Symmetry-based structure entropy of complex networks

Precisely quantifying the heterogeneity or disorder of network systems is important and desired in studies of behaviors and functions of network systems. Although various degree-based entropies have been available to measure the heterogeneity of real networks, heterogeneity implicated in the structures of networks can not be precisely quantified yet. Hence, we propose a new structure entropy based on automorphism partition. Analysis of extreme cases shows that entropy based on automorphism partition can quantify the structural heterogeneity of networks more precisely than degree-based entropies. We also summarized symmetry and heterogeneity statistics of many real networks, finding that real networks are more heterogeneous in the view of automorphism partition than what have been depicted under the measurement of degree-based entropies; and that structural heterogeneity is strongly negatively correlated to symmetry of real networks.     

七芯光子晶体光纤结构优化的数值分析

从耦合模方程出发,推导了描述七芯光子晶体光纤模场特性的本征方程,得到了这种结构光纤的本征值和本征矢.使用频域有限差分法数值模拟,通过改变传输波长、纤芯间距、包层孔大小分析了纤芯之间的强度分布与耦合系数的关系,分别实现了模式整形和高阶超模截止.掌握了具有强耦合特性的七芯光子晶体光纤设计规律,和这种结构光子晶体光纤的同相位超模的选取方法. 关键词： 光子晶体光纤 大模场面积光纤 多芯结构 频域有限差分法     

Defect and lattice structure for air-silica index-guiding holey fibers

We propose new design parameters for index-guiding holey fiber that can provide flexibility in defect and lattice design and adiabatic mode transformation capability. The new defect consists of a central air hole and a germanosilicate ring surrounding it, which results in a large-area annulus mode profile, low splice losses to standard fiber, 0.7 dB at 1.55 microm, and chromatic dispersion with a low slope, 0.002 ps/km nm2.     

Surface structure and adsorption properties of ultrafine porous carbon fibers

Ultrafine porous carbon fibers (UPCFs) were successfully synthesized by chemical activation of electrospun polyacrylonitrile fibers. In the current approach, potassium hydroxide was adopted as activation reagent. UPCFs were systematically evaluated by scanning electron microscope and nitrogen adsorption. The mass ratio of potassium hydroxide to preoxidized fibers, activation temperature and activation time are crucial for producing high quality UPCFs. The relationships between porous structure and process parameters are explored. UPCFs were applied as adsorbent for nitrogen monoxide to be compared with commercial porous carbon fibers.     

Effects of thermal contraction on structure and properties of PET fibers

A series of poly(ethylene terephthalate) fibers of various molecular weights was first drawn to a practical maximum draw ratio and then allowed to contract thermally under tension for 10, 20, and 38%. These contracted fibers exhibit a high degree of plasticity even when tested at—100°C and extension rates of 1300%/sec. An attempt is made to explain this behavior by means of a systematic study of morphological changes which occur during thermal contraction. The interpretation of the results of small-angle and wide-angle x-ray diffractions, infrared spectroscopy, and birefringence suggest the existence of two types of amorphous domains; those separating the adjacent crystallites in the microfibril and those separating the microfibrils. It is speculated that the molecules in these two domains respond differently to thermal effects and stress, and that the interfibrillar amorphous domain consists of highly extended molecules.

It is shown that the thermal contraction, which does not involve major changes in the degree of crystallinity, proceeds by several mechanisms. At low degrees of contraction, the most important mechanism is the contraction of the microfibrils. At high levels of contraction, the shrinkage proceeds to a large degree via relative displacement of the microfibrils and the contraction of extended interfibrillar tie molecules.

The conclusions regarding the structure of these fibers are corroborated by means of transmission electron microscopy of thin cross-sections.     

Electronic structure of phosphate glasses with a complex oxygen sublattice structure

Calculations of the electronic structure of phosphate glasses are performed in the MO LCAO x _α approximation discrete variation method. On the basis of an analysis of theoretical and experimental electronic spectra of the system BeO-P₂O₅ regularities are found in the formation of the valence band of alkaline-earth phosphate glasses with different types of anion sublattice. Data on the electronic structure are used to refine the models of short-range order; in particular, the possibility of oxygen in the threefold coordination state is confirmed. With the features of the spectrum of electronic states taken into account, localization of charge carriers, the nature of the optical transitions, and hole-transport phenomena are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1366–1372 (August 1997)     

Optical and mechanical properties of hollow-core fibers with cobweb cladding structure

This paper explores the possibility of realizing a hollow-core optical fiber, whose cladding is composed of cylindrical alternating layers of air and high-index material with supporting structure. The optical properties and the design criteria of the proposed fiber are evaluated by the compact two-dimensional (2D) finite-difference time-domain (FDTD) method. In particular, the influence of the number and width of supporting strips on the leakage loss of the fiber is investigated. Furthermore, the mechanical performances of the fiber are estimated by finite-element method, confirming that hollow-core fibers with a reasonable size and number of supporting strips are reliable.     

Towards all-fiber structure pulsed mid-infrared laser by gas-filled hollow-core fibers

We report here on a diode-pumped pulsed mid-infrared laser source based on gas-filled hollow-core fibers(HCFs)towards an all-fiber structure by the tapering method. The pump laser is coupled into an acetylene-filled HCF through a tapered single-mode fiber. By precisely tuning the wavelength of the diode to match different absorption lines of acetylene near 1.5 μm, mid-infrared emission around 3.1–3.2 μm is generated. With 2 m HCFs and3 mbar acetylene gas, a maximum average power of 130 m W is obtained with a laser slope efficiency of ～24%.This work provides a potential scheme for all-fiber mid-infrared fiber gas lasers.     

Fabrication of composite structure of carbon fibers and high density SiC nanowires

Composite structure of carbon fibers and SiC nanowires was fabricated by a simple chemical vapor deposition process, using commercial silicon dioxide and graphite powders as raw materials. The analysis of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction indicates that the synthesized SiC nanowires distribute uniformly with high density in the used carbon fiber preform, which are perpendicular to and around each carbon fiber in a radial array. The SiC nanowires located at the interface of advanced composites is very favorable to the interfacial bonding between composites matrix and carbon fibers, thereby increasing the strength of composites greatly.     

Effect of annealing on the structure and morphology of nylon 6 fibers

Changes in the structure of nylon 6 fibers annealed in dry and wet atmospheres were studied by small-and wide-angle x-ray diffraction. In the presence of water or saturated steam, fibers can be annealed to the same strucutral state at temperatures 70°C lower than in dry atmosphere. This is due to the enhanced mobility of the molecular segments in the amorphous region, a mechanism which is also known to lower the T_g by the same amount. Upon annealing under unconstrained conditions, lamellar spacing, crystallite size in the equatorial plane, crystalline as well as fiber density, and the chain-axis repeat increase with annealing-temperature; whereas crystalline orientation and the Van der Waals separation of the hydrogen-bonded sheets decrease. The monoclinic angle 8 remains constant at 66.7° (σ = 0.3°) and might depend on the starting fiber rather than on the treatment of the fiber. Most of these changes occur above a critical temperature of 170°C if dry, or 100°C if wet; rate of crystallization is also the highest under these conditions in nylon. The effect of these changes on such fiber properties as dyeing and the role of micro voids in dye diffusion and in dye uptake are discussed. Surface premelting and the accompanying changes in the surface structure of the lamellae, selective melting, and more importantly, the longitudinal motion of the nylon 6 chains and the resulting folding of interfibrillar extended amorphous chains are invoked to explain the shrinkage of the fiber, disorientation of the crystallites, increase in crystalline perfection, and the increase in lamellar spacing.     

Fabrication of composite structure of carbon fibers and high density SiC nanowires

Composite structure of carbon fibers and SiC nanowires was fabricated by a simple chemical vapor deposition process, using commercial silicon dioxide and graphite powders as raw materials. The analysis of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction indicates that the synthesized SiC nanowires distribute uniformly with high density in the used carbon fiber preform, which are perpendicular to and around each carbon fiber in a radial array. The SiC nanowires located at the interface of advanced composites is very favorable to the interfacial bonding between composites matrix and carbon fibers, thereby increasing the strength of composites greatly.     

Physics of complex semiconductor crystals and their structure

We review the results obtained in recent years by various authors with regard to the physics of complex semiconductor crystals, superlattices, and structures with quantum wells and barriers. We discuss the production processes for nonlinear optic crystals which produce high quality materials, and the changes in the electrophysical properties of a large group of materials when intrinsic defects are introduced. We analyze the characteristics of lateral semiconductor structure growth and its relationship to the characteristics of the usual epitaxial growth, as well as the theory of electron quantum processes in various quantum well structures, and we present an analysis of the vibrational states of a large group of crystals and superstructures. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 26–38, August, 1998.     

Electronic structure of neutral silicon-vacancy complex in diamond

Quantum-chemical open-shell method with parametrization INDO is used to construct the model of a large unit cell for the [Si-V]⁰ impurity complex in diamond. It is shown that the ground spin-triplet state of the complex exerts tunnel (rather than Jahn-Teller) splitting associated with off-center shift of Si atom along the trigonal axis of the fully symmetric atomic configuration D _3d . Therefore, this complex, being a source of electron spin resonance KUL1 S=1, may appear to be the known optical 1.68-eV center with a typical (≈1 meV) splitting of the zero-phonon line. The intracenter optical excitation occurs from the filled orbital doublet localized at Si to the orbital doublet localized at three of six carbon atoms neighboring Si and has a multiplet structure.     

Microwave spectra and structure of an isoxazole-water complex

The rotational spectrum of a hydrogen-bonded isoxazole-water complex has been measured between 6–18 GHz with a pulsed nozzle Fourier transform microwave spectrometer. In addition to isoxazole-H₂O, the complexes with HDO and D₂O as well as isoxazole-¹⁵ N-H₂O have been investigated in order to determine the structure of the complex. Rotational constants, quartic centrifugal distortion constants and quadrupole coupling constants, where applicable, have been fitted to the measured transition frequencies of the isotopomers. Structural data, which have been deduced from the planar moments of inertia and the quadrupole coupling constants of the isotopomers, have established conclusively that water binds to nitrogen in the ring plane of isoxazole. Ab initio calculations have revealed that complexes with a hydrogen-bond to nitrogen or to oxygen are both stable. The complex with water attached to nitrogen has been found to be more strongly bound than that with water attached to oxygen. Small splittings of the rotational transitions of the two complexes with H₂O have been interpreted as being the result of an internal rotation of water with respect to isoxazole.     

Band structure of photonic crystal fibers with silica rings in triangular lattice

The characteristics of the cladding band structure of air-core photonic crystal fibers with silica rings in triangular lattice are investigated by using a standard plane wave method. The numerical results show that light can be localized in the air core by the photonic band gaps of the fiber. By increasing the air-filling fraction, the band gap edges of the low frequency photonic band gaps shift to shorter wavelength, whereas the band gap width decreases linearly. In order to make a specified light fall in the low frequency band gaps of the fiber, the interplay of the silica ring spacing and the air-filling fraction is also analyzed. It shows that the silica ring spacing increases monotonously when the air-filling fraction is increased, and the spacing range increases exponentially. This type fiber might have potential in infrared light transmission.     

The mode structure and spectral properties of supercontinuum emission from microstructure fibers

The mode structure and spectral properties of supercontinuum emission generated by femtosecond pulses of Ti: sapphire laser radiation in microstructure fibers are studied. The long-wavelength (720–900 nm) and visible (400–600 nm) parts of supercontinuum emission are shown to be spatially separated in microstructure-fiber modes, which can be isolated with an appropriate spectral filtering. The spatial modes thus isolated in spectrally sliced supercontinuum emission possess a spatial quality sufficient for further efficient frequency conversion. The possibility of achieving a high spectral quality of supercontinuum emission is also demonstrated. We explore the ways to control the spectrum of supercontinuum emission by matching parameters of the pump pulse with the parameters of a microstructure fiber and by tuning the initial chirp of the pump pulse. The results of our studies show that supercontinua produced in microstructure fibers offer new approaches to designing a new generation of optical parametric amplifiers and broadband radiation sources for spectroscopic, metrological, and biomedical applications.     

Index structure of fiber Bragg gratings in Ge-SiO(2) fibers

The behavior of the concentration of photoinduced color centers in Ge-SiO(2) optical fibers was compared with that of the index modulation associated with fiber Bragg gratings (FBG's) written in the same fibers. We find that the fluence dependence of the photoinduced Ge E? center, its thermal annealing behavior, and its reaction with H(2) are similar to that of the index modulation generated in both H(2)-loaded and unloaded Ge-SiO(2) fibers. The much higher photosensitivity of H(2)-loaded Ge-SiO(2) fibers is attributed to the much higher formation efficiency of Ge E? centers, with an additional contribution from GeH. A diamagnetic structure, possibly densification, is also found to contribute to the index modulation of FBG's.     

Electrospun ceramic fibers: Composition, structure and the fate of precursors

Fibers are electrospun from aluminum acetate/polymer mixtures and characterized by an array of techniques before and after annealing at 1200 °C. We demonstrate that sodium and boron present in the initial starting materials as adducts and stabilizers remain incorporated into the resulting fibers after annealing and pyrolysis of the host polymer. The influence of these minor constituents on the surfaces of the fibers is suggested by infrared and X-ray photoelectron spectroscopic data. The presence of these species may impact potential chemical applications of small diameter ceramic fibers, such as their use as catalytic supports or for chemical decomposition.     

Surface modification and submicron structure of carbon fibers through high current pulse

This paper investigated the behavior of carbon fibers subjected to a ∼20 kA, ∼5 μs high current pulse. It was found that the broken fibers and submicron particles were generated by electrical explosion process. After high current pulsed discharge, the fiber diameter increased significantly, from 5-7 μm to ∼13 μm. Also, the surface rupture of carbon fibers with valleys of hundreds of nanometers was observed. Most notably, the submicron particles appeared with two typical shapes (near-sphere and square). The high current pulsed discharge of carbon fibers can be divided into three stages, namely, heating stage, phase change stage, and explosion stage. Indeed, the electrical explosion process occurred in the last stage of ∼200 ns. The nature behind these results is closely related to the plasma development during the explosion process. The plasma expansion due to a large plasma thermal stress leads to the incomplete explosion. In the explosion stage, the current passing through the fibers exhibited a huge fluctuation, indicating plasma instabilities. Finally, the physical mechanisms, how to affect the surface morphology of carbon fibers, are presented.