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  • Wiley-Blackwell  (955)
  • American Institute of Physics (AIP)  (522)
Document type
  • 1
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 74 (1999), S. 3326-3328 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present real-time surface x-ray scattering measurements during homoepitaxial growth of GaN by metal-organic chemical vapor deposition. We observed intensity oscillations corresponding to the completion of each monolayer during layer-by-layer growth. The growth rate was found to be temperature independent and Ga-transport limited. Transitions between step-flow, layer-by-layer, and three-dimensional growth modes were determined as a function of temperature and growth rate. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2013-08-13
    Description: Current interferon-based therapy for hepatitis C virus (HCV) infection is inadequate, prompting a shift towards combinations of direct-acting antivirals (DAA) with the first protease-targeted drugs licensed in 2012. Many compounds are in the pipeline yet primarily target only three viral proteins, namely NS3/4A protease, NS5B polymerase and NS5A. With concerns growing over resistance, broadening the repertoire for DAA targets is a major priority. Here we describe the complete structure of the HCV p7 protein as a monomeric hairpin, solved using a novel combination of chemical shift and NOE based methods. This represents atomic resolution information for a full-length virus-coded ion channel, or “viroporin”, whose essential functions represent a clinically proven class of antiviral target exploited previously for influenza A virus therapy. Specific drug-protein interactions validate an allosteric site on the channel periphery and its relevance is demonstrated by the selection of novel, structurally diverse inhibitory small molecules with nanomolar potency in culture. Hit compounds represent a 10,000-fold improvement over prototypes, suppress rimantadine resistance polymorphisms at sub-micromolar concentrations and show activity against other HCV genotypes. Conclusions : This proof-of-principle that structure-guided design can lead to drug-like molecules affirms p7 as a much needed new target in the burgeoning era of HCV DAA. (H epatology 2013;)
    Print ISSN: 0270-9139
    Electronic ISSN: 1527-3350
    Topics: Medicine
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  • 3
    Publication Date: 2014-01-31
    Description: [1]  The nitrogen and oxygen isotopic composition of nitrate in ice cores offers unique potential for examining reactive nitrogen oxide (NO x ) budgets and oxidation chemistry of past atmospheres. A low-latitude record is of particular interest given that the dominant natural sources of NO x and production of hydroxyl radical are most prevalent in the Tropics. Any interpretation of nitrate in ice cores, however, must first consider that nitrate in snow is vulnerable to post-depositional loss and isotopic alteration. We report and assess the integrity of nitrate–δ 15 N, –δ 18 O and –Δ 17 O in a 30 m ice core from a high-elevation site in the central Andes. Clear seasonality in δ 15 N, δ 18 O and nitrate concentration exists throughout most of the record and cannot be explained by photolysis or evaporation based on our current understanding of these processes. In contrast, nitrate in the upper ~12 m of the core and in a snowpit shows very different behavior. This may reflect alteration facilitated by recent melting at the surface. The relationships between δ 15 N, δ 18 O, Δ 17 O and concentration in the unaltered sections can be interpreted in terms of mixing of nitrate from discrete sources. Transport effects and an englacial contribution from nitrification cannot be ruled out at this time, but the observed isotopic compositions are consistent with expected signatures of known NO x sources and atmospheric oxidation pathways. Specifically, nitrate deposited during the wet season reflects biogenic soil emissions and hydroxyl/peroxy radical chemistry in the Amazon while dry season deposition reflects a lightning source and ozone chemistry at higher levels in the Troposphere.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 4
    Publication Date: 2017-09-19
    Description: Data collected between 1974 and 2016 from snow pits and core samples from two Peruvian ice fields demonstrate the effect of the recent warming over the tropical Andes, augmented by El Niño, on the preservation of the climate record. As the 0 o C isotherm is approaching the summit of the Quelccaya ice cap in the Andes of southern Peru (5670 masl), the distinctive seasonal δ 18 O oscillations in the fresh snow deposited within each thermal year are attenuated at depth due to melting and percolation through the firn. This has become increasingly pronounced over 43 years. In the Andes of northern Peru, the ice field on the col of Nevado Huascarán (6050 masl) has retained its seasonal δ 18 O variations at depth due to its higher elevation. During the 2015/16 El Niño, snow on Quelccaya and Huascarán was isotopically (δ 18 O) enriched and the net sum of accumulation over the previous year (NSA) was below the mean for non-El Niño years, particularly on Quelccaya (up to 64% below the mean) which was more pronounced than the NSA decrease during the comparable 1982/83 El Niño. Interannual large-scale oceanic and middle to upper level atmospheric temperatures influence δ 18 O in precipitation on both ice fields, although the influences are variably affected by strong El Niño-Southern Oscillation events, especially on Quelccaya. The rate of ice wastage along Quelccaya's margin was dramatically higher during 2015/16 compared with that of the previous 15 years, suggesting that warming from future El Niños may accelerate mass loss on Peruvian glaciers.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 3725-3733 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The solidification of Si-As alloys induced by pulsed-laser melting was studied at regrowth velocities where the partition coefficient is close to unity. The congruent melting temperatures T0 of Si-As alloys were determined using a temperature measurement technique developed for this work, and was confirmed with T0 measurements using three other methods. The time-resolved temperature measurement uses a thin-film platinum thermistor, below and electrically isolated from the Si-As alloy layer, to directly measure the temperature during solidification. The other techniques compared the results of heat flow simulations with the fluence dependence of the peak melt depth obtained by transient conductance, the fluence dependence of the melt duration determined from time-resolved reflectivity and transient conductance, and the fluence threshold for the initiation of melting. This combination of measurements in conjunction with Rutherford backscattering spectrometry permitted the determination of the solid-liquid interface temperature, velocity and partition coefficient, the latent heat of fusion and T0 for Si-4.5 at. % As and Si-9 at. % As alloys. The values of T0 determined by all four independent methods were consistent, indicating overall agreement between the direct experimental measurements and the analyses based on heat flow simulations. T0 was determined to be 1565±25 K for 4.5 at. % As and 1425±25 K for 9 at. % As. In addition, the enthalpy of fusion was determined to be independent of composition for the range studied. The values obtained in this work are compared with previous measurements.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new low-field resonance technique for determining the initial hard-axis permeability (μ') of materials used in thin film and magnetoresistive heads has been developed. The initial μ' is found to be less than 4πM/Hk due to the internal effective magnetostatic field caused by magnetization ripple, in agreement with the theory and transverse biased initial susceptibility measurements of Hoffmann and others.1 A proper consideration of the in-plane susceptibility for real films includes the Stoner–Wohlfarth anisotropy field (Hk)−1 dependence, modified by the factors due to ripple (B) and skew (E) as defined by Hoffmann. The latter produce internal fields that shift the Kittel resonance versus field plots (fr2 vs H) leftward from their ideal symmetric positions for angles of 0° and 90° between bias field and preferred axis. When resonance data from both orientations are computer-fit simultaneously, we determine the parameters B and E as well as Hk and Ms. The coefficients are believed to have more consistency than those obtained using Hk found from hard-axis magnetization curves. The reduction of the hard-axis μ' is by the factor (1+B+E)−1. The high frequency properties (300 kHz–3 GHz) were measured using a test jig consisting of a shorted-shield strip-line section with a one-turn inductively coupled loop containing the sample and located at the shorted end. Bias fields from zero to 100 Oe were employed. We measure fR reproducibly with an uncertainty of less than 1%. This analysis determines the parameters of interest from measurements under dynamic conditions. Results include Permalloy and amorphous TM-metalloid films. When annealed to reduce Hk, their B parameters increase in accord with theory, assuming invariant structure constants, S. Beyond head applications, the method relates to thin film microwave devices operating near fr in zero field.2
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 72 (1992), S. 4477-4479 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A compensation of stress induced in silicon epitaxial films heavily doped with boron was investigated. Addition of a controlled amount of germanium during the film growth allows one to attain a desired compressive or tensile stress in the film, or its complete elimination. The data shows very good correlation with a theoretical model adapted from Herzog et al. [J. Electrochem. Soc. 131, 2969 (1984)]. A 6.45 : 1 ratio of atomic concentrations of Ge and B completely eliminates stress in these films. We determined a critical amount of strain in the films, (approximately-equal-to)2–4 μm thick, beyond which misfit dislocations are generated.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 5930-5940 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present atomic force microscopy images of diamond films grown by chemical vapor deposition epitaxially on diamond (100), (110), and (111) substrates. The films were grown from 0.2%–1.6% mixtures of CH4 and C2H2 in H2 in a hot-filament reactor at a total pressure of 25 Torr. The substrate and filament temperatures were held at 810–1000 and 2000–2150 °C, respectively. A (100)-oriented diamond film grown with 0.3% CH4 at a substrate temperature of 810 °C was rough on the μm scale, exhibiting pyramidal features, terraces, and penetration twins, while films grown at higher substrate temperatures and hydrocarbon flow rates were smooth on the nm scale and showed evidence of a (2×1) reconstruction. A (110)-oriented film was very rough on the μm scale but nearly atomically smooth on the 0.5–5 nm scale and exhibited local slopes higher than 40° with no evidence of faceting. A film grown on a diamond (111) substrate underwent spontaneous fracture due to tensile stress and exhibited a roughness of ≈10–50 nm on the ≈100 nm lateral scale in regions far away from any cracks. The implications of the morphological features for diamond growth mechanisms are discussed.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 1708-1712 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The electrical activation and carrier mobility of InP implanted with the group-IV elements at MeV energies has been studied as a function of implanted atom (C, Si, Ge, and Sn) and rapid thermal annealing temperature (500–800 °C). In addition, electrical results have been correlated with photoluminescence (PL) measurements. In general, for a dose of 5×1014/cm2 and a projected range of ∼1.0 μm, the electrical activation and carrier mobility increase then saturate with increasing annealing temperature. Similarily, PL emission intensity increases with increasing annealing temperature. At a temperature of 750 °C, the electrically active fraction increases from C, Ge, Si, to Sn, respectively, while carrier mobility and PL emission intensity decreases with increasing atomic mass. Thus, Sn exhibits the highest electrical activation yet lowest carrier mobility with little optically observable, postanneal lattice recovery.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 665-673 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Gold films were deposited on quartz-crystal microbalances by decomposing C7H7F6O2Au (dimethyl gold hexafluoroacetylacetonate) with 2–10-keV Xe+, Kr+, Ar+, Ne+, or He+ ion beams. The number of molecules decomposed per incident ion (i.e., the total decomposition yield) was determined as a function of ion mass and energy. The total decomposition yield increases with increasing ion mass and ion energy, and is approximately proportional to the nuclear stopping power. A binary collision model and a thermal spike model are developed that relate the energy deposited by the ion, at the substrate surface, to the total number of molecules decomposed. Both models predict total decomposition yields that are in reasonable agreement with the experimental measurements; however, the variation of total yield with changes in ion mass and energy are best described by the binary collision model. The success of both models demonstrates that the energy deposited into the substrate surface through the ion-solid interaction is responsible for the decomposition of adsorbed molecules.
    Type of Medium: Electronic Resource
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