編譯 | 李言
Nature, 3 February 2022, Volume 602 Issue 7895
《自然》2022年2月3日，第602卷，7895期
物理學Physics
Search for magnetic monopoles produced via the Schwinger mechanism
尋找透過施溫格機制產生的磁單極子
▲ 作者：B. Acharya, J. Alexandre, P. Benes, B. Bergmann et al.
▲ 連結：
https://www.nature.com/articles/s41586-021-04298-1
▲ 摘要
在此，我們展示了在大型強子對撞機Pb-Pb重離子碰撞中，施溫格機制產生磁單極子的研究，這產生了目前的宇宙中已知最強的磁場。
這是由MoEDAL實驗進行的，該實驗的捕獲探測器在2018年11月暴露在每奈米庫內0.235個Pb-Pb碰撞中，或約1.8 × 109個Pb-Pb碰撞中，每次碰撞的質心能量為5.02 兆兆電子伏。超導量子干涉裝置（SQUID）磁強計掃描MoEDAL的捕獲探測器以尋找磁荷的存在，磁荷會在SQUID中誘導持續電流。
磁荷基本單位為1、2和3個基本單位的輕於每光速平方75吉電子伏的單極子的存在被排除在95%置信水平的分析之外。這提供了一個較低的質量限制，有限大小的磁單極子從對撞機搜尋，並大大擴充套件了以前的質量界限。
▲ Abstract
Here we present a search for magnetic monopole production by the Schwinger mechanism in Pb–Pb heavy ion collisions at the Large Hadron Collider, producing the strongest known magnetic fields in the current Universe. It was conducted by the MoEDAL experiment, whose trapping detectors were exposed to 0.235 per nanobarn, or approximately 1.8 × 109, of Pb–Pb collisions with 5.02-teraelectronvolt center-of-mass energy per collision in November 2018. A superconducting quantum interference device (SQUID) magnetometer scanned the trapping detectors of MoEDAL for the presence of magnetic charge, which would induce a persistent current in the SQUID. Magnetic monopoles with integer Dirac charges of 1, 2 and 3 and masses up to 75 gigaelectronvolts per speed of light squared were excluded by the analysis at the 95% confidence level. This provides a lower mass limit for finite-size magnetic monopoles from a collider search and greatly extends previous mass bounds.
Domain-wall dynamics in Bose–Einstein condensates with synthetic gauge fields
合成規範場中玻色-愛因斯坦凝聚的疇壁動力學
▲ 作者：Kai-Xuan Yao, Zhendong Zhang & Cheng Chin
▲ 連結：
https://www.nature.com/articles/s41586-021-04250-3
▲ 摘要
在此，我們證明了在穩定的玻色-愛因斯坦凝聚與由原子密度決定的規範場中，疇壁的確定性形成。密度相關規範場是由光學晶格勢和原子間相互作用的同時調製產生的，並導致原子域凝聚成兩個不同的動量。
將疇壁建模為基本激發，我們發現疇壁響應於電荷質量比大於或相反於裸原子的合成電場。我們的工作對用動態規範場模擬量子系統中先前未描述的激發的動力學和相互作用進行了有前途的展望。
▲ Abstract
Here we demonstrate deterministic formation of domain walls in a stable Bose–Einstein condensate with a gauge field that is determined by the atomic density. The density-dependent gauge field is created by simultaneous modulations of an optical lattice potential and interatomic interactions, and results in domains of atoms condensed into two different momenta. Modelling the domain walls as elementary excitations, we find that the domain walls respond to synthetic electric field with a charge-to-mass ratio larger than and opposite to that of the bare atoms. Our work offers promising prospects to simulate the dynamics and interactions of previously undescribed excitations in quantum systems with dynamical gauge fields.
Polarized phonons carry angular momentum in ultrafast demagnetization
圓極化聲子在超快退磁中具有角動量
▲ 作者：S. R. Tauchert, M. Volkov, D. Ehberger, D. Kazenwadel et al.
▲ 連結：
https://www.nature.com/articles/s41586-021-04306-4
▲ 摘要
在這裡，我們使用超快電子衍射揭示了鎳中幾乎瞬時的、持久的、各向異性高頻聲子的非平衡分佈，出現在150-750 fs。各向異性平面垂直於初始磁化方向，原子振盪振幅為2pm。
我們認為這是在材料旋轉之前迅速吸收角動量的自旋系統的圓極化聲子造成的。退磁所需的時間與原子加速所需的時間有關。這些結果提供了愛因斯坦-德哈斯效應的原子圖象，表明了圓極化聲子在非平衡動力學和相變中的一般重要性。
▲ Abstract
Here we use ultrafast electron diffraction to reveal in nickel an almost instantaneous, long-lasting, non-equilibrium population of anisotropic high-frequency phonons that appear within 150–750 fs. The anisotropy plane is perpendicular to the direction of the initial magnetization and the atomic oscillation amplitude is 2 pm. We explain these observations by means of circularly polarized phonons that quickly absorb the angular momentum of the spin system before macroscopic sample rotation. The time that is needed for demagnetization is related to the time it takes to accelerate the atoms. These results provide an atomistic picture of the Einstein–de Haas effect and signify the general importance of polarized phonons for non-equilibrium dynamics and phase transitions.
化學Chemistry
Selective sulfidation of metal compounds
金屬化合物的選擇性硫化
▲ 作者：Caspar Stinn & Antoine Allanore
▲ 連結：
https://www.nature.com/articles/s41586-021-04321-5
▲ 摘要
在此，我們提出了一種基於選擇性陰離子交換的金屬加工方法。幾種簡單的工藝槓桿（氣體分壓、氣體流量和碳新增）被證明可以從混合金屬-氧化物原料中選擇性地硫化目標金屬。硫化物和氧化物之間的物理和化學差異（例如密度、磁化率和表面化學）可大大改進液-液分離方法。
我們給出了56種元素的硫化工藝條件，並對其中15種元素進行了驗證。對環境和經濟影響的評估表明，與液態-液態溼法冶金相比，該方法可以減少60-90%的溫室氣體排放，同時節省大量成本。
▲ Abstract
Here a metal-processing approach based on selective anion exchange is proposed. Several simple process levers (gas partial pressure, gas flowrate and carbon addition) are demonstrated to selectively sulfidize a target metal from a mixed metal-oxide feed. The physical and chemical differences between the sulfide and oxide compounds (for example, density, magnetic susceptibility and surface chemistry) can then be exploited for vastly improved separation compared with liquid–liquid methods. The process conditions of sulfidation are provided for 56 elements and demonstrated for 15 of them. An assessment of the environmental and economic impacts suggests a path towards 60–90% reductions in greenhouse gas emissions while offering substantial capital cost savings compared with liquid–liquid hydrometallurgy.
材料科學Material Science
Irreversible synthesis of an ultrastrong two-dimensional polymeric material
超強二維聚合物材料的不可逆合成
▲ 作者：Yuwen Zeng, Pavlo Gordiichuk, Takeo Ichihara, Ge Zhang et al.
▲ 連結：
https://www.nature.com/articles/s41586-021-04296-3
▲ 摘要
二維共價延伸的聚合物作為一種將傳統二維材料的機械強度和麵內能量傳導與一維材料的低密度、合成加工效能和有機組成相結合的手段，在近期引起了人們的關注。
在此，我們展示了一種二維不可逆聚合，它可以生成一種化學穩定且易加工的二維聚合材料。
進一步加工可得到高度定向的獨立薄膜，其二維彈性模量和屈服強度分別為12.7±3.8 GPa和488±57 GPa。這一合成路線為二維材料的應用提供了機會，從複合結構到屏障塗層材料。
▲ Abstract
Polymers that extend covalently in two dimensions have attracted recent attention as a means of combining the mechanical strength and in-plane energy conduction of conventional two-dimensional (2D) materials with the low densities, synthetic processability and organic composition of their one-dimensional counterparts. Here we demonstrate a homogenous 2D irreversible polycondensation that results in a covalently bonded 2D polymeric material that is chemically stable and highly processable. Further processing yields highly oriented, free-standing films that have a 2D elastic modulus and yield strength of 12.7 ± 3.8 gigapascals and 488 ± 57 megapascals, respectively. This synthetic route provides opportunities for 2D materials in applications ranging from composite structures to barrier coating materials.
動物學Zoology
Novel flight style and light wings boost flight performance of tiny beetles
新穎的飛行方式和輕巧翅膀提高了小型甲蟲的飛行效能
▲ 作者：Sergey E. Farisenkov, Dmitry Kolomenskiy, Pyotr N. Petrov, Thomas Engels, Nadezhda A. Lapina, Fritz-Olaf Lehmann, Ryo Onishi, Hao Liu & Alexey A. Polilov
▲ 連結：
https://www.nature.com/articles/s41586-021-04303-7
▲ 摘要
動物的飛行速度與體型呈正相關。然而，微型羽翼甲蟲可以以三倍於自身大小的速度和加速度飛行。在此，我們展示了這一表現正是源於更小的翅膀以及一種以前未知的振翅方式。
我們的實驗結合了對最小昆蟲之一的纓甲（體長395 μm）的形態和運動學的三維重建。撲動的纓翅遵循一個明顯的8字形迴圈，包括次垂直的上下拍擊，以及在身體上下振動時的拍擊。鞘翅充當慣性制動器，防止身體過度振盪。
計算分析表明，振翅迴圈由兩次動力半程拍擊產生很大的向上的力、隨後兩次較慢的回覆拍擊產生較小的向下的力。與較重的膜翅相比，同樣大小的纓翅的運動需要很少的慣性功率。因此，肌肉的機械動力需求在整個振翅週期保持積極，使彈效能量儲存過時。這些適應性有助於解釋為什麼極小的昆蟲能在小型化過程中保持良好的飛行效能，這是它們進化成功的因素之一。
▲ Abstract
Flight speed is positively correlated with body size in animals. However, miniature featherwing beetles can fly at speeds and accelerations of insects three times their size. Here we show that this performance results from a reduced wing mass and a previously unknown type of wing-motion cycle. Our experiment combines three-dimensional reconstructions of morphology and kinematics in one of the smallest insects, the beetle Paratuposa placentis (body length 395 μm). The flapping bristled wings follow a pronounced figure-of-eight loop that consists of subperpendicular up and down strokes followed by claps at stroke reversals above and below the body. The elytra act as inertial brakes that prevent excessive body oscillation. Computational analyses suggest functional decomposition of the wingbeat cycle into two power half strokes, which produce a large upward force, and two down-dragging recovery half strokes. In contrast to heavier membranous wings, the motion of bristled wings of the same size requires little inertial power. Muscle mechanical power requirements thus remain positive throughout the wingbeat cycle, making elastic energy storage obsolete. These adaptations help to explain how extremely small insects have preserved good aerial performance during miniaturization, one of the factors of their evolutionary success.
