编译 | 未玖
Nature, 24 March 2022, VOL 603, ISSUE 7902
《自然》2022年3月24日，第603卷，7902期
天文学Astronomy
A time-resolved picture of our Milky Way’s early formation history
银河系早期形成史的时间分辨图
▲ 作者：Maosheng Xiang & Hans-Walter Rix
▲ 链接：
https://www.nature.com/articles/s41586-022-04496-5
▲ 摘要
我们银河系的形成可以定性地分为不同的阶段，这导致了其结构不同的恒星群：晕族恒星和盘族恒星的组成。要想对银河系的整体结构进行定量分析，需要一个具有极精确年龄的大型恒星样本。
研究组报道了使用亚巨星对该类样本的分析。他们发现恒星年龄-金属丰度分布p（τ，[Fe/H]）分裂成两个几乎不相交的部分，在年龄τ≃ 80亿年处分离。较年轻部分反映了动态静止银盘形成的晚期阶段，有证据表明恒星径向轨道迁移；另一部分反映了恒星晕和旧α-过程增强（厚）盘形成的早期阶段。
研究结果表明，银河系旧（厚）盘的形成始于大约130亿年前，仅比宇宙大爆炸晚了8亿年，且比内银晕的最后集成早20亿年。
这些恒星大多形成于约110亿年前，当时盖亚·恩克多拉斯伴星系正与我们的银河系合并。在接下来的50-60亿年中，银河系经历了持续的化学元素富集，最终富集了10倍，而恒星形成气体则保持着良好的混合。
▲ Abstract
The formation of our Milky Way can be split up qualitatively into different phases that resulted in its structurally different stellar populations: the halo and the disk components. Revealing a quantitative overall picture of our Galaxy’s assembly requires a large sample of stars with very precise ages. Here we report an analysis of such a sample using subgiant stars. We find that the stellar age–metallicity distribution p(τ, [Fe/H]) splits into two almost disjoint parts, separated at age τ≃8Gyr. The younger part reflects a late phase of dynamically quiescent Galactic disk formation with manifest evidence for stellar radial orbit migration; the other part reflects the earlier phase, when the stellar halo and the old α-process-enhanced (thick) disk formed. Our results indicate that the formation of the Galaxy’s old (thick) disk started approximately 13 Gyr ago, only 0.8Gyr after the Big Bang, and 2Gyr earlier than the final assembly of the inner Galactic halo. Most of these stars formed around 11Gyr ago, when the Gaia-Sausage-Enceladus satellite merged with our Galaxy. Over the next 5–6Gyr, the Galaxy experienced continuous chemical element enrichment, ultimately by a factor of 10, while the star-forming gas managed to stay well mixed.
物理学Physics
Emergence of Fermi arcs due to magnetic splitting in an antiferromagnet
反铁磁体中的磁分裂产生费米弧
▲ 作者：Benjamin Schrunk, Yevhen Kushnirenko, Brinda Kuthanazhi, Junyeong Ahn, Lin-Lin Wang, Evan O’Leary, et al.
▲ 链接：
https://www.nature.com/articles/s41586-022-04412-x
▲ 摘要
费米面在控制材料的电子、输运和热力学性质等方面起着重要作用。由于费米面由动量空间中定义明确的能带闭合轮廓组成，因此所谓费米弧的断开部分，可能是不寻常电子态的标志，例如赝隙。
另一种获得费米弧的方法是打破三维狄拉克半金属的时间反演对称性或反演对称性，从而形成一对手性相反的韦尔节点，它们的投影通过体边界处的费米弧连接。
研究组提供了实验证据，证明在立方NdBi的反铁磁状态中，由于一种新的磁分裂效应，在尼尔温度（TN）以下出现了成对的类空穴和类电子的费米弧。
观察到的磁分裂很不寻常，因为它产生了相反曲率带，这些带随温度变化，并遵循反铁磁序参数。这不同于之前理论上考虑和实验上报道的磁分裂情况，如传统的塞曼和拉什巴，在这些情况中，带曲率得以保留。
研究结果表明，在存在长程反铁磁序的情况下，出现了一种新型磁能带分裂，而现有理论观点无法解释。
▲ Abstract
The Fermi surface plays an important role in controlling the electronic, transport and thermodynamic properties of materials. As the Fermi surface consists of closed contours in the momentum space for well-defined energy bands, disconnected sections known as Fermi arcs can be signatures of unusual electronic states, such as a pseudogap. Another way to obtain Fermi arcs is to break either the time-reversal symmetry or the inversion symmetry of a three-dimensional Dirac semimetal, which results in formation of pairs of Weyl nodes that have opposite chirality, and their projections are connected by Fermi arcs at the bulk boundary. Here, we present experimental evidence that pairs of hole- and electron-like Fermi arcs emerge below the Neel temperature (TN) in the antiferromagnetic state of cubic NdBi due to a new magnetic splitting effect. The observed magnetic splitting is unusual, as it creates bands of opposing curvature, which change with temperature and follow the antiferromagnetic order parameter. This is different from previous theoretically considered and experimentally reported cases of magnetic splitting, such as traditional Zeeman and Rashba, in which the curvature of the bands is preserved. Therefore, our findings demonstrate a type of magnetic band splitting in the presence of a long-range antiferromagnetic order that is not readily explained by existing theoretical ideas.
材料科学Materials Science
Single fibre enables acoustic fabrics via nanometre-scale vibrations
单根纤维通过纳米级振动实现声学织物
▲ 作者：Wei Yan, Grace Noel, Gabriel Loke, Elizabeth Meiklejohn, Tural Khudiyev, Juliette Marion, et al.
▲ 链接：
https://www.nature.com/articles/s41586-022-04476-9
▲ 摘要
织物由于其组成和结构，传统上被用作吸声材料。但受听觉系统的启发，研究组发明了一种织物，它可作为灵敏的听觉麦克风工作，同时保留织物的传统品质，如机洗性和悬垂性。
织物介质由高杨氏模量的纺织纬纱与棉质经纱编织而成，将微弱的10−7-大气压力波以可听频率转换为低阶机械振动模式。织入织物的是一种热拉伸的复合压电纤维，它与织物相匹配，并将机械振动转换为电信号。
光纤灵敏度的关键是弹性聚合物包层，它将机械应力集中在压电复合材料层中，压电系数高达46 pC/N，这是一种热拉伸工艺的结果。
对声音激励下的电输出和空间振动模式的同时测量表明，具有纳米振幅位移的织物振动模式是纤维电输出的来源。由于纤维占织物体积的比例不到0.1%，单根纤维拉伸就可实现数十平方米的织物麦克风。
三个不同的应用举例充分说明了这项研究的实用性：带有双声纤维的编织衬衫可测量声脉冲的精确方向，在两种用作声音发射器和接收器的织物之间建立双向通信，以及编织衬衫可听诊心音信号。
▲ Abstract
Fabrics, by virtue of their composition and structure, have traditionally been used as acoustic absorbers. Here, inspired by the auditory system, we introduce a fabric that operates as a sensitive audible microphone while retaining the traditional qualities of fabrics, such as machine washability and draping. The fabric medium is composed of high-Young’s modulus textile yarns in the weft of a cotton warp, converting tenuous10−7-atmosphere pressure waves at audible frequencies into lower-order mechanical vibration modes. Woven into the fabric is a thermally drawn composite piezoelectric fibre that conforms to the fabric and converts the mechanical vibrations into electrical signals. Key to the fibre sensitivity is an elastomeric cladding that concentrates the mechanical stress in a piezocomposite layer with a high piezoelectric charge coefficient of approximately 46 picocoulombs per newton, a result of the thermal drawing process. Concurrent measurements of electric output and spatial vibration patterns in response to audible acoustic excitation reveal that fabric vibrational modes with nanometre amplitude displacement are the source of the electrical output of the fibre. With the fibre subsuming less than 0.1% of the fabric by volume, a single fibre draw enables tens of square metres of fabric microphone. Three different applications exemplify the usefulness of this study: a woven shirt with dual acoustic fibres measures the precise direction of an acoustic impulse, bidirectional communications are established between two fabrics working as sound emitters and receivers, and a shirt auscultates cardiac sound signals.
High-brightness all-polymer stretchable LED with charge-trapping dilution
具有电荷捕获稀释功能的高明亮度全聚合物可拉伸LED
▲ 作者：Zhitao Zhang, Weichen Wang, Yuanwen Jiang, Yi-Xuan Wang, Yilei Wu, Jian-Cheng Lai, et al.
▲ 链接：
https://www.nature.com/articles/s41586-022-04400-1
▲ 摘要
下一代皮肤发光显示器应该是柔软、可拉伸且明亮的。先前报道的可拉伸发光器件大多基于无机纳米材料，如发光电容器、量子点或钙钛矿。它们要么需要高工作电压，要么在压力下可拉伸性、明亮度、分辨率或稳健性均受限。
另一方面，本质上可拉伸的聚合物材料具有良好的应变耐受性。但对于本质上可拉伸的发光二极管来说，实现高明亮度仍是一个巨大的挑战。
研究组报道了一种材料设计策略和制造工艺，以实现具有高明亮度（约7450坎德拉/平方米）、高电流效率（约5.3坎德拉/安培）和高拉伸性（约100%应变）的可拉伸全聚合物发光二极管。
他们制作了红色、绿色和蓝色的可拉伸全聚合物发光二极管，实现了皮肤无线供电和脉冲信号实时显示。这项工作标志着人们向高性能可拉伸显示器的方向迈进了一大步。
▲ Abstract
Next-generation light-emitting displays on skin should be soft, stretchable and bright. Previously reported stretchable light-emitting devices were mostly based on inorganic nanomaterials, such as light-emitting capacitors, quantum dots or perovskites. They either require high operating voltage or have limited stretchability and brightness, resolution or robustness under strain. On the other hand, intrinsically stretchable polymer materials hold the promise of good strain tolerance. However, realizing high brightness remains a grand challenge for intrinsically stretchable light-emitting diodes. Here we report a material design strategy and fabrication processes to achieve stretchable all-polymer-based light-emitting diodes with high brightness (about 7,450 candela per square metre), current efficiency (about 5.3candela per ampere) and stretchability (about 100 per cent strain). We fabricate stretchable all-polymer light-emitting diodes coloured red, green and blue, achieving both on-skin wireless powering and real-time displaying of pulse signals. This work signifies a considerable advancement towards high-performance stretchable displays.
Metastable hexagonal close-packed palladium hydride in liquid cell TEM
液体池TEM中的亚稳六方密排氢化钯
▲ 作者：Jaeyoung Hong, Jee-Hwan Bae, Hyesung Jo, Hee-Young Park, Sehyun Lee, Sung Jun Hong, et al.
▲ 链接：
https://www.nature.com/articles/s41586-021-04391-5
▲ 摘要
亚稳相（动力学上的优势结构）在自然界中普遍存在。由高能前驱体生长的晶体通常最初采用亚稳态结构，而非形成热力学稳定的基态结构，这取决于初始条件，例如温度、压力或晶体尺寸。随着晶体进一步生长，它们通常会经历一系列转变，从亚稳相到低能和最终能量稳定相。
亚稳相有时表现出优越的物理化学性质，因此，新亚稳相的发现和合成是材料科学创新颇有前景的途径。然而，亚稳材料的探索主要是启发式的，基于经验、直觉甚至推测性预测，即经验法则。这种局限性迫切要求出现一种基于理性设计的新模式，来发现新亚稳相。
研究组通过液体池透射电子显微镜合成亚稳态六方密排（hcp）氢化钯（PdHx），充分体现了这种设计规则。亚稳态hcp结构通过溶液中前驱体浓度之间的独特相互作用来稳定：氢（H）的充足供应有利于亚纳米尺度上的hcp结构，钯的不足则抑制进一步生长和后续向热力学稳定的面心立方结构的转变。
这些发现为亚稳态工程策略提供了热力学见解，可用于发现新亚稳相。
▲ Abstract
Metastable phases—kinetically favoured structures—are ubiquitous in nature. Rather than forming thermodynamically stable ground-state structures, crystals grown from high-energy precursors often initially adopt metastable structures depending on the initial conditions, such as temperature, pressure or crystal size. As the crystals grow further, they typically undergo a series of transformations from metastable phases to lower-energy and ultimately energetically stable phases. Metastable phases sometimes exhibit superior physicochemical properties and, hence, the discovery and synthesis of new metastable phases are promising avenues for innovations in materials science. However, the search for metastable materials has mainly been heuristic, performed on the basis of experiences, intuition or even speculative predictions, namely ‘rules of thumb’. This limitation necessitates the advent of a new paradigm to discover new metastable phases based on rational design. Such a design rule is embodied in the discovery of a metastable hexagonal close-packed (hcp) palladium hydride (PdHx) synthesized in a liquid cell transmission electron microscope. The metastable hcp structure is stabilized through a unique interplay between the precursor concentrations in the solution: a sufficient supply of hydrogen (H) favours the hcp structure on the subnanometre scale, and an insufficient supply of Pd inhibits further growth and subsequent transition towards the thermodynamically stable face-centred cubic structure. These findings provide thermodynamic insights into metastability engineering strategies that can be deployed to discover new metastable phases.
地球科学Earth Science
Geophysical imaging of the Yellowstone hydrothermal plumbing system
黄石公园热液管道系统的地球物理成像
▲ 作者：Carol A. Finn, Paul A. Bedrosian, W. Steven Holbrook, Esben Auken, Benjamin R. Bloss & Jade Crosbie
▲ 链接：
https://www.nature.com/articles/s41586-021-04379-1
▲ 摘要
黄石公园的管道系统将深层热液与其传奇的地热特征联系起来，但其本质几乎未知。黄石公园水文学和化学的主流概念是，流体存在于几何形状未知的储层中，从远端源头横向流动，并出现在熔岩流的边缘。
研究组通过航空地球物理数据的电阻率和磁化率模型，得出了黄石公园热液系统路径的高分辨率概观图像。含有大量溶解性固体的地下水和热液显著降低了多孔火山岩的电阻率，且可根据电阻率特征进行区分。
热区和钻孔中绘制的粘土层序通常形成于深度不到1000米处，在断层控制的热液和/或气体管道上方。研究组发现，大多数地热特征位于高通量管道上方，沿着被低电阻率和低磁化率粘土覆盖的埋藏断层。
浅层近水平通道将地下水注入盆地，与垂直管道中的热液混合。这些混合流体出现在地表，受地表渗透性控制，并沿着更深的角砾岩层向外流动。这些流出物持续在间歇泉盆地之间，与局部地下水和热液混合，产生了观测到的地球化学特征。
研究组的高保真图像为全球热液系统的地球化学和地下水模型提供了宝贵信息。
▲ Abstract
The nature of Yellowstone National Park’s plumbing system linking deep thermal fluids to its legendary thermal features is virtually unknown. The prevailing concepts of Yellowstone hydrology and chemistry are that fluids reside in reservoirs with unknown geometries, flow laterally from distal sources and emerge at the edges of lava flows. Here we present a high-resolution synoptic view of pathways of the Yellowstone hydrothermal system derived from electrical resistivity and magnetic susceptibility models of airborne geophysical data. Groundwater and thermal fluids containing appreciable total dissolved solids significantly reduce resistivities of porous volcanic rocks and are differentiated by their resistivity signatures. Clay sequences mapped in thermal areas and boreholes typically form at depths of less than 1,000 metres over fault-controlled thermal fluid and/or gas conduits. We show that most thermal features are located above high-flux conduits along buried faults capped with clay that has low resistivity and low susceptibility. Shallow subhorizontal pathways feed groundwater into basins that mixes with thermal fluids from vertical conduits. These mixed fluids emerge at the surface, controlled by surficial permeability, and flow outwards along deeper brecciated layers. These outflows, continuing between the geyser basins, mix with local groundwater and thermal fluids to produce the observed geochemical signatures. Our high-fidelity images inform geochemical and groundwater models for hydrothermal systems worldwide.
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