743 research outputs found
Quantum effects in linear and non-linear transport of T-shaped ballistic junction
We report low-temperature transport measurements of three-terminal T-shaped
device patterned from GaAs/AlGaAs heterostructure. We demonstrate the mode
branching and bend resistance effects predicted by numerical modeling for
linear conductance data. We show also that the backscattering at the junction
area depends on the wave function parity. We find evidence that in a non-linear
transport regime the voltage of floating electrode always increases as a
function of push-pull polarization. Such anomalous effect occurs for the
symmetric device, provided the applied voltage is less than the Fermi energy in
equilibrium
Effects of an Early-Time Impact Generated Vapor Blast in the Martian Atmosphere: Formation of High-Latitude Pedestal Craters
Following impact, vapor expansion creates an intense airblast that interacts with the ambient atmosphere. The resulting hemi-spherical shock wave leaves a signature on the surface that is dependent on initial atmospheric and surface conditions. Here we propose that the formation of pedestal craters (craters surrounded by an erosion-resistant pedestal) may be a direct consequence of extreme winds and elevated temperatures generated by such an impact-induced atmospheric blast. Pedestal craters, first recognized in Mariner 9 data, are a unique feature on Mars and likely a signature of near-surface volatiles. They are found at high latitudes (small pedestals, Amazonian to Late Hesperian in age) and in thick equatorial mantling deposits (larger pedestals, early Hesperian to Noachian in age). Previously suggested mechanisms for pedestal crater formation (e.g., wind: ejecta curtain vortices or vapor blast; and ejecta dust: armoring) do not provide a complete picture. The clear evidence for near-surface volatiles at high latitudes requires a re-evaluation of these alternative models. The results presented here suggest that a combined atmospheric blast/thermal model provides a plausible formation hypothesis
Could humans recognize odor by phonon assisted tunneling?
Our sense of smell relies on sensitive, selective atomic-scale processes that
are initiated when a scent molecule meets specific receptors in the nose.
However, the physical mechanisms of detection are not clear. While odorant
shape and size are important, experiment indicates these are insufficient. One
novel proposal suggests inelastic electron tunneling from a donor to an
acceptor mediated by the odorant actuates a receptor, and provides critical
discrimination. We test the physical viability of this mechanism using a simple
but general model. Using values of key parameters in line with those for other
biomolecular systems, we find the proposed mechanism is consistent both with
the underlying physics and with observed features of smell, provided the
receptor has certain general properties. This mechanism suggests a distinct
paradigm for selective molecular interactions at receptors (the swipe card
model): recognition and actuation involve size and shape, but also exploit
other processes.Comment: 10 pages, 1 figur
Development and Validation of a New Bitumen Fume Generation System which Generates Polycyclic Aromatic Hydrocarbon Concentrations Proportional to Fume Concentrations
Bitumen fumes emitted during road paving and roofing contain polycyclic aromatic compounds (PACs) of potential health concern. Little information is available for an experimental device devoted to inhalation experiments with animals exposed to bitumen fumes, and in all studies the systems were never validated for a range of fume concentrations, which prohibited their use for toxicological concentration-effect studies. Therefore, the purpose of this study was to validate a new experimental device able to generate bitumen fumes at different total particulate matter (TPM) concentrations with a linear correlation between TPM and the concentrations of different PACs, thus allowing toxicological dose-response studies with fumes representative of those in the field. Atmosphere samples collected from an animal exposure chamber allowed the determination of TPM, toluene soluble matter, polycyclic aromatic hydrocarbons (PAHs) and semi-volatiles. The particulate size distributions were determined in order to assess the deposition pattern in the respiratory tract. The temperature of 170°C was chosen by analogy with the upper range of the temperature used during paving operations. The temperature of the air passing over the fume emission area was regulated to 20°C and stirring of the heated bitumen was restricted to 90 r.p.m. The data show that the objective of developing a static fume generation system that reproducibly produces fumes in the inhalation chamber for specified target concentrations (TPM) were successful. The within-day variation coefficients for TPM were between 2.5 and 6.1%. The day-to-day variations for TPM concentration were between 4.1 and 5.8%. The concentrations of the 4-5 ring PAHs and the polycyclic aromatic sulphur heterocycles were proportional to the TPM concentration. The 2 and 3 ring PAH concentrations showed a deviation from proportionality with the TPM, probably due to their re-evaporation during sampling. The mass median aerodynamic diameter of airborne particles varied from 1.4 µm at a fume concentration of 5 mg/m3 to 3.2 µm at 100 mg/m3. In conclusion, this equipment was suitable for nose-only inhalation studies in the 5-100 mg/m3 range of TPM. Bitumen fumes were generated with a good reproducibility under well-controlled conditions. Finally, the PAH profiles from atmospheric samples were in good agreement with those measured during road pavin
The Shroud Around the Twin Radio Jets in NGC 1052
(Abridged) We discuss multiple VLBI continuum and spectral line observations
and WSRT spectroscopy of NGC 1052. Sub-parsec scale features move outward at
approximately 0.26c in bi-symmetric jets, most likely oriented near the plane
of the sky. Absorption and emission signatures reveal ionised, atomic, and
molecular components of the surrounding medium.
Seven-frequency (1.4 to 43 GHz) VLBA observations show free-free absorption
in the inner parsec, probably together with synchrotron self-absorption. There
is apparently a geometrically thick but patchy structure oriented roughly
orthogonal to the jets. The western jet is receding: it is covered more deeply
and extensively. HI spectral line VLBI reveals atomic gas in front of both
jets. There appear to be three velocity systems. The deepest, at "high
velocities" (receding by 125 to 200 km/s), seems restricted to a shell 1 to 2
pc away from the core, within which this gas might be largely ionised. WSRT
spectroscopy has revealed 1667 and 1665 MHz OH absorption with their line ratio
varying roughly from 1:1 to 2:1 between -35 and 200 km/s. In the high velocity
system the OH profiles are similar to HI, suggesting co-location of that atomic
and molecular gas, and leaving unclear the connection to the H2O masing gas
seen elsewhere. We have also detected both 18cm OH satellite lines in the high
velocity system. They have conjugate profiles: 1612 MHz is in absorption, and
1720 MHz in emission.Comment: 16 pages, 14 figures, LaTeX, includes aa.cls, accepted for
publication in Astronomy and Astrophysic
Soft X-Ray Sources at the Centers of the Elliptical Galaxies NGC 4472 and NGC 4649
Analysis of recent Chandra observations of the elliptical galaxies NGC 4472
and NGC 4649 has revealed faint soft X-ray sources at their centers. The
sources are located to within 1'' of the optical centers of the galaxies. They
are most likely associated with the central supermassive black holes. Interest
in these and several other similar objects stems from the unusually low
luminosity of the supermassive black holes embedded in dense interstellar
medium. Our Chandra sources have very soft spectra. They are detectable only
below ~0.6 keV and have luminosities in the 0.2-2.5 keV energy band of ~ 6 *
10^{37} erg/s and ~1.7 * 10^{38} erg/s in NGC 4649 and NGC 4472, respectively.Comment: Shortened version of the paper published in Astronomy Letter
Complex Visibilities of Cosmic Microwave Background Anisotropies
We study the complex visibilities of the cosmic microwave background
anisotropies that are observables in interferometric observations of the cosmic
microwave background, using the multipole expansion methods commonly adopted in
analyzing single-dish experiments. This allows us to recover the properties of
the visibilities that is obscured in the flat-sky approximation. Discussions of
the window function, multipole resolution, instrumental noise, pixelization,
and polarization are given.Comment: 22 pages, 1 figure include
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