145 research outputs found
Auger decay of degenerate and Bose-condensed excitons in CuO
We study the non-radiative Auger decay of excitons in CuO, in which two
excitons scatter to an excited electron and hole. The exciton decay rate for
the direct and the phonon-assisted processes is calculated from first
principles; incorporating the band structure of the material leads to a
relatively shorter lifetime of the triplet state ortho excitons. We compare our
results with the Auger decay rate extracted from data on highly degenerate
triplet excitons and Bose-condensed singlet excitons in CuO.Comment: 15 pages, revtex, figures available from G. Kavoulaki
Fusion of secretory vesicles isolated from rat liver
Secretory vesicles isolated from rat liver were found to fuse after exposure to Ca2+. Vescle fusion is characterized by the occurrence of twinned vesicles with a continuous cleavage plane between two vesicles in freeze-fracture electron microscopy. The number of fused vesicles increases with increasing Ca2+-concentrations and is half maximal around 10â6 m. Other divalent cations (Ba2+, Sr2+, and Mg2+) were ineffective. Mg2+ inhibits Ca2+-induced fusion. Therefore, the fusion of secretory vesiclesin vitro is Ca2+ specific and exhibits properties similar to the exocytotic process of various secretory cells.
Various substances affecting secretionin vivo (microtubular inhibitors, local anethetics, ionophores) were tested for their effect on membrane fusion in our system.
The fusion of isolated secretory vesicles from liver was found to differ from that of pure phospholipid membranes in its temperature dependence, in its much lower requirement for Ca2+, and in its Ca2+-specificity. Chemical and enzymatic modifications of the vesicle membrane indicate that glycoproteins may account for these differences
Fine structure of excitons in CuO
Three experimental observations on 1s-excitons in CuO are not consistent
with the picture of the exciton as a simple hydrogenic bound state: the
energies of the 1s-excitons deviate from the Rydberg formula, the total exciton
mass exceeds the sum of the electron and hole effective masses, and the
triplet-state excitons lie above the singlet. Incorporating the band structure
of the material, we calculate the corrections to this simple picture arising
from the fact that the exciton Bohr radius is comparable to the lattice
constant. By means of a self-consistent variational calculation of the total
exciton mass as well as the ground-state energy of the singlet and the
triplet-state excitons, we find excellent agreement with experiment.Comment: Revised abstract; 10 pages, revtex, 3 figures available from G.
Kavoulakis, Physics Department, University of Illinois, Urban
Quantum saturation and condensation of excitons in CuO: a theoretical study
Recent experiments on high density excitons in CuO provide evidence for
degenerate quantum statistics and Bose-Einstein condensation of this nearly
ideal gas. We model the time dependence of this bosonic system including
exciton decay mechanisms, energy exchange with phonons, and interconversion
between ortho (triplet-state) and para (singlet-state) excitons, using
parameters for the excitonic decay, the coupling to acoustic and low-lying
optical phonons, Auger recombination, and ortho-para interconversion derived
from experiment. The single adjustable parameter in our model is the
optical-phonon cooling rate for Auger and laser-produced hot excitons. We show
that the orthoexcitons move along the phase boundary without crossing it (i.e.,
exhibit a ``quantum saturation''), as a consequence of the balance of entropy
changes due to cooling of excitons by phonons and heating by the non-radiative
Auger two-exciton recombination process. The Auger annihilation rate for
para-para collisions is much smaller than that for ortho-para and ortho-ortho
collisions, explaining why, under the given experimental conditions, the
paraexcitons condense while the orthoexcitons fail to do so.Comment: Revised to improve clarity and physical content 18 pages, revtex,
figures available from G. Kavoulakis, Physics Department, University of
Illinois, Urban
Evidence of Color Coherence Effects in W+jets Events from ppbar Collisions at sqrt(s) = 1.8 TeV
We report the results of a study of color coherence effects in ppbar
collisions based on data collected by the D0 detector during the 1994-1995 run
of the Fermilab Tevatron Collider, at a center of mass energy sqrt(s) = 1.8
TeV. Initial-to-final state color interference effects are studied by examining
particle distribution patterns in events with a W boson and at least one jet.
The data are compared to Monte Carlo simulations with different color coherence
implementations and to an analytic modified-leading-logarithm perturbative
calculation based on the local parton-hadron duality hypothesis.Comment: 13 pages, 6 figures. Submitted to Physics Letters
Schmidt-hammer exposure ages from periglacial patterned ground (sorted circles) in Jotunheimen, Norway, and their interpretative problems
© 2016 Swedish Society for Anthropology and Geography Periglacial patterned ground (sorted circles and polygons) along an altitudinal profile at Juvflya in central Jotunheimen, southern Norway, is investigated using Schmidt-hammer exposure-age dating (SHD). The patterned ground surfaces exhibit R-value distributions with platycurtic modes, broad plateaus, narrow tails, and a negative skew. Sample sites located between 1500 and 1925 m a.s.l. indicate a distinct altitudinal gradient of increasing mean R-values towards higher altitudes interpreted as a chronological function. An established regional SHD calibration curve for Jotunheimen yielded mean boulder exposure ages in the range 6910 ± 510 to 8240 ± 495 years ago. These SHD ages are indicative of the timing of patterned ground formation, representing minimum ages for active boulder upfreezing and maximum ages for the stabilization of boulders in the encircling gutters. Despite uncertainties associated with the calibration curve and the age distribution of the boulders, the early-Holocene age of the patterned ground surfaces, the apparent cessation of major activity during the Holocene Thermal Maximum (HTM) and continuing lack of late-Holocene activity clarify existing understanding of the process dynamics and palaeoclimatic significance of large-scale sorted patterned ground as an indicator of a permafrost environment. The interpretation of SHD ages from patterned ground surfaces remains challenging, however, owing to their diachronous nature, the potential for a complex history of formation, and the influence of local, non-climatic factors
The uses and abuses of power: teaching school leadership through children's literature
There are relatively few studies of how representations of teachers, schools and educational administrators in popular films and television might be, and are, used in leadership preparation. This paper seeks to add to this small body of work; it reports on an exploratory study of the representation of headteachers in contemporary children's fiction. Thirty-one texts are analysed to ascertain key themes and the major characterisations. The paper draws on children's literature scholars to argue that both the historical school story and its contemporary counterpart focus heavily on the power of the head to control the micro-world of the school. Because these fictional accounts deal with issues of power and justice more openly than many mainstream educational administration texts, this makes them particularly useful in the preparation of potential school leaders
Glucosylated cholesterol in mammalian cells and tissues: formation and degradation by multiple cellular ÎČ-glucosidases
The membrane lipid glucosylceramide (GlcCer) is continuously formed and degraded. Cells express two GlcCer-degrading ÎČ-glucosidases, GBA and GBA2, located in and outside the lysosome, respectively. Here we demonstrate that through transglucosylation both GBA and GBA2 are able to catalyze in vitro the transfer of glucosyl-moieties from GlcCer to cholesterol, and vice versa. Furthermore, the natural occurrence of 1-O-cholesteryl-ÎČ-D-glucopyranoside (GlcChol) in mouse tissues and human plasma is demonstrated using LC-MS/MS and 13C6-labelled GlcChol as internal standard. In cells the inhibition of GBA increases GlcChol, whereas inhibition of GBA2 decreases glucosylated sterol. Similarly, in GBA2-deficient mice GlcChol is reduced. Depletion of GlcCer by inhibition of GlcCer synthase decreases GlcChol in cells and likewise in plasma of inhibitor-treated Gaucher disease patients. In tissues of mice with Niemann-Pick type C, a condition characterized by intralysosomal accumulation of cholesterol, marked elevations in GlcChol occur as well. When lysosomal accumulation of cholesterol is induced in cultured cells, GlcChol is formed via lysosomal GBA. This illustrates that reversible transglucosylation reactions are highly dependent on local availability of suitable acceptors. In conclusion, mammalian tissues contain GlcChol formed by transglucosylation through ÎČ-glucosidases using GlcCer as donor. Our findings reveal a novel metabolic function for GlcCer.Bio-organic SynthesisMedical Biochemistr
Virtual Ontogeny of Cortical Growth Preceding Mental Illness
Background: Morphology of the human cerebral cortex differs across psychiatric disorders, with neurobiology and developmental origins mostly undetermined. Deviations in the tangential growth of the cerebral cortex during pre/perinatal periods may be reflected in individual variations in cortical surface area later in life. Methods: Interregional profiles of group differences in surface area between cases and controls were generated using T1-weighted magnetic resonance imaging from 27,359 individuals including those with attention-deficit/hyperactivity disorder, autism spectrum disorder, bipolar disorder, major depressive disorder, schizophrenia, and high general psychopathology (through the Child Behavior Checklist). Similarity of interregional profiles of group differences in surface area and prenatal cell-specific gene expression was assessed. Results: Across the 11 cortical regions, group differences in cortical area for attention-deficit/hyperactivity disorder, schizophrenia, and Child Behavior Checklist were dominant in multimodal association cortices. The same interregional profiles were also associated with interregional profiles of (prenatal) gene expression specific to proliferative cells, namely radial glia and intermediate progenitor cells (greater expression, larger difference), as well as differentiated cells, namely excitatory neurons and endothelial and mural cells (greater expression, smaller difference). Finally, these cell types were implicated in known pre/perinatal risk factors for psychosis. Genes coexpressed with radial glia were enriched with genes implicated in congenital abnormalities, birth weight, hypoxia, and starvation. Genes coexpressed with endothelial and mural genes were enriched with genes associated with maternal hypertension and preterm birth. Conclusions: Our findings support a neurodevelopmental model of vulnerability to mental illness whereby prenatal risk factors acting through cell-specific processes lead to deviations from typical brain development during pregnancy
Search for electroweak production of single top quarks in collisions.
We present a search for electroweak production of single top quarks in the electron+jets and muon+jets decay channels. The measurements use ~90 pb^-1 of data from Run 1 of the Fermilab Tevatron collider, collected at 1.8 TeV with the DZero detector between 1992 and 1995. We use events that include a tagging muon, implying the presence of a b jet, to set an upper limit at the 95% confidence level on the cross section for the s-channel process ppbar->tb+X of 39 pb. The upper limit for the t-channel process ppbar->tqb+X is 58 pb. (arXiv
- âŠ