自然（20220908出版）一周论文导读

　　编译 | 未玖
　　Nature , 8 September 2022, VOL 609, ISSUE 7926
　　《自然》 2022年9月8日，第609卷，7926期
　　天文学 Astronomy
　　Rapid quasi-periodic oscillations in the relativistic jet of BL Lacertae
　　蝎虎座BL相对论射流的快速准周期振荡
　　▲ 作者：S. G. Jorstad, A. P. Marscher, C. M. Raiteri, M. Villata, Z. R. Weaver, H. Zhang, et al.
　　▲ 链接：
　　https://www.nature.com/articles/s41586-022-05038-9
　　▲ 摘要：
　　Blazars是一类具有相对论喷流的活动星系核（AGN），其非热辐射在不同时间尺度上变化极大。这种变化似乎大部分是随机的，尽管在blazars和其他AGN中已报道过一些暗示系统过程的准周期振荡（QPOs）。时间尺度为天或小时的QPO在AGN中特别罕见，其性质备受争议，这可用射流中发射螺旋运动等离子体、等离子体不稳定性或吸积盘中的轨道运动来解释。
　　研究组报道了蝎虎座BL（BL Lac）在2020年一次剧烈爆发期间的强光学和γ射线通量监测结果。BL Lac是blazars亚类的原型，由一个椭圆星系（距离= 313兆秒差距）中的一个1.7×108 MSun 黑洞提供能量。研究组的观测结果显示，在爆发的最高状态下，光通量、线偏振和γ射线通量的QPOs周期短至约13 h。
　　在距随明显超光速特征沿射流向下移动的黑洞约5秒差距（pc）的再准直冲击附近，QPO的特性与流驱动的扭结不稳定性的预期相符。这种扭结在微波甚长基线阵列（VLBA）图像中很明显。
　　▲ Abstract：
　　Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales. This variability seems mostly random, although some quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGN. QPOs with timescales of days or hours are especially rare in AGN and their nature is highly debated, explained by emitting plasma moving helically inside the jet, plasma instabilities or orbital motion in an accretion disc. Here we report results of intense optical and γ-ray flux monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020. BL Lac, the prototype of a subclass of blazars, is powered by a 1.7 × 108 MSun  black hole in an elliptical galaxy (distance = 313 megaparsecs). Our observations show QPOs of optical flux and linear polarization, and γ-ray flux, with cycles as short as approximately 13 h during the highest state of the outburst. The QPO properties match the expectations of current-driven kink instabilities near a recollimation shock about 5 parsecs (pc) from the black hole in the wake of an apparent superluminal feature moving down the jet. Such a kink is apparent in a microwave Very Long Baseline Array (VLBA) image.
　　物理学 Physics
　　A sustained high-temperature fusion plasma regime facilitated by fast ions
　　快离子促进的持续高温聚变等离子体机制
　　▲ 作者：H. Han, S. J. Park, C. Sung, J. Kang, Y. H. Lee, J. Chung, et al.
　　▲ 链接：
　　https://www.nature.com/articles/s41586-022-05008-1
　　▲ 摘要：
　　核聚变是碳依赖能源最具吸引力的替代品之一。然而，尽管磁约束方法已经有了多年研究和稳步进展，但在大型反应堆中驾驭核聚变产生的能量仍面临许多科学挑战。目前最先进的磁聚变装置尚不能实现可持续的聚变性能，这需要超过1亿开尔文的高温和对不稳定性的充分控制，以确保在数十秒量级上的稳态运行。
　　研究组报道了在韩国超导托卡马克先进研究装置上进行的实验，产生了满足上述大多数要求的等离子体聚变机制：由于大量快离子稳定了核心等离子体湍流，他们在1亿开尔文的温度下产生等离子体，并持续了20秒，且没有等离子体边缘不稳定或杂质累积。
　　低等离子体密度与适当的操作输入功率相结合是通过保持高比例快离子来建立该机制的关键。这种机制很少受到干扰，即使没有精密控制也能稳定维持，因此代表了一条走向商业聚变反应堆的希望之路。
　　▲ Abstract：
　　Nuclear fusion is one of the most attractive alternatives to carbon-dependent energy sources. Harnessing energy from nuclear fusion in a large reactor scale, however, still presents many scientific challenges despite the many years of research and steady advances in magnetic confinement approaches. State-of-the-art magnetic fusion devices cannot yet achieve a sustainable fusion performance, which requires a high temperature above 100 million kelvin and sufficient control of instabilities to ensure steady-state operation on the order of tens of seconds. Here we report experiments at the Korea Superconducting Tokamak Advanced Research device producing a plasma fusion regime that satisfies most of the above requirements: thanks to abundant fast ions stabilizing the core plasma turbulence, we generate plasmas at a temperature of 100 million kelvin lasting up to 20 seconds without plasma edge instabilities or impurity accumulation. A low plasma density combined with a moderate input power for operation is key to establishing this regime by preserving a high fraction of fast ions. This regime is rarely subject to disruption and can be sustained reliably even without a sophisticated control, and thus represents a promising path towards commercial fusion reactors.
　　Imaging hydrodynamic electrons flowing without Landauer–Sharvin resistance
　　无Landauer-Sharvin电阻的流体动力学电子流成像
　　▲ 作者：C. Kumar, J. Birkbeck, J. A. Sulpizio, D. Perello, T. Taniguchi, K. Watanabe, et al.
　　▲ 链接：
　　https://www.nature.com/articles/s41586-022-05002-7
　　▲ 摘要：
　　电阻通常源于晶格缺陷。然而，即使是完美晶格也有一个基本电阻极限，由有限数量的传播电子模式引起的Landauer电导给出。Sharvin揭示的出现在电子设备触点处的电阻，设定了非相互作用电子的最终传导极限。近年来，流体动力学电子现象的证据越来越多，促使最近的理论开始质疑电子流体是否能从根本上突破基础Landauer-Sharvin极限。
　　研究组使用高迁移率石墨烯科比诺圆盘设备中的电子流实现单电子晶体管成像，回答了这个问题。首先，通过对液氦温度下的弹道流成像，他们观察到一个Landauer-Sharvin电阻，它未出现在触点，而是分布在整个块体中。这支持了从传导模式数量空间梯度中出现电阻的相空间起源。
　　在高温下，通过识别和解释电子-声子散射，研究组展示了纯流体动力学流动的细节。值得注意的是，他们发现电子流体动力学消除了块体Landauer-Sharvin电阻。最后，通过对螺旋磁流体动力学科比诺流的成像，研究组展示了流体动力学理论预测的关键涌现长度尺度——Gurzhi长度。
　　这些观察表明，电子流体可以极大地超越弹道电子的基本限制，这对基础科学和未来技术具有重要意义。
　　▲ Abstract：
　　Electrical resistance usually originates from lattice imperfections. However, even a perfect lattice has a fundamental resistance limit, given by the Landauer conductance caused by a finite number of propagating electron modes. This resistance, shown by Sharvin to appear at the contacts of electronic devices, sets the ultimate conduction limit of non-interacting electrons. Recent years have seen growing evidence of hydrodynamic electronic phenomena, prompting recent theories to ask whether an electronic fluid can radically break the fundamental Landauer–Sharvin limit. Here, we use single-electron-transistor imaging of electronic flow in high-mobility graphene Corbino disk devices to answer this question. First, by imaging ballistic flows at liquid-helium temperatures, we observe a Landauer–Sharvin resistance that does not appear at the contacts but is instead distributed throughout the bulk. This underpins the phase-space origin of this resistance—as emerging from spatial gradients in the number of conduction modes. At elevated temperatures, by identifying and accounting for electron–phonon scattering, we show the details of the purely hydrodynamic flow. Strikingly, we find that electron hydrodynamics eliminates the bulk Landauer–Sharvin resistance. Finally, by imaging spiralling magneto-hydrodynamic Corbino flows, we show the key emergent length scale predicted by hydrodynamic theories—the Gurzhi length. These observations demonstrate that electronic fluids can dramatically transcend the fundamental limitations of ballistic electrons, with important implications for fundamental science and future technologies.
　　材料科学 Materials Science
　　Exciton-coupled coherent magnons in a 2D semiconductor
　　二维半导体中激子耦合相干磁子
　　▲ 作者：Youn Jue Bae, Jue Wang, Allen Scheie, Junwen Xu, Daniel G. Chica, Geoffrey M. Diederich, et al.
　　▲ 链接：
　　https://www.nature.com/articles/s41586-022-05024-1
　　▲ 摘要：
　　最近发现的二维（2D）磁体及其堆叠为范德华结构扩大了2D现象的范围。一个令人振奋的应用是利用相干磁子作为自旋电子学和磁子学中能效信息载体或作为混合量子系统中的互连。
　　当2D磁体同时也是半导体时，就会出现一个特殊机会，如最近报道的CrSBr和NiPS3 ，其特征是具有大振荡器强度的紧密束缚激子和由于带隙和空间约束而潜在的长寿命相干磁子。虽然磁子和激子在能量上失配了几个数量级，但二者耦合可导致对自旋信息的有效光学存取。
　　研究组报道了2D A型反铁磁半导体CrSBr中的强磁子-激子耦合。通过上述间隙激发发射的相干磁子调制激子能量。时间分辨激子传感显示磁子可以相干移动超过7微米，相干时间超过5纳秒。
　　研究组在偶数层和奇数层中观察到这些激子-耦合相干磁子（无论是否有补偿磁化），直至双层极限。考虑到范德华异质结构的多功能性，这些相干2D磁子可能是光学可存取的自旋电子学、磁子学和量子互连的基础。
　　▲ Abstract：
　　The recent discoveries of two-dimensional (2D) magnets and their stacking into van der Waals structures have expanded the horizon of 2D phenomena. One exciting application is to exploit coherent magnons as energy-efficient information carriers in spintronics and magnonics or as interconnects in hybrid quantum systems. A particular opportunity arises when a 2D magnet is also a semiconductor, as reported recently for CrSBr and NiPS3  that feature both tightly bound excitons with a large oscillator strength and potentially long-lived coherent magnons owing to the bandgap and spatial confinement. Although magnons and excitons are energetically mismatched by orders of magnitude, their coupling can lead to efficient optical access to spin information. Here we report strong magnon–exciton coupling in the 2D A-type antiferromagnetic semiconductor CrSBr. Coherent magnons launched by above-gap excitation modulate the exciton energies. Time-resolved exciton sensing reveals magnons that can coherently travel beyond seven micrometres, with a coherence time of above five nanoseconds. We observe these exciton-coupled coherent magnons in both even and odd numbers of layers, with and without compensated magnetization, down to the bilayer limit. Given the versatility of van der Waals heterostructures, these coherent 2D magnons may be a basis for optically accessible spintronics, magnonics and quantum interconnects.
　　化学 Chemistry
　　Intrinsically unidirectional chemically fuelled rotary molecular motors
　　内在单向化学驱动的旋转分子马达
　　▲ 作者：Ke Mo, Yu Zhang, Zheng Dong, Yuhang Yang, Xiaoqiang Ma, Ben L. Feringa, et al.
　　▲ 链接：
　　https://www.nature.com/articles/s41586-022-05033-0
　　▲ 摘要：
　　生物系统主要利用化学能驱动自主分子马达，使系统脱离平衡状态。从细菌鞭毛马达和三磷酸腺苷合酶等旋转马达中获得灵感，并在光驱动单向旋转分子马达获得成功的基础上，科学家们致力于设计完全由化学能驱动的合成分子马达。
　　然而，设计化学燃料自主驱动、具有内在结构设计元素以允许像三磷酸腺苷合酶那样进行360°单向旋转运动的人工旋转分子马达，仍极具挑战性。
　　研究组展示了一种具有三种不同立体化学元素的同手性联芳基马达-3，它是一种旋转马达，在化学燃料的驱动下，两个芳基围绕单键轴进行重复和单向360°旋转。它依次经历了酯环化、螺旋反转和开环，在自主旋转循环中实现了高达99%的单向性。
　　分子旋转马达可以在两种模式下运行：与化学燃料脉冲和酸碱振荡的同步运动；在微碱性水溶液条件下，在化学燃料存在时进行自主运动。这种旋转马达设计具有对旋转方向的内在控制、用于自主运动的简单化学燃料和近乎完美的单向性，表明了未来几代多组件机器执行机械功能的潜力。
　　▲ Abstract：
　　Biological systems mainly utilize chemical energy to fuel autonomous molecular motors, enabling the system to be driven out of equilibrium. Taking inspiration from rotary motors such as the bacterial flagellar motor and adenosine triphosphate synthase, and building on the success of light-powered unidirectional rotary molecular motors, scientists have pursued the design of synthetic molecular motors solely driven by chemical energy. However, designing artificial rotary molecular motors operating autonomously using a chemical fuel and simultaneously featuring the intrinsic structural design elements to allow full 360° unidirectional rotary motion like adenosine triphosphate synthase remains challenging. Here we show that a homochiral biaryl Motor-3, with three distinct stereochemical elements, is a rotary motor that undergoes repetitive and unidirectional 360° rotation of the two aryl groups around a single-bond axle driven by a chemical fuel. It undergoes sequential ester cyclization, helix inversion and ring opening, and up to 99% unidirectionality is realized over the autonomous rotary cycle. The molecular rotary motor can be operated in two modes: synchronized motion with pulses of a chemical fuel and acid–base oscillations; and autonomous motion in the presence of a chemical fuel under slightly basic aqueous conditions. This rotary motor design with intrinsic control over the direction of rotation, simple chemical fuelling for autonomous motion and near-perfect unidirectionality illustrates the potential for future generations of multicomponent machines to perform mechanical functions.
　　地球科学 Earth Science
　　Delayed use of bioenergy crops might threaten climate and food security
　　推迟利用生物能源作物可能会威胁气候和粮食安全
　　▲ 作者：Siqing Xu, Rong Wang, Thomas Gasser, Philippe Ciais, Josep Peñuelas, Yves Balkanski, et al.
　　▲ 链接：
　　https://www.nature.com/articles/s41586-022-05055-8
　　▲ 摘要：
　　将全球升温控制在2℃以内缓解行动的潜力或依赖于充足的生物质供应，用于大规模（假设在未来显著扩大）生物能源的碳捕获和存储（BECCS）。然而，气候变化对作物产量的不利影响可能会降低BECCS的能力，威胁粮食安全，从而对全球变暖形成一个易忽略的正反馈循环。
　　研究组通过在一个紧凑的地球系统模型中实施作物产量对增加生长季温度、大气CO2 浓度和氮（N）施肥强度的响应来量化这种反馈的强度。超过气候变化阈值将危及气候稳定并威胁粮食安全，从而导致社会-生态系统的变革性变化。
　　如果将伴随大规模BECCS的全球减缓推迟到2060年（届时全球升温超过约2.5℃），那么BECCS的农业剩余物产量将过低，以至于无法实现2200年2℃的《巴黎协定》目标。持续的粮食需求放大了这种失败的风险，导致扩大耕地或强化氮肥，以弥补气候导致的产量损失。
　　因此，该研究结果强调了尽早缓解气候变化的紧迫性（最好是在2040年前），以避免不可逆转的气候变化和严重的粮食危机，除非在不久的将来出现其他负排放技术能弥补BECCS能力的下降。
　　▲ Abstract：
　　The potential of mitigation actions to limit global warming within 2 °C might rely on the abundant supply of biomass for large-scale bioenergy with carbon capture and storage (BECCS) that is assumed to scale up markedly in the future. However, the detrimental effects of climate change on crop yields may reduce the capacity of BECCS and threaten food security, thus creating an unrecognized positive feedback loop on global warming. We quantified the strength of this feedback by implementing the responses of crop yields to increases in growing-season temperature, atmospheric CO2  concentration and intensity of nitrogen (N) fertilization in a compact Earth system model. Exceeding a threshold of climate change would cause transformative changes in social–ecological systems by jeopardizing climate stability and threatening food security. If global mitigation alongside large-scale BECCS is delayed to 2060 when global warming exceeds about 2.5 °C, then the yields of agricultural residues for BECCS would be too low to meet the Paris goal of 2 °C by 2200. This risk of failure is amplified by the sustained demand for food, leading to an expansion of cropland or intensification of N fertilization to compensate for climate-induced yield losses. Our findings thereby reinforce the urgency of early mitigation, preferably by 2040, to avoid irreversible climate change and serious food crises unless other negative-emission technologies become available in the near future to compensate for the reduced capacity of BECCS.