Lenghu on the Tibetan Plateau as an astronomical observing site.

Compilation | Weijiu

Nature, 19 August 2021, VOL 596, ISSUE 7872

《Nature》August 19, 2021, Volume 596, Issue 7872

AstronomyAstronomy

Lenghu on the Tibetan Plateau as an astronomical observing site

New Astronomical Observation Station is located in Lenghu Town, Qinghai-Tibet Plateau

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

▲ Link:

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

▲ Abstract

On the surface of the earth, only a few high-quality astronomical sites can meet the requirements of the next generation of large-scale facilities. Against the backdrop of scientific opportunities in time-domain astronomy, an excellent location on Qinghai-Tibet Plateau will fill the longitudinal gap between known best locations (all located in the Western Hemisphere).

Qinghai-Tibet Plateau is the highest plateau on earth, with an average altitude of more than 4,000 meters, which is likely to provide a very good opportunity for astronomy and particle astrophysics.

Research team reported the results of monitoring in a experimental zone on the top of Saishteng Mountain near Lenghu Town, Qinghai Province for three consecutive years. The altitude of the area is between 4200 and 4500 meters. The area around Lenghu Town is more than 100,000 square kilometers, with an altitude of less than 3,000 meters. The climate is extremely arid and the local sky is extremely clear (day and night).

The test area has clear photometric conditions at 70% of the nights, with an average visibility of 0.75 arc seconds. The median night temperature change is only 2.4℃, indicating that the local surface air is very stable. 55% of the nighttime precipitation can be less than 2 mm.

▲ Abstract 3

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 partial 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 millionimetres for 55 per cent of the night.

PhysicsPhysics

Two-dimensional supersolidity in a dipolar quantum gas

2D ultra-solid state dipole quantum gas

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

▲ Link:

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

▲ Abstract

supersolid state has both typical solid and superfluid-related characteristics.Like solids, they are crystalline in an orderly manner, manifested as periodic modulation of particle density; but unlike typical solids, they also have superfluid properties, which is caused by the delocalization of coherent particles throughout the system.

This state was originally conceived against the background of large solid helium and was used to explore whether solids have superfluid properties. Despite the efforts of scientists, ultrasolids have not been observed in solid helium, ultracold atomic gases offer another method that makes it possible to observe and study ultrasolids with dipole atoms in recent times. However, unlike the phenomenon proposed in helium, these gaseous systems have so far shown only supersolid states in a single direction.

Research team prepared dysprosium-atom supersolid state quantum gas on both sides of the structural phase transition, prostrating that the properties of supersolid state can be expanded to two-dimensional, similar to ion chains, quantum lines and theoretical single dipole particle chains. This provides the possibility for studying rich excitation characteristics in highly flexible and controllable systems, including vortex formation and ground-state phases with different geometric structures.

▲ Abstract 3

Supersolid states simply feature properties typically associated with a solid and with a superfluid. Like a solid, they possess crystalline order, manifesting as a periodic module of the partial density; but unlike a typical solid, they also have superfluid properties, resulting from coherent partial 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 phenomenon in helium, these gaseous systems have so far only show 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 diplomal particles. This opens the possibility of studying rich excitation properties, including vortex formation, and ground-state phases with varied geometric structure in a highly flexible and controlled system.

Materials ScienceMaterials Science

Ghost hyperbolic surface polaritons in bulk anisotropic crystals

Advanced hyperbolic polarized elements in bulk anisotropic crystals

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

▲ Link:

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

▲ Abstract

Polarized excitons in anisotropic materials produce strange optical properties, which provides an opportunity to control light on the nanoscale. So far, these polarized antonyms are limited to two categories: bulk polarized antonyms that propagate inside the material and surface polarized antonyms that attenuate exponentially at a distance from the interface.

Research group reported a near-field observation of ghost phonon polarized elements, which propagates in in-plane hyperbolic dispersion on the surface of polar uniaxial crystals, while presenting a ramped wavefront in the bulk.

ghost polarized exciton is an atypical nonuniform surface wave solution of Maxwell's system of equations, and it is generated from the surface of a uniaxial material whose optical axis is inclined relative to the interface. They exhibit an unusual bimorphic property that propagates simultaneously within the crystal ( phase propulsion) and disappears (attenuation), which is in sharp contrast to traditional surface waves that completely disappear from the interface.

real-space near-field imaging experiments reveal the long-distance subwavelength ghost polarized elements on the surface (more than 20 microns) and ray-like propagation, verifying the long-range, directional and diffraction-free polarized elements propagation.

At the same time, the research team also showed that controlling the out-of-plane angle of the optical axis can achieve the topology conversion of hyperbolic to ellipse at a fixed frequency, which provides a way to customize the band graph topology of surface polarized exciter waves.

This study results demonstrate a polarized wave phenomenon, providing a unique opportunity for customizing nanoscale light in natural anisotropic crystals.

▲ Abstract 3

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 average non-uniform surface wave solution of Maxwell’s equations, arising at the surface of uniaxial materials in which the optical 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 deep 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 optical 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.

ChemistryChemistry

Mobility gradients yield rubbery surfaces on top of polymer glasses

Mobility gradients yield rubbery surfaces on top of polymer glasses

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

▲ Link:

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

▲ Abstract

Many emerging materials, such as ultra-stable glass for mobile phone displays and OLED TV screens, are attributed to the gradient of enhanced surface fluidity of glass molded liquids. This discovery of enhanced surface fluidity has changed people's understanding of glass molding agent behavior and how it can be made into improved materials. In polymer glass, these interface modifications become complicated by the presence of a second length scale (size of the polymer chain) and the length scale of the interface fluidity gradient.

Research team revealed this biscale property of the glassy polymer surface through simulation, theoretical and time-resolved surface nanocreep experiments, and even in polymers composed of short sub-entangled chains, it will also lead to transient rubber-like and tangled surface behavior. They found that this effect arises from superposition gradients in segmented dynamics and chain conformation statistics.

The lifespan of this rubbery sexual behavior extends with the cooling of the material, which will have a wide range of effects in limiting surface relaxation for applications such as tribology, adhesion and surface healing of polymer glass.The surface layer suffers general damage in time-temperature superposition (TTS), which is the fundamental principle of polymer physics and rheology .

This finding may require a re-evaluation of the prediction strategies for the long-term performance of polymer glasses with high interface areas. The research team predicts that such interfacial transient elastomer effects and TTS breakdown should usually occur in macromolecular systems from nanocomposites to thin films, where interfaces determine the properties of the material.

▲ Abstract 3

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 reshaded our understanding of the behavior of glass formers and of how to fashion them into improved materials. In polymer 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 rubber, enangled-like surface behavior even in polymers compiled 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 behavior, 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 polymer glasses with high interfacial areas. We expect that this interfacial transient elasticer effect and TTS breakdown should normally occur in macromolecular systems ranged from nanocomposites to thin films, where interfaces dominate material properties.

Earth Science

The Montreal Protocol protects the terrestrial carbon sink

" Montreal Protocol " protects land carbon sink

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

▲ Link:

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

▲ Abstract

Control of the production of substances that consume ozone layer through the Montreal Protocol means that the stratospheric ozone layer is recovering, thus avoiding the increase of harmful surface ultraviolet radiation.

The Montreal Protocol has common benefits for mitigating climate change because substances that deplete the ozone layer are powerful greenhouse gases. Avoiding UV radiation and climate change also has common benefits for plants and their ability to store carbon through photosynthesis with , but this has not been studied before. Using a model framework that combines ozone consumption, climate change, damage to plants by UV radiation and carbon cycle, the study group explores the benefits of avoiding increased UV radiation and climate change on the terrestrial biosphere and its carbon sink capacity.

Considering a range of advantages of UV radiation on plant growth, the research team estimated that without the Montreal Protocol (compared to climate forecasts for controlling ozone-depleting substances), by the end of this century (2080-2099), the carbon content in plants and soils could be reduced by 325-690 billion tons by the end of this century (2080-2099).

This change can cause an increase in the atmospheric carbon dioxide content of to 115-235 ppm, which may cause the global average surface temperature to rise by 0.50-1.0℃. The findings suggest that the Montreal Protocol may also help mitigate climate change by avoiding reducing land carbon sinks.

▲ Abstract t

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 harmonious surface ultraviolet radiation are being avoided. The Montreal Protocol has co-benefits for climate change mitigation, because ozone-depleting substances are potential 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 projects with controls on ozone-depleting substances). This change could have resulted in an additional 115–235 parts per million of atmosphere carbon dietide, 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

Implementation of net negative carbon economy

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

▲ Link:

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

▲ Summary

The remaining carbon budget that limits global warming to 1.5℃ may be exhausted within this decade. The carbon debts that arise thereafter will need to be compensated through net negative emissions. However, economic policy tools have not been designed to guarantee potentially very expensive net carbon dioxide removal (CDR).

Research Group proposed cross-period tools to provide the basis for the widely used carbon tax and emission trading system and to finance the net negative carbon economy. They examined an idealized market approach to identifying the responsibility of emitters to incentivize repayment of previously accrued carbon debt through the “carbon removal obligation” (CRO) net carbon dioxide removal responsibility. Inherent risks, such as the risk of default of carbon debtors, can be solved by pricing atmospheric carbon dioxide storage with interest in carbon debt.

Contrary to the current literature on emission pathways, the research team found that interest payments for CROs lead to larger near-term decarbonization targets, which would complement earlier and less aggressive CDR deployments. The study group concluded that CRO would need to be an integral part of the global climate policy portfolio to ensure the feasibility of ambitious climate goals and to mitigate the equitable distribution of efforts across generations.

▲ Abstract 3

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 designed. 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 investigated an idealized market approach to incentivize the repayment of previously accrusted carbon debt by establishing the responsibility of emitters for the net removal of carbon dieteous through ‘carbon removal obligations’ (CROs). Inherent risks, such as the risk of default by carbon debtors, are addressed by pricing atmosphere 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 completed 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.