11,917 research outputs found
Excitonic Effects in Quantum Wires
We review the effects of Coulomb correlation on the linear and non-linear
optical properties of semiconductor quantum wires, with emphasis on recent
results for the bound excitonic states. Our theoretical approach is based on
generalized semiconductor Bloch equations, and allows full three-dimensional
multisubband description of electron-hole correlation for arbitrary confinement
profiles. In particular, we consider V- and T-shaped structures for which
significant experimental advances were obtained recently. Above band gap, a
very general result obtained by this approach is that electron-hole Coulomb
correlation removes the inverse-square-root single-particle singularity in the
optical spectra at band edge, in agreement with previous reports from purely
one-dimensional models. Strong correlation effects on transitions in the
continuum are found to persist also at high densities of photoexcited carriers.
Below bandgap, we find that the same potential- (Coulomb) to kinetic-energy
ratio holds for quite different wire cross sections and compositions. As a
consequence, we identify a shape- and barrier-independent parameter that
governs a universal scaling law for exciton binding energy with size. Previous
indications that the shape of the wire cross-section may have important effects
on exciton binding are discussed in the light of the present results.Comment: Proc. OECS-5 Conference, G\"ottingen, 1997 (To appear in Phys. Stat.
Sol. (b)
Multidomain switching in the ferroelectric nanodots
Controlling the polarization switching in the ferroelectric nanocrystals,
nanowires and nanodots has an inherent specificity related to the emergence of
depolarization field that is associated with the spontaneous polarization. This
field splits the finite-size ferroelectric sample into polarization domains.
Here, based on 3D numerical simulations, we study the formation of 180 polarization domains in a nanoplatelet, made of uniaxial ferroelectric
material, and show that in addition to the polarized monodomain state, the
multidomain structures, notably of stripe and cylindrical shapes, can arise and
compete during the switching process. The multibit switching protocol between
these configurations may be realized by temperature and field variations
Gravity duals of supersymmetric gauge theories on three-manifolds
We study gravity duals to a broad class of N=2 supersymmetric gauge theories
defined on a general class of three-manifold geometries. The gravity
backgrounds are based on Euclidean self-dual solutions to four-dimensional
gauged supergravity. As well as constructing new examples, we prove in general
that for solutions defined on the four-ball the gravitational free energy
depends only on the supersymmetric Killing vector, finding a simple closed
formula when the solution has U(1) x U(1) symmetry. Our result agrees with the
large N limit of the free energy of the dual gauge theory, computed using
localization. This constitutes an exact check of the gauge/gravity
correspondence for a very broad class of gauge theories with a large N limit,
defined on a general class of background three-manifold geometries.Comment: 74 pages, 2 figures; v2: minor change
Targeting the phosphatidylinositol 3-kinase/Akt/mechanistic target of rapamycin signaling pathway in B-lineage acute lymphoblastic leukemia: An update
Despite considerable progress in treatment protocols, B-lineage acute lymphoblastic leukemia (B-ALL) displays a poor prognosis in about 15–20% of pediatric cases and about 60% of adult patients. In addition, life-long irreversible late effects from chemo- and radiation therapy, including secondary malignancies, are a growing problem for leukemia survivors. Targeted therapy holds promising perspectives for cancer treatment as it may be more effective and have fewer side effects than conventional therapies. The phosphatidylinositol 3-phosphate kinase (PI3K)/Akt/mechanistic target of rapamycin (mTOR) signaling pathway is a key regulatory cascade which controls proliferation, survival and drug-resistance of cancer cells, and it is frequently upregulated in the different subtypes of B-ALL, where it plays important roles in the pathophysiology, maintenance and progression of the disease. Moreover, activation of this signaling cascade portends a poorer prognosis in both pediatric and adult B-ALL patients. Promising preclinical data on PI3K/Akt/mTOR inhibitors have documented their anticancer activity in B-ALL and some of these novel drugs have entered clinical trials as they could lead to a longer event-free survival and reduce therapy-associated toxicity for patients with B-ALL. This review highlights the current status of PI3K/Akt/mTOR inhibitors in B-ALL, with an emphasis on emerging evidence of the superior efficacy of synergistic combinations involving the use of traditional chemotherapeutics or other novel, targeted agents
Literature review of lead-lithium thermophysical properties
Abstract In recent years, the scientific community of nuclear fusion raised the issue of thermophysical properties of lead lithium alloys. These alloys are foreseen to be used in several Breeding Blanket concepts in an almost eutectic composition, but only few data on the properties are available in literature and large differences on the same property exist between different authors. Moreover, apparently each organization used different available properties correlations, making practically pointless every comparison of results with the other organizations involved in the design of Breeding Blankets. The aim of this paper is to identify the properties to be used in the design of the Breeding Blankets, performing a literature review of the available data and suggesting a correlation for each of the main properties. These correlations were chosen based on the accurateness of the paper and on the similarities between different authors, where it was possible (e.g., density). The table with the correlations should represent a starting point for a discussion to reach a general consensus on the property database, which should be mandatory in order to allow a comparison of the results from different organizations. Very likely new experiments will be necessary to definitely measure at least the properties with the biggest scattering of the data (e.g., specific heat), encouraging a consensus and reducing the errors in the design activities
R-charges from toric diagrams and the equivalence of a-maximization and Z-minimization
We conjecture a general formula for assigning R-charges and multiplicities
for the chiral fields of all gauge theories living on branes at toric
singularities. We check that the central charge and the dimensions of all the
chiral fields agree with the information on volumes that can be extracted from
toric geometry. We also analytically check the equivalence between the volume
minimization procedure discovered in hep-th/0503183 and a-maximization, for the
most general toric diagram. Our results can be considered as a very general
check of the AdS/CFT correspondence, valid for all superconformal theories
associated with toric singularities.Comment: 43 pages, 17 figures; minor correction
The -semantics approach; theory and applications
AbstractThis paper is a general overview of an approach to the semantics of logic programs whose aim is to find notions of models which really capture the operational semantics, and are, therefore, useful for defining program equivalences and for semantics-based program analysis. The approach leads to the introduction of extended interpretations which are more expressive than Herbrand interpretations. The semantics in terms of extended interpretations can be obtained as a result of both an operational (top-down) and a fixpoint (bottom-up) construction. It can also be characterized from the model-theoretic viewpoint, by defining a set of extended models which contains standard Herbrand models. We discuss the original construction modeling computed answer substitutions, its compositional version, and various semantics modeling more concrete observables. We then show how the approach can be applied to several extensions of positive logic programs. We finally consider some applications, mainly in the area of semantics-based program transformation and analysis
M-theory and Seven-Dimensional Inhomogeneous Sasaki-Einstein Manifolds
Seven-dimensional inhomogeneous Sasaki-Einstein manifolds
present a challenging example of AdS/CFT correspondence. At present, their
field theory duals for base are proposed only within a
restricted range as quiver Chern-Simons-matter
theories with gauge group, nine bifundamental
chiral multiplets interacting through a cubic superpotential. To further
elucidate this correspondence, we use particle approximation both at classical
and quantum level. We setup a concrete AdS/CFT mapping of conserved quantities
using geodesic motions, and turn to solutions of scalar Laplace equation in
. The eigenmodes also provide an interesting subset of Kaluza-Klein
spectrum for supergravity in , and are dual
to protected operators written in terms of matter multiplets in the dual
conformal field theory.Comment: v2 refs added. 19 pages 1 figur
Redistribution of DNA topoisomerase II beta after in vitro stabilization of human erythroleukemic nuclei by heat or Cu++ revealed by confocal microscopy.
Using confocal laser scanning microscope and a monoclonal antibody we have examined by means of indirect immunofluorescence techniques the distribution of DNA topoisomerase II beta (the 180-kDa nucleolar isoform of topoisomerase II) following stabilization of isolated nuclei by exposure to moderate heat (37 degrees or 42 degrees C) or Cu++. In intact cells the antibody specifically decorated the nucleoli. The same pattern was maintained if nuclei were incubated at 0 degree C in a buffer containing spermine/spermidine/KCl or stabilized by means of 0.5 mM Cu++ for 10 minutes at 0 degree C in the same buffer. On the contrary, if stabilization was performed by incubating the nuclei either at 37 degrees or 42 degrees C, the immunoreactivity dispersed all over the nucleus, forming numerous speckles. This phenomenon was not detected if, in addition to spermine/spermidine/KCl, the incubation buffer also contained 5 mM Mg++ and the temperature was 37 degrees C. If the stabilization was performed at 42 degrees C, Mg++ failed to maintain the original distribution of DNA topoisomerase II beta, as seen in intact cells. The analysis on 2-D optical section showed the alteration of the nucleolar profile, particularly at 37 degrees C, even when the samples were treated with Mg++. The 3-D reconstruction figured out the irregularity of the surface at 37 degrees C and the variations of the volume occupied by the fluorescent figures. These were in close proximity to each other both in intact cells and in 0 degree C incubated nuclei; they showed a certain degree of shrinkage in 0 degree C plus Cu++ exposed samples (-20\% of the volume), and, on the contrary, the labeled structures were scattered in a volume increased two- or threefold when exposed to 37 degrees or 42 degrees C, respectively. The addition of Mg++ restored the original spatial relationship and volume at 37 degrees C, but not at 42 degrees C, where the volumetric analysis showed an increase of about 50\%. Our results demonstrate that heat stabilization of isolated nuclei in a buffer without Mg++ (i.e., a technique often employed to prepare the nuclear matrix or scaffold) cannot be considered an optimal procedure to maintain the original distribution of protein within the nucleus
Further studies on the use of molecular probes to grapevine closterovirus A
Two cloned cDNA probes to genomic RNA of grapevine closterovirus A (GVA) were utilized successfully for the detection of viral sequences in infected herbaceous hosts (Nicotiana benthamiana) and grapevines. One of the probes (pGA112) was complementary to the central part of the viral genome and gave light false positive signals with healthy grapevine extracts, whereas the other (pGA240), which is presumably colinear with the 3' terminus, was virus-specific and hybridized only with infected sample extracts. The two probes recognized smaller than genome RNAs in electrophoresed N. benthamiana extracts and hybridized differentially with the bands, thus suggesting that these represent subgenomic RNAs. Probe pGA240 may be used for GVA detection, but the preparation of samples for hybridization needs further improvement for routine testing.Weitere Untersuchungen über die Verwendung von Molekülsonden beim Grapevine-Closterovirus AZwei klonierte cDNA-Sonden für genomische RNA des Grapevine-Closterovirus A (GVA} wurden mit Erfolg zum Nachweis von Virussequenzen in infizierten krautigen Wirtspflanzen (Nicotiana benthamiana) und Reben benützt. Die eine Sonde (pGA112} war komplementär zum zentralen Teil des Virusgenoms und lieferte schwache falsch-positive Nachweisreaktionen mit Extrakten aus gesunden Reben; die andere Sonde (pGA240}, die vermutlich kolinear mit dem 3'-Terminus ist, war dagegen virusspezifisch und hybridisierte nur mit Extrakten aus infizierten Proben. In elektrophoretisch aufgetrennten N. benthamiana-Extrakten "erkannten" die beiden Sonden RNAs von weniger als Genomgröße; sie hybridisierten differenziert mit den Banden, so daß diese subgenomische RNAs darstellen könnten. Die Sonde pGA240 kann für den GVA-Nachweis verwendet werden; für Routinetests muß die Vorbereitung der Proben für die Hybridisierung noch verbessert werden
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