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《自然》（20240711出版）一周論文導讀
編譯|馮維維
Nature,? Volume 631 Issue 8020, 11 July 2024
《自然》第631卷8020期，2024年7月11日
物理學Physics
Observation of Bose–Einstein condensation of dipolar molecules
偶極分子玻色-愛因斯坦凝聚的觀察
▲ 作者：Niccolò Bigagli, Weijun Yuan, Siwei Zhang, Boris Bulatovic, Tijs Karman, Ian Stevenson & Sebastian Will
▲鏈接：
https://www.nature.com/articles/s41586-024-07492-z
▲摘要：
受量子力學定律支配的粒子群表現出有趣的涌現行為。量子材料中的原子量子氣體、液氦和電子由于它們的組成和相互作用都表現出不同的性質。超冷偶極分子的量子簡并樣品有望實現物質的新相，并為量子模擬和量子計算提供新的途徑。但即使通過碰撞屏蔽技術減少了快速損失，到目前為止也阻礙了蒸發冷卻到玻色-愛因斯坦凝聚體（BEC）。
研究者報道了偶極分子的BEC的實現。通過增強碰撞屏蔽來抑制二體和三體損失，他們蒸發冷卻鈉-銫分子到量子簡并并跨越相變到BEC。當相空間密度超過1時，BEC表現為雙峰分布。生成了冷凝分數為60(10)%，溫度為6(2)nK的BECs，并發現其穩定壽命接近2秒。這項工作為探索迄今為止無法進入的體制中的偶極量子物質打開了大門，有望在光學晶格中創造奇異的偶極液滴，自組織晶體相和偶極自旋液體。
▲ Abstract：
Ensembles of particles governed by quantum mechanical laws exhibit intriguing emergent behaviour. Atomic quantum gases, liquid helium and electrons in quantum materials all exhibit distinct properties because of their composition and interactions. Quantum degenerate samples of ultracold dipolar molecules promise the realization of new phases of matter and new avenues for quantum simulation and quantum computation. However, rapid losses, even when reduced through collisional shielding techniques, have so far prevented evaporative cooling to a Bose–Einstein condensate (BEC). Here we report on the realization of a BEC of dipolar molecules. By strongly suppressing two- and three-body losses via enhanced collisional shielding, we evaporatively cool sodium–caesium molecules to quantum degeneracy and cross the phase transition to a BEC. The BEC reveals itself by a bimodal distribution when the phase-space density exceeds 1. BECs with a condensate fraction of 60(10)% and a temperature of 6(2) nK are created and found to be stable with a lifetime close to 2 s. This work opens the door to the exploration of dipolar quantum matter in regimes that have been inaccessible so far, promising the creation of exotic dipolar droplets, self-organized crystal phases and dipolar spin liquids in optical lattices.
Tunable entangled photon-pair generation in a liquid crystal
液晶中可調諧糾纏光子對的產生
▲ 作者：Vitaliy Sultanov, Alja? Kav?i?, Emmanouil Kokkinakis, Nerea Sebastián, Maria V. Chekhova & Matja? Humar?
▲鏈接：
https://www.nature.com/articles/s41586-024-07543-5
▲摘要：
液晶具有自組裝、對電場的強響應和可集成到復雜系統中的能力，是光束操縱的關鍵材料。最近發現的鐵電向列液晶也具有相當大的二階光學非線性，使其成為非線性光學的潛在材料。它們作為量子光源的使用可以大大擴展光子量子技術的邊界。然而，自發參數下轉換，糾纏光子的基本來源，預示的單光子和壓縮光，到目前為止還沒有在液晶或任何液體或有機材料中觀察到。
研究者在鐵電向列液晶中實現了自發參數下轉換，并展示了電場可調諧的糾纏光子寬帶產生，其效率可與最佳非線性晶體相媲美。施加幾伏電壓或沿樣品方向扭轉分子取向，光子對的發射速率和偏振態會發生顯著變化。液晶源可以實現一種特殊類型的準相位匹配，這種匹配基于分子扭曲結構，因此可以根據光子對的所需光譜和偏振特性進行重新配置。
這種光源有望在功能、亮度和所產生量子態的可調性方面優于標準非線性光學材料。這一概念可以擴展到復雜的拓撲結構，宏觀器件和多像素可調量子光源。
▲ Abstract：
Liquid crystals, with their ability to self-assemble, strong response to an electric field and integrability into complex systems, are key materials in light-beam manipulation1. The recently discovered ferroelectric nematic liquid crystals also have considerable second-order optical nonlinearity, making them a potential material for nonlinear optics. Their use as sources of quantum light could considerably extend the boundaries of photonic quantum technologies6. However, spontaneous parametric down-conversion, the basic source of entangled photons, heralded single photons and squeezed light, has so far not been observed in liquid crystals—or in any liquids or organic materials. Here we implement spontaneous parametric down-conversion in a ferroelectric nematic liquid crystal and demonstrate electric-field tunable broadband generation of entangled photons, with an efficiency comparable to that of the best nonlinear crystals. The emission rate and polarization state of photon pairs is markedly varied by applying a few volts or twisting the molecular orientation along the sample. A liquid-crystal source enables a special type of quasi-phase matching, which is based on the molecular twist structure and is therefore reconfigurable for the desired spectral and polarization properties of photon pairs. Such sources promise to outperform standard nonlinear optical materials in terms of functionality, brightness and the tunability of the generated quantum state. The concepts developed here can be extended to complex topological structures, macroscopic devices and multi-pixel tunable quantum light sources.
Fast-moving stars around an intermediate-mass black hole in ω Centauri
圍繞半人馬ω球狀星團內一個中等質量黑洞的快速移動恒星
▲ 作者：Maximilian H?berle, Nadine Neumayer, Anil Seth, Andrea Bellini, Mattia Libralato, Holger Baumgardt, Matthew Whitaker, Antoine Dumont, Mayte Alfaro-Cuello, Jay Anderson, Callie Clontz, Nikolay Kacharov, Sebastian Kamann, Anja Feldmeier-Krause, Antonino Milone, Maria Selina Nitschai, Renuka Pechetti & Glenn van de Ven
▲鏈接：
https://www.nature.com/articles/s41586-024-07511-z
▲摘要：
科學家在銀河系半人馬座歐米茄星團（半人馬ω球狀星團）內觀測到的快速移動恒星為一個中等質量黑洞的存在提供了證據。該研究基于哈勃空間望遠鏡20多年的觀測結果，或有助于今后研究人員搜索這類難以發現的黑洞。
科學家已發現各種質量的黑洞，從5~150倍太陽質量（M⊙）的恒星質量黑洞到星系中央的超大質量黑洞（超過10萬倍太陽質量；105M⊙）。不過，目前只發現了少數存在爭議的候選中等質量黑洞，這類黑洞的質量在太陽質量的150倍到10萬倍不等。
鑒于半人馬ω球狀星團高質量和復雜星族的特性，它是尋找中等質量黑洞的一個很有希望的目標，但之前對該區域發現了一個黑洞的報道一直面臨爭議。德國馬普學會天文研究所的MaximilianHaberle和同事利用哈勃空間望遠鏡20年的影像，監測了該星團中央附近恒星的運動。他們提出有7個恒星在中央區域快速移動，并認為這說明半人馬ω球狀星團的中央有一個中等質量黑洞。他們推測該黑洞質量的下限為8200M⊙左右。
“這些新發現的恒星是半人馬ω球狀星團中存在一個中等質量黑洞的最佳證據。”加拿大麥吉爾大學的DarylHaggard和美國舊金山州立大學的AdrienneCool在一篇同期發表的新聞觀點文章中寫道，研究結果或表明在其他球狀星團中搜尋中等質量黑洞是合理的。
歐空局哈勃拍攝的半人馬座歐米茄星團新彩色圖像。圖片來自：ESA哈勃&NASA，M.Haberle
▲ Abstract：
Black holes have been found over a wide range of masses, from stellar remnants with masses of 5–150 solar masses (M☉), to those found at the centres of galaxies with M>105M☉. However, only a few debated candidate black holes exist between 150M☉ and 105M☉. Determining the population of these intermediate-mass black holes is an important step towards understanding supermassive black hole formation in the early universe. Several studies have claimed the detection of a central black hole in ω Centauri, the most massive globular cluster of the Milky Way. However, these studies have been questioned because of the possible mass contribution of stellar mass black holes, their sensitivity to the cluster centre and the lack of fast-moving stars above the escape velocity. Here we report the observations of seven fast-moving stars in the central 3 arcsec (0.08 pc) of ω Centauri. The velocities of the fast-moving stars are significantly higher than the expected central escape velocity of the star cluster, so their presence can be explained only by being bound to a massive black hole. From the velocities alone, we can infer a firm lower limit of the black hole mass of about 8,200M☉, making this a good case for an intermediate-mass black hole in the local universe.
Tunable superconductivity in electron- and hole-doped Bernal bilayer graphene
電子和空穴摻雜Bernal雙層石墨烯的可調諧超導性
▲ 作者：Chushan Li, Fan Xu, Bohao Li, Jiayi Li, Guoan Li, Kenji Watanabe, Takashi Taniguchi, Bingbing Tong, Jie Shen, Li Lu, Jinfeng Jia, Fengcheng Wu, Xiaoxue Liu & Tingxin Li
▲鏈接：
https://www.nature.com/articles/s41586-024-07584-w
▲摘要：
基于石墨烯的高質量二維電子系統已經成為研究超導性的高度可調平臺。具體來說，在電子和空穴摻雜的扭曲石墨烯渦流體系中都觀察到了超導性，而在晶體石墨烯體系中，迄今為止只在空穴摻雜的菱形三層石墨烯(RTG)和空穴摻雜的Bernal雙層石墨烯(BBG)中觀察到超導性。最近，由于接近單層WSe2, BBG中的超導性得到了增強。
研究者報道了通過靜電摻雜在電子和空穴摻雜的BBG/WSe2器件中觀察到的超導性和一系列風味對稱性破缺相。觀察到的超導性的強度可以通過施加垂直電場來調節。電子摻雜和空穴摻雜超導的最大berezinski - kosterlitz - Thouless轉變溫度分別約為210 mK和400 mK。只有當外加電場驅動BBG電子或空穴波函數向WSe2層移動時，超導性才會出現，這強調了WSe2層在觀察到的超導性中的重要性。空穴摻雜的超導違反了泡利順磁極限，符合伊辛類超導體。相比之下，電子摻雜的超導性服從泡利極限，盡管在導帶中鄰近誘導的伊辛自旋軌道耦合也很明顯。該發現突出了與BBG中傳導帶相關的豐富物理特性，為進一步研究晶體石墨烯的超導機制和基于BBG的超導體器件的開發鋪平了道路。
▲ Abstract：
Graphene-based, high-quality, two-dimensional electronic systems have emerged as a highly tunable platform for studying superconductivity. Specifically, superconductivity has been observed in both electron- and hole-doped twisted graphene moiré systems, whereas in crystalline graphene systems, superconductivity has so far been observed only in hole-doped rhombohedral trilayer graphene (RTG) and hole-doped Bernal bilayer graphene (BBG). Recently, enhanced superconductivity has been demonstrated in BBG because of the proximity to a monolayer WSe2. Here we report the observation of superconductivity and a series of flavour-symmetry-breaking phases in electron- and hole-doped BBG/WSe2 devices by electrostatic doping. The strength of the observed superconductivity is tunable by applied vertical electric fields. The maximum Berezinskii–Kosterlitz Thouless transition temperature for the electron- and hole-doped superconductivity is about 210mK and 400mK, respectively. Superconductivities emerge only when the applied electric fields drive the BBG electron or hole wavefunctions towards the WSe2 layer, underscoring the importance of the WSe2 layer in the observed superconductivity. The hole-doped superconductivity violates the Pauli paramagnetic limit, consistent with an Ising-like superconductor. By contrast, the electron-doped superconductivity obeys the Pauli limit, although the proximity-induced Ising spin–orbit coupling is also notable in the conduction band. Our findings highlight the rich physics associated with the conduction band in BBG, paving the way for further studies into the superconducting mechanisms of crystalline graphene and the development of superconductor devices based on BBG.
化學Chemistry
Glassy gels toughened by solvent
用溶劑增韌的玻璃狀凝膠
▲ 作者：Meixiang Wang, Xun Xiao, Salma Siddika, Mohammad Shamsi, Ethan Frey, Wen Qian, Wubin Bai, Brendan T. O’Connor & Michael D. Dickey
▲鏈接：
https://www.nature.com/articles/s41586-024-07564-0
▲摘要：
玻璃狀聚合物通常堅硬而堅固，但延展性有限。通過溶脹，玻璃狀聚合物可以變成柔軟而脆弱的凝膠，但具有增強的延展性。性能的顯著變化是由于溶劑增加了鏈間的自由體積，同時減弱了聚合物與聚合物之間的相互作用。研究者展示了極性聚合物與離子液體——離子凝膠，在適當濃度下的溶劑化可以產生一種獨特的材料，稱為玻璃凝膠，具有玻璃和凝膠的理想性能。
離子液體增加了自由體積，因此，盡管缺乏傳統的溶劑(例如，水)，可擴展性。但在室溫下，離子液體在聚合物鏈之間形成強而豐富的非共價交聯，形成堅硬、堅韌、玻璃狀和均勻的網絡(即沒有相分離)。盡管液體含量超過54wt %，但玻璃凝膠具有巨大的斷裂強度(42 MPa)、韌性(110 MJ m?3)、屈服強度(73 MPa)和楊氏模量(1 GPa)。這些值與熱塑性塑料(如聚乙烯)相似，但與熱塑性塑料不同的是，玻璃狀凝膠可以變形到670%的應變，并且在加熱時可以完全快速恢復。這些透明材料通過一步聚合形成，具有令人印象深刻的粘合、自愈和形狀記憶特性。
▲ Abstract：
Glassy polymers are generally stiff and strong yet have limited extensibility. By swelling with solvent, glassy polymers can become gels that are soft and weak yet have enhanced extensibility. The marked changes in properties arise from the solvent increasing free volume between chains while weakening polymer–polymer interactions. Here we show that solvating polar polymers with ionic liquids (that is, ionogels) at appropriate concentrations can produce a unique class of materials called glassy gels with desirable properties of both glasses and gels. The ionic liquid increases free volume and therefore extensibility despite the absence of conventional solvent (for example, water). Yet, the ionic liquid forms strong and abundant non-covalent crosslinks between polymer chains to render a stiff, tough, glassy, and homogeneous network (that is, no phase separation), at room temperature. Despite being more than 54wt% liquid, the glassy gels exhibit enormous fracture strength (42 MPa), toughness (110 MJ m?3), yield strength (73 MPa) and Young’s modulus (1 GPa). These values are similar to those of thermoplastics such as polyethylene, yet unlike thermoplastics, the glassy gels can be deformed up to 670% strain with full and rapid recovery on heating. These transparent materials form by a one-step polymerization and have impressive adhesive, self-healing and shape-memory properties.
Direct radical functionalization of native sugars
天然糖的直接自由基功能化
▲ 作者：Yi Jiang, Yi Wei, Qian-Yi Zhou, Guo-Quan Sun, Xia-Ping Fu, Nikita Levin, Yijun Zhang, Wen-Qiang Liu, NingXi Song, Shabaz Mohammed, Benjamin G. Davis & Ming Joo Koh
▲鏈接：
https://www.nature.com/articles/s41586-024-07548-0
▲摘要：
天然存在的糖和碳水化合物含有許多具有相似反應活性的羥基。因此，化學家通常依靠費力的、多步驟的保護基團策略，將這些可再生原料轉化為制造聚糖的試劑(糖基供體)。
將天然糖直接轉化為復合糖仍然是一個顯著的挑戰。研究者描述了一種光誘導方法，通過均溶(單電子)化學繞過不必要的羥基掩膜和操縱，從廣泛可用的天然糖構建塊實現位點和立體選擇性化學糖基化。這一過程使人聯想到自然界在區域控制下產生瞬時糖基供體，在光激活下與親電試劑進行基于自由基的交叉偶聯。
通過選擇性的單糖和寡糖的頭聚糖功能化，這種無保護基團的“帽和糖基酸”方法提供了一種直接的途徑，可以獲得大量代謝旺盛的糖基化合物。由于其生物相容性，該方法被擴展到蛋白質的直接翻譯后糖基化。
▲ Abstract：
Naturally occurring (native) sugars and carbohydrates contain numerous hydroxyl groups of similar reactivity. Chemists, therefore, rely typically on laborious, multi-step protecting-group strategies3 to convert these renewable feedstocks into reagents (glycosyl donors) to make glycans. The direct transformation of native sugars to complex saccharides remains a notable challenge. Here we describe a photoinduced approach to achieve site- and stereoselective chemical glycosylation from widely available native sugar building blocks, which through homolytic (one-electron) chemistry bypasses unnecessary hydroxyl group masking and manipulation. This process is reminiscent of nature in its regiocontrolled generation of a transient glycosyl donor, followed by radical-based cross-coupling with electrophiles on activation with light. Through selective anomeric functionalization of mono- and oligosaccharides, this protecting-group-free ‘cap and glycosylate’ approach offers straightforward access to a wide array of metabolically robust glycosyl compounds. Owing to its biocompatibility, the method was extended to the direct post-translational glycosylation of proteins.
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