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Caltech Authors (167.915 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 41 - 50 de 50

  1. Improved electrical properties of wafer-bonded p-GaAs/n-InP interfaces with sulfide passivation

    Nakayama, Keisuke; Tanabe, Katsuaki; Atwater, Harry A.
    Sulfide-passivated GaAs and InP wafers were directly bonded to explore the efficiency of sulfide passivation on the bonded interfacial properties. We find that the bonded GaAs/InP interfaces after sulfide passivation contain sulfur atoms and a decreased amount of oxide species relative to the pairs bonded after conventional acid treatment; however, the residual sulfur atoms have no effect on the bonding strength. The electrical properties of the bonded p-GaAs/n-InP heterojunctions were studied for different acceptor concentrations in p-GaAs. A reduced interfacial trap state density enhances the tunnel current flow across the depletion layer in the sulfide-passivated case. A directly bonded tunnel...

  2. Expression of an atrial G-protein-activated potassium channel in Xenopus oocytes

    Dascal, Nathan; Lim, Nancy F.; Schreibmayer, Wolfgang; Wang, Weizhen; Davidson, Norman; Lester, Henry A.
    Injection of rat atrial RNA into Xenopus oocytes resulted in the expression of a guanine nucleotide binding (G) protein-activated K+ channel. Current through the channel could be activated by acetylcholine or, if RNA encoding a neuronal 5HT1A receptor was coinjected with atrial RNA, by serotonin (5HT). A 5HT-evoked current (I5HT) was observed in oocytes injected with ventricle RNA fractions (of 2.5-5.5 kb) and 5HT1A receptor RNA. I5HT displayed strong inward rectification with very little conductance above the K+ equilibrium potential, was highly selective for K+ over Na+, and was blocked by 5-300 µM Ba2+. I5HT was suppressed by intracellular injection...

  3. A synthetic Escherichia coli predator–prey ecosystem

    Balagaddé, Frederick K.; Song, Hao; Ozaki, Jun; Collins, Cynthia H.; Barnet, Matthew; Arnold, Frances H.; Quake, Stephen R.; You, Lingchong
    We have constructed a synthetic ecosystem consisting of two Escherichia coli populations, which communicate bi-directionally through quorum sensing and regulate each other's gene expression and survival via engineered gene circuits. Our synthetic ecosystem resembles canonical predator–prey systems in terms of logic and dynamics. The predator cells kill the prey by inducing expression of a killer protein in the prey, while the prey rescue the predators by eliciting expression of an antidote protein in the predator. Extinction, coexistence and oscillatory dynamics of the predator and prey populations are possible depending on the operating conditions as experimentally validated by long-term culturing of...

  4. Direct thermal conductance measurements on suspended monocrystalline nanostructures

    Tighe, T. S.; Worlock, J. M.; Roukes, M. L.
    We describe and demonstrate a new class of devices that enable direct thermal conductance measurements on monocrystalline nanostructures. These are possible through our newly developed techniques for three-dimensional, successive surface nanomachining of GaAs-based heterostructures. Our methods allow the patterning of complex devices comprising electrically insulating, mesoscopic thermal conductors with separate, thermal transducers in situ. Intimate thermal contact between these elements is provided by their epitaxial registry. Low-temperature thermal conductance measurements indicate that phonon boundary scattering in these initial nanometer is scale structures is partially specular. These devices offer promise for ultrasensitive bolometry and calorimetry.

  5. Evolution of a curved vortex filament into a vortex ring

    Moin, Parviz; Leonard, Anthony; Kim, John
    The deformation of a hairpin-shaped vortex filament under self-induction and in the presence of shear is studied numerically using the Biot–Savart law. It is shown that the tip region of an elongated hairpin vortex evolves into a vortex ring and that the presence of mean shear impedes the process. In addition, evolution of a finite-thickness vortex sheet under self-induction is investigated using the Navier–Stokes equations. The layer evolves into a hairpin vortex, which in turn produces a vortex ring of high Reynolds stress content. These results indicate a mechanism for the generation of ring vortices in turbulent shear flows, and...

  6. Evolution of a curved vortex filament into a vortex ring

    Moin, Parviz; Leonard, Anthony; Kim, John
    The deformation of a hairpin-shaped vortex filament under self-induction and in the presence of shear is studied numerically using the Biot–Savart law. It is shown that the tip region of an elongated hairpin vortex evolves into a vortex ring and that the presence of mean shear impedes the process. In addition, evolution of a finite-thickness vortex sheet under self-induction is investigated using the Navier–Stokes equations. The layer evolves into a hairpin vortex, which in turn produces a vortex ring of high Reynolds stress content. These results indicate a mechanism for the generation of ring vortices in turbulent shear flows, and...

  7. Efficient multiphoton dissociation of CF3I+ in the metastable X-tilde 2E1/2 excited state using cw infrared laser radiation

    Thorne, L. R.; Beauchamp, J. L.
    Evidence for the multiphoton dissociation of an electronic metastable state of CF3I+ with CO2 laser radiation is presented. Only those ions possessing sufficient internal excitation before irradiation can be dissociated. Photodissociation results primarily from resonant absorption by the nu1 vibrational mode of CF3I+ in the higher of the two, spin–orbit levels of the ground electronic state Xbar^2 E1/2. Only the lowest energy decomposition channel is observed; this yields CF + 3 and I. Spin–orbit and vibrational relaxation compete with photodissociation. At CF3I pressures below 2×10^−7 Torr, the collisional relaxation rate is proportional to CF3I pressure kc = (3.1±0.4)×10^−9 cm^3 molecule^−3...

  8. Vibrational overtone initiated unimolecular dissociation of HOCH_2OOH and HOCD_2OOH: Evidence for mode selective behavior

    Matthews, Jamie; Fry, Julie L.; Roehl, Coleen M.; Wennberg, Paul O.; Sinha, Amitabha
    The vibrational overtone induced unimolecular dissociation of HMHP (HOCH2OOH) and HMHP-d2 (HOCD2OOH) into OH and HOCH2O (HOCD2O) fragments is investigated in the region of the 4nuOH and 5nuOH bands. The unimolecular dissociation rates in the threshold region, corresponding to the 4nuOH band, exhibit measurable differences associated with excitation of the OH stretch of the alcohol versus the peroxide functional group, with the higher energy alcohol OH stretching state exhibiting a slower dissociation rate compared to the lower energy peroxide OH stretch in both HMHP and HMHP-d2. Predictions using the Rice–Ramsperger–Kassel–Marcus theory give rates that are in reasonably good agreement with...

  9. Synthesis of immunoglobulin µ chain gene products precedes synthesis of light chains during B-lymphocyte development

    Siden, E.; Alt, F. W.; Shinefeld, L.; Sato, V.; Baltimore, D.
    Immunoglobulin (Ig) gene expression has been followed during the later stages of development of the murine fetal liver. Biosynthetic labeling and immunoprecipitation were used to isolate Ig-related polypeptides from fetal and neonatal livers. By examination of the specific immune precipitates, the earliest detectable Ig was shown to consist only of µ heavy chain. At about the time of birth, when light chain synthesis became evident, separation of surface Ig-positive cells from surface Ig-negative cells by using anti-Ig-coated dishes showed that cells lacking surface Ig (pre-B lymphocytes) synthesized only µ chains. Thus, commencement of light chain synthesis was closely coordinated with...

  10. Acidity enhances the formation of a persistent ozonide at aqueous ascorbate/ozone gas interfaces

    Enami, Shinichi; Hoffmann, M. R.; Colussi, A. J.
    The pulmonary epithelium, like most aerial biosurfaces, is naturally protected against atmospheric ozone (O3) by fluid films that contain ascorbic acid (AH2) and related scavengers. This mechanism of protection will fail, however, if specific copollutants redirect AH2 and O3(g) to produce species that can transduce oxidative damage to underlying tissues. Here, the possibility that the synergistic adverse health effects of atmospheric O3(g) and acidic particulate matter revealed by epidemiological studies could be mediated by hitherto unidentified species is investigated by electrospray mass spectrometry of aqueous AH2 droplets exposed to O3(g). The products of AH2 ozonolysis at the relevant air–water interface...

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