The behavior of liquid crystalline polymers in the interfacial region between the isotropic and nematic phases is investigated based on an inhomogeneous free-energy functional. A mean-field approximation is used for t...
The behavior of liquid crystalline polymers in the interfacial region between the isotropic and nematic phases is investigated based on an inhomogeneous free-energy functional. A mean-field approximation is used for the system of semiflexible polymers obeying the Saito-Takahashi-Yunoki description and interacting via the Onsager-type repulsive interaction. The density distribution of the polymers crossing the interface is computed by using a spherical-harmonics expansion. The calculated interfacial tension is consistent with the results of the scaling argument which is also presented in this paper.
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.
Interfacial structure in the isotropic phase of a liquid-crystalline material near a wall is studied by a mean-field density-functional theory. With increasing strength of the wall anchoring potential, the theory pred...
Interfacial structure in the isotropic phase of a liquid-crystalline material near a wall is studied by a mean-field density-functional theory. With increasing strength of the wall anchoring potential, the theory predicts a first-order transition from incomplete to complete wetting by the smectic-A phase at bulk isotropic-smectic coexistence, with an associated prewetting transition occurring away from bulk coexistence. The incomplete wetting case is accompanied by a small number (between 0 and and 2) of discrete layer transitions, while an infinite number of such transitions occurs at complete wetting. An analysis of the underlying physical mechanisns for layer transitions reveals that these transitions tend to disappear as the system is moved both sufficiently close to and sufficiently far from the bulk isotropic–nematic–smectic-A triple point by varying the model coupling parameters. These results reconcile findings from previous theories and experiments.
We examine a three-dimensional vector lattice model for water, oil, and amphiphile mixtures, using the mean-field and Bethe approximations as well as Monte Carlo methods. The Monte Carlo results exhibit three-phase co...
We examine a three-dimensional vector lattice model for water, oil, and amphiphile mixtures, using the mean-field and Bethe approximations as well as Monte Carlo methods. The Monte Carlo results exhibit three-phase coexistence between water-rich, oil-rich, and microemulsion phases, where the microemulsion has low amphiphile concentration (≊15%) in agreement with experiment. Also present is a floating incommensurate lamellar phase which coexists with the microemulsion at slightly higher amphiphile concentration, similar to lamellar phases observed experimentally. These features are absent in the two mean-field-like approximations and we conclude that, in general, these types of approximations are inappropriate for describing mixtures containing efficient amphiphiles.
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.
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.
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.
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.
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.
The Raman and infrared wavenumbers of solid HCONH2 and DCOND2 were measured at 20 K and high temperatures. More peaks were observed in both the lattice and internal mode regions than were expected from the known struc...
The Raman and infrared wavenumbers of solid HCONH2 and DCOND2 were measured at 20 K and high temperatures. More peaks were observed in both the lattice and internal mode regions than were expected from the known structure, so the structure was redetermined. The resulting structure is in agreement with earlier determinations, so the extra peaks are attributed to second-order effects produced by a large anharmonicity associated with the hydrogen bonds. Attempts to model the structural details and the lattice dynamics had only a limited success, which may again be attributed to anharmonic effects and to the basic inadequacies of the models.
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