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

Graphene is one of the most promising materials in nanotechnology. The electronic and mechanical properties of graphene samples with high perfection of the atomic lattice are outstanding, but structural defects, which may appear during growth or processing, deteriorate the performance of graphene-based devices. However, deviations from perfection can be useful in some applications, as they make it possible to tailor the local properties of graphene and to achieve new functionalities. In this article, the present knowledge about point and line defects in graphene are reviewed. Particular emphasis is put on the unique ability of graphene to reconstruct its lattice around intrinsic defects, leading to interesting effects and potential applications. Extrinsic defects such as foreign atoms which are of equally high importance for designing graphene-based devices with dedicated properties are also discussed.

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Structural defects in graphene

■ Abstract The phase-field method has recently emerged as a powerful computational approach to modeling and predicting mesoscale morphological and microstructure evolution in materials. It describes a microstructure using a set of conserved and nonconserved field variables that are continuous across the interfacial regions. The temporal and spatial evolution of the field variables is governed by the Cahn-Hilliard nonlinear diffusion equation and the Allen-Cahn relaxation equation. With the fundamental thermodynamic and kinetic information as the input, the phase-field method is able to predict the evolution of arbitrary morphologies and complex microstructures without explicitly tracking the positions of interfaces. This paper briefly reviews the recent advances in developing phase-field models for various materials processes including solidification, solid-state structural phase transformations, grain growth and coarsening, domain evolution in thin films, pattern formation on surfaces, dislocation microstructures, crack propagation, and electromigration.

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Phase-Field Models for Microstructure Evolution

IN two previous notes1, Prof. Max Born and I have shown that one can obtain a theory of superconductivity by taking account of the fact that the interaction of the electrons with the ionic lattice is appreciable only near the boundaries of Brillouin zones, and particularly strong near the corners of these. This leads to the criterion that the metal should be superconductive if a set of corners of a Brillouin zone is lying very near the Fermi surface, considered as a sphere, which limits the region in the momentum space completely filled with electrons.

On the theory of superconductivity

인공고관절의 세라믹 볼 헤드의 기계적 안정성 평가를 위한 3 차원 유한요소 해석

We present a new way of nanoengineering graphene by using defect domains. These regions have ring structures that depart from the usual honeycomb lattice, though each carbon atom still has three nearest neighbors. A set of stable domain structures is identified by using density functional theory, including blisters, ridges, ribbons, and metacrystals. All such structures are made solely out of carbon; the smallest encompasses just 16 atoms. Blisters, ridges, and metacrystals rise up out of the sheet, while ribbons remain flat. In the vicinity of vacancies, the reaction barriers to formation are sufficiently low that such defects could be synthesized through the thermally activated restructuring of coalesced adatoms.

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Nanoengineering defect structures on graphene.

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Nano-Engineering Defect Structures on Graphene

An extended one-dimensional defect that has the potential to act as a conducting wire has been embedded in another perfect graphene sheet

Defect engineering: Graphene gets designer defects.

It is commonly held that a sufficient condition for the rule of additivity to be valid is that the transformation rate depend only on temperature and volume fraction. This is not true in general.

On the rule of additivity in phase transformation kinetics

We analyze a fundamental building block for monolithic nanoengineering on graphene: the Inverse-Stone-Thrower-Wales (ISTW) defect. The ISTW is formed from a pair of joined pentagonal carbon rings placed between a pair of heptagonal rings; the well-known Stone-Thrower-Wales defect is the same arrangement, but with the heptagonal rather than pentagonal rings joined. When removed and passivated with hydrogen, the structure constitutes a molecule, diazulene, which may be viewed as the result of an ad-dimer defect on anthracene. Embedding diazulene in the honeycomb lattice, we study the effect of ad-dimers on planar graphene. Because the ISTW defect has yet to be experimentally identified, we examine several synthesis routes and find one for which the barrier is only slightly higher than that associated with adatom hopping on graphene. ISTW and STW defects may be viewed as fundamental building blocks for monolithic structures on graphene. We show how to construct extended defect domains on the surface of graphene in the form of blisters, bubbles, and ridges on a length scale as small as $2\text{ }\text{\AA{}}\ifmmode\times\else\texttimes\fi{}7\text{ }\text{\AA{}}$. Our primary tool in these studies is density functional theory.

Mark T. Lusk

Papers

Nanoengineering defect structures on graphene.

Nano-Engineering Defect Structures on Graphene

Defect engineering: Graphene gets designer defects.

On the rule of additivity in phase transformation kinetics

Graphene nanoengineering and the inverse Stone-Thrower-Wales defect