735 research outputs found
Influence of the Characteristics of the STM-tip on the Electroluminescence Spectra
We analyze the influence of the characteristics of the STM-tip (applied
voltage, tip radius) on the electroluminescence spectra from an STM-tip-induced
quantum dot taking into account the many-body effects. We find that positions
of electroluminescence peaks, attributed to the electron-hole recombination in
the quantum dot, are very sensitive to the shape and size of the confinement
potential as determined by the tip radius and the applied voltage. A critical
value of the tip radius is found, at which the luminescence peak positions as a
function of the tip radius manifest a transition from decreasing behavior for
smaller radii to increasing behavior for larger radii. We find that this
critical value of the tip radius is related to the confinement in the lateral
and normal direction.Comment: 15 pages, 5 figure
Dissipation of Magnetohydrodynamic Waves on Energetic Particles: Impact on Interstellar Turbulence and Cosmic Ray Transport
The physical processes involved in diffusion of Galactic cosmic rays in the
interstellar medium are addressed. We study the possibility that the nonlinear
MHD cascade sets the power-law spectrum of turbulence which scatters charged
energetic particles. We find that the dissipation of waves due to the resonant
interaction with cosmic ray particles may terminate the Kraichnan-type cascade
below wavelengths 10^13 cm. The effect of this wave dissipation has been
incorporated in the GALPROP numerical propagation code in order to asses the
impact on measurable astrophysical data. The energy-dependence of the
cosmic-ray diffusion coefficient found in the resulting self-consistent model
may explain the peaks in the secondary to primary nuclei ratios observed at
about 1 GeV/nucleon.Comment: 15 pages, 20 figures, 1 table, emulateapj.cls; To be published in ApJ
10 May 2006, v.64
A circular polarimeter for the Cosmic Microwave Background
A primordial degree of circular polarization of the Cosmic Microwave
Background is not observationally excluded. The hypothesis of primordial
dichroism can be quantitatively falsified if the plasma is magnetized prior to
photon decoupling since the initial V-mode polarization affects the evolution
of the temperature fluctuations as well as the equations for the linear
polarization. The observed values of the temperature and polarization angular
power spectra are used to infer constraints on the amplitude and on the
spectral slope of the primordial V-mode. Prior to photon decoupling magnetic
fields play the role of polarimeters insofar as they unveil the circular
dichroism by coupling the V-mode power spectrum to the remaining brightness
perturbations. Conversely, for angular scales ranging between 4 deg and 10 deg
the joined bounds on the magnitude of circular polarization and on the magnetic
field intensity suggest that direct limits on the V-mode power spectrum in the
range of 0.01 mK could directly rule out pre-decoupling magnetic fields in the
range of 10-100 nG. The frequency dependence of the signal is located, for the
present purposes, in the GHz range.Comment: 28 pages, 12 included figures
The influence of fractional diffusion in Fisher-KPP equations
We study the Fisher-KPP equation where the Laplacian is replaced by the
generator of a Feller semigroup with power decaying kernel, an important
example being the fractional Laplacian. In contrast with the case of the stan-
dard Laplacian where the stable state invades the unstable one at constant
speed, we prove that with fractional diffusion, generated for instance by a
stable L\'evy process, the front position is exponential in time. Our results
provide a mathe- matically rigorous justification of numerous heuristics about
this model
A Study of the Scintillation Induced by Alpha Particles and Gamma Rays in Liquid Xenon in an Electric Field
Scintillation produced in liquid xenon by alpha particles and gamma rays has
been studied as a function of applied electric field. For back scattered gamma
rays with energy of about 200 keV, the number of scintillation photons was
found to decrease by 64+/-2% with increasing field strength. Consequently, the
pulse shape discrimination power between alpha particles and gamma rays is
found to reduce with increasing field, but remaining non-zero at higher fields.Comment: 15 pages, 12 figures, accepted by Nuclear Instruments and Methods in
Physics Research
New evidence for strong nonthermal effects in Tycho's supernova remnant
For the case of Tycho's supernova remnant (SNR) we present the relation
between the blast wave and contact discontinuity radii calculated within the
nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. It is
demonstrated that these radii are confirmed by recently published Chandra
measurements which show that the observed contact discontinuity radius is so
close to the shock radius that it can only be explained by efficient CR
acceleration which in turn makes the medium more compressible. Together with
the recently determined new value erg of the SN
explosion energy this also confirms our previous conclusion that a TeV
gamma-ray flux of erg/(cms) is to be expected from
Tycho's SNR. Chandra measurements and the HEGRA upper limit of the TeV
gamma-ray flux together limit the source distance to kpc.Comment: 5 pages, 4 figures. Accepted for publication in Astrophysics and
Space Science, Proc. of "The Multi-Messenger Approach to High-Energy
Gamma-ray Sources (Third Workshop on the Nature of Unidentified High-Energy
Sources)", Barcelona, July 4-7, 200
Renormalization scale uncertainty in tne DIS 2+1 jet cross-section
The deep inelastic scattering 2+1 jet cross- section is a useful observable
for precision tests of QCD, e.g. measuring the strong coupling constant
alpha(s). A consistent analysis requires a good understanding of the
theoretical uncertainties and one of the most fundamental ones in QCD is due to
the renormalization scheme and scale ambiguity. Different methods, which have
been proposed to resolve the scale ambiguity, are applied to the 2+1 jet
cross-section and the uncertainty is estimated. It is shown that the
uncertainty can be made smaller by choosing the jet definition in a suitable
way.Comment: 24 pages, uuencoded compressed tar file, DESY 94-082, TSL-ISV-94-009
Monte Carlo simulation for jet fragmentation in SUSY-QCD
We present results from a new Monte Carlo simulation for jet fragmentation in
QCD and SUSY QCD for large primary energies up to GeV. In
the case of SUSY QCD the simulation takes into account not only gluons and
quarks as cascading particles, but also their supersymmetric partners. A new
model-independent hadronization scheme is developed, in which the hadronization
functions are found from LEP data. An interesting feature of SUSY QCD is the
prediction of a sizeable flux of the lightest supersymmetric particles (LSPs),
if R-parity is conserved. About 10% of the jet energy is transferred to LSPs
which, owing to their harder spectra, constitute an important part of the
spectra for large . Spectra of protons and of secondary particles,
photons and neutrinos, are also calculated. These results have implications for
the decay of superheavy particles with masses up to the GUT scale, which have
been suggested as a source of ultrahigh energy cosmic rays.Comment: latex, 25 pages with 17 eps figure
The ZEPLIN II dark matter detector: data acquisition system and data reduction
ZEPLIN-II is a two-phase (liquid/gas) xenon dark matter detector searching
for WIMP-nucleon interactions. In this paper we describe the data acquisition
system used to record the data from ZEPLIN-II and the reduction procedures
which parameterise the data for subsequent analysis.Comment: 11 pages, 10 figure
The ZEPLIN II dark matter detector: data acquisition system and data reduction
ZEPLIN-II is a two-phase (liquid/gas) xenon dark matter detector searching
for WIMP-nucleon interactions. In this paper we describe the data acquisition
system used to record the data from ZEPLIN-II and the reduction procedures
which parameterise the data for subsequent analysis.Comment: 11 pages, 10 figure
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