Search Results - (Author, Cooperation:X. Chu)

2018-01-03

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Biological Evolution ; Geologic Sediments/chemistry ; History, Ancient ; Iron/*analysis/chemistry ; Oceans and Seas ; Seawater/*chemistry ; Sulfur/analysis/chemistry ; Sulfur Isotopes ; Time Factors

2018-10-12

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2018-06-02

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2018-03-06

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2018-01-20

The American Society for Pharmacology and Experimental Therapeutics (ASPET)

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Chemistry and Pharmacology

Medicine

1089-7550

AIP Digital Archive

Physics

Magnetotransport experiments on several n-type Hg1−xZnxTe-CdTe superlattices reveal the existence of two species of charge carriers which dominate in low and in high magnetic fields. These superlattices exhibit low-magnetic-field Shubnikov–de Haas oscillations and Hall plateaus in one superlattice. They also exhibit a metal-insulator transition due to an energy gap induced by the high magnetic fields. The induced energy gap is estimated using a simplified parallel-conduction model. When most of the electrons in the superlattices have been magnetically frozen out, a second species of charge carriers starts dominating the charge transport. The second species of charge carriers shows large amplitude Shubnikov–de Haas oscillations and Hall plateaus. The standard determination of effective masses, mobilities, and carrier densities from the Shubnikov–de Haas oscillations in low and in high magnetic fields also confirms the presence of two species of charge carriers of different origin. It is proposed that the high-magnetic-field Shubnikov–de Haas oscillations are due to electrons residing at the buffer-superlattice interface on the superlattice side.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We consider chemical reactions which at equilibrium have multiple stationary states due to nonidealities of chemical species. When such reactions are included in a simple reaction mechanism open to mass flow, without autocatalysis or feedback steps, there may occur complex dynamics such as relaxation oscillations, as reported earlier for regular solutions. Here we consider both regular solution and ionic species (Debye–Hückel nonideality), show that chemical oscillations may occur arbitrarily close to chemical equilibrium, and trace the topological structure of the complex dynamics of relaxation oscillations, sustained oscillations, stable focus, and stable nodes to the multiplicity of equilibrium states, for stated constraints. Relaxation oscillations occur around an unstable stationary state which, on approach to equilibrium, connects to an unstable equilibrium state. Thus, there is no bifurcation to oscillations on removing the systems from equilibrium. Neither is there a region where linear irreversible thermodynamics is valid close to equilibrium. Earlier work on ionic systems is found to be in error.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The microstructures of chemically polymerized polypyrrole films (100–800 A(ring) thick) were studied by transmission electron microscopy, electron diffraction, and x-ray photoelectron spectroscopy. In contrast to previous work where only amorphous polypyrrole was found, crystalline fiber structures were observed in the chemically polymerized thin films. The fibers are embedded in an amorphous matrix which forms a self-reinforced composite. The shape of the fibers ranged from thin rods to ellipsoids depending on the preparation conditions. The density and size of the fibers were affected by the polymerization time and the concentration ratio of pyrrole and oxidants. Polypyrrole fibers were aligned along the thin-film plane and were randomly oriented in the plane. The two-dimensional orientation of the crystalline fibers produced strongly anisotropic electrical properties in the thin films. © 1995 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Advancement of a fine slit along a planar grain boundary in an electric field E0, applied parallel to the slit, is investigated by considering electromigration along both the grain boundary and the slit surface. Electrically induced flux in the grain boundary Igb (+ toward the slit tip) and both electrically and curvature-induced fluxes on the slit surfaces are considered assuming 2Is〉Igb, where Is is the flux (+ away from the slit tip) on each of the parallel slit surfaces far removed from the tip. Steady-state solutions of the transport equations are classified according to the value of a parameter β=tan−1 (2Is/Igb) which, under reasonable assumptions, depends on material parameters only. For 5π/4≥β≥β2, unique steady-state solutions exist; for β2〉β〉β1, multiple steady-state solutions occur; below β1≥π/4, no steady-state solution is possible. Since β1〈π/2, Igb〉0 (flux exiting the grain boundary into the slit) for all cases in which no steady-state solution is possible. In the case of multiple solutions, those corresponding to smallest width (and hence largest velocity) are determined. For all steady-state solutions, slit width and tip velocity scale as E−1/20 and E3/20, respectively. Results also apply to the propagation of a slit within a grain or along a passivation layer. Generally, tip velocities can approach 1 nm/s (3.6 μm/h), thereby representing a likely failure mechanism in fine-line (near bamboo structure) interconnects. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A transmission electron microscopy study of CrN0.6/TiN superlattices deposited by reactive magnetron sputtering is described. The stable structure of CrN0.60 is hexagonal, but high resolution transmission electron microscopy images of the superlattices showed that CrN0.6 layers ≤10 nm thick were cubic, while 50 nm thick layers were hexagonal. That is, the cubic CrN structure was "epitaxially stabilized" by the cubic TiN, with which there is a 2.4% lattice mismatch. The superlattices with hexagonal CrN0.6 showed high strains and defect densities within (approximate)5 nm of each interface, presumably due to the 5.4% volume decrease associated with the cubic-to-hexagonal transformation. The effect of this strain on the transformation is discussed. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report on the observation of stimulated emission from a (100) oriented Hg1−xCdxTe epilayer grown by molecular beam epitaxy. The cleaved epilayers were cooled and optically pumped by a Nd:YAG laser and were found to lase continuously up to 40 K.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Hg1−xCdxTe films with 2 in. diameters have been grown by molecular beam epitaxy on GaAs (100) substrates. These films were grown in both the (100) and (∼(111)) B crystallographic orientations and in both conduction types. They were characterized by in situ electron diffraction, infrared absorption, and van der Pauw dc Hall measurements. Their surfaces were shiny and mirrorlike from center to edge. The Cd concentrations (x) of these films were very uniform, exhibiting standard deviations (Δx) as low as 0.7% of the mean (x¯). Their thicknesses also were uniform within 0.6%. These films were completely uniform in their conduction types; that is, the n-type films were entirely n type, and likewise for the p-type films. The Hall mobilities of these films show them to be of high quality, with values as high as 6.7×102 cm2 V−1 s−1 for the p-type (x=0.22) and 1.8×105 cm2 V−1 s−1 for the n-type films (x=0.21). These results represent an important achievement toward the future of infrared detector technology.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

We investigate here the influence of the crystallographic orientation of the CdTe substrate on the condensation coefficients of Hg, Cd, and Te during the growth of Hg1−xCdxTe and CdTe by molecular-beam epitaxy. We show that the Hg condensation coefficient is strongly influenced by the orientation. A CdTe (1¯1¯1¯)B face requires about one order of magnitude less mercury than a (111)A face when growth occurs at 185 °C. Whereas for a CdTe(100) face, the Hg condensation coefficient falls in between. Even though the effect is less dramatic for the condensation coefficient of cadmium, a similar tendency is observed. These results can be explained in terms of the bonding of the surface atoms and confirm once again that the surface plays an important role in the molecular-beam-epitaxial growth process.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The structural properties of [001] ZnTe epitaxial layers grown on [001] GaAs substrates were investigated by transmission electron microscopy. The layers were deposited by molecular-beam epitaxy with thicknesses ranging from 50 to 1800 nm. The layers can be divided into three distinct regions of dislocation arrangements above the interface. The interface consists of an array of misfit dislocations. The separation of these dislocations is about 54 A(ring) and is independent of layer thickness. The first region extends 300 nm towards the surface and consists of a tangle of dislocations. The density of these dislocations increases with layer thickness. The second region, between 600 and 1300 nm above the interface, was found to contain a low density of dislocations. Above 1300 nm, the third region, the dislocation density surprisingly increases again and in addition Te precipitates are detected near the top surface.

Electronic Resource