Nature, 11 February 2021, VOL 590, ISSUE 7845
《自然》2021年2月11日，第590卷，7845期
物理学Physics
A quantum enhanced search for dark matter axions
量子增强搜索暗物质轴子
▲ 作者：K. M. Backes, D. A. Palken, S. Al Kenany, B. M. Brubaker, S. B. Cahn, A. Droster, et al.
▲ 链接：
https://www.nature.com/articles/s41586-021-03226-7
▲ 摘要
在暗物质轴子搜索中，量子不确定性表现为一个基本噪声源，限制了用于探测的正交可观测值的测量。对暗物质的研究很少接近这个极限，到目前为止也无人超越。
研究组利用真空压缩来突破量子极限寻找暗物质。通过制备一个压缩状态下的微波频率电磁场，并以近乎无噪声的方式读出压缩正交曲线，研究组可在质量范围内将轴子的搜索速度提高一倍。在16.96-17.12和17.14-17.28微伏的轴子剩余能量窗口中，研究组没有发现暗物质存在的证据。
突破量子极限带来了一个基础物理探索的时代，与接近量子极限的收益递减相比，降噪技术将带来极大益处。
▲ Abstract
In dark matter axion searches, quantum uncertainty manifests as a fundamental noise source, limiting the measurement of the quadrature observables used for detection. Few dark matter searches have approached this limit, and until now none has exceeded it. Here we use vacuum squeezing to circumvent the quantum limit in a search for dark matter. By preparing a microwave-frequency electromagnetic field in a squeezed state and near-noiselessly reading out only the squeezed quadrature, we double the search rate for axions over a mass range favoured by some recent theoretical projections. We find no evidence of dark matter within the axion rest energy windows of 16.96–17.12 and 17.14–17.28 microelectronvolts. Breaking through the quantum limit invites an era of fundamental physics searches in which noise reduction techniques yield unbounded benefit compared with the diminishing returns of approaching the quantum limit.
A universal 3D imaging sensor on a silicon photonics platform
基于硅光子学平台的通用三维成像传感器
▲ 作者：Christopher Rogers, Alexander Y. Piggott, David J. Thomson, Robert F. Wiser, Ion E. Opris, Steven A. Fortune, et al.
▲ 链接：
https://www.nature.com/articles/s41586-021-03259-y
▲ 摘要
精确的三维（3D）成像对于机器绘制地图和与物理世界交互至关重要。由于难以为每个像素提供电子和光子连接，以前的系统限制在20个像素以下。
研究组演示了一个由512个像素组成的大规模相干探测器阵列在3D成像系统中的操作。利用光子和电子电路单片集成的最新进展，将密集的光学外差探测器阵列与集成的电子读出结构相结合，可直接扩展到任意大的阵列。
双轴固态光束转向消除了视野和距离之间的任何权衡。在量子噪声极限下，研究组的系统仅使用4毫瓦的光时，在75米的距离可达到3.1毫米的精度，比现有固态系统在该距离内的精度高出一个数量级。
未来使用最先进的组件缩小像素尺寸，可为消费者相机传感器大小的阵列提供超过2000万像素的分辨率。该研究成果为低成本、紧凑和高性能的3D成像相机的开发和普及铺平了道路，这些相机可应用于从机器人技术和自主导航到增强现实和医疗保健等领域。
▲ Abstract
Accurate three-dimensional (3D) imaging is essential for machines to map and interact with the physical world. Owing to difficulties in providing electrical and photonic connections to every pixel, previous systems have been restricted to fewer than 20 pixels. Here we demonstrate the operation of a large-scale coherent detector array, consisting of 512 pixels, in a 3D imaging system. Leveraging recent advances in the monolithic integration of photonic and electronic circuits, a dense array of optical heterodyne detectors is combined with an integrated electronic readout architecture, enabling straightforward scaling to arbitrarily large arrays. Two-axis solid-state beam steering eliminates any trade-off between field of view and range. Operating at the quantum noise limit, our system achieves an accuracy of 3.1 millimetres at a distance of 75 metres when using only 4 milliwatts of light, an order of magnitude more accurate than existing solid-state systems at such ranges. Future reductions of pixel size using state-of-the-art components could yield resolutions in excess of 20 megapixels for arrays the size of a consumer camera sensor. This result paves the way for the development and proliferation of low-cost, compact and high-performance 3D imaging cameras that could be used in applications from robotics and autonomous navigation to augmented reality and healthcare.
材料科学Materials Science
Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene
魔角扭曲三层石墨烯中可调谐的强耦合超导
▲ 作者：Jeong Min Park, Yuan Cao, Kenji Watanabe, Takashi Taniguchi & Pablo Jarillo-Herrero
▲ 链接：
https://www.nature.com/articles/s41586-021-03192-0
▲ 摘要
魔角扭曲双层石墨烯仍然是唯一一种可重现强超导性的体系。研究组在魔角扭曲三层石墨烯（MATTG）中实现了莫尔超导体，它比魔角扭曲双层石墨烯具有更好的电子结构和超导性能。
测量霍尔效应和量子振荡作为密度和电场的函数，研究组能够确定系统在正常金属状态下的可调谐相界。零磁场电阻率测量表明，超导性的存在与每个莫尔晶胞中两个载流子所形成的破缺对称相密切相关。
研究组发现超导相被抑制，并被限制在部分环绕着破缺对称相的范霍夫奇点处，这很难与弱耦合Bardeen-Cooper-Schrieffer理论相吻合。此外，该系统广泛的原位可调谐性使其能够达到超强耦合状态，其特征是金兹堡-朗道相干长度达到平均粒子间距离，以及非常大的TBKT/TF值，超过0.1。
观察结果表明，MATTG可电调谐至接近二维玻色-爱因斯坦凝聚体的交叉点。研究结果建立了一系列可调谐莫尔超导体，它们有可能彻底改变人们对强耦合超导的基本认识和应用。
▲ Abstract
Magic-angle twisted bilayer graphene remains the only one in which robust superconductivity has been reproducibly measured. Here we realize a moiré superconductor in magic-angle twisted trilayer graphene (MATTG), which has better tunability of its electronic structure and superconducting properties than magic-angle twisted bilayer graphene. Measurements of the Hall effect and quantum oscillations as a function of density and electric field enable us to determine the tunable phase boundaries of the system in the normal metallic state. Zero-magnetic-field resistivity measurements reveal that the existence of superconductivity is intimately connected to the broken-symmetry phase that emerges from two carriers per moiré unit cell. We find that the superconducting phase is suppressed and bounded at the Van Hove singularities that partially surround the broken-symmetry phase, which is difficult to reconcile with weak-coupling Bardeen–Cooper–Schrieffer theory. Moreover, the extensive in situ tunability of our system allows us to reach the ultrastrong-coupling regime, characterized by a Ginzburg–Landau coherence length that reaches the average inter-particle distance, and very large TBKT/TFvalues, in excess of 0.1. These observations suggest that MATTG can be electrically tuned close to the crossover to a two-dimensional Bose–Einstein condensate. Our results establish a family of tunable moiré superconductors that have the potential to revolutionize our fundamental understanding of and the applications for strongly coupled superconductivity.
Facile route to bulk ultrafine-grain steels for high strength and ductility
一种大规模制备高强度高塑性超细晶钢的简易方法
▲ 作者：Junheng Gao, Suihe Jiang, Huairuo Zhang, Yuhe Huang, Dikai Guan, Yidong Xu, et al.
▲ 链接：
https://www.nature.com/articles/s41586-021-03246-3
▲ 摘要
亚微米晶粒尺寸的钢通常具有较高的韧性和强度，这使其在轻量化技术和节能战略方面具有广阔的应用前景。迄今为止，超细晶（UFG）合金的工业制备通常依赖于扩散相变的控制，因此仅限于制备奥氏体-铁素体相变的钢。此外，这些UFG钢有限的加工硬化和均匀延伸阻碍了其广泛应用。
研究组报道了一种在Fe-22Mn-0.6C孪晶诱导塑性钢中大量制备UFG结构的简易方法，即通过微量铜合金化，以及30秒内相干无序富Cu相的晶内纳米析出控制再结晶过程。快速而大量的纳米析出不仅阻止了新的亚微米级再结晶晶粒的生长，而且还通过齐纳钉扎机制提高了所获得的UFG结构的热稳定性。
此外，由于析出相完全的相干性和无序性，在载荷条件下，析出相与位错的相互作用较弱。这种方法能够制备晶粒尺寸为800±400纳米的完全再结晶UFG结构，而不会引入有害的晶格缺陷，如脆性颗粒和晶界偏析。
与未添加Cu的钢相比，UFG结构的屈服强度提高了一倍，达到710兆帕左右，均匀延展性为45%，抗拉强度为2000兆帕左右。这种晶粒细化的概念亦可扩展到其他合金系统，并且制造工艺较易应用到现有的工业生产线。
▲ Abstract
Steels with sub-micrometre grain sizes usually possess high toughness and strength, which makes them promising for lightweighting technologies and energy-saving strategies. So far, the industrial fabrication of ultrafine-grained (UFG) alloys, which generally relies on the manipulation of diffusional phase transformation, has been limited to steels with austenite-to-ferrite transformation. Moreover, the limited work hardening and uniform elongation of these UFG steels hinder their widespread application. Here we report the facile mass production of UFG structures in a typical Fe–22Mn–0.6C twinning-induced plasticity steel by minor Cu alloying and manipulation of the recrystallization process through the intragranular nanoprecipitation (within 30 seconds) of a coherent disordered Cu-rich phase. The rapid and copious nanoprecipitation not only prevents the growth of the freshly recrystallized sub-micrometre grains but also enhances the thermal stability of the obtained UFG structure through the Zener pinning mechanism. Moreover, owing to their full coherency and disordered nature, the precipitates exhibit weak interactions with dislocations under loading. This approach enables the preparation of a fully recrystallized UFG structure with a grain size of 800 ± 400 nanometres without the introduction of detrimental lattice defects such as brittle particles and segregated boundaries. Compared with the steel to which no Cu was added, the yield strength of the UFG structure was doubled to around 710 megapascals, with a uniform ductility of 45 per cent and a tensile strength of around 2,000 megapascals. This grain-refinement concept should be extendable to other alloy systems, and the manufacturing processes can be readily applied to existing industrial production lines.
Thermally reconfigurable monoclinic nematic colloidal fluids
热可重构单斜向列相胶体液
▲ 作者：Haridas Mundoor, Jin-Sheng Wu, Henricus H. Wensink & Ivan I. Smalyukh
▲ 链接：
https://www.nature.com/articles/s41586-021-03249-0
▲ 摘要
迄今为止，除简单结构外，具有很少或没有对称操作的结构已被证明仅是固体的一种性质，而不是它们的完全流体凝聚态对应物的性质，尽管这种对称性在理论上被考虑并在磁胶体中被观察到。
研究组证明了在由分子棒组成的向列相主体中分散高各向异性的带电胶体盘，为观察许多低对称相提供了一个平台。根据盘的温度、浓度和表面电荷，研究组发现向列相、近晶相和柱状组织的对称性从单轴转向正交和单斜。
随着温度的升高，研究组观察到了从低序状态到高序状态、以及重入相的异常转变。最重要的是，研究组证明了可重构单斜相胶体向列相序的存在，以及低对称性自组装的热控制和磁控制的可能性。
研究组的实验结果得到了向列相主体中圆盘间胶体相互作用的理论模型的支持，并有望为在具有不同形状和尺寸的构建块的系统中实现许多低对称凝聚相及其技术应用提供一条途径。
▲ Abstract
To have a structure with few or no symmetry operations apart from trivial ones has so far been demonstrated to be a property of solids alone, but not of their fully fluid condensed matter counterparts, even though such symmetries have been considered theoretically and observed in magnetic colloids. Here we show that dispersing highly anisotropic charged colloidal disks in a nematic host composed of molecular rods provides a platform for observing many low-symmetry phases. Depending on the temperature, concentration and surface charge of the disks, we find nematic, smectic and columnar organizations with symmetries ranging from uniaxial to orthorhombic and monoclinic. With increasing temperature, we observe unusual transitions from less- to more-ordered states and re-entrant phases. Most importantly, we demonstrate the presence of reconfigurable monoclinic colloidal nematic order, as well as the possibility of thermal and magnetic control of low-symmetry self-assembly. Our experimental findings are supported by theoretical modelling of the colloidal interactions between disks in the nematic host and may provide a route towards realizing many low-symmetry condensed matter phases in systems with building blocks of dissimilar shapes and sizes, as well as their technological applications.
化学Chemistry
Complex structures arising from the self-assembly of a simple organic salt
简单有机盐自组装形成的复杂结构
▲ 作者：Riccardo Montis, Luca Fusaro, Andrea Falqui, Michael B. Hursthouse, Nikolay Tumanov, Simon J. Coles, et al.
▲ 链接：
https://www.nature.com/articles/s41586-021-03194-y
▲ 摘要
虽然分子自组装已经被广泛研究，但理解控制这种现象的规则仍具有挑战性。
研究组报道了一种简单的氨吡啶盐酸盐结晶为四种不同的结构，其中两种采用了不寻常的自组装组成了氯离子和吡啶离子的多面体团簇。这两种结构代表了刚性有机小分子的Frank–Kasper（FK）相。
尽管FK相在60多年前就已在金属合金中发现，但最近已在几类超分子软物质和金纳米晶体超晶格中观察到FK相，并持续至今。在这些体系中，原子或分子的球形组件被组装成配位数为12、14、15或16的多面体。
该文报道的两种FK结构是从致密液相结晶出来的，显示出一种在刚性有机小分子中通常无法观察到的复杂性。通过低温电子显微镜对前驱体致密液相的研究，揭示了球形聚集体的存在，其尺寸在1.5到4.6纳米之间。
这些结构，连同用于制备它们的实验程序，引起了人们对其形成的有趣猜测，并为有机晶体材料的设计开辟了不同的视角。
▲ Abstract
Although molecular self-assembly has been extensively investigated, understanding the rules governing this phenomenon remains challenging. Here we report on a simple hydrochloride salt of fampridine that crystallizes as four different structures, two of which adopt unusual self-assemblies consisting of polyhedral clusters of chloride and pyridinium ions. These two structures represent Frank–Kasper (FK) phases of a small and rigid organic molecule. Although discovered in metal alloys more than 60 years ago, FK phases have recently been observed in several classes of supramolecular soft matter and in gold nanocrystal superlattices and remain the object of recent discoveries. In these systems, atoms or spherical assemblies of molecules are packed to form polyhedra with coordination numbers 12, 14, 15 or 16. The two FK structures reported here crystallize from a dense liquid phase and show a complexity that is generally not observed in small rigid organic molecules. Investigation of the precursor dense liquid phase by cryogenic electron microscopy reveals the presence of spherical aggregates with sizes ranging between 1.5 and 4.6 nanometres. These structures, together with the experimental procedure used for their preparation, invite interesting speculation about their formation and open different perspectives for the design of organic crystalline materials.