Organometallic chiral liquid crystals: bis{4-[ω-(cholest-5-en-3-yloxycarbonyl)alkoxy]phenylethynyl}mercury complexes

We report the synthesis and thermal behaviour of the first chiral organometallic trimesogens having bis[(4-phenyl)ethynyl]mercury as the central core connected on either side to cholesteryl ester moieties through either even-even (C₄) or odd-odd (C₃, C₅ and C₇) central alkylene spacers. All the organomercury trimesogens exhibit a chiral nematic phase. These mercury complexes are extremely sensitive to heat, in a manner reminiscent of other reports.     

Uniaxial compression testing of bulk nanoporous gold

Abstract

The mechanical properties of bulk nanoporous gold (np–Au) with a relative density of 0.35 were investigated by compression testing of millimetre-scale specimens. Young’s modulus, Poisson’s ratio and yield strength values were determined from uniaxial, quasi-static experiments using a custom-built mechanical testing system. The cuboid-shaped specimens were fabricated following a specific, controlled process (including cutting and grinding) that guaranteed a precise and repeatable geometry. The np–Au structure was created from a silver–gold alloy by electrochemical dealloying in nitric acid. Mechanical properties obtained from compression testing are compared to values reported in the literature and to scaling relations. Values are found to agree with a recently proposed scaling relation for the yield strength of np–Au that incorporates a ligament size effect and a modified scaling exponent.     

鸟类领域和领域行为

本文综述了国内外鸟类领域性研究的有关文献,就鸟类领域的研究概况,领域的定义,功能,种类,面积和影响因素以及领域行为等问题进行了讨论。     

中国人口分布规律及演化机理研究

近20年来,中国的总人口近似地按指数规律增加,但增加的速度有减缓的趋势.省、市、县三级的人口分布具有相似的规律：人口数k小的各级人口分布随k缓慢地变化;而k较大时,人口分布随k急剧变化,满足齐普夫定律.研究结果表明中国人口系统的各级分布具有自相似性.采用无标度网络上的聚集体迁移模型较好地解释了中国人口分布的演化规律. 关键词： 人口分布 齐普夫定律 迁移 动力学行为     

Dielectric and impedance properties of Bi(Zn2/3V1/3)O3 electronic material

A polycrystalline vanadium doped lead free dielectric material of Bi(Zn_2/3V_1/3)O₃ (BZV) has been prepared using a standard high-temperature solid state reaction technique. Its temperature and frequency dependent capacitive, conductive and resistive characteristics are outlined though experimental investigation. The formation of single phase compound of BZV material with orthorhombic crystal symmetry is identified through X-ray diffraction data analysis, and the homogeneous distribution of grains are realized through scanning electron micrograph. The acquaintance of frequency–temperature dependent electrical parameters with the obtained micrograph provides the experimental evidence of contributions of grain as well as grain boundary in its capacitive and resistive characteristics. The negative temperature coefficient of resistance behaviour of the material is revealed from impedance characteristic, and non-Debye type relaxation has been realized from the Nyquist plot. The charge carriers of this electronic compound have both long & short range order that has been validated from the complex modulus and impedance analysis. The prepared electronic material substantiate some important dielectric features which props up the material as promising component for electronic devices.     

Solvable Catalyzed Birth-Death-Exchange Competition Model of Three Species

A competition model of three species in exchange-driven aggregation growth is proposed. In the model, three distinct aggregates grow by exchange of monomers and in parallel, birth of species A is catalyzed by species B and death of species A is catalyzed by species C. The rates for both catalysis processes are proportional to kj^υ and kj^ω respectively, where υ(ω) is a parameter reflecting the dependence of the catalysis reaction rate of birth (death) on the catalyst aggregate's size. The kinetic evolution behaviors of the three species are investigated by the rate equation approach based on the mean-field theory. The form of the aggregate size distribution ofA-species a_k(t) is found to be dependent crucially on the two catalysis rate kernel parameters. The results show that (i) in case of υ ≤ 0, the form of a_k(t) mainly depends on the competition between self-exchange of species A andspecies-C-catalyzed death of species A; (ii) in case of υ>0, the form of a_k(t) mainly depends on the competition between species-B-catalyzed birth of species A and species-C-catalyzed death of species A.     

Propagation of ultrashort pulsed beams in dispersive media

Starting from the Rayleigh diffraction integral, the propagation equation of ultrashort pulsed beams in dispersive media is derived without making the paraxial approximation and slowly varying envelope approximation (SVEA). The spatiotemporal properties of ultrashort pulsed beams in dispersive media, such as spectrum redshifting, narrowing and pulse distortion are illustrated with pulsed Gaussian beams. It is stressed that the “antibeam“ behaviour of ultrashort pulsed beams can be avoided, if a suitable truncation function is chosen.     

Quantum gravity and matter: counting graphs on causal dynamical triangulations

An outstanding challenge for models of non-perturbative quantum gravity is the consistent formulation and quantitative evaluation of physical phenomena in a regime where geometry and matter are strongly coupled. After developing appropriate technical tools, one is interested in measuring and classifying how the quantum fluctuations of geometry alter the behaviour of matter, compared with that on a fixed background geometry. In the simplified context of two dimensions, we show how a method invented to analyze the critical behaviour of spin systems on flat lattices can be adapted to the fluctuating ensemble of curved spacetimes underlying the causal dynamical triangulations (CDT) approach to quantum gravity. We develop a systematic counting of embedded graphs to evaluate the thermodynamic functions of the gravity-matter models in a high- and low-temperature expansion. For the case of the Ising model, we compute the series expansions for the magnetic susceptibility on CDT lattices and their duals up to orders 6 and 12, and analyze them by ratio method, Dlog Padé and differential approximants. Apart from providing evidence for a simplification of the model’s analytic structure due to the dynamical nature of the geometry, the technique introduced can shed further light on criteria à la Harris and Luck for the influence of random geometry on the critical properties of matter systems.     

A generic intermittency model and its 1-D meta-map: Power laws, invariants and the succession of laminar sequences

Intermittent behaviour has been found in many systems able to switch between two different dynamic states, e.g. between long laminar phases and short chaotic bursts. Despite the apparently high-dimensional complexity, certain one-dimensional (1-D) maps are known to mimic properties of such dynamics. To these belongs the iterative map x_n+1,i = (x_n,i + (x_n,i)^z + ε) mod 1, giving rise to long laminar lengths. The statistics of the laminar lengths are of special interest.

Starting from this map, we are interested in the values of x_0,i which arise after passing through the modulo operation. These determine the laminar lengths uniquely. A 1-D meta-map x_0,i = f(x_0,i−1) is derived heuristically. It is used to calculate statistical properties of the laminar phases. Our results show an improvement in the behaviour of short and very long laminar phases as compared to earlier analytical results. Introducing the concept of the generic starting value, we find laminar phases not to be strictly independent of their predecessors.     

Bursting behaviours in cascaded stimulated Brillouin scattering

Stimulated Brillouin scattering is studied by numerically solving the Vlasov-Maxwell system. A cascade of stimulated Brillouin scattering can occur when a linearly polarized laser pulse propagates in a plasma. It is found that a stimulated Brillouin scattering cascade can reduce the scattering and increase the transmission of light, as well as introduce a bursting behaviour in the evolution of the laser-plasma interaction. The bursting time in the reflectivity is found to be less than half the ion acoustic period. The ion temperature can affect the stimulated Brillouin scattering cascade, which can repeat several ×at low ion temperatures and can be completely eliminated at high ion temperatures. For stimulated Brillouin scattering saturation, higher-harmonic generation and wave-wave interaction of the excited ion acoustic waves can restrict the amplitude of the latter. In addition, stimulated Brillouin scattering cascade can restrict the amplitude of the scattered light.