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 stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

This overview highlights very recent achievements and new trends in one of the most active and developing fields in modern materials science: the production of bulk ultrafine-grained (UFG) materials using severe plastic deformation (SPD). The article also summarizes the chronology of early work in SPD processing and presents clear and definitive descriptions of the terminology currently in use in this research area. Special attention is given to the principles of the various SPD processing techniques as well as the major structural features and unique properties of bulk UFG materials that underlie their prospects for widespread practical utilization.

https://link.springer.com/content/pdf/10.1007%2Fs11837-006-0213-7.pdf

Producing bulk ultrafine-grained materials by severe plastic deformation

A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

IN view of the interest attaching to the vaporisation and diffusion of solids, the following observations may be worthy of record.

The Diffusion of Solids

Crowdion presents an interstitial mobile defect propagating in close-packed crystallographic directions and plays an important role in energy and mass transfer processes occurring in bcc tungsten lattice in non-equilibrium conditions. In the present day, tungsten remains one of the most promising plasma-oriented material, which saves its protective features even under high-intensive irradiation influence in nuclear reactors. Crowdions can be one of most possible and effective nonlinear channels of energy dissipation obtaining under irradiation. The dynamics of the crowdion in the lattice with the zero or very low temperature has been already deeply studied. At the same time, influence of thermal oscillations of atoms on the crowdion dynamics has not been studied in detail, despite the processes of irradiation always occur under finite nonzero temperature. In the present work, we try to reveal changing the crowdion features under different finite temperature of the tungsten lattice.

Propagation of 2-crowdion in W bcc lattice at finite temperature

The effect of severe plastic deformation by high-pressure torsion (HPT) and subsequent annealing on the microstructure and microhardness of squeeze casting Al–22%Si–3%Cu–1.7%Ni alloy was investigated. HPT was performed at room temperature with 5 rotations under the pressure of 4 GPa. Annealing temperature range varied from 300 to 500°C for 5 min. HPT resulted in refinement and partial dissolution of the primary silicon and intermetallic particles in aluminum matrix and structure fragmentation that caused the microhardness increase. Subsequent annealing lead to the decomposition of the supersaturated solid solution that took place simultaneously with recovery and recrystallization of the fragmented structure. Increase of annealing temperature resulted in decrease of microhardness values.

Structure and microhardness of Al–Si–Cu–Ni alloy after severe plastic deformation and high-temperature annealing

A beam of single-layer carbon nanotubes (CNTs) subjected to a shock load in the transverse direction was studied using a chain model with a reduced number of degrees of freedom. The compression shock wave is initiated by a piston moving at a constant speed. Two different scenarios of shock wave propagation and their influence on the structure of the CNT beam depending on the piston velocity were found. At lower speeds, only a faster wave front propagates, which leads to elliptization of the CNT, whereas at higher speeds this is accompanied by a slower wave front of collapse. The evolution of the CNT beam structure in time during compression is studied in detail. The energy absorption rate is estimated as a function of the piston velocity. The results obtained can be useful in the development of new types of elastic shock absorbers.

Propagation of supersonic excitations through the carbon nanotube forest

Behaviour of carbon nanotube bundle under quasistatic and dynamic transverse compression

This chapter presents a comprehensive overview of investigations devoted to the atomistic simulations of nonlinear excitations in two-dimensional (2-D) materials with an emphasis on graphene and its derivatives. The role of interatomic potentials in molecular dynamics studies of nonlinear effects is discussed. A detailed analysis of the dynamics of one- and two-component delocalized vibrational modes is given, and their possible effect on the mechanical properties of graphene is presented. The interrelation between spatially localized and delocalized vibrational modes is discussed, and new types of localized vibrational states, also called discrete breathers (DBs), obtained for graphene as a result of this analysis are presented. Other recent achievements in the study of nonlinear lattice dynamics of 2-D materials are presented, and open problems are listed.

Anharmonic Excitations in Graphene and Other 2-D Nanomaterials

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