编译 | 未玖
Nature ,16 July 2020, VOL 583, ISSUE 7816
《自然》 2020年7月16日，第583卷，7816期
材料学 Materials Science
Superconductivity in metallic twisted bilayer graphene stabilized by WSe2
WSe2稳定的金属扭曲双层石墨烯的超导电性
▲ 作者：Harpreet Singh Arora, Robert Polski, Yiran Zhang, Alex Thomson, Youngjoon Choi, Hyunjin Kim, et al.
▲ 链接：https://www.nature.com/articles/s41586-020-2473-8
▲ 摘要
魔角扭曲双层石墨烯（TBG）的旋转角度接近1.1度，具有孤立的平坦电子能带，包含丰富的相关绝缘、超导、铁磁和拓扑相的相图。此前仅在魔角0.1度范围内观测到相关绝缘体和超导性，并出现在相邻或重叠的电子密度范围内。
除了扭转角和应变，TBG相图还取决于封装石墨烯片的绝缘六方氮化硼（hBN）的排列和厚度，这表明了微观介电环境的重要性。研究组展示了在hBN和TBG之间添加绝缘的二硒化钨（WSe2）单层，可在扭转角远小于魔角的情况下稳定超导性。
对于0.79度的最小扭转角，尽管TBG在整个电子密度范围内都表现出金属行为，但仍观察到超导性。有限磁场测量进一步揭示了弱的反定位特征，以及四重自旋-谷对称性的破坏，这与TBG通过靠近WSe2引起的自旋-轨道耦合一致。
研究结果限制了TBG中出现超导性的理论解释，并为莫尔系统中的工程量子相开辟了道路。
▲ Abstract
Magic-angle twisted bilayer graphene (TBG), with rotational misalignment close to 1.1 degrees, features isolated flat electronic bands that host a rich phase diagram of correlated insulating, superconducting, ferromagnetic and topological phases. Correlated insulators and superconductivity have been previously observed only for angles within 0.1 degree of the magic angle and occur in adjacent or overlapping electron-density ranges. Beyond twist angle and strain, the dependence of the TBG phase diagram on the alignment and thickness of the insulating hexagonal boron nitride (hBN) used to encapsulate the graphene sheets indicates the importance of the microscopic dielectric environment. Here we show that adding an insulating tungsten diselenide (WSe2) monolayer between the hBN and the TBG stabilizes superconductivity at twist angles much smaller than the magic angle. For the smallest twist angle of 0.79 degrees, superconductivity is still observed despite the TBG exhibiting metallic behaviour across the whole range of electron densities. Finite-magnetic-field measurements further reveal weak antilocalization signatures as well as breaking of fourfold spin–valley symmetry, consistent with spin–orbit coupling induced in the TBG via its proximity to WSe2. Our results constrain theoretical explanations for the emergence of superconductivity in TBG and open up avenues towards engineering quantum phases in moiré systems.
Untying the insulating and superconducting orders in magic-angle graphene
解开魔角石墨烯中绝缘态和超导态关系
▲ 作者：Petr Stepanov, Ipsita Das, Xiaobo Lu, Ali Fahimniya, Kenji Watanabe, Takashi Taniguchi, et al.
▲ 链接：https://www.nature.com/articles/s41586-020-2459-6
▲ 摘要
魔角扭曲双层石墨烯中超导态和相关绝缘态的共存，引发了人们对它们之间关系的关注。独立控制这些相的微观机制有助于揭示它们各自的作用，并阐明它们之间错综复杂的相互作用。
研究组报道了通过改变石墨烯与金属屏蔽层之间的分隔距离来直接调节该系统中电子相互作用的方法。他们观察到屏蔽层间距小于15纳米的典型Wannier轨道尺寸且扭转角略偏离1.10±0.05度魔角的装置中相关绝缘态的消除。绝缘等级消失后，空出的相空间将由超导圆顶取代，其临界温度可与具有强绝缘子的设备相媲美。
此外，研究组发现半填充的绝缘态会重新出现在0.4特斯拉的小平面外磁场中，并生成Chern数为2的量子化霍尔态。研究组建议重新审视莫尔石墨烯中绝缘相和超导相之间常被假定的“亲子关系”，并提出一种直接探测强相互作用超导体系的微观机制的方法。
▲ Abstract
The coexistence of superconducting and correlated insulating states in magic-angle twisted bilayer graphene prompts fascinating questions about their relationship. Independent control of the microscopic mechanisms that govern these phases could help uncover their individual roles and shed light on their intricate interplay. Here we report on direct tuning of electronic interactions in this system by changing the separation distance between the graphene and a metallic screening layer. We observe quenching of correlated insulators in devices with screening layer separations that are smaller than the typical Wannier orbital size of 15 nanometres and with twist angles that deviate slightly from the magic angle of 1.10 ± 0.05 degrees. Upon extinction of the insulating orders, the vacated phase space is taken over by superconducting domes that feature critical temperatures comparable to those in devices with strong insulators. In addition, we find that insulators at half-filling can reappear in small out-of-plane magnetic fields of 0.4 tesla, giving rise to quantized Hall states with a Chern number of 2. Our study suggests re-examination of the often-assumed ‘parent-and-child’ relation between the insulating and superconducting phases in moiré graphene, and suggests a way of directly probing the microscopic mechanisms of superconductivity in strongly correlated systems.
物理学 Physics
A subradiant optical mirror formed by a single structured atomic layer
由单个原子层构成的亚辐射光学镜
▲ 作者：Jun Rui, David Wei, Antonio Rubio-Abadal, Simon Hollerith, Johannes Zeiher, Dan M. Stamper-Kurn, et al.
▲ 链接：https://www.nature.com/articles/s41586-020-2463-x
▲ 摘要
具有强大且可调的光-物质相互作用的多功能界面对于量子科学至关重要，因为它们能够映射光和物质之间的量子特性。最近的研究提出了一种控制光-物质相互作用的方法，即利用在量子发射器的结构化亚波长阵列中光子介导的偶极子-偶极子间的丰富相互作用。
研究组报告了光学晶格中二维方形原子阵列的协同亚辐射响应的直接观测。他们观察到整体原子响应的光谱变窄远低于单个原子进入自由空间的量子限制衰减。通过空间分辨光谱测量，研究组表明该阵列可作为由数百个原子的单层膜形成的高效镜。
通过调整阵列中的原子密度并更改粒子的排列顺序，他们能够控制阵列的协同响应，并阐明空间有序相互作用和偶极相互作用对系统整体性质的影响。阵列外部原子的布洛赫振荡使研究组能够动态控制原子镜的反射率。
研究组的工作证明了基于原子结构化集成的高效光学超材料工程，并为在单量子水平上控制光和光-物质界面的多体物理学铺平了道路。
▲ Abstract
Versatile interfaces with strong and tunable light–matter interactions are essential for quantum science because they enable mapping of quantum properties between light and matter. Recent studies have proposed a method of controlling light–matter interactions using the rich interplay of photon-mediated dipole–dipole interactions in structured subwavelength arrays of quantum emitters. Here we report the direct observation of the cooperative subradiant response of a two-dimensional square array of atoms in an optical lattice. We observe a spectral narrowing of the collective atomic response well below the quantum-limited decay of individual atoms into free space. Through spatially resolved spectroscopic measurements, we show that the array acts as an efficient mirror formed by a single monolayer of a few hundred atoms. By tuning the atom density in the array and changing the ordering of the particles, we are able to control the cooperative response of the array and elucidate the effect of the interplay of spatial order and dipolar interactions on the collective properties of the ensemble. Bloch oscillations of the atoms outside the array enable us to dynamically control the reflectivity of the atomic mirror. Our work demonstrates efficient optical metamaterial engineering based on structured ensembles of atoms and paves the way towards controlling many-body physics with light and light–matter interfaces at the single-quantum level.
Monolithic piezoelectric control of soliton microcombs
孤子微梳的单片压电控制
▲ 作者：Junqiu Liu, Hao Tian, Erwan Lucas, Arslan S. Raja, Grigory Lihachev, Rui Ning Wang, et al.
▲ 链接：https://www.nature.com/articles/s41586-020-2465-8
▲ 摘要
在使用激光器和频率梳的应用中，激光频率的高速驱动至关重要，是相位锁定、频率稳定和光学载波间稳定性转移的前提。孤子微梳已成为芯片级频率梳源，并已用于系统级演示。
研究组演示了使用集成压电元件的高速孤子微梳驱动。通过在超低损耗Si3N4光子电路上单片集成AlN驱动器，研究组实现了具有兆赫兹带宽的压控孤子启动、调谐和稳定化。AlN驱动器使用300纳瓦的功率，并具有双向调谐、高线性度和低滞后性的特点。它们展现出高达1兆赫的平坦驱动响应，这大大超过了整体压电调谐带宽。
通过克服与芯片的声学轮廓模式的耦合，可以扩展到更高的频率。通过激光和微谐振器的同步调谐，研究组利用这种能力对光梳频谱进行频移，并使其偏移出孤子的存在范围。这为激光雷达提供了一个大规模并行的调频引擎，具有更高的频率偏移、更低的功率并消除了孤子拉曼自频移所导致的信道失真。
此外，通过以与高泛音体声波共振频率相匹配的速率进行调制，体声波能量的共振积累可使所需驱动电压降低14倍，使其与CMOS电子器件兼容。研究组的方法使孤子微梳具有集成、超低功耗和快速驱动的特性，从而扩大了微梳的技术应用范围。
▲ Abstract
High-speed actuation of laser frequency is critical in applications using lasers and frequency combs, and is a prerequisite for phase locking, frequency stabilization and stability transfer among optical carriers. Soliton microcombs have emerged as chip-scale frequency comb sources, and have been used in system-level demonstrations. Here we demonstrate high-speed soliton microcomb actuation using integrated piezoelectric components. By monolithically integrating AlN actuators on ultralow-loss Si3N4 photonic circuits, we demonstrate voltage-controlled soliton initiation, tuning and stabilization with megahertz bandwidth. The AlN actuators use 300 nanowatts of power and feature bidirectional tuning, high linearity and low hysteresis. They exhibit a flat actuation response up to 1 megahertz—substantially exceeding bulk piezo tuning bandwidth—that is extendable to higher frequencies by overcoming coupling to acoustic contour modes of the chip. Via synchronous tuning of the laser and the microresonator, we exploit this ability to frequency-shift the optical comb spectrum and make excursions beyond the soliton existence range. This enables a massively parallel frequency-modulated engine for lidar, with increased frequency excursion, lower power and elimination of channel distortions resulting from the soliton Raman self-frequency shift. Moreover, by modulating at a rate matching the frequency of high-overtone bulk acoustic resonances, resonant build-up of bulk acoustic energy allows a 14-fold reduction of the required driving voltage, making it compatible with CMOS electronics. Our approach endows soliton microcombs with integrated, ultralow-power and fast actuation, expanding the repertoire of technological applications of microcombs.
化学 Chemistry
Vacancy-enabled N2 activation for ammonia synthesis on an Ni-loaded catalyst
负载镍催化剂上空穴活化N 2合成氨
▲ 作者：Tian-Nan Ye, Sang-Won Park, Yangfan Lu, Jiang Li, Masato Sasase, Masaaki Kitano, et al.
▲ 链接：https://www.nature.com/articles/s41586-020-2464-9
▲ 摘要
氨（NH3）对化肥工业至关重要，是最普遍的化学品之一。直接使用大气中的氮（N2）极具挑战性，因为其键能很高（每摩尔945千焦耳），直到哈伯-博世制造工艺的出现。随后，人们探索了许多策略来降低N≡N键的活化势垒，并提高反应效率。
电子载体因其低功函和高电子密度增强了电子向过渡金属的转移，进一步降低了活化势垒。该策略促进了N2解离合成氨，并在温和条件下实现了催化操作。
但它需要使用昂贵的钌。有研究显示含表面氮空穴的氮化物可以活化N2。
研究组报道了镍负载的氮化镧（LaN）能够稳定高效地合成氨，因其避免了常见结垢关系的双位机制。动力学和同位素标记实验以及密度泛函理论计算证实，氮空穴是在LaN上以较低的形成能生成的，并可有效结合和活化N2。此外，负载在氮化物上的镍金属会解离H2。
使用不同空穴位点活化两种反应物及其协同作用，导致负载镍的LaN催化剂显示出远远超过传统的钴基和镍基催化剂的活性，并且与钌基催化剂相媲美。研究结果证明了在反应循环中使用空穴位点的潜力，介绍了一种使用自然丰富元素合成氨的催化剂的设计理念。
▲ Abstract
Ammonia (NH3) is pivotal to the fertilizer industry and one of the most commonly produced chemicals. The direct use of atmospheric nitrogen (N2) had been challenging, owing to its large bond energy (945 kilojoules per mole), until the development of the Haber–Bosch process. Subsequently, many strategies have been explored to reduce the activation barrier of the N≡N bond and make the process more efficient. An electride support further lowers the activation barrier because its low work function and high electron density enhance electron transfer to transition metals. This strategy has facilitated ammonia synthesis from N2 dissociation and enabled catalytic operation under mild conditions;however, it requires the use of ruthenium, which is expensive. Alternatively, it has been shown that nitrides containing surface nitrogen vacancies can activate N2. Here we report that nickel-loaded lanthanum nitride (LaN) enables stable and highly efficient ammonia synthesis, owing to a dual-site mechanism that avoids commonly encountered scaling relations. Kinetic and isotope-labelling experiments, as well as density functional theory calculations, confirm that nitrogen vacancies are generated on LaN with low formation energy, and efficiently bind and activate N2. In addition, the nickel metal loaded onto the nitride dissociates H2. The use of distinct sites for activating the two reactants, and the synergy between them, results in the nickel-loaded LaN catalyst exhibiting an activity that far exceeds that of more conventional cobalt- and nickel-based catalysts, and that is comparable to that of ruthenium-based catalysts. Our results illustrate the potential of using vacancy sites in reaction cycles, and introduce a design concept for catalysts for ammonia synthesis, using naturally abundant elements.
Self-assembled poly-catenanes from supramolecular toroidal building blocks
超分子环形构造块自组装聚索烃
▲ 作者：Sougata Datta, Yasuki Kato, Seiya Higashiharaguchi, Keisuke Aratsu, Atsushi Isobe, Takuho Saito, et al.
▲ 链接：https://www.nature.com/articles/s41586-020-2445-z
▲ 摘要
在现代合成化学和材料科学中，分子的机械互锁（连锁）是一项不小的挑战。实现连锁的一种策略是设计前环分子，这种分子既能有效环化，又能预组织另一种前体参与后续连锁。当环形靶由大分子整体组成，即它是个超分子组装体时，该任务特别困难。
然而，这种空前的组装构造将使得通过显微镜技术可观察到非凡的纳米拓扑结构，这不仅满足了学术界的好奇心，还为开发具有纳米拓扑结构特性的材料铺平了道路。
研究组报告了使用具有固有曲率的纤维超分子组装体来合成这种纳米拓扑。使用溶剂混合策略，研究组动态地组织了一个可拉长为半径约13纳米的环形分子。原子力显微镜观察到该纳米级环状体显示出很高的连锁率，这足以产生“纳米链烷”，即由五个相互连锁的圆环体组成的纳米级索烃。
光谱和理论研究表明，这种异常高的连锁度是由于圆环体周围的前体分子二次成核所致。通过修改自组装方案以促进环的闭合和二次成核，原子力显微镜确认最大连锁数为22。
▲ Abstract
Mechanical interlocking of molecules (catenation) is a nontrivial challenge in modern synthetic chemistry and materials science. One strategy to achieve catenation is the design of pre-annular molecules that are capable of both efficient cyclization and of pre-organizing another precursor to engage in subsequent interlocking. This task is particularly difficult when the annular target is composed of a large ensemble of molecules, that is, when it is a supramolecular assembly. However, the construction of such unprecedented assemblies would enable the visualization of nontrivial nanotopologies through microscopy techniques, which would not only satisfy academic curiosity but also pave the way to the development of materials with nanotopology-derived properties. Here we report the synthesis of such a nanotopology using fibrous supramolecular assemblies with intrinsic curvature. Using a solvent-mixing strategy, we kinetically organized a molecule that can elongate into toroids with a radius of about 13 nanometres. Atomic force microscopy on the resulting nanoscale toroids revealed a high percentage of catenation, which is sufficient to yield ‘nanolympiadane’, a nanoscale catenane composed of five interlocked toroids. Spectroscopic and theoretical studies suggested that this unusually high degree of catenation stems from the secondary nucleation of the precursor molecules around the toroids. By modifying the self-assembly protocol to promote ring closure and secondary nucleation, a maximum catenation number of 22 was confirmed by atomic force microscopy.