There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to one-dimensional materials, it is relatively easy to fabricate complex structures from them. The most widely studied two-dimensional material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained silicon films or requires high voltages. Although single layers of MoS(2) have a large intrinsic bandgap of 1.8 eV (ref. 16), previously reported mobilities in the 0.5-3 cm(2) V(-1) s(-1) range are too low for practical devices. Here, we use a halfnium oxide gate dielectric to demonstrate a room-temperature single-layer MoS(2) mobility of at least 200 cm(2) V(-1) s(-1), similar to that of graphene nanoribbons, and demonstrate transistors with room-temperature current on/off ratios of 1 × 10(8) and ultralow standby power dissipation. Because monolayer MoS(2) has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS(2) could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.

/pdf/single-layer-mos2-transistors-6go64w4t3p.pdf

Single-layer MoS2 transistors

Ultralong beltlike (or ribbonlike) nanostructures (so-called nanobelts) were successfully synthesized for semiconducting oxides of zinc, tin, indium, cadmium, and gallium by simply evaporating the desired commercial metal oxide powders at high temperatures. The as-synthesized oxide nanobelts are pure, structurally uniform, and single crystalline, and most of them are free from defects and dislocations. They have a rectanglelike cross section with typical widths of 30 to 300 nanometers, width-to-thickness ratios of 5 to 10, and lengths of up to a few millimeters. The beltlike morphology appears to be a distinctive and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures. The nanobelts could be an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts.

https://science.sciencemag.org/content/sci/291/5510/1947.full.pdf

Nanobelts of Semiconducting Oxides

This review summarizes recent developments in the theory of spin glasses, as well as pertinent experimental data. The most characteristic properties of spin glass systems are described, and related phenomena in other glassy systems (dielectric and orientational glasses) are mentioned. The Edwards-Anderson model of spin glasses and its treatment within the replica method and mean-field theory are outlined, and concepts such as "frustration," "broken replica symmetry," "broken ergodicity," etc., are discussed. The dynamic approach to describing the spin glass transition is emphasized. Monte Carlo simulations of spin glasses and the insight gained by them are described. Other topics discussed include site-disorder models, phenomenological theories for the frozen phase and its excitations, phase diagrams in which spin glass order and ferromagnetism or antiferromagnetism compete, the Ne\'el model of superparamagnetism and related approaches, and possible connections between spin glasses and other topics in the theory of disordered condensed-matter systems.

Spin glasses: Experimental facts, theoretical concepts, and open questions

Atomic-scale three-dimensional kinetic Monte Carlo simulation of organometallic vapor-phase epitaxy of ordered films

Material science and technology annual external review panel.

Morphology evolution of asperity contacts in microelectromechanical systems switches is examined in the framework of the phase field method. Surface mass diffusion, driven by capillarity, rapidly increases the asperity contact radius at early times, decreases it slowly at later times, and, possibly, pinches off the contact due to a Rayleigh instability. Electromigration accelerates this process but retards or prevents the pinch off at late times. The evolution occurs faster when the asperity size and/or density are smaller. Approximate analytical results for the evolution kinetics are provided.

Capillarity and electromigration effects on asperity contact evolution in microelectromechanical systems switches

The structure and properties of grain boundaries in the intermetallic alloy Ni3A1 have received considerable attention in the past several years due to their interesting mechanical properties. The aim in many of these studies [1-13] was to explain the increase in ductility that occurs when polycrystalline Ni3AI is doped with boron. Experimental observations [1-7] have shown that there is a strong correlation between nickel enrichment at grain boundaries and the ductilization of boron-doped polycrystalline Ni3AI. This grain boundary nickel enrichment has only been observed in Nirich Ni3-xAll+x (i.e., x<0). Some experimental studies have shown that grain boundary nickel enrichment also occurs in Ni-rich, boron-free polycrystalline Ni3-xAll+x [3,7]. Two models have been proposed [14,15] to explain the observed ductilization of polycrstalline Ni3AI. In the first model [e.g. 14], an increase in grain boundary cohesive energy has been attributed to nickel enrichment at the grain boundaries, while in the other model [15], it is suggested that the transmittal of slip across the grain boundary becomes easier in chemically disordered grain boundaries, where the disorder is induced by the nickel enrichment at grain boundaries. While these two purported effects of nickel segregation are not necessarily exclusive, experimental studies have been unable to conclusively validate either. There have been several atomistic simulation studies [8-13] that have been carded out to explore this subject. Most of these simulation studies [8-11] have been done at zero temperature and without direct consideration of the important segregation effects, These studies have shown that the cohesive energy for grain boundaries rich in nickel is lower than that for the same boundaries with aluminum-rich or stoichiometric compositions. One of these same studies [9] has shown that the grain boundary cohesive energy can be reduced even further by placing boron atoms in interstitial sites in the boundary, These results seem to give some credibility to the first model, since these changes in the cohesive energy occur for boundaries that are well ordered, One should note, however, that these simulations are carried out with stoichiometric bulk compositions, not at the nickel rich hulk compositions, where nickel enrichment of the grain boundaries and ductilization are experimentally observed. Due to the nature of the zero-temperature, composition simulation method used in these studies, it was not possible to study the variation in grain boundary cohesive energy that occurs with segregation and equilibrium disordering. Monte Carlo simulation studies [12] at I000 K have shown that grain boundaries become enriched with nickel and aluminum for Ni-rich and Al-rich bulk composition, respectively. In this letter, we report the results of our investigations of segregation effects on grain boundary cohesive energy in ordered, boron-fzee Ni3-xAll+x within ~e framework of ~ atomistic simulation procedure. Segregation to two high angle (001) twist grain boundaries ~ (36.9) and 5".13 (22.6) and to (001) free surfaces are studied using a newly developed, free energy minimization method [16-18]. These simulations are carded out for a range of alloy compositions corresponding to nickel concentrations in the bulk of 73.5, 75.0, and 76.6 atomic percent in the temperature range 300-900 K. This model is also used to investigate the degree of chemical disordering that occurs at the grain boundaries. The two existing models f~ the role of Ni segregation on the ductilization of Ni3AI are discussed in light of the present simulation results. Method The local harmonic CL,I-I) model has been applied with considerable success to both perfect and defected single component solids [16,17]. In this model, the classical vibrational contribution to the free energy for a single component system is given by

The effects of segregation on grain boundary cohesive energies in Ni3−xAl1+x

We develop an approach to determining Lennard-Jones embedded-atom method potentials for alloys and use these to determine the solid-liquid phase diagrams for binary metallic alloys using Kofke's Gibbs-Duhem integration technique combined with semigrand canonical Monte Carlo simulations. We demonstrate that it is possible to produce a wide range of experimentally observed binary phase diagrams (with no intermetallic phases) by reference to the atomic sizes and cohesive energies of the two elemental materials. In some cases, it is useful to employ a single adjustable parameter to adjust the phase diagram (we provided a good choice for this free parameter). Next, we perform a systematic investigation of the effect of relative atomic sizes and cohesive energies of the elements on the binary phase diagrams. We then show that this approach leads to good agreement with several experimental binary phase diagrams. The main benefit of this approach is not the accurate reproduction of experimental phase diagrams, but rather to provide a method by which material properties can be continuously changed in simulation studies. This is one of the keys to the use of atomistic simulations to understand mechanisms and properties in a manner not available to experiment.

David J. Srolovitz

Papers

Atomic-scale three-dimensional kinetic Monte Carlo simulation of organometallic vapor-phase epitaxy of ordered films

Material science and technology annual external review panel.

Capillarity and electromigration effects on asperity contact evolution in microelectromechanical systems switches

The effects of segregation on grain boundary cohesive energies in Ni3−xAl1+x

Solid-Liquid Phase Diagrams for Binary Metallic Alloys: Adjustable Interatomic Potentials