478 research outputs found
Microscopic interface phonon modes in structures of GaAs quantum dots embedded in AlAs shells
By means of a microscopic valence force field model, a series of novel
microscopic interface phonon modes are identified in shell quantum dots(SQDs)
composed of a GaAs quantum dot of nanoscale embedded in an AlAs shell of a few
atomic layers in thickness. In SQDs with such thin shells, the basic principle
of the continuum dielectric model and the macroscopic dielectric function are
not valid any more. The frequencies of these microscopic interface modes lie
inside the gap between the bulk GaAs band and the bulk AlAs band, contrary to
the macroscopic interface phonon modes. The average vibrational energies and
amplitudes of each atomic shell show peaks at the interface between GaAs and
AlAs. These peaks decay fast as their penetrating depths from the interface
increase.Comment: 13 pages, 4 figure
Composition of receptor tyrosine kinase-mediated lipid micro-domains controlled by adaptor protein interaction
Receptor tyrosine kinases (RTKs) are highly regulated, single pass transmembrane proteins, fundamental to cellular function and survival. Aberrancies in regulation lead to corruption of signal transduction and a range of pathological outcomes. Although control mechanisms associated with the receptors and their ligands are well understood, little is known with respect to the impact of lipid/lipid and lipid/protein interactions in the proximal plasma membrane environment. Given that the transmembrane regions of RTKs change in response to extracellular ligand binding, the lipid interactions have important consequences in influencing signal transduction. Fibroblast growth factor receptor 2 (FGFR2) is a highly regulated RTK, including under basal conditions. Binding of the adaptor protein, growth factor receptor-bound protein 2 (GRB2) to FGFR2 prevents full activation and recruitment of downstream signalling effector proteins in the absence of extracellular stimulation. Here we demonstrate that the FGFR2-GRB2 complex is sustained in a defined lipid environment. Dissociation of GRB2 from this complex due to ligand binding, or reduced GRB2 expression, facilitates the dispersion of FGFR2 into detergent-resistant membrane (DRM) micro-domains. This modification of the plasma membrane proximal to FGFR2 provides a further regulatory checkpoint which controls receptor degradation, recycling and recruitment of intracellular signalling proteins
Arsenic in the cerebrospinal fluid of a patient receiving arsenic trioxide for relapsed acute promyelocytic leukemia with CNS involvement
We report on a 42-year-old patient whose relapse of acute promyelocytic leukaemia (APL) included meningeal infiltration. Since he had previously experienced ATRA syndrome, he received arsenic trioxide (ATO) plus intrathecal therapy with cytarabine, prednisone, and methotrexate. We measured the concentration of arsenic in his cerebrospinal fluid (CSF). Arsenic showed a peak CSF concentration of 0.008mg/l (0.11mumol/l) and a nadir of 0.002mg/l (0.027mumol/l), both representing about 14% of blood levels. ATO thus crosses the blood-CSF-barrier when administered intravenously, but the concentration in CSF is probably not sufficient for treatment of meningeal leukemia
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Space physics for graduate students: an activities-based approach
The geospace environment is controlled largely by events on the Sun, such as solar flares and coronal mass ejections, which generate significant geomagnetic and upper atmospheric disturbances. The study of this Sun-Earth system, which has become known as space weather, has both intrinsic scientific interest and practical applications. Adverse conditions in space can damage satellites and disrupt communications, navigation, and electric power grids, as well as endanger astronauts. The Center for Integrated Space Weather Modeling (CISM), a Science and Technology Center (STC) funded by the U.S. National Science Foundation (see http://www.bu.edu/cism/), is developing a suite of integrated physics-based computer models that describe the space environment from the Sun to the Earth for use in both research and operations [Hughes and Hudson, 2004, p. 1241]. To further this mission, advanced education and training programs sponsored by CISM encourage students to view space weather as a system that encompasses the Sun, the solar wind, the magnetosphere, and the ionosphere/thermosphere. This holds especially true for participants in the CISM space weather summer school [Simpson, 2004]
Polar optical phonons in wurtzite spheroidal quantum dots: Theory and application to ZnO and ZnO/MgZnO nanostructures
Polar optical-phonon modes are derived analytically for spheroidal quantum
dots with wurtzite crystal structure. The developed theory is applied to a
freestanding spheroidal ZnO quantum dot and to a spheroidal ZnO quantum dot
embedded into a MgZnO crystal. The wurtzite (anisotropic) quantum dots are
shown to have strongly different polar optical-phonon modes in comparison with
zincblende (isotropic) quantum dots. The obtained results allow one to explain
and accurately predict phonon peaks in the Raman spectra of wurtzite
nanocrystals, nanorods (prolate spheroids), and epitaxial quantum dots (oblate
spheroids).Comment: 11 page
Classical and quantum chaos in a circular billiard with a straight cut
We study classical and quantum dynamics of a particle in a circular billiard
with a straight cut. This system can be integrable, nonintegrable with soft
chaos, or nonintegrable with hard chaos, as we vary the size of the cut. We use
a quantum web to show differences in the quantum manifestations of classical
chaos for these three different regimes.Comment: LaTeX2e, 8 pages including 3 Postscript figures and 4 GIF figures,
submitted to Phys. Rev.
Wavelength-scale stationary-wave integrated Fourier-transform spectrometry
Spectrometry is a general physical-analysis approach for investigating
light-matter interactions. However, the complex designs of existing
spectrometers render them resistant to simplification and miniaturization, both
of which are vital for applications in micro- and nanotechnology and which are
now undergoing intensive research. Stationary-wave integrated Fourier-transform
spectrometry (SWIFTS)-an approach based on direct intensity detection of a
standing wave resulting from either reflection (as in the principle of colour
photography by Gabriel Lippmann) or counterpropagative interference
phenomenon-is expected to be able to overcome this drawback. Here, we present a
SWIFTS-based spectrometer relying on an original optical near-field detection
method in which optical nanoprobes are used to sample directly the evanescent
standing wave in the waveguide. Combined with integrated optics, we report a
way of reducing the volume of the spectrometer to a few hundreds of cubic
wavelengths. This is the first attempt, using SWIFTS, to produce a very small
integrated one-dimensional spectrometer suitable for applications where
microspectrometers are essential
Tetrathiotetracene thin film morphology and electrical properties
The electrical properties of organic thin films are determined by their chemical constituents and the morphology of the films deposited. In this paper the morphology of vacuum sublimed (7∙10-6 mbar) tetrathiotetracene (TTT) thin films is shown to be strongly affected by the thermal deposition temperature (222-350 K) and rate of deposition. Mostly needle-like morphologies are identified by scanning electron microscopy. Optimal TTT purity (a pre-requisite for device preparation via subsequent oxidation) is evidenced by their initially low electrical conductivity. Altering the TTT morphology, by variation of the evaporation parameters, strongly affects this base electrical conductivity. Four probe conductivity measurements and charge extraction by linear increasing voltage methods are used to characterize film electrical properties. In-plane conductivity of up to 7.03∙10-5 S/cm is achieved for pure TTT thin films. Subsequent aerial oxidation resulted in a 3.4-fold increase in electrical conductivity
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