5,958 research outputs found
Effects of magnetic field and disorder on electronic properties of Carbon Nanotubes
Electronic properties of metallic and semiconducting carbon nanotubes are
investigated in presence of magnetic field perpendicular to the CN-axis, and
disorder introduced through energy site randomness. The magnetic field field is
shown to induce a metal-insulator transition (MIT) in absence of disorder, and
surprisingly disorder does not affect significantly the MIT. These results may
find confirmation through tunneling experimentsComment: 4 pages, 6 figures. Phys. Rev. B (in press
Galaxy Clustering Evolution in the UH8K Weak Lensing Fields
We present measurements of the two-point galaxy angular correlation function
as a function of apparent magnitude, color, and morphology. We present new
galaxy number counts to limiting magnitudes of I=24.0 and V=25.0. We find
to be well described by a power-law of slope -0.8. We find the
amplitude of the correlation function to decrease monotonically with
increasingly faint apparent magnitude. We compare with predictions utilizing
redshift distributions based on deep spectroscopic observations. We conclude
that simple redshift-dependent models which characterize evolution by means of
the epsilon parameter inadequately describe the observations. We find a strong
clustering dependence on V-I color because galaxies of extreme color lie at
similar redshifts and the angular correlation functions for these samples are
minimally diluted by chance projections.
We then present the first attempt to investigate the redshift evolution of
clustering, utilizing a population of galaxies of the same morphological type
and absolute luminosity. We study the dependence of on
redshift for Lstar early-type galaxies in the redshift range 0.2<z<0.9.
Although uncertainties are large, we find the evolution in the clustering of
these galaxies to be consistent with stable clustering [epsilon=0]. We find
Lstar early-type galaxies to cluster slightly more strongly (rnought =
5.25\pm0.28 \hMpc assuming epsilon=0) than the local full field population.
This is in good agreement with the 2dFGRS value for Lstar early-type galaxies
in the local universe (abridged).Comment: 41 pages, including 12 figs, 10 tables, to appear in Ap
Competition between magnetic field dependent band structure and coherent backscattering in multiwall carbon nanotubes
Magnetotransport measurements in large diameter multiwall carbon nanotubes
(20-40 nm) demonstrate the competition of a magnetic-field dependent
bandstructure and Altshuler-Aronov-Spivak oscillations. By means of an
efficient capacitive coupling to a backgate electrode, the magnetoconductance
oscillations are explored as a function of Fermi level shift. Changing the
magnetic field orientation with respect to the tube axis and by ensemble
averaging, allows to identify the contributions of different Aharonov-Bohm
phases. The results are in qualitative agreement with numerical calculations of
the band structure and the conductance.Comment: 4 figures, 5 page
Mesoscale Equipartition of kinetic energy in Quantum Turbulence
The turbulence of superfluid helium is investigated numerically at finite
temperature. Direct numerical simulations are performed with a "truncated HVBK"
model, which combines the continuous description of the
Hall-Vinen-Bekeravich-Khalatnikov equations with the additional constraint that
this continuous description cannot extend beyond a quantum length scale
associated with the mean spacing between individual superfluid vortices. A good
agreement is found with experimental measurements of the vortex density.
Besides, by varying the turbulence intensity only, it is observed that the
inter-vortex spacing varies with the Reynolds number as , like the
viscous length scale in classical turbulence. In the high temperature limit,
Kolmogorov's inertial cascade is recovered, as expected from previous numerical
and experimental studies. As the temperature decreases, the inertial cascade
remains present at large scales while, at small scales, the system evolves
towards a statistical equipartition of kinetic energy among spectral modes,
with a characteristic velocity spectrum. The accumulation of superfluid
excitations on a range of mesoscales enables the superfluid to keep dissipating
kinetic energy through mutual friction with the residual normal fluid, although
the later becomes rare at low temperature. It is found that most of the
superfluid vorticity can concentrate on these mesoscales at low temperature,
while it is concentrated in the inertial range at higher temperature. This
observation should have consequences on the interpretation of decaying
turbulence experiments, which are often based on vortex line density
measurements.Comment: 6 pages, 5 figure
Dust Properties and Star-Formation Rates in Star-Forming Dwarf Galaxies
We have used the Spitzer Space Telescope to study the dust properties of a
sample of star-forming dwarf galaxies. The differences in the mid-infrared
spectral energy distributions for these galaxies which, in general, are low
metallicity systems, indicate differences in the physical properties, heating,
and/or distribution of the dust. Specifically, these galaxies have more hot
dust and/or very small grains and less PAH emission than either spiral or
higher luminosity starburst galaxies. As has been shown in previous studies,
there is a gradual decrease in PAH emission as a function of metallicity.
Because much of the energy from star formation in galaxies is re-radiated in
the mid-infrared, star-formation rate indicators based on both line and
continuum measurements in this wavelength range are coming into more common
usage. We show that the variations in the interstellar medium properties of
galaxies in our sample, as measured in the mid-infrared, result in over an
order of magnitude spread in the computed star-formation rates.Comment: 25 pages, 11 figures, 4 tables, ApJ accepte
Persistent currents in carbon nanotubes based rings
Persistent currents in rings constructed from carbon nanotubes are
investigated theoretically. After studying the contribution of finite
temperature or quenched disorder on covalent rings, the complexity due to the
bundle packing is addressed. The case of interacting nanotori and
self-interacting coiled nanotubes are analyzed in details in relation with
experiments.Comment: 7 sections, 9 figure
Prevalence of hepatitis B immunity and infection in home self-sampling HIV service users
OBJECTIVES: Although hepatitis B virus (HBV) vaccination for high-risk groups including gay, bisexual and other men who have sex with men (MSM) is recommended in the UK, data on HBV immunisation coverage are limited. This study aimed to understand the prevalence of HBV infection, susceptibility and immunity due to immunisation among a high-risk population of MSM and heterosexuals who are less likely to attend sexual health services. METHODS: Residual HIV-negative serology samples archived from a national HIV self-sampling service in 2016 were tested for HBV markers using an unlinked anonymous approach. Prevalence of HBV infection, evidence of immunisation and susceptibility were calculated and stratified by individuals' characteristics. Multinomial logistic regression was used to estimate relative risk ratios (RRRs) associated with covariates. RESULTS: Of 2172 samples tested, 1497 (68.9%) were from MSM and 657 (30.2%) were from heterosexuals. Susceptibility to HBV infection was 66.1% among MSM and 77.0% among heterosexuals. Only 29.9% of MSM and 17.4% of heterosexuals had serological evidence of immunisation. Current infection was 1.1% in heterosexuals and 0.2% in MSM. Adjusted analysis showed evidence of immunisation was lower among heterosexuals (RRR 0.66, 95% CI 0.50 to 0.86) and those with no previous HIV test (RRR 0.41, 95% CI 0.31 to 0.54), and higher in those of other white or other ethnicity. CONCLUSIONS: Among MSM and heterosexual users of a self-sampling HIV service, evidence of immunisation to HBV infection was low and susceptibility to infection was comparatively high, suggesting suboptimal delivery of HBV immunisation in sexual health services
Residual stresses and practical adhesion: effect of organo-metallic complex formation and crystallization
Epoxy-amine liquid pre-polymers are often applied onto metallic substrates and cured to obtain painted materials or bonded joint structures. The overall performance of such systems depends on the interphase created between the epoxy amine polymer and the metallic substrate. When epoxyamine liquid mixtures are applied onto a metallic oxide layer, concomitant amine chemisorption and oxide dissolution occur leading to organo-metallic complex formation. Depending on the amine nature, as soon as the organo-metallic complex concentration is higher than the solubility product (e.g., isophoronediamine (IPDA)), these organo-metallic complexes crystallize as sharp needles. At the same time, the uncrystallized organo-metallic complexes react with the epoxy monomer to form, after curing cycle, a new network. Moreover, the crystal size increases with the solid/liquid contact time leading to an increase of intrinsic residual stresses and Young's modulus. When aliphatic diethylenetriamine (DETA) was used, no crystallization occurred, but the interphase formation was observed. The aim of this study was to understand and to establish the role of crystallization of organo-metallic complexes formed within the interphase on the practical adhesion performance. As the crystallization of the organo-metallic complex depends on the nature of the amine, two amine hardeners were used (IPDA inducing the formation of crystals and DETA without formation of crystals). For DGEBA-IPDA systems, the ultimate load decreases while residual stresses increase when the liquid/solid contact time increases. When no crystal formation was observed (e.g., DGEBADETA system), residual stresses, coating Young's modulus and ultimate load values all remained nearly constant irrespective of the liquid/solid contact time
9.7 um Silicate Features in AGNs: New Insights into Unification Models
We describe observations of 9.7 um silicate features in 97 AGNs, exhibiting a
wide range of AGN types and of X-ray extinction toward the central nuclei. We
find that the strength of the silicate feature correlates with the HI column
density estimated from fitting the X-ray data, such that low HI columns
correspond to silicate emission while high columns correspond to silicate
absorption. The behavior is generally consistent with unification models where
the large diversity in AGN properties is caused by viewing-angle-dependent
obscuration of the nucleus. Radio-loud AGNs and radio-quiet quasars follow
roughly the correlation between HI columns and the strength of the silicate
feature defined by Seyfert galaxies. The agreement among AGN types suggests a
high-level unification with similar characteristics for the structure of the
obscuring material. We demonstrate the implications for unification models
qualitatively with a conceptual disk model. The model includes an inner
accretion disk (< 0.1 pc in radius), a middle disk (0.1-10 pc in radius) with a
dense diffuse component and with embedded denser clouds, and an outer clumpy
disk (10-300 pc in radius).Comment: Accepted for publication in ApJ, 14 pages, 5 figures. The on-line
table is available at http://cztsy.as.arizona.edu/~yong/silicate_tab1.pd
Quantum partition noise of photo-created electron-hole pairs
We show experimentally that even when no bias voltage is applied to a quantum
conductor, the electronic quantum partition noise can be investigated using GHz
radiofrequency irradiation of a reservoir. Using a Quantum Point Contact
configuration as the ballistic conductor we are able to make an accurate
determination of the partition noise Fano factor resulting from the
photo-assisted shot noise. Applying both voltage bias and rf irradiation we are
able to make a definitive quantitative test of the scattering theory of
photo-assisted shot noise.Comment: 4 pages, 4 figure
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