采用適當的銷毀技術有可能使全球HFC-23排放量至少減少84%（69~100%），即14（12~16）Gg/yr-1。這相當于2019年航空業17%的二氧化碳排放量。

研究組還演示了另一種副產品和溫室氣體PFC-318的協同銷毀。該研究結果凸顯了《蒙特利爾議定書》2016年基加利修正案的重要性，該修正案要求各締約方從2020年起“在可行的范圍內”銷毀生產氯氟烴和氟代烴生產設施排放的HFC-23。

▲ Abstract：

HFC-23 is a potent greenhouse gas, predominantly emitted as an undesired by-product during the synthesis and processing of HCFC-22. Previously, the Clean Development Mechanism and national efforts called for the implementation of abatement technology for reducing HFC-23 emissions. Nevertheless, between 2015 and 2019, a divergence was found between the global emissions derived from atmospheric observations and those expected from reported abatement. Primarily, this points to insufficient implementation of abatement strategies, calling for independent verification of the emissions at the individual chemical facility level. Here we use regional atmospheric observations and a new, deliberately released tracer to quantify the HFC-23 emissions from an HCFC-22 and fluoropolymer production facility, which is equipped with waste gas destruction technology. We find that our inferred HFC-23/HCFC-22 emission factor of 0.19% (0.13–0.24%) broadly fits within the emission factor considered practicable for abatement projects. Extrapolation to global HCFC-22 production underscores that the operation of appropriate destruction technology has the potential to reduce global HFC-23 emissions by at least 84% (69–100%) (14 (12–16)Gg yr-1). This reduction is equivalent to 17% CO2emissions from aviation in 2019. We also demonstrate co-destruction of PFC-318, another by-product and greenhouse gas. Our findings show the importance of the 2016 Kigali Amendment to the Montreal Protocol, which obligates parties to destroy HFC-23 emissions from facilities manufacturing hydrochlorofluorocarbons and hydrofluorocarbons “to the extent practicable” from 2020 onwards.

Highly variable magmatic accretion at the ultraslow-spreading Gakkel Ridge

超低速擴張蓋克爾洋脊上高度變化的巖漿增生

▲ 作者：Tao Zhang, Jiabiao Li, Xiongwei Niu, Weiwei Ding, Yinxia Fang, Jian Lin, et al.

▲ 鏈接：https://www.nature.com/articles/s41586-024-07831-0

▲摘要：

洋中脊的地殼增生支配著海洋巖石圈的形成和演化。普遍接受的被動地幔上涌和熔融模型預測，地殼厚度以小于20 mm/年的速度顯著減少。

研究組在北冰洋的蓋克爾洋脊率先進行了高分辨率海底地震儀（OBS）實驗，并對地球上擴張最慢的洋脊地殼結構進行了成像。出乎意料的是，他們發現沿洋脊軸的地殼厚度在3.3~8.9千米之間，在過去的5 Myr中，跨軸區域內的地殼厚度從約4.5千米增加到7.5千米。地殼厚度的高度變化和相對較大的平均值與被動地幔上涌模型的預測不一致。相反，它可用一個由熔體萃取引起的熱和成分密度變化驅動的浮力主動地幔流模型來解釋。

研究組預測主動和被動上涌的影響隨擴張速率的降低而增加。預計主動地幔上涌主要受地幔溫度和成分的影響。這表明觀測到的地殼增生的變化，包括地殼厚度的顯著變化，可能是超低速擴張脊的固有特征。

▲ Abstract：

Crustal accretion at mid-ocean ridges governs the creation and evolution of the oceanic lithosphere. Generally accepted models of passive mantle upwelling and melting predict notably decreased crustal thickness at a spreading rate of less than 20 mm year 1. We conducted the first, to our knowledge, high-resolution ocean-bottom seismometer (OBS) experiment at the Gakkel Ridge in the Arctic Ocean and imaged the crustal structure of the slowest-spreading ridge on the Earth. Unexpectedly, we find that crustal thickness ranges between 3.3 km and 8.9 km along the ridge axis and it increased from about 4.5 km to about 7.5 km over the past 5 Myr in an across-axis profile. The highly variable crustal thickness and relatively large average value does not align with the prediction of passive mantle upwelling models. Instead, it can be explained by a model of buoyant active mantle flow driven by thermal and compositional density changes owing to melt extraction. The influence of active versus passive upwelling is predicted to increase with decreasing spreading rate. The process of active mantle upwelling is anticipated to be primarily influenced by mantle temperature and composition. This implies that the observed variability in crustal accretion, which includes notably varied crustal thickness, is probably an inherent characteristic of ultraslow-spreading ridges.