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

Origin of friction and the new frictionless technology—Superlubricity: Advancements and future outlook

Miniaturized or microscale generators that can effectively convert weak and random mechanical energy into electricity have significant potential to provide solutions for the power supply problem of distributed devices. However, owing to the common occurrence of friction and wear, all such generators developed so far have failed to simultaneously achieve sufficiently high current density and sufficiently long lifetime, which are crucial for real-world applications. To address this issue, we invent a microscale Schottky superlubric generator (S-SLG), such that the sliding contact between microsized graphite flakes and n-type silicon is in a structural superlubric state (an ultra-low friction and wearless state). The S-SLG not only generates high current (~210 Am−2) and power (~7 Wm−2) densities, but also achieves a long lifetime of at least 5,000 cycles, while maintaining stable high electrical current density (~119 Am−2). No current decay and wear are observed during the experiment, indicating that the actual persistence of the S-SLG is enduring or virtually unlimited. By excluding the mechanism of friction-induced excitation in the S-SLG, we further demonstrate an electronic drift process during relative sliding using a quasi-static semiconductor finite element simulation. Our work may guide and accelerate the future use of S-SLGs in real-world applications. It is difficult to achieve high current density and long lifetime for micro generators owing to the common friction and wear. Here the authors invent a microscale Schottky superlubric generator to tackle this issue, reporting not only high current and power densities but also long lifetime of at least 5,000 cycles.

/pdf/microscale-schottky-superlubric-generator-with-high-direct-nvhmmanee2.pdf

Microscale Schottky superlubric generator with high direct-current density and ultralong life.

More than thirty years ago, it was theoretically predicted that friction for incommensurate contacts between atomically smooth, infinite, crystalline materials (e.g., graphite, ${\mathrm{MoS}}_{2}$) is vanishing in the low speed limit, and this corresponding state was called structural superlubricity (SSL). However, experimental validation of this prediction has met challenges, since real contacts always have a finite size, and the overall friction arises not only from the atoms located within the contact area, but also from those at the contact edges which can contribute a finite amount of friction even when the incommensurate area does not. Here, we report, using a novel method, the decoupling of these contributions for the first time. The results obtained from nanoscale to microscale incommensurate contacts of graphite under ambient conditions verify that the average frictional contribution of an inner atom is no more than ${10}^{\ensuremath{-}4}$ that of an atom at the edge. Correspondingly, the total friction force is dominated by friction between the contact edges for contacts up to $10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ in lateral size. We discuss the physical mechanisms of friction observed in SSL contacts, and provide guidelines for the rational design of large-scale SSL contacts.

Origin of Friction in Superlubric Graphite Contacts.

Abstract Superlubricity has drawn substantial attention worldwide while the energy crisis is challenging human beings. Hence, numerous endeavors are bestowed to design materials for superlubricity achievement at multiple scales. Developments in graphene-family materials, such as graphene, graphene oxide, and graphene quantum dots, initiated an epoch for atomically thin solid lubricants. Nevertheless, superlubricity achieved with graphene-family materials still needs fundamental understanding for being applied in engineering in the future. In this review, the fundamental mechanisms for superlubricity that are achieved with graphene-family materials are outlined in detail, and the problems concerning graphene superlubricity and future progress in superlubricity are proposed. This review concludes the fundamental mechanisms for graphene superlubricity and offers guidance for utilizing graphene-family materials in superlubricity systems. 

/pdf/graphene-superlubricity-a-review-cflrjx18.pdf

Graphene superlubricity: A review

Direct characterizations of the two component surfaces of a solid-solid interface are essential for understanding its various interfacial mechanical, physical, and electrical behaviors. Particularly, the fascinating phenomenon termed structural superlubricity, a state of nearly zero friction and wear, is sensitively dependent on the interface structure. Here we report a controllable pick-and-flip technique to separate a microscale contact pair for the characterization of its two component surfaces for van der Waals layered materials. With this technique, the interface of a graphite superlubric contact is characterized with resolution from microscale down to the atomic level. Imaging of the graphite lattice provides direct proof that this superlubric interface consists of two monocrystalline surfaces incommensurate with each other. More importantly, the structure-property relationship for this contact is investigated. Friction measurements combined with fully atomistic molecular dynamics reveal that internal structures [internals steps, pits, and bulges buried underneath the topmost graphene sheet(s)] have negligible contribution to the total friction; in contrast, external defects lead to a high friction. These results help us to better understand the structure of highly oriented pyrolytic graphite and the fundamental mechanisms of structural superlubricity, as well as to guide the design of superlubricity-based devices.

Characterization of a Microscale Superlubric Graphite Interface.

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Origin of Friction in Superlubric Graphite Contacts.

Characterization of a Microscale Superlubric Graphite Interface.