462 research outputs found
On The Non Thermal Emission and Acceleration of Electrons in Coma and Other Clusters of Galaxies
Some clusters of galaxies in addition to thermal bremsstrahlung (TB), emit
diffuse radiation from the intercluster medium (ICM) at radio, EUV and hard
x-ray (HXR) ranges. The radio radiation is due to synchrotron by relativistic
electrons, and the inverse Compton (IC) scattering by the cosmic microwave
background radiation of the same electrons is the most natural source for the
HXR and perhaps the EUV emissions. However, simple estimates give a weaker
magnetic field than that suggested by Faraday rotation measurements.
Consequently, non-thermal bremsstrahlung (NTB) and TB have also been suggested
as sources of these emissions. We show that NTB cannot be the source of the
HXRs and that the difficulty with the low magnetic field in the IC model is
alleviated if we take into account the effects of observational bias,
nonisotropic pitch angle distribution and spectral breaks. We derive a spectrum
for the radiating electrons and discuss acceleration scenarios. We show that
continuous and in situ acceleration in the ICM of the background thermal
electrons requires unreasonably high energy input and acceleration of injected
relativistic electrons gives rise to a much flatter spectrum than desired,
unless a large fraction of electrons escape the ICM, in which case one obtains
EUV and HXR emissions extending well beyond the boundaries of the cluster. A
continuous emission by a cooling spectrum resulting from interaction with ICM
of electrons accelerated elsewhere also suffers from similar shortcomings. The
most likely scenario appears to be an episodic injection-acceleration model,
whereby one obtains a time dependent spectrum that for certain phases of its
evolution satisfies all the requirements.Comment: 27 pages, one Table, Four Figures. Latex AAS v5.0. Accepted by Ap
Gate tunability of stray-field-induced electron spin precession in a GaAs/InGaAs quantum well below an interdigitated magnetized Fe grating
Time-resolved Faraday rotation is used to measure the coherent electron spin
precession in a GaAs/InGaAs quantum well below an interdigitated magnetized Fe
grating. We show that the electron spin precession frequency can be modified by
applying a gate voltage of opposite polarity to neighboring bars. A tunability
of the precession frequency of 0.5 GHz/V has been observed. Modulating the gate
potential with a gigahertz frequency allows the electron spin precession to be
controlled on a nanosecond timescale
Extreme Ultraviolet Emission from Clusters of Galaxies: Inverse Compton Radiation from a Relic Population of Cosmic Ray Electrons?
We suggest that the luminous extreme ultraviolet (EUV) emission which has
been detected recently from clusters of galaxies is Inverse Compton (IC)
scattering of Cosmic Microwave Background (CMB) radiation by low energy cosmic
ray electrons in the intracluster medium. The cosmic ray electrons would have
Lorentz factors of gamma ~ 300, and would lose energy primarily by emitting EUV
radiation. These particles have lifetimes comparable to the Hubble time; thus,
the electrons might represent a relic population of cosmic rays produced by
nonthermal activity over the history of the cluster. The IC model naturally
explains the observed increase in the ratio of EUV to X-ray emission with
radius in clusters. The required energy in cosmic ray electrons is typically
1--10% of the thermal energy content of the intracluster gas. We suggest that
the cosmic ray electrons might have been produced by supernovae in galaxies, by
radio galaxies, or by particle acceleration in intracluster shocks.Comment: ApJ Letters, in press, 4 pages with 1 embedded figure, Latex in
emulateapj styl
Measurements of higher order noise correlations in a quantum dot with a finite bandwidth detector
We present measurements of the fourth and fifth cumulants of the distribution
of transmitted charge in a tunable quantum dot. We investigate how the measured
statistics is influenced by the finite bandwidth of the detector and by the
finite measurement time. By including the detector when modeling the system, we
use the theory of full counting statistics to calculate the noise levels for
the combined system. The predictions of the finite-bandwidth model are in good
agreement with measured data
Spatially Resolved Raman Spectroscopy of Single- and Few-Layer Graphene
We present Raman spectroscopy measurements on single- and few-layer graphene
flakes. Using a scanning confocal approach we collect spectral data with
spatial resolution, which allows us to directly compare Raman images with
scanning force micrographs. Single-layer graphene can be distinguished from
double- and few-layer by the width of the D' line: the single peak for
single-layer graphene splits into different peaks for the double-layer. These
findings are explained using the double-resonant Raman model based on ab-initio
calculations of the electronic structure and of the phonon dispersion. We
investigate the D line intensity and find no defects within the flake. A finite
D line response originating from the edges can be attributed either to defects
or to the breakdown of translational symmetry
Raman imaging of doping domains in graphene on SiO2
We present spatially resolved Raman images of the G and 2D lines of
single-layer graphene flakes. The spatial fluctuations of G and 2D lines are
correlated and are thus shown to be affiliated with local doping domains. We
investigate the position of the 2D line -- the most significant Raman peak to
identify single-layer graphene -- as a function of charging up to |n|~4 10^12
cm^-2. Contrary to the G line which exhibits a strong and symmetric stiffening
with respect to electron and hole-doping, the 2D line shows a weak and slightly
asymmetric stiffening for low doping. Additionally, the line width of the 2D
line is, in contrast to the G line, doping-independent making this quantity a
reliable measure for identifying single-layer graphene
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