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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Macromolecules 17 (1984), S. 1998-2005 
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Journal of Polymer Science: Polymer Chemistry Edition 22 (1984), S. 2777-2788 
    ISSN: 0360-6376
    Keywords: Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In contradiction to reports from this and other laboratories, this study reports that the integrated Mayo-Lewis equation, or Meyer-Lowery equation, adequately describes the high-conversion free radical copolymerization of styrene and methylmethacrylate. The copolymerization was monitored by following the changes in the feed composition using NMR, as well as determination of the resulting copolymer compositions by NMR and UV. “Error in all Variables” statistical techniques were used to produce estimates of the reactivity ratios. The reactivity ratios estimated were, from feed composition, NMR, r1 (styrene) = 0.472, r2 = 0.454, from copolymer composition, UV, r1 = 0.497, r2 = 0.464, and NMR, r1 = 0.432, r2 = 0.422.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Publication Date: 1984-10-01
    Print ISSN: 0024-9297
    Electronic ISSN: 1520-5835
    Topics: Chemistry and Pharmacology , Physics
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  • 4
    Publication Date: 2015-12-31
    Description: The energy levels created in supersaturated n-type silicon substrates with titanium implantation in the attempt to create an intermediate band in their band-gap are studied in detail. Two titanium ion implantation doses (10 13  cm -2 and 10 14  cm -2 ) are studied in this work by conductance transient technique and admittance spectroscopy. Conductance transients have been measured at temperatures of around 100 K. The particular shape of these transients is due to the formation of energy barriers in the conduction band, as a consequence of the band-gap narrowing induced by the high titanium concentration. Moreover, stationary admittance spectroscopy results suggest the existence of different energy level configuration, depending on the local titanium concentration. A continuum energy level band is formed when titanium concentration is over the Mott limit. On the other hand, when titanium concentration is lower than the Mott limit, but much higher than the donor impurity density, a quasi-continuum energy level distribution appears. Finally, a single deep center appears for low titanium concentration. At the n-type substrate, the experimental results obtained by means of thermal admittance spectroscopy at high reverse bias reveal the presence of single levels located at around E c -425 and E c -275 meV for implantation doses of 10 13  cm −2 and 10 14  cm −2 , respectively. At low reverse bias voltage, quasi-continuously distributed energy levels between the minimum of the conduction bands, E c and E c -450 meV, are obtained for both doses. Conductance transients detected at low temperatures reveal that the high impurity concentration induces a band gap narrowing which leads to the formation of a barrier in the conduction band. Besides, the relationship between the activation energy and the capture cross section values of all the energy levels fits very well to the Meyer-Neldel rule. As it is known, the Meyer-Neldel rule typically appears in processes involving multiple excitations, like carrier capture and emission in deep levels, and it is generally observed in disordered systems. The obtained Meyer-Neldel energy value, 15.19 meV, is very close to the value obtained in multicrystalline silicon samples contaminated with iron (13.65 meV), meaning that this energy value could be associated to the phonons energy in this kind of substrates.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 5
    Publication Date: 2015-10-20
    Description: Some geometric and topological implications of noncommutative Wilson loops are explored via the Seiberg-Witten map. In the abelian Chern-Simons theory on a three-dimensional manifold, it is shown that the effect of noncommutativity is the appearance of new knots at the th order of the Seiberg-Witten expansion. These knots are trivial homology cycles which are Poincaré dual to the higher-order Seiberg-Witten potentials. Moreover the linking number of a standard 1-cycle with the Poincaré dual of the gauge field is shown to be written as an expansion of the linking number of this 1-cycle with the Poincaré dual of the Seiberg-Witten gauge fields. In the process we explicitly compute the noncommutative “Jones-Witten” invariants up to first order in the noncommutative parameter. Finally in order to exhibit a physical example, we apply these ideas explicitly to the Aharonov-Bohm effect. It is explicitly displayed at first order in the noncommutative parameter; we also show the relation to the noncommutative Landau levels.
    Print ISSN: 1687-9120
    Electronic ISSN: 1687-9139
    Topics: Mathematics , Physics
    Published by Hindawi
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  • 6
    Publication Date: 2015-04-18
    Description: Author(s): Diego Oliver, Jose H. Garcia, Tatiana G. Rappoport, N. M. R. Peres, and Felipe A. Pinheiro We consider resonant scatterers with large scattering cross sections in graphene that are produced by a gated disk or a vacancy, and show that a gated ring can be engineered to produce an efficient electron cloak. We also demonstrate that this same scheme can be applied to tune the direction of elec... [Phys. Rev. B 91, 155416] Published Fri Apr 17, 2015
    Keywords: Surface physics, nanoscale physics, low-dimensional systems
    Print ISSN: 1098-0121
    Electronic ISSN: 1095-3795
    Topics: Physics
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  • 7
    Publication Date: 2013-01-17
    Description: Pool-riffle sequences are geomorphological features of many streams, thought to contribute to the hydrodynamic variability necessary to support healthy habitat conditions. Due to this fact, the addition of artificial pools and riffles is a common alternative for restoration projects on channelized streams. In this paper, detailed three-dimensional (3-D) flow measurements conducted on a scale model of an existing pool-riffle design implemented as part of a restoration project is presented. The design incorporated the basic features of natural pool-riffle sequences but maintained the deepest part of the pool in the center of the cross section and away from the banks. Results showed that the 3-D flow patterns were qualitatively different for two discharge conditions tested. The lower discharge case was strongly affected by the topography, displaying a pattern consistent with a secondary flow generated by the curvature of the streamlines. The higher discharge case was less affected by the topography, presenting a secondary flow pattern similar to that observed over a flat bed and typically associated with turbulence anisotropy. Self-maintenance and flow variability were also investigated. Even though convergence of the values of bed shear stresses at pool and riffle sections with increasing discharge did take place, reversal conditions did not occur. The difference in flow structure with flow stage was also reflected in the spatial flow variability, the lower discharge displaying larger variability than the higher discharge. The higher discharge generated a level of variability comparable with the values obtained over a flat bed.
    Print ISSN: 0043-1397
    Electronic ISSN: 1944-7973
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
    Publication Date: 2012-04-15
    Description: The lake levels in Lake Michigan-Huron have recently fallen to near historical lows, as has the elevation difference between Lake Michigan-Huron compared to Lake Erie. This decline in lake levels has the potential to cause detrimental impacts on the lake ecosystems, together with social and economic impacts on communities in the entire Great Lakes region. Results from past work suggest that morphological changes in the St Clair River, which is the only natural outlet for Lake Michigan-Huron, could be an appreciable factor in the recent trends of lake level decline. A key research question is whether bed erosion within the river has caused an increase in water conveyance, therefore, contributed to the falling lake level. In this paper, a numerical modeling approach with field data is used to investigate the possibility of sediment movement in the St Clair River and assess the likelihood of morphological change under the current flow regime. A two-dimensional numerical model was used to study flow structure, bed shear stress, and sediment mobility/armoring over a range of flow discharges. Boundary conditions for the numerical model were provided by detailed field measurements that included high-resolution bathymetry and three-dimensional flow velocities. The results indicate that, without considering other effects, under the current range of flow conditions, the shear stresses produced by the river flow are too low to transport most of the coarse bed sediment within the reach and are too low to cause substantial bed erosion or bed scour. However, the detailed maps of the bed show mobile bedforms in the upper St Clair River that are indicative of sediment transport. Relatively high shear stresses near a constriction at the upstream end of the river and at channel bends could cause local scour and deposition. Ship-induced propeller wake erosion also is a likely cause of sediment movement in the entire reach. Other factors that may promote sediment movement, such as ice cover and dredging in the lower river, require further investigation. Copyright © 2012 John Wiley & Sons, Ltd.
    Print ISSN: 0197-9337
    Electronic ISSN: 1096-9837
    Topics: Geography , Geosciences
    Published by Wiley
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  • 9
    Publication Date: 2012-09-13
    Description: Past analytical studies of meander planform development have mostly focused on the complexity of the governing equations, i.e., hydrodynamics, and less so on the stream bank resistance to erosion, whose spatial heterogeneity is difficult to describe deterministically. This motivated the use of a Monte Carlo approach to examine the effects of floodplain soils and their distribution on planform development, with the goal of including bank erosion properties in the analysis. Simulated bank erosion rates are controlled by the resistance to hydraulic erosion of the bank soils using an excess shear stress approach. The spatial distribution of critical shear stress across the floodplain is delineated on a rectangular, equidistant grid with varying degrees of variability. The corresponding erodibility coefficient is computed using a field-derived empirical relation. For a randomly disturbed distribution, in which the mean resistance to erosion exponentially increases away from the valley centerline, two relevant parameters are identified: the standard deviation of the critical shear stress distribution, which controls skewness and variability of the channel centerline, and the cross-valley increase in soil resistance, which constrains lateral migration and also affects bend skewness. For a purely random distribution, migrated centerlines exhibit larger variability for increasing spatial scales of floodplain soil heterogeneity. For equal stochastic variability of the corresponding governing parameters, relating meander migration to hydraulic erosion of the bank soils produces more variability and shape complexity than the “classic” bank migration approach of Ikeda et al. (1981), which relates migration rate to excess velocity at the outer bank. Finally, the proposed stochastic approach provides a foundation for estimating a suitable spatial density of measurements to characterize the physical properties of floodplain soils and vegetation.
    Print ISSN: 0043-1397
    Electronic ISSN: 1944-7973
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 10
    Publication Date: 2012-05-19
    Description: Wind stress over the ocean depends on the sea surface roughness which is determined by the sea state. On one hand, underdeveloped wind seas, rougher than their fully developed counterpart, increase the drag. On the other hand, the presence of swell can modify wind stress by modifying the wind sea roughness. This latter mechanism is believed to have a great impact at high winds whenever underdeveloped local waves coexist with swell. Detailed measurements of wind stress and wavefield in fetch-limited growth conditions were made in an area subjected to strong and persistent winds. Through the analysis of wavefield observations, it is found that the presence of swell dampens the short wind waves. The observed attenuation is greater for younger wind seas and decreases as the wind waves become older. Results obtained from modeling the interaction of wind waves and the air flow above point out that the attenuation of short wind waves causes a reduction of the wave-supported stress, which in turn decreases the total wind stress.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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