Experimental characterization of vibrated granular rings

We report an experimental study of the statistical properties of vibrated granular rings. In this system, a linked rod and bead metallic chain in the form of a ring is collisionally excited by a vertically oscillating plate. The dynamics are driven primarily by inelastic bead-plate collisions and are simultaneously constrained by the rings' physical connectedness. By imaging many instances of the ring configurations, we measure the ensemble averages and distributions of several physical characteristics on the scale of individual beads and composite ring. We study local properties such as inter-bead separation and inter-bonds angles, and global properties such as the radius of gyration and center-of-mass motion. We characterize scaling with respect to the size of the chain.     

Cubic InN on -plane sapphire

InN has been grown directly on r-plane sapphire substrates by plasma-induced molecular beam epitaxy. X-ray diffraction and transmission electron microscopy investigations have shown that the InN layers consist of a predominant zinc-blende (cubic) structure along with a fraction of the wurtzite (hexagonal) phase whose content increases with proceeding growth. The InN layer is defect rich with a high number of stacking faults and twins. As a consequence a very high residual doping of was estimated. The lattice constant for the zinc-blende phase of InN was found to be a=4.986 Å. The optical investigations were strongly affected by a high number of defects, but nevertheless indicated an absorption edge below 0.6 eV. For this unusual growth of the metastable cubic phase on a noncubic substrate an epitaxial relationship was proposed, where the metastable zinc-blende phase grows directly on the r-plane of sapphire.     

Interpretation of sputter depth profiles by mixing simulations

The interpretation of sputter depth profiles can be simplified by use of computer simulations. Distortions caused by mixing effects and distortions caused by the information depth of the analytical method have to be distinguished. Atomic mixing and the information depth distort the depth profile simultaneously. Therefore, it is necessary to take into consideration a superposition of both distortion effects. The sputtering of a GaAs/A1As multilayer has been calculated on a personal computer with the binary collision approximation code T-DYN by Biersack and with an own layer model. A new computer code LAMBDA has been used for the investigation of the influence of the AES information depth in addition to atomic mixing and preferential sputtering. A comparison of the calculated and the measured depth profile explains the observed effects. Therefore conclusions can be drawn about the original elemental distribution in the sample from the measured depth profile.     

Pair dispersion and doubling time statistics in two-dimensional turbulence

Experimental measurements of pair separation statistics in two-dimensional turbulence are reported for an electromagnetically forced stratified-layer system with simultaneous ranges of direct-enstrophy and inverse-energy transfer separated by a well-defined spatial injection scale. Data for pair separation as a function of time are analyzed to determine the dependence of separation statistics in both regimes. Using doubling-time statistics, we show how the measured scalings of the mean quantities are consistent with exponential behavior in the enstrophy range and power-law behavior in the inverse-energy range. Exponential scaling of the doubling-time probability distribution function agrees well with theoretical predictions. Finite size effects are shown to play an important role in the interpretation of the data.     

Two scenarios for avalanche dynamics in inclined granular layers

We report experimental measurements of avalanche behavior of thin granular layers on an inclined plane for low volume flow rate. The dynamical properties of avalanches were quantitatively and qualitatively different for smooth glass beads compared to irregular granular materials such as sand. Two scenarios for granular avalanches on an incline are identified, and a theoretical explanation for these different scenarios is developed based on a depth-averaged approach that takes into account the differing rheologies of the granular materials.     

Optical fiber sensors using hollow glass spheres and CCD spectrometer interrogator

Hollow glass micro-spheres, firstly used to make fiber optic sensors for high hydrostatic pressure, have been interrogated using a high-resolution CCD-based spectrometer, to give far better precision than conventional spectrometric read out. It is found that these simple, low-cost micro-sensors have excellent sensitivity to both static and dynamic pressure, and have the advantage of being hermetically sealed. Many other application areas are foreseen for these low-cost sensors.     

Thermally stable optical fibre Bragg grating wavelength reference

The inherent temperature dependence of optical fibre Bragg gratings is caused mainly by the positive thermo-optic effect of the fibre core material, and it results in an increase of Bragg wavelength with temperature. Special mounting techniques may be designed in order to compensate this thermo-optic effect by counter-acting effects: decreasing mechanical strain, as well as a decreasing effective refractive index for the guided light wave by evanescent field interaction with a liquid of negative thermo-optic effect. Because of the non-linear interaction characteristics, exact temperature compensation is obtained for a certain temperature, which depends on the design parameters. Such stabilised fibre Bragg gratings find application as wavelength references in Bragg grating sensor networks.