4,263 research outputs found
Characterization of the Local Density of States Fluctuations near the Integer Quantum Hall Transition in a Quantum Dot Array
We present a calculation for the second moment of the local density of states
in a model of a two-dimensional quantum dot array near the quantum Hall
transition. The quantum dot array model is a realistic adaptation of the
lattice model for the quantum Hall transition in the two-dimensional electron
gas in an external magnetic field proposed by Ludwig, Fisher, Shankar and
Grinstein. We make use of a Dirac fermion representation for the Green
functions in the presence of fluctuations for the quantum dot energy levels. A
saddle-point approximation yields non-perturbative results for the first and
second moments of the local density of states, showing interesting fluctuation
behaviour near the quantum Hall transition. To our knowledge we discuss here
one of the first analytic characterizations of chaotic behaviour for a
two-dimensional mesoscopic structure. The connection with possible experimental
investigations of the local density of states in the quantum dot array
structures (by means of NMR Knight-shift or single-electron-tunneling
techniques) and our work is also established.Comment: 11 LaTeX pages, 1 postscript figure, to appear in Phys.Rev.
Production cross section of At radionuclides from Li+Pb and Be+Tl reactions
Earlier we reported theoretical studies on the probable production of
astatine radionuclides from Li and Be-induced reactions on
natural lead and thalliun targets, respectively. For the first time, in this
report, production of astatine radionuclides has been investigated
experimentally with two heavy ion induced reactions:
Be+Tl and Li+Pb. Formation cross
sections of the evaporation residues, At, produced in (HI,
xn) channel, have been measured by the stacked-foil technique followed by the
off-line -spectrometry at the low incident energies (50 MeV).
Measured excitation functions have been explained in terms of compound nuclear
reaction mechanism using Weisskopf-Ewing and Hauser-Feshbach model. Absolute
cross section values are lower than the respective theoretical predictions.Comment: 11 pages 6 figure
Structural defects induced by Fe-ion implantation in TiO2
X-ray photoelectron spectroscopy (XPS) and resonant x-ray emission
spectroscopy (RXES) measurements of pellet and thin film forms of TiO with
implanted Fe ions are presented and discussed. The findings indicate that
Fe-implantation in a TiO pellet sample induces heterovalent cation
substitution (Fe Ti) beneath the surface region. But
in thin film samples, the clustering of Fe atoms is primarily detected. In
addition to this, significant amounts of secondary phases of Fe are
detected on the surface of all doped samples due to oxygen exposure. These
experimental findings are compared with density functional theory (DFT)
calculations of formation energies for different configurations of structural
defects in the implanted TiO:Fe system. According to our calculations, the
clustering of Fe-atoms in TiO:Fe thin films can be attributed to the
formation of combined substitutional and interstitial defects. Further, the
differences due to Fe doping in pellet and thin film samples can ultimately be
attributed to different surface to volume ratios.Comment: 7+ pages, 3 Figure, to appear in J. Appl. Phy
Helicobacter pylori and cancer among adults in Uganda
Data from Africa on infection with Helicobacter pylori (H. pylori) are sparse. Therefore, as part of an epidemiological study of cancer in Uganda, we investigated the prevalence and determinants of antibodies against H. pylori among 854 people with different cancer types and benign tumours. Patients were recruited from hospitals in Kampala, Uganda, interviewed about various demographic and lifestyle factors and tested for antibodies against H. pylori. In all patients combined, excluding those with stomach cancer (which has been associated with H. pylori infection), the prevalence of antibodies was 87% (723/833) overall, but declined with increasing age (p = 0.02) and was lower among people who were HIV seropositive compared to seronegative (p <0.001). Otherwise, there were few consistent epidemiological associations. Among those with stomach cancer, 18/21 (86%) had anti-H. pylori antibodies (odds ratio 0.8, 95% confidence intervals 0.2–2.9, p = 0.7; estimated using all other patients as controls, with adjustment for age, sex and HIV serostatus). No other cancer site or type was significantly associated with anti-H. pylori antibodies. The prevalence of H. pylori reported here is broadly in accord with results from other developing countries, although the determinants of infection and its' role in the aetiology of gastric cancer in Uganda remain unclear
Measurement of (α,n) reaction cross sections of erbium isotopes for testing astrophysical rate predictions
Date of Acceptance: 30/01/2015The γ-process in core-collapse and/or type Ia supernova explosions is thought to explain the origin of the majority of the so-called p nuclei (the 35 proton-rich isotopes between Se and Hg). Reaction rates for γ-process reaction network studies have to be predicted using Hauser-Feshbach statistical model calculations. Recent investigations have shown problems in the prediction of α-widths at astrophysical energies which are an essential input for the statistical model. It has an impact on the reliability of abundance predictions in the upper mass range of the p nuclei. With the measurement of the 164,166Er(α,n)167,169Yb reaction cross sections at energies close to the astrophysically relevant energy range we tested the recently suggested low energy modification of the α+nucleus optical potential in a mass region where γ-process calculations exhibit an underproduction of the p nuclei. Using the same optical potential for the α-width which was derived from combined 162Er(α,n) and 162Er(α,γ) measurement makes it plausible that a low-energy modification of the optical α+nucleus potential is needed.Peer reviewedFinal Accepted Versio
The formation of Ti-O tetrahedra and band gap reduction in SiO2 via pulsed ion implantation
Titanium ions are implanted into amorphous SiO2 at two different fluences using pulsed ion implantation, and the resulting samples are annealed. Bulk sensitive soft X-ray absorption spectroscopy of the Ti L 2, 3 edge reveal strikingly different spectra for the two fluences. Spectral simulations using multiplet crystal field theory show clearly that for low fluence the Ti ions have a local octahedral coordination, while at higher fluence the formation of Ti4-O tetrahedra dominates. Using O K-edge X-ray absorption and emission, the effect of the Ti states on the valence and conduction bands of the host SiO2 is revealed. With the introduction of Ti tetrahedra, the band gap reduces from about 8 eV to just over 4 eV, due entirely to the Ti 3d conduction band states. These results demonstrate the possibility to obtain Ti-O tetrahedra in silica by Ti ion implantation and a suitable thermal treatment, clarify the mechanism of band gap reduction with Ti doping in SiO2, and demonstrate the sensitivity of L-edge X-ray absorption with a multiplet crystal field theory analysis to the Ti coordination of novel materials. © 2013 American Institute of Physics
Investigating the History of Aubrites Using X-Ray Computed Tomography and Bulk Partition Coefficients
The aubrites are a unique group of differentiated meteorites that formed on parent bodies with oxygen fugacities (O2) from ~2 to ~6 log units below the iron-wustite buffer. At these highly reduced condi- tions, elements deviate from the geochemical behavior exhibited at terrestrial O2, and may form FeO-poor silicates, Si-bearing metals, and exotic sulfides. Geochemical examinations of aubrites, such as mineral major-element compositions, bulk-rock compositions, O isotopes, and crystallization ages, are crucial to understand their formation and evolution at extreme O2 conditions. In this study, we determine partitioning relationships of elements between bulk silicate, sulfide, and metal phases within aubrites, and compare the results to partition coefficients determined from petrologic experiments run under mercurian conditions. While previous studies have described the petrology and 2D modal abundances of aubrites, this work provides the first 3D view of aubritic mineralogies, which are com- pared to the available 2D data. Constraints of 3D modal abundances will increase the accuracy of computed bulk distribution coefficients; therefore, 3D scans of aubrite samples are imperative. We utilize X-ray computed tomogra- phy (XCT) to non-destructively analyze the distribution and abundances of mineral phases in aubrites and locate composite clasts of sulfide grains for future analysis
Risk factors for high anti-HHV-8 antibody titers (≥1:51,200) in black, HIV-1 negative South African cancer patients: a case control study
Background: Infection with human herpesvirus 8 (HHV-8) is the necessary causal agent in the
development of Kaposi's sarcoma (KS). Infection with HIV-1, male gender and older age all increase
risk for KS. However, the geographic distribution of HHV-8 and KS both prior to the HIV/AIDS
epidemic and with HIV/AIDS suggest the presence of an additional co-factor in the development of
KS.
Methods: Between January 1994 and October 1997, we interviewed 2576 black in-patients with
cancer in Johannesburg and Soweto, South Africa. Blood was tested for antibodies against HIV-1
and HHV-8 and the study was restricted to 2191 HIV-1 negative patients. Antibodies against the
latent nuclear antigen of HHV-8 encoded by orf73 were detected with an indirect
immunofluorescence assay. We examined the relationship between high anti-HHV-8 antibody
titers (≥1:51,200) and sociodemographic and behavioral factors using unconditional logistic
regression models. Variables that were significant at p = 0.10 were included in multivariate analysis.
Results: Of the 2191 HIV-1 negative patients who did not have Kaposi's sarcoma, 854 (39.0%)
were positive for antibodies against HHV-8 according to the immunofluorescent assay. Among
those seropositive for HHV-8, 530 (62.1%) had low titers (1:200), 227 (26.6%) had medium titers
(1:51,200) and 97 (11.4%) had highest titers (1:204,800). Among the 2191 HIV-1 negative patients,
the prevalence of high anti-HHV-8 antibody titers (≥1:51,200) was independently associated with
increasing age (ptrend = 0.04), having a marital status of separated or divorced (p = 0.003), using
wood, coal or charcoal as fuel for cooking 20 years ago instead of electricity (p = 0.02) and
consuming traditional maize beer more than one time a week (p = 0.02; p-trend for increasing
consumption = 0.05) although this may be due to chance given the large number of predictors
considered in this analysis.
Conclusions: Among HIV-negative subjects, patients with high anti-HHV-8 antibody titers are
characterized by older age. Other associations that may be factors in the development of high anti-
HHV-8 titers include exposure to poverty or a low socioeconomic status environment and
consumption of traditional maize beer. The relationship between these variables and high anti-
HHV-8 titers requires further, prospective study
Universal Constraints on Low-Energy Flavour Models
It is pointed out that in a general class of flavour models one can identify
certain universally present FCNC operators, induced by the exchange of heavy
flavour messengers. Their coefficients depend on the rotation angles that
connect flavour and fermion mass basis. The lower bounds on the messenger scale
are derived using updated experimental constraints on the FCNC operators. The
obtained bounds are different for different operators and in addition they
depend on the chosen set of rotations. Given the sensitivity expected in the
forthcoming experiments, the present analysis suggests interesting room for
discovering new physics. As the highlights emerge the leptonic processes,
, and
conversion in nuclei.Comment: 18 pages, 3 figures; v2 matches published versio
Investigations of three, four, and five-particle exit channels of levels in light nuclei created using a 9C beam
The interactions of a E/A=70-MeV 9C beam with a Be target was used to
populate levels in Be, B, and C isotopes which undergo decay into many-particle
exit channels. The decay products were detected in the HiRA array and the level
energies were identified from their invariant mass. Correlations between the
decay products were examined to deduce the nature of the decays, specifically
to what extent all the fragments were created in one prompt step or whether the
disintegration proceeded in a sequential fashion through long-lived
intermediate states. In the latter case, information on the spin of the level
was also obtained. Of particular interest is the 5-body decay of the 8C ground
state which was found to disintegrate in two steps of two-proton decay passing
through the 6Beg.s. intermediate state. The isobaric analog of 8Cg.s. in 8B was
also found to undergo two-proton decay to the isobaric analog of 6Beg.s. in
6Li. A 9.69-MeV state in 10C was found to undergo prompt 4-body decay to the
2p+2alpha exit channel. The two protons were found to have a strong
enhancementin the diproton region and the relative energies of all four p-alpha
pairs were consistent with the 5Lig.s. resonance
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