编译|未玖

Nature,  5 September 2024, Volume 633, Issue 8028

《自然》2024年9月5日，第633卷，8028期

天文学Astronomy

Gravitational instability in a planet-forming disk

行星形成盘的引力不稳定性

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

▲链接：

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

▲摘要：

关于行星在星周盘中形成的正则理论提出，行星最初是由小得多的种子长成的。长期考虑的替代理论提出，巨大的原行星可由引力不稳定引起的巨大旋臂的坍塌碎片直接形成——如果星周盘引力不稳定的话。

要实现这一点，星周盘的质量必须比中心恒星大得多：星周盘与恒星的质量比为1:10被广泛认为是触发引力不稳定性、激发大量非开普勒动力学、产生明显旋臂的粗略阈值。虽然估计星周盘质量历来颇具挑战性，但气体的运动可通过其对盘速度结构的影响揭示引力不稳定性的存在。

研究组利用阿塔卡马大型毫米波/亚毫米波阵列（ALMA）对13CO和C18O谱线发射的深入观测，提出了AB Aurigae星周盘引力不稳定的运动学证据。观测到的运动信号与模拟和分析建模的预测非常相似。通过定量比较，研究组推断出在天空中1″到5″范围内的星周盘质量高达恒星质量的三分之一。

▲ 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.

物理学Physics

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

魔角扭曲双层石墨烯的长寿命同位旋激发

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

▲链接：

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

▲摘要：

人们已报道了魔角扭曲双层石墨烯（MATBG）诸多相关的多体相，包括传统相和奇异相。与这些相关态有关的动力学对于理解潜在的物理学至关重要，但尚未得到探索。

研究组结合激子传感和光泵浦-探针光谱来研究具有WSe2基底的MATBG在整个平带上的同位旋序动力学，达到了亚皮秒分辨率。结果观察到，在ν=2附近和ν=-3和-2之间的宽填充范围内，同位旋动力学非常缓慢，寿命高达300 ps，与电子温度更快的冷却（约10 ps）解耦。这种非热行为表明在同位旋自由度中存在异常的长寿命模式。

这一出乎理论预测的观察结果，表明了长程传播集体模式、强同位旋涨落和记忆效应的存在，并可能与谷间相干或不匹配的凯库勒螺旋基态有关。研究组进一步证明了先前在整数填充周围发现的同位旋序的非平衡控制。具体来说，通过超快操作，可暂时偏离整数填充。该研究展示了MATBG中集体激发的独特探索，并为主动控制莫尔系统中的非平衡现象铺平了道路。

▲ 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.

材料科学Materials Science

Photoredox phase engineering of transition metal dichalcogenides

过渡金属二硫族化物的光氧化还原相工程

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

▲链接：

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

▲摘要：

晶体相工程在精确控制材料的物理和电子特性方面起着重要作用。在二维过渡金属二硫族化物（2D TMDs）中，利用有机金属化剂正丁基锂（n-BuLi）进行化学锂化，将半导体2H（三角）相转化为金属1T（八面体）相的相工程已在晶体管、催化和电池等领域得到了广泛探索应用。虽然这种化学相工程可在环境温度和压力下进行，但其潜在机制尚不清楚，并且n-BuLi的使用引发了明显的安全问题。

研究组光学可视化了单层和双层二维TMDs中从2H相到1T相的典型相变，并发现使用455 nm的低功率照明可将该反应加速多达6个数量级。结果发现，间隙上方的照明通过光氧化还原过程改善了限速电荷转移动力学。研究组使用该方法来实现快速和高质量的相位工程，并证明其可利用衍射限制边缘分辨率将任意相图案刻入少层TMDs中。

最后，他们用更安全的多环芳香族有机锂试剂取代了自燃性n-BuLi，并表明其作为相变剂的性能优于n-BuLi。该工作为探索电化学过程的原位表征提供了机会，并为通过光氧化还原相工程可持续地扩大材料和器件的规模铺平了道路。

▲ 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.

化学Chemistry

Catalytic asymmetric synthesis of meta benzene isosteres

间苯等排体的催化不对称合成

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

▲链接：

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

▲摘要：

虽然芳香环是药物活性化合物中的常见元素，但这些基序的存在给药物的可开发性带来了一些不利因素。用非芳香族等排基序取代芳香环可改善药物化合物的非最佳效价、代谢稳定性、溶解度和亲脂性。

此外，芳香环是平面的，缺乏三维性，但大多数药物靶点的结合口袋是手性的。因此，等排替代的立体化学构型可能为提高衍生配体对靶受体的亲和力提供一个额外机会。这种方法的一个显著障碍是缺乏从现成前体中简单和可扩展地催化对映选择性合成候选同位异构体。

研究组提出了一种以前未知的钯催化反应，将烃类衍生的前体转化为手性含硼的去甲三环烃，并表明这些去甲三环烃的形状使其成为间二取代芳环的合理等排异构体。使用手性催化剂，钯催化的反应能够以对映选择性的方式完成，后续的硼基转化提供了获得广泛结构的途径。

研究组还表明，将去甲三环类掺入药物基序可改善生物物理性质以及立体化学依赖的活性。他们预计这些特征加上简单、廉价的功能化去甲三环支架的合成，将使该平台成为组装新生物活性物质的坚实基础。

▲ 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.

地球科学Earth Science

Effective realization of abatement measures can reduce HFC-23 emissions

有效实施减排措施可减少HFC-23的排放

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

▲链接：

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

▲摘要：

HFC-23是一种强效温室气体，主要在HCFC-22的合成和加工过程中作为有害副产品排放。此前，清洁发展机制和各国努力呼吁实施减排技术以减少HFC-23排放。然而，2015至2019年间，大气观测得出的全球排放量与报告减排的预期排放量之间存在差异。这从根本上表明减排战略执行不足，亟需对单个化工设施层面的排放进行独立核查。

研究组使用区域大气观测和一种新型、故意释放的示踪剂来量化一个配备了废气销毁技术的HCFC-22和含氟聚合物生产设施的HFC-23排放量。结果发现推断的HFC-23/HCFC-22排放系数为0.19%（0.13~0.24%），大致符合减排项目认为可行的排放系数。对全球HCFC-22产量的外推强调，采用适当的销毁技术有可能使全球HFC-23排放量至少减少84%（69~100%），即14（12~16）Gg/yr-1。这相当于2019年航空业17%的二氧化碳排放量。

研究组还演示了另一种副产品和温室气体PFC-318的协同销毁。该研究结果凸显了《蒙特利尔议定书》2016年基加利修正案的重要性，该修正案要求各缔约方从2020年起“在可行的范围内”销毁生产氯氟烃和氟代烃生产设施排放的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

超低速扩张盖克尔洋脊上高度变化的岩浆增生

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

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

▲摘要：

洋中脊的地壳增生支配着海洋岩石圈的形成和演化。普遍接受的被动地幔上涌和熔融模型预测，地壳厚度以小于20 mm/年的速度显著减少。

研究组在北冰洋的盖克尔洋脊率先进行了高分辨率海底地震仪（OBS）实验，并对地球上扩张最慢的洋脊地壳结构进行了成像。出乎意料的是，他们发现沿洋脊轴的地壳厚度在3.3~8.9千米之间，在过去的5 Myr中，跨轴区域内的地壳厚度从约4.5千米增加到7.5千米。地壳厚度的高度变化和相对较大的平均值与被动地幔上涌模型的预测不一致。相反，它可用一个由熔体萃取引起的热和成分密度变化驱动的浮力主动地幔流模型来解释。

研究组预测主动和被动上涌的影响随扩张速率的降低而增加。预计主动地幔上涌主要受地幔温度和成分的影响。这表明观测到的地壳增生的变化，包括地壳厚度的显著变化，可能是超低速扩张脊的固有特征。

▲ 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.