Abstract: The dielectric polarizations induced by impurity ions in both high- and low-resistivity nematic liquid crystals (NLCs) were investigated. Upon adding carbon nanotubes (CNTs) as a dopant, the former showed no distinct change in the low-frequency (< 102 Hz) dielectric spectrum, whereas the latter exhibited dramatic decrease in dielectric constant in that the CNTs remarkably trapped the impurity ions in the NLC of high ionic content. Consequently, the dopant raised the voltage holding ratio by 26% and prolonged the lifetime of the cell. Also investigated were ionic behaviors in the low-resistivity NLC confined in cells with three different configurations. The diffusion constant of the ions in homeotropic cells was found to be the greatest, and the dc conductivity, determined by the diffusion constant, was also higher in the homeotropic ones. PubDate: Mon, 13 May 2013 12:04:05 +000
Abstract: Time-resolved macromolecular crystallography unifies structure determination with chemical kinetics, since the structures of transient states and chemical and kinetic mechanisms can be determined simultaneously from the same data. To start a reaction in an enzyme, typically, an initially inactive substrate present in the crystal is activated. This has particular disadvantages that are circumvented when active substrate is directly provided by diffusion. However, then it is prohibitive to use macroscopic crystals because diffusion times become too long. With small micro- and nanocrystals diffusion times are adequately short for most enzymes and the reaction can be swiftly initiated. We demonstrate here that a time-resolved crystallographic experiment becomes feasible by mixing substrate with enzyme nanocrystals which are subsequently injected into the X-ray beam of a pulsed X-ray source. PubDate: Sun, 12 May 2013 17:33:14 +000
Abstract: The electric and magnetic properties of layered perovskites have been investigated systematically over the doping range . It was found that both Sr1.5Y0.5CoO4 and Sr1.4Y0.6CoO4 undergo ferromagnetic (FM) transition around 145 K and 120 K, respectively. On the other hand, Sr1.3Y0.7CoO4 and Sr1.2Y0.8CoO4 compounds showed paramagnetic behavior over a wide range of temperatures. In addition, spin-glass transition () was observed at 10 K for Sr1.3Y0.7CoO4. All investigated samples are semiconducting-like within the temperature range of 10–300 K. The temperature dependence of the electrical resistivity, , was described by two-dimensional variable range hopping (2D-VRH) model at 50 K < ≤ 300 K. Comparison with other layered perovskites was discussed in this work. PubDate: Tue, 07 May 2013 08:30:23 +000
Abstract: The two-band hybridized superconductor which the pairing occurred by conduction electron band and other-electron band are considered within a mean-field approximation. The critical temperature, zero-temperature order parameter, gap-to- ratio, and isotope effect coefficient are derived. We find that the hybridization coefficient shows a little effect on the superconductor that conduction electron band has the same energy as other-electron band but shows more effect on the superconductor that conduction electron band coexists with lower-energy other-electron band. The critical temperature is decreased as the hybridization coefficient increases. The higher value of hybridization coefficient, lower value of gap-to- ratio, and higher value of isotope effect coefficient are found. PubDate: Thu, 18 Apr 2013 10:19:14 +000
Abstract: Multiferroic hexagonal manganites ReMnO3 studied by optics are reviewed. Their electronic structures were revealed by static linear and nonlinear spectra. Two transitions located at 1.7 eV and 2.3 eV have been observed and attributed to the interband transitions from the lower-lying Mn3+ and states to the Mn3+ state, respectively. These so-called d-d transitions exhibit a blueshift as decreasing temperatures and an extra blueshift near . This dramatic change indicates that the magnetic ordering seriously influences the electronic structure. On the other hand, the ultrafast optical pump-probe spectroscopy has provided the important information on spin-charge coupling and spin-lattice coupling. Because of the strongly correlation between electronic structure and magnetic ordering, the amplitude of the initial rising component in shows striking changes at the vicinity of . Moreover, the coherent optical and acoustic phonons were observed on optical pump-probe spectroscopy. Both the amplitude and dephasing time of coherent phonons also exhibit significant changes at , which provide the evidence for spin-lattice interaction in these intriguing materials. PubDate: Thu, 11 Apr 2013 17:51:23 +000
Abstract: We study meniscus driven locking of point defects of nematic liquid crystals confined within a cylindrical tube with free ends. Curvilinear coordinate system is introduced in order to focus on the phenomena of both (convex and concave) types of menisci. Frank's description in terms of the nematic director field is used. The resulting Euler-Lagrange differential equation is solved numerically. We determine conditions for the defects to be trapped by the meniscus.
Abstract: Systematic series of binary zinc tellurite glasses in the form (where to 0.4 with an interval of 0.05 mole fraction) have been successfully prepared via conventional melt cast-quenching technique. Their density was determined by Archimedes method with acetone as buoyant liquid. The thermal expansion coefficient of each zinc tellurite glasses was measured using L75D1250 dilatometer, while their glass transition temperature () was determined by the SETARAM Labsys DTA/6 differential thermogravimetric analysis at a heating rate of 20 K min−1. The acoustic Debye temperature and the softening temperature () were estimated based on the longitudinal () and shear ultrasonic () wave velocities propagated in each glass sample. For ultrasonic velocity measurement of the glass sample, MATEC MBS 8000 Ultrasonic Data Acquisition System was used. All measurements were taken at 10 MHz frequency and at room temperature. All the thermal properties of such binary tellurite glasses were measured as a function of ZnO composition. The composition dependence was discussed in terms of ZnO modifiers that were expected to change the thermal properties of tellurite glasses. Experimental results show their density, and the thermal expansion coefficient increases as more ZnO content is added to the tellurite glass network, while their glass transition, Debye temperature, and the softening temperature decrease due to a change in the coordination number (CN) of the network forming atoms and the destruction of the network structure brought about by the formation of some nonbridging oxygen (NBO) atoms.
Abstract: Cation and/or molecule transfer within nanoporous materials is utilized in lithium-ion secondary battery, ion exchange, hydrogen storage, molecular sensors, molecular filters, and so on. Here, we performed ab initio total energy calculation to derive the alkali cation potential in the Prussian blue analogues, (, Na, K, Rb, and Cs; , Ni, Mn, and Cd), with jungle-gym-type nanoporous framework. The potential curves of larger cations, that is, K+, Rb+ and Cs+, exhibit a barrier at the window of the host framework, while those of the smaller cations, that is, Li+ and Na+, exhibit no barrier. We will discuss the useful functionalities observed in the Prussian blue analogues, that is, (a) battery properties mediated by Li+ intercalation/de-intercalation, (b) electrochromism mediated by Na+ transfer in all solid device, and (c) the elimination of Cs+ from aqueous solution by precipitation, in terms of the alkali cation potentials.
Abstract: The paper presents the analysis of the electromagnetic wave propagation through liquid crystal optical fibers (LCOFs) of two different types—conventional guides loaded with liquid crystals (addressed as LCOFs) and those with additional twists due to conducting helical windings (addressed as HCLCOFs). More precisely, the three-layer optical waveguide structures are considered along with its outermost region being loaded with radially anisotropic liquid crystal material and the inner regions being made of usual silica, as used in conventional optical fibers. In addition to that, LCOF with twists introduced in the form of conducting helical windings at the interface of the silica core and the liquid crystal clad is also taken into account. Emphasis has been put on the power confinements by the lower-order TE modes sustained in the different sections of the LCOF structure. The results demonstrate useful applications of these guides in integrated optics as the power sustained in the liquid crystal section by the excited TE modes remains very high. In the case of twisted clad liquid crystal guides, descriptions are limited to the nature of dispersion relation only under the TE mode excitation, and corresponding to the cases of helix orientations being parallel and perpendicular to the optical axis.
Abstract: A self-made automobile heterodyne interferometer with a closed control loop is presented. In addition, we will apply the self-made automobile heterodyne interferometer to measure the parameters of optically compensated twisted nematic liquid crystal (TN LC). It is worthy to be mentioned that the self-made automobile heterodyne interferometer has a compact size because all the optical components are installed in the mechanism of the system. Besides, the self-made automobile heterodyne interferometer has some much more merits, for example, automobile control, a simple setup, in real-time test, reliability, and so forth. Besides, it is very stable because of its common optical path insensitive to environmental disturbances. And its feasibility is demonstrated.
Abstract: Liquid crystal thermography is an advanced nonintrusive measurement technique, which is capable of providing a high-accuracy continuous temperature field measurement, especially for a complex structured heat transfer surface. The first part of the paper presents a comprehensive introduction to the thermochromic liquid crystal material and the related liquid crystal thermography technique. Then, based on the aythors' experiences in using the liquid crystal thermography for the heat transfer measurement, the parameters affecting the measurement uncertainty of the liquid crystal thermography have been discussed in detail through an experimental study. The final part of the paper describes the applications of the steady and transient liquid crystal thermography technique in the study of the turbulent flow heat transfer related to the aeroengine turbine blade cooling.
Abstract: The temperature dependence of the static dielectric constant () is calculated close to the smectic A-smectic B () transition ( = 71.3°C) for the liquid crystal compound B5. By expanding the free energy in terms of the order parameter in the mean field theory, the expression for the dielectric susceptibility (dielectric constant) is derived and is fitted to the experimental data for which was obtained at the field strengths of 0 and 67 kV/cm from literature. Coefficients in the free energy expansion are determined from our fit for the transition of B5. Our results show that the observed behaviour of the dielectric constant close to the transition in B5 can be described satisfactorily by our mean field model.
Abstract: The superconductivity in antiperovskite compound MgCNi3 was discovered in 2001 following the discovery of the superconducting MgB2. In spite of its lower superconducting transition temperature (8 K) than MgB2 (39 K), MgCNi3 has attracted considerable attention due to its high content of magnetic element Ni and the cubic structure analogous to the perovskite cuprates. After years of extensive investigations both theoretically and experimentally, however, it is still not clear whether the mechanism for superconductivity is conventional or not. The central issue is if and how the ferromagnetic spin fluctuations contribute to the cooper paring. Recently, the experimental results on the single crystals firstly reported in 2007 trend to indicate a conventional s-wave mechanism. Meanwhile many compounds neighboring to MgCNi3 were synthesized and the physical properties were investigated, which enriches the physics of the Ni-based antiperovskite compounds and help understand the superconductivity in MgCNi3. In this paper, we summarize the research progress in these two aspects. Moreover, a universal phase diagram of these compounds is presented, which suggests a phonon-mediated mechanism for the superconductivity, as well as a clue for searching new superconductors with the antiperovskite structure. Finally, a few possible scopes for future research are proposed.
Abstract: Various ZnNyNi3−xCox compounds with differing Co content, x, were synthesized, and their magnetic properties were investigated. Uniform solid solutions could not be obtained at low Co content (); instead micrometer-scaled ferromagnetic ZnNyNi0.6Co2.4 domains formed embedded within a superconductive ZnNNi3 bulk, showing chemical phase separation of superconductive ZnNNi3 and ferromagnetic ZnNyNi0.6Co2.4. At intermediate levels of Co concentration (), this two-phase separation might persist, and the superconductive behavior was strongly suppressed in this composition region. Only at high Co concentration () the uniform ferromagnetic solid solution ZnNyNi3−xCox (with most likely ) formed. The phase separation behavior is intrinsic to the system, reflecting the existence of a miscibility gap in ZnNyNi3−xCox for the samples with , and was shown not to be attributable to incomplete synthesis. In the two-phased samples, high-quality granular contact between the superconductor and ferromagnet has been realized, suggesting that the production of useful devices requiring high-quality contacts between superconductors and ferromagnets may be possible by making use of this two-phase situation.
Abstract: The temperature dependence of the tilt angle is studied in the smectic phase near the smectic A-smectic tricritical point for a mixture of 70PD0B in the ferroelectric liquid crystal C7 (). The mean-field models with the biquadratic ( is the spontaneous polarization) and (bilinear) coupling terms in the free energy expansion are used to analyze the experimental data for the tilt angle in this binary mixture. From our analysis, the coefficients given in the free energy expansion of the mean-field models are determined.
Our results show that the mean-field theory explains adequately the observed behaviour of the C7-70PD0B mixture near the tricritical point.
Abstract: The physical properties of polycrystalline Mn3SbN were investigated using measurements of the magnetic, calorimetric, and electronic transport properties. At room temperature, the phase crystallizes in a tetragonal structure with symmetry. A remarkably sharp peak in the heat capacity versus temperature curve was found near 353 K. The peak reaches 723 J mol−1 K−1 at its highest, which corresponds to a transition entropy of 10.2 J mol−1 K−1. The majority of the large entropy change appears to be due to lattice distortion from the high-temperature cubic structure to the room-temperature tetragonal structure and the accompanying Ferrimagnetic transition.
Abstract: We theoretically investigated antiperovskite chromium-based carbides ACCr3 through the first-principles calculation based on density functional theory (DFT). The structure optimization shows that the lattice parameter of ACCr3 is basically proportional to the radius of A-site elements. The calculated formation energies show that AlCCr3 and GaCCr3
can be synthesized at ambient pressure and are stable with nonmagnetic ground states. Based on the calculation of elastic constants, some elastic, mechanical, and thermal parameters are derived and discussed. AlCCr3 and GaCCr3 show ductile natures and may have similar thermal properties. From the analysis of the electronic structures, it was found that there are electron and hole bands that cross the Fermi level for AlCCr3 and GaCCr3, indicating multiple-band natures. The Fermi level locates at the vicinity of the density of states (DOSs) peak, which leads to a large DOS at Fermi level dominated by Cr-3d electrons. The band structures of AlCCr3 and GaCCr3 are very similar to those of the superconducting antiperovskite MgCNi3. The similarity may make AlCCr3 and GaCCr3 behave superconductively, which needs to be further investigated in theoretical and experimental studies.
Abstract: We report the effect of Cr doping on the structural, magnetic, and electrical transport properties in Cr-doped antiperovskite compounds (). With increasing the Cr content , the lattice constant increases while both the Curie temperature and the saturated magnetization decrease gradually. The electrical resistivity shows different behaviors as a function of . For , there exists a semiconductor-like behavior below a certain temperature which decreases with increasing . In contrast, for , the resistivity shows a metallic behavior in the whole temperature measured (2–350 K). In particular, the Fermi-liquid behavior is obtained below 70 K. Finally, based on the magnetic and electrical properties of () a magnetic/electrical phase diagram was plotted.
Abstract: We synthesized Mn3Zn0.9Cu0.1N by solid state reaction, and magnetic as well as electrical transport properties were investigated. It is found that Mn3Zn0.9Cu0.1N exhibits a first-order antiferromagnetism (AFM) to paramagnetic (PM) transition with the Néel temperature ~163 K, and substitution of Cu for Zn would favor ferromagnetism (FM) state and weaken AFM ground state, leading to a convex curvature character of curve. With high external fields 10 kOe–50 kOe, magnetic transition remains a robust AFM-PM feature while FM phase is completely suppressed. Thermal hysteresis of under 500 Oe is also suppressed when the magnetic field exceeds 10 kOe. Mn3Zn0.9Cu0.1N exhibits a good metallic behavior except for a slope change around , which is closely related to AFM-PM magnetic transition. Compared with the first differential of resistivity with respect to temperature for in transition temperature range, the absolute value of is much lower which is close to zero.
Abstract: We investigate the high-temperature characteristics of InGaN/GaN multiple quantum well light-emitting devices with and without multiple quantum barriers (MQBs) in depth. The electroluminescence measurements were carried out over a temperature range from 200 to 380 K and an injection current level from 1 to 100 mA. Enhanced carrier confinement and stronger carrier localization in the active layer are achieved for the sample with MQBs. Furthermore, it is found that the external quantum efficiency of the sample possessing MQBs is higher than that of the sample with GaN barriers. The MQB structure improves the high-temperature operation of light-emitting devices.
Abstract: We calculate for La2CuO4 the phonon-induced redistribution of the electronic charge density in the insulating, the underdoped pseudogap, and the more conventional metallic state as obtained for optimal and overdoping, respectively. The investigation is performed for the anomalous high-frequency oxygen bond-stretching modes, which experimentally are known to display a strong softening of the frequencies upon doping in the cuprates. This most likely generic anomalous behaviour of these modes is shown to be due to a strong nonlocal electron-phonon interaction mediated by charge fluctuations on the ions. We demonstrate that the softening of the modes is caused by nonlocally induced dynamic charge inhomogeneities in form of charge stripes along the CuO bonds with different orbital character. The dynamic charge inhomogeneities may in turn be considered as precursors of static charge stripe order as recently observed in in a broad range of doping
around . The latter may trigger a reconstruction of the Fermi surface into
small pockets with reduced doping. We argue that the incompressibility of the orbital and simultaneously the compressibility of the orbital in the pseudogap state seem to be required to nucleate dynamic stripes.
Abstract: The dielectric and thermal properties of Bi (bismuth)-layered perovskite SrBi2Ta2O9 (SBT) are discussed in comparison with ferroelectric thin BaTiO3 films. Although these two perovskites exhibit quite a different nature, the dielectric properties of BaTiO3 thin film are similar to those in bulk SBT. The dielectric properties and pseudo-two-dimensional structure between SBT and thin film suggest that the bulk layered ferroelectric SBT is a good model of ultra-thin ferroelectric film with two perovskite layers, free from any misfit lattice strain with substrate and surface charge at the interface with electrodes. Based on the mechanism of ferroelectric phase transition of SBT, it seems plausible that the ferroelectric interaction is still prominent but shows a crossover from ferroelectric to antiferroelectric interaction in perovskite ultra-thin films along the tetragonal axis.
Abstract: A flexible bistable smectic-A liquid crystal (SmA LC) device using pixel-isolated mode was demonstrated, in which SmA LC molecules were isolated in pixels by vertical polymer wall and horizontal polymer layer. The above microstructure was achieved by using ultraviolet (UV) photolithography and photoinduced phase separation. The polymer wall was fabricated by photolithography, and then the SmA LC was encapsulated in pixels between polymer wall through UV-induced phase separation, in which the polymer wall acts as supporting structure from mechanical pressure and maintains the cell gap from bending, and the polymer layer acts as adhesive for tight attachment of two substrates. The results demonstrated that all the intrinsic bistable properties of the SmA LC are preserved, and good electrooptical characteristics such as high contrast ratio and excellent stability of the bistable states were characterized. This kind of SmA bistable flexible display has high potential to be used as electronic paper, smart switchable reflective windows, and so forth.
Abstract: This study presents a compound optical film to improve luminance and uniformity to apply in side-LED (light-emitting diode) backlight module. LIGA (lithographie galvanoformung abformung) technology, soft lithography, and homemade gray scale mask were combined to fabricate microlens array. Optical film with variable size microlens array embedded with microvoids was designed and manufactured. FRED software was used to simulate optical performance. Microvoids were quantitatively embedded in the PDMS (polydimethylsiloxane) optical film. Under the quantitative control of air pressure, those microvoids inside the optical film can cause light diffusion. The compound optical film with embedded microvoids, multiaspect ratio, and variable size microlens array can be fabricated quickly without substrate. Luminance colorimeter BM-7A from TOPCON was used to carry out the optical measurement. According to the measured data, the compound optical film with embedded microvoids can enhance the luminance up to 5% and the uniformity up to
6.5% ~ 8.4%. The optical efficiency can be improved via the compound optical film.
Abstract: Coalescence induced by deposition of different Cu clusters on an epitaxial Co cluster supported on a Cu(001) substrate is studied by constant-temperature molecular dynamics simulations. The degree of epitaxy of the final system increases with increasing separation between the centres of mass of the projectile and target clusters during the collision. Structure, roughness, and epitaxial order of the supported cluster also influence the degree of epitaxy. The effect of energy and temperature is determinant on the epitaxial condition of the coalesced cluster, especially both factors modify the generation, growth and interaction among grains. A higher temperature favours the epitaxial growth for low impact parameters. A higher energy contributes to the epitaxial coalescence for any initial separation between the projectile and target clusters. The influence of projectile energy is notably greater than the influence of temperature since higher energies allow greater and instantaneous atomic reorganizations, so that the number of arisen grains just after the collision becomes smaller. The appearance of grain boundary dislocations is, therefore, a decisive factor in the epitaxial growth of the coalesced cluster.
Abstract: This paper presents a method which combines the statistical analysis with texture structural analysis called Local Binary Gray Level Cooccurrence Matrix (LBGLCM) to investigate the phase transition temperatures of thermotropic -alkyloxy benzoic acid (OBA, and 12) liquid crystals. Textures of the homeotropically aligned liquid crystal compounds are recorded as a function of temperature using polarizing optical microscope attached to the hot stage and high resolution camera. In this method, second-order statistical parameters (contrast, energy, homogeneity, and correlation) are extracted from the LBGLCM of the textures. The changes associated with the values of extracted parameters as a function of temperature are a helpful process to identify the phases and phase transition temperatures of the samples. Results obtained from this method have validity and are in good agreement with the literature.
Abstract: Two methods are used to simulate electronic structure of gallium arsenide nanocrystals. The cluster full geometrical optimization procedure which is suitable for small nanocrystals and large unit cell that simulates specific parts of larger nanocrystals preferably core part as in the present work. Because of symmetry consideration, large unit cells can reach sizes that are beyond the capabilities of first method. The two methods use ab initio Hartree-Fock and density functional theory, respectively. The results show that both energy gap and lattice constant decrease in their value as the nanocrystals grow in size. The inclusion of surface part in the first method makes valence band width wider than in large unit cell method that simulates the core part only. This is attributed to the broken symmetry and surface passivating atoms that split surface degenerate states and adds new levels inside and around the valence band. Bond length and tetrahedral angle result from full geometrical optimization indicate good convergence to the ideal zincblende structure at the centre of hydrogenated nanocrystal. This convergence supports large unit cell methodology. Existence of oxygen atoms at nanocrystal surface melts down density of states and reduces energy gap.
Abstract: Charge order affects most of the electronic properties but is believed not to alter the spin arrangement since the magnetic susceptibility remains unchanged. We present electron-spin-resonance experiments on quasi-one-dimensional salts ( PF6, AsF6, and SbF6), which reveal that the magnetic properties are modified below when electronic ferroelectricity sets in. The coupling of anions and organic molecules rotates the g-tensor out of the molecular plane creating magnetically nonequivalent sites on neighboring chains at domain walls. Due to anisotropic Zeeman interaction a novel magnetic interaction mechanism in the charge-ordered state is observed as a doubling of the rotational periodicity of .
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