New class of T′-structure cuprate superconductors

High-temperature superconductivity has been discovered in La_2−xBa_xCuO₄ [J.G. Bednorz, K.A. Müller, Z. Phys. B 64 (1986) 189. [1]], a compound that derives from the undoped La₂CuO₄ crystallizing in the perovskite T-structure. In this structure oxygen octahedra surround the copper ions. It is common knowledge that charge carriers induced by doping in such an undoped antiferromagnetic Mott-insulator lead to high-temperature superconductivity [V.J. Emery, Phys. Rev. Lett. 58 (1987) 2794; C.M. Varma, S. Schmitt-Rink, E. Abrahams, Solid State Commun. 62 (1987) 681; E. Dagotto, Rev. Mod. Phys. 66 (1994) 763. [2], [3] and [4]]. The undoped material La₂CuO₄ is also the basis of the electron-doped cuprate superconductors [Y. Tokura, H. Takagi, S. Uchida, Nature (London) 337 (1989) 345. [5]] of the form La_2−xCe_xCuO_4+y [M. Naito, M. Hepp, Jpn. J. Appl. Phys. 39 (2000) L485; A. Sawa, M. Kawasaki, H. Takagi, Y. Tokura, Phys. Rev. B 66 (2002) 014531. [6] and [7]] which, however, crystallize in the so-called T′-structure, i.e. without apical oxygen above or below the copper ions of the CuO₂-plane. It is well known that for La_2−xCe_xCuO_4+y the undoped T′-structure parent compound cannot be prepared due to the structural phase transition back into the T-structure occurring around x∼0.05. Here, we report that if La is substituted by RE=Y, Lu, Sm, Eu, Gd, or Tb, which have smaller ionic radii but have the same valence as La, nominally undoped La_2−xRE_xCuO₄ can be synthesized by molecular beam epitaxy in the T′-structure. The second important result is that all these new T′-compounds are superconductors with fairly high critical temperatures up to 21 K. For this new class of cuprates La_2−xRE_xCuO₄, which forms the T′-parent compounds of the La-based electron doped cuprates, we have not been able to obtain the Mott-insulating ground state for small x before the structural phase transition into the T-structure takes place.     

Tie2 is tied at the cell-cell contacts and to extracellular matrix by Angiopoietin-1

Angiopoietin-1 (Ang1) binds to and activates Tie2 receptor tyrosine kinase. Ang1-Tie2 signal has been proposed to exhibit two opposite roles in the controlling blood vessels. One is vascular stabilization and the other is vascular angiogenesis. There has been no answer to the question as to how Tie2 induces two opposite responses to the same ligand. Our group and Dr. Alitalo''s group have demonstrated that trans-associated Tie2 at cell-cell contacts and extracellular matrix (ECM)-anchored Tie2 play distinct roles in the endothelial cells. The complex formation depends on the presence or absence of cell-cell adhesion. Here, we review how Ang1-Tie2 signal regulates vascular maintenance and angiogenesis. We further point to the unanswered questions that must be clarified to extend our knowledge of vascular biology and to progress basic knowledge to the treatment of the diseases in which Ang1-Tie2-mediated signal is central.     

A crack in a piezoelectric layer bonded to a dissimilar elastic layer under transient load

Summary  A piezoelectric layer bonded to the surface of an elastic structure is considered. The piezoelectric and the elastic layers are infinite along the x-axis and have finite thickness in the y-direction. The polarization direction of the piezoelectric material is along the y-axis. By means of the method of singular integral equations, the solution in a Laplace transform plane is demonstrated. Laplace inversion yields the results in the time domain. Numerical values of the crack tip fields under in-plane transient electromechanical loading are obtained. The influence of layers thickness on stress and electric displacement intensity factors is investigated. Received 16 March 2000; accepted for publication 16 August 2000     

Synthesis and properties of fluorene or carbazole‐based alternating copolymers containing Si and vinylene units in the main chain

Alternating copolymerization of 9,9‐dihexyl‐2,7‐dibromofluorene (HFl), N‐hexyl‐2,7‐dibromocarbazole (2,7‐HCz) or N‐hexyl‐3,6‐dibromocarbazole (3,6‐HCz) with Si‐containing divinyl or diallyl compounds, divinyldimethylsilane, divinyldiphenylsilane, 1,3‐divinyltetramethyldisiloxane, 1,4‐bis(dimethylvinylsilyl)benznene, diallyldimethylsilane, or diallyldiphenylsilane has been investigated using Mizoroki‐Heck reaction with a Pd catalyst. The corresponding alternating copolymers were obtained in good yield. The alternating copolymers from HFl or 2,7‐HCz showed good solubility in typical organic solvents. On the other hand, the copolymer with 3,6‐HCz became insoluble due to the crosslinking. Photophysical properties of the resulting copolymers were investigated with UV–vis absorption and photoluminescence spectroscopy. All the copolymers showed absorption peak derived from π–π* transition at around 340 nm, which was blue shifted in comparison with that of the corresponding homopolymer. Whereas emission peaks of the copolymers of the cast film were red shifted in comparison with that of the homopolymers. Multiple broad absorption peaks, which would be derived from intramolecular charge transfer through σ–π moiety, were also detected in the range from 390 to 450 nm in the spectra of the copolymers containing Si‐vinylene unit. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4513–4521, 2008     

Efficient and stable tin-based perovskite solar cells by introducing π-conjugated Lewis base

Science China Chemistry - Tin-based perovskite solar cells (TPSCs) as the most promising candidate for lead-free PSCs have incurred extensive researches all over the world. However, the...     

Transient thermal stresses in a transversely isotropic semi infinite circular cylinder with an asymmetric temperature distribution on the end surface and a convective heat loss

Summary In this paper, the three-dimensional transient thermal stress problem in a transversely isotropic semi-infinite circular cylinder subjected to an asymmetric temperature distribution on the end surface and convective heat losses into the surrounding medium from the surfaces is analyzed by means of the modified transversely isotropic potential function method.As a numerical example, the temperature and thermal stress distributions are calculated for a graphite which belongs under a transverse isotropy, and compared with those for isotropy. The effects of various heating conditions on the temperature and thermal stresses are presented.

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Transiente Wärmespannungen in einem transvers-isotropen, halbunendlichen Kreiszylinder mit asymmetrischer Temperaturverteilung an der Stirnfläche und mit Wärmestrahlung

Übersicht In dieser Arbeit wird das dreidimensionale Wärmespannungsproblem in einem transvers-isotropen, halbunendlichen Kreiszzylinder mit asymmetrischer Temperaturverteilung an der Stirnfläche und mit Wärmestrahlung untersucht. Dabei wird eine modifizierte Potentialfunktionsmethode angewendet.In einem numerischen Beispiel werden die Temperatur- und die Spannungsverteilungen für Graphit berechnet und mit dem isotropen Fall verglichen. Der Einfluß von verschiedener Erwärmung auf die Temperatur und die Wärmespannungen wird ebenfalls angegeben.

     

Gravitational effects on global hemodynamics in different postures: A closed-loop multiscale mathematical analysis

We present a novel methodology and strategy to predict pressures and flow rates in the global cardio-vascular network in different postures varying from supine to upright. A closed-loop, multiscale mathematical model of the entire cardiovascular system (CVS) is developed through an integration of one-dimensional (1D) modeling of the large systemic arteries and veins, and zero-dimensional (0D) lumped-parameter modeling of the heart, the cardiac-pulmonary circulation, the cardiac and venous valves, as well as the microcirculation. A versatile junction model is proposed and incorporated into the 1D model to cope with splitting and/or merging flows across a multibranched junc-tion, which is validated to be capable of estimating both subcritical and supercritical flows while ensuring the mass conservation and total pressure continuity. To model grav-itational effects on global hemodynamics during postural change, a robust venous valve model is further established for the 1D venous flows and distributed throughout the entire venous network with consideration of its anatomically real-istic numbers and locations. The present integrated model is proven to enable reasonable prediction of pressure and flow rate waveforms associated with cardiopulmonary circu-lation, systemic circulation in arteries and veins, as well as microcirculation within normal physiological ranges, partic-ularly in mean venous pressures, which well match the in vivo measurements. Applications of the cardiovascular model at different postures demonstrate that gravity exerts remarkable influence on arterial and venous pressures, venous returns and cardiac outputs whereas venous pressures below the heart level show a specific correlation between central venous and hydrostatic pressures in right atrium and veins.     

Fiber devices using polarization- maintaining fibers

Fiber devices using polarization-maintaining fibers called PANDA fibers are presented. They are polarization-maintaining couplers and polarization-splitting couplers, optical isolators and optical circulators, and multi/demultiplexers designed for 1.3 μm wavelength. Crosstalk of-32 dB and excess loss of 0.03 dB for the polarization-maintaining coupler, and polarization-splitting ratio of 17 dB and excess loss of 0.5 dB for the polarization-splitting coupler have been fabricated by a fusion-elongation method. A fiber polarizer with the extinction ratio of more than 40 dB has been presented by using the difference of bending loss between the orthogonal modes in the PANDA fibers. Multi/demultiplexer with narrow band-pass of 1.4 nm utilizing combination of PANDA fiber polarization dispersion and the polarization-splitting coupler has been realized. An optical isolator consisting of fiber polarizers and a spherical YIG with a lens function and an optical circulator consisting of polarizing-splitting couplers and spherical YIGs have been achieved.     

Dual nature of the electronic structure of (La(2--x--y)Nd(y)Sr(x))CuO(4) and La(1.85)Sr(0.15)CuO(4).

High resolution angle-resolved photoemission measurements have been carried out on (La(1.4--x)-Nd(0.6)Sr(x))CuO(4), a model system with static one-dimensional (1D) charge ordering (stripe), and (La(1.85)-Sr(0.15))CuO(4), a high temperature superconductor (T(c) = 40 K) with possible dynamic stripes. In addition to the straight segments near ( pi,0) and ( 0,pi) antinodal regions, we have identified the existence of spectral weight along the [1,1] nodal direction in the electronic structure of both systems. This observation of nodal state, together with the straight segments near antinodal regions, reveals the dual nature of the electronic structure of stripes due to the competition of order and disorder.     

Structural properties of GaAs nanostructures formed by a supply of intense As4 flux in droplet epitaxy

We investigated detailed structural properties of GaAs nanostructures formed by a supply of intense As₄ flux to Ga droplets. Scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (TEM) revealed that whisker-like nanostructures had formed on the truncated cone-shaped bases after crystallization. Moreover, electron energy loss spectroscopy in scanning transmission electron microscopy (STEM-EELS) revealed that elemental Ga atoms remained inside the nanostructures while outside, some had crystallized into GaAs. These findings suggest that crystallization started at the edges of the droplets and the GaAs grew upward along the periphery of the droplets until the droplets were completely covered with crystallized GaAs.