Average Coherence Length Research Articles

Study of phase transitions and structural order in perturbed nematic liquid crystals

We studied the typical domain size and configuration character of randomly perturbed nematic liquid crystal system, or in general the system of rod-like objects which interact via a Lebwohl–Lasher-type interaction. We described their local direction with a headless unit director field. We introduced into the system the impurities of concentration p, which impose the random anisotropy field-type disorder to directors. We studied the domain-type pattern of molecules as a function of p, anchoring strength W between a neighboring director and impurity, temperature, and history of samples. In simulations, we quenched the directors either from the random or from homogeneous initial configuration. Our results show that a history of system strongly influences: i) the average domain coherence length; and ii) the range of orientational ordering in the system. In the random case, the obtained order is always short-range (SRO). On the contrary, in the homogeneous case, SRO is obtained only for strong enough anchoring W and large enough concentration p. In other cases, the ordering is either quasi long range (QLRO) or long range (LRO). We further studied field-induced memory effects for the random initial configuration. With increasing external ordering field, either QLRO or LRO is realized. This ordering is preserved even if the field is switched off, and its degree saturates for a large enough value of the field. Therefore, one can control the degree of global ordering and average domain coherence size by temporarily exposing the system to an external ordering field. Such systems could be exploited as soft matter based phase-memory devices.

Superconducting Polycrystalline Silicon Layer Obtained by Boron Implantation and Nanosecond Laser Annealing

In this work, we report on the material properties of superconducting heavily boron-doped polycrystalline Silicon-On-Insulator (SOI) thin layers fabricated by pulsed laser induced recrystallization under experimental conditions compatible with high volume CMOS integration. This approach combines boron implantation and ultra-violet nanosecond laser annealing (UV-NLA) to reach maximum dopant activation by exceeding boron solid solubility in silicon. For our process conditions, material characterizations revealed five laser annealing regimes, including the SOI full-melt, which leads to the formation of superconducting polycrystalline layers. The average critical temperature was found to be around 170 mK, neither influenced by energy density nor the number of laser pulses. In addition, thanks to low temperature measurements coupled with magnetic field variations, we highlighted a type II superconductor behavior due to strong impurity effect. The deducted average effective coherence length of hole pairs in our layers was estimated around 85 nm.

Capacity of communication link with carrier of vortex localized wave in absorptive turbulent seawater

We explore the influence of absorption, anisotropy and unstable layered turbulence on the channel capacity of horizontal seawater link. By developing the spectrum average coherence length of collimated beam wave in the absorptive seawater turbulence, we propose a model of the channel capacity of links with carrier Bessel-Gaussian vortex localized wave. Our model shows that the capacity of links increases with the increasing of the half-modulated pulse width of the input pulse, the rate of dissipation of kinetic energy per unit mass of fluid and the anisotropic factor of turbulence, but it decreases with the increasing of the Bessel cone angle and the dissipation rate of the mean-squared temperature. We also find that there is a maximum effective beam waist for a given receiving aperture, the channel capacity is more sensitive to salinity fluctuations than to temperature fluctuations in turbulence, and the influences of the inner scale and outer scale of turbulence on channel capacity of links in anisotropic turbulence and isotropic turbulence are different.

Graphene Oxide Concentration Effect on the Optoelectronic Properties of ZnO/GO Nanocomposites.

In this work, the effects of graphene oxide (GO) concentrations (1.5 wt.%, 2.5 wt.%, and 5 wt.%) on the structural, morphological, optical, and luminescence properties of zinc oxide nanorods (ZnO NRs)/GO nanocomposites, synthesized by a facile hydrothermal process, were investigated. X-ray diffraction (XRD) patterns of NRs revealed the hexagonal wurtzite structure for all composites with an average coherence length of about 40–60 nm. A scanning electron microscopy (SEM) study confirmed the presence of transparent and wrinkled, dense GO nanosheets among flower-like ZnO nanorods, depending on the GO amounts used in preparation. Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–Vis) absorption spectroscopy, and photoluminescence (PL) measurements revealed the impact of GO concentration on the optical and luminescence properties of ZnO NRs/GO nanocomposites. The energy band gap of the ZnO nanorods was independent of GO concentration. Photoluminescence spectra of nanocomposites showed a significant decrease in the intensities in the visible light range and red shifted suggesting a charge transfer process. The nanocomposites’ chromaticity coordinates for CIE 1931 color space were estimated to be (0.33, 0.34), close to pure white ones. The obtained results highlight the possibility of using these nanocomposites to achieve good performance and suitability for optoelectronic applications.

Received Probability of Vortex Modes Carried by Localized Wave of Bessel–Gaussian Amplitude Envelope in Turbulent Seawater

By using the two-frequency coherence function model of a beam in a turbulent medium and the localized wave theory of the polychromatic beam, we develop the spectrum average mutual coherence function of the localized wave of Bessel–Gaussian amplitude envelope and the spectrum average coherence length of spherical wave. By the spectrum average coherence length and the spectrum average mutual coherence function, we construct a received probability of vortex modes carried by localized wave of Bessel–Gaussian amplitude envelope in anisotropic turbulent seawater. Our results show that the received probability of signal vortex modes increases with the increase of half-modulated pulse width of the input pulse, turbulent inner scale, anisotropic factor of turbulence and rate of dissipation of kinetic energy per unit mass of fluid, but it increases with the decrease of the Bessel cone angle and the dissipation rate of the mean-squared temperature. We also find that there is a maximum effective beam waist for a given receiving aperture, and the vortex mode is more sensitive to salinity fluctuations than to temperature fluctuations in turbulence. Our conclusions show that localized wave of Bessel–Gaussian amplitude envelope is a more suitable beam for the vortex mode communication than conventional vortex waves.

The morphology and kinematics of neutral hydrogen in the vicinity ofz= 0 galaxies with Milky Way masses – a study with the Illustris simulation

We analyze the properties of the circumgalactic gas (CGM) around 120 galaxies with stellar and dark matter halo masses similar to that of the Milky Way. We focus on the morphology and kinematics of the neutral hydrogen and how this depends on f_g, the ratio of gas-to-stellar mass within the optical radius. In gas-rich galaxies with f_g > 0.1, gas temperatures rise slowly from center of the halo out to the virial radius and average neutral gas column densities remain above 10^19 atoms cm^-2 out to radii of 50-70 kpc. In gas-poor galaxies with f_g < 0.1, gas temperatures rise quickly outside the edge of the disk to 10^6 K, and then remain fixed out to radii of 100 kpc. The column density of neutral gas quickly drops below 10^19 atoms cm^-2 at radii of 10 kpc. Neutral gas distributions are also more asymmetric in gas-poor galaxies. Most of the differences between gas-poor and gas-rich galaxies in the Illustris simulation can be attributed to the effects of "radio-mode" AGN feedback. In the Illustris simulation, the circumgalactic gas is found to rotate coherently about the center of the galaxy with a maximum rotational velocity of around 200 km/s. In gas-rich galaxies, the average coherence length of the rotating gas is 40 kpc, compared to 10 kpc in gas-poor galaxies. In the very most gas-rich systems, the CGM can rotate coherently over scales of 70-100 kpc. We discuss our results in the context of recent observations of the CGM in low mass galaxies via UV absorption-line spectroscopy and deep 21cm observations of edge-on spiral galaxies.

Rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

Rubrene polycrystalline films growth from vacuum deposition (with a fixed source temperature of 300°C) were characterized with respect to various substrate temperatures (Tsub=103–221°C). First, the growth behavior of these as-deposited polycrystalline films is confirmed to follow an activated surface-adsorption process with an activation energy EA=0.69±0.01eV. A comparison of EA for the growth of some other small organic molecular solid films is given. Then, the surface morphology and the temporal evolution of the grain size in these polycrystalline films with respect to Tsub are described and discussed. Furthermore, by X-ray diffraction, these rubrene crystalline grains are confirmed to have an orthorhombic structure, and the average coherent length and lattice microstrain of the crystallites deposited at high Tsub (189–221°C) are estimated and compared. This experimental work reveals that Tsub has a strong influence on the growth rate, the surface morphology, and the structural properties of the as-deposited rubrene polycrystalline films.

Characterization of facial and meridional Alq3 thin films fabricated from physical vapor transport at high substrate temperatures

During a thermal annealing process, the isomeric transformation from meridional (mer) into facial (fac) tris(8-hydroxyquinoline)-aluminum (Alq3) based on the infrared absorption was observed to be nearly complete around 375 °C. Then, in a physical vapor transport process with post-annealed fac-Alq3 as source powder (at source temperature of 375 °C), the effects from various substrate temperatures (Tsub from 285 to 346 °C) on the transformation from fac-Alq3 into mer-Alq3 isomers re-condensed on the substrate were studied again, and an isomeric transformation occurs at Tsub = 316 ± 2 °C. For Tsub below 316 ± 2 °C, the re-condensed mer-Alq3 isomers tend to aggregate to form close-packed micron-rods, which are composed of α-phase crystallites with an average coherent length L ∼ 46 nm. The photoluminescence (PL) spectra for these as-fabricated mer-Alq3 films show typical green emission. However, for Tsub above 316 ± 2 °C, the adsorbed fac-Alq3 molecules on the substrate preserve their corresponding isomeric structure from the post-annealed fac-Alq3 source and self-organize into flexible submicron-fibers. These samples are dominated by δ-phase crystallites with L ∼ 57 nm. The PL spectra for these as-fabricated fac-Alq3 films do emit blue luminescence. Besides, Raman scattering for these mer-Alq3 and fac-Alq3 molecules are also compared.

Second harmonic generation measurements at high pressures on powder samples

This article reviews the most recent results con- cerning second harmonic generation (SHG) experiments of non-phase matchable and phase matchable powder samples at high pressures and explains the pressure de- pendence of the intensity of the SHG signal by correlat- ing it to the ratio between the average coherence length and the average particle size. The examples discussed here include pressure-induced structural changes in quartz, ZnO, ice VII and KIO3. It is shown that the second harmonic generation technique is a unique tool for the detection of pressure-induced structural phase transi- tions. It is laboratory based and allows fast measure- ments. It is complementary to X-ray diffraction and pro- vides additional information about the presence of an inversion center for unknown or controversially dis- cussed structures at high pressure.

The coherence of low-frequency sound in shallow water in the presence of internal waves

The coherence time and transverse coherence length of a low-frequency (100–300 Hz) sound field that is formed by an omnidirectional point source at a distance of 10–30 km in a shallow-water acoustic waveguide, which is characteristic of an open ocean shelf, were estimated analytically and in a numerical experiment. An anisotropic field of background internal waves is considered as a source of spatiotemporal fluctuations. It is shown that the coherence time decreases as the frequency increases, and strongly depends on the perturbation-movement direction. The transverse coherence length is primarily determined by phase incursions that are related to the cylindrical shape of the acoustic-wave front. In the case of transverse propagation, background internal waves may lead to significant variations in this length. The introduction of compensating phase corrections during processing provides a considerable increase in the average transverse coherence length.

Stochastic method for determining the coherence length of a TEA CO2 laser

Abstract In this work the results of the measurement of the coherence length of a TEA CO 2 laser developed in DEILAP are presented. A Michelson interferometer of about 5 m of optical path length difference was built for this purpose. A motorized translation optical mount was placed in one arm in order to enable the variation of the optical path length in many wavelengths. The length of the other arm was varied manually in steps of several tens of centimeters. Spatial filtering forced the laser to oscillate in a single transverse mode, TEM 00 . An innovative stochastic technique for data analysis allowed determining the amount of longitudinal modes, the amplitude and the individual average coherence length of each mode as well as the global coherence length of the laser. This technique was later compared with a method based on the analysis of the Fourier transform of the time series obtained at the interferometer input.

Enhancement in superconducting transition temperature (Tc) and upper critical field (Hc2) in new Yb-doped Ce1−xYbxO0.9F0.1FeAs superconductors

We report the synthesis and characterization of new oxypnictides of the type Ce1−xYbxO0.9F0.1FeAs (‘x’=0.3, 0.4 and 0.5) for the first time. All these compounds crystallize in the tetragonal ZrCuSiAs type structure (space group: P4/nmm). Reduction in both the lattice parameters (a and c) was observed on substitution of smaller Yb ions at Ce sites in CeO0.9F0.1FeAs. All these compounds were found to be superconducting with maximum Tc of 48.7K for ‘x’=0.5 composition which is highest among Ce(O/F)FeAs based superconductors. Temperature dependence of resistivity under magnetic field has been studied to evaluate the upper critical field (Hc2(0)) of these superconductors. Enhancement in upper critical field (Hc2) was observed for these Yb doped Ce1−xYbxO0.9F0.1FeAs superconductors (max Hc2(0)=∼142T for ‘x’=0.3). The average coherence length of all Yb doped samples was estimated to be ∼23Å.

Trap-limited and percolation conduction mechanisms in amorphous oxide semiconductor thin film transistors

The electron conduction mechanism in the above-threshold regime in amorphous oxide semiconductor thin film transistors is shown to be controlled by percolation and trap-limited conduction. The band tail state slope controls the field effect mobility, while the average spatial coherence length and potential fluctuation control percolation conduction. In these limits, the field effect mobility is found to follow a power law, from which a universal mobility versus carrier concentration dependence is extracted.

Lineshape Analysis of the Beat Signal Between Optical Carrier and Delayed Sidebands

This paper presents the lineshape analysis of the beat signal between the optical carrier and the shifted and delayed sidebands produced by sinusoidal amplitude modulation. It is shown that the beat signal has a typical lineshape with a very narrow ? -peak superposed on a quasi-Lorentzian profile. Theoretical explanation for the appearance of this peak has been given based on optical spectral structure constructed by a large number of optical wave trains. It is predicted that the ?-peak is originated from the beat between the wave trains in the carrier and those in the delayed sidebands when their average coherence length is longer than the delay line. Experiments carried out using different delay lines clearly show that the ? -peak is always located at the modulation frequency and decreases with the increasing delay line. Our analysis explicitly indicates that the linewidth is related to the observation time. It is also suggested that the disappearance of the ? -peak can be used as the criterion of coherence elimination.

Optical Turbulence Measurements and Models for Mount John University Observatory

AbstractSite measurements were collected at Mount John University Observatory in 2005 and 2007 using a purpose-built scintillation detection and ranging system. C2N(h) profiling indicates a weak layer located at 12–14 km above sea level and strong low altitude turbulence extending up to 5 km. During calm weather conditions, an additional layer was detected at 6–8 km above sea level. V(h) profiling suggests that tropopause layer velocities are nominally 12–30m s−1, and near-ground velocities range between 2 and 20m s−1, dependent on weather. Little seasonal variation was detected in either C2N(h) and V(h) profiles. The average coherence length, r0, was found to be 7±1 cm for the full profile at a wavelength of 589 nm. The average isoplanatic angle, θ0, was 1.0±0.1 arcsec. The mean turbulence altitude, , was found to be 2.0±0.7 km above sea level. No average in the Greenwood frequency, fG, could be established due to the gaps present in the V(h) profiles obtained. A modified Hufnagel-Valley model was developed to describe the C2N(h) profiles at Mount John, which estimates r0 at 6 cm and θ0 at 0.9 arcsec. A series of V(h) models were developed, based on the Greenwood wind model with an additional peak located at low altitudes. Using the C2N(h) model and the suggested V(h) model for moderate ground wind speeds, fG is estimated at 79 Hz.

Structural changes in the trichocyte intermediate filaments accompanying the transition from the reduced to the oxidized form

Earlier studies established that substantial changes take place in the three-dimensional structure of the newly assembled trichocyte keratin intermediate filament (IF) during the oxidation process (Wang, H., Parry, D.A.D., Jones, L.N., Idler, W.W., Marekov, L.N., Steinert, P.M. 2000. In vitro assembly and structure of trichocyte keratin intermediate filaments: A novel role for stabilization by disulfide bonding. J. Cell Biol. 151, 1459–1468). The present contribution describes a re-examination of previous data in which more accurate values for the axial dispositions of the molecules have been obtained to yield the most detailed picture yet available of the structural changes that occur in vivo. In particular, it is shown that in the newly assembled (reduced) IF the crosslinking data are consistent with the detailed (8 + 0) model suggested earlier (Fraser, R.D.B., Parry, D.A.D. 2005. The three-dimensional structure of trichocyte (hard α-) keratin intermediate filaments: Features of the molecular packing deduced from the sites of induced crosslinks. J. Struct. Biol. 151, 171–181), in which eight four-chain protofilaments are arranged on an annular ring. For oxidized IF, however, the existing X-ray data require a periodic imperfection in the surface lattice which is substantial in the case of an (8 + 0) model and hence difficult to explain. In contrast, an alternative (7 + 1) model (Fraser, R.D.B., MacRae, T.P., Parry, D.A.D., Suzuki, E. 1986. Intermediate filaments in α-keratin. Proc. Natl. Acad. Sci. USA 83, 1179–1183) requires only a minor imperfection, and it is suggested that this is associated with the central protofilament. This suggestion is shown to be compatible with both the crosslinking data and a model for the axial distribution of electron density derived from the meridional X-ray pattern. In addition, evidence from an X-ray diffraction study of the follicle (Er Rafik, M., Briki, F., Burghammer, M., Doucet, J. 2006. In vivo formation of the hard α-keratin intermediate filament along a hair follicle: Evidence for structural polymorphism. J. Struct. Biol. 154, 79–88) and electron microscope studies of isolated reduced IF (Watts, N.R., Jones, L.N., Cheng, N., Wall, J.S., Parry, D.A.D., Steven, A.C. 2002. Cryo-electron microscopy of trichocyte (hard α-keratin) intermediate filaments reveals a low-density core. J. Struct. Biol. 137, 109–118) have been combined with earlier X-ray studies to give an estimate of the reduction in diameter that occurs in the IF due to the lateral reorganization of the protofilaments during the oxidation process. It has also been shown that the local coiled-coil geometry in the immediate vicinity of the contributing cysteine residues is necessarily disrupted, a feature consistent with the breadths of the near-equatorial layer lines in the X-ray diffraction pattern that indicate an average coherent length of coiled coil of only about 5 nm.

Photochromism induced magnetization changes in Prussian Blue ultrathin films fabricated into the Langmuir–Blodgett films composed of an amphiphilic azobenzene and a deoxyribonucleic acid

A photomagnetic ultrathin film consisting of an azobenzene, a deoxyribonucleic acid, and Prussian Blue has been fabricated by the Langmuir–Blodgett method. Prussian Blue layers in the films were a two-dimensional structure with the height of 50 Å and the average coherence length of 210 Å. Reversible photoisomerization of the azobenzenes was observed even in the films. The films exhibit the long-range ferromagnetic order below 4.2 K and moreover, upon photoillumination at 2 K, reversible changes in the magnetization were realized with the value of ca. 1.0%. This photoswitching in the magnetization is due to changes in the electrostatic field induced by photoisomerization.

Size Distribution of Gel and Fluid Clusters in DMPC/DSPC Lipid Bilayers. A Monte Carlo Simulation Study

Proteins and lipid components are organized into domains in many biological membranes. With different experimental techniques vastly different cluster sizes have been measured in an equimolar mixture of a DMPC/DSPC two-component lipid bilayer: very small ones in the nanometer range and very large ones of size comparable with the size of the bilayer. In this paper the lateral distribution of gel and fluid lipid molecules in a DMPC/DSPC bilayer is simulated by using a two-state, Ising type model with the application of Monte Carlo methods. The same model has been able to predict the excess heat capacity curves, FRAP threshold temperatures, average coherence length between DSPC clusters, and the fractal dimension of gel clusters, in agreement with the respective experimental data. In this work, similarly to the experimental results, the calculated equilibrium distributions of cluster size show that, between the onset temperature of the gel-to-fluid transition and the percolation threshold temperature of the...

Effect of annealing on the microstructure of Ni 80Fe 20/Cu multilayers

[Ni 80Fe 20/Cu] 15 multilayers grown by DC-magnetron sputtering and annealed at different temperatures and/or times were investigated by low- and high-angle X-ray diffraction. Structural parameters such as superlattice period, interplane distance, average multilayer coherence length and interfacial roughness were obtained. It was found that as the annealing temperature increases the superlattice period, interplane distance, average multilayer coherence length decrease, while (1 1 1) preferred orientation of the superlattices was improved slightly. The interfacial roughness increases with increasing annealing temperature and/or time. A significant intermixing layer located in the interlayer region between the Ni 80Fe 20 and Cu layers was revealed by simulating the high-angle X-ray diffraction profiles. The thickness of the intermixing layer increases as the annealing temperature or annealing time increases.

Synthesis and optical properties of nanosized powders: lanthanide-doped Y 2O 3

Y 1.8Nd 0.2O 3 and Y 1.8Er 0.2O 3 powder samples were prepared by combustion synthesis. X-ray diffraction showed that the materials are nanostructured, with average coherence length 5–10 nm. The visible and infrared luminescence spectra of nanometric cubic Y 1.8Nd 0.2O 3 and Y 1.8Er 0.2O 3 show some important differences from those of the bulk materials. In the case of Y 1.8Er 0.2O 3, faster relaxation of some excited states occurs, leading to changes in the relative intensities of several optical transitions. As far as nanostructured Y 1.8Nd 0.2O 3 is concerned, the 4 F 3/2→ 4 I 9/2 transition is characterized by a perturbed intensity pattern of the Stark components and by an anomalous integrated intensity. Possible explanations for this behaviour are proposed.