《科学》(20210402出版)一周论文导读
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Science , 02 APRIL 2021, VOL 372, ISSUE 6537
《科学》 2021年4月2日，第372卷，6537期
天文学 Astronomy
Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust
地壳中封存了海洋规模的水使火星长期干燥
作者：E. L. Scheller, B. L. Ehlmann, Renyu Hu, D. J. Adams, Y. L. Yung
链接：
https://science.sciencemag.org/content/372/6537/56
摘要
地质证据表明古代火星有大量液态水。过去的火星氢逃逸到太空的模型，用当前逃逸速率的观测值来校准，不能解释现在的氘-氢同位素比率（D/H）。
研究组模拟了火星上的火山脱气、大气逃逸和地壳水合作用，并结合了来自航天器、探测器和陨石的观测*。结果发现，相当于100至1500米火星地层的古代水量与地质证据、损失率估计和D/H测量值同时一致。
在研究组的模型中，参与水文循环的水量在诺亚时期（约37至41亿年前）减少了40%至95%，在约30亿年前达到今天的数值。火星上30%到99%的水通过地壳水合作用被隔离，这表明不可逆的化学风化可增加类地行星的干旱程度。
Abstract
Geological evidence shows that ancient Mars had large volumes of liquid water. Models of past hydrogen escape to space, calibrated with observations of the current escape rate, cannot explain the present-day deuterium-to-hydrogen isotope ratio (D/H). We simulated volcanic degassing, atmospheric escape, and crustal hydration on Mars, incorporating observational constraints from spacecraft, rovers, and meteorites. We found that ancient water volumes equivalent to a 100 to 1500 meter global layer are simultaneously compatible with the geological evidence, loss rate estimates, and D/H measurements. In our model, the volume of water participating in the hydrological cycle decreased by 40 to 95% over the Noachian period (~3.7 billion to 4.1 billion years ago), reaching present-day values by ~3.0 billion years ago. Between 30 and 99% of martian water was sequestered through crustal hydration, demonstrating that irreversible chemical weathering can increase the aridity of terrestrial planets.
物理学 Physics
Observation of a non-Hermitian phase transition in an optical quantum gas
光学量子气体中非厄米相变的观察
作者：Fahri Emre Öztürk, Tim Lappe, Göran Hellmann, Julian Schmitt, Jan Klaers, Frank Vewinger, et al.
链接：
https://science.sciencemag.org/content/372/6537/88
摘要
光的量子气体，例如光学微腔中的光子或极化激元凝聚，是一种集合量子系统，能够通过诸如空腔损耗来调节耗散。这一特性使它们成为研究耗散相的工具，耗散相是量子多体物理学中的一个新兴课题。
研究组实验证明了光子玻色-爱因斯坦凝聚的非厄米相转到二阶相干双指数衰减的耗散相。相变的发生是因为量子气体中出现了一个异常点。
虽然玻色-爱因斯坦凝聚常通过平滑交叉连接到激光上，但观察到的相变将双指数相位从激光和一个中间振荡凝聚区中分离出来。研究组的方法可用于研究拓扑或晶格系统中的大量耗散量子相。
Abstract
Quantum gases of light, such as photon or polariton condensates in optical microcavities, are collective quantum systems enabling a tailoring of dissipation from, for example, cavity loss. This characteristic makes them a tool to study dissipative phases, an emerging subject in quantum many-body physics. We experimentally demonstrate a non-Hermitian phase transition of a photon Bose-Einstein condensate to a dissipative phase characterized by a biexponential decay of the condensate’s second-order coherence. The phase transition occurs because of the emergence of an exceptional point in the quantum gas. Although Bose-Einstein condensation is usually connected to lasing by a smooth crossover, the observed phase transition separates the biexponential phase from both lasing and an intermediate, oscillatory condensate regime. Our approach can be used to study a wide class of dissipative quantum phases in topological or lattice systems.
Nonlinear tuning of PT symmetry and non-Hermitian topological states
PT对称与非厄米拓扑态的非线性调谐
作者：Shiqi Xia, Dimitrios Kaltsas, Daohong Song, Ioannis Komis, Jingjun Xu, Alexander Szameit, et al.
链接：
https://science.sciencemag.org/content/372/6537/72
摘要
在自然科学的复杂系统中，拓扑结构、宇称时间（PT）对称性和非线性是许多基本现象的起源，但它们之间的相互作用尚未明确。
研究组建立了一个非线性非厄米拓扑平台，用于PT对称性和拓扑态的主动调谐。他们发现，非厄米光子晶格中拓扑缺陷势的损耗可完全通过非线性调谐，实现PT对称和非PT对称区域之间的转换，并引导拓扑零模。
研究组还揭示了两种明显对抗效应间的相互作用，即对异常点的敏感性和非厄米拓扑态的鲁棒性。该方案利用局部非线性的PT对称和拓扑结构的单通道控制，为非常规的光操作和器件应用提供了机会。
Abstract
Topology, parity-time (PT) symmetry, and nonlinearity are at the origin of many fundamental phenomena in complex systems across the natural sciences, but their mutual interplay remains unexplored. We established a nonlinear non-Hermitian topological platform for active tuning of PT symmetry and topological states. We found that the loss in a topological defect potential in a non-Hermitian photonic lattice can be tuned solely by nonlinearity, enabling the transition between PT-symmetric and non–PT-symmetric regimes and the maneuvering of topological zero modes. The interaction between two apparently antagonistic effects is revealed: the sensitivity close to exceptional points and the robustness of non-Hermitian topological states. Our scheme using single-channel control of global PT symmetry and topology via local nonlinearity may provide opportunities for unconventional light manipulation and device applications.
材料科学 Materials Science
A van der Waals interface that creates in-plane polarization and a spontaneous photovoltaic effect
产生面内极化和自发光伏效应的范德华界面
作者：Takatoshi Akamatsu, Toshiya Ideue, Ling Zhou, Yu Dong, Sota Kitamura, Mao Yoshii, et al.
链接：
https://science.sciencemag.org/content/372/6537/68
摘要
范德华界面可以通过层堆叠形成，而不考虑单个构建块的晶格常数或对称性。
研究组设计了硒化钨和黑磷的范德华界面的对称性，实现了面内电子极化，从而产生自发光伏效应。沿极性方向可观察到自发光电流，垂直于极性方向则没有自发光电流。
所观察到的自发光电流可以用量子力学位移电流来解释，该位移电流反映了这个涌现界面的几何和拓扑电子性质。研究结果为适用于各种范德华界面的对称工程提供了一个简单指导。
Abstract
Van der Waals interfaces can be formed by layer stacking without regard to lattice constants or symmetries of inpial building blocks. We engineered the symmetry of a van der Waals interface of tungsten selenide and black phosphorus and realized in-plane electronic polarization that led to the emergence of a spontaneous photovoltaic effect. Spontaneous photocurrent was observed along the polar direction and was absent in the direction perpendicular to it. The observed spontaneous photocurrent was explained by a quantum-mechanical shift current that reflects the geometrical and topological electronic nature of this emergent interface. The present results offer a simple guideline for symmetry engineering that is applicable to a variety of van der Waals interfaces.
地球科学 Earth Science
Extinction at the end-Cretaceous and the origin of modern Neotropical rainforests
白垩纪末期大灭绝和现代新热带雨林的起源
作者：Mónica R. Carvalho, Carlos Jaramillo, Felipe de la Parra, Dayenari Caballero-Rodríguez, Fabiany Herrera, Scott Wing, et al.
链接：
https://science.sciencemag.org/content/372/6537/63
摘要
白垩纪末期事件对全球陆生群落来说是灾难性的，然而它对热带森林的长期影响在很大程度上仍然未知。
研究组使用哥伦比亚地区的化石花粉（>50000次出现）和树叶（>6000个标本），对白垩纪末期事件导致的热带森林植物灭绝和生态变化进行了量化。
晚白垩世（马斯特里赫特）雨林的特征是开放的树冠和多样的植物-昆虫相互作用。白垩纪-古近纪界线的植物多样性下降了45%，约600万年没有恢复。
古新世森林与现代新热带雨林相似，具有封闭的树冠和以被子植物为主的多层结构。白垩纪末期事件造成很长一段时间内新热带地区植物多样性较低，并导致了今天最具多样性的陆地生态系统的进化组合。
Abstract
The end-Cretaceous event was catastrophic for terrestrial communities worldwide, yet its long-lasting effect on tropical forests remains largely unknown. We quantified plant extinction and ecological change in tropical forests resulting from the end-Cretaceous event using fossil pollen (>50,000 occurrences) and leaves (>6000 specimens) from localities in Colombia. Late Cretaceous (Maastrichtian) rainforests were characterized by an open canopy and perse plant–insect interactions. Plant persity declined by 45% at the Cretaceous–Paleogene boundary and did not recover for ~6 million years. Paleocene forests resembled modern Neotropical rainforests, with a closed canopy and multistratal structure dominated by angiosperms. The end-Cretaceous event triggered a long interval of low plant persity in the Neotropics and the evolutionary assembly of today’s most perse terrestrial ecosystem.
At-risk marine biopersity faces extensive, expanding, and intensifying human impacts
濒危海洋生物多样性面临着广泛、扩大和加剧的人类影响
作者：Casey C. O’Hara, Melanie Frazier, Benjamin S. Halpern
链接：
https://science.sciencemag.org/content/372/6537/84
摘要
人类活动和气候变化威胁着全世界的海洋生物多样性，尽管不同物种和分类群对这些应激源的敏感性存在很大差异。
将2003-2013年14种人为应激源的空间分布映射到1271种对其敏感的濒危海洋物种，研究组发现，平均而言，这些物种在它们57%的活动范围内面临潜在影响，该足迹随着时间的推移而扩大，而在37%的活动范围内影响加剧。
虽然渔业活动在国家水域的影响足迹中占主导地位，但气候压力源也推动了影响的扩大和加剧。减轻对濒危生物多样性的影响对于支持具有恢复力的海洋生态系统至关重要，同时，识别跨多个分类群的影响共现现象突出了扩大保护管理效益的机会。
Abstract
Human activities and climate change threaten marine biopersity worldwide, though sensitivity to these stressors varies considerably by species and taxonomic group. Mapping the spatial distribution of 14 anthropogenic stressors from 2003 to 2013 onto the ranges of 1271 at-risk marine species sensitive to them, we found that, on average, species faced potential impacts across 57% of their ranges, that this footprint expanded over time, and that the impacts intensified across 37% of their ranges. Although fishing activity dominated the footprint of impacts in national waters, climate stressors drove the expansion and intensification of impacts. Mitigating impacts on at-risk biopersity is critical to supporting resilient marine ecosystems, and identifying the co-occurrence of impacts across multiple taxonomic groups highlights opportunities to amplify the benefits of conservation management.
