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

The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 06 eV This leads to a crossover to a direct-gap material in the limit of the single monolayer Unlike the bulk material, the MoS₂ monolayer emits light strongly The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 10⁴ compared with the bulk material

Atomically thin MoS2: a new direct-gap semiconductor

Graphene is a wonder material with many superlatives to its name. It is the thinnest known material in the universe and the strongest ever measured. Its charge carriers exhibit giant intrinsic mobility, have zero effective mass, and can travel for micrometers without scattering at room temperature. Graphene can sustain current densities six orders of magnitude higher than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and reconciles such conflicting qualities as brittleness and ductility. Electron transport in graphene is described by a Dirac-like equation, which allows the investigation of relativistic quantum phenomena in a benchtop experiment. This review analyzes recent trends in graphene research and applications, and attempts to identify future directions in which the field is likely to develop.

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Graphene: Status and Prospects

Problems associated with large-scale pattern growth of graphene constitute one of the main obstacles to using this material in device applications. Recently, macroscopic-scale graphene films were prepared by two-dimensional assembly of graphene sheets chemically derived from graphite crystals and graphene oxides. However, the sheet resistance of these films was found to be much larger than theoretically expected values. Here we report the direct synthesis of large-scale graphene films using chemical vapour deposition on thin nickel layers, and present two different methods of patterning the films and transferring them to arbitrary substrates. The transferred graphene films show very low sheet resistance of approximately 280 Omega per square, with approximately 80 per cent optical transparency. At low temperatures, the monolayers transferred to silicon dioxide substrates show electron mobility greater than 3,700 cm(2) V(-1) s(-1) and exhibit the half-integer quantum Hall effect, implying that the quality of graphene grown by chemical vapour deposition is as high as mechanically cleaved graphene. Employing the outstanding mechanical properties of graphene, we also demonstrate the macroscopic use of these highly conducting and transparent electrodes in flexible, stretchable, foldable electronics.

Large-scale pattern growth of graphene films for stretchable transparent electrodes

There is intense interest in graphene in fields such as physics, chemistry, and materials science, among others. Interest in graphene's exceptional physical properties, chemical tunability, and potential for applications has generated thousands of publications and an accelerating pace of research, making review of such research timely. Here is an overview of the synthesis, properties, and applications of graphene and related materials (primarily, graphite oxide and its colloidal suspensions and materials made from them), from a materials science perspective.

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Graphene and Graphene Oxide: Synthesis, Properties, and Applications

An implantable medical device for increasing coronary blood flow in heart failure patients. The device includes an elongate tubular body having a distal end and a proximal end and a lumen therebetween, a valve disposed within the lumen of the elongate tubular body, and a pressure sensor attached to the elongate tubular body and operativelv engaged to the valve, in which the pressure sensor actuates opening and closing of the valve.

Left ventricular coronary conduit to increase coronary blood flow in heart failure patients

Disclosed is a conduit that is adapted to be positioned in the myocardium to provide a passage for blood to flow from a heart chamber to a coronary artery. The conduit has a sphincter to open or close the passage in response to an electric current applied from a control unit and a one-way valve positioned therein to prevent the backflow of blood from the coronary artery into the heart chamber. Also disclosed is an implantable system or device comprising the conduit, a sensor unit to measure at least one physiological condition and a control unit to analyze signals form the sensor unit and open the sphincter for improving coronary blood flow in order to increase myocardial oxygen delivery during periods of high myocardial demand.

Conduit to increase coronary blood flow

Micro scale laser shock peening (lLSP) is a process in which compressive residual stresses are induced in a material surface to improve fatigue life and wear resistance under cyclic loading. Since the diameter of the laser spot used during the process is the same order of magnitude as grain size, the effects of anisotropy and heterogeneity have to be explicitly taken into account in any model of the process. In this study experimental and numerical studies have been performed in order to investigate the response of an aluminum bicrystal under laser shock peening. The grain boundary is shocked to investigate heterogeneity, and single crystals are shocked to study the effect of anisotropy in the absence of heterogeneity. The orientations of the crystals in the bicrystal as well as the reference single crystals have been chosen such that an approximate plane strain condition is achieved. A finite element model which accounts for the anisotropy, heterogeneity and inertia has also been developed based on single crystal micromechanics. Simulation results are compared with experimental findings. The potential benefit of lLSP as a surface treatment for improvement of fatigue life is also discussed.

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Grain Boundary Response of Aluminum Bicrystal Under Micro Scale Laser Shock Peening

The criteria which determine the initial energy dissipation mechanism that is activated at or near a crack tip are derived. The possible mechanisms considered are cleavage, crack tip dislocation nucleation and also Frank-Read source activation near the tip. The criteria can be succinctly expressed in graphical form. It is suggested that a change in the initial energy dissipation mechanism may determine the conditions under which the brittle to ductile transition occurs. The criteria compare favourably to several experiments in the literature which address the competition between the various energy dissipation mechanisms.

Jeffrey W. Kysar

Papers

Left ventricular coronary conduit to increase coronary blood flow in heart failure patients

Conduit to increase coronary blood flow

Grain Boundary Response of Aluminum Bicrystal Under Micro Scale Laser Shock Peening

Dependence of Ductile and Brittle Response on Initial Energy Dissipation Mechanism at Crack Tip

Anatomic, physiologic, and proteomic consequences of repeated microneedle-mediated perforations of the round window membrane