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

We have calculated the crystal-melt interfacial stiffnesses using simulations with three different interatomic potentials for Al, and from these derived the anisotropic crystal-melt interfacial free energies. We find that there is a strong dependence of the results on the potential, and that this dependence cannot be explained by the usual Turnbull relation between the interfacial free energy and the latent heat. The potentials which produce liquid structures in closer agreement with experiments give free energies in good agreement with nucleation data.

A comparison of crystal-melt interfacial free energies using different Al potentials

We report observations of two distinct types of phase‐separated microstructures in co‐deposited Al‐Ge films. In the initial stages of growth, lateral phase separation is observed, with a temperature dependence consistent with surface diffusion. As the film grows thicker, the Ge‐rich phase becomes increasingly buried, and a transverse phase‐separated microstructure results, consisting of an Al‐rich layer covering a Ge‐rich layer. This observation is explained in terms of the competition between surface and interfacial free energies. We discuss the kinetic aspects of the phase separation process, and the resulting behavior in the thick‐film limit.

/pdf/transition-from-lateral-to-transverse-phase-separation-24uzalk8nf.pdf

Transition from lateral to transverse phase separation during film co‐deposition

Atomistic simulations of segregation to the (100) free surface in Ni-Cu alloys have been performed for a wide range of temperatures and compositions within the solid-solution region of the alloy phase diagram. In addition to the surface-segregation profile, surface structures, free energies, enthalpies, and entropies were determined. These simulations were performed within the framework of the free-energy simulation method, in which an approximate free-energy functional is minimized with respect to atomic coordinates and atomic-site occupation. For all alloy bulk compositions (0.05\ensuremath{\le}C\ensuremath{\le}0.95) and temperatures (400\ensuremath{\le}T\ensuremath{\le}1000 K) examined, Cu segregates strongly to the surface and Ni segregates to the planes just below the surface. The width of the segregation profile is limited to approximately three atomic planes. The resultant segregation profiles are shown to be in good agreement with an empirical segregation theory. A simpler method for determining the equilibrium segregation in terms of the properties of unrelaxed pure Ni and pure Cu surface data is proposed and shown to be more accurate than the existing empirical segregation analyses. The surface thermodynamic properties depend sensitively on the magnitude of the surface segregation. The enthalpy, entropy of segregation, and the change in the interlayer spacing adjacent to the surface are shown to vary linearly with the magnitude of the surface segregation.

/pdf/100-surface-segregation-in-cu-ni-alloys-28hm4kooyy.pdf

(100) surface segregation in Cu-Ni alloys

The development of order for the Q-state Potts model for 2lQ\ensuremath{\le}48 in the presence of static, random impurities is studied following a quench from high temperature (T\ensuremath{\gg}${T}_{c}$) to very low temperatures. We find that the domain growth becomes pinned for quenches to T=0 and the average pinned domain area ${A}_{f}$ varies inversely with the concentration c of impurities. This value of the proportionality factor between ${A}_{f}$ and ${c}^{\mathrm{\ensuremath{-}}1}$ can be understood in terms of a simple topological analysis for large Q.

Impurity effects on domain-growth kinetics. II. Potts model

Molecular dynamics simulations are performed to examine the effects of ion beams in selecting MgO island orientations during ion beam assisted deposition. Sputter yields are determined as a function of ion beam orientation for MgO islands of different sizes. While channeling does occur in small islands, the difference in sputter yields between channeling and nonchanneling orientations is much smaller in small islands than in larger islands or a continuous film. The ion beam is capable of disrupting the entire island structure at small island sizes, while channeling dependent sputtering occurs at larger island size. Since less sputtering occurs for channeling orientations, the ion beam produces an orientation-dependent island growth rate. This leads to island orientation-dependent nucleation rates. A new analytical model parameterized using the molecular dynamics sputter yield data is developed which predicts how growth conditions affect these island-orientation nucleation rates. Based upon these simulations and model results, we suggest a strategy to efficiently grow thin, highly textured MgO films.

David J. Srolovitz

Papers

A comparison of crystal-melt interfacial free energies using different Al potentials

Transition from lateral to transverse phase separation during film co‐deposition

(100) surface segregation in Cu-Ni alloys

Impurity effects on domain-growth kinetics. II. Potts model

Effects of ion beams on the early stages of MgO growth