編譯 | 未玖
Nature, 19 August 2021, VOL 596, ISSUE 7872
《自然》2021年8月19日，第596卷，7872期
天文學Astronomy
Lenghu on the Tibetan Plateau as an astronomical observing site
新天文觀測台址坐落在青藏高原冷湖鎮
▲ 作者：Licai Deng, Fan Yang, Xiaodian Chen, Fei He, Qili Liu, Bo Zhang, et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-021-03711-z
▲ 摘要
在地球表面，只有少數高質量天文站點能夠滿足下一代大型設施的要求。在時域天文學的科學機遇背景下，青藏高原上的一個絕佳地點將填補已知最佳地點（均位於西半球）之間的縱向差距。
青藏高原是地球上最高的高原，平均海拔超過4000米，很有可能為天文學和粒子天體物理學提供非常好的機會。
研究組報道了在青海省冷湖鎮附近的賽什騰山山頂上某個試驗區連續三年監測的結果。該地的海拔高度在4200至4500米之間。冷湖鎮周圍面積超過10萬平方公里，海拔低於3000米，氣候極端乾旱，當地天空異常晴朗（晝夜）。
該試驗區70%的夜晚具有清晰的光度條件，平均能見度為0.75弧秒。夜間氣溫變化中值僅為2.4℃，表明當地地表空氣非常穩定。55%的夜間可降水量低於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.
物理學Physics
Two-dimensional supersolidity in a dipolar quantum gas
二維超固態偶極量子氣體
▲ 作者：Matthew A. Norcia, Claudia Politi, Lauritz Klaus, Elena Poli, Maximilian Sohmen, Manfred J. Mark, et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-021-03725-7
▲ 摘要
超固體狀態同時具有典型的固體和超流體相關的特性。像固體一樣，它們晶態有序，表現為粒子密度的周期性調製；但與典型固體不同，它們也具有超流體性質，這是由整個系統中的相干粒子離域作用造成的。
這種狀態最初是在大塊固體氦的背景下設想的，用於探討固體是否具有超流體性質。儘管科學家付出了很多努力，在固體氦中仍未觀察到超固體，但超冷原子氣體提供了另一種方法，使得近期觀察和研究具有偶極原子的超固體成為可能。然而，與氦中提出的現象不同，到目前為止，這些氣態系統僅顯示出單一方向上的超固態。
研究組通過在結構相變的兩側製備鏑原子超固態量子氣體，證明了超固態性質可擴展到二維，類似於離子鏈、量子線以及理論上的單個偶極粒子鏈。這為在高度靈活和可控系統中研究豐富激勵特性提供了可能，包括渦旋形成和具有不同幾何結構的基態相。
▲ 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.
材料科學Materials Science
Ghost hyperbolic surface polaritons in bulk anisotropic crystals
體塊各向異性晶體中的幽靈雙曲極化激元
▲ 作者：Weiliang Ma, Guangwei Hu, Debo Hu, Runkun Chen, Tian Sun, Xinliang Zhang, et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-021-03755-1
▲ 摘要
各向異性材料中的極化激元會產生奇異的光學特性，這為在納米尺度上控制光提供了機會。到目前為止，這些極化激元僅限於兩類：在材料內部傳播的體極化激元和在遠離界面處呈指數衰減的表面極化激元。
研究組報道了一種幽靈聲子極化激元的近場觀測，其在極性單軸晶體表面以面內雙曲型色散傳播，同時在體塊中呈現斜波陣面。
幽靈極化激元是麥克斯韋方程組的非典型非均勻表面波解，產生於光軸相對於界面傾斜的單軸材料表面。它們表現出一種不尋常的雙態性質，在晶體內同時傳播（相位推進）與消逝（衰減），這與從界面完全消失的傳統表面波形成了鮮明對比。
真實空間近場成像實驗揭示了深亞波長幽靈極化激元在表面的長距離（超過20微米）、類射線傳播，驗證了長程、定向和無衍射的極化激元傳播。
同時，研究組還表明，控制光軸的平面外角度可以在固定頻率下實現雙曲線到橢圓的拓撲轉換，這為定製表面極化激元波的能帶圖拓撲提供了一條途徑。
該研究結果證明了一種極化波現象，為在自然各向異性晶體中定製納米尺度光提供了獨特機會。
▲ 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.
化學Chemistry
Mobility gradients yield rubbery surfaces on top of polymer glasses
聚合物玻璃頂部的橡膠表面產生流動性梯度
▲ 作者：Zhiwei Hao, Asieh Ghanekarade, Ningtao Zhu, Katelyn Randazzo, Daisuke Kawaguchi, Keiji Tanaka, et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-021-03733-7
▲ 摘要
許多新興材料，如用於手機顯示器和OLED電視屏幕的超穩定玻璃，其性能歸功於玻璃成型液體表面流動性增強的梯度。這種表面流動性增強的發現改變了人們對玻璃成型劑行為以及如何將其製成改良材料的理解。在聚合物玻璃中，由於存在第二長度尺度（聚合物鏈的大小）以及界面流動性梯度的長度尺度，這些界面改性變得複雜。
研究組通過模擬、理論和時間分辨表面納米蠕變實驗，來揭示玻璃態聚合物表面的這種雙尺度性質，甚至在由短的亞纏結鏈組成的聚合物中，也會導致出現瞬態橡膠狀、纏結狀表面行為。他們發現，這種效應產生於分段動力學和鏈構象統計中的疊加梯度。
這種橡膠性行為的壽命隨著材料的冷卻而延長，這將在限制聚合物玻璃的摩擦學、粘附和表面癒合等應用的表面鬆弛方面產生廣泛的影響。表層在時間-溫度疊加（TTS）中遭受普遍破壞，這是聚合物物理學和流變學的基本原理。
這一發現可能需要重新評估具有高界面面積的聚合物玻璃長期性能的預測策略。研究組預計，這種界面瞬態彈性體效應和TTS擊穿通常應發生在從納米複合材料到薄膜的大分子系統中，在這些系統中，界面決定了材料的性能。
▲ 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.
地球科學Earth Science
The Montreal Protocol protects the terrestrial carbon sink
《蒙特利爾議定書》保護陸地碳匯
▲ 作者：Paul J. Young, Anna B. Harper, Chris Huntingford, Nigel D. Paul, Olaf Morgenstern, Paul A. Newman, et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-021-03737-3
▲ 摘要
通過《蒙特利爾議定書》對消耗臭氧層物質的生產進行控制意味著平流層臭氧層正在恢復，從而避免了有害的表面紫外線輻射的增加。
《蒙特利爾議定書》對減緩氣候變化具有共同益處，因為消耗臭氧層的物質是強效溫室氣體。避免紫外線輻射和氣候變化對植物及其通過光合作用儲存碳的能力也有共同益處，但這此前尚未被研究過。
研究組使用一個將臭氧消耗、氣候變化、紫外線輻射對植物的損害和碳循環結合起來的模型框架，探討了避免紫外線輻射增加和氣候變化對陸地生物圈及其碳匯能力的益處。
考慮到紫外線輻射對植物生長影響的一系列優勢，研究組估計，如果沒有《蒙特利爾議定書》（與控制臭氧消耗物質的氣候預測相比），到本世紀末（2080-2099年），植物和土壤中的碳含量可能會減少3250-6900億噸。
該變化可導致大氣中二氧化碳含量增加115-235 ppm，這可能導致全球平均地表升溫0.50-1.0℃。研究結果表明，《蒙特利爾議定書》也可能通過避免減少土地碳匯來幫助緩解氣候變化。
▲ 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
實施凈負碳經濟
▲ 作者：Johannes Bednar, Michael Obersteiner, Artem Baklanov, Marcus Thomson, Fabian Wagner, Oliver Geden, et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-021-03723-9
▲ 摘要
將全球變暖限制在1.5℃的剩餘碳預算可能在本十年內耗盡。此後產生的碳債務將需要通過凈負排放來補償。然而，還沒有設計出經濟政策工具來保證可能非常昂貴的凈二氧化碳去除（CDR）。
研究組提出跨期工具，為廣泛應用的碳稅和排放交易系統提供基礎，為凈負碳經濟融資。他們研究了一種理想化的市場方法，即確定排放者通過「碳清除義務」（CRO）凈清除二氧化碳的責任來激勵償還先前應計的碳債務。固有風險，如碳債務人的違約風險，可通過碳債務利息定價大氣二氧化碳儲存來解決。
與當前關於排放途徑的文獻相反，研究組發現，CRO的利息支付導致了更大規模的近期脫碳目標，而早期和較不積極的CDR部署則會對其起補充作用。研究組得出的結論是，若要確保雄心勃勃的氣候目標的可行性和緩解各代人努力的公平分配，CRO將需要成為全球氣候政策組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.
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