Heteroclinic Ratchets in Networks of Coupled Oscillators

We analyze an example system of four coupled phase oscillators and discover a novel phenomenon that we call a “heteroclinic ratchet”; a particular type of robust heteroclinic network on a torus where connections wind in only one direction. The coupling structure has only one symmetry, but there are a number of invariant subspaces and degenerate bifurcations forced by the coupling structure, and we investigate these. We show that the system can have a robust attracting heteroclinic network that responds to a specific detuning Δ between certain pairs of oscillators by a breaking of phase locking for arbitrary Δ>0 but not for Δ≤0. Similarly, arbitrary small noise results in asymmetric desynchronization of certain pairs of oscillators, where particular oscillators have always larger frequency after the loss of synchronization. We call this heteroclinic network a heteroclinic ratchet because of its resemblance to a mechanical ratchet in terms of its dynamical consequences. We show that the existence of heteroclinic ratchets does not depend on symmetry or number of oscillators but depends on the specific connection structure of the coupled system.     

A micro-curricular analysis of unified mathematics curricula in Turkey

This article aims to give a detailed micro-level curricular analysis of the extent to which the intended mathematics curriculum matches the potentially implemented curriculum using the case of Turkey. The article makes inferences about what it means to have a match or mismatch between these two types of curricula. As a result, it is clear that even though there is a close match between the intended and the potentially implemented mathematics curricula, such a match does not seem to be enough to help students to have a solid understanding of targeted mathematical concepts outlined in the overall Turkish curricular standards.     

Two‐Photon Absorption Cross‐Sections of Reference Dyes: A Critical Examination

The electronic structure and one‐ and two‐photon absorption spectra of four fluorophores, p‐bis(o‐methoxystyryl)benzene (Bis‐MSB), coumarin 307, fluorescein and rhodamine B, commonly used as reference compounds for two‐photon absorption spectra, have been theoretically calculated and compared with available experimental data. The possible reasons for the wide discrepancies in two‐photon absorption cross‐sections reported in the literature are discussed on the basis of the theoretical findings. The role of a solvent environment on the electronic one‐ and two‐photon absorption spectra is also studied. We highlight some necessary precautions that one needs to take when comparing literature results of two‐photon absorption cross‐sections.     

Investigation of bacterial adherence to a non-precious alloy with radiolabeling method

Summary The objective of this study was to investigate the bacterial adherence to a non-precious alloy with radiolabeling method. S. mutans, E. coliand C. albicanswere labeled with ^99mTc by using stannous chloride and their radiolabeling yields were calculated. After the labeling procedure, metal disks (3 mm×10 mm) were treated with microorganisms. The amount of labeled microorganisms adhered on metal surfaces was determined by activity measurements. The labeling yields for S. mutans, E. coliand C. albicanswere 69.95±7.58%, 78.84±0.44% and 79.71±10.17%, respectively. The mean values for adherence for S. mutans, E. coliand C. albicanson metal samples were 7.02±2.18%, 0.96±0.49% and 8.80±8.24%, respectively. The radiolabeling method could be considered as safe and precise for determining the adherence of microorganisms.     

Electrochemical Oxidation of ds‐DNA on Polypyrrole Nanofiber Modified Pencil Graphite Electrode

Preparation of a polypyrrole nanofiber (PPyNF) modified pencil graphite (PG) electrode and its usage in the electrochemical DNA biosensors was investigated. The electrodes (PPyNF/PG1 and PPyNF/PG2) were prepared from a solution containing 0.1 M pyrrole, 0.1 M Na₂CO₃ and 0.1 M LiCIO₄ by using potentiostatic and potentiodynamic methods. PPyNF/PG2 electrodes which were prepared by potentiostatic procedure showed higher responses for the oxidation of ds‐DNA than the PPyNF/PG1 electrodes prepared by potentiodynamic methods. Immobilization of the ds‐DNA on PG and PPyNF/PG surfaces was performed at a constant potential, +0.5 V, for 300 s in 0.5 M ABS (pH 4.8) containing 15 μg mL^?1 ds‐DNA and 20 mM LiCIO₄. The oxidation peak potentials of the ds‐DNA bases, guanine and adenine, were shifted to more cathodic values by using PPyNF/PG electrodes. The oxidation signal of the guanine base of ds‐DNA was decreased in the presence of methylene blue.     

Forced convective heat transfer in porous medium of wire screen meshes

The hydrodynamic and heat transfer characteristics of a porous medium consisting of 20 wire screen meshes are examined theoretically and experimentally. The hydrodynamic experiments are conducted for the range of Reynolds number based on mean velocity and wire diameter from 1.5 to 12. The Ergun's constants and thermal dispersion coefficients are calculated in this range. Nusselt number variation is determined in both thermally developing and fully developed flows by the help of forced convection heat transfer experiments conducted for the uniform heat flux boundary condition. Correlation functions of Nusselt number in the range of fully developed and thermally developing, and of thermal entrance length are obtained from experimental data. Solutions of momentum and energy equations simulating the experimental model are obtained numerically with variable porosity and the anticipated thermal dispersion coefficients. The thermal dispersion coefficients well-adjusted to the experimental data are determined by numerical solution of the energy equation. Received on 22 November 1996     

A localization method for the measurement of fast relaxing13C NMR signals in humans at high magnetic fields

Nuclear magnetic resonance (NMR) signals with shortT ₁ andT ₂, such as the¹³C signal of glycogen, are difficult to localize in three dimensions without major signal loss. A pulse sequence that accomplishes the spatial localization of¹H-decoupled¹³C NMR signals on a whole-body scanner within the Food and Drug Administration guidelines for specific absorption rates was designed. The method uses an optimized three-dimensional outer volume suppression scheme combined with one-dimensional image-selected in vivo spectroscopy and surface coil detection. The localization performance of the sequence was validated at 4 T with double chambered phantoms and¹³C magnetic resonance imaging. Localized¹³C spectra were acquired from human brain and muscle.     

Localized States of a Narrow Single-Walled Carbon Nanotube by Using Su-Schrieffer-Heeger Model Hamiltonian

As a continuation of our previous work [N. Sünel and O. Özsoy: Int. J. Quant. Chem., Vol. 99 (2004)] on deriving the most general matrix representations, we investigate localized states of a carbon sheet and those of a single-walled carbon nanotube and of a nanotorus within the extended Su-Schrieffer-Heeger model.     

Three-dimensional flow and turbulence structure in electrostatic precipitator by stereo PIV

Stereo PIV was employed to study the three-dimensional velocity and turbulence fields in a laboratory model of a negative corona, barbed-wire, smooth-plate electrostatic precipitator. The calculated electric drift velocity of charged seeding particles is subtracted from the measured particle velocity to obtain the gas velocity. Results show how the strength of the secondary flow (in the form of longitudinal rolls) and the level of turbulence (as well as its degree of anisotropy) increase with increasing axial position, increasing current density, and decreasing bulk velocity. Both of these features of the gas flow are crucial to the performance of precipitators.     

Efficiency improvement of quantum well solar cell with the AuGeNi metallization and Si3N4 ARC design

This study demonstrates the power conversion efficiency enhancement on In_0.19Ga_0.81As/GaAs quantum well solar cells (QWSC). The solar cell structure was grown on n-type (100)-oriented GaAs substrate by using solid-source molecular beam epitaxy technique and divided into square pieces. In order to understand whether the eight quantum wells were grown or not, scanning electron microscopy (SEM), and secondary ion mass spectrometry characterizations were done at room temperature. After that, the Si₃N₄ antireflection layers were coated onto both two square pieces of In_0.19Ga_0.81As/GaAs QWSC structure and p-GaAs substrate at different temperatures by the radio frequency magnetron sputtering system. The optical properties of the Si₃N₄ coated and uncoated p-GaAs samples have been evaluated by means of ultraviolet-visible spectrometry measurements at room temperature. According to ultraviolet-visible spectrometry results, the best Si₃N₄ antireflection coated one was obtained at 100 °C substrate temperature. Thus, the In_0.19Ga_0.81As/GaAs QWSC structure with and without Si₃N₄ layer, which was coated at 100 °C substrate temperature, was selected for other measurements and processes. Moreover, the In_0.19Ga_0.81As/GaAs QWSC samples with and without Si₃N₄ antireflection coating were separately fabricated with different metallization materials for obtaining the solar cell electrical output parameters. AuGe and AuGeNi metallization materials were used for the fabrication processes. After fabrication, the electrical output parameters were extracted from the current-voltage measurements at room temperature both in dark and under AM1.5 – 1 Sun illumination. The proposed design which includes the AuGeNi metallization and Si₃N₄ antireflection layer enhanced the power conversion efficiency by 44.40%.