自然（20230323出版）一周论文导读

　　编译 | 未玖
　　Nature , 23 March 2023, VOL 615, ISSUE 7953
　　《自然》 2023年3月23日，第615卷，7953期
　　天文学 Astronomy
　　A helium-burning white dwarf binary as a supersoft X-ray source
　　氦燃烧白矮星双星可作为超软X射线源
　　作者：J. Greiner, C. Maitra, F. Haberl, R. Willer, J. M. Burgess, N. Langer, et al.
　　链接：
　　https://www.nature.com/articles/s41586-023-05714-4
　　摘要：
　　Ia型超新星是宇宙距离的指示器，也是宇宙中铁的主要来源，但它们的形成路径仍有争议。几十个超软X射线源（其中一颗白矮星从一颗非简并供体恒星中吸积富氢物质）已被观测到，并被认为是Ia型超新星的前体。
　　然而人们预期的氢在超新星爆炸期间从供体恒星上剥离的观测证据仍然缺乏。30多年前就有人预测，吸积氦的白矮星可避开这个问题，包括它们作为超软X射线源的出现，但迄今为止尚未探测到。
　　研究组报道了一个吸积盘光谱完全由氦主导的超软X射线源，这表明供体恒星不含氢。他们认为明亮的超软X射线因吸积白矮星表面附近氦燃烧产生。
　　该系统的性质为基于氦吸积的钱德拉塞卡质量爆炸扩展路径提供了证据，尤其是在白矮星中以低于目前预期的吸积率稳定燃烧。在这种情况下，人们有望恢复亚高能被称为Iax型超新星的数量，高达所有Ia型超新星总数的30%。
　　Abstract：
　　Type Ia supernovae are cosmic distance indicators, and the main source of iron in the Universe, but their formation paths are still debated. Several dozen supersoft X-ray sources, in which a white dwarf accretes hydrogen-rich matter from a non-degenerate donor star, have been observed and suggested as Type Ia supernovae progenitors. However, observational evidence for hydrogen, which is expected to be stripped off the donor star during the supernova explosion, is lacking. Helium-accreting white dwarfs, which would circumvent this problem, have been predicted for more than 30 years, including their appearance as supersoft X-ray sources, but have so far escaped detection. Here we report a supersoft X-ray source with an accretion disk whose optical spectrum is completely dominated by helium, suggesting that the donor star is hydrogen-free. We interpret the luminous and supersoft X-rays as resulting from helium burning near the surface of the accreting white dwarf. The properties of our system provide evidence for extended pathways towards Chandrasekhar-mass explosions based on helium accretion, in particular for stable burning in white dwarfs at lower accretion rates than expected so far. This may allow us to recover the population of the sub-energetic so-called Type Iax supernovae, up to 30% of all Type Ia supernovae, within this scenario.
　　物理学 Physics
　　Diode effect in Josephson junctions with a single magnetic atom
　　单个磁原子约瑟夫森结中的二极管效应
　　作者：Martina Trahms, Larissa Melischek, Jacob F. Steiner, Bharti Mahendru, Idan Tamir, Nils Bogdanoff, et al.
　　链接：
　　https://www.nature.com/articles/s41586-023-05743-z
　　摘要：
　　电子器件中的电流流动可能与偏置方向不对称，这是二极管效用的一种潜在现象，被称为非互易电荷传输。最近，无耗散电子学的前景刺激了人们对超导二极管的追求，非互易超导器件已在各种非中心对称系统中实现。
　　研究组通过在扫描隧道显微镜中创建原子尺度的Pb-Pb约瑟夫森结来研究微型化的最终极限。由单个Pb原子稳定的原始结表现出迟滞行为，证实了结的高质量，但偏置方向之间没有不对称性。当将单个磁原子嵌入结中时，会出现非互易超电流，其首选方向取决于原子种类。
　　借助理论建模，研究组追踪了超导能隙内电子空穴不对称Yu-Shiba-Rusinov态流动的准粒子电流的非互易性，并确定了约瑟夫森结中二极管行为的新机制。该研究结果为创造原子级约瑟夫森二极管和通过单原子操作调整其特性开辟了新途径。
　　Abstract：
　　Current flow in electronic devices can be asymmetric with bias direction, a phenomenon underlying the utility of diodes and known as non-reciprocal charge transport. The promise of dissipationless electronics has recently stimulated the quest for superconducting diodes, and non-reciprocal superconducting devices have been realized in various non-centrosymmetric systems. Here we investigate the ultimate limits of miniaturization by creating atomic-scale Pb–Pb Josephson junctions in a scanning tunnelling microscope. Pristine junctions stabilized by a single Pb atom exhibit hysteretic behaviour, confirming the high quality of the junctions, but no asymmetry between the bias directions. Non-reciprocal supercurrents emerge when inserting a single magnetic atom into the junction, with the preferred direction depending on the atomic species. Aided by theoretical modelling, we trace the non-reciprocity to quasiparticle currents flowing by means of electron–hole asymmetric Yu–Shiba–Rusinov states inside the superconducting energy gap and identify a new mechanism for diode behaviour in Josephson junctions. Our results open new avenues for creating atomic-scale Josephson diodes and tuning their properties through single-atom manipulation.
　　信息科学 Information Science
　　Dense reinforcement learning for safety validation of autonomous vehicles
　　密集强化学习助力自动驾驶汽车安全性验证
　　作者：Shuo Feng, Haowei Sun, Xintao Yan, Haojie Zhu, Zhengxia Zou, Shengyin Shen, et al.
　　链接：
　　https://www.nature.com/articles/s41586-023-05732-2
　　摘要：
　　阻碍自动驾驶汽车开发和部署的一个关键瓶颈是，由于安全关键事件很罕见，在自然驾驶环境中验证其安全性需要高昂的经济和时间成本。
　　研究组开发了一个智能测试环境，其中基于人工智能的后台代理经过训练，以验证自动驾驶汽车在加速模式下的安全性能，而不损失无偏性。从自然驾驶数据中，后台代理通过密集深度强化学习（D2RL）方法学习要执行何种对抗性操作，通过删除非安全关键状态并重新连接关键状态来编辑马尔可夫决策过程，从而使训练数据中的信息密集化。
　　D2RL使神经网络能够从具有安全关键事件的密集信息中学习，并完成传统深度强化学习方法难以解决的任务。研究组通过在增强现实环境下在高速公路和城市测试轨道上测试高度自动化的车辆，并将模拟背景车辆与物理道路基础设施和真实自动驾驶测试车辆相结合，证明了该方法的有效性。
　　研究结果表明，经过D2RL训练的代理可将评估过程加快多个数量级（快103到105 倍）。此外，D2RL将实现与其他安全关键自主系统一起加速测试和培训。
　　Abstract：
　　One critical bottleneck that impedes the development and deployment of autonomous vehicles is the prohibitively high economic and time costs required to validate their safety in a naturalistic driving environment, owing to the rarity of safety-critical events. Here we report the development of an intelligent testing environment, where artificial-intelligence-based background agents are trained to validate the safety performances of autonomous vehicles in an accelerated mode, without loss of unbiasedness. From naturalistic driving data, the background agents learn what adversarial manoeuvre to execute through a dense deep-reinforcement-learning (D2RL) approach, in which Markov decision processes are edited by removing non-safety-critical states and reconnecting critical ones so that the information in the training data is densified. D2RL enables neural networks to learn from densified information with safety-critical events and achieves tasks that are intractable for traditional deep-reinforcement-learning approaches. We demonstrate the effectiveness of our approach by testing a highly automated vehicle in both highway and urban test tracks with an augmented-reality environment, combining simulated background vehicles with physical road infrastructure and a real autonomous test vehicle. Our results show that the D2RL-trained agents can accelerate the evaluation process by multiple orders of magnitude (103 to 105  times faster). In addition, D2RL will enable accelerated testing and training with other safety-critical autonomous systems.
　　化学 Chemistry
　　Catalytic asymmetric synthesis of cannabinoids and menthol from neral
　　从柠檬醛催化不对称合成大麻素和薄荷醇
　　作者：Joyce A. A. Grimm, Hui Zhou, Roberta Properzi, Markus Leutzsch, Giovanni Bistoni, Johanna Nienhaus, et al.
　　链接：
　　https://www.nature.com/articles/s41586-023-05747-9
　　摘要：
　　将天然或合成的柠檬醛选择性转化为（ 1R,6S ）-反式异戊醇将使薄荷醇和大麻素的可持续途径得以实现和加速。然而，人们一直认为该反应无法实现，因为其产物对所需酸性催化剂的反应性比原料更强，导致一些副产物生成。
　　研究组表明，一种不对称、强而空间受限的手性酸（即高度氟化的亚胺-亚胺二磷酸）能以优异的效率和选择性催化这一过程。将该策略扩展到其他α、β-不饱和醛，可获得此前不易获得的新型大麻素和薄荷醇衍生物。
　　机理研究表明，空间受限催化剂通过将产物以非活性构象结合，阻止其分解，从而完成该反应。研究组还展示了（ 1R,6S ）-反式异戊醇如何高效地转化为药用大麻素和薄荷醇，每种策略都是迄今为止最短和最原子经济性的路线。
　　Abstract：
　　The selective conversion of natural or synthetic neral to ( 1R,6S )-trans-isopiperitenol would enable and expedite sustainable routes to menthol and cannabinoids. However, this reaction has been considered impossible because its product is more reactive to the required acid catalysts than its starting material, resulting in several side products. We now show that an unsymmetric, strong and confined chiral acid, a highly fluorinated imino-imidodiphosphate, catalyses this process with excellent efficiency and selectivity. Expanding the method to other α,β-unsaturated aldehydes could enable access to new cannabinoids and menthol derivatives not readily accessible previously. Mechanistic studies suggest that the confined catalyst accomplishes this reaction by binding the product in an unreactive conformation, thereby preventing its decomposition. We also show how ( 1R,6S )-trans-isopiperitenol can be readily converted to pharmaceutically useful cannabinoids and menthol, each in the shortest and most atom-economic routes so far.
　　地球科学 Earth Science
　　Basin-scale reconstruction of euxinia and Late Devonian mass extinctions
　　缺氧硫化与晚泥盆纪大灭绝的盆地尺度重建
　　作者：Swapan K. Sahoo, Geoffrey J. Gilleaudeau, Kathleen Wilson, Bruce Hart, Ben D. Barnes, Tytrice Faison, et al.
　　链接：
　　https://www.nature.com/articles/s41586-023-05716-2
　　摘要：
　　泥盆纪-石炭纪的过渡标志着地表环境的根本转变，主要与海洋-大气氧化态的变化有关，这是由于维管陆生植物的持续增殖刺激了水文循环和大陆风化、冰川海面升降、陆表海富营养化和缺氧扩张，以及大规模灭绝事件。
　　研究组展示了整个巴肯页岩（北美威利斯顿盆地）90个岩芯地球化学数据的综合时空汇编。该数据集详细记录了有毒的缺氧硫化水逐步侵入浅海，导致了一系列晚泥盆纪灭绝事件。
　　显生宙的其他灭绝也与浅水缺氧化扩张有关，这表明硫化氢毒性是显生宙生物多样性的关键驱动因素。
　　Abstract：
　　The Devonian–Carboniferous transition marks a fundamental shift in the surface environment primarily related to changes in ocean–atmosphere oxidation states, resulting from the continued proliferation of vascular land plants that stimulated the hydrological cycle and continental weathering, glacioeustasy, eutrophication and anoxic expansion in epicontinental seas, and mass extinction events. Here we present a comprehensive spatial and temporal compilation of geochemical data from 90 cores across the entire Bakken Shale (Williston Basin, North America). Our dataset allows for the detailed documentation of stepwise transgressions of toxic euxinic waters into the shallow oceans that drove a series of Late Devonian extinction events. Other Phanerozoic extinctions have also been related to the expansion of shallow-water euxinia, indicating that hydrogen sulfide toxicity was a key driver of Phanerozoic biopersity.
　　Core origin of seismic velocity anomalies at Earth’s core–mantle boundary
　　地核-地幔边界地震速度异常的地核起源
　　作者：Suyu Fu, Stella Chariton, Vitali B. Prakapenka & Sang-Heon Shim
　　链接：
　　https://www.nature.com/articles/s41586-023-05713-5
　　摘要：
　　地震研究在地核-地幔边界（CMB）发现了精细尺度异常，如超低速带（ULVZs）和地核刚性带。ULVZs被归因于地幔相关过程，但对可能的地核起源知之甚少。人们提议用外核轻元素沉淀来解释核心刚性区，但仍不清楚是什么过程导致了这种沉淀。尽管Fe-Si-H对外核很重要，但其在相关压力-温度条件下的熔化行为尚未明确。
　　研究组报道了在125 GPa和3700 K的氢气存在下，通过激光加热的金刚石压砧从Fe-9wt %Si熔体中观察到B2 FeSi结晶。氢极大地增加了B2晶体中Si的浓度，使Si:Fe摩尔比达到 1，而Si大部分保留在共存的Fe液中。
　　B2相中的高Si含量使其在最外层地核温度下以固体形式稳定存在，且密度小于周围的液体。因此，富硅晶体可在CMB界面下方形成、漂浮和沉积，这很好地解释了地核侧刚性异常。如果少量FeSi晶体能被地幔吸收，它们将在CMB上方形成致密的低速结构，这可能是一些ULVZs形成的原因。
　　最外层地核H促进的B2 FeSi沉淀为CMB的两类异常提供了单一地核驱动来源。这种情况也可用于解释，与沉淀物平衡的物质通过与ULVZ相连的地幔羽流被夹带到最上面地幔后，海洋岛屿玄武岩中的类核钨同位素特征。
　　Abstract：
　　Seismic studies have found fine-scale anomalies at the core–mantle boundary (CMB), such as ultralow velocity zones (ULVZs) and the core rigidity zone. ULVZs have been attributed to mantle-related processes, but little is known about a possible core origin. The precipitation of light elements in the outer core has been proposed to explain the core rigidity zone, but it remains unclear what processes can lead to such precipitation. Despite its importance for the outer core, the melting behaviour of Fe–Si–H at relevant pressure–temperature conditions is not well understood. Here we report observations of the crystallization of B2 FeSi from Fe–9wt%Si melted in the presence of hydrogen up to 125 GPa and 3,700 K by using laser-heated diamond anvil cells. Hydrogen dramatically increases the Si concentration in the B2 crystals to a molar ratio of Si:Fe   1, whereas it mostly remains in the coexisting Fe liquid. The high Si content in the B2 phase makes it stable in a solid form at the outermost core temperatures and less dense than the surrounding liquids. Consequently, the Si-rich crystallites could form, float and be sedimented to the underside of the CMB interface, and that well explains the core side rigidity anomalies. If a small amount of the FeSi crystals can be incorporated into the mantle, they would form dense low-velocity structures above the CMB, which may account for some ULVZs. The B2 FeSi precipitation promoted by H in the outermost core provides a single core-driven origin for two types of anomalies at the CMB. Such a scenario could also explain the core-like tungsten isotope signatures in ocean island basalts, after the materials equilibrated with the precipitates are entrained to the uppermost mantle by the mantle plumes connected to ULVZs.