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

Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.

Metal oxide gas sensors: Sensitivity and influencing factors

The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited

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

Course Description This is an advanced course in which we explore the field of Statistical Optics. Topics covered include such subjects as the statistical properties of natural (thermal) and laser light, spatial and temporal coherence, effects of partial coherence on optical imaging instruments, effects on imaging due to randomly inhomogeneous media, and a statistical treatment of the detection of light. Development of this more comprehensive model of the behavior of light draws upon the use of tools traditionally available to the electrical engineer, such as linear system theory and the theory of stochastic processes.

http://ieeexplore.ieee.org/iel5/3/23094/01073023.pdf

Statistical optics

The sensing ability of individual SnO2 nanowires and nanobelts configured as gas sensors was measured before and after functionalization with Pd catalyst particles. In situ deposition of Pd in the same reaction chamber in which the sensing measurements were carried out ensured that the observed modification in behavior was due to the Pd functionalization rather than the variation in properties from one nanowire to another. Changes in the conductance in the early stages of metal deposition (i.e., before metal percolation) indicated that the Pd nanoparticles on the nanowire surface created Schottky barrier-type junctions resulting in the formation of electron depletion regions within the nanowire, constricting the effective conduction channel and reducing the conductance. Pd-functionalized nanostructures exhibited a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion ...

Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles.

The diffraction of electrons through a nanoscale hologram that imprints a certain phase modulation on the electrons’ wavefunction produces a non-spreading electron Airy beam that follows a parabolic trajectory and can reconstruct its original shape after passing an obstacle. Light, as is widely known, travels in straight lines. Yet a few years ago it was shown that specially tailored light beams can follow a curved trajectory, without spreading. Such beams follow a waveform known from quantum mechanics, called the Airy function, a concept originally developed by the astronomer Sir George Biddell Airy in work on the trajectories of light in rainbows. Now, with the demonstration of Airy beams consisting of free electrons, new possibilities for manipulating electrons are in prospect. Airy electron beam arcs were generated by the diffraction of electrons through a nanoscale hologram, which imprints a specific phase modulation on the electrons' wavefunction. These beams can bend in space without any external force, stay localized over distances of up to 100 metres and self-heal after passing an obstacle. Possible applications include use in high-performance electron microscopes and as a basis for a new type of electron interferometer. Within the framework of quantum mechanics, a unique particle wave packet exists1 in the form of the Airy function2,3. Its counterintuitive properties are revealed as it propagates in time or space: the quantum probability wave packet preserves its shape despite dispersion or diffraction and propagates along a parabolic caustic trajectory, even though no force is applied. This does not contradict Newton’s laws of motion, because the wave packet centroid propagates along a straight line. Nearly 30 years later, this wave packet, known as an accelerating Airy beam, was realized4 in the optical domain; later it was generalized to an orthogonal and complete family of beams5 that propagate along parabolic trajectories, as well as to beams that propagate along arbitrary convex trajectories6. Here we report the experimental generation and observation of the Airy beams of free electrons. These electron Airy beams were generated by diffraction of electrons through a nanoscale hologram7,8,9, which imprinted on the electrons’ wavefunction a cubic phase modulation in the transverse plane. The highest-intensity lobes of the generated beams indeed followed parabolic trajectories. We directly observed a non-spreading electron wavefunction that self-heals10, restoring its original shape after passing an obstacle. This holographic generation of electron Airy beams opens up new avenues for steering electronic wave packets like their photonic counterparts, because the wave packets can be imprinted with arbitrary shapes5 or trajectories6.

Generation of electron Airy beams

Using temperature driven sharp metal−insulator phase transition in single crystal VO2 nanowires, the realization of a novel gas sensing concept has been tested. Varying the temperature of the nanowire close to the transition edge, the conductance of the nanowire becomes extremely responsive to the tiny changes in molecular composition, pressure, and temperature of the ambient gas environment. This gas sensing analog of the transition edge sensor radiometry used in astrophysics opens new opportunities in gas sensorics.

Gas Sensor Based on Metal−Insulator Transition in VO2 Nanowire Thermistor

Using temperature driven sharp metal-insulator phase transition in single crystal VO(2) nanowires, the realization of a novel gas sensing concept has been tested. Varying the temperature of the nanowire close to the transition edge, the conductance of the nanowire becomes extremely responsive to the tiny changes in molecular composition, pressure, and temperature of the ambient gas environment. This gas sensing analog of the transition edge sensor radiometry used in astrophysics opens new opportunities in gas sensorics.

Gas Sensor Based on Metal−Insulator Transition in VO 2 Nanowire Thermistor

Tin oxide single nanowires configured as field effect transistors were shown to be operable and tunable alternately as gas sensors or as catalysts under a gaseous atmosphere that simulated realistic ambient conditions. The unusually large surface-to-volume ratio available with nanowires causes adsorption or desorption of donor or acceptor molecules on the nanowire's surface to greatly alter its bulk electron density at relatively small values of the gate voltage. This process can be sensitively monitored as changes in the nanowire's conductivity. The potentially radical change in carrier density can lead to significant changes in the nanowire's sensitivity as a sensor or reciprocally as a catalyst in reactions that involve charge exchange across the nanowire's surface. This leads to the prospect of tuning catalysis or other surface reactions entirely through electronic means.

Yigal Lilach

Papers

Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles.

Generation of electron Airy beams

Gas Sensor Based on Metal−Insulator Transition in VO2 Nanowire Thermistor

Gas Sensor Based on Metal−Insulator Transition in VO 2 Nanowire Thermistor

Electronic control of chemistry and catalysis at the surface of an individual tin oxide nanowire.