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Observations of Electron Vortex at the Dipolarization Front 期刊论文
GEOPHYSICAL RESEARCH LETTERS, 2020, 47 (13)
作者:  Jiang, K.;  Huang, S. Y.;  Yuan, Z. G.;  Deng, X. H.;  Xu, S. B.;  Wei, Y. Y.;  He, L. H.;  Zhang, J.;  Zhang, Z. H.
收藏  |  浏览/下载:35/0  |  提交时间:2020/06/22
dipolarization front  electron vortex  small-scale plateau  asymmetric electric field  
Synthesis and properties of free-standing monolayer amorphous carbon 期刊论文
NATURE, 2020, 577 (7789) : 199-+
作者:  Toh, Chee-Tat;  Zhang, Hongji;  Lin, Junhao;  Mayorov, Alexander S.;  Wang, Yun-Peng;  Orofeo, Carlo M.;  Ferry, Darim Badur;  Andersen, Henrik;  Kakenov, Nurbek;  Guo, Zenglong;  Abidi, Irfan Haider;  Sims, Hunter;  Suenaga, Kazu;  Pantelides, Sokrates T.;  Ozyilmaz, Barbaros
收藏  |  浏览/下载:22/0  |  提交时间:2020/07/03

Bulk amorphous materials have been studied extensively and are widely used, yet their atomic arrangement remains an open issue. Although they are generally believed to be Zachariasen continuous random networks(1), recent experimental evidence favours the competing crystallite model in the case of amorphous silicon(2-4). In two-dimensional materials, however, the corresponding questions remain unanswered. Here we report the synthesis, by laser-assisted chemical vapour deposition(5), of centimetre-scale, free-standing, continuous and stable monolayer amorphous carbon, topologically distinct from disordered graphene. Unlike in bulk materials, the structure of monolayer amorphous carbon can be determined by atomic-resolution imaging. Extensive characterization by Raman and X-ray spectroscopy and transmission electron microscopy reveals the complete absence of long-range periodicity and a threefold-coordinated structure with a wide distribution of bond lengths, bond angles, and five-, six-, seven- and eight-member rings. The ring distribution is not a Zachariasen continuous random network, but resembles the competing (nano)crystallite model(6). We construct a corresponding model that enables density-functional-theory calculations of the properties of monolayer amorphous carbon, in accordance with observations. Direct measurements confirm that it is insulating, with resistivity values similar to those of boron nitride grown by chemical vapour deposition. Free-standing monolayer amorphous carbon is surprisingly stable and deforms to a high breaking strength, without crack propagation from the point of fracture. The excellent physical properties of this stable, free-standing monolayer amorphous carbon could prove useful for permeation and diffusion barriers in applications such as magnetic recording devices and flexible electronics.


  
Short-range order and its impact on the CrCoNi medium-entropy alloy 期刊论文
NATURE, 2020, 581 (7808) : 283-+
作者:  Tan, Hwei-Ee;  Sisti, Alexander C.;  Jin, Hao;  Vignovich, Martin;  Villavicencio, Miguel;  Tsang, Katherine S.;  Goffer, Yossef;  Zuker, Charles S.
收藏  |  浏览/下载:20/0  |  提交时间:2020/07/03

Traditional metallic alloys are mixtures of elements in which the atoms of minority species tend to be distributed randomly if they are below their solubility limit, or to form secondary phases if they are above it. The concept of multiple-principal-element alloys has recently expanded this view, as these materials are single-phase solid solutions of generally equiatomic mixtures of metallic elements. This group of materials has received much interest owing to their enhanced mechanical properties(1-5). They are usually called medium-entropy alloys in ternary systems and high-entropy alloys in quaternary or quinary systems, alluding to their high degree of configurational entropy. However, the question has remained as to how random these solid solutions actually are, with the influence of short-range order being suggested in computational simulations but not seen experimentally(6,7). Here we report the observation, using energy-filtered transmission electron microscopy, of structural features attributable to short-range order in the CrCoNi medium-entropy alloy. Increasing amounts of such order give rise to both higher stacking-fault energy and hardness. These findings suggest that the degree of local ordering at the nanometre scale can be tailored through thermomechanical processing, providing a new avenue for tuning the mechanical properties of medium- and high-entropy alloys.


Metal alloys consisting of three or more major elemental components show enhanced mechanical properties, which are now shown to be correlated with short-range order observed with electron microscopy.


  
Laser spectroscopy of pionic helium atoms 期刊论文
NATURE, 2020, 581 (7806) : 37-+
作者:  Shang, Jian;  Ye, Gang;  Shi, Ke;  Wan, Yushun;  Luo, Chuming;  Aihara, Hideki;  Geng, Qibin;  Auerbach, Ashley;  Li, Fang
收藏  |  浏览/下载:36/0  |  提交时间:2020/07/03

Charged pions(1) are the lightest and longest-lived mesons. Mesonic atoms are formed when an orbital electron in an atom is replaced by a negatively charged meson. Laser spectroscopy of these atoms should permit the mass and other properties of the meson to be determined with high precision and could place upper limits on exotic forces involving mesons (as has been done in other experiments on antiprotons(2-9)). Determining the mass of the pi(-) meson in particular could help to place direct experimental constraints on the mass of the muon antineutrino(10-13). However, laser excitations of mesonic atoms have not been previously achieved because of the small number of atoms that can be synthesized and their typically short (less than one picosecond) lifetimes against absorption of the mesons into the nuclei(1). Metastable pionic helium (pi He-4(+)) is a hypothetical(14-16) three-body atom composed of a helium-4 nucleus, an electron and a pi(-) occupying a Rydberg state of large principal (n approximate to 16) and orbital angular momentum (l approximate to n - 1) quantum numbers. The pi He-4(+) atom is predicted to have an anomalously long nanosecond-scale lifetime, which could allow laser spectroscopy to be carried out(17). Its atomic structure is unique owing to the absence of hyperfine interactions(18,19) between the spin-0 pi(-) and the He-4 nucleus. Here we synthesize pi He-4(+) in a superfluid-helium target and excite the transition (n, l) = (17, 16) -> (17, 15) of the pi(-)-occupied pi He-4(+) orbital at a near-infrared resonance frequency of 183,760 gigahertz. The laser initiates electromagnetic cascade processes that end with the nucleus absorbing the pi(-) and undergoing fission(20,21). The detection of emerging neutron, proton and deuteron fragments signals the laser-induced resonance in the atom, thereby confirming the presence of pi He-4(+). This work enables the use of the experimental techniques of quantum optics to study a meson.


Long-lived pionic helium atoms (composed of a helium-4 nucleus, an electron and a negatively charged pion) are synthesized in a superfluid-helium target, as confirmed by laser spectroscopy involving the pion-occupied orbitals.


  
Operation of a silicon quantum processor unit cell above one kelvin 期刊论文
NATURE, 2020, 580 (7803) : 350-+
作者:  Han, Kyuho;  Pierce, Sarah E.;  Li, Amy;  Spees, Kaitlyn;  Anderson, Grace R.;  Seoane, Jose A.;  Lo, Yuan-Hung;  Dubreuil, Michael;  Olivas, Micah;  Kamber, Roarke A.;  Wainberg, Michael;  Kostyrko, Kaja;  Kelly, Marcus R.;  Yousefi, Maryam;  Simpkins, Scott W.;  Yao, David
收藏  |  浏览/下载:28/0  |  提交时间:2020/07/03

Quantum computers are expected to outperform conventional computers in several important applications, from molecular simulation to search algorithms, once they can be scaled up to large numbers-typically millions-of quantum bits (qubits)(1-3). For most solid-state qubit technologies-for example, those using superconducting circuits or semiconductor spins-scaling poses a considerable challenge because every additional qubit increases the heat generated, whereas the cooling power of dilution refrigerators is severely limited at their operating temperature (less than 100 millikelvin)(4-6). Here we demonstrate the operation of a scalable silicon quantum processor unit cell comprising two qubits confined to quantum dots at about 1.5 kelvin. We achieve this by isolating the quantum dots from the electron reservoir, and then initializing and reading the qubits solely via tunnelling of electrons between the two quantum dots(7-9). We coherently control the qubits using electrically driven spin resonance(10,11) in isotopically enriched silicon(12 28)Si, attaining single-qubit gate fidelities of 98.6 per cent and a coherence time of 2 microseconds during '  hot'  operation, comparable to those of spin qubits in natural silicon at millikelvin temperatures(13-16). Furthermore, we show that the unit cell can be operated at magnetic fields as low as 0.1 tesla, corresponding to a qubit control frequency of 3.5 gigahertz, where the qubit energy is well below the thermal energy. The unit cell constitutes the core building block of a full-scale silicon quantum computer and satisfies layout constraints required by error-correction architectures(8),(17). Our work indicates that a spin-based quantum computer could be operated at increased temperatures in a simple pumped He-4 system (which provides cooling power orders of magnitude higher than that of dilution refrigerators), thus potentially enabling the integration of classical control electronics with the qubit array(18,19).


  
Nagaoka ferromagnetism observed in a quantum dot plaquette 期刊论文
NATURE, 2020, 579 (7800) : 528-533
作者:  Yu, Yong;  Ma, Fei;  Luo, Xi-Yu;  Jing, Bo;  Sun, Peng-Fei;  Fang, Ren-Zhou;  Yang, Chao-Wei;  Liu, Hui;  Zheng, Ming-Yang;  Xie, Xiu-Ping;  Zhang, Wei-Jun;  You, Li-Xing;  Wang, Zhen;  Chen, Teng-Yun;  Zhang, Qiang;  Bao, Xiao-Hui;  Pan, Jian-Wei
收藏  |  浏览/下载:53/0  |  提交时间:2020/07/03

A quantum dot device designed to host four electrons is used to demonstrate Nagaoka ferromagnetism-a model of itinerant magnetism that has so far been limited to theoretical investigation.


Engineered, highly controllable quantum systems are promising simulators of emergent physics beyond the simulation capabilities of classical computers(1). An important problem in many-body physics is itinerant magnetism, which originates purely from long-range interactions of free electrons and whose existence in real systems has been debated for decades(2,3). Here we use a quantum simulator consisting of a four-electron-site square plaquette of quantum dots(4) to demonstrate Nagaoka ferromagnetism(5). This form of itinerant magnetism has been rigorously studied theoretically(6-9) but has remained unattainable in experiments. We load the plaquette with three electrons and demonstrate the predicted emergence of spontaneous ferromagnetic correlations through pairwise measurements of spin. We find that the ferromagnetic ground state is remarkably robust to engineered disorder in the on-site potentials and we can induce a transition to the low-spin state by changing the plaquette topology to an open chain. This demonstration of Nagaoka ferromagnetism highlights that quantum simulators can be used to study physical phenomena that have not yet been observed in any experimental system. The work also constitutes an important step towards large-scale quantum dot simulators of correlated electron systems.


  
In situ NMR metrology reveals reaction mechanisms in redox flow batteries 期刊论文
NATURE, 2020, 579 (7798) : 224-+
作者:  Ma, Jianfei;  You, Xin;  Sun, Shan;  Wang, Xiaoxiao;  Qin, Song;  Sui, Sen-Fang
收藏  |  浏览/下载:41/0  |  提交时间:2020/07/03

Large-scale energy storage is becoming increasingly critical to balancing renewable energy production and consumption(1). Organic redox flow batteries, made from inexpensive and sustainable redox-active materials, are promising storage technologies that are cheaper and less environmentally hazardous than vanadium-based batteries, but they have shorter lifetimes and lower energy density(2,3). Thus, fundamental insight at the molecular level is required to improve performance(4,5). Here we report two in situ nuclear magnetic resonance (NMR) methods of studying redox flow batteries, which are applied to two redox-active electrolytes: 2,6-dihydroxyanthraquinone (DHAQ) and 4,4 '  -((9,10-anthraquinone-2,6-diyl)dioxy) dibutyrate (DBEAQ). In the first method, we monitor the changes in the H-1 NMR shift of the liquid electrolyte as it flows out of the electrochemical cell. In the second method, we observe the changes that occur simultaneously in the positive and negative electrodes in the full electrochemical cell. Using the bulk magnetization changes (observed via the H-1 NMR shift of the water resonance) and the line broadening of the H-1 shifts of the quinone resonances as a function of the state of charge, we measure the potential differences of the two single-electron couples, identify and quantify the rate of electron transfer between the reduced and oxidized species, and determine the extent of electron delocalization of the unpaired spins over the radical anions. These NMR techniques enable electrolyte decomposition and battery self-discharge to be explored in real time, and show that DHAQ is decomposed electrochemically via a reaction that can be minimized by limiting the voltage used on charging. We foresee applications of these NMR methods in understanding a wide range of redox processes in flow and other electrochemical systems.


  
NEDD8 nucleates a multivalent cullin-RING-UBE2D ubiquitin ligation assembly 期刊论文
NATURE, 2020, 578 (7795) : 461-+
作者:  Fruchart, Michel;  Zhou, Yujie;  Vitelli, Vincenzo
收藏  |  浏览/下载:24/0  |  提交时间:2020/07/03

Eukaryotic cell biology depends on cullin-RING E3 ligase (CRL)-catalysed protein ubiquitylation(1), which is tightly controlled by the modification of cullin with the ubiquitin-like protein NEDD8(2-6). However, how CRLs catalyse ubiquitylation, and the basis of NEDD8 activation, remain unknown. Here we report the cryo-electron microscopy structure of a chemically trapped complex that represents the ubiquitylation intermediate, in which the neddylated CRL1(beta-TRCP) promotes the transfer of ubiquitin from the E2 ubiquitin-conjugating enzyme UBE2D to its recruited substrate, phosphorylated I kappa B alpha. NEDD8 acts as a nexus that binds disparate cullin elements and the RING-activated ubiquitin-linked UBE2D. Local structural remodelling of NEDD8 and large-scale movements of CRL domains converge to juxtapose the substrate and the ubiquitylation active site. These findings explain how a distinctive ubiquitin-like protein alters the functions of its targets, and show how numerous NEDD8-dependent interprotein interactions and conformational changes synergistically configure a catalytic CRL architecture that is both robust, to enable rapid ubiquitylation of the substrate, and fragile, to enable the subsequent functions of cullin-RING proteins.


A cryo-electron microscopy structure provides insights into the activation of cullin-RING E3 ligases by NEDD8 and the consequent catalysis of ubiquitylation reactions.


  
Electron Density Distributions in Saturn's Ionosphere 期刊论文
GEOPHYSICAL RESEARCH LETTERS, 2019, 46 (6) : 3061-3068
作者:  Persoon, A. M.;  Kurth, W. S.;  Gurnett, D. A.;  Groene, J. B.;  Sulaiman, A. H.;  Wahlund, J. -E.;  Morooka, M. W.;  Hadid, L. Z.;  Nagy, A. F.;  Waite, J. H.;  Cravens, T. E.
收藏  |  浏览/下载:30/0  |  提交时间:2019/11/26
Saturn  Saturn'  s ionosphere  ionospheric layers  electron densities  plasma scale height  scale height density model  
Rippled Electron-Scale Structure of a Dipolarization Front 期刊论文
GEOPHYSICAL RESEARCH LETTERS, 2018, 45 (22) : 12116-12124
作者:  Pan, Dong-Xiao;  Khotyaintsev, Yuri V.;  Graham, Daniel B.;  Vaivads, Andris;  Zhou, Xu-Zhi;  Andre, Mats;  Lindqvist, Per-Arne;  Ergun, Robert E.;  Le Contel, Olivier;  Russell, Christopher T.;  Torbert, Roy B.;  Giles, Barbara;  Burch, James L.
收藏  |  浏览/下载:27/0  |  提交时间:2019/04/09
dipolarization fronts  LHDI  ripples  electron-scale structures  MMS  turbulence