Isotropic properties of the photonic band gap in quasicrystals with low-index contrast

We report on the formation and development of the photonic band gap in two-dimensional 8-, 10- and 12-fold symmetry quasicrystalline lattices of low index contrast. Finite size structures made of dielectric cylindrical rods were studied and measured in the microwave region, and their properties compared with a conventional hexagonal crystal. Band gap characteristics were investigated by changing the direction of propagation of the incident beam inside the crystal. Various angles of incidence from 0 \degree to 30\degree were used in order to investigate the isotropic nature of the band gap. The arbitrarily high rotational symmetry of aperiodically ordered structures could be practically exploited to manufacture isotropic band gap materials, which are perfectly suitable for hosting waveguides or cavities.Comment: 16 pages, 7 figures, submitted to PR

Covariant nucleon wave function with S, D, and P-state components

Expressions for the nucleon wave functions in the covariant spectator theory (CST) are derived. The nucleon is described as a system with a off-mass-shell constituent quark, free to interact with an external probe, and two spectator constituent quarks on their mass shell. Integrating over the internal momentum of the on-mass-shell quark pair allows us to derive an effective nucleon wave function that can be written only in terms of the quark and diquark (quark-pair) variables. The derived nucleon wave function includes contributions from S, P and D-waves.Comment: 13 pages and 1 figur

Wind tunnel performance results of an aeroelastically scaled 2/9 model of the PTA flight test prop-fan

High speed wind tunnel aerodynamic performance tests of the SR-7A advanced prop-fan have been completed in support of the Prop-Fan Test Assessment (PTA) flight test program. The test showed that the SR-7A model performed aerodynamically very well. At the cruise design condition, the SR-7A prop fan had a high measured net efficiency of 79.3 percent

Donor-strand exchange in chaperone-assisted pilus assembly revealed in atomic detail by molecular dynamics

Adhesive multi-subunit fibres are assembled on the surface of many pathogenic bacteria via the chaperone-usher pathway. In the periplasm, a chaperone donates a β-strand to a pilus subunit to complement its incomplete immunoglobulin-like fold. At the outer membrane, this is replaced with a β-strand formed from the N-terminal extension (Nte) of an incoming pilus subunit by a donorstrand exchange (DSE) mechanism. This reaction has previously been shown to proceed via a concerted mechanism, in which the Nte interacts with the chaperone:subunit complex before the chaperone has been displaced, forming a ternary intermediate. Thereafter, the pilus and chaperone β-strands have been postulated to undergo a strand swap by a ‘zip-in-zip-out’ mechanism, whereby the chaperone strand zips out, residue by residue, as the Nte simultaneously zips in. Here, molecular dynamics simulations have been used to probe the DSE mechanism during formation of the Salmonella enterica Saf pilus at an atomic level, allowing the direct investigation of the zip-inzip- out hypothesis. The simulations provide an explanation of how the incoming Nte is able to dock and initiate DSE due to inherent dynamic fluctuations within the chaperone:subunit complex. The chaperone donor-strand is shown to unbind from the pilus subunit residue by residue, in direct support of the zip-in-zip-out hypothesis. In addition, an interaction of a residue towards the Nterminus of the Nte with a specific binding pocket (P*) on the adjacent pilus subunit is shown to stabilise the DSE product against unbinding, which also proceeds by a zippering mechanism. Together, the study provides an in-depth picture of DSE, including the first insights into the molecular events occurring during the zip-in-zip-out mechanism

Observations of the J = 2→1 transitions of ¹²C¹⁶O and ¹²C¹⁸O towards galactic H II regions

Observations are reported of the J = 2→1 transitions of CO and 12C18O at 230 and 219 GHz respectively from a number of galactic sources. A map of the central 1/2° × 1/2° of the Orion A molecular cloud is presented. The spectra are interpreted to derive molecular densities and abundance ratios in the molecular clouds observed

Material dependence of Casimir forces: gradient expansion beyond proximity

A widely used method for estimating Casimir interactions [H. B. G. Casimir, Proc. K. Ned. Akad. Wet. 51, 793 (1948)] between gently curved material surfaces at short distances is the proximity force approximation (PFA). While this approximation is asymptotically exact at vanishing separations, quantifying corrections to PFA has been notoriously difficult. Here we use a derivative expansion to compute the leading curvature correction to PFA for metals (gold) and insulators (SiO$_2$) at room temperature. We derive an explicit expression for the amplitude $\hat\theta_1$ of the PFA correction to the force gradient for axially symmetric surfaces. In the non-retarded limit, the corrections to the Casimir free energy are found to scale logarithmically with distance. For gold, $\hat\theta_1$ has an unusually large temperature dependence.Comment: 4 pages, 2 figure

Competition of mixing and segregation in rotating cylinders

Using discrete element methods, we study numerically the dynamics of the size segregation process of binary particle mixtures in three-dimensional rotating drums, operated in the continuous flow regime. Particle rotations are included and we focus on different volume filling fractions of the drum to study the interplay between the competing phenomena of mixing and segregation. It is found that segregation is best for a more than half-filled drum due to the non-zero width of the fluidized layer. For different particle size ratios, it is found that radial segregation occurs for any arbitrary small particle size difference and the final amount of segregation shows a linear dependence on the size ratio of the two particle species. To quantify the interplay between segregation and mixing, we investigate the dynamics of the center of mass positions for each particle component. Starting with initially separated particle groups we find that no mixing of the component is necessary in order to obtain a radially segregated core.Comment: 9 pages, 12 figures (EPIC/EEPIC & EPS, macros included), submitted to Physics of Fluid

A Solution of the Maxwell-Dirac Equations in 3+1 Dimensions

We investigate a class of localized, stationary, particular numerical solutions to the Maxwell-Dirac system of classical nonlinear field equations. The solutions are discrete energy eigenstates bound predominantly by the self-produced electric field.Comment: 12 pages, revtex, 2 figure