Johannes Roth

Archimedean-like tiling on decagonal quasicrystalline surfaces

Monolayers on crystalline surfaces often form complex structures with physical and chemical properties that differ strongly from those of their bulk phases. Such hetero-epitactic overlayers are currently used in nanotechnology and understanding their growth mechanism is important for the development of new materials and devices. In comparison with crystals, quasicrystalline surfaces exhibit much larger structural and chemical complexity leading, for example, to unusual frictional, catalytical or optical properties. Deposition of thin films on such substrates can lead to structures that may have typical quasicrystalline properties. Recent experiments have indeed showed 5-fold symmetries in the diffraction pattern of metallic layers adsorbed on quasicrystals. Here we report a real-space investigation of the phase behaviour of a colloidal monolayer interacting with a quasicrystalline decagonal substrate created by interfering five laser beams. We find a pseudomorphic phase that shows both crystalline and quasicrystalline structural properties. It can be described by an archimedean-like tiling consisting of alternating rows of square and triangular tiles. The calculated diffraction pattern of this phase is in agreement with recent observations of copper adsorbed on icosahedral Al70Pd21Mn9 surfaces. In addition to establishing a link between archimedean tilings and quasicrystals, our experiments allow us to investigate in real space how single-element monolayers can form commensurate structures on quasicrystalline surfaces.

A MOLECULAR DYNAMICS RUN WITH 5 180 116 000 PARTICLES

We report on the development of IMD, a scalable program for classical molecular dynamics simulations on massively parallel supercomputers. New features like online-visualization and metacomputing are described.

Proliferation of anomalous symmetries in colloidal monolayers subjected to quasiperiodic light fields

Quasicrystals provide a fascinating class of materials with intriguing properties. Despite a strong potential for numerous technical applications, the conditions under which quasicrystals form are still poorly understood. Currently, it is not clear why most quasicrystals hold 5- or 10-fold symmetry but no single example with 7- or 9-fold symmetry has ever been observed. Here we report on geometrical constraints which impede the formation of quasicrystals with certain symmetries in a colloidal model system. Experimentally, colloidal quasicrystals are created by subjecting micron-sized particles to two-dimensional quasiperiodic potential landscapes created by n = 5 or seven laser beams. Our results clearly demonstrate that quasicrystalline order is much easier established for n = 5 compared to n = 7. With increasing laser intensity we observe that the colloids first adopt quasiperiodic order at local areas which then laterally grow until an extended quasicrystalline layer forms. As nucleation sites where quasiperiodicity originates, we identify highly symmetric motifs in the laser pattern. We find that their density strongly varies with n and surprisingly is smallest exactly for those quasicrystalline symmetries which have never been observed in atomic systems. Since such high-symmetry motifs also exist in atomic quasicrystals where they act as preferential adsorption sites, this suggests that it is indeed the deficiency of such motifs which accounts for the absence of materials with e.g., 7-fold symmetry.

Simulation of shear stress in icosahedral quasicrystals

By a numerical relaxation scheme we have simulated the plastic deformation of a quasicrystal. Shear stress was applied to a quasiperiodic icosahedral atomic configuration of Lennard-Jones particles, and nucleation of dislocation dipoles was observed which move in a slip plane. We also found climb motion and dissociation of a perfect dislocation into a pair of partials. A plane of phasonic defects is left in the wake of the propagating dislocations.

Phason elastic constants of a binary tiling quasicrystal

For a two-dimensional binary tiling model quasicrystal, the full set of (zero temperature) elastic constants is determined. It is found that the elastic energy is a perfect quadratic form in the phonon and phason strains. One of the phason elastic constants turns out to be negative, implying that the quasicrystal is only metastable at zero temperature.

Direct Wolf summation of a polarizable force field for silica

We extend the Wolf direct, pairwise r(-1) summation method with spherical truncation to dipolar interactions in silica. The Tangney-Scandolo interatomic force field for silica takes regard of polarizable oxygen atoms whose dipole moments are determined by iteration to a self-consistent solution. With Wolf summation, the computational effort scales linearly in the system size and can easily be distributed among many processors, thus making large-scale simulations of dipoles possible. The details of the implementation are explained. The approach is validated by estimations of the error term and simulations of microstructural and thermodynamic properties of silica.

Brownian particles in random and quasicrystalline potentials: how they approach the equilibrium.

Abstract.We study the Brownian motion of an ensemble of single colloidal particles in a random square and a quasicrystalline potential when they start from non-equlibrium. For both potentials, Brownian dynamics simulations reveal a widespread subdiffusive regime before the diffusive long-time limit is reached in thermal equilibrium. We develop a random trap model based on a distribution for the depths of trapping sites that reproduces the results of the simulations in detail. Especially, it gives analytic formulas for the long-time diffusion constant and the relaxation time into the diffusive regime. Aside from detailed differences, our work demonstrates that quasicrystalline potentials can be used to mimic aspects of random potentials.

Classical interaction potentials for diverse materials from ab initio data : a review of potfit

Force matching is an established technique to generate effective potentials for molecular dynamics simulations from first-principles data. This method has been implemented in the open source code potfit. Here, we present a review of the method and describe the main features of the code. Particular emphasis is placed on the features added since the initial release: interactions represented by analytical functions, differential evolution as optimization method, and a greatly extended set of interaction models. Beyond the initially present pair and embedded-atom method potentials, potfit can now also optimize angular dependent potentials, charge and dipolar interactions, and electron-temperature-dependent potentials. We demonstrate the functionality of these interaction models using three example systems: phonons in type I clathrates, fracture of {\alpha}-alumina, and laser-irradiated silicon.

Motion of dislocations in two-dimensional decagonal quasicrystals

Abstract To analyse the motion of dislocations in quasicrystals a numerical simulation method is developed by which the shear stress is applied to a two-dimensional atomic configuration of Lennard-Jones particles. The method is based on a refined gradient relaxation procedure. In this paper first results of simulations on a binary tiling are presented. Creation of dislocations is observed. A mechanism for the motion of dislocations is proposed which is connected with grid directions of the underlying Penrose pattern.

Statistical investigations on nitrogen-vacancy center creation

Quantum information technologies require networks of interacting defect bits. Color centers, especially the nitrogen vacancy (NV−) center in diamond, represent one promising avenue, toward the realisation of such devices. The most successful technique for creating NV− in diamond is ion implantation followed by annealing. Previous experiments have shown that shallow nitrogen implantation (<10 keV) results in NV− centers with a yield of 0.01%–0.1%. We investigate the influence of channeling effects during shallow implantation and statistical diffusion of vacancies using molecular dynamics and Monte Carlo simulation techniques. Energy barriers for the diffusion process were calculated using density functional theory. Our simulations show that 25% of the implanted nitrogens form a NV center, which is in good agreement with our experimental findings.