编译 | 未玖
Science, 25 JUNE 2021, VOL 372, ISSUE 6549
《科学》2021年6月25日 ， 第372卷 ， 6549期
本文图片
物理学Physics
Discovery of a Cooper-pair density wave state in a transition-metal dichalcogenide
过渡金属二硫化物中发现库珀对密度波
作者：Xiaolong Liu, Yi Xue Chong, Rahul Sharma, J. C. Séamus Davis.
链接：
https://science.sciencemag.org/content/372/6549/1447
摘要
双密度波（PDW）态由空间调制超导序参量定义 。 为了在过渡金属二硫化物（TMDs）中寻找这种状态 ， 研究组使用了高速原子分辨扫描的约瑟夫森隧道显微镜 。
研究组检测到一个PDW态 ， 其电子对密度和能隙在预先存在的电荷密度波（CDW）态的波矢上进行空间调制 。 PDW与s波超导体和CDW呈线性耦合 ， 并在边界处呈现出公度错相滑移的相称畴 ， 与晶格锁定的相称CDW一致 。
然而 ， 研究组发现PDW态和CDW态之间的整体相位差为δΦ?±2π/3 ， 这可能与库珀对波函数轨道有关 。 研究组的发现预示着PDW物理学在许多其他维持CDW和超导态的TMDs中普遍存在 。
Abstract
Pair density wave (PDW) states are defined by a spatially modulating superconductive order parameter. To search for such states in transition-metal dichalcogenides (TMDs), we used high-speed atomic-resolution scanned Josephson-tunneling microscopy. We detected a PDW state whose electron-pair density and energy gap modulate spatially at the wave vectors of the preexisting charge density wave (CDW) state. The PDW couples linearly to both the s-wave superconductor and the CDW and exhibits commensurate domains with discommensuration phase slips at the boundaries, conforming those of the lattice-locked commensurate CDW. Nevertheless, we found a global δΦ?±2π/3 phase difference between the PDW and CDW states, possibly owing to the Cooper-pair wave function orbital content. Our findings presage pervasive PDW physics in the many other TMDs that sustain both CDW and superconducting states.
材料科学Materials Science
Stacking-engineered ferroelectricity in bilayer boron nitride
双层氮化硼的堆叠工程铁电性
作者：Kenji Yasuda, Xirui Wang, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero.
链接：
https://science.sciencemag.org/content/372/6549/1458
摘要
二维（2D）铁电体具有稳健极化强度 ， 可达到原子厚度 ， 为功能异质结构提供了基础 。 由于需要层状极性晶体 ， 实验实现仍然具有挑战性 。
研究组展示了一个从非铁电母体化合物通过使用范德华组装工程二维铁电体的合理设计方法 。 平行堆叠的双层氮化硼表现出面外电极化 ， 并随堆叠顺序发生反转 。
通过相邻堆叠的石墨烯片的电阻来探测极化转换 。 由于形成了交错极化的莫尔铁电体 ， 将氮化硼片扭转小角度即可改变转换动力学 。
这种铁电性在保持石墨烯高迁移率的同时可持续至室温 ， 为超薄非易失性存储器的潜在应用奠定了基础 。
Abstract
Two-dimensional (2D) ferroelectrics with robust polarization down to atomic thicknesses provide building blocks for functional heterostructures. Experimental realization remains challenging because of the requirement of a layered polar crystal. Here, we demonstrate a rational design approach to engineering 2D ferroelectrics from a nonferroelectric parent compound by using van der Waals assembly. Parallel-stacked bilayer boron nitride exhibits out-of-plane electric polarization that reverses depending on the stacking order. The polarization switching is probed through the resistance of an adjacently stacked graphene sheet. Twisting the boron nitride sheets by a small angle changes the dynamics of switching because of the formation of moiré ferroelectricity with staggered polarization. The ferroelectricity persists to room temperature while keeping the high mobility of graphene, paving the way for potential ultrathin nonvolatile memory applications.
Interfacial ferroelectricity by van der Waals sliding
范德华滑动的界面铁电性
作者：M. Vizner Stern, Y. Waschitz, W. Cao, I. Nevo, K. Watanabe, T. Taniguchi, et al.
链接：
https://science.sciencemag.org/content/372/6549/1462
摘要
虽然有部分离子性质 ， 但许多层状双原子晶体通过在其最佳范德华堆叠处形成中心对称晶格来避免内部电极化 。
研究组报告了出现在两个自然生长的六方氮化硼片之间的稳定铁电有序 ， 在亚稳非中心对称平行方向堆叠在一起 。
他们观察到反向常规极化的交替域 ， 这是由域之间一个晶格位的横向移动引起的 。 通过扫描表面上方的偏压尖端 ， 实现了与横向滑动耦合的可逆极化转换 。
研究组的计算将这种现象的起源追溯到电荷再分配和离子位移之间的微妙相互作用 ， 并为探索界面极化及其独特的“滑动电子学”转换机制提供了直观见解 。
Abstract
Despite their partial ionic nature, many-layered diatomic crystals avoid internal electric polarization by forming a centrosymmetric lattice at their optimal van der Waals stacking. Here, we report a stable ferroelectric order emerging at the interface between two naturally grown flakes of hexagonal boron nitride, which are stacked together in a metastable non-centrosymmetric parallel orientation. We observe alternating domains of inverted normal polarization, caused by a lateral shift of one lattice site between the domains. Reversible polarization switching coupled to lateral sliding is achieved by scanning a biased tip above the surface. Our calculations trace the origin of the phenomenon to a subtle interplay between charge redistribution and ionic displacement and provide intuitive insights to explore the interfacial polarization and its distinctive “slidetronics” switching mechanism.
Pressure-driven fusion of amorphous particles into integrated monoliths
压力驱动无定形颗粒融合为完整块体
作者：Zhao Mu, Kangren Kong, Kai Jiang, Hongliang Dong, Xurong Xu, Zhaoming Liu, et al.
链接：
https://science.sciencemag.org/content/372/6549/1466
摘要
生物有机体可以利用无定形前体 ， 通过完全的颗粒融合 ， 产生具有连续结构的无机骨架 。 由于烧结技术会破坏连续性并限制机械强度 ， 合成块体要困难得多 。
研究组在调节结构结合水和外部压力的同时 ， 通过颗粒融合制备了无定形碳酸钙无机块体 。 该块体是透明的 ， 由于其结构的连续性 ， 机械强度接近单晶方解石 。 非晶体内部的动态水通道受水含量和外部压力协同控制 ， 促进颗粒融合的质量传递 。
【科学|《科学》（20210625出版）一周论文导读】研究组提供了一种替代传统烧结方法的策略 ， 将助力于构建温度敏感的生物矿物和生物材料块体 。
Abstract
Biological organisms can use amorphous precursors to produce inorganic skeletons with continuous structures through complete particle fusion. Synthesizing monoliths is much more difficult because sintering techniques can destroy continuity and limit mechanical strength. We manufactured inorganic monoliths of amorphous calcium carbonate by the fusion of particles while regulating structurally bound water and external pressure. Our monoliths are transparent, owing to their structural continuity, with a mechanical strength approaching that of single-crystal calcite. Dynamic water channels within the amorphous bulk are synergistically controlled by water content and applied pressure and promote mass transportation for particle fusion. Our strategy provides an alternative to traditional sintering methods that should be attractive for constructing monoliths of temperature-sensitive biominerals and biomaterials.
考古学Archaeology
A Middle Pleistocene Homo from Nesher Ramla, Israel
以色列内舍尔拉姆拉遗址的中更新世人类
作者：Israel Hershkovitz, Hila May, Rachel Sarig, Ariel Pokhojaev, Dominique Grimaud-Hervé, Emiliano Bruner, et al.
链接：
https://science.sciencemag.org/content/372/6549/1424
摘要
长期以来 ， 人们认为尼安德特人起源于欧洲大陆 ， 并在那里繁衍生息 。 然而 ， 最近的形态学和遗传学研究表明 ， 他们可能从一个未知的非欧洲群体获得了遗传贡献 。
研究组报道了最近在以色列内舍尔拉姆拉遗址发现的远古智人化石 ， 可以追溯到14万到12万年前 。 对其顶骨、下颌骨和下第二磨牙的综合定性和定量分析表明 ， 这一类人呈现出尼安德特人和古人类特征的独特组合 。
研究组认为 ， 这些标本代表了黎凡特中更新世镶嵌模式的晚期幸存者 ， 他们很可能参与了欧洲和东亚中更新世人类的进化 。
Abstract
It has long been believed that Neanderthals originated and flourished on the European continent. However, recent morphological and genetic studies have suggested that they may have received a genetic contribution from a yet unknown non-European group. Here we report on the recent discovery of archaic Homo fossils from the site of Nesher Ramla, Israel, which we dated to 140,000 to 120,000 years ago. Comprehensive qualitative and quantitative analyses of the parietal bones, mandible, and lower second molar revealed that this Homo group presents a distinctive combination of Neanderthal and archaic features. We suggest that these specimens represent the late survivors of a Levantine Middle Pleistocene paleodeme that was most likely involved in the evolution of the Middle Pleistocene Homo in Europe and East Asia.
气候学Climatology
Transformative climate adaptation in the United States: Trends and prospects
美国转型气候适应的趋势分析
作者：Linda Shi, Susanne Moser
链接：
https://science.sciencemag.org/content/372/6549/eabc8054
摘要
随着气候变化加剧 ， 民间社会对转型适应的呼声越来越高 ， 以纠正气候脆弱性的驱动因素 。
研究组分析了美国联邦政府、私营企业和民间社会如何规划气候适应的趋势 ， 但发现他们在做法和影响上越来越不一致 。 这种不协调增加了对气候盲目的基础设施的不适应性投资 ， 金融和专业部门的司法盲目改革 ， 并加剧了气候影响的社会脆弱性 。
如果这些行动者想要积极审慎地参与转型适应 ， 他们就需要解决维持现有制度的物质、关系和规范因素 。 通过回顾转型和集体影响文献 ， 研究组总结了研究和政策参与的方向 ， 以支持更多转型适应向前发展 。
Abstract
As climate change intensifies, civil society is increasingly calling for transformative adaptation that redresses drivers of climate vulnerability. We review trends in how US federal government, private industry, and civil society are planning for climate adaptation. We find growing divergence in their approaches and impacts. This incoherence increases maladaptive investment in climate-blind infrastructure, justice-blind reforms in financial and professional sectors, and greater societal vulnerability to climate impacts. If these actors were to proactively and deliberatively engage in transformative adaptation, they would need to address the material, relational, and normative factors that hold current systems in place. Drawing on a review of transformation and collective impact literatures, we conclude with directions for research and policy engagement to support more transformative adaptation moving forward.
编辑｜宗华
排版｜李言
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