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 demonstrate that a monolayer graphene membrane is impermeable to standard gases including helium. By applying a pressure difference across the membrane, we measure both the elastic constants and the mass of a single layer of graphene. This pressurized graphene membrane is the world's thinnest balloon and provides a unique separation barrier between 2 distinct regions that is only one atom thick.

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Impermeable atomic membranes from graphene sheets.

SINCE the invention of the scanning tunnelling microscope1, the value of establishing a physical connection between the macroscopic world and individual nanometre-scale objects has become increasingly evident, both for probing these objects2–4 and for direct manipulation5–7 and fabrication8–10 at the nanometre scale. While good progress has been made in controlling the position of the macroscopic probe of such devices to sub-angstrom accuracy, and in designing sensitive detection schemes, less has been done to improve the probe tip itself4. Ideally the tip should be as precisely defined as the object under investigation, and should maintain its integrity after repeated use not only in high vacuum but also in air and water. The best tips currently used for scanning probe microscopy do sometimes achieve sub-nanometre resolution, but they seldom survive a 'tip crash' with the surface, and it is rarely clear what the atomic configuration of the tip is during imaging. Here we show that carbon nanotubes11,12 might constitute well defined tips for scanning probe microscopy. We have attached individual nanotubes several micrometres in length to the silicon cantilevers of conventional atomic force microscopes. Because of their flexibility, the tips are resistant to damage from tip crashes, while their slenderness permits imaging of sharp recesses in surface topography. We have also been able to exploit the electrical conductivity of nanotubes by using them for scanning tunnelling microscopy.

Nanotubes as nanoprobes in scanning probe microscopy

The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited

A concept for molecular electronics exploiting carbon nanotubes as both molecular device elements and molecular wires for reading and writing information was developed. Each device element is based on a suspended, crossed nanotube geometry that leads to bistable, electrostatically switchable ON/OFF states. The device elements are naturally addressable in large arrays by the carbon nanotube molecular wires making up the devices. These reversible, bistable device elements could be used to construct nonvolatile random access memory and logic function tables at an integration level approaching 10 12 elements per square centimeter and an element operation frequency in excess of 100 gigahertz. The viability of this concept is demonstrated by detailed calculations and by the experimental realization of a reversible, bistable nanotube-based bit.

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Carbon nanotube-based nonvolatile random access memory for molecular computing

Breaking individual chemical bonds via STM-induced excitations

STM-induced H atom desorption from Si(100): isotope effects and site selectivity

We have used laser‐induced fluorescence to measure the energy distributions of Al atoms and AlO molecules produced by excimer laser ablation of Al2O3. Excimer laser fluences close to the threshold for ablation were used to minimize the effects of gas phase collisions. The kinetic energies of both species were high, ∼4 eV for Al and ∼1 eV for AlO, but the AlO rotational and vibrational energies were quite low, corresponding to a temperature of ∼600 K. These results rule out thermal vaporization and provide indirect support for an electronic ablation mechanism.

Laser‐induced fluorescence studies of excimer laser ablation of Al2O3

The high current density obtainable with the scanning tunneling microscope can be used to promote multiple vibrational excitations of an adsorbate via inelastic electron tunneling. Calculations indicate that this effect plays an important role in the transfer of Xe atoms between a sample and tip by the application of current pulses. Directionality of atom transfer and applications to other systems are discussed.

Role of multiple inelastic transitions in atom transfer with the scanning tunneling microscope.

Real space studies of the interaction of the two‐dimensional electron gas provided by metal surface states with localized scatterers are presented. The results involve electron scattering by steps and point defects (adsorbates) at Au(111) and Ag(111) surfaces. These scattering events lead, through interference, to an oscillatory local density of states (LDOS), which is imaged in maps of (dI/dV)/(I/V). Analysis of the LDOS oscillations provides insights into the scattering phenomena involved. We show that the decay of the amplitude of the oscillations as a function of distance from the scatterer can be accounted for by a model that describes the loss of coherence as a result of the wave number (k∥) spread of the states probed by the STM. This model also explains the energy dependence of the amplitude of the oscillations and provides a basis for comparing results from different metal surfaces. Analysis of the properties of the oscillations shows that at low k∥, steps act very much like hard walls isolating ...

Robert E. Walkup

Papers

Breaking individual chemical bonds via STM-induced excitations

STM-induced H atom desorption from Si(100): isotope effects and site selectivity

Laser‐induced fluorescence studies of excimer laser ablation of Al2O3

Role of multiple inelastic transitions in atom transfer with the scanning tunneling microscope.

Real space imaging of electron scattering phenomena at metal surfaces