Cesium ordering and electron localization-delocalization phenomena in the quasi-one-dimensional diphosphate tungsten bronzes: Cs1‒xP8W8O40

We present a structural investigation of the family of quasi-one-dimensional (quasi-1D) conductors, which exhibit intriguing charge transport properties where, for x small, the conductivity exhibits a crossover from a semiconducting to a metallic like regime when the temperature decreases. In these materials the double zig-zag chains, together with the diphosphate groups, delimit channels which are partially filled with the ions. It is found, from an X-ray diffuse scattering investigation, that at room temperature the ions are locally ordered on a lattice of well-defined sites in the channel direction and not ordered between neighboring channels. These ions form 1D incommensurate concentration waves whose periodicity depends on the stoichiometry. In upon cooling, the intrachannel order increases significantly, and an interchannel order between the 1D concentration waves develops. But, probably because of kinetic effects, no tridimensional (3D) long range order of the ions is achieved at low temperature. The 3D low-temperature local order has been determined and it is found that the phase shift between the concentration waves minimizes their Coulomb repulsions. This local order is increasingly reduced as the Cs concentration diminishes. We interpret the intriguing features of the electrical conductivity in relationship with the thermal evolution of the Cs ordering effects. We suggest that in , for x small, a localization-delocalization transition of the Anderson type occurs due to the thermal variation of the Cs disorder. When x increases, the enhancement of the disorder leads to a localization of the electronic wave function in the whole temperature range measured. Finally, and probably because of the disorder, no charge density wave instability is revealed by our X-ray diffuse scattering investigation. Received: 10 October 1997 / Received in final form: 11 December 1997 / Accepted: 16 December 1997     

Optimal coupled-cluster approximation for the ground-state properties of a spin-boson model

In the present work we have developed an optimal coupled-cluster approximation, which can take care of both the accuracies of the ground-state energy and the wavefunction estimates, for the ground state of a two-state system coupled to a dispersionless boson bath. This new approach is also able to give a tight upper bound to the ground-state energy of the system. Up to the fourth level of this approximation our results show excellent agreement with the numerical exact diagonalization results. In particular, our results suggest no discontinuous localization-delocalization transition of the two-state system. This is consistent with the exact result. Received: 11 March 1998 / Accepted: 23 June 1998     

The analytical investigation of the super-Gaussian pump source on the thermal, stress and thermo-optics properties of double-clad Yb:glass fiber lasers

Fiber lasers have attracted considerable attention when their power can realistically be scaled to kilowatt level and beyond. In this paper, we assumed that the fiber core and first clad are exposed to a pump source with a super-Gaussian profile of order four. The effects of this non-uniform heat deposition on thermal, stress and thermooptics properties such as temperature-dependent change of refractive index and thermally induced stress have been comprehensively studied and their equations analytically derived.     

Effect of surface electric field on the anchoring of nematic liquid crystals

We analyse the influence of adsorbed ions and the resulting surface electric field and its gradient on the anchoring properties of nematics with ionic conductivity. We take into account two physical mechanisms for the coupling of the nematic director with the surface electric field: (i) the dielectric anisotropy and (ii) the coupling of the quadrupolar component of the flexoelectric coefficient with the field gradient. It is shown that for sufficiently large fields near saturated coverage of the adsorbed ions, there can be a spontaneous curvature distortion in the cell even when the anchoring energy is infinitely strong. We also discuss the director distortion when the anchoring energy of the surface is finite. Received: 29 September 1997 / Received in final from: 10 November 1997 / Accepted: 18 November 1997     

Synthesis,photochemical and phase behavior of linear and hyperbranched photoactive benzylidene liquid‐crystalline polyesters

Photoactive hyperbranched benzylidene liquid‐crystalline polyester (PAHBP) and photoactive linear benzylidene liquid‐crystalline polyester (PALBP) were synthesized by solution polycondensation with pyridine as an acid acceptor. PAHBP and PALBP were thoroughly characterized with Fourier transform infrared, ¹H and ¹³C NMR, ultraviolet–visible spectrophotometry, fluorescent spectrophotometry, gel permeation chromatography, thermogravimetric analysis, differential scanning calorimetry, and polarized optical microscopy. Both polymers exhibited nematic mesophase. The glass‐transition temperature and liquid‐crystalline isotropic temperature of PAHBP were higher than those of PALBP. During photolysis under ultraviolet light, both polymers underwent an intermolecular photocycloaddition reaction, and the photoactivity of PAHBP was faster than that of PALBP; this was further confirmed by photoviscosity studies. PALBP and PAHBP were fluorescent in nature. An increase in the fluorescence intensity with the time of ultraviolet‐light irradiation was observed for both PAHBP and PALBP. The rate of increase in the fluorescence intensity of the linear analogue (PALBP) was higher than that of the hyperbranched polymer (PAHBP). This behavior could be attributed to the attainment of better planarity in the case of the linear one but not in the case of PAHBP because of the rapid crosslinking of PAHBP leading to an irregular architecture. This behavior was further confirmed by the calculation of the steric energy from corresponding model compounds. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3986–3994, 2006     

Quantum states of hydrogen (H, D, T) atoms on Cu(1 0 0) and (1 1 0) surfaces

We investigate the quantum mechanical behavior of adsorbed hydrogen (H, D, T) on Cu(1 0 0) and (1 1 0) surfaces. We construct potential energy surfaces (PESs) for the motion of the hydrogen H atom on Cu(1 0 0) and (1 1 0) surfaces within the framework of density functional theory. The potential energy takes a minimum value on the hollow site of Cu(1 0 0) and on the short bridge site of Cu(1 1 0). Moreover, we calculate the quantum states of hydrogen atom motion on these calculated PESs. The ground state wave function of the hydrogen atom motion is strongly localized around the hollow site on the Cu(1 0 0) surface. On the other hand, the ground state wave function of the hydrogen atom motion on Cu(1 1 0) is distributed from the short bridge site to two neighboring pseudo-threefold sites. We finally show isotope effects on the quantum states of the motion of hydrogen on both surfaces.     

On rates of convergence of information theoretic criterion in rank determination of one-way random effects models

In this paper, upper bounds on the probabilities of wrong determination of the rank of covariance matrix of random effects in one-way random effects models are given, based on the information theoretic criterion. Under weak conditions, the bounds are shown of exponential-type.The work was partially supported by Natural Sciences and Engineering Research Council of Canada and Faculty Research Grant, Faculty of Arts, York University.     

Cavitation in superfluid helium-4 at low temperature

We have studied the nucleation of bubbles in pure superfluid helium-4 at temperatures down to 65 mK. We have found that the nucleation is a stochastic process, and that at temperatures below 600 mK the nucleation rate is independent of temperature. These results are consistent with the assumption that the nucleation takes place via quantum tunneling. Received: 15 November 1997 / Received in final form: 19 December 1997 / Accepted: 22 January 1998     

Residual thermal strain of metals in heating-cooling cycles

Summary Metallic samples subjected to heating-cooling cycles undergo a permanent change in their length. The difference between the initial length and the length at the end of the cycle (residual strain) depends on the maximum temperature attained during the cycle. This dependence of residual strain on the maximum temperature of the cycle is similar for all of the examined metals (polycrystalline copper, aluminium, iron, gold, lead and single-crystal copper). Upon further increasing this temperature, the metal residual strain changes from positive to negative values. The temperature at which the maximum positive residual strain occurs is characteristic for each metal. The value of the residual strain and the temperature at which it changes its sign depend on the previous thermal history of the metal.     

纳米材料二氧化锡的制备和激子态光学特性研究

报道纳米材料二氧化锡的制备和室温下激子态光学特征，应用弱量子限域下激子响应模型对结果进行了解释。