1,768 research outputs found
Kinetics of natural aging in Al-Mg-Si alloys studied by positron annihilation lifetime spectroscopy
The process of natural aging in pure ternary Al-Mg-Si alloys was studied by
positron annihilation lifetime spectroscopy in real time in order to clarify
the sequence and kinetics of clustering and precipitation. It was found that
natural aging takes place in at least five stages in these alloys, four of
which were directly observed. This is interpreted as the result of complex
interactions between vacancies and solute atoms or clusters. One of the early
stages of positron lifetime evolution coincides with a clustering process
observed by differential scanning calorimetry (DSC) and involves the formation
of a positron trap with \sim 0.200 ns lifetime. In later stages, a positron
trap with a higher lifetime develops in coincidence with the DSC signal of a
second clustering reaction. Mg governs both the kinetics and the lifetime
change in this stage. Within the first 10 min after quenching, a period of
nearly constant positron lifetime was found for those Mg-rich alloys that later
show an insufficient hardness response to artificial aging, the so-called
"negative effect." The various processes observed could be described by two
effective activation energies that were found by varying the aging temperature
from 10 to 37\degree C.Comment: arXiv admin note: same as v2, to correct mistaken v
Ag on Ge(111): 2D X-ray structure analysis of the (Wurzel)3 x (Wurzel)3 superstructure
We have studied the Ag/Ge(111)(Wurzel)3 x (Wurzel)3 superstructure by grazing-incidence X-ray diffraction. In our structural analysis we find striking similarities to the geometry of Au on Si(111). The Ag atoms form trimer clusters with an Ag-Ag distance of 2.94+-0.04°A with the centers of the trimers being located at the origins of the (Wurzel)3 x (Wurzel)3 lattice. The Ag layer is incomplete and at least one substrate layer is distorted
Density of states and electron concentration of double heterojunctions subjected to an in-plane magnetic field
We calculate the electronic states of
AlGaAs/GaAs/AlGaAs double heterojunctions subjected to
a magnetic field parallel to the quasi two-dimensional electron gas. We study
the energy dispersion curves, the density of states, the electron concentration
and the distribution of the electrons in the subbands. The parallel magnetic
field induces severe changes in the density of states, which are of crucial
importance for the explanation of the magnetoconductivity in these structures.
However, to our knowledge, there is no systematic study of the density of
states under these circumstances. We attempt a contribution in this direction.
For symmetric heterostructures, the depopulation of the higher subbands, the
transition from a single to a bilayer electron system and the domination of the
bulk Landau levels in the centre the wide quantum well, as the magnetic field
is continuously increased, are presented in the ``energy dispersion picture''
as well as in the ``electron concentration picture'' and in the ``density of
states picture''.Comment: J. Phys.: Condens. Matter 11 No 26 (5 July 1999) 5131-5141 Figures
(three) embedde
Finite Temperature Behavior of the Quantum Hall Effect in Bilayer Electron Systems
An effective field theoretic description of bilayer electron systems
stabilized by Coulomb repulsion in a single wide quantum well is examined using
renormalization group techniques. The system is found to undergo a crossover
from a low temperature strongly correlated quantum Hall state to a high
temperature compressible state. This picture is used to account for the recent
experimental observation of an anomalous transition in bilayer electron systems
(T. S. Lay, {\em et al.} Phys. Rev. B {\bf 50}, 17725 (1994)). An estimate for
the crossover temperature is provided, and it is shown that its dependence on
electron density is in reasonable agreement with i the experiment.Comment: Corrected typos, and changed content, 5 pages and 2 figures, accepted
in Phys. Rev.
Interferometric Observations of the Nuclear Region of Arp220 at Submillimeter Wavelengths
We report the first submillimeter interferometric observations of an
ultraluminous infrared galaxy. We observed Arp220 in the CO J=3-2 line and
342GHz continuum with the single baseline CSO-JCMT interferometer consisting of
the Caltech Submillimeter Observatory (CSO) and the James Clerk Maxwell
Telescope (JCMT). Models were fit to the measured visibilities to constrain the
structure of the source. The morphologies of the CO J=3-2 line and 342GHz
continuum emission are similar to those seen in published maps at 230 and
110GHz. We clearly detect a binary source separated by about 1 arcsec in the
east-west direction in the 342GHz continuum. The CO J=3-2 visibility
amplitudes, however, indicate a more complicated structure, with evidence for a
compact binary at some velocities and rather more extended structure at others.
Less than 30% of the total CO J=3-2 emission is detected by the interferometer,
which implies the presence of significant quantities of extended gas. We also
obtained single-dish CO J=2-1, CO J=3-2 and HCN J=4-3 spectra. The HCN J=4-3
spectrum, unlike the CO spectra, is dominated by a single redshifted peak. The
HCN J=4-3/CO J=3-2, HCN J=4-3/HCN J=1-0 and CO J=3-2/2-1 line ratios are larger
in the redshifted (eastern) source, which suggests that the two sources may
have different physical conditions. This result might be explained by the
presence of an intense starburst that has begun to deplete or disperse the
densest gas in the western source, while the eastern source harbors undispersed
high density gas.Comment: 17 pages, 9 figures, 4 Tables. accepted by Ap
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Macrophage migration inhibitory factor downregulation: a novel mechanism of resistance to anti-angiogenic therapy.
Anti-angiogenic therapies for cancer such as VEGF neutralizing antibody bevacizumab have limited durability. While mechanisms of resistance remain undefined, it is likely that acquired resistance to anti-angiogenic therapy will involve alterations of the tumor microenvironment. We confirmed increased tumor-associated macrophages in bevacizumab-resistant glioblastoma patient specimens and two novel glioblastoma xenograft models of bevacizumab resistance. Microarray analysis suggested downregulated macrophage migration inhibitory factor (MIF) to be the most pertinent mediator of increased macrophages. Bevacizumab-resistant patient glioblastomas and both novel xenograft models of resistance had less MIF than bevacizumab-naive tumors, and harbored more M2/protumoral macrophages that specifically localized to the tumor edge. Xenografts expressing MIF-shRNA grew more rapidly with greater angiogenesis and had macrophages localizing to the tumor edge which were more prevalent and proliferative, and displayed M2 polarization, whereas bevacizumab-resistant xenografts transduced to upregulate MIF exhibited the opposite changes. Bone marrow-derived macrophage were polarized to an M2 phenotype in the presence of condition-media derived from bevacizumab-resistant xenograft-derived cells, while recombinant MIF drove M1 polarization. Media from macrophages exposed to bevacizumab-resistant tumor cell conditioned media increased glioma cell proliferation compared with media from macrophages exposed to bevacizumab-responsive tumor cell media, suggesting that macrophage polarization in bevacizumab-resistant xenografts is the source of their aggressive biology and results from a secreted factor. Two mechanisms of bevacizumab-induced MIF reduction were identified: (1) bevacizumab bound MIF and blocked MIF-induced M1 polarization of macrophages; and (2) VEGF increased glioma MIF production in a VEGFR2-dependent manner, suggesting that bevacizumab-induced VEGF depletion would downregulate MIF. Site-directed biopsies revealed enriched MIF and VEGF at the enhancing edge in bevacizumab-naive patients. This MIF enrichment was lost in bevacizumab-resistant glioblastomas, driving a tumor edge M1-to-M2 transition. Thus, bevacizumab resistance is driven by reduced MIF at the tumor edge causing proliferative expansion of M2 macrophages, which in turn promotes tumor growth
Spin-squeezed Ground States in the Bilayer Quantum Hall Ferromagnet
A "squeezed-vacuum" state considered in quantum optics is shown to be
realized in the ground-state wavefunction for the bilayer quantum Hall system
at the total Landau level filling of (m: odd integer). This is
derived in the boson approximation, where a particle-hole pair creation across
the symmetric-antisymmetric gap, , is regarded as a boson. In
terms of the pseudospin describing the layers, the state is a spin-squeezed
state, where the degree of squeezing is controlled by the layer separation and
. An exciton condensation, which amounts to a rotated
spin-squeezed state, has a higher energy due to the degraded SU(2) symmetry for
.Comment: 4 pages, revtex, one figure, to appear in PRB Rapid Communicatio
Wigner crystallization and metal-insulator transition of two-dimensional holes in GaAs/AlGaAs at B=0
We report the transport properties of a low disorder two-dimensional hole
system (2DHS) in the GaAs/AlGaAs heterostructure, which has an unprecedentedly
high peak mobility of , with hole density of in the temperature range of
. From their T, p, and electric field dependences, we find that
the metal-insulator transition in zero magnetic field in this exceptionally
clean 2DHS occurs at , which is in good agreement with the
critical for Wigner crystallization , predicted by
Tanatar and Ceperley for an ideally clean 2D system.Comment: 4 pages, 4 Postscript figure
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