编译 李言 Nature, 7 May 2020, Volume 581 Issue 7806 《自然》2020年5月7日，第581卷，7805期 材料科学 Materials Science Mapping the twist-angle disorder and Landau levels in magic-angle graphene 魔角石墨烯中的扭曲角无序和朗道能级 ▲ 作者：A. Uri, S. Grover, Y. Cao, J. A. Crosse, K. Bagani, D. Rodan-Legrain, et al. ▲ 链接：https://www.nature.com/articles/s41586-020-2255-3 ▲ 摘要 最近发现的扁平电子能带和强烈的相关性和超导阶段魔角扭曲双层石墨烯（MATBG）最关键的是取决于中间层扭曲角θ。虽然整体θ的控制精度约为0.1度已经被证明，但对局部扭曲角分布所知的信息还很少。 在此，我们使用纳米级针尖扫描超导量子干涉装置（SQUID-on-tip）获得处于量子霍尔态的朗道能级的断层图像，并绘制六方氮化硼（hBN）封装的MATBG器件的局部θ变化图，其相对精度达到0.002度，并且空间分辨率为几个莫尔周期。 我们发现发现了θ无序程度与MATBG传输特性的质量之间的相关性，即使最先进的设备，其θ局部变化也高于0.1°，表现出大量的梯度和迁跃网络，并可能包含没有MATBG行为的局部区域。 我们观察到，相对于扭曲角，MATBG中的相关状态特别脆弱。我们还发现，θ的梯度会产生在金属区域也不会被屏蔽的大栅极可调谐的平面内电场，通过形成边缘通道而改变量子霍尔态，并可能会影响相关态和超导态的相图。 因此，我们建立了θ无序作为一种非传统的无序类型的重要性，使扭曲角梯度能够用于带状结构工程，以用于器件应用的门可调谐内置平面电场。 ▲ Abstract The recently discovered flat electronic bands and strongly correlated and superconducting phases in magic-angle twisted bilayer graphene (MATBG) crucially dep on the interlayer twist angle, θ. Although control of the global θ with a precision of about 0.1 degrees has been demonstrated, little information is available on the distribution of the local twist angles. Here we use a nanoscale on-tip scanning superconducting quantum interference device (SQUID-on-tip) to obtain tomographic images of the Landau levels in the quantum Hall state and to map the local θ variations in hexagonal boron nitride (hBN)-encapsulated MATBG devices with relative precision better than 0.002 degrees and a spatial resolution of a few moiré periods. We find a correlation between the degree of θ disorder and the quality of the MATBG transport characteristics and show that even state-of-the-art devices—which exhibit correlated states, Landau fans and superconductivity—display considerable local variation in θ of up to 0.1 degrees, exhibiting substantial gradients and networks of jumps, and may contain areas with no local MATBG behaviour. We observe that the correlated states in MATBG are particularly fragile with respect to the twist-angle disorder. We also show that the gradients of θ generate large gate-tunable in-plane electric fields, unscreened even in the metallic regions, which profoundly alter the quantum Hall state by forming edge channels in the bulk of the sample and may affect the phase diagram of the correlated and superconducting states. We thus establish the importance of θ disorder as an unconventional type of disorder enabling the use of twist-angle gradients for bandstructure engineering, for realization of correlated phenomena and for gate-tunable built-in planar electric fields for device applications. Iron-based binary ferromagnets for transverse thermoelectric conversion 用于横向热电转换的铁基二元铁磁体 ▲ 作者：Akito Sakai, Susumu Minami, Takashi Koretsune, Taishi Chen, Tomoya Higo, et al. ▲ 链接：https://www.nature.com/articles/s41586-020-2230-z ▲ 摘要 在电子结构中，反常能斯特效应（ANE）来源于费米能量附近传导电子的贝里曲率。为了设计一个大的贝里曲率，几种方式正在考虑使用动量空间中的节点和线。 在此，我们演示了一种高通量计算搜索，并发现在α铁中掺杂25％的铝/镓能够将ANE的强度提高十倍以上，在室温下分别达到约4和6 μV/K，这接近目前报道的最高值。 理论与实验结果的对比表明，节点网的费米能调谐是横向热电系数强增强的关键（节点网是由相互连接的节点线构成的扁平带结构），其值达到约5A每开尔文每米，与温度呈对数关系。 我们还成功地制作了在零场中显示出大ANE的薄膜，这可能适用于设计低成本、灵活的微电子热电发电机。 ▲ Abstract In terms of the electronic structure, the anomalous Nernst effect (ANE) originates from the Berry curvature of the conduction electrons near the Fermi energy. To design a large Berry curvature, several approaches have been considered using nodal points and lines in momentum space. Here we perform a high-throughput computational search and find that 25 percent doping of aluminium and gallium in alpha iron, a naturally abundant and low-cost element, dramatically enhances the ANE by a factor of more than ten, reaching about 4 and 6 microvolts per kelvin at room temperature, respectively, close to the highest value reported so far. The comparison between experiment and theory indicates that the Fermi energy tuning to the nodal web—a flat band structure made of interconnected nodal lines—is the key for the strong enhancement in the transverse thermoelectric coefficient, reaching a value of about 5 amperes per kelvin per metre with a logarithmic temperature depence. We have also succeeded in fabricating thin films that exhibit a large ANE at zero field, which could be suitable for designing low-cost, flexible microelectronic thermoelectric generators. 物理学Physics Laser spectroscopy of pionic helium atoms π介子氦原子的激光光谱分析 ▲ 作者：Masaki Hori, Hossein Aghai-Khozani, Anna Sótér, Andreas Dax & Daniel Barna ▲ 链接：https://www.nature.com/articles/s41586-020-2240-x#citeas ▲ 摘要 带电介子是最轻和寿命最长的介子。当原子中的轨道电子被带负电荷的介子取代时，就形成了介子原子。π4He+原子被预测拥有一个异常长的纳秒量“寿命”，可以用激光光谱测量。 其原子结构由于缺乏自旋为0的π−和氦核的超精细相互作用而独特。此次，我们将π4He+设为超流体氦的目标，并在近红外共振频率为183,760千兆赫时激发π4He+ 的轨道(n, l) = (17, 16) → (17, 15) 。激光启动电磁级联过程结尾原子核吸收π−并发生裂变。 对产生的种子、质子及氘核片段的检测中发现了激光诱导的原子共振的信号，因此证实了π4He+的存在。这项工作使得利用量子光学的实验技术来研究介子成为可能。 ▲ Abstract Charged pions are the lightest and longest-lived mesons. Mesonic atoms are formed when an orbital electron in an atom is replaced by a negatively charged meson. The π4He+ atom is predicted to have an anomalously long nanosecond-scale lifetime, which could allow laser spectroscopy to be carried out. Its atomic structure is unique owing to the absence of hyperfine interactions between the spin-0 π− and the 4He nucleus. Here we synthesize π4He+ in a superfluid-helium target and excite the transition (n, l) = (17, 16) → (17, 15) of the π—occupied π4He+ orbital at a near-infrared resonance frequency of 183,760 gigahertz. The laser initiates electromagnetic cascade processes that with the nucleus absorbing the π− and undergoing fission. The detection of emerging neutron, proton and deuteron fragments signals the laser-induced resonance in the atom, thereby confirming the presence of π4He+. This work enables the use of the experimental techniques of quantum optics to study a meson. Detection of metastable electronic states by Penning trap mass spectrometry 用潘宁阱质谱法检测亚稳态电子态 ▲ 作者：R. X. Schüssler, H. Bekker, M. Braß, H. Cakir, J. R. Crespo López-Urrutia, et al. ▲ 链接：https://www.nature.com/articles/s41586-020-2221-0 ▲ 摘要 不够精确的原子结构计算阻碍了HCIs中适当跃迁的识别。在此，我们报告了通过测量铼的基态和激发态之间的质量差，观察到HCI中一个长期存在的亚稳态电子态，提供了一种非破坏性的、直接测定电子激发态能量的方法。其计算结果与高级计算结果一致。 我们使用高精度潘宁阱质谱仪PENTATRAP测量离子基态与亚稳态的回旋频率比，精度为10−11，比以前的测量提高了10倍。寿命约130天，潜在的软x射线频率参考在4.96×1016赫兹（对应于202电子伏特的转换能量）只有5×10−8赫兹的线宽，是迄今为止实验测量的最高电子质量因子之一（1024）。 我们的方法的低不确定性将使搜索在HCIs中进一步的软x射线时钟跃迁，这是基础物理的精确研究所需要的。 ▲ Abstract Insufficiently accurate atomic structure calculations hinder the identification of suitable transitions in HCIs. Here we report the observation of a long-lived metastable electronic state in an HCI by measuring the mass difference between the ground and excited states in rhenium, providing a non-destructive, direct determination of an electronic excitation energy. The result is in agreement with advanced calculations. We use the high-precision Penning trap mass spectrometer PENTATRAP to measure the cyclotron frequency ratio of the ground state to the metastable state of the ion with a precision of 10−11—an improvement by a factor of ten compared with previous measurements. With a lifetime of about 130 days, the potential soft-X-ray frequency reference at 4.96 × 1016 hertz (corresponding to a transition energy of 202 electronvolts) has a linewidth of only 5 × 10−8 hertz and one of the highest electronic quality factors (1024) measured experimentally so far. The low uncertainty of our method will enable searches for further soft-X-ray clock transitions in HCIs, which are required for precision studies of fundamental physics. Liquid flow and control without solid walls 无固体壁的液体流动与控制 ▲ 作者：Peter Dunne, Takuji Adachi, Arvind Arun Dev, Alessandro Sorrenti, Lucas Giacchetti, et al. ▲ 链接：https://www.nature.com/articles/s41586-020-2254-4 ▲ 摘要 当将流体电路小型化时，流体通道的固体壁变得越来越重要，因为它们限制了给定压降下的流速，并且容易产生污垢。在这里，我们演示了一种方法，在水的液体通道被非混相磁液体包围，两者都被一个四极磁场稳定。 这就创造了自愈合、不堵塞、防污和几乎无摩擦的液中液流体通道。对磁场的控制可以控制水流，例如分开水流、聚合水流或泵水。后者是通过移动永磁体实现的，永磁体与液体通道没有物理接触。 我们证明，这种磁势泵送方法可以用来传输人类血液，并几乎不产生因剪切力导致的损伤。溶血（血细胞破裂）与传统的蠕动泵送相比减少了一个数量级，传统蠕动泵送是通过塑料管机械地挤压血液。 ▲ Abstract When miniaturizing fluidic circuitry, the solid walls of the fluid channels become increasingly important because they limit the flow rates achievable for a given pressure drop, and they are prone to fouling. Here we demonstrate an approach in which aqueous liquid channels are surrounded by an immiscible magnetic liquid, both of which are stabilized by a quadrupolar magnetic field. This creates self-healing, non-clogging, anti-fouling and near-frictionless liquid-in-liquid fluidic channels. Manipulation of the field provides flow control, such as valving, splitting, merging and pumping. The latter is achieved by moving permanent magnets that have no physical contact with the liquid channel. We show that this magnetostaltic pumping method can be used to transport whole human blood with very little damage due to shear forces. Haemolysis (rupture of blood cells) is reduced by an order of magnitude compared with traditional peristaltic pumping, in which blood is mechanically squeezed through a plastic tube. 环境学Environmentology Millennial-scale hydroclimate control of tropical soil carbon storage 热带土壤碳储量的千年尺度水文气候控制 ▲ 作者：Christopher J. Hein, Muhammed Usman, Timothy I. Eglinton, et al. ▲ 链接：https://www.nature.com/articles/s41586-020-2233-9 ▲ 摘要 陆地生物圈中有机碳的储存在很大的时间尺度上直接影响大气中二氧化碳的浓度。在陆地生物圈内，碳储量的大小会随着环境（如温度或水文气候）的变化而变化，并对大气中的二氧化碳储量产生反馈。 在此，我们展示了过去18000年中印度夏季季风降雨的变化对恒河-雅鲁藏布江流域土壤碳停留时间的控制。放射性碳年代的大块有机碳和陆生高等植物生物标记物的共定位古水文记录的对比，揭示了季风降雨与土壤有机碳储量在千禧年尺度上的负相关关系。 在整个冰消期，降雨增加和相关的土壤呼吸速率增强引起了全流域土壤碳储量的耗竭。我们的结果表明，未来热带地区的水文气候变化可能会加速土壤碳的不稳定，从而进一步增加大气中的二氧化碳浓度。 ▲ Abstract The storage of organic carbon in the terrestrial biosphere directly affects atmospheric concentrations of carbon dioxide over a wide range of timescales. Within the terrestrial biosphere, the magnitude of carbon storage can vary in response to environmental perturbations such as changing temperature or hydroclimate, potentially generating feedback on the atmospheric inventory of carbon dioxide. Here we show that changes in Indian Summer Monsoon rainfall have controlled the residence time of soil carbon in the Ganges–Brahmaputra basin over the past 18,000 years. Comparison of radiocarbon ages of bulk organic carbon and terrestrial higher-plant biomarkers with co-located palaeohydrological records reveals a negative relationship between monsoon rainfall and soil organic carbon stocks on a millennial timescale. Across the deglaciation period, a depletion of basin-wide soil carbon stocks was triggered by increasing rainfall and associated enhanced soil respiration rates. Our results suggest that future hydroclimate changes in tropical regions are likely to accelerate soil carbon destabilization, further increasing atmospheric carbon dioxide concentrations. 古生物学Paleobiology Tail-propelled aquatic locomotion in a theropod dinosaur 兽脚亚目恐龙具有可用于水上运动的尾部 ▲ 作者：Nizar Ibrahim, Simone Maganuco, Cristiano Dal Sasso, Matteo Fabbri, et al. ▲ 链接：https://www.nature.com/articles/s41586-020-2190-3 ▲ 摘要 近期有人认为，至少有一些棘龙（白垩纪时期的大型肢体兽脚类群）是半水生的，这一观点在解剖学、生物力学和分类学上都受到了挑战，至今仍有争议。 在此，我们明确证明了恐龙（即巨型兽脚亚目恐龙埃及棘龙）有水生推进结构。这种恐龙有一个形状独特的尾巴，由非常高的神经棘和细长的线形组成，形成一个大而灵活的鳍状器官，能够广泛的横向移动。 使用机器人拍打设备测量不同尾巴形状物理模型的起伏力后，我们发现，棘龙的微博形状在水中比陆生恐龙的尾形产生更大的推力和效率，并且这些性能指标与现存的水生脊椎动物的可比性更强，后者使用垂直扩展的尾巴在游泳时产生正向推进力。这些结果与之前文献记载的棘龙适应水生生活方式和鱼食性饮食的系列相一致。 ▲ Abstract It has recently been argued that at least some of the spinosaurids—an unusual group of large-bodied theropods of the Cretaceous era—were semi-aquatic, but this idea has been challenged on anatomical, biomechanical and taphonomic grounds, and remains controversial. Here we present unambiguous evidence for an aquatic propulsive structure in a dinosaur, the giant theropod Spinosaurus aegyptiacus. This dinosaur has a tail with an unexpected and unique shape that consists of extremely tall neural spines and elongate chevrons, which forms a large, flexible fin-like organ capable of extensive lateral excursion. Using a robotic flapping apparatus to measure undulatory forces in physical models of different tail shapes, we show that the tail shape of Spinosaurus produces greater thrust and efficiency in water than the tail shapes of terrestrial dinosaurs and that these measures of performance are more comparable to those of extant aquatic vertebrates that use vertically expanded tails to generate forward propulsion while swimming. These results are consistent with the suite of adaptations for an aquatic lifestyle and piscivorous diet that have previously been documented for Spinosaurus. 医学Medical Science Childhood vaccines and antibiotic use in low- and middle-income countries 低收入和中等收入国家的儿童疫苗和抗生素使用情况 ▲ 作者：Joseph A. Lewnard, Nathan C. Lo, Nimalan Arinaminpathy, Isabel Frost & Ramanan Laxminarayan ▲ 链接：https://www.nature.com/articles/s41586-020-2238-4 ▲ 摘要 疫苗可以减轻抗菌素耐药性的负担，部分原因是通过预防感染（治疗往往包括使用抗生素）。 然而，接种疫苗对抗生素消费的影响仍然缺乏认识，特别是在低收入和中等收入国家（LMICs），那里的抗菌素耐药性负担最大。在此，我们发现，世界卫生组织近期的扩大免疫规划中实施的疫苗大大减少了中低收入国家5岁以下儿童的抗生素消耗。 从家庭的大规模研究通过分析数据，我们估计在急性呼吸道感染和腹泻发生最严重的年龄组中，肺炎球菌结合疫苗和减毒轮状病毒活疫苗可提供19.7％（95％置信区间，3.4-43.4％）和11.4％（4.0-18.6％）的抗生素治疗保护。 在目前的覆盖水平下，肺炎球菌结合疫苗和减毒轮状病毒活疫苗每年在中低收入国家5岁以下儿童中分别预防了2380万起和1360万起抗生素治疗疾病。由于实现了这些疫苗的普遍覆盖目标而产生的直接保护，可以预防额外出现4000万次经抗生素治疗的疾病。 ▲ Abstract Vaccines may reduce the burden of antimicrobial resistance, in part by preventing infections for which treatment often includes the use of antibiotics. However, the effects of vaccination on antibiotic consumption remain poorly understood—especially in low- and middle-income countries (LMICs), where the burden of antimicrobial resistance is greatest. Here we show that vaccines that have recently been implemented in the World Health Organization’s Expanded Programme on Immunization reduce antibiotic consumption substantially among children under five years of age in LMICs. By analysing data from large-scale studies of households, we estimate that pneumococcal conjugate vaccines and live attenuated rotavirus vaccines confer 19.7% (95% confidence interval, 3.4–43.4%) and 11.4% (4.0–18.6%) protection against antibiotic-treated episodes of acute respiratory infection and diarrhoea, respectively, in age groups that experience the greatest disease burden attributable to the vaccine-targeted pathogens. Under current coverage levels, pneumococcal and rotavirus vaccines prevent 23.8 million and 13.6 million episodes of antibiotic-treated illness, respectively, among children under five years of age in LMICs each year. Direct protection resulting from the achievement of universal coverage targets for these vaccines could prevent an additional 40.0 million episodes of antibiotic-treated illness.