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Nature, 19 December 2019,Volume 576 Issue 7787 《自然》2019年12月19日，第576卷，7787期 拓扑物理学 Topological physics Prediction and observation of an antiferromagnetic topological insulator 反铁磁拓扑绝缘子的预测与观测 ▲ 作者：M. M. Otrokov, I. I. Klimovskikh, E. V. Chulkov, etc. ▲ 链接： https://www./articles/s41586-019-1840-9 ▲ 摘要： 磁拓扑绝缘子是将非平凡带拓扑结构与磁序结合起来的窄间隙半导体材料。 作者通过预测并利用各种实验技术进一步证实了反铁磁拓扑绝缘体在层状范德瓦尔斯化合物MnBi2Te4中的实现。MnBi2Te4的反铁磁序使它在时间反转和原晶格平移对称的组合下保持不变，从而证实了它的拓扑非平凡性。 实验表明，MnBi2Te4的对称断裂表面在拓扑表面状态下表现出较大的带隙。 作者期望这一特性最终能让他们观察到许多基本现象，其中包括量子化的磁电耦合和轴离子电动力学。 ▲ Abstract Magnetic topological insulators are narrow-gap semiconductor materials that combine non-trivial band topology and magnetic order. Here we predict by ab initio calculations and further confirm using various experimental techniques the realization of an antiferromagnetic topological insulator in the layered van der Waals compound MnBi2Te4. The antiferromagnetic ordering that MnBi2Te4 shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, confirming its topologically nontrivial nature. Our experiments indicate that the symmetry-breaking surface of MnBi2Te4 exhibits a large bandgap in the topological surface state. We expect this property to eventually enable the observation of a number of fundamental phenomena, among them quantized magnetoelectric coupling and axion electrodynamics. Large magnetic gap at the Dirac point in Bi2Te3/MnBi2Te4 heterostructures 掺杂锰的Bi2Te3的磁隙 ▲ 作者：E. D. L. Rienks, S. Wimmer, J. Sánchez-Barriga, O. Caha, G. Springholz ▲ 链接： https://www./articles/s41586-019-1826-7 ▲ 摘要： 磁掺杂拓扑绝缘子使量子反常霍尔效应（QAHE）成为可能，它为无损电荷传输应用提供了量子化的边缘状态。 作者使用低温光电子能谱来明确地揭示掺杂锰的Bi2Te3的磁隙，它表现出铁磁的平面外自旋结构，只在TC以下打开。 令人惊讶的是，分析显示，在1开尔文高达90毫电子伏特的情况下，会出现大的间隙，这比理论预测的要大5倍。作者表示这一发现为推动拓扑绝缘体的无损传输向室温应用方向发展提供了重要的见解。 ▲ Abstract Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE), which provides quantized edge states for lossless charge-transport applications. Here we use low-temperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi2Te3, which displays ferromagnetic out-of-plane spin texture and opens up only below TC. Surprisingly, our analysis reveals large gap sizes at 1 kelvin of up to 90 millielectronvolts, which is five times larger than theoretically predicted. Our findings provide insights that will be crucial in pushing lossless transport in topological insulators towards room-temperature applications. 材料物理 Material physics Cooperative elastic fluctuations provide tuning of the metal–insulator transition 金属—绝缘体跃迁的调谐 ▲ 作者：G. G. Guzmán-Verri, R. T. Brierley, P. B. Littlewood ▲ 链接： https://www./articles/s41586-019-1824-9 ▲ 摘要： 由强电子关联驱动的金属到绝缘体的跃迁在凝聚态系统中经常发生，并与固体中的显著的集体现象有关，包括超导性和磁性。 这项研究表明，现有的关于钙钛矿过渡金属氧化物中金属—绝缘体转变的数据通过离子尺寸效应提供了证据，证明了由于弹性应变的动态波动而对相变产生的大的系统影响，而这通常被忽略。 作者重现了在锰酸盐和镍酸盐材料中观察到的转变温度对阳离子半径的依赖关系。 由于弹性耦合通常是强的，作者预计这些结论将推广到所有的金属到绝缘体的跃迁耦合的晶格对称性的变化。 ▲ Abstract Metal-to-insulator transitions driven by strong electronic correlations occur frequently in condensed matter systems, and are associated with remarkable collective phenomena in solids, including superconductivity and magnetism. Here we show that existing data on the tuning of metal-to-insulator transitions in perovskite transition-metal oxides through ionic size effects provides evidence of large systematic effects on the phase transition owing to dynamical fluctuations of the elastic strain, which have usually been neglected. We reproduce the observed dependence of the transition temperature on the cation radius in the well studied manganite and nickelate materials. Because elastic couplings are generally strong, we anticipate that these conclusions will generalize to all metal-to-insulator transitions that couple to a change in lattice symmetry. Molecular heterogeneity drives reconfigurable nematic liquid crystal drops 分子非均质性驱动可重构向列液晶滴 ▲ 作者：Wei-Shao Wei, Yu Xia, Sophie Ettinger, Shu Yang & A. G. Yodh ▲ 链接： https://www./articles/s41586-019-1809-8 ▲ 摘要： 物质的多分散性或分子非均质性会阻碍自组装和状态转换，几乎没有例外。 作者发现低聚物链长度分布、温度和表面活性剂浓度的变化改变了NLCO弹性能和界面能之间的平衡，并驱动向列相结构的形成，这些向列相结构的范围从粗糙的球体到“花”形，再到具有可控直径的分枝丝状网络。 在至少1平方厘米的面积上，可以可逆地产生具有有限液晶指向场的支链结构，并可通过紫外线固化转化为液晶弹性体。 观察和建模显示，通过空间分离，链长多分散性在驱动这些形态现象中起着至关重要的作用。这一发现为软材料网络结构和功能的编码提供了新的途径。 ▲ Abstract With few exceptions, polydispersity or molecular heterogeneity in matter tends to impede self-assembly and state transformation. We find that variation of oligomer chain length distribution, temperature, and surfactant concentration alters the balance between NLCO elastic energy and interfacial energy, and drives formation of nematic structures that range from roughened spheres to ‘flower’ shapes to branched filamentous networks with controllable diameters. The branched structures with confined liquid crystal director fields can be produced reversibly over areas of at least one square centimetre and can be converted into liquid crystal elastomers by ultraviolet curing. Observations and modelling reveal that chain length polydispersity plays a crucial role in driving these morphogenic phenomena, via spatial segregation. This insight suggests new routes for encoding network structure and function in soft materials. 天体物理学 Astrophysics A statistical solution to the chaotic, non-hierarchical three-body problem 混沌三体问题的统计解决方案 ▲ 作者：Nicholas C. Stone & Nathan W. C. Leigh ▲ 链接： https://www./articles/s41586-019-1833-8#citeas ▲ 摘要： 三体问题是天体物理学中最古老的悬而未决的问题，几个世纪以来一直无法形成一个一般的解析。各种摄动理论在部分参数空间中提供了解决方案，但却仅在质量或分离的层次结构存在的地方。 作者报告了一个非层次的三体问题的统计解决方案，它是用遍历假设推导出来的，当给定守恒的运动积分时，它提供了封闭式分布（例如，二元轨道元素）结果。 作者将结果分布与数值三体积分的大集合进行比较，发现了很好的一致性，只要把自己限制在“共振”的环境中（约50%的散射经历了混沌演化）。 在分析散射实验中，作者将“扰频”（不存在成对双星的时间段）确定为遍历一个非层次三重系统的关键动力学状态。他们预测的双星偏心率“幸存者”的普遍超热分布在许多天体物理场景中有显著的应用。 ▲ Abstract The three-body problem is arguably the oldest open question in astrophysics and has resisted a general analytic solution for centuries. Various implementations of perturbation theory provide solutions in portions of parameter space, but only where hierarchies of masses or separations exist. Here we report a statistical solution to the non-hierarchical three-body problem that is derived using the ergodic hypothesis and that provides closed-form distributions of outcomes (for example, binary orbital elements) when given the conserved integrals of motion. We compare our outcome distributions to large ensembles of numerical three-body integrations and find good agreement, so long as we restrict ourselves to ‘resonant’ encounters10 (the roughly 50 per cent of scatterings that undergo chaotic evolution). In analysing our scattering experiments, we identify ‘scrambles’ (periods of time in which no pairwise binaries exist) as the key dynamical state that ergodicizes a non-hierarchical triple system. The generally super-thermal distributions of survivor binary eccentricity that we predict have notable applications to many astrophysical scenarios. 粒子物理学 Particle physics Atomic-scale imaging of a 27-nuclear-spin cluster using a quantum sensor 量子传感器实现原子尺度成像 ▲ 作者：M. H. Abobeih, J. Randall, C. E. Bradley, H. P. Bartling, etc. ▲ 链接： https://www./articles/s41586-019-1834-7 ▲ 摘要： 核磁共振（NMR）是一种测定分子和蛋白质结构的强有力的方法。 作者利用单氮空位中心作为量子传感器实现了这种原子尺度成像，并在27个耦合13C核自旋的金刚石模型系统上进行了演示。 他们提出了一种多维光谱学方法，分离出高光谱分辨率（小于80毫赫）和高精度（2毫赫）的单个核—核自旋相互作用，证明了这些相互作用编码了集群的组成和互联性，并开发了以亚埃（比纳米还小的数量级）分辨率提取集群三维结构的方法。 作者认为这一结果证明了一种对单个分子和其他复杂自旋系统磁成像的关键能力。 ▲ Abstract Nuclear magnetic resonance (NMR) is a powerful method for determining the structure of molecules and proteins. Here we realize such atomic-scale imaging using a single nitrogen vacancy centre as a quantum sensor, and demonstrate it on a model system of 27 coupled 13C nuclear spins in diamond. We present a multidimensional spectroscopy method that isolates individual nuclear–nuclear spin interactions with high spectral resolution (less than 80 millihertz) and high accuracy (2 millihertz). We show that these interactions encode the composition and inter-connectivity of the cluster, and develop methods to extract the three-dimensional structure of the cluster with sub-ångström resolution. Our results demonstrate a key capability towards magnetic imaging of individual molecules and other complex spin systems. 核结构物理学 Nuclear-structure physics Probing thecritical nucleus size for ice formation with graphene oxide nanosheets 冰核的临界尺寸 ▲ 作者：Guoying Bai, Dong Gao, Zhang Liu, Xin Zhou & Jianjun Wang ▲ 链接： https://www./articles/s41586-019-1827-6 ▲ 摘要： 水冻结无处不在，影响着气候、化学工业、低温生物学和材料科学等各种领域。冰核是水冻结的控制步骤，近一个世纪以来，人们一直认为会形成临界冰核。 作者报告了含有受控尺寸氧化石墨烯纳米薄片的水滴中的冰核，并表明只有当水滴过冷的程度不同时，其对冰核的影响才显著。 作者推断，氧化石墨烯的临界尺寸反映了临界冰核的大小，在足够大的氧化石墨烯的情况下，临界冰核位于其表面，并产生与经典成核理论一致的成冰行为。 相比之下，当氧化石墨烯的尺寸小于临界冰核的尺寸时，氧化石墨烯的边缘会随着冰核的生长而发生钉扎变形。 这使得成核的自由能壁垒大大提高，并抑制了氧化石墨烯的促进作用。这些结果提供了有关临界冰核存在和随温度变化的大小的实验信息，在此之前只是通过理论和模拟来探索。 ▲ Abstract Waterfreezing is ubiquitous and affects areas as diverse as climate, the chemicalindustry, cryobiology and materials science. Ice nucleation is the controllingstep in water freezing and has, for nearly a century, been assumed to requirethe formation of a critical ice nucleus. Here we report ice nucleation in waterdroplets containing graphene oxide nanosheets of controlled sizes and show thatthey have a notable impact on ice nucleation only above a certain size thatvaries with the degree of supercooling of the droplets. We infer from ourexperimental data and theoretical calculations that the critical size of thegraphene oxide reflects the size of the critical ice nucleus, which in the caseof sufficiently large graphene oxides sits on their surface and gives rise toice formation behaviour consistent with classical nucleation theory. Bycontrast, when the graphene oxide size is smaller than that of the critical icenucleus, pinning at the periphery of the graphene oxide deforms the ice nucleusas it grows. This gives rise to a much higher free-energy barrier fornucleation and suppresses the promoting effect of the graphene oxide. Theresults provide experimental information on the existence andtemperature-dependent size of the critical ice nucleus, which has previouslyonly been explored theoretically and through simulations. 考古学Archeology Earliest hunting scene in prehistoric art 史前绘画中的早期狩猎场景 ▲ 作者：Maxime Aubert, Rustan Lebe, Adhi Agus Oktaviana, Muhammad Tang, etc. ▲ 链接： https://www./articles/s41586-019-1806-y ▲ 摘要： 人类似乎有一种创造、讲述和消费故事的适应倾向。史前洞穴艺术为人们提供了对最早故事叙述的最直接的了解，其形式是叙事构图或“场景”，其特点是对空间上相邻的几组人物进行清晰的形象描绘，从这些形象中可以推断出人物之间发生的动作。 作者描述了一块来自印度尼西亚苏拉威西岛，良布鲁四世石灰岩洞穴的精美的岩石艺术面板，它描绘了几个人物，似乎代表了狩猎野猪和矮牛科动物的类人猿。 根据对上覆的洞穴化石的铀系列分析，这幅画的年代距今至少为43900年。作者表示，这个狩猎的场景是目前世界上最古老的图画记录和最早的比喻艺术作品。 ▲ Abstract Humans seem to have an adaptive predisposition for inventing, telling and consuming stories. Prehistoric cave art provides the most direct insight that we have into the earliest storytelling, in the form of narrative compositions or ‘scenes’ that feature clear figurative depictions of sets of figures in spatial proximity to each other, and from which one can infer actions taking place among the figures. Here we describe an elaborate rock art panel from the limestone cave of Leang Bulu’ Sipong 4 (Sulawesi, Indonesia) that portrays several figures that appear to represent therianthropes hunting wild pigs and dwarf bovids; this painting has been dated to at least 43.9 ka on the basis of uranium-series analysis of overlying speleothems. This hunting scene is—to our knowledge—currently the oldest pictorial record of storytelling and the earliest figurative artwork in the world.
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