Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature-carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 1011 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.

Unconventional superconductivity in magic-angle graphene superlattices

BACKGROUND Materials by design is an appealing idea that is very hard to realize in practice. Combining the best of different ingredients in one ultimate material is a task for which we currently have no general solution. However, we do have some successful examples to draw upon: Composite materials and III-V heterostructures have revolutionized many aspects of our lives. Still, we need a general strategy to solve the problem of mixing and matching crystals with different properties, creating combinations with predetermined attributes and functionalities. ADVANCES Two-dimensional (2D) materials offer a platform that allows creation of heterostructures with a variety of properties. One-atom-thick crystals now comprise a large family of these materials, collectively covering a very broad range of properties. The first material to be included was graphene, a zero-overlap semimetal. The family of 2D crystals has grown to includes metals (e.g., NbSe 2 ), semiconductors (e.g., MoS 2 ), and insulators [e.g., hexagonal boron nitride (hBN)]. Many of these materials are stable at ambient conditions, and we have come up with strategies for handling those that are not. Surprisingly, the properties of such 2D materials are often very different from those of their 3D counterparts. Furthermore, even the study of familiar phenomena (like superconductivity or ferromagnetism) in the 2D case, where there is no long-range order, raises many thought-provoking questions. A plethora of opportunities appear when we start to combine several 2D crystals in one vertical stack. Held together by van der Waals forces (the same forces that hold layered materials together), such heterostructures allow a far greater number of combinations than any traditional growth method. As the family of 2D crystals is expanding day by day, so too is the complexity of the heterostructures that could be created with atomic precision. When stacking different crystals together, the synergetic effects become very important. In the first-order approximation, charge redistribution might occur between the neighboring (and even more distant) crystals in the stack. Neighboring crystals can also induce structural changes in each other. Furthermore, such changes can be controlled by adjusting the relative orientation between the individual elements. Such heterostructures have already led to the observation of numerous exciting physical phenomena. Thus, spectrum reconstruction in graphene interacting with hBN allowed several groups to study the Hofstadter butterfly effect and topological currents in such a system. The possibility of positioning crystals in very close (but controlled) proximity to one another allows for the study of tunneling and drag effects. The use of semiconducting monolayers leads to the creation of optically active heterostructures. The extended range of functionalities of such heterostructures yields a range of possible applications. Now the highest-mobility graphene transistors are achieved by encapsulating graphene with hBN. Photovoltaic and light-emitting devices have been demonstrated by combining optically active semiconducting layers and graphene as transparent electrodes. OUTLOOK Currently, most 2D heterostructures are composed by direct stacking of individual monolayer flakes of different materials. Although this method allows ultimate flexibility, it is slow and cumbersome. Thus, techniques involving transfer of large-area crystals grown by chemical vapor deposition (CVD), direct growth of heterostructures by CVD or physical epitaxy, or one-step growth in solution are being developed. Currently, we are at the same level as we were with graphene 10 years ago: plenty of interesting science and unclear prospects for mass production. Given the fast progress of graphene technology over the past few years, we can expect similar advances in the production of the heterostructures, making the science and applications more achievable.

2D materials and van der Waals heterostructures

Graphene-Like Two-Dimensional Materials

Because they can be isolated down to single unit cells and recombined almost arbitrarily through layerby-layer stacking, two-dimensional (2D) materials offer a unique opportunity to study the structureproperty relationship of materials with high precision using electron microscopy. Due to their atomically thin structure, 2D materials are predicted to be among the most flexible, high-quality electronic materials known. However, efforts to experimentally measure their bending stiffness, using conventional techniques such as nanoindentation and electrostatic actuation, have yielded conflicting results that range over orders of magnitude [1-3]. Here, we show that electron microscopy is an exceptionally precise, yet flexible technique for measuring the intrinsic bending stiffness of 2D materials. We use these techniques to

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Probing the Intrinsic Bending Stiffness of 2D Multilayers and Heterostructures Using Aberration-corrected STEM

Understanding graphene's role as a protective substrate for atomic-resolution electron microscopy of small organic molecules

Atomic-resolution and Atomic-scale Imaging of Small Organic Molecules

Antiferromagnets are a category of magnetic materials with near-zero net magnetization. New magnetic devices involving antiferromagnets may rely on manipulating patterns of magnetic moments such as domain walls or skyrmions, which have been imaged using techniques such as x-ray photoemission electron microscopy or differential phase contrast scanning transmission electron microscopy (DPC STEM) [1, 2]. DPC STEM with an atomically sharp probe and unit-cell averaging has been used to map the magnetic structure of an antiferromagnet in real space [3]. However, electron microscopy studies of antiferromagnets need to isolate and detect the relatively weak signal due to scattering from the magnetic structure and remain limited in signal-to-noise ratio. Here, we propose using antiferromagnetic reflections to perform phase-contrast imaging of magnetic structure at few-angstrom resolution. Such reflections are commonly measured in neutron scattering [4] and, more recently, have been detected in transmission electron diffraction, where they are around 10 4 times less intense than structural reflections [5]. In the regions of convergent beam electron diffraction (CBED) patterns where two disks overlap, interference between the two beams causes the intensity to vary sinusoidally with probe position [6]. Thus, a periodic magnetic structure can be visualized as lattice fringes.

Measuring Antiferromagnetism at the Angstrom Scale using 4D-STEM

Soft-hard interfaces at the surface of nanoparticles determine interaction potentials, including the mechanisms of growth, spatial reactivity, colloidal stability, and nanoparticle functionality [1]. For example, soft molecular ligands are thought to guide growth and symmetry breaking in anisotropic nanoparticles. These ligands can also act as soft templates for site-selective deposition of functional coatings [2-4]. Quantitative details regarding the local attachment, distribution, and structure of these softhard interfaces would enable the development of methods for high-yield, monodisperse nanoparticle synthesis for applications ranging from catalysis [5] to drug delivery [6].

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Pinshane Y. Huang

Papers

Probing the Intrinsic Bending Stiffness of 2D Multilayers and Heterostructures Using Aberration-corrected STEM

Understanding graphene's role as a protective substrate for atomic-resolution electron microscopy of small organic molecules

Atomic-resolution and Atomic-scale Imaging of Small Organic Molecules

Measuring Antiferromagnetism at the Angstrom Scale using 4D-STEM

Quantitative Chemical Mapping of Soft-Hard Interfaces on Gold Nanorods