Library

X
feed icon rss

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • 1
    Electronic Resource
    Electronic Resource
    Time-dependent simulation of helix traveling wave tubes (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 5182-5194 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A time-dependent nonlinear analysis of a helix traveling wave tube (TWT) is presented for a configuration where an electron beam propagates through a three-dimensional tape helix surrounded by a conducting wall. The effects of dielectric- and vane-loading are included as is efficiency enhancement by tapering the helix pitch and external focusing by means of either a uniform solenoidal magnetic field or a periodic field produced by a periodic permanent magnet stack. Dielectric-loading is described under the assumption that the gap between the helix and the wall is uniformly filled by a dielectric material. Vane-loading describes the insertion of an arbitrary number of vanes running the length of the helix. The electromagnetic field is represented as a superposition of waves in a vacuum tape helix. The propagation of each wave as well as the interaction of each wave with the electron beam is included by allowing the amplitudes of the waves to vary in z and t. The field equations are solved in conjunction with the three-dimensional Lorentz force equations for an ensemble of electrons. Collective effects are included by both a finite-difference grid formulation of Poisson's equation and an analysis based upon a modal superposition of solutions of the Helmholtz equation. The time-dependent simulation is compared with a steady-state simulation for a model corresponding to a TWT built at Northrop–Grumman Corporation. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1319338
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 2
    Electronic Resource
    Electronic Resource
    Model for stress generated upon contact of neighboring islands on the surface of a substrate (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 4866-4873 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In the early stage of growth of a metal film on a substrate by the Volmer–Weber mechanism, a tensile stress in the film is observed to arise at about the point in the process when islands of deposited material begin to coalesce. The mechanism commonly proposed as the origin of this tensile stress is that the coalescing islands deform in order to form a relatively low energy grain boundary, at the expense of some surface energy by surface area reduction, and that this proceeds until a stress is generated that has magnitude sufficient to prevent further area reduction. Several models have been proposed for this process, but the inferred tensile stress estimates have been much larger than observed stress magnitudes in many cases. The purpose here is to introduce a model for the process based on the theory of contact of elastic solids with cohesion. A description of the process is developed on this premise for one-dimensional, two-dimensional, and three-dimensional states of deformation of coalescing islands. It is found that the latter case leads to an estimate of film stress generally consistent with observations. The features of the model for different dimensionalities are compared and contrasted with each other, as well as with other models which have been proposed for this process. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1359437
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 3
    Electronic Resource
    Electronic Resource
    A molecular beam/surface spectroscopy apparatus for the study of reactions on complex model catalysts (2000)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 71 (2000), S. 4395-4408 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We describe a newly developed ultrahigh vacuum (UHV) experiment which combines molecular beam techniques and in situ surface spectroscopy. It has been specifically designed to study the reaction kinetics and dynamics on complex model catalysts. The UHV system contains: (a) a preparation compartment providing the experimental techniques which are required to prepare and characterize single-crystal based model catalysts such as ordered oxide surfaces or oxide supported metal particles; and (b) the actual scattering chamber, where up to three molecular beams can be crossed on the sample surface. Two beams are produced by newly developed differentially pumped sources based on multichannel arrays. The latter are capable of providing high intensity and purity beams and can be modulated by means of a vacuum-motor driven and computer-controlled chopper. The third beam is generated in a continuous or pulsed supersonic expansion and is modulated via a variable duty-cycle chopper. Angular and time-resolved measurements of desorbing and scattered molecules are performed with a rotatable doubly differentially pumped quadrupole mass spectrometer with a liquid-nitrogen cooled ionizer housing. Time-resolved but angle-integrated measurements are realized with a second nondifferentially pumped quadrupole mass spectrometer. In situ measurements of adsorbed species under reaction conditions are performed by means of an adapted vacuum Fourier transform infrared spectrometer. The spectrometer provides the possibility of time-resolved measurements and can be synchronized with any of the beam sources. This contribution provides a general overview of the system and a description of all new components and their interplay. We also present test data for all components employing simple adsorption/desorption and reaction systems. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1318919
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 4
    Electronic Resource
    Electronic Resource
    Messenger RNAs Located in Myelin Sheath Assembly Sites (2000)
    Oxford UK : Blackwell Science Ltd.
    Journal of neurochemistry 75 (2000), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: The targeting of mRNAs to specific subcellular locations is believed to facilitate the rapid and selective incorporation of their protein products into complexes that may include membrane organelles. In oligodendrocytes, mRNAs that encode myelin basic protein (MBP) and select myelin-associated oligodendrocytic basic proteins (MOBPs) locate in myelin sheath assembly sites (MSAS). To identify additional mRNAs located in MSAS, we used a combination of subcellular fractionation and suppression subtractive hybridization. More than 50% of the 1,080 cDNAs that were analyzed were derived from MBP or MOBP mRNAs, confirming that the method selected mRNAs enriched in MSAS. Of 90 other cDNAs identified, most represent one or more mRNAs enriched in rat brain myelin. Five cDNAs, which encode known proteins, were characterized for mRNA size(s), enrichment in myelin, and tissue and developmental expression patterns. Two of these, peptidylarginine deiminase and ferritin heavy chain, have recognized roles in myelination. The corresponding mRNAs were of different sizes than the previously identified mRNA, and they had tissue and development expression patterns that were indistinguishable from those of MBP mRNA. Three other cDNAs recognize mRNAs whose proteins (SH3p13, KIF1A, and dynein light intermediate chain) are involved in membrane biogenesis. Although enriched in myelin, the tissue and developmental distribution patterns of these mRNAs differed from those of MBP mRNA. Six other cDNAs, which did not share significant sequence homology to known mRNAs, were also examined. The corresponding mRNAs were highly enriched in myelin, and four had tissue and developmental distribution patterns indistinguishable from those of MBP mRNA. These studies demonstrate that MSAS contain a diverse population of mRNAs, whose locally synthesized proteins are placed to contribute to myelin sheath assembly and maintenance. Characterization of these mRNAs and proteins will help provide a comprehensive picture of myelin sheath assembly.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1471-4159.2000.0751834.x
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 5
    Electronic Resource
    Electronic Resource
    Cellular distribution and bioactivity of the key steroidogenic enzyme, cytochrome P450side chain cleavage, in sensory neural pathways (2003)
    Oxford, UK : Blackwell Science Ltd
    Journal of neurochemistry 86 (2003), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Neurosteroids are steroids produced within the nervous system. Based on behavioural responses evoked in animals by synthetic steroid injections, several studies suggested neurosteroid involvement in important neurophysiological processes. These observations should be correlated only to neuroactive effects of the injected steroids. Neurosteroids mostly control the CNS activity through allosteric modulation of neurotransmitter receptors within concentration ranges used by neurotransmitters themselves. Therefore, neurosteroid production within pathways controlling a neurophysiological process is necessary to consider neurosteroid involvement in that process. Because of the increasing speculation about pain modulation by neurosteroids based on pharmacological observations, we decided to clarify the situation by investigating neurosteroidogenesis occurrence in sensory pathways, particularly in nociceptive structures. We studied the presence and activity of cytochrome P450side chain cleavage (P450scc) in rat pain pathways. P450scc-immunoreactive cells were localized in dorsal root ganglia (DRG), spinal cord (SC) dorsal horn, nociceptive supraspinal nuclei (SSN) and somatosensory cortex. Incubation of DRG, SSN or SC tissue homogenates with [3H]cholesterol yielded the formation of radioactive metabolites including [3H]pregnenolone of which the synthesis was reduced in presence of aminogluthetimide, a P450scc inhibitor. These first neuroanatomical and neurochemical results demonstrate the occurrence of neurosteroidogenesis in nociceptive pathways and strongly suggest that neurosteroids may control pain mechanisms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1471-4159.2003.01935.x
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 6
    Electronic Resource
    Electronic Resource
    Strain induced stabilization of stepped Si and Ge surfaces near (001) (2002)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 81 (2002), S. 364-366 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on calculations of the formation energies of several [100] and [110] oriented step structures on biaxially stressed Si and Ge (001) surfaces. It is shown that a novel rebonded [100] oriented single-height step is strongly stabilized by compressive strain compared to most well-known step structures. We propose that the side walls of "hut"-shaped quantum dots observed in recent experiments on SiGe/Si films are made up of these steps. Our calculations provide an explanation for the nucleationless growth of shallow mounds, with steps along the [100] and [110] directions in low- and high-misfit films, respectively, and for the stability of the (105) facets under compressive strain. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1491611
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 7
    Electronic Resource
    Electronic Resource
    Letter to the Editor (2000)
    Oxford, UK and Boston, USA : Blackwell Publishers Ltd
    Teaching statistics 22 (2000), S. 0 
    Details
    ISSN: 1467-9639
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mathematics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/1467-9639.00015
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 8
    Electronic Resource
    Electronic Resource
    Electro-osmosis in a nanometer-scale channel studied by atomistic simulation (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 2194-2200 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An atomistic simulation of an electro-osmotic flow in a 65.3 Å-wide channel is performed to study its physical details and evaluate continuum models. The working fluid is a 0.01 M solution (at midchannel) of Cl− in water. For simplicity and computational efficiency, only negatively charged ions are included. The water is modeled by the SPC/E potential and the Cl− are modeled as point charges plus an established Lennard-Jones potential. The channel walls are fixed lattices of positively charged Lennard-Jones atoms. In one case an appropriate fraction of the wall atoms is given elementary charges; for comparison, another case is simulated with uniformly distributed partial charges on the wall atoms. For the distributed elementary charge case the Cl− concentration at the wall is 80 percent higher than predicted by the Poisson–Boltzmann theory. It is over 100 percent higher for the uniformly charged wall case. In both cases, the waters in the 10 Å closest to the walls are preferentially oriented. Their respective orientations are similar except in the first monolayer. However, the effect of this orientational bias on the permittivity and subsequently the Cl− distribution is shown to be minor by Monte Carlo simulations, which predict an ion distribution in agreement with the dynamic simulation using only cursive-epsilon=80 to model the water. Computed, one-dimensional self-diffusivities of the waters match accepted values greater than 10 Å from the walls, but decay significantly close to the walls. The decay is not monotonic for the wall-normal diffusivity. The atoms near the walls are not fixed in a Stern layer, as typically assumed in models, but the viscosity is found to increase by over a factor of 6 in the 10 Å closest to the wall. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1431543
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 9
    Electronic Resource
    Electronic Resource
    Rotational alignment in the photodesorption of CO from Cr2O3(0001): A systematic three-dimensional ab initio study (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 762-773 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In recent experiments, the rotational alignment of the laser induced nonthermal desorption of CO adsorbed on an epitaxially grown film of Cr2O3(0001) has been studied [Beauport, Al-Shamery, and Freund, Chem. Phys. Lett. 256, 641 (1996)]. At low-rotational quantum numbers J, the molecules desorb like helicopters (J-vector perpendicular to the surface) while at high J-values cartwheel motion is preferred (J-vector parallel to the surface). These stereodynamic effects and the experimental state resolved velocity distributions of the desorbing species are simulated by means of an exact time-dependent wave packet method in three dimensions. As a basis for this quantum-mechanical treatment of the CO desorption ab initio potential energy surfaces (PES) were used. The PES for the electronic ground state of the CO–Cr2O3(0001) system has been calculated previously by Pykavy et al. [Surf. Sci. 479, 11 (2001)] in an embedded cluster approach. As the intermediate state, generated by the laser irradiation, an internal CO excited state (5σ→2π*) was considered. The PES of this a 3Π-like state of CO adsorbed on Cr2O3(0001) was calculated at the ab initio CASSCF-level. Our key findings in the subsequent wave packet calculations are (1) a high-dimensional treatment of the photodesorption process is very important in this system, essentially the angular coordinates, very often neglected in similar studies, are responsible for a "successful" desorption event; (2) the change from the strongly tilted equilibrium geometry in the electronic ground state to the preferred upright position in the electronically excited state after laser irradiation is essential for the mechanistic picture of the desorption process; (3) the experimental phenomemon of rotational alignment can only be explained if the topologies of the PES of both the electronically excited and the ground state are accounted for; (4) the lifetime of the CO*-intermediate is in the order of 10 fs; (5) the molecule–surface vibrations in the electronic ground state do not much influence the asymptotic results at the experimental temperature of T=100 K. However, the inclusion of excited levels of the hindered rotation helps to gain insight into the desorption mechanism on a microscopic level. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1425383
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 10
    Electronic Resource
    Electronic Resource
    Hypervascularization in the Magnocellular Nuclei of the Rat Hypothalamus: Relationship with the Distribution of Aquaporin-4 and Markers of Energy Metabolism (2000)
    Oxford, UK : Blackwell Science Ltd
    Journal of neuroendocrinology 12 (2000), S. 0 
    Details
    ISSN: 1365-2826
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: In the magnocellular nuclei of the hypothalamus, there is a rich vascular network for which the function remains to be established. In the supraoptic nucleus, the high vascular density may be one element, which together with the water channel aquaporin-4 expressed in the astrocytes, is related to a role in osmoreception. We tested the osmoreception hypothesis by studying the correlation between vascular and cellular densities in the paraventricular nucleus and the supraoptic nucleus. Whether aquaporin-4 is likely to contribute to osmoreception was tested by studying the distribution in the magnocellular nuclei of the hypothalamus. The high vascular density may also reflect a high metabolic activity due to the synthesis of vasopressin and oxytocin. This metabolic hypothesis was tested by studying the regional cytochrome oxidase histochemistry, the local cerebral blood flow, and the density of glucose transporter type-1 in the supraoptic and paraventricular nuclei. All the magnocellular nuclei were characterized by an extended and intense aquaporin-4 labelling and a weak cytochrome oxidase histochemistry. The highest vascular density was found in the supraoptic nucleus and the magnocellular regions of the paraventricular nucleus. The local cerebral blood flow rates were surprisingly low in the paraventricular nucleus and the supraoptic nucleus in comparison to the cerebral cortex. Furthermore in these nuclei, the antibody for glucose transporter type-1 revealed two populations of vessels differing by their labelling intensity. The similarities observed between the different nuclei suggest that, in the hypothalamus, all magnocellular regions sense the plasma osmolarity. The low local cerebral blood flow, and the patterns of glucose transporter type-1 labelling and cytochrome oxidase histochemistry suggest that the high vascularization of these hypothalamic nuclei is not related to a high metabolic capacity in basal conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2826.2000.00539.x
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...