6,324 research outputs found
Non trivial generalizations of the Schwinger pair production result
We present new, non trivial generalizations of the recent Tomaras, Tsamis and
Woodard extension of the original Schwinger formula for charged pair production
in a constant field.Comment: 11 page
Determination of complex dielectric functions of ion implanted and implantedâannealed amorphous silicon by spectroscopic ellipsometry
Measuring with a spectroscopic ellipsometer (SE) in the 1.8â4.5 eV photon energy region we determined the complex dielectric function (Ï” = Ï”1 + iÏ”2) of different kinds of amorphous silicon prepared by selfâimplantation and thermal relaxation (500â°C, 3 h). These measurements show that the complex dielectric function (and thus the complex refractive index) of implanted aâSi (iâaâSi) differs from that of relaxed (annealed) aâSi (râaâSi). Moreover, its Ï” differs from the Ï” of evaporated aâSi (eâaâSi) found in the handbooks as Ï” for aâSi. If we use this Ï” to evaluate SE measurements of ion implanted silicon then the fit is very poor. We deduced the optical band gap of these materials using the DavisâMott plot based on the relation: (Ï”2E2)1/3 ⌠(Eâ Eg). The results are: 0.85 eV (iâaâSi), 1.12 eV (eâaâSi), 1.30 eV (râaâSi). We attribute the optical change to annihilation of point defects
Ion-implantation-caused special damage profiles determined by spectroscopic ellipsometry in crystalline and in relaxed (annealed) amorphous silicon
We previously developed a fitting method of several parameters to evaluate ion-implantation-caused damage profiles from spectroscopic ellipsometry (SE) (M. Fried et al., J. Appl. Phys., 71 (1992) 2835). Our optical model consists of a stack of layers with fixed and equal thicknesses and damage levels described by a depth profile function (coupled half Gaussians). The complex refractive index of each layer is calculated from the actual damage level by Bruggeman effective medium approximation (EMA) using crystalline (c-Si) and amorphous (a-Si) silicon as end-points. Two examples are presented of the use of this method with modified optical models. First, we investigated the surface damage formed by room temperature B+ and N+ implantation into silicon. For the analysis of the SE data we added a near surface amorphous layer to the model with variable thickness. Second, we determined 20 keV B+ implantation-caused damage profiles in relaxed (annealed) amorphous silicon. In this special case, the complex refractive index of each layer was calculated from the actual damage level by the EMA using relaxed a-Si and implanted a-Si as end-points. The calculated profiles are compared with Monte Carlo simulations (TRIM code); good agreement is obtained
Atmospheric image blur with finite outer scale or partial adaptive correction
Seeing-limited resolution in large telescopes working over wide wavelength
range depends substantially on the turbulence outer scale and cannot be
adequately described by one "seeing" value. We attempt to clarify frequent
confusions on this matter. We study the effects of finite turbulence outer
scale and partial adaptive corrections by means of analytical calculations and
numerical simulations. If a von Karman turbulence model is adopted, a simple
approximate formula captures the dependence of atmospheric long-exposure
resolution on the outer scale over the entire practically interesting range of
telescope diameters and wavelengths. In the infrared (IR), the difference with
the standard Kolmogorov seeing formula can exceed a factor of two. We find that
low-order adaptive turbulence correction produces residual wave-fronts with
effectively small outer scale, so even very low compensation order leads to a
substantial improvement in resolution over seeing, compared to the standard
theory. Seeing-limited resolution of large telescopes, especially in the IR, is
currently under-estimated by not accounting for the outer scale. On the other
hand, adaptive-optics systems designed for diffraction-limited imaging in the
IR can improve the resolution in the visible by as much as two times.Comment: A&A accepte
Nonlocal spectral properties of disordered alloys
A general method is proposed for calculating a fully k-dependent, continuous,
and causal spectral function A(k,E) within the recently introduced nonlocal
version of the coherent-potential approximation (NLCPA). The method involves
the combination of both periodic and anti-periodic solutions to the associated
cluster problem and also leads to correct bulk quantities for small cluster
sizes. We illustrate the method by investigating the Fermi surface of a
two-dimensional alloy. Dramatically, we find a smeared electronic topological
transition not predicted by the conventional CPA.Comment: 17 pages, 5 figures, Submitted to: J. Phys.: Condens. Matter
Editorial receipt 25 May 200
The Double Quasar Q2138-431: Lensing by a Dark Galaxy?
We report the discovery of a new gravitational lens candidate Q2138-431AB,
comprising two quasar images at a redshift of 1.641 separated by 4.5 arcsecs.
The spectra of the two images are very similar, and the redshifts agree to
better than 115 km.sec. The two images have magnitudes and
, and in spite of a deep search and image subtraction procedure, no
lensing galaxy has been found with . Modelling of the system
configuration implies that the mass-to-light ratio of any lensing galaxy is
likely to be around , with an absolute lower limit of
for an Einstein-de Sitter universe. We conclude that
the most likely explanation of the observations is gravitational lensing by a
dark galaxy, although it is possible we are seeing a binary quasar.Comment: 17 pages (Latex), 8 postscript figures included, accepted by MNRA
Sequential Relational Decomposition
The concept of decomposition in computer science and engineering is
considered a fundamental component of computational thinking and is prevalent
in design of algorithms, software construction, hardware design, and more. We
propose a simple and natural formalization of sequential decomposition, in
which a task is decomposed into two sequential sub-tasks, with the first
sub-task to be executed before the second sub-task is executed. These tasks are
specified by means of input/output relations. We define and study decomposition
problems, which is to decide whether a given specification can be sequentially
decomposed. Our main result is that decomposition itself is a difficult
computational problem. More specifically, we study decomposition problems in
three settings: where the input task is specified explicitly, by means of
Boolean circuits, and by means of automatic relations. We show that in the
first setting decomposition is NP-complete, in the second setting it is
NEXPTIME-complete, and in the third setting there is evidence to suggest that
it is undecidable. Our results indicate that the intuitive idea of
decomposition as a system-design approach requires further investigation. In
particular, we show that adding a human to the loop by asking for a
decomposition hint lowers the complexity of decomposition problems
considerably
The optically-powerful quasar E1821+643 is associated with a 300-kpc scale FRI radio structure
We present a deep image of the optically-powerful quasar E1821+643 at 18cm
made with the Very Large Array (VLA). This image reveals radio emission, over
280 kpc in extent, elongated way beyond the quasar's host galaxy. Its radio
structure has decreasing surface brightness with increasing distance from the
bright core, characteristic of FRI sources (Fanaroff & Riley 1974). Its radio
luminosity at 5GHz falls in the classification for `radio-quiet' quasars (it is
only 10^23.9 W/Hz/sr; see e.g. Kellermann et al 1994). Its radio luminosity at
151MHz (which is 10^25.3 W/Hz/sr) is at the transition luminosity observed to
separate FRIs and FRIIs. Hitherto, no optically-powerful quasar had been found
to have a conventional FRI radio structure. For searches at low-frequency this
is unsurprising given current sensitivity and plausible radio spectral indices
for radio-quiet quasars. We demonstrate the inevitability of the extent of any
FRqI radio structures being seriously under-estimated by existing targetted
follow-up observations of other optically-selected quasars, which are typically
short exposures of z > 0.3 objects, and discuss the implications for the
purported radio bimodality in quasars.
The nature of the inner arcsec-scale jet in E1821+643, together with its
large-scale radio structure, suggest that the jet-axis in this quasar is
precessing (cf. Galactic jet sources such as SS433). A possible explanation for
this is that its central engine is a binary whose black holes have yet to
coalesce. The ubiquity of precession in `radio-quiet' quasars, perhaps as a
means of reducing the observable radio luminosity expected in highly-accreting
systems, remains to be established.Comment: Accepted by ApJ Letters; higher quality versions of figures available
at http://www-astro.physics.ox.ac.uk/~km
Optical properties and spatial distribution of MgII absorbers from SDSS image stacking
We present a statistical analysis of the photometric properties and spatial
distribution of more than 2,800 MgII absorbers with 0.37<z<1 and rest
equivalent width W_0(\lambda2796)>0.8\AA detected in SDSS quasar spectra. Using
an improved image stacking technique, we measure the cross-correlation between
MgII gas and light (in the g, r, i and z-bands) from 10 to 200 kpc and infer
the light-weighted impact parameter distribution of MgII absorbers. Such a
quantity is well described by a power-law with an index that strongly depends
on W_0, ranging from ~-1 for W_0~ 1.5\AA. At redshift
0.37<z<0.55, we find the average luminosity enclosed within 100 kpc around MgII
absorbers to be M_g=-20.65+-0.11 mag, which is ~0.5 L_g*. The global
luminosity-weighted colors are typical of present-day intermediate type
galaxies. However, while the light of weaker absorbers originates mostly from
red passive galaxies, stronger systems display the colors of blue star-forming
galaxies. Based on these observations, we argue that the origin of strong MgII
absorber systems might be better explained by models of metal-enriched gas
outflows from star-forming/bursting galaxies. Our analysis does not show any
redshift dependence for both impact parameter and rest-frame colors up to z=1.
However, we do observe a brightening of the absorbers related light at high
redshift (~50% from z~0.4 to 1). We argue that MgII absorbers are a phenomenon
typical of a given evolutionary phase that more massive galaxies experience
earlier than less massive ones, in a downsizing fashion. (abridged)Comment: ApJ in press, 28 pages, 16 figures, using emulateapj. Only typo
corrections wrt the original submission (v1
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