《自然》（20210819出版）一周论文导读

编译 | 未玖

Nature, 19 August 2021, VOL 596, ISSUE 7872

《自然》2021年8月19日，第596卷，7872期

天文学Astronomy

Lenghu on the Tibetan Plateau as an astronomical observing site

新天文观测台址坐落在青藏高原冷湖镇

▲ 作者：Licai Deng, Fan Yang, Xiaodian Chen, Fei He, Qili Liu, Bo Zhang, et al.

▲ 链接：

https://www.nature.com/articles/s41586-021-03711-z

▲ 摘要

在地球表面，只有少数高质量天文站点能够满足下一代大型设施的要求。在时域天文学的科学机遇背景下，青藏高原上的一个绝佳地点将填补已知最佳地点（均位于西半球）之间的纵向差距。

青藏高原是地球上最高的高原，平均海拔超过4000米，很有可能为天文学和粒子天体物理学提供非常好的机会。

研究组报道了在青海省冷湖镇附近的赛什腾山山顶上某个试验区连续三年监测的结果。该地的海拔高度在4200至4500米之间。冷湖镇周围面积超过10万平方公里，海拔低于3000米，气候极端干旱，当地天空异常晴朗（昼夜）。

该试验区70%的夜晚具有清晰的光度条件，平均能见度为0.75弧秒。夜间气温变化中值仅为2.4℃，表明当地地表空气非常稳定。55%的夜间可降水量低于2毫米。

▲ Abstract

On Earth’s surface, there are only a handful of high-quality astronomical sites that meet the requirements for very large next-generation facilities. In the context of scientific opportunities in time-domain astronomy, a good site on the Tibetan Plateau will bridge the longitudinal gap between the known best sites (all in the Western Hemisphere). The Tibetan Plateau is the highest plateau on Earth, with an average elevation of over 4,000 metres, and thus potentially provides very good opportunities for astronomy and particle astrophysics. Here we report the results of three years of monitoring of testing an area at a local summit on Saishiteng Mountain near Lenghu Town in Qinghai Province. The altitudes of the potential locations are between 4,200 and 4,500 metres. An area of over 100,000 square kilometres surrounding Lenghu Town has a lower altitude of below 3,000 metres, with an extremely arid climate and unusually clear local sky (day and night). Of the nights at the site, 70 per cent have clear, photometric conditions, with a median seeing of 0.75 arcseconds. The median night temperature variation is only 2.4 degrees Celsius, indicating very stable local surface air. The precipitable water vapour is lower than 2 millimetres for 55 per cent of the night.

物理学Physics

Two-dimensional supersolidity in a dipolar quantum gas

二维超固态偶极量子气体

▲ 作者：Matthew A. Norcia, Claudia Politi, Lauritz Klaus, Elena Poli, Maximilian Sohmen, Manfred J. Mark, et al.

▲ 链接：

https://www.nature.com/articles/s41586-021-03725-7

▲ 摘要

超固体状态同时具有典型的固体和超流体相关的特性。像固体一样，它们晶态有序，表现为粒子密度的周期性调制；但与典型固体不同，它们也具有超流体性质，这是由整个系统中的相干粒子离域作用造成的。

这种状态最初是在大块固体氦的背景下设想的，用于探讨固体是否具有超流体性质。尽管科学家付出了很多努力，在固体氦中仍未观察到超固体，但超冷原子气体提供了另一种方法，使得近期观察和研究具有偶极原子的超固体成为可能。然而，与氦中提出的现象不同，到目前为止，这些气态系统仅显示出单一方向上的超固态。

研究组通过在结构相变的两侧制备镝原子超固态量子气体，证明了超固态性质可扩展到二维，类似于离子链、量子线以及理论上的单个偶极粒子链。这为在高度灵活和可控系统中研究丰富激励特性提供了可能，包括涡旋形成和具有不同几何结构的基态相。

▲ Abstract

Supersolid states simultaneously feature properties typically associated with a solid and with a superfluid. Like a solid, they possess crystalline order, manifesting as a periodic modulation of the particle density; but unlike a typical solid, they also have superfluid properties, resulting from coherent particle delocalization across the system. Such states were initially envisioned in the context of bulk solid helium, as a possible answer to the question of whether a solid could have superfluid properties. Although supersolidity has not been observed in solid helium (despite much effort), ultracold atomic gases provide an alternative approach, recently enabling the observation and study of supersolids with dipolar atoms. However, unlike the proposed phenomena in helium, these gaseous systems have so far only shown supersolidity along a single direction. Here we demonstrate the extension of supersolid properties into two dimensions by preparing a supersolid quantum gas of dysprosium atoms on both sides of a structural phase transition similar to those occurring in ionic chains, quantum wires and theoretically in chains of individual dipolar particles. This opens the possibility of studying rich excitation properties, including vortex formation, and ground-state phases with varied geometrical structure in a highly flexible and controllable system.

材料科学Materials Science

Ghost hyperbolic surface polaritons in bulk anisotropic crystals

体块各向异性晶体中的幽灵双曲极化激元

▲ 作者：Weiliang Ma, Guangwei Hu, Debo Hu, Runkun Chen, Tian Sun, Xinliang Zhang, et al.

▲ 链接：

https://www.nature.com/articles/s41586-021-03755-1

▲ 摘要

各向异性材料中的极化激元会产生奇异的光学特性，这为在纳米尺度上控制光提供了机会。到目前为止，这些极化激元仅限于两类：在材料内部传播的体极化激元和在远离界面处呈指数衰减的表面极化激元。

研究组报道了一种幽灵声子极化激元的近场观测，其在极性单轴晶体表面以面内双曲型色散传播，同时在体块中呈现斜波阵面。

幽灵极化激元是麦克斯韦方程组的非典型非均匀表面波解，产生于光轴相对于界面倾斜的单轴材料表面。它们表现出一种不寻常的双态性质，在晶体内同时传播（相位推进）与消逝（衰减），这与从界面完全消失的传统表面波形成了鲜明对比。

真实空间近场成像实验揭示了深亚波长幽灵极化激元在表面的长距离（超过20微米）、类射线传播，验证了长程、定向和无衍射的极化激元传播。

同时，研究组还表明，控制光轴的平面外角度可以在固定频率下实现双曲线到椭圆的拓扑转换，这为定制表面极化激元波的能带图拓扑提供了一条途径。

该研究结果证明了一种极化波现象，为在自然各向异性晶体中定制纳米尺度光提供了独特机会。

▲ Abstract

Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale. So far these polaritons have been limited to two classes: bulk polaritons, which propagate inside a material, and surface polaritons, which decay exponentially away from an interface. Here we report a near-field observation of ghost phonon polaritons, which propagate with in-plane hyperbolic dispersion on the surface of a polar uniaxial crystal and, at the same time, exhibit oblique wavefronts in the bulk. Ghost polaritons are an atypical non-uniform surface wave solution of Maxwell’s equations, arising at the surface of uniaxial materials in which the optic axis is slanted with respect to the interface. They exhibit an unusual bi-state nature, being both propagating (phase-progressing) and evanescent (decaying) within the crystal bulk, in contrast to conventional surface waves that are purely evanescent away from the interface. Our real-space near-field imaging experiments reveal long-distance (over 20 micrometres), ray-like propagation of deeply subwavelength ghost polaritons across the surface, verifying long-range, directional and diffraction-less polariton propagation. At the same time, we show that control of the out-of-plane angle of the optic axis enables hyperbolic-to-elliptic topological transitions at fixed frequency, providing a route to tailor the band diagram topology of surface polariton waves. Our results demonstrate a polaritonic wave phenomenon with unique opportunities to tailor nanoscale light in natural anisotropic crystals.

化学Chemistry

Mobility gradients yield rubbery surfaces on top of polymer glasses

聚合物玻璃顶部的橡胶表面产生流动性梯度

▲ 作者：Zhiwei Hao, Asieh Ghanekarade, Ningtao Zhu, Katelyn Randazzo, Daisuke Kawaguchi, Keiji Tanaka, et al.

▲ 链接：

https://www.nature.com/articles/s41586-021-03733-7

▲ 摘要

许多新兴材料，如用于手机显示器和OLED电视屏幕的超稳定玻璃，其性能归功于玻璃成型液体表面流动性增强的梯度。这种表面流动性增强的发现改变了人们对玻璃成型剂行为以及如何将其制成改良材料的理解。在聚合物玻璃中，由于存在第二长度尺度（聚合物链的大小）以及界面流动性梯度的长度尺度，这些界面改性变得复杂。

研究组通过模拟、理论和时间分辨表面纳米蠕变实验，来揭示玻璃态聚合物表面的这种双尺度性质，甚至在由短的亚缠结链组成的聚合物中，也会导致出现瞬态橡胶状、缠结状表面行为。他们发现，这种效应产生于分段动力学和链构象统计中的叠加梯度。

这种橡胶性行为的寿命随着材料的冷却而延长，这将在限制聚合物玻璃的摩擦学、粘附和表面愈合等应用的表面松弛方面产生广泛的影响。表层在时间-温度叠加（TTS）中遭受普遍破坏，这是聚合物物理学和流变学的基本原理。

这一发现可能需要重新评估具有高界面面积的聚合物玻璃长期性能的预测策略。研究组预计，这种界面瞬态弹性体效应和TTS击穿通常应发生在从纳米复合材料到薄膜的大分子系统中，在这些系统中，界面决定了材料的性能。

▲ Abstract

Many emerging materials, such as ultrastable glasses of interest for phone displays and OLED television screens, owe their properties to a gradient of enhanced mobility at the surface of glass-forming liquids. The discovery of this surface mobility enhancement has reshaped our understanding of the behaviour of glass formers and of how to fashion them into improved materials. In polymeric glasses, these interfacial modifications are complicated by the existence of a second length scale—the size of the polymer chain—as well as the length scale of the interfacial mobility gradient. Here we present simulations, theory and time-resolved surface nano-creep experiments to reveal that this two-scale nature of glassy polymer surfaces drives the emergence of a transient rubbery, entangled-like surface behaviour even in polymers comprised of short, subentangled chains. We find that this effect emerges from superposed gradients in segmental dynamics and chain conformational statistics. The lifetime of this rubbery behaviour, which will have broad implications in constraining surface relaxations central to applications including tribology, adhesion, and surface healing of polymeric glasses, extends as the material is cooled. The surface layers suffer a general breakdown in time−temperature superposition (TTS), a fundamental tenet of polymer physics and rheology. This finding may require a reevaluation of strategies for the prediction of long-time properties in polymeric glasses with high interfacial areas. We expect that this interfacial transient elastomer effect and TTS breakdown should normally occur in macromolecular systems ranging from nanocomposites to thin films, where interfaces dominate material properties.

地球科学Earth Science

The Montreal Protocol protects the terrestrial carbon sink

《蒙特利尔议定书》保护陆地碳汇

▲ 作者：Paul J. Young, Anna B. Harper, Chris Huntingford, Nigel D. Paul, Olaf Morgenstern, Paul A. Newman, et al.

▲ 链接：

https://www.nature.com/articles/s41586-021-03737-3

▲ 摘要

通过《蒙特利尔议定书》对消耗臭氧层物质的生产进行控制意味着平流层臭氧层正在恢复，从而避免了有害的表面紫外线辐射的增加。

《蒙特利尔议定书》对减缓气候变化具有共同益处，因为消耗臭氧层的物质是强效温室气体。避免紫外线辐射和气候变化对植物及其通过光合作用储存碳的能力也有共同益处，但这此前尚未被研究过。

研究组使用一个将臭氧消耗、气候变化、紫外线辐射对植物的损害和碳循环结合起来的模型框架，探讨了避免紫外线辐射增加和气候变化对陆地生物圈及其碳汇能力的益处。

考虑到紫外线辐射对植物生长影响的一系列优势，研究组估计，如果没有《蒙特利尔议定书》（与控制臭氧消耗物质的气候预测相比），到本世纪末（2080-2099年），植物和土壤中的碳含量可能会减少3250-6900亿吨。

该变化可导致大气中二氧化碳含量增加115-235 ppm，这可能导致全球平均地表升温0.50-1.0℃。研究结果表明，《蒙特利尔议定书》也可能通过避免减少土地碳汇来帮助缓解气候变化。

▲ Abstract

The control of the production of ozone-depleting substances through the Montreal Protocol means that the stratospheric ozone layer is recovering and that consequent increases in harmful surface ultraviolet radiation are being avoided. The Montreal Protocol has co-benefits for climate change mitigation, because ozone-depleting substances are potent greenhouse gases. The avoided ultraviolet radiation and climate change also have co-benefits for plants and their capacity to store carbon through photosynthesis, but this has not previously been investigated. Here, using a modelling framework that couples ozone depletion, climate change, damage to plants by ultraviolet radiation and the carbon cycle, we explore the benefits of avoided increases in ultraviolet radiation and changes in climate on the terrestrial biosphere and its capacity as a carbon sink. Considering a range of strengths for the effect of ultraviolet radiation on plant growth, we estimate that there could have been 325–690 billion tonnes less carbon held in plants and soils by the end of this century (2080–2099) without the Montreal Protocol (as compared to climate projections with controls on ozone-depleting substances). This change could have resulted in an additional 115–235 parts per million of atmospheric carbon dioxide, which might have led to additional warming of global-mean surface temperature by 0.50–1.0 degrees. Our findings suggest that the Montreal Protocol may also be helping to mitigate climate change through avoided decreases in the land carbon sink.

Operationalizing the net-negative carbon economy

实施净负碳经济

▲ 作者：Johannes Bednar, Michael Obersteiner, Artem Baklanov, Marcus Thomson, Fabian Wagner, Oliver Geden, et al.

▲ 链接：

https://www.nature.com/articles/s41586-021-03723-9

▲ 摘要

将全球变暖限制在1.5℃的剩余碳预算可能在本十年内耗尽。此后产生的碳债务将需要通过净负排放来补偿。然而，还没有设计出经济政策工具来保证可能非常昂贵的净二氧化碳去除（CDR）。

研究组提出跨期工具，为广泛应用的碳税和排放交易系统提供基础，为净负碳经济融资。他们研究了一种理想化的市场方法，即确定排放者通过“碳清除义务”（CRO）净清除二氧化碳的责任来激励偿还先前应计的碳债务。固有风险，如碳债务人的违约风险，可通过碳债务利息定价大气二氧化碳储存来解决。

与当前关于排放途径的文献相反，研究组发现，CRO的利息支付导致了更大规模的近期脱碳目标，而早期和较不积极的CDR部署则会对其起补充作用。研究组得出的结论是，若要确保雄心勃勃的气候目标的可行性和缓解各代人努力的公平分配，CRO将需要成为全球气候政策组合不可分割的一部分。

▲ Abstract

The remaining carbon budget for limiting global warming to 1.5 degrees Celsius will probably be exhausted within this decade. Carbon debt generated thereafter will need to be compensated by net-negative emissions. However, economic policy instruments to guarantee potentially very costly net carbon dioxide removal (CDR) have not yet been devised. Here we propose intertemporal instruments to provide the basis for widely applied carbon taxes and emission trading systems to finance a net-negative carbon economy. We investigate an idealized market approach to incentivize the repayment of previously accrued carbon debt by establishing the responsibility of emitters for the net removal of carbon dioxide through ‘carbon removal obligations’ (CROs). Inherent risks, such as the risk of default by carbon debtors, are addressed by pricing atmospheric CO2 storage through interest on carbon debt. In contrast to the prevailing literature on emission pathways, we find that interest payments for CROs induce substantially more-ambitious near-term decarbonization that is complemented by earlier and less-aggressive deployment of CDR. We conclude that CROs will need to become an integral part of the global climate policy mix if we are to ensure the viability of ambitious climate targets and an equitable distribution of mitigation efforts across generations.