Randomly branched polymers with quenched branchings, in which monomers interact with each other through the excluded volume interaction, are investigated by using an approach of conformation-space renormalization grou...
Randomly branched polymers with quenched branchings, in which monomers interact with each other through the excluded volume interaction, are investigated by using an approach of conformation-space renormalization group theory combined with the replica technique. The perturbation expansion for the mean-square radius of gyration is found and the scaling variable is identified. The renormalization group analysis shows that the critical exponents of the radius of gyration (ν=1/4+ε/40) and of the total number of configurations (θ=5/2-3ε/20) no longer obey the scaling law θ-1=(D-2)ν, where ε=8-D and D is the space dimensionality, which is tenable for randomly branched polymers with annealed branchings.
Reports on the feasibility of applying neural networks to the problem of forecasting the Canada/US spot exchange rate. The inputs to the network consist of the short-term trend in the spot rate (Monday-Thursday) and t...
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Reports on the feasibility of applying neural networks to the problem of forecasting the Canada/US spot exchange rate. The inputs to the network consist of the short-term trend in the spot rate (Monday-Thursday) and the change in interest rate spread between the two countries (Wednesday-Thursday). The output is a prediction of the exchange rate on Friday. The model is able to explain 6.6% of the variance in the data between November 1991 and June 1993 (out of the sample), yielding a success rate of 59% on directional forecasts.
The equilibrium state of a triangular pile of particles interconnected by linear springs and subjected to the force of gravity is investigated numerically. The three strain components describing the deformation of the...
The equilibrium state of a triangular pile of particles interconnected by linear springs and subjected to the force of gravity is investigated numerically. The three strain components describing the deformation of the pile, the density change, and the supporting stress field are calculated. The results (a) exhibit an unexpected richness in behavior and (b) show evidence of a thermodynamic limit in small (∼100 layers) piles.
The effects of many-body interactions in the static structure of dense gaseous krypton (for 0.9-2.1 rho(c), where rho(c) is the critical density) and xenon (for 2.2-2.6 rho(c)) were investigated in the momentum transf...
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The effects of many-body interactions in the static structure of dense gaseous krypton (for 0.9-2.1 rho(c), where rho(c) is the critical density) and xenon (for 2.2-2.6 rho(c)) were investigated in the momentum transfer range 4 less than or equal to Q less than or equal to 42 nm(-1), through comparisons of the measured structure factors S(Q) with those calculated through the MHNC-CRS integral equation by Reatto and Tau. One property investigated is the small increase in the effective atomic diameter with increasing density. While the observed variation, as determined through the position of the main diffraction maxima, is similar to that given by the inclusion of the Axilrod-Teller triple-diploe interaction in the calculation, further small deviations exist which indicate the need for additional many-body terms.
An improvement of our previous extended mean-field theory of the liquid-vapour interface of water is described. This revision extends the range of validity of the theory to room temperature. The current theory is impr...
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An improvement of our previous extended mean-field theory of the liquid-vapour interface of water is described. This revision extends the range of validity of the theory to room temperature. The current theory is improved in three aspects: (1) the effective angle-averaged pair potential is obtained by the RAM (reference-averaged Mayer) function approximation, avoiding expansion in inverse powers of temperature;(2) die soft repulsive core of the TIP4P water model pair potential is treated accurately;and (3) the anisotropic interactions are represented by a higher-order (l = 5) multipole expansion. The current theory includes no adjustable parameters;the critical temperature T(c) is found to be 609 K. Detailed calculations are given for room temperature, where molecular-dynamics simulation results are available for comparison. A more detailed representation of the orientational distribution function is reported.
Hexagonal ZnO films with different observed orientations were deposited on glass substrates at temperatures below 200-degrees-C by r.f. sputtering of a ZnO target using an oxygen and argon gas mixture. Orientations wi...
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Hexagonal ZnO films with different observed orientations were deposited on glass substrates at temperatures below 200-degrees-C by r.f. sputtering of a ZnO target using an oxygen and argon gas mixture. Orientations with the (002) planes of the crystallites parallel to the substrate, or with the (100) planes parallel to the substrate, or films in which the (101) peak is dominant in their X-ray diffraction spectra have been obtained by selecting the deposition parameters. The partial pressure of the oxygen in the sputtering gas, the distance between target and substrate, the substrate temperature during film nucleation and the input power each affect (to varying degrees) the observed crystallite orientation obtained. Polycrystalline ZnO films made in the presence of excess oxygen or at a high sputtering gas pressure tend to have the c axis parallel to the substrate and they exhibit a large photoresponse, while films sputter-deposited with a low partial pressure of oxygen tend to have the c axis perpendicular to the substrate and may have no detectable d.c. photoresponse. The availability of oxygen during sample preparation has a significant affect on the slow photoresponse of the resulting ZnO films.
Annealing ZnO films prepared by ion-beam-assisted deposition (IBAD) in air above 773 K can change the resistivity from almost-equal-to 10(-4) OMEGA cm to more than 10(6) OMEGA cm, changing the films from a highly dege...
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Annealing ZnO films prepared by ion-beam-assisted deposition (IBAD) in air above 773 K can change the resistivity from almost-equal-to 10(-4) OMEGA cm to more than 10(6) OMEGA cm, changing the films from a highly degenerate transparent material to a thermally activated semiconductor. No change in the transmission through the visible region of the spectrum (more than 90%) is observed but the optical band gap can change from almost-equal-to 3.41 to almost-equal-to 3.24 eV, as a result of a Burstein-Moss shift. However, for films that are not initially very transparent, the average transmission can be increased from almost-equal-to 28% to more than 85%. High temperature annealing increases the average size of crystallites by almost-equal-to 15%. After annealing, a large photoresponse is observed, with the conductivity increasing by more than a factor of 10(5) when the film is illuminated with UV light (almost-equal-to 4 x 10(16) photons cm-2 s-1). The rise and decay time constants are very large, indicating that previously reported surface structural changes occur. These constants strongly depend on the intensity of the light and the ambient gases present. Any true photoconductiity present appears to be very small. Encapsulating the free surface of a ZnO film with a layer of amorphous SiN(x) can help to stabilize the film, and it decreases the photoresponse time as well as the magnitude of the photoresponse.
Amorphous hydrogenated silicon germanium thin films with 20% Ge content (a-Si0.8Ge0.2:H) were deposited by ion beam-assisted deposition (IBAD), using SiH4 as the source gas. Moderate ion energies of similar to 125 to ...
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Amorphous hydrogenated silicon germanium thin films with 20% Ge content (a-Si0.8Ge0.2:H) were deposited by ion beam-assisted deposition (IBAD), using SiH4 as the source gas. Moderate ion energies of similar to 125 to 135 eV and an ion-to-atom ratio of similar to 1, produced samples with a reduced number of SiH2 and (SiH2)(n) bonding configurations, an increased hydrogen content and a decreased porosity compared with non-bombarded samples. This reduces the disorder in the a-SiGe network as well as the defect density. The preferential attachment coefficient for these films is near unity. The optical and electrical properties of the a-SiGe:H alloy have been studied as a function of the ion beam energy and the ion-to-atom ratio at the substrate. The best films had a photosensitivity of sigma(ph)/sigma(d) = 1.7 x 10(4) and E(g)= 1.65 eV. The ion beam increases the optical gap, lowers the dark conductivity and increases the electrical activation energy.
An electron cyclotron resonance plasma-assisted deposition method has been used to prepare polycrystalline GaN films. These GaN films can have dark conductivities as low as 10(-10) S cm(-1) and they exhibit well-defin...
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An electron cyclotron resonance plasma-assisted deposition method has been used to prepare polycrystalline GaN films. These GaN films can have dark conductivities as low as 10(-10) S cm(-1) and they exhibit well-defined conductivity activation energies. The sample conductivity is decreased by surface adsorption of gases and it increases again when the adsorbed species is photodesorbed from the surface using UV light. As a result, the photoresponse is very slow, especially the decay process, which may require several days for a sample to recover when it is mounted in a vacuum. This recovery time can be reduced significantly if the sample temperature is increased or the sample is exposed to air. In this respect, these GaN films have a number of features in common with ZnO.
Dynamic light scattering is an established technique for measuring the average size and size distribution of particles in a suspension. The technique has the advantage of being fast and non-invasive, but it does requi...
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Dynamic light scattering is an established technique for measuring the average size and size distribution of particles in a suspension. The technique has the advantage of being fast and non-invasive, but it does require low particle concentrations. As well, dynamic light scattering results are often open to misinterpretation if one is unaware of the state of the sample and the method of data analysis. The following discussion reviews some of the basic concepts of dynamic light scattering and outlines some of the pitfalls that are often encountered in data interpretation. A modification of dynamic light scattering, diffusing wave spectroscopy, can be used to obtain approximate size information at higher particle concentrations. The fundamentals of this new technique are summarized.
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