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

Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

J. Appl. Cryst.の発刊に際して

Diamond-like carbon (DLC) is a metastable form of amorphous carbon with significant sp3 bonding. DLC is a semiconductor with a high mechanical hardness, chemical inertness, and optical transparency. This review will describe the deposition methods, deposition mechanisms, characterisation methods, electronic structure, gap states, defects, doping, luminescence, field emission, mechanical properties and some applications of DLCs. The films have widespread applications as protective coatings in areas, such as magnetic storage disks, optical windows and micro-electromechanical devices (MEMs).

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Diamond-like amorphous carbon

Mechanical properties of nanocrystalline materials

Ionized cluster beam deposition is studied by molecular dynamics simulations in which the atomic size of incident cluster atoms is different from the size of substrate atoms. Lennard-Jones interatomic potentials are used for the two-component system. The results provide the morphologies of the overlayers for various atomic sizes and are compared to simulation results of molecular beam epitaxy.

Atomic size effect on the formation of ionized cluster beam epitaxy in Lennard-Jones systems

Phase evolution in FCC metals with strongly interacting alloy components during severe plastic deformation is investigated using molecular dynamics simulations. Specifically, we study the alloy microstructure in steady state, nucleation and growth of precipitates in supersaturated alloys, and the decomposition of precipitates in undersaturated alloys. The results are compared to a modified effective temperature model, providing a physical understanding for the atomic processes underlying the model and a perspective on its strengths and weaknesses. Key observations in this work are nucleation and growth of precipitates during SPD at a temperature of 100 K; Gibbs-Thomson-like behavior relating solubility to precipitate size under steady-state shearing; a direct relationship between the effective temperature and the shear modulus; and the importance of cluster agglomeration during precipitate growth. The study also reveals that the mechanisms of forced chemical mixing depends on precipitate size, adding complications for effective temperature models describing inhomogeneous systems. The simulations are shown to provide good semiquantitative agreement with experimental findings reported in the literature.

Phase Evolution in Two-Phase Alloys During Severe Plastic Deformation

Ion mixing of thin markers in Zr was investigated by irradiating with 660 keV Kr++ ions at temperatures between 300 to 423 K. Very thin films of vacuum evaporated Ti, Cr, Fe, Co, Ni, Cu+ Hf, W, and Au served as markers. The samples were analyzed by 2 MeV He backscattering spectrometry. The marker elements that are likely to dissolve interstitially in Zr have higher mixing efficiencies at elevated irradiation temperature than the markers that are likely to dissolve substitutionally. The results are explained by radiation-enhanced diffusion theory.

Temperature Dependence of Ion Mixing Of Markers in Zr

Defect production and electronic stopping for light ions in metals

Tracer impurity diffusion coefficients for Co and Hf were measured in a series of amorphous NixZr(1-x) alloys in the range 0.30 < x < 0.70 and at temperatures between 250 °C and 350° C. The diffusion coefficients for the Co atoms are between 2 and 4 orders of magnitude greater than those for Hf, depending on alloy composition and temperature. Diffusion coefficients for Hf are independent of alloy composition, whereas those for Co decrease strongly with increasing Ni content. The activation enthalpies of diffusion and the pre-exponential factors were both found to be significantly smaller for the Hf than the Co tracer impurity atoms. These results are discussed in terms of possible diffusion mechanisms in amorphous metallic alloys.

Robert S Averback

Papers

Atomic size effect on the formation of ionized cluster beam epitaxy in Lennard-Jones systems

Phase Evolution in Two-Phase Alloys During Severe Plastic Deformation

Temperature Dependence of Ion Mixing Of Markers in Zr

Defect production and electronic stopping for light ions in metals

Tracer Diffusion in Amorphous Ni-Zr Alloys