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 isolation of graphene has triggered an avalanche of studies into the spin-dependent physical properties of this material, as well as graphene-based spintronic devices Here we review the experimental and theoretical state-of-art concerning spin injection and transport, defect-induced magnetic moments, spin-orbit coupling and spin relaxation in graphene Future research in graphene spintronics will need to address the development of applications such as spin transistors and spin logic devices, as well as exotic physical properties including topological states and proximity-induced phenomena in graphene and other 2D materials

Graphene Spintronics

Graphene is a two-dimensional (2D) material with over 100-fold anisotropy of heat flow between the in-plane and out-of-plane directions. High in-plane thermal conductivity is due to covalent sp2 bonding between carbon atoms, whereas out-of-plane heat flow is limited by weak van der Waals coupling. Herein, we review the thermal properties of graphene, including its specific heat and thermal conductivity (from diffusive to ballistic limits) and the influence of substrates, defects, and other atomic modifications. We also highlight practical applications in which the thermal properties of graphene play a role. For instance, graphene transistors and interconnects benefit from the high in-plane thermal conductivity, up to a certain channel length. However, weak thermal coupling with substrates implies that interfaces and contacts remain significant dissipation bottlenecks. Heat flow in graphene or graphene composites could also be tunable through a variety of means, including phonon scattering by substrates, edges or interfaces. Ultimately, the unusual thermal properties of graphene stem from its 2D nature, forming a rich playground for new discoveries of heat flow physics and potentially leading to novel thermal management applications.

Thermal properties of graphene: Fundamentals and applications

Achieving mastery over the synthesis of metal nanocrystals has emerged as one of the foremost scientific endeavors in recent years. This intense interest stems from the fact that the composition, size, and shape of nanocrystals not only define their overall physicochemical properties but also determine their effectiveness in technologically important applications. Our aim is to present a comprehensive review of recent research activities on bimetallic nanocrystals. We begin with a brief introduction to the architectural diversity of bimetallic nanocrystals, followed by discussion of the various synthetic techniques necessary for controlling the elemental ratio and spatial arrangement. We have selected key examples from the literature that exemplify critical concepts and place a special emphasis on mechanistic understanding. We then discuss the composition-dependent properties of bimetallic nanocrystals in terms of catalysis, optics, and magnetism and conclude the Review by highlighting applications that h...

Bimetallic Nanocrystals: Syntheses, Properties, and Applications

Graphene is a two-dimensional (2D) material with over 100-fold anisotropy of heat flow between the in-plane and out-of-plane directions. High in-plane thermal conductivity is due to covalent sp2 bonding between carbon atoms, whereas out-of-plane heat flow is limited by weak van der Waals coupling. Herein, we review the thermal properties of graphene, including its specific heat and thermal conductivity (from diffusive to ballistic limits) and the influence of substrates, defects, and other atomic modifications. We also highlight practical applications in which the thermal properties of graphene play a role. For instance, graphene transistors and interconnects benefit from the high in-plane thermal conductivity, up to a certain channel length. However, weak thermal coupling with substrates implies that interfaces and contacts remain significant dissipation bottlenecks. Heat flow in graphene or graphene composites could also be tunable through a variety of means, including phonon scattering by substrates, edges, or interfaces. Ultimately, the unusual thermal properties of graphene stem from its 2D nature, forming a rich playground for new discoveries of heat-flow physics and potentially leading to novel thermal management applications.

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Thermal conductance of graphene nanoribbons (GNRs) with the width varying from 0.5 to 35 nm is systematically investigated using nonequilibrium Green’s function method. Anisotropic thermal conductance is observed with the room temperature thermal conductance of zigzag GNRs up to ∼30% larger than that of armchair GNRs. At room temperature, the anisotropy is found to disappear when the width is larger than 100 nm. This intrinsic anisotropy originate from different boundary condition at ribbon edges, and can be used to tune thermal conductance, which have important implications for the applications of GNRs in nanoelectronics and thermoelectricity.

Intrinsic anisotropy of thermal conductance in graphene nanoribbons

Based on ab initio calculations of both the ABC- and AB-stacked graphites, interlayer potentials (i.e., graphene-graphene interaction) are obtained as a function of the interlayer spacing using a modified Mobius inversion method, and are used to calculate basic physical properties of graphite. Excellent consistency is observed between the calculated and experimental phonon dispersions of AB-stacked graphite, showing the validity of the interlayer potentials. More importantly, layer-related properties for nonideal structures (e.g., the exfoliation energy, cleave energy, stacking fault energy, surface energy, etc.) can be easily predicted from the interlayer potentials, which promise to be extremely efficient and helpful in studying van der Waals structures.

/pdf/interlayer-interactions-in-graphites-3mvwwtivhx.pdf

Interlayer interactions in graphites

Topological Phononics: From Fundamental Models to Real Materials

The magnetic properties of the low-temperature phase of MnBi, prepared by rapid solidification, have been measured in a pulsed field over the temperature range 80--625 K. The anisotropy field, obtained by the singular-point-detection technique, is found to increase with temperature and has a maximum value of 9 T at 530 K. A fit to the saturation-magnetization measurement gives a virtual Curie point of 775 K. The coercive field is fitted by a hybrid domain-wall-pinning theory which yields, at 300 K, a domain-wall energy of 15.6 erg/${\mathrm{cm}}^{2}$ and a wall thickness of 70 \AA{}. Below 200 K, the presence of a ferrimagnetic phase is detected. Its critical field associated with spin reversal is a convex function of temperature having a maximum value of 8 T at 120 K.

Magnetic properties of MnBi prepared by rapid solidification.

Two dimensional (2D) materials are promising candidates for developing next-generation electronics. Monolayer tin(II) selenide (SnSe), which can be obtained by exfoliating bulk SnSe crystals at a low cleavage energy, is shown to be a nearly direct band gap semiconductor using first-principles calculations. By incorporating the anisotropic characteristics of effective masses, elastic modulus, and deformation potential with the longitudinal acoustic deformation potential scattering mechanism, we demonstrate that the charge carrier mobilities of monolayer SnSe strongly depend on the carrier type, valley index, transport direction, and biaxial strain. In particular, electron mobility is generally higher than hole mobility, and exhibits anisotropy of up to 241%. With increasing biaxial tensile strain, both electron mobility and hole mobility decline gradually, which is mainly attributed to a strain-induced heavier effective mass. Surprisingly, it is found that carrier mobilities can be enhanced by about 147% (electrons) and 968% (holes) via a small biaxial compressive strain, which effectively produces smaller effective masses of carriers and larger elastic modulus, suggesting the possibility of achieving high mobility of monolayer SnSe over a large number of substrates. Our results highlight a new promising 2D tin-based chalcogenide material with high thermal stability and controllable superior carrier mobility, having great potential in future flexible nano-electronic devices.

Xiaobin Chen

Papers

Intrinsic anisotropy of thermal conductance in graphene nanoribbons

Interlayer interactions in graphites

Topological Phononics: From Fundamental Models to Real Materials

Magnetic properties of MnBi prepared by rapid solidification.

Widely tunable and anisotropic charge carrier mobility in monolayer tin(II) selenide using biaxial strain: a first-principles study