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

We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ~3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.

Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers

The fact that some of solid materials with suitable phases possess higher thermal conductivity than liquids gave rise to their utilization in heat transfer processes. Fluids that contain nanopartic...

A Review on Graphene Derivatives-Based Nanofluids: Investigation on Properties and Heat Transfer Characteristics

The effect of graphene nanoribbon twist on its lateral buckling resistance to axial compression and on its thermal conductivity is analyzed with the help of molecular dynamics simulations. It is shown that the nanoribbon twisted by an angle close to π can withstand three times greater compressive force as compared to a flat nanoribbon. The explanation lies in the fact that such twist increases the effective area moment of inertia of the nanoribbon cross section. It is also found that the thermal conductivity coefficient of the nanoribbon increases monotonically up to 10% with increasing twist angle, in the regime of uniform twisting. This effect is due to the introduction of tensile strain in the twisted nanoribbon, which increases the contribution of the acoustic out-of plane (ZA) phonon modes to thermal conductivity. Our results demonstrate that twist deformation of nanoribbons can improve their mechanical and physical properties. The reported effects can be observed for 2D materials other than graphene because they have simple mechanical explanation not related to a particular crystal structure.

Longitudinal stiffness and thermal conductivity of twisted carbon nanoribbons

Graphene is a two-dimensional crystal in which sp2-hybridized carbon atoms have valence bonds with three neighbors. Theoretically, other two-dimensional carbon structures were predicted, in which each carbon atom has valence bonds with three neighbors. In this paper, the molecular dynamics method is used to analyze the mechanical properties and structural transformations of such materials under uniaxial and biaxial stretching. The dependences of the tensile membrane forces on the applied tensile strain are constructed, the limiting values of the membrane forces and strains are determined. The three structures studied differ in their density, and it could be expected that the strength of the structures should decrease with decreasing density. However, it turned out that such a correlation did not manifest itself in all cases: a less dense structure may turn out to be stronger due to the fact that all interatomic bonds in it turn out to be loaded more uniformly. The results can be useful in analyzing the potentialities of application of sp2-carbon membranes in various technologies.

Ragesh Chellattoan

Papers

The effect of torsional deformation on thermal conductivity of mono-, bi- and trilayer graphene nanoribbon