Nature, 11 February 2021, VOL 590, ISSUE 7845

《自然》2021年2月11日，第590卷，7845期

物理學Physics

A quantum enhanced search for dark matter axions

量子增強搜尋暗物質軸子

▲ 連結：

https://www.nature.com/articles/s41586-021-03226-7

▲ 摘要

在暗物質軸子搜尋中，量子不確定性表現為一個基本噪聲源，限制了用於探測的正交可觀測值的測量。對暗物質的研究很少接近這個極限，到目前為止也無人超越。

研究組利用真空壓縮來突破量子極限尋找暗物質。透過製備一個壓縮狀態下的微波頻率電磁場，並以近乎無噪聲的方式讀出壓縮正交曲線，研究組可在質量範圍內將軸子的搜尋速度提高一倍。在16.96-17.12和17.14-17.28微伏的軸子剩餘能量視窗中，研究組沒有發現暗物質存在的證據。

突破量子極限帶來了一個基礎物理探索的時代，與接近量子極限的收益遞減相比，降噪技術將帶來極大益處。

▲ 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

基於矽光子學平臺的通用三維成像感測器

▲ 作者：Christopher Rogers, Alexander Y. Piggott, David J. Thomson, Robert F. Wiser, Ion E. Opris, Steven A. Fortune, et al.

▲ 連結：

https://www.nature.com/articles/s41586-021-03259-y

▲ 摘要

精確的三維（3D）成像對於機器繪製地圖和與物理世界互動至關重要。由於難以為每個畫素提供電子和光子連線，以前的系統限制在20個畫素以下。

研究組演示了一個由512個畫素組成的大規模相干探測器陣列在3D成像系統中的操作。利用光子和電子電路單片整合的最新進展，將密集的光學外差探測器陣列與整合的電子讀出結構相結合，可直接擴充套件到任意大的陣列。

雙軸固態光束轉向消除了視野和距離之間的任何權衡。在量子噪聲極限下，研究組的系統僅使用4毫瓦的光時，在75米的距離可達到3.1毫米的精度，比現有固態系統在該距離內的精度高出一個數量級。

未來使用最先進的元件縮小畫素尺寸，可為消費者相機感測器大小的陣列提供超過2000萬畫素的解析度。該研究成果為低成本、緊湊和高效能的3D成像相機的開發和普及鋪平了道路，這些相機可應用於從機器人技術和自主導航到增強現實和醫療保健等領域。

▲ 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.

材料科學Materials Science

Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene

魔角扭曲三層石墨烯中可調諧的強耦合超導

▲ 連結：

https://www.nature.com/articles/s41586-021-03192-0

▲ 摘要

魔角扭曲雙層石墨烯仍然是唯一一種可重現強超導性的體系。研究組在魔角扭曲三層石墨烯（MATTG）中實現了莫爾超導體，它比魔角扭曲雙層石墨烯具有更好的電子結構和超導效能。

測量霍爾效應和量子振盪作為密度和電場的函式，研究組能夠確定系統在正常金屬狀態下的可調諧相界。零磁場電阻率測量表明，超導性的存在與每個莫爾晶胞中兩個載流子所形成的破缺對稱相密切相關。

研究組發現超導相被抑制，並被限制在部分環繞著破缺對稱相的範霍夫奇點處，這很難與弱耦合Bardeen-Cooper-Schrieffer理論相吻合。此外，該系統廣泛的原位可調諧性使其能夠達到超強耦合狀態，其特徵是金茲堡-朗道相干長度達到平均粒子間距離，以及非常大的TBKT/TF值，超過0.1。

觀察結果表明，MATTG可電調諧至接近二維玻色-愛因斯坦凝聚體的交叉點。研究結果建立了一系列可調諧莫爾超導體，它們有可能徹底改變人們對強耦合超導的基本認識和應用。

▲ 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

一種大規模製備高強度高塑性超細晶鋼的簡易方法

▲ 連結：

https://www.nature.com/articles/s41586-021-03246-3

▲ 摘要

亞微米晶粒尺寸的鋼通常具有較高的韌性和強度，這使其在輕量化技術和節能戰略方面具有廣闊的應用前景。迄今為止，超細晶（UFG）合金的工業製備通常依賴於擴散相變的控制，因此僅限於製備奧氏體-鐵素體相變的鋼。此外，這些UFG鋼有限的加工硬化和均勻延伸阻礙了其廣泛應用。

研究組報道了一種在Fe-22Mn-0.6C孿晶誘導塑性鋼中大量製備UFG結構的簡易方法，即透過微量銅合金化，以及30秒內相干無序富Cu相的晶內奈米析出控制再結晶過程。快速而大量的奈米析出不僅阻止了新的亞微米級再結晶晶粒的生長，而且還透過齊納釘扎機制提高了所獲得的UFG結構的熱穩定性。

此外，由於析出相完全的相干性和無序性，在載荷條件下，析出相與位錯的相互作用較弱。這種方法能夠製備晶粒尺寸為800±400奈米的完全再結晶UFG結構，而不會引入有害的晶格缺陷，如脆性顆粒和晶界偏析。

與未新增Cu的鋼相比，UFG結構的屈服強度提高了一倍，達到710兆帕左右，均勻延展性為45%，抗拉強度為2000兆帕左右。這種晶粒細化的概念亦可擴充套件到其他合金系統，並且製造工藝較易應用到現有的工業生產線。

▲ 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

熱可重構單斜向列相膠體液

▲ 連結：

https://www.nature.com/articles/s41586-021-03249-0

▲ 摘要

迄今為止，除簡單結構外，具有很少或沒有對稱操作的結構已被證明僅是固體的一種性質，而不是它們的完全流體凝聚態對應物的性質，儘管這種對稱性在理論上被考慮並在磁膠體中被觀察到。

研究組證明了在由分子棒組成的向列相主體中分散高各向異性的帶電膠體盤，為觀察許多低對稱相提供了一個平臺。根據盤的溫度、濃度和表面電荷，研究組發現向列相、近晶相和柱狀組織的對稱性從單軸轉向正交和單斜。

隨著溫度的升高，研究組觀察到了從低序狀態到高序狀態、以及重入相的異常轉變。最重要的是，研究組證明了可重構單斜相膠體向列相序的存在，以及低對稱性自組裝的熱控制和磁控制的可能性。

研究組的實驗結果得到了向列相主體中圓盤間膠體相互作用的理論模型的支援，並有望為在具有不同形狀和尺寸的構建塊的系統中實現許多低對稱凝聚相及其技術應用提供一條途徑。

▲ 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.

化學Chemistry

Complex structures arising from the self-assembly of a simple organic salt

簡單有機鹽自組裝形成的複雜結構

▲ 作者：Riccardo Montis, Luca Fusaro, Andrea Falqui, Michael B. Hursthouse, Nikolay Tumanov, Simon J. Coles, et al.

▲ 連結：

https://www.nature.com/articles/s41586-021-03194-y

▲ 摘要

雖然分子自組裝已經被廣泛研究，但理解控制這種現象的規則仍具有挑戰性。

儘管FK相在60多年前就已在金屬合金中發現，但最近已在幾類超分子軟物質和金奈米晶體超晶格中觀察到FK相，並持續至今。在這些體系中，原子或分子的球形元件被組裝成配位數為12、14、15或16的多面體。

這些結構，連同用於製備它們的實驗程式，引起了人們對其形成的有趣猜測，併為有機晶體材料的設計開闢了不同的視角。

▲ 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.