編譯 | 未玖
Nature, 10 February 2022, VOL 602, ISSUE 7896
《自然》2022年2月10日，第602卷，7896期
天文學Astronomy
A white dwarf accreting planetary material determined from X-ray observations
X射線觀測到白矮星吸積行星物質
▲ 作者：Tim Cunningham, Peter J. Wheatley, Pier-Emmanuel Tremblay, Boris T. Gänsicke, George W. King, Odette Toloza, et al.
▲ 連結：
https://www.nature.com/articles/s41586-021-04300-w
▲ 摘要
大多數白矮星的大氣受到重元素的污染，這些重元素預計會在短時間内從可見層下沉。這被解釋為白矮星不斷吸積小行星、彗星和巨行星碎片的标志。探測一些白矮星周圍的碎片盤和行星碎片淩星現象支援了這一設想。
然而，光球金屬隻是持續吸積的間接證據，推斷的吸積率和母體成分在很大程度上取決于白矮星大氣中擴散和混合過程的模型。
研究組報道了被污染白矮星G29–38的X射線4.4σ探測。根據測得的X射線光度，他們推導出瞬時吸積率MX=1.63×109 g s−1，且獨立于恒星大氣模型。該比率高于過去對G29–38光球豐度研究所作的估計，表明可能需要對流過沖來模拟白矮星碎片聚集的光譜。
研究組測量出kBT= 0.5 ± 0.2 keV的低等離子體溫度，證明了此前預測的白矮星以低吸積率吸積的爆炸方式。
▲ Abstract
The atmospheres of a large proportion of white dwarf stars are polluted by heavy elements that are expected to sink out of visible layers on short timescales. This has been interpreted as a signature of ongoing accretion of debris from asteroids, comets and giant planets. This scenario is supported by the detection of debris discs and transits of planetary fragments around some white dwarfs. However, photospheric metals are only indirect evidence for ongoing accretion, and the inferred accretion rates and parent body compositions heavily depend on models of diffusion and mixing processes within the white dwarf atmosphere. Here we report a 4.4σ detection of X-rays from a polluted white dwarf, G29–38. From the measured X-ray luminosity, we derive an instantaneous accretion rate of MX=1.63×109 g s−1, which is independent of stellar atmosphere models. This rate is higher than estimates from past studies of the photospheric abundances of G29–38, suggesting that convective overshoot may be needed to model the spectra of debris-accreting white dwarfs. We measure a low plasma temperature of kBT = 0.5 ± 0.2 keV, corroborating the predicted bombardment solution for white dwarfs accreting at low accretion rates.
實體學Physics
Real-space visualization of intrinsic magnetic fields of an antiferromagnet
反鐵磁體内禀磁場的實空間可視化
▲ 作者：Yuji Kohno, Takehito Seki, Scott D. Findlay, Yuichi Ikuhara & Naoya Shibata
https://www.nature.com/articles/s41586-021-04254-z
原子尺度的磁性結構表征，是材料和器件中納米磁性設計與調控的核心。然而，在該次元上，磁場的實空間可視化一直頗具挑戰性。近年來，原子分辨率差分相襯掃描透射電子顯微鏡（DPC STEM）已能夠直接成像單個原子内部的電場分布。
研究組展示了在無磁場環境中使用原子分辨率DPC STEM實作反鐵磁赤鐵礦（α-Fe2O3）内部磁場分布的實空間可視化。
在去除原子電場引起的相移分量并通過單元平均法提高信噪比後，研究組實作了α-Fe2O3内禀磁場的實空間可視化。這些研究結果為許多磁性結構的實空間表征提供了新途徑。
Characterizing magnetic structures down to atomic dimensions is central to the design and control of nanoscale magnetism in materials and devices. However, real-space visualization of magnetic fields at such dimensions has been extremely challenging. In recent years, atomic-resolution differential phase contrast scanning transmission electron microscopy (DPC STEM) has enabled direct imaging of electric field distribution even inside single atoms. Here we show real-space visualization of magnetic field distribution inside antiferromagnetic haematite (α-Fe2O3) using atomic-resolution DPC STEM in a magnetic-field-free environment. After removing the phase-shift component due to atomic electric fields and improving the signal-to-noise ratio by unit-cell averaging, real-space visualization of the intrinsic magnetic fields in α-Fe2O3 is realized. These results open a new possibility for real-space characterization of many magnetic structures.
Ferroelectric incommensurate spin crystals
鐵電不相稱自旋晶體
▲ 作者：Dorin Rusu, Jonathan J. P. Peters, Thomas P. A. Hase, James A. Gott, Gareth A. A. Nisbet, Jörg Strempfer, et al.
https://www.nature.com/articles/s41586-021-04260-1
鐵性材料，尤其是鐵磁體，在特定的電和力學邊界條件下，可形成複雜的拓撲自旋結構，如漩渦和斯格明子。
在專用鐵電系統中，尤其是在PbTiO3/SrTiO3等鐵電-絕緣體超晶格中，人們已觀察到簡單的渦狀電偶極子拓撲結構，但後來由于其高去極化場，被證明是一個模型系統。迄今為止，還沒有實驗觀察到由Dzyaloshinskii–Moriya互相作用（DMi）驅動的有序磁自旋晶格的等效電偶極。
研究組探讨了夾在SrRuO3電極間的單一PbTiO3外延層的疇結構。他們觀察到周期性的順時針和逆時針鐵電旋渦，沿其環形核心受到二階調制。計算結果支援的拓撲結構是具有兩個正交周期調制的迷宮狀圖案，形成了一個不相稱的極性晶體，其鐵電性類似于最近在鐵磁材料中發現的不相稱自旋晶體。
這些發現進一步模糊了突現鐵磁和鐵電拓撲之間的邊界，為未來磁DMi驅動相的電對應物的實驗實作掃清了道路。
Ferroics, especially ferromagnets, can form complex topological spin structures such as vortices and skyrmions when subjected to particular electrical and mechanical boundary conditions. Simple vortex-like, electric-dipole-based topological structures have been observed in dedicated ferroelectric systems, especially ferroelectric–insulator superlattices such as PbTiO3/SrTiO3, which was later shown to be a model system owing to its high depolarizing field. To date, the electric dipole equivalent of ordered magnetic spin lattices driven by the Dzyaloshinskii–Moriya interaction (DMi) has not been experimentally observed. Here we examine a domain structure in a single PbTiO3 epitaxial layer sandwiched between SrRuO3 electrodes. We observe periodic clockwise and anticlockwise ferroelectric vortices that are modulated by a second ordering along their toroidal core. The resulting topology, supported by calculations, is a labyrinth-like pattern with two orthogonal periodic modulations that form an incommensurate polar crystal that provides a ferroelectric analogue to the recently discovered incommensurate spin crystals in ferromagnetic materials. These findings further blur the border between emergent ferromagnetic and ferroelectric topologies, clearing the way for experimental realization of further electric counterparts of magnetic DMi-driven phases.
材料科學Materials Science
A highly distorted ultraelastic chemically complex Elinvar alloy
一種高度變形超彈性化學複雜的艾林瓦合金
▲ 作者：Q. F. He, J. G. Wang, H. A. Chen, Z. Y. Ding, Z. Q. Zhou, L. H. Xiong, et al.
https://www.nature.com/articles/s41586-021-04309-1
高性能超彈性金屬的研發具有超高強度、大彈性應變極限和溫度不敏感的彈性模量（艾林瓦效應），對于從緻動器、醫療裝置到高精度儀器的各種工業應用都至關重要。由于位錯易滑動，體晶金屬的彈性應變極限通常小于1%。
形狀記憶合金（包括膠質金屬和應變玻璃合金）的彈性應變極限可高達幾個百分點，雖然這是僞彈性的結果，且伴随着巨大的能量耗散。近年來，化學性質複雜的合金，如“高熵”合金，因其良好性能引發了人們廣泛的研究興趣。
在這項工作中，研究組報道了一種化學複雜的合金，其原子尺寸錯配較大，正常合金通常無法承受。該合金在室溫下具有較高的彈性應變極限（約2%）和極低的内耗（小于2 × 10−4）。
更有趣的是，這種合金表現出非凡的艾林瓦效應，在室溫和627℃（900K）之間的彈性模量近乎恒定，迄今為止報道的現有合金均無法與之比拟。
The development of high-performance ultraelastic metals with superb strength, a large elastic strain limit and temperature-insensitive elastic modulus (Elinvar effect) are important for various industrial applications, from actuators and medical devices to high-precision instruments. The elastic strain limit of bulk crystalline metals is usually less than 1 per cent, owing to dislocation easy gliding. Shape memory alloys—including gum metals and strain glass alloys—may attain an elastic strain limit up to several per cent, although this is the result of pseudo-elasticity and is accompanied by large energy dissipation. Recently, chemically complex alloys, such as ‘high-entropy’ alloys, have attracted tremendous research interest owing to their promising properties. In this work we report on a chemically complex alloy with a large atomic size misfit usually unaffordable in conventional alloys. The alloy exhibits a high elastic strain limit (approximately 2 per cent) and a very low internal friction (less than 2 × 10−4) at room temperature. More interestingly, this alloy exhibits an extraordinary Elinvar effect, maintaining near-constant elastic modulus between room temperature and 627 degrees Celsius (900 kelvin), which is, to our knowledge, unmatched by the existing alloys hitherto reported.
人工智能Artificial Intelligence
Outracing champion Gran Turismo drivers with deep reinforcement learning
通過深度強化學習超越Gran Turismo冠軍級賽車手
▲ 作者：Peter R. Wurman, Samuel Barrett, Kenta Kawamoto, James MacGlashan, Kaushik Subramanian, Thomas J. Walsh, et al.
https://www.nature.com/articles/s41586-021-04357-7
人工智能的許多潛在應用涉及與人類互動時在實體系統中做出實時決策。賽車就是這種情況的一個典型代表；賽車手必須執行複雜的戰術性操作以超越或阻擋對手，同時在牽引力極限下駕駛賽車。
賽車模拟，比如PlayStation遊戲Gran Turismo，忠實地再現了真實賽車的非線性控制挑戰，同時也封裝了複雜的多代理互動。
研究組介紹了他們如何訓練Gran Turismo的AI代理，使其能夠與世界上最優秀的電子競技賽車手相匹敵。他們将最先進的、無模型的深度強化學習算法與混合場景訓練相結合，來學習一種綜合控制政策，将卓越的速度與令人印象深刻的戰術相結合。
此外，研究組還建構了一個獎勵函數，使AI代理能夠在遵守重要但靈活的賽車規則的同時保持競争力。最終研究組的AI代理Gran Turismo Sophy嶄露頭角，在正面競争中戰勝了四名世界頂級Gran Turismo賽車手。
通過描述如何訓練冠軍級别的賽車手，研究組展示了使用這些技術來控制複雜動态系統的機遇和挑戰，在這些領域中，AI代理必須尊重靈活定義的人類規則。
Many potential applications of artificial intelligence involve making real-time decisions in physical systems while interacting with humans. Automobile racing represents an extreme example of these conditions; drivers must execute complex tactical manoeuvres to pass or block opponents while operating their vehicles at their traction limits. Racing simulations, such as the PlayStation game Gran Turismo, faithfully reproduce the non-linear control challenges of real race cars while also encapsulating the complex multi-agent interactions. Here we describe how we trained agents for Gran Turismo that can compete with the world’s best e-sports drivers. We combine state-of-the-art, model-free, deep reinforcement learning algorithms with mixed-scenario training to learn an integrated control policy that combines exceptional speed with impressive tactics. In addition, we construct a reward function that enables the agent to be competitive while adhering to racing’s important, but under-specified, sportsmanship rules. We demonstrate the capabilities of our agent, Gran Turismo Sophy, by winning a head-to-head competition against four of the world’s best Gran Turismo drivers. By describing how we trained championship-level racers, we demonstrate the possibilities and challenges of using these techniques to control complex dynamical systems in domains where agents must respect imprecisely defined human norms.
地球科學Earth Science
Superionic iron alloys and their seismic velocities in Earth’s inner core
超離子鐵合金及其在地球核心中的地震速度
▲ 作者：Yu He, Shichuan Sun, Duck Young Kim, Bo Gyu Jang, Heping Li & Ho-kwang Mao
https://www.nature.com/articles/s41586-021-04361-x
地球核心（IC）的密度低于純鐵，這表明其内部存在輕元素。矽、硫、碳、氧和氫被認為是候選元素，人們研究了鐵-輕元素合金的性能以限制IC成分。輕元素對鐵合金的地震速度、熔化溫度和熱導率有很大影響。然而，人們很少考慮IC中輕元素的狀态。
研究組利用第一性原理分子動力學模拟，發現在IC條件下，六方密排鐵中的氫、氧和碳轉變為超離子态，表現出像流體一樣的高擴散系數。這表明IC可以處于超離子态，而非正常固态。
流體輕元素導緻地震速度大幅降低，接近IC的地震學觀測值。橫波波速的大幅降低為軟IC提供了一種解釋。此外，輕元素對流對IC地震學結構和磁場也有潛在影響。
Earth’s inner core (IC) is less dense than pure iron, indicating the existence of light elements within it. Silicon, sulfur, carbon, oxygen and hydrogen have been suggested to be the candidates, and the properties of iron–light-element alloys have been studied to constrain the IC composition. Light elements have a substantial influence on the seismic velocities, the melting temperatures and the thermal conductivities of iron alloys. However, the state of the light elements in the IC is rarely considered. Here, using ab initio molecular dynamics simulations, we find that hydrogen, oxygen and carbon in hexagonal close-packed iron transform to a superionic state under the IC conditions, showing high diffusion coefficients like a liquid. This suggests that the IC can be in a superionic state rather than a normal solid state. The liquid-like light elements lead to a substantial reduction in the seismic velocities, which approach the seismological observations of the IC. The substantial decrease in shear-wave velocity provides an explanation for the soft IC. In addition, the light-element convection has a potential influence on the IC seismological structure and magnetic field.