Molecular Dynamics Computer Simulation of the Dynamics of Supercooled Silica

We present the results of a large scale computer simulation of supercooled silica. We find that at high temperatures the diffusion constants show a non-Arrhenius temperature dependence whereas at low temperature this dependence is also compatible with an Arrhenius law. We demonstrate that at low temperatures the intermediate scattering function shows a two-step relaxation behavior and that it obeys the time temperature superposition principle. We also discuss the wave-vector dependence of the nonergodicity parameter and the time and temperature dependence of the non-Gaussian parameter.Comment: 5 pages, Latex, 6 postscript figure

Path integral Monte Carlo simulations of silicates

We investigate the thermal expansion of crystalline SiO$_2$ in the $\beta$-- cristobalite and the $\beta$-quartz structure with path integral Monte Carlo (PIMC) techniques. This simulation method allows to treat low-temperature quantum effects properly. At temperatures below the Debye temperature, thermal properties obtained with PIMC agree better with experimental results than those obtained with classical Monte Carlo methods.Comment: 27 pages, 10 figures, Phys. Rev. B (in press

Wireless communication with chaos

Peer reviewedPublisher PD

A combined XAS and XRD Study of the High-Pressure Behaviour of GaAsO4 Berlinite

Combined X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) experiments have been carried out on GaAsO4 (berlinite structure) at high pressure and room temperature. XAS measurements indicate four-fold to six-fold coordination changes for both cations. The two local coordination transformations occur at different rates but appear to be coupled. A reversible transition to a high pressure crystalline form occurs around 8 GPa. At a pressure of about 12 GPa, the system mainly consists of octahedral gallium atoms and a mixture of arsenic in four-fold and six-fold coordinations. A second transition to a highly disordered material with both cations in six-fold coordination occurs at higher pressures and is irreversible.Comment: 8 pages, 5 figures, LaTeX2

Perturbation theory of the dynamic inverse spin Hall effect with charge conservation

We present gauge-invariant theory of the dynamic inverse spin Hall effect driven by the spin--orbit interaction in metallic systems. Charge conservation is imposed diagrammatically by including vertex corrections. We show the charge current is induced by an effective electric field that is proportional to the spin current pumped by the magnetization dynamics. The result is consistent with recent experiments.Comment: 16pages, 5figure

Identification of novel clostridium perfringens type E strains that carry an iota toxin plasmid with a functional enterotoxin gene

Clostridium perfringens enterotoxin (CPE) is a major virulence factor for human gastrointestinal diseases, such as food poisoning and antibiotic associated diarrhea. The CPE-encoding gene (cpe) can be chromosomal or plasmid-borne. Recent development of conventional PCR cpe-genotyping assays makes it possible to identify cpe location (chromosomal or plasmid) in type A isolates. Initial studies for developing cpe genotyping assays indicated that all cpe-positive strains isolated from sickened patients were typable by cpe-genotypes, but surveys of C. perfringens environmental strains or strains from feces of healthy people suggested that this assay might not be useful for some cpe-carrying type A isolates. In the current study, a pulsed-field gel electrophoresis Southern blot assay showed that four cpe-genotype untypable isolates carried their cpe gene on a plasmid of ~65 kb. Complete sequence analysis of the ~65 kb variant cpe-carrying plasmid revealed no intact IS elements and a disrupted cytosine methyltransferase (dcm) gene. More importantly, this plasmid contains a conjugative transfer region, a variant cpe gene and variant iota toxin genes. The toxin genes encoded by this plasmid are expressed based upon the results of RT-PCR assays. The ~65 kb plasmid is closely related to the pCPF4969 cpe plasmid of type A isolates. MLST analyses indicated these isolates belong to a unique cluster of C. perfringens. Overall, these isolates carrying a variant functional cpe gene and iota toxin genes represent unique type E strains. © 2011 Miyamoto et al

Crystal-like high frequency phonons in the amorphous phases of solid water

The high frequency dynamics of low- (LDA) and high-density amorphous-ice (HDA) and of cubic ice (I_c) has been measured by inelastic X-ray Scattering (IXS) in the 1-15 nm^{-1} momentum transfer (Q) range. Sharp phonon-like excitations are observed, and the longitudinal acoustic branch is identified up to Q = 8nm^{-1} in LDA and I_c and up to 5nm^{-1} in HDA. The narrow width of these excitations is in sharp contrast with the broad features observed in all amorphous systems studied so far. The "crystal-like" behavior of amorphous ices, therefore, implies a considerable reduction in the number of decay channels available to sound-like excitations which is assimilated to low local disorder.Comment: 4 pages, 3 figure

The electronic structure of amorphous silica: A numerical study

We present a computational study of the electronic properties of amorphous SiO2. The ionic configurations used are the ones generated by an earlier molecular dynamics simulations in which the system was cooled with different cooling rates from the liquid state to a glass, thus giving access to glass-like configurations with different degrees of disorder [Phys. Rev. B 54, 15808 (1996)]. The electronic structure is described by a tight-binding Hamiltonian. We study the influence of the degree of disorder on the density of states, the localization properties, the optical absorption, the nature of defects within the mobility gap, and on the fluctuations of the Madelung potential, where the disorder manifests itself most prominently. The experimentally observed mismatch between a photoconductivity threshold of 9 eV and the onset of the optical absorption around 7 eV is interpreted by the picture of eigenstates localized by potential energy fluctuations in a mobility gap of approximately 9 eV and a density of states that exhibits valence and conduction band tails which are, even in the absence of defects, deeply located within the former band gap.Comment: 21 pages of Latex, 5 eps figure

Shrinking Point Bifurcations of Resonance Tongues for Piecewise-Smooth, Continuous Maps

Resonance tongues are mode-locking regions of parameter space in which stable periodic solutions occur; they commonly occur, for example, near Neimark-Sacker bifurcations. For piecewise-smooth, continuous maps these tongues typically have a distinctive lens-chain (or sausage) shape in two-parameter bifurcation diagrams. We give a symbolic description of a class of "rotational" periodic solutions that display lens-chain structures for a general $N$-dimensional map. We then unfold the codimension-two, shrinking point bifurcation, where the tongues have zero width. A number of codimension-one bifurcation curves emanate from shrinking points and we determine those that form tongue boundaries.Comment: 27 pages, 6 figure