1,298 research outputs found
Electron transport through multilevel quantum dot
Quantum transport properties through some multilevel quantum dots sandwiched
between two metallic contacts are investigated by the use of Green's function
technique. Here we do parametric calculations, based on the tight-binding
model, to study the transport properties through such bridge systems. The
electron transport properties are significantly influenced by (a) number of
quantized energy levels in the dots, (b) dot-to-electrode coupling strength,
(c) location of the equilibrium Fermi energy and (d) surface disorder. In
the limit of weak-coupling, the conductance () shows sharp resonant peaks
associated with the quantized energy levels in the dots, while, they get
substantial broadening in the strong-coupling limit. The behavior of the
electron transfer through these systems becomes much more clearly visible from
our study of current-voltage (-) characteristics. In this context we also
describe the noise power of current fluctuations () and determine the Fano
factor () which provides an important information about the electron
correlation among the charge carriers. Finally, we explore a novel transport
phenomenon by studying the surface disorder effect in which the current
amplitude increases with the increase of the surface disorder strength in the
strong disorder regime, while, the amplitude decreases in the limit of weak
disorder. Such an anomalous behavior is completely opposite to that of bulk
disordered system where the current amplitude always decreases with the
disorder strength. It is also observed that the current amplitude strongly
depends on the system size which reveals the finite quantum size effect.Comment: 12 pages, 7 figure
Physical Conditoins in Orion's Veil II: A Multi-Component Study of the Line of Sight Toward the Trapezium
Orion's Veil is an absorbing screen that lies along the line of sight to the
Orion H II region. It consists of two or more layers of gas that must lie
within a few parsecs of the Trapezium cluster. Our previous work considered the
Veil as a whole and found that the magnetic field dominates the energetics of
the gas in at least one component. Here we use high-resolution STIS UV spectra
that resolve the two velocity components in absorption and determine the
conditions in each. We derive a volume hydrogen density, 21 cm spin
temperature, turbulent velocity, and kinetic temperature, for each. We combine
these estimates with magnetic field measurements to find that magnetic energy
significantly dominates turbulent and thermal energies in one component, while
the other component is close to equipartition between turbulent and magnetic
energies. We observe molecular hydrogen absorption for highly excited v, J
levels that are photoexcited by the stellar continuum, and detect blueshifted S
III and P III. These ions must arise from ionized gas between the mostly
neutral portions of the Veil and the Trapezium and shields the Veil from
ionizing radiation. We find that this layer of ionized gas is also responsible
for He I absorption in the Veil, which resolves a 40-year-old debate on the
origin of He I absorption towards the Trapezium. Finally, we determine that the
ionized and mostly atomic layers of the Veil will collide in less than 85,000
years.Comment: 43 pages, 15 figures, to be published in Ap
Physical Conditions in Orion's Veil
Orion's veil consists of several layers of largely neutral gas lying between
us and the main ionizing stars of the Orion nebula. It is visible in 21cm H I
absorption and in optical and UV absorption lines of H I and other species.
Toward the Trapezium, the veil has two remarkable properties, high magnetic
field (~100 microGauss) and a surprising lack of molecular hydrogen given its
total hydrogen column density. Here we compute photoionization models of the
veil to establish its gas density and its distance from the Trapezium. We use a
greatly improved model of the hydrogen molecule that determines level
populations in ~1e5 rotational/vibrational levels and provides improved
estimates of molecular hydrogen destruction via the Lyman-Werner bands. Our
best fit photoionization models place the veil 1-3 pc in front of the star at a
density of 1e3-1e4 cubic centimeters. Magnetic energy dominates the energy of
non-thermal motions in at least one of the 21cm H I velocity components.
Therefore, the veil is the first interstellar environment where magnetic
dominance appears to exist. We find that the low ratio of molecular to atomic
hydrogen (< 1e-4) is a consequence of high UV flux incident upon the veil due
to its proximity to the Trapezium stars and the absence of small grains in the
region.Comment: 45 pages, 20 figures, accepted for publication in Ap
A model for atomic and molecular interstellar gas: The Meudon PDR code
We present the revised ``Meudon'' model of Photon Dominated Region (PDR
code), presently available on the web under the Gnu Public Licence at:
http://aristote.obspm.fr/MIS. General organisation of the code is described
down to a level that should allow most observers to use it as an interpretation
tool with minimal help from our part. Two grids of models, one for low
excitation diffuse clouds and one for dense highly illuminated clouds, are
discussed, and some new results on PDR modelisation highlighted.Comment: accepted in ApJ sup
Contribution des rejets de dĂ©versoirs dâorage aux flux de micropolluants Ă©mis dans un milieu rĂ©cepteur urbain
Colloque avec actes et comité de lecture. Internationale.International audienc
Cancer and renal insufficiency results of the BIRMA study
Background: Half of anticancer drugs are predominantly excreted in urine. Dosage adjustment in renal insufficiency (RI) is, therefore, a crucial issue. Moreover, patients with abnormal renal function are at high risk for drug-induced nephrotoxicity. The Belgian Renal Insufficiency and Anticancer Medications (BIRMA) study investigated the prevalence of RI in cancer patients, and the profile/dosing of anticancer drugs prescribed. Methods:Primary end point: to estimate the prevalence of abnormal glomerular filtration rate (GFR; estimated with the abbreviated Modification of Diet in Renal Disease formula) and RI in cancer patient. Secondary end point: to describe the profile of anticancer drugs prescribed (dose reduction/nephrotoxicity). Data were collected for patients presenting at one of the seven Belgian BIRMA centres in March 2006. Results: A total of 1218 patients were included. The prevalence of elevated SCR (1.2 mg per 100 ml) was 14.9%, but 64.0% had a GFR90 ml min 1 per 1.73 m 2. In all, 78.6% of treated patients (n1087) were receiving at least one drug needing dosage adjustment and 78.1% received at least one nephrotoxic drug. In all, 56.5% of RI patients receiving chemotherapy requiring dose reduction in case of RI did not receive dose adjustment. Conclusions: The RI is highly frequent in cancer patients. In all, 80% of the patients receive potentially nephrotoxic drugs and/or for which dosage must be adjusted in RI. Oncologists should check the appropriate dose of chemotherapeutic drugs in relation to renal function before prescribing. © 2010 Cancer Research UK.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
Infrared Spectroscopy of Molecular Supernova Remnants
We present Infrared Space Observatory spectroscopy of sites in the supernova
remnants W28, W44, and 3C391, where blast waves are impacting molecular clouds.
Atomic fine-structure lines were detected from C, N, O, Si, P, and Fe. The S(3)
and S(9) lines of H2 were detected for all three remnants. The observations
require both shocks into gas with moderate (~ 100 /cm3) and high (~10,000 /cm3)
pre-shock densities, with the moderate density shocks producing the ionic lines
and the high density shock producing the molecular lines. No single shock model
can account for all of the observed lines, even at the order of magnitude
level. We find that the principal coolants of radiative supernova shocks in
moderate-density gas are the far-infrared continuum from dust grains surviving
the shock, followed by collisionally-excited [O I] 63.2 and [Si II] 34.8 micron
lines. The principal coolant of the high-density shocks is
collisionally-excited H2 rotational and ro-vibrational line emission. We
systematically examine the ground-state fine structure of all cosmically
abundant elements, to explain the presence or lack of all atomic fine lines in
our spectra in terms of the atomic structure, interstellar abundances, and a
moderate-density, partially-ionized plasma. The [P II] line at 60.6 microns is
the first known astronomical detection. There is one bright unidentified line
in our spectra, at 74.26 microns. The presence of bright [Si II] and [Fe II]
lines requires partial destruction of the dust. The required gas-phase
abundance of Fe suggests 15-30% of the Fe-bearing grains were destroyed. The
infrared continuum brightness requires ~1 Msun of dust survives the shock,
suggesting about 1/3 of the dust mass was destroyed, in agreement with the
depletion estimate and with theoretical models for dust destruction.Comment: 40 pages; 10 figures; accepted by ApJ July 11, 200
An Interstellar Conduction Front Within a Wolf-Rayet Ring Nebula Observed with the GHRS
With the High Resolution Spectrograph aboard the Hubble Space Telescope we
obtained high signal-to-noise (S/N > 200 - 600 per 17 km/s resolution element)
spectra of narrow absorption lines toward the Wolf-Rayet star HD 50896. The
ring nebula S308 that surrounds this star is thought to be caused by a
pressure-driven bubble bounded by circumstellar gas (most likely from a red
supergiant or luminous blue variable progenitor) pushed aside by a strong
stellar wind. Our observation has shown for the first time that blueshifted
(approximately 70 km/s relative to the star) absorption components of C IV and
N V arise in a conduction front between the hot interior of the bubble and the
cold shell of swept-up material. These lines set limits on models of the
conduction front. Nitrogen in the shell appears to be overabundant by a factor
~10. The P Cygni profiles of N V and C IV are variable, possibly due to a
suspected binary companion to HD 50896.Comment: 32 pages, Latex, to appear in the Astrophysical Journal, April, 199
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