340 research outputs found
Nonadiabatic scattering of a quantum particle in an inhomogenous magnetic field
We investigate the quantum effects, in particular the Landau-level
quantization, in the scattering of a particle the nonadiabatic classical
dynamics of which is governed by an adiabatic invariant. As a relevant example,
we study the scattering of a drifting particle on a magnetic barrier in the
quantum limit where the cyclotron energy is much larger than a broadening of
the Landau levels induced by the nonadiabatic transitions. We find that,
despite the level quantization, the exponential suppression (barrier width , orbital shift per cyclotron revolution )
of the root-mean-square transverse displacement experienced by the particle
after the scattering is the same in the quantum and the classical regime.Comment: 4 page
The quest for hot gas in the halo of NGC 1511
XMM-Newton observations of the starburst galaxy NGC 1511 reveal the presence
of a previously unknown extended hot gaseous phase of its ISM, which partly
extends out of the disk plane. The emission distribution is asymmetric, being
brightest in the eastern half of the galaxy, where also radio continuum
observations suggest the highest level of star formation. Spectral analysis of
the integral 0.2-12 keV X-ray emission from NGC 1511 indicates a complex
emission composition. A model comprising a power law plus thermal plasma
component, both absorbed by foreground gas, cannot explain all details of the
observed spectrum, requiring a third spectral component to be added. This
component can be a second thermal plasma, but other spectral models can be
fitted as well. Its X-ray properties characterize NGC 1511 as a starburst
galaxy. The X-ray-to-infrared luminosity ratio is consistent with this result.
Together with the X-ray data, XMM-Newton obtained UV images of NGC 1511,
tracing massive stars heating the ambient gas, which is then seen in H\alpha
emission. UV, H\alpha and near-infrared imagery suggest that NGC 1511 is
disturbed, most likely by its two small companions, NGC 1511a and NGC 1511b.Comment: 7 pages, 7 figures, accepted for publication in A&
Spatially-Correlated Microstructure and Superconductivity in Polycrystalline Boron-Doped Diamond
Scanning tunneling spectroscopies are performed below 100~mK on
nano-crystalline boron-doped diamond films characterized by Transmission
Electron Microscopy and transport measurements. We demonstrate a strong
correlation between the local superconductivity strength and the granular
structure of the films. The study of the spectral shape, amplitude and
temperature dependence of the superconductivity gap enables us to differentiate
intrinsically superconducting grains that follow the BCS model, from grains
showing a different behavior involving the superconducting proximity effect
Absence of boron aggregates in superconducting silicon confirmed by atom probe tomography
Superconducting boron-doped silicon films prepared by gas immersion laser
doping (GILD) technique are analyzed by atom probe tomography. The resulting
three-dimensional chemical composition reveals that boron atoms are
incorporated into crystalline silicon in the atomic percent concentration
range, well above their solubility limit, without creating clusters or
precipitates at the atomic scale. The boron spatial distribution is found to be
compatible with local density of states measurements performed by scanning
tunneling spectroscopy. These results, combined with the observations of very
low impurity level and of a sharp two-dimensional interface between doped and
undoped regions show, that the Si:B material obtained by GILD is a well-defined
random substitutional alloy endowed with promising superconducting properties.Comment: 4 page
The mysterious HI deficiency of NGC 3175
Australia Telescope Compact Array HI observations reveal the existence of
5.8x10^8 M_sun of HI gas in the central 7 kpc of the edge-on spiral galaxy NGC
3175. The detected HI and CO gas can explain why star formation, as traced by
other emission processes, is going on in the inner part of its disk. On the
other hand, the entire outer disk, beyond 3.5 kpc radius, shows no HI emission,
has a very red colour and exhibits neither radio continuum nor H-alpha
emission. This indicates that the outer part of NGC 3175 is quiescent, i.e. not
forming stars at a measurable rate. Its HI deficiency and the small extent of
the HI layer, which is confined to the boundaries of the optically visible
disk, make NGC 3175 a peculiar spiral galaxy. No intergalactic HI gas in the
NGC 3175 group was detected in our interferometric observations. Earlier Parkes
telescope single dish HI observations put an upper limit on the amount of
diffuse gas that might have been missed by the interferometer at 2x10^8 M_sun.
On DSS plates no galaxy in the NGC 3175 group of galaxies (Garcia 1993) is
close enough to it and none exhibits disturbances that could indicate a close
interaction which might have led to the stripping of large parts of its HI gas.
Thus, despite an extensive multi-wavelength investigation, the reason for the
unusual absence of HI and star formation activity in the outer disk of NGC 3175
remains an intriguing mystery.Comment: 8 pages, 6 figures, uses aa.cls. Accepted for publication in
Astronomy & Astrophysic
Subkelvin tunneling spectroscopy showing Bardeen-Cooper-Schrieffer superconductivity in heavily boron-doped silicon epilayers
Scanning tunneling spectroscopies in the subKelvin temperature range were
performed on superconducting Silicon epilayers doped with Boron in the atomic
percent range. The resulting local differential conductance behaved as expected
for a homogeneous superconductor, with an energy gap dispersion below +/- 10%.
The spectral shape, the amplitude and temperature dependence of the
superconductivity gap follow the BCS model, bringing further support to the
hypothesis of a hole pairing mechanism mediated by phonons in the weak coupling
limit.Comment: 4 pages, 3 figure
An X-ray Mini-survey of Nearby Edge-on Starburst Galaxies II. The Question of Metal Abundance
(abbreviated) We have undertaken an X-ray survey of a far-infrared flux
limited sample of seven nearby edge-on starburst galaxies. Here, we examine the
two X-ray-brightest sample members NGC 253 and M 82 in a self-consistent
manner, taking account of the spatial distribution of the X-ray emission in
choosing our spectral models. There is significant X-ray absorption in the disk
of NGC 253. When this is accounted for we find that multi-temperature thermal
plasma models with significant underlying soft X-ray absorption are more
consistent with the imaging data than single-temperature models with highly
subsolar abundances or models with minimal absorption and non-equilibrium
thermal ionization conditions. Our models do not require absolute abundances
that are inconsistent with solar values or unusually supersolar ratios of the
alpha-burning elements with respect to Fe (as claimed previously). We conclude
that with current data, the technique of measuring abundances in starburst
galaxies via X-ray spectral modeling is highly uncertain.
Based on the point-like nature of much of the X-ray emission in the PSPC
hard-band image of NGC 253, we suggest that a significant fraction of the
``extended'' X-ray emission in the 3-10 keV band seen along the disk of the
galaxy with ASCA and BeppoSAX (Cappi et al.) is comprised of discrete sources
in the disk, as opposed to purely diffuse, hot gas. This could explain the low
Fe abundances of ~1/4 solar derived for pure thermal models.Comment: (accepted for publication in the Astrophysical Journal
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