《自然》（20190509出版）一周論文導讀

編譯 | 宗華

Nature, 09 MAY 2019, VOL 569, ISSUE 7755

《自然》2019年5月9日，第7755期569卷

物理/ 天文Physics/Astronomy

Imaging individual barium atoms in solid xenon for barium tagging in nEXO

對固體氙中單個鋇原子成像用於nEXO實驗中的鋇標記

作者：nEXO Collaboration

鏈接：

https://www.nature.com/articles/s41586-019-1169-4

摘要：

雙β衰變涉及兩個中子同步轉換成兩個質子，以及兩個電子和兩個中微子的發射。無中微子的過程雖然尚未被觀察到，但被認為涉及兩個電子的發射且沒有中微子。

雙β衰變探測器大而昂貴，因此在每項研究中實現可能的最高靈敏度是必需的，同時從探測到的信號中刪除虛假的貢獻（「背景」）至關重要。

在nEXO的無中微子雙β衰變試驗中，對¹³⁶氙雙β衰變產生的¹³⁶鋇子系原子進行辨別，或者說「貼標籤」，提供了一種背景鑒別技術。

這裡，我們演示了通過掃描在藍寶石窗口上沉積的固體氙基體上的聚焦激光，對固體氙中單個鋇原子進行了成像和計數。當激光照射單個原子時，熒光持續了約30秒，然後突然下降到背景水平——這清楚地證實了單原子成像。

Abstract

Double-β-decay involves the simultaneous conversion of two neutrons into two protons, and the emission of two electrons and two neutrinos; the neutrinoless process, although not yet observed, is thought to involve the emission of the two electrons but no neutrinos. Double-β-decay detectors are large and expensive, so it is essential to achieve the highest possible sensitivity with each study, and removing spurious contributions (『background』) from detected signals is crucial. In the nEXO neutrinoless-double-β-decay experiment, the identification, or 『tagging』, of the ¹³⁶Ba daughter atom resulting from the double-β decay of¹³⁶Xe provides a technique for discriminating background. Here we demonstrate the imaging and counting of individual barium atoms in solid xenon by scanning a focused laser across a solid xenon matrix deposited on a sapphire window. When the laser irradiates an individual atom, the fluorescence persists for about 30 seconds before dropping abruptly to the background level—a clear confirmation of one-atom imaging.

Collapsars as a major source of r-process elements

黑洞是r-過程元素的主要來源

作者：Daniel M. Siegel, Jennifer Barnes & Brian D. Metzger

鏈接：

https://www.nature.com/articles/s41586-019-1136-0

摘要：

快速中子俘獲（r-過程）在中子星合併過程中產生元素，在理論上是可以預期的，並且得到了重力波事件GW170817多體觀測結果的支持：這種產生路線原則上足以解釋宇宙中大多數的r-過程元素。

這裡，我們報告的模擬結果表明，黑洞吸積盤產生足夠的r-過程元素，以解釋宇宙中被觀測到的丰度。

雖然這些超新星比中子星合併更罕見，但每次事件噴射出的物質量越多，其發生率就越低。我們計算得出，黑洞可能提供了宇宙中超過80%的r-過程。

Abstract

The production of elements by rapid neutron capture (r-process) in neutron-star mergers is expected theoretically and is supported by multimessenger observations of gravitational-wave event GW170817: this production route is in principle sufficient to account for most of the r-process elements in the Universe. Here we report simulations that show that collapsar accretion disks yield sufficient r-process elements to explain observed abundances in the Universe. Although these supernovae are rarer than neutron-star mergers, the larger amount of material ejected per event compensates for the lower rate of occurrence. We calculate that collapsars may supply more than 80 per cent of the r-process content of the Universe.

生物學Biology

The bone marrow microenvironment at single-cell resolution

單細胞解析度的骨髓微環境

作者：Anastasia N. Tikhonova, Igor Dolgalev, Hai Hu, et al

鏈接：

https://www.nature.com/articles/s41586-019-1104-8

摘要：

骨髓微環境在調節造血過程中起著關鍵作用，但其分子複雜性和應激反應尚不完全清楚。

這裡，我們繪製了單細胞解析度下小鼠骨髓血管、血管周圍和成骨細胞群在穩態和應激誘導造血條件下的轉錄圖譜。

這項分析揭示了骨髓生態位中以前未被認識到的細胞異質性水平，並且解決了促造血生長因子、趨化因子和膜結合配體的細胞來源。

Abstract

The bone marrow microenvironment has a key role in regulating haematopoiesis, but its molecular complexity and response to stress are incompletely understood. Here we map the transcriptional landscape of mouse bone marrow vascular, perivascular and osteoblast cell populations at single-cell resolution, both at homeostasis and under conditions of stress-induced haematopoiesis. This analysis revealed previously unappreciated levels of cellular heterogeneity within the bone marrow niche and resolved cellular sources of pro-haematopoietic growth factors, chemokines and membrane-bound ligands.

Visualization of clustered protocadherin neuronal self-recognition complexes

簇狀原鈣粘蛋白神經元自識別複合物的可視化

作者：Julia Brasch, Kerry M. Goodman, Alex J. Noble, Micah Rapp, et al

鏈接：

https://www.nature.com/articles/s41586-019-1089-3

摘要：

神經突的自我識別和迴避是所有神經系統的基本特徵。這些過程促進樹突狀樹枝化，阻止自身突觸的形成，並允許非自我神經元之間的自由互動。

這裡，我們利用X射線晶體學和低溫電子層析技術，研究了單異構體自識別複合物中全長聚簇原鈣粘蛋白外域的分子排列。

我們確定了簇狀原鈣粘附因子γB4胞外域的晶體結構。這揭示了一個由交替的順式和反式交互形成的拉鏈狀晶格。

Abstract

Neurite self-recognition and avoidance are fundamental properties of all nervous systems. These processes facilitate dendritic arborization, prevent formation of autapses and allow free interaction among non-self neurons. Here we determine the molecular arrangement of full-length clustered protocadherin ectodomains in single-isoform self-recognition complexes, using X-ray crystallography and cryo-electron tomography. We determine the crystal structure of the clustered protocadherin γB4 ectodomain, which reveals a zipper-like lattice that is formed by alternating cis and trans interactions.

化學Chemistry

Aqueous Li-ion battery enabled by halogen conversion–intercalation chemistry in graphite

利用石墨的鹵素轉化—插入化學反應製備水性鋰離子電池

作者：Chongyin Yang, Ji Chen, Xiao Ji, et al

鏈接：

https://www.nature.com/articles/s41586-019-1175-6

摘要：

「鹽中水」電解質的使用將水性鋰離子電池的電化學窗口擴展至3~4伏特，從而使高壓陰極與低電位石墨陽極耦合成為可能。

這裡，我們報告了石墨中的鹵素轉化—嵌入化學。它能產生每克（電極總重量）243毫安時的複合電極，相對於Li/Li⁺的平均電位為4.2伏特。

實驗表徵和建模將這種高比容歸因於密集填充的I級石墨插層化合物C_3.5[Br_0.5Cl_0.5]。它可以在雙水電解質中可逆地形成。

Abstract

The use of 『water-in-salt』 electrolytes has considerably expanded the electrochemical window of aqueous lithium-ion batteries to 3 to 4 volts, making it possible to couple high-voltage cathodes with low-potential graphite anodes. Here we report a halogen conversion–intercalation chemistry in graphite that produces composite electrodes with a capacity of 243 milliampere-hours per gram (for the total weight of the electrode) at an average potential of 4.2 volts versus Li/Li⁺. Experimental characterization and modelling attribute this high specific capacity to a densely packed stage-I graphite intercalation compound, C_3.5[Br_0.5Cl_0.5], which can form reversibly in water-in-bisalt electrolyte.

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