编译 | 李言
nature, 3 Dec++ember 2020，Volume 588 Issue 7836
《自然》2020年12月3日，第588卷，7836期
物理学Physics
Determination of the fine-structure constant with an accuracy of 81 parts per trillion
测定精细结构常数，精度为万亿分之81
▲ 作者：Léo Morel, Zhibin Yao, Pierre Cladé & Saïda guellati-Khélifa
▲ 链接：
https://www.nature.com/articles/s41586-020-2964-7
▲ 摘要
粒子物理学的标准模型非常成功，因为它（几乎）与所有的实验结果一致。然而，它不能解释暗物质、暗能量以及宇宙中物质和反物质之间的不平衡。
在这里，我们使用物波干涉测量法来测量一个铷原子吸收一个光子的反冲速度，并确定了精细结构常数α−1 = 137.035999206(11)，相对精度为万亿分之81。
11位数字的准确性导致了电子g因子——标准模型中最精确的预测——极大地减少了不确定性。我们的精细结构常数值与铯反冲测量的最佳结果相差5个标准差以上。
我们的结果修正了对可能的候选暗物质粒子的限制，这些暗物质粒子被提议用来解释8Be原子核激发态的异常衰变，并且为测试电子部门中观测到的介子磁矩异常的差异铺平了道路。
▲ Abstract
The standard model of particle physics is remarkably successful because it is consistent with (almost) all experimental results. However, it fails to explain dark matter, dark energy and the imbalance between matter and antimatter in the Universe. here we use matter-wave interferometry to measure the recoil velocity of a rubidium atom that absorbs a photon, and determine the fine-structure constant α−1 = 137.035999206(11) with a relative accuracy of 81 parts per trillion. The accuracy of eleven digits in α leads to an electron g factor—the most precise prediction of the standard model—that has a greatly reduced uncertainty. Our value of the fine-structure constant differs by more than 5 standard deviations from the best available result from caesium recoil measurements. Our result modifies the constraints on possible candidate dark-matter particles proposed to explain the anomalous decays of excited states of 8Be nuclei and paves the way for testing the discrepancy observed in the magnetic moment anomaly of the muon in the electron sector.
材料科学material science
Electrical switching of magnetic order in an orbital Chern insulator
轨道Chern绝缘子中的超常规磁性切换
▲ 作者：H. Polshyn, J. Zhu, M. A. Kumar, Y. Zhang, F. Yang, C. L. Tschirhart, M. Serlin, K. Watanabe, T. Taniguchi, A. H. macDonald & A. F. Young
▲ 链接：
https://www.nature.com/articles/s41586-020-2963-8
▲ 摘要
磁性通常是由费米统计和排斥性库仑相互作用共同产生的，这有利于基态与非零电子自旋。因此，只能间接地实现利用电场控制自旋磁性——自旋电子学和多铁学的长期技术目标。
在此，我们实验演示了在一个非普通带拓扑有利于轨道角动量的长程顺序，但自旋被认为是无序的磁系统中的磁态的直接电场控制。在带内每莫尔元胞中填充一和三个电子时，我们发现通过对化学势的场效应控制可以逆转量子反常霍尔效应的符号；此外，这种跃迁具有迟滞性，我们用迟滞性来演示非挥发性电场诱导的磁态反转。
理论分析表明，这种效应是由拓扑边缘状态引起的，它引起磁化符号的变化，从而使有利的磁态发生逆转。
▲ Abstract
Magnetism typically arises from the joint effect of Fermi statistics and repulsive Coulomb interactions, which favours ground states with non-zero electron spin. As a result, controlling spin magnetism with electric fields—a longstanding technological goal in spintronics and multiferroics—can be achieved only indirectly. Here we experimentally demonstrate direct electric-field control of magnetic states in an orbital Chern insulator, a magnetic system in which non-trivial band topology favours long-range order of orbital angular momentum but the spins are thought to remain disordered. At a filling of three electrons per moiré unit cell, we find that the sign of the quantum anomalous Hall effect can be reversed via field-effect control of the chemical potential; moreover, this transition is hysteretic, which we use to demonstrate non-volatile electric-field-induced reversal of the magnetic state. A theoretical analysis indicates that the effect arises from the topological edge states, which drive a change in sign of the magnetization and thus a reversal in the favoured magnetic state.
Unconventional ferroelectricity in moiré heterostructures
莫尔异质结中的反常铁电性
▲ 作者：Zhiren Zheng, Qiong Ma, Zhen Bi, Sergio de la Barrera, Ming-Hao Liu et al.
▲ 链接：
https://www.nature.com/articles/s41586-020-2970-9
▲ 摘要
组成物质的粒子可以以不同的方式排列自己，产生多种奇异的现象，这些现象往往无法通过研究单个的组成部分来理解。
在此，我们报告了在石墨烯型莫尔异质结构中意外出现的铁电现象。铁电材料表现出电可转换的电偶极子，通常是由单元内正电荷和负电荷平均中心的空间分离而形成的。
在这个基础上，很难想象石墨烯——一种仅由碳原子组成的材料——会表现出铁电性。然而，在这项工作中，我们实现了夹在两个六方氮化硼层之间的伯纳层双层石墨烯的可切换铁电性。
通过引入莫尔超晶格势（通过使双层石墨烯与顶部和/或底部氮化硼晶体对齐），我们观察到石墨烯电阻在外部施加平面外位移场时显著而强劲的磁滞行为。
▲ Abstract
The constituent particles of matter can arrange themselves in various ways, giving rise to emergent phenomena that can be surprisingly rich and often cannot be understood by studying only the individual constituents. Here we report on the surprising observation of emergent ferroelectricity in graphene-based moiré heterostructures. Ferroelectric materials show electrically switchable electric dipoles, which are usually formed by spatial separation between the average centres of positive and negative charge within the unit cell. On this basis, it is difficult to imagine graphene—a material composed of only carbon atoms—exhibiting ferroelectricity. However, in this work we realize switchable ferroelectricity in Bernal-stacked bilayer graphene sandwiched between two hexagonal boron nitride layers. By introducing a moiré superlattice potential (via aligning bilayer graphene with the top and/or bottom boron nitride crystals), we observe prominent and robust hysteretic behaviour of the graphene resistance with an externally applied out-of-plane displacement field.
人工智能Artificial Intelligence
Autonomous navigation of stratospheric balloons using reinforcement learning
基于强化学习的平流层气球自主导航系统
▲ 作者：Marc G. Bellemare, Salvatore Candido, Pablo Samuel Castro, Jun Gong, Marlos C. Machado, subhodeep Moitra, Sameera S. Ponda & Ziyu Wang
▲ 链接：
https://www.nature.com/articles/s41586-020-2939-8
▲ 摘要
在此，我们使用强化学习来创建一个高性能的飞行控制器。我们的算法使用数据增强和自校正设计，以克服关键技术挑战——使用不完善的数据进行强化学习，这已被证明是将之应用于物理系统时会碰到的主要障碍。
我们在全球多个地方部署了Loon超压气球控制器，包括在太平洋上空进行的为期39天的控制实验。分析表明，该控制器优于Loon之前的算法，对平流层风的自然多样性具有鲁棒性。
这些结果表明，强化学习是一种有效的解决现实世界中传统方法和人工干预都不够的自主控制问题的方法，它提供了一些线索，以说明创建持续与真实动态环境交互的人工智能代理可能需要什么。
▲ Abstract
Here we describe the use of reinforcement learning to create a high-performing flight controller. Our algorithm uses data augmentation and a self-correcting design to overcome the key technical challenge of reinforcement learning from imperfect data, which has proved to be a major obstacle to its application to physical systems. We deployed our controller to station Loon superpressure balloons at multiple locations across the globe, including a 39-day controlled experiment over the Pacific Ocean. Analyses show that the controller outperforms Loon’s previous algorithm and is robust to the natural diversity in stratospheric winds. These results demonstrate that reinforcement learning is an effective solution to real-world autonomous control problems in which neither conventional methods nor human intervention suffice, offering clues about what may be needed to create artificially intelligent agents that continuously interact with real, dynamic environments.
地球科学Geoscience
The future of food from the sea
海洋食品的未来
▲ 作者：Christopher Costello, Ling Cao, Stefan Gelcich, et al.
▲ 链接：
https://www.nature.com/articles/s41586-020-2616-y
▲ 摘要
全球粮食需求正在上升，而供应能否持续增长仍然是一个严重的问题。土地产出的食物总量可能会继续增长，但同时也可能会加剧气候变化和生物多样性丧失，并危及其他生态系统。
由于来自海洋的食物只占目前可食用肉类产量的17%，我们不禁要问，到2050年，海洋还能可持续地生产多少食物？在此，我们研究了海洋中的主要食品来源——野生渔业、海鱼养殖和双壳类海鱼养殖业——以估计“可持续供应曲线”，从而解释生态、经济、监管和技术方面的限制。
我们将这些供给曲线与需求情景叠加，以估计未来的海产品产量。我们发现，在估计的需求变化和供应情景下（考虑到政策改革和技术改进），到2050年，海洋可食用食品将增加2100 - 4400万吨，与当前产量相比增加36-74%。这相当于到2050年养活98亿人所需的所有肉类估计增量的12-25%。
三类海洋食品来源都有可能增加，但最明显的是海水养殖业。这些生产潜力能否持续实现将取决于政策改革、技术革新和未来需求变化的程度等因素。
▲ Abstract
Global food demand is rising, and serious questions remain about whether supply can increase sustainably. Land-based expansion is possible but may exacerbate climate change and biodiversity loss, and compromise the delivery of other ecosystem services. As food from the sea represents only 17% of the current production of edible meat, we ask how much food we can expect the ocean to sustainably produce by 2050. Here we examine the main food-producing sectors in the ocean—wild fisheries, finfish mariculture and bivalve mariculture—to estimate ‘sustainable supply curves’ that account for ecological, economic, regulatory and technological constraints. We overlay these supply curves with demand scenarios to estimate future seafood production. We find that under our estimated demand shifts and supply scenarios (which account for policy reform and technology improvements), edible food from the sea could increase by 21–44 million tonnes by 2050, a 36–74% increase compared to current yields. This represents 12–25% of the estimated increase in all meat needed to feed 9.8 billion people by 2050. Increases in all three sectors are likely, but are most pronounced for mariculture. Whether these production potentials are realized sustainably will depend on factors such as policy reforms, technological innovation and the extent of future shifts in demand.
Thermochemical lithosphere differentiation and the origin of cratonic mantle
热化学岩石圈分异与克拉通地幔的起源
▲ 作者：Fabio A. Capitanio, Oliver Nebel & Peter A. Cawood
▲ 链接：
https://www.nature.com/articles/s41586-020-2976-3
▲ 摘要
克拉通记录了大陆岩石圈形成的早期历史，但它们是如何成为地球上岩石圈最持久的部分仍不得而知。
本文提出了大量熔融耗尽克拉通岩石圈地幔（CLM）的形成及其演化为稳定克拉通的机制。数值模型显示，在一个伸展的岩石圈之下，一个炎热的早期地球地幔在减压作用下发生了大范围的融化，融化的过程中，在深处留下了大量耗尽的地幔。
脱水、硬化的地幔阻止进一步的变形，迫使应变迁移和冷却，从而将耗尽的地幔同化到岩石圈中。应变局部化和加强之间的负反馈维持长期的扩散延伸和大量耗尽的CLM的就位。低压下CLM的形成及其深层的重新平衡再现了受深度-温度条件限制的太古岩石圈的演化，而岩石圈强度较低的模型则最能匹配太古岩石圈中大量的损耗和熔融体积。
在这些条件下，这些条件对板块构造来说是可行的，热化学分异有效地阻止了边缘的产生和形成:裂谷作用和岩石圈俯冲是短暂的，作为残余结构嵌入到冷却的CLM中，重现了在太古代克拉通中发现的循环和再加工环境。
▲ Abstract
Cratons record the early history of continental lithosphere formation, yet how they became the most enduring part of the lithosphere on Earth remains unknown. Here we propose a mechanism for the formation of large volumes of melt-depleted cratonic lithospheric mantle (CLM) and its evolution to stable cratons. Numerical models show large decompression melting of a hot, early Earth mantle beneath a stretching lithosphere, where melt extraction leaves large volumes of depleted mantle at depth. The dehydrated, stiffer mantle resists further deformation, forcing strain migration and cooling, thereby assimilating depleted mantle into the lithosphere. The negative feedback between strain localization and stiffening sustains long-term diffused extension and emplacement of large amounts of depleted CLM. The formation of CLM at low pressure and its deeper re-equilibration reproduces the evolution of Archaean lithosphere constrained by depth–temperature conditions, whereas large degrees of depletion and melt volumes in Archaean cratons are best matched by models with lower lithospheric strength. Under these conditions, which are otherwise viable for plate tectonics, thermochemical differentiation effectively prevents yielding and formation of margins: rifting and lithosphere subduction are short lived and embedded in the cooling CLM as relict structures, reproducing the recycling and reworking environments that are found in Archaean cratons.
