323 research outputs found
Why does wurtzite form in nanowires of III-V zinc-blende semiconductors?
We develop a nucleation-based model to explain the formation of the wurtzite
(WZ) crystalline phase during the vapor-liquid-solid growth of free-standing
nanowires of zinc-blende (ZB) semiconductors. We first show that, in nanowires,
nucleation generally occurs at the outer edge of the solid/liquid interface
(the triple phase line) rather than elsewhere at the solid/liquid interface. In
the present case, this entails major differences between ZB and WZ nuclei.
Depending on the pertinent interface energies, WZ nucleation is favored at high
liquid supersaturation. This explains our systematic observation of ZB during
the early stages of nanowire growth.Comment: 4 pages with 4 figures Submitted to Physical Review Letter
Giant spin-dependent photo-conductivity in GaAsN dilute nitride semiconductor
A theoretical and experimental study of the spin-dependent photoconductivity
in dilute Nitride GaAsN is presented. The non linear transport model we develop
here is based on the rate equations for electrons, holes, deep paramagnetic and
non paramagnetic centers both under CW and pulsed optical excitation. Emphasis
is given to the effect of the competition between paramagnetic centers and non
paramagnetic centers which allows us to reproduce the measured characteristics
of the spin-dependent recombination power dependence. Particular attention is
paid to the role of an external magnetic field in Voigt geometry. The
photoconductivity exhibits a Hanle-type curve whereas the spin polarization of
electrons shows two superimposed Lorentzian curves with different widths,
respectively related to the recombination of free and trapped electrons. The
model is capable of reproducing qualitatively and quantitatively the most
important features of photoluminescence and photocurrent experiments and is
helpful in providing insight on the various mechanisms involved in the electron
spin polarization and filtering in GaAsN semiconductors.Comment: 10 pages, 5 figure
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Flow measurement using micro-PIV and related temperature distributions within evaporating sessile drops of self-rewetting mixtures of 1-pentanol and water
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Recently interest has arisen in the use of so-called self-rewetting mixtures for micro-scale heat
transfer systems. Such fluids, in which the surface tension can increase with increasing temperature, are
expected to offer superior evaporative cooling performance by extending the region of operation before dryout
of the heated surface sets in. Whilst improved performance has been shown in some practical situations
using these fluids, it is not entirely clear as to the mechanism of such improvements.
We have studied the flow within evaporating sessile drops of 1-pentanol-water mixtures using micro-PIV
and have observed three stages in the evaporation process. During the first stage there appears to be a single
toroidal vortex with flow inwards along the base of the drop. The vortex only occupies the central region of
the drop and appears to pulsate, reducing in size during evaporation. This is followed by a second transition
stage to a third stage in which the flow is directed radially outward, as observed by us for pure water droplet
evaporation and in the latter stages of ethanol/water drop evaporation. Temperature measurements, using IR
thermography suggest that the initial stage of evaporation may be controlled by thermal Marangoni effects as
opposed to the concentration driven Marangoni flows postulated for ethanol-water mixtures
Room temperature Giant Spin-dependent Photoconductivity in dilute nitride semiconductors
By combining optical spin injection techniques with transport spectroscopy
tools, we demonstrate a spin-photodetector allowing for the electrical
measurement and active filtering of conduction band electron spin at room
temperature in a non-magnetic GaAsN semiconductor structure. By switching the
polarization of the incident light from linear to circular, we observe a Giant
Spin-dependent Photoconductivity (GSP) reaching up to 40 % without the need of
an external magnetic field. We show that the GSP is due to a very efficient
spin filtering effect of conduction band electrons on Nitrogen-induced Ga
self-interstitial deep paramagnetic centers.Comment: 4 pages, 3 figure
The mathematical analysis of a syntrophic relationship between two microbial species in a chemostat
A mathematical model involving a syntrophic relationship between two populations of bacteria in a continuous culture is proposed. A detailed qualitative analysis is carried out as well as the analysis of the local and global stability of the equilibria. We demonstrate, under general assumptions of monotonicity which are relevant from an applied point of view, the asymptotic stability of the positive equilibrium point which corresponds to the coexistence of the two bacteria. A syntrophic relationship in the anaerobic digestion process is proposed as a real candidate for this model
Bifurcation and stability analysis of a two step model for monitoring anaerobic digestion processes
This paper deals with the equilibria and stability analysis of the two step anaerobic model initially proposed by [12] to describe the dynamical behavior of an anaerobic fixed-bed wastewater treatment process. In a first part, the model is analyzed: its equilibria and their stability are established considering qualitative properties of the kinetics. In a second part, it is shown that the overloading tolerance (denoted herein OT), a parameter proposed in [9] to monitor anaerobic processes on-line, may not be suitable for monitoring the system and even causes serious problems under certain functioning conditions. Based on the analysis results established in the first part, a modified OT is proposed and evaluated in simulation
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Interfacial Heat Transfer Measurements During Flow Boiling in a PDMS Rectangular Channel
Analysis of serum lipid profile in adult female smokers in Erbil city, Kurdistan region of Iraq
Background: There are few studies demonstrated the association between smoking and lipid profile in female adult smokers. Materials and Methods: This study conducted to determine and compare the serum lipid profile of female adult smokers with non-female smokers, known as controls. In 180 female subjects, the level of serum lipid profile measured. Results: Of these, 110 were smokers and 70 non-smokers (control) aged between 25 and 50 years. The study involved only smokers who had smoked for more than 5 years. Our result revealed that mean serum of total cholesterol (275.2 ± 32.6 mg/dl), triacylglycerol (188.4 ± 56.42 mg/dl), very low density lipoprotein (36.6 ± 14.2 mg/dl), low density lipoprotein (133.21 ± 9.81 mg/dl) were significantly higher in female smokers as compared to non-female smokers with mean of serum total cholesterol (172.3 ± 18.6 mg/dl), very low density lipoprotein (21.8 ± 9.6 mg/dl), triglyceride (108 ± 8.84 mg/dl), low density lipoprotein (94.54 ± 8.5 mg/dl). However, the mean of serum value for high density lipoprotein cholesterol in chronic female smokers was lower (44.6 ± 4.6 mg/dl) than in non-female smokers (55.3 ± 8.2 mg/dl). Conclusion: This study indicated that smoking cigarettes in female causeâs dyslipidemia, resulting in increased risk of cardiovascular disease among smokers
Ultrafast observation of lattice dynamics in laser-irradiated gold foils
We have observed the lattice expansion before the onset of compression in an optical-laser-driven target, using diffraction of femtosecond X-ray beams generated by the SPring-8 Angstrom Compact Free-electron Laser. The change in diffraction angle provides a direct measure of the lattice spacing, allowing the density to be calculated with a precision of ±1%. From the known equation of state relations, this allows an estimation of the temperature responsible for the expansion as <1000 K. The subsequent ablation-driven compression was observed with a clear rise in density at later times. This demonstrates the feasibility of studying the dynamics of preheating and shock formation with unprecedented detail.N. J. Hartley, N. Ozaki, T. Matsuoka, B. Albertazzi, A. Faenov, Y. Fujimoto, H. Habara, M. Harmand, Y. Inubushi, T. Katayama, M. Koenig, A. Krygier, P. Mabey, Y. Matsumura, S. Matsuyama, E. E. McBride, K. Miyanishi, G. Morard, T. Okuchi, T. Pikuz, O. Sakata, Y. Sano, T. Sato, T. Sekine, Y. Seto, K. Takahashi, K. A. Tanaka, Y. Tange, T. Togashi, Y. Umeda, T. Vinci, M. Yabashi, T. Yabuuchi, K. Yamauchi, and R. Kodama , "Ultrafast observation of lattice dynamics in laser-irradiated gold foils", Appl. Phys. Lett. 110, 071905 (2017) https://doi.org/10.1063/1.4976541
Indirect monitoring shot-to-shot shock waves strength reproducibility during pump-probe experiments
We present an indirect method of estimating the strength of a shock wave, allowing on line monitoring of its reproducibility in each laser shot. This method is based on a shot-to-shot measurement of the X-ray emission from the ablated plasma by a high resolution, spatially resolved focusing spectrometer. An optical pump laser with energy of 1.0 J and pulse duration of âŒ660 ps was used to irradiate solid targets or foils with various thicknesses containing Oxygen, Aluminum, Iron, and Tantalum. The high sensitivity and resolving power of the X-ray spectrometer allowed spectra to be obtained on each laser shot and to control fluctuations of the spectral intensity emitted by different plasmas with an accuracy of âŒ2%, implying an accuracy in the derived electron plasma temperature of 5%-10% in pump-probe high energy density science experiments. At nano- and sub-nanosecond duration of laser pulse with relatively low laser intensities and ratio Z/A ⌠0.5, the electron temperature follows Te ⌠Ilas2/3. Thus, measurements of the electron plasma temperature allow indirect estimation of the laser flux on the target and control its shot-to-shot fluctuation. Knowing the laser flux intensity and its fluctuation gives us the possibility of monitoring shot-to-shot reproducibility of shock wave strength generation with high accuracy.T. A. Pikuz, A. Ya. Faenov, N. Ozaki, N. J. Hartley, B. Albertazzi, T. Matsuoka, K. Takahashi, H. Habara, Y. Tange, S. Matsuyama, K. Yamauchi, R. Ochante, K. Sueda, O. Sakata, T. Sekine, T. Sato, Y. Umeda, Y. Inubushi, T. Yabuuchi, T. Togashi, T. Katayama, M. Yabashi, M. Harmand, G. Morard, M. Koenig, V. Zhakhovsky, N. Inogamov, A. S. Safronova, A. Stafford, I. Yu. Skobelev, S. A. Pikuz, T. Okuchi, Y. Seto, K. A. Tanaka, T. Ishikawa, and R. Kodama, "Indirect monitoring shot-to-shot shock waves strength reproducibility during pumpâprobe experiments", Journal of Applied Physics 120, 035901 (2016) https://doi.org/10.1063/1.4958796
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