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rss_1.0 Recursos de colección

Caltech Authors (99,106 recursos)
Repository of works by Caltech published authors.

Type = Article

Mostrando recursos 1 - 20 de 38,651

1. Metal contact double injection in GaAs - Mead, C. A.
Recent studies on the GaAs injection laser have indicated certain problems associated with the localization of the region of population inversion to the immediate vicinity of the p-n junction. It has also been pointed out that very heavy doping is necessary of laser action. However, in heavily doped material the absorption of light is large, raising the threshold current for laser action. These problems can be overcome simultaneously by using a metal-GaAs-metal structure operating in the double injection mode described by Lampert. A metal of low work function is used to inject electrons at one surface and one of high...

2. Physical properties of artificial extracellular matrix protein films prepared by isocyanate crosslinking - Nowatzki, Paul J.; Tirrell, David A.
Artificial extracellular matrix proteins, genetically engineered from elastin- and fibronectin-derived repeating units, were crosslinked with hexamethylene diisocyanate in dimethylsulfoxide. The resulting hydrogel films were transparent, uniform, and highly extensible. Their tensile moduli depended on crosslinker concentration and spanned the range characteristic of native elastin. The water content of the films was low (∼27%), but the temperature-dependent swelling behavior of the crosslinked materials was reminiscent of the lower critical solution temperature property of the soluble polymers.

3. Endothelial cell adhesion to the fibronectin CS5 domain in artificial extracellular matrix proteins - Heilshorn, Sarah C.; DiZio, Kathleen A.; Welsh, Eric R.; Tirrell, David A.
This study examines the spreading and adhesion of human umbilical vein endothelial cells (HUVEC) on artificial extracellular matrix (aECM) proteins containing sequences derived from elastin and fibronectin. Three aECM variants were studied: aECM 1 contains lysine residues periodically spaced within the protein sequence and three repeats of the CS5 domain of fibronectin, aECM 2 contains periodically spaced lysines and three repeats of a scrambled CS5 sequence, and aECM 3 contains lysines at the protein termini and five CS5 repeats. Comparative cell binding and peptide inhibition assays confirm that the tetrapeptide sequence REDV is responsible for HUVEC adhesion to aECM proteins...

4. A Hierarchical Timing Simulation Model - Lin, Tzu-Mu; Mead, Carver A.
A hierarchical timing simulation model has been developed to deal with VLSI designs at any level of representation. A set of physically based parameters are used to characterize the behavior and timing of a semantic design object (cell) independent of its composition environment. As cells are composed, the parameters of the composite cell can be determined from those of the component cells either analytically or by simulation. Based on this model, a behavior-level simulator has been developed and combined with other tools to form an integrated design system that fully supports the structured design methodology.

5. Immunomicrospheres: reagents for cell labeling and separation - Rembaum, A.; Dreyer, W. J.
Immunomicrospheres are specially designed microscopic particles that have antibodies or similar molecules chemically bound to their surfaces. The antibody-coated microspheres react in a highly specific way with target cells, viruses, or other antigenic agents. Immunomicropheres may be synthesized so that they incorporate compounds that are highly radioactive, intensely fluorescent, magnetic, electron opaque, highly colored, or pharmacologically active. These various types of microspheres may be coated with pure, highly specific monoclonal antibodies obtained by the new hybridoma cell cloning techniques or with conventional antibody preparations. Some of the many present and potential applications for these new reagents are (i) new types...

6. Silicon strain gages for cryostatic temperature use - Hamill, G. P.; Vreeland, T., Jr.
Doped silicon piezo-resistive strain gages have been used to monitor torsional stress waves at temperatures down to 44 K. These gages had the commonly employed dopant level of about 1.3 X 10^(18) acceptors/cm^3. At temperatures below approximately 38 K, the gage resistance increases as electrons freeze out of the conduction band, and the resistivity becomes too high to make use of the piezo-resistive effect. At approximately 1 X 10^(19) acceptors/cm^3, the silicon loses its semiconductor character and becomes a pure resistor with piezo-resistive properties. The room temperature gage constant decreases with an increase in doping level, but data on the gage constant down to 4.2 K were not available. This note reports the use of...

7. Etching patterns in high purity zinc - Frank, F. C.; Wills, H. H.; Vreeland, Thad, Jr.
Servi (1958) has observed certain ridges on an otherwise smooth surface produced by the action of a special etching agent on zinc. He interprets these ridges as showing the course of dislocation lines pre-existing in the specimen, suggesting that accumulated solute near the dislocation lines has made the metal more etch resistant. No other interpretation seems probable, but Servi confesses to being unable to interpret certain features in his second picture, which appears to show a 'Frank-Read source' in operation.

8. Preyield plastic and anelastic microstrain in low-carbon steel - Vreeland, T., Jr.; Wood, D. S.; Clark, D. S.
This paper presents the results of an experimental investigation of the behavior of an annealed low-carbon steel subjected to rapidly applied constant stresses less than the static upper yield stress. Sensitive measurements of plastic and anelastic microstrain were made and the relationship between stress and equilibrium microstrain determined. The experimentally observed microstrain is quantitatively explained by a dislocation-generating mechanism. The equilibrium number of dislocation loops produced by a Frank-Read source is estimated as a function of the applied stress and the dislocation source length. Comparison of the theory and the experimental data leads to the determination of a characteristic length of...

9. Deep Hubble Space Telescope observations of blue star clusters in NGC 3597 - Carlson, Matthew N.; Holtzman, Jon A.; Grillmair, Carl J.; Mould, Jeremy R.; Griffiths, Richard E.; Ballester, Gilda E.; Burrows, Christopher J.; Clarke, John T.; Crisp, David; Evans, Robin W.; Gallagher, John S., III; Hester, J. Jeff; Hoessel, John G.; Scowen, Paul A.; Stapelfeldt, Karl R.; Trauger, John T.; Watson, Alan M.; Westphal, James A.
We have analyzed HST/WFPC2 images of NGC 3597 and find ~700 compact objects surrounding the galaxy with an average (B-R)_0~0.6. We propose that the majority of these objects are young globular clusters. They have a spread in colors that is consistent with that expected for a population of young clusters with a common age and spread induced by photometric errors and reddening within NGC 3597. If these objects were similar to the Galactic globular cluster system seen at a younger age, we would predict a turnover in the luminosity function at B~23. However, we find that the luminosity function for...

10. Partially pinned tilt boundaries - Vreeland, T., Jr.
This paper presents the results of a theoretical study of the conditions under which small angle tilt boundaries in a crystal can be moved when subjected to a stress which does not produce general slip. The dislocations in the boundary are assumed to be partially pinned by Cottrell atmospheres, intersecting substructure, or precipitates along the dislocation lines. The results show that partially pinned dislocation boundaries may move at lower stresses than similarly pinned isolated dislocations if the density of pinned segments is sufficiently low. Stress concentrations at pinned segments on a small angle boundary are discussed. A “yield condition” for...

11. Structure/property evaluation and comparison between shock-wave consolidated and hot-isostatically pressed compacts of RSP pyromet 718 alloy powders - Thadhani, N. N.; Mutz, A. H.; Vreeland, T., Jr.
Shock-wave processing of a nickel-based superalloy (Pyromet 718) powder is explored and compared to conventional hot-isostatic processing. The shock consolidated powder has extensively deformed particle interiors, A fine microcrystalline phase was observed at interparticle regions, and is attributed to rapid solidification of melt formed during the consolidation process. The shock consolidated material has a yield strength of 1 GPa prior to heat treatment, which is at least 10% higher than that of the hot isostatically pressed alloy. A high density of very fine disk-shaped γ″ precipitates nucleate in the shock consolidated material after solution treatment and upon aging at 620°C...

12. Stereographic X-ray reflection topography of dislocations in zinc - Vreeland, T., Jr.
X-ray reflection topographs were taken of a zinc surface oriented about 1° from a basal plane. Basal dislocations are revealed in the topographs, and their apparent depth was determined using stereo pairs of topographs. The apparent depths observed in a complimentary pair of topographs using 1013 and 1013 reflections were significantly greater than those observed in an asymmetric pair of topographs in which the same 1013 reflection was used. This difference is attributed to shifts of the image with respect to the dislocation position. Quantitative estimates of the image shifts and the actual depths of the observed dislocations are obtained...

13. Comparisons Between Seismic Earth Structures and Mantle Flow Models Based on Radial Correlation Functions - Jordan, Thomas H.; Puster, Peter; Glatzmaier, Gary A.; Tackley, Paul J.
Three-dimensional numerical simulations were conducted of mantle convection in which flow through the transition zone is impeded by either a strong chemical change or an endothermic phase change. The temperature fields obtained from these models display a well-defined minimum in the vertical correlation length at or near the radius where the barrier is imposed, even when the fields were filtered to low angular and radial resolutions. However, evidence for such a feature is lacking in the shear-velocity models derived by seismic tomography. This comparison suggests that any stratification induced by phase or chemical changes across the mid-mantle transition zone has...

14. Information processing in the primate visual system: an integrated systems perspective - Van Essen, David C.; Anderson, Charles H.; Felleman, Daniel J.
The primate visual system contains dozens of distinct areas in the cerebral cortex and several major subcortical structures. These subdivisions are extensively interconnected in a distributed hierarchical network that contains several intertwined processing streams. A number of strategies are used for efficient information processing within this hierarchy. These include linear and nonlinear filtering, passage through information bottlenecks, and coordinated use of multiple types of information. In addition, dynamic regulation of information flow within and between visual areas may provide the computational flexibility needed for the visual system to perform a broad spectrum of tasks accurately and at high resolution.

15. Membrane Transport Mechanisms Probed by Capacitance Measurements With Megahertz Voltage Clamp - Lu, Chin-Chih; Kabakov, Anatoli; Markin, Vladislav S.; Mager, Sela; Frazier, Gary A.; Hilgemann, Donald W.
We have used capacitance measurements with a 1-µs voltage clamp technique to probe electrogenic ion-transporter interactions in giant excised membrane patches, The hydrophobic ion dipicrylamine was used to test model predictions for a simple charge-moving reaction. The voltage and frequency dependencies of the apparent dipicrylamine-induced capacitance, monitored by 1-mV sinusoidal perturbations, correspond to single charges moving across 76% of the membrane field at a rate of 9500 s^(-1) at 0 mV. For the cardiac Na,K pump, the combined presence of cytoplasmic ATP-and sodium induces an increase of apparent membrane capacitance which requires the presence of extracellular sodium, The dependencies of...

16. Diffusive Isotopic Contamination of Mafic Magma by Coexisting Silicic Liquid in the Muskox Intrusion - Stewart, Brian W.; DePaolo, Donald J.
Shifts in ^(87)Sr/^(86)Sr and ^(143)Nd/^(144)Nd ratios measured in cumulates from the upper levels of the Muskox mafic intrusion indicate that isotopic and bulk chemical exchange were decoupled across a mafic-silicic liquid interface during crystallization of the intrusion. Modeling of diffusive exchange between liquid layers demonstrates that isotopic compositions of silicate liquids in layered magma chambers may be strongly affected by this process on time scales of 10^3 to 10^4 years. Diffusive contamination can be used to place constraints on the physical processes and time scales of magmatic systems.

17. Shock wave consolidation of a Ni-Cr-Si-B metallic glass powder - Bach, J.; Krueger, B.; Fultz, B.
We studied parameters for the shock wave consolidation of a metallic glass powder, such as the effects of the shock wave energy and the shock wave duration. Microstructural characterizations of the compacts showed that some of them remained amorphous with porosity of about 1%, and that the shock wave propagated in a planar fashion causing large particle deformations. For our experimental parameters the energy range for a successful consolidation was between 140 and 200 J/g.

18. Structure of Complex of Synthetic HIV-1 Protease with a Substrate-Based Inhibitor at 2.3 Å Resolution - Miller, M.; Schneider, J.; Sathyanarayana, Bangalore K.; Toth, Mihaly V.; Marshall, Garland R.; Clawson, Leigh; Selk, Linda; Kent, Stephen B. H.; Wlodawer, Alexander
The structure of a complex between a peptide inhibitor with the sequence N-aceti-Thr-Ile-Nle-t[CH2-NH]-Nle-ψ[CH_2-NH]Nel-Gln-Arg.amide (Nle,norleucine) with chemically synthesized HIV-1 (human immunodeficiency virus 1) protease was determined at 2.3 Å resolution (R factor of 0.176). Despite the symmetric nature of the unliganded enzyme, the asymmetric inhibitor lies in a single orientation and makes extensive interactions at the interface between the two subunits of the homodimeric protein. Compared with the unliganded enzyme, the protein molecule underwent substancial changes, particularly in an extended region corresponding to the "flaps"(residues 35 to 57 in each chain), where backbone movements as large as 7 Å are observed.

19. Network Codes Resilient to Jamming and Eavesdropping - Yao, Hongyi; Silva, Danilo; Jaggi, Sidharth; Langberg, Michael
We consider the problem of communicating information over a network secretly and reliably in the presence of a hidden adversary who can eavesdrop and inject malicious errors. We provide polynomial-time distributed network codes that are information-theoretically rate-optimal for this scenario, improving on the rates achievable in prior work by Ngai et al. Our main contribution shows that as long as the sum of the number of links the adversary can jam (denoted by Z_O) and the number of links he can eavesdrop on (denoted by Z_I) is less than the network capacity (denoted by C) (i.e., Z(O) + Z_I <...

20. A Requirement for Local Protein Synthesis in Neurotrophin-Induced Hippocampal Synaptic Plasticity - Kang, Hyejin; Schuman, Erin M.
Two neurotrophic factors, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), are able to produce a long-lasting enhancement of synaptic transmission in the hippocampus. Unlike other forms of plasticity, neurotrophin-induced plasticity exhibited an immediate requirement for protein synthesis. Plasticity in rat hippocampal slices in which the synaptic neuropil was isolated from the principal cell bodies also required early protein synthesis. Thus, the neurotrophins may stimulate the synthesis of proteins in either axonal or dendritic compartments, allowing synapses to exert local control over the complement of proteins expressed at individual synaptic sites.

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