4,755 research outputs found
Resummations in the Bloch-Nordsieck model
We studied different levels of resummations of the exactly solvable
Bloch-Nordsieck model in order to be able to compare the approximations with an
exact result. We studied one-loop perturbation theory, 2PI resummation and
Schwinger-Dyson equations truncated in a way to maintain Ward-identities. At
all levels we carefully performed renormalization. We found that although the
2PI resummation does not exhibit infrared sensitivity at the mass shell (the
one-loop perturbation theory does), but it is still far from the exact
solution. The method of truncated Schwinger-Dyson equations, however, is exact
in this model, so it provides a new way of solving the Bloch-Nordsieck model.
This method can also be generalized to other, more complicated theories.Comment: 12 pages, 3 figures, revtex
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
FORMATION AND CHARACTERIZATION OF NITROGEN IMPLANTED SILICON-ON-INSULATOR STRUCTURE
Silicon wafer has been implanted with 200keV14N+ ions to a dose of 0.75 x 10 18N+ /cm2 at a temperature of 600°C and has been annealed at 1300°C for 2 hours. During post-annealing rapid redistribution of the implanted nitrogen results in formation of buried polycrystalline nitride layer under the damage-free (except for few dislocations < 10⁵/cm2) single crystal silicon layer, which is characterized by n type conduction. The buried dielectric has a resistivity of approximately 10⁸ Ωcm. P channel integrated circuit transistors have been fabricated in the buried nitrid area. The measurements of these transistor devices demonstrate the suitability of nitrogen implanted SOl structure for integrated circuit application
Stellar Dynamics and the implications on the merger evolution in NGC6240
We report near-infrared integral field spectroscopy of the luminous merging
galaxy NGC 6240. Stellar velocities show that the two K-band peaks separated by
1.6arcsec are the central parts of inclined, rotating disk galaxies with equal
mass bulges. The dynamical masses of the nuclei are much larger than the
stellar mass derived from the K-band light, implying that the progenitor
galaxies were galaxies with massive bulges. The K-band light is dominated by
red supergiants formed in the two nuclei in starbursts, triggered ~2x10^7 years
ago, possibly by the most recent perigalactic approach. Strong feedback effects
of a superwind and supernovae are responsible for a short duration burst
(~5x10^6 years) which is already decaying. The two galaxies form a
prograde-retrograde rotating system and from the stellar velocity field it
seems that one of the two interacting galaxies is subject to a prograde
encounter. Between the stellar nuclei is a prominent peak of molecular gas
(H_2, CO). The stellar velocity dispersion peaks there indicating that the gas
has formed a local, self-gravitating concentration decoupled from the stellar
gravitational potential. NGC 6240 has previously been reported to fit the
paradigm of an elliptical galaxy formed through the merger of two galaxies.
This was based on the near-infrared light distribution which follows a
r^1/4-law. Our data cast strong doubt on this conclusion: the system is by far
not relaxed, rotation plays an important role, as does self-gravitating gas,
and the near-infrared light is dominated by young stars.Comment: 34 pages, 11 figures, using AASTEX 5.0rc3.1, paper submitted to the
Astrophysical Journal, revised versio
Probability distributions for quantum stress tensors in four dimensions
We treat the probability distributions for quadratic quantum fields, averaged
with a Lorentzian test function, in four-dimensional Minkowski vacuum. These
distributions share some properties with previous results in two-dimensional
spacetime. Specifically, there is a lower bound at a finite negative value, but
no upper bound. Thus arbitrarily large positive energy density fluctuations are
possible. We are not able to give closed form expressions for the probability
distribution, but rather use calculations of a finite number of moments to
estimate the lower bounds, the asymptotic forms for large positive argument,
and possible fits to the intermediate region. The first 65 moments are used for
these purposes. All of our results are subject to the caveat that these
distributions are not uniquely determined by the moments. However, we also give
bounds on the cumulative distribution function that are valid for any
distribution fitting these moments.We apply the asymptotic form of the
electromagnetic energy density distribution to estimate the nucleation rates of
black holes and of Boltzmann brains.Comment: 26 pages, 2 figure
Ray-based calculations of backscatter in laser fusion targets
A 1D, steady-state model for Brillouin and Raman backscatter from an
inhomogeneous plasma is presented. The daughter plasma waves are treated in the
strong damping limit, and have amplitudes given by the (linear) kinetic
response to the ponderomotive drive. Pump depletion, inverse-bremsstrahlung
damping, bremsstrahlung emission, Thomson scattering off density fluctuations,
and whole-beam focusing are included. The numerical code DEPLETE, which
implements this model, is described. The model is compared with traditional
linear gain calculations, as well as "plane-wave" simulations with the paraxial
propagation code pF3D. Comparisons with Brillouin-scattering experiments at the
OMEGA Laser Facility [T. R. Boehly et al., Opt. Commun. 133, p. 495 (1997)]
show that laser speckles greatly enhance the reflectivity over the DEPLETE
results. An approximate upper bound on this enhancement, motivated by phase
conjugation, is given by doubling the DEPLETE coupling coefficient. Analysis
with DEPLETE of an ignition design for the National Ignition Facility (NIF) [J.
A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, p. 755
(1994)], with a peak radiation temperature of 285 eV, shows encouragingly low
reflectivity. Re-absorption of Raman light is seen to be significant in this
design.Comment: 16 pages, 19 figure
Implementation of Rare Isotopologues into Machine Learning of the Chemical Inventory of the Solar-Type Protostellar Source IRAS 16293-2422
Machine learning techniques have been previously used to model and predict
column densities in the TMC-1 dark molecular cloud. In interstellar sources
further along the path of star formation, such as those where a protostar
itself has been formed, the chemistry is known to be drastically different from
that of largely quiescent dark clouds. To that end, we have tested the ability
of various machine learning models to fit the column densities of the molecules
detected in source B of the Class 0 protostellar binary IRAS 16293-2422. By
including a simple encoding of isotopic composition in our molecular feature
vectors, we also examine for the first time how well these models can replicate
the isotopic ratios. Finally, we report the predicted column densities of the
chemically relevant molecules that may be excellent targets for
radioastronomical detection in IRAS 16293-2422B.Comment: Accepted for publication in Digital Discovery. 18 pages, 8 figures, 5
table
Haptoglobin frequencies in Jewish communities *
Haptoglobin and transferrin types have been determined by starch gel electrophoresis on blood from 929 subjects belonging to various Jewish communities. The frequency of the Hp 1 gene in 499 Ashkenazic Jews is 0.29 and does not differ significantly from the value of 0–26 found in 345 Jews of Oriental origin. The Hp 1 frequency of Ashkenazic Jews is significantly lower than that reported for the autochthonous populations of Central and Western Europe. Two small samples collected among Sephardic Jews and among the offspring of intercommunity marriages exhibit somewhat higher frequencies of the Hp 1 gene. The modified 2-1 phenotype was found in a single subject from Baghdad. There were three cases of ahaptoglobinaemia among Ashkenazic Jews and three among the Oriental groups. No ahaptoglobinaemia was discovered in a family sample of ninety-two Jews from Kurdistan among whom thalassaemia minor was common and the majority of whom were affeeted with G-6-P-D deficiency. All transferrins were of type C.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66130/1/j.1469-1809.1962.tb01307.x.pd
Stopping of Charged Particles in a Magnetized Classical Plasma
The analytical and numerical investigations of the energy loss rate of the
test particle in a magnetized electron plasma are developed on the basis of the
Vlasov-Poisson equations, and the main results are presented. The Larmor
rotation of a test particle in a magnetic field is taken into account. The
analysis is based on the assumption that the energy variation of the test
particle is much less than its kinetic energy. The obtained general expression
for stopping power is analyzed for three cases: (i) the particle moves through
a collisionless plasma in a strong homogeneous magnetic field; (ii) the fast
particle moves through a magnetized collisionless plasma along the magnetic
field; and (iii) the particle moves through a magnetized collisional plasma
across a magnetic field. Calculations are carried out for the arbitrary test
particle velocities in the first case, and for fast particles in the second and
third cases. It is shown that the rate at which a fast test particle loses
energy while moving across a magnetic field may be much higher than the loss in
the case of motion through plasma without magnetic field.Comment: 14 pages, 3 figures, LaTe
- …