607 research outputs found
Magnetic Field Seeding by Galactic Winds
The origin of intergalactic magnetic fields is still a mystery and several
scenarios have been proposed so far: among them, primordial phase transitions,
structure formation shocks and galactic outflows. In this work we investigate
how efficiently galactic winds can provide an intense and widespread "seed"
magnetisation. This may be used to explain the magnetic fields observed today
in clusters of galaxies and in the intergalactic medium (IGM). We use
semi-analytic simulations of magnetised galactic winds coupled to high
resolution N-body simulations of structure formation to estimate lower and
upper limits for the fraction of the IGM which can be magnetised up to a
specified level. We find that galactic winds are able to seed a substantial
fraction of the cosmic volume with magnetic fields. Most regions affected by
winds have magnetic fields in the range -12 < Log B < -8 G, while higher seed
fields can be obtained only rarely and in close proximity to wind-blowing
galaxies. These seed fields are sufficiently intense for a moderately efficient
turbulent dynamo to amplify them to the observed values. The volume filling
factor of the magnetised regions strongly depends on the efficiency of winds to
load mass from the ambient medium. However, winds never completely fill the
whole Universe and pristine gas can be found in cosmic voids and regions
unaffected by feedback even at z=0. This means that, in principle, there might
be the possibility to probe the existence of primordial magnetic fields in such
regions.Comment: 14 pages, 5 figures. Accepted for publications by MNRAS. A high
resolution version of the paper is available at
http://astronomy.sussex.ac.uk/~sb207/Papers/bb.ps.g
Measuring current by counting electrons in a nanowire quantum dot
We measure current by counting single electrons tunneling through an InAs
nanowire quantum dot. The charge detector is realized by fabricating a quantum
point contact in close vicinity to the nanowire. The results based on electron
counting compare well to a direct measurements of the quantum dot current, when
taking the finite bandwidth of the detector into account. The ability to detect
single electrons also opens up possibilities for manipulating and detecting
individual spins in nanowire quantum dots
SZ effect from radio-galaxy lobes: astrophysical and cosmological relevance
We derive the SZ effect arising in radio-galaxy lobes that are filled with
high-energy, non-thermal electrons. We provide here quantitative estimates for
SZ effect expected from the radio galaxy lobes by normalizing it to the
Inverse-Compton light, observed in the X-ray band, as produced by the
extrapolation to low energies of the radio emitting electron spectrum in these
radio lobes. We compute the spectral and spatial characteristics of the SZ
effect associated to the radio lobes of two distant radio galaxies (3C294 and
3C432) recently observed by Chandra, and we further discuss its detectability
with the next generation microwave and sub-mm experiments with arcsec and K sensitivity. We finally highlight the potential use of the SZE from
radio-galaxy lobes in the astrophysical and cosmological context.Comment: 8 pages, 5 figures, MNRAS in pres
Results from a Second RXTE Observation of the Coma Cluster
The RXTE satellite observed the Coma cluster for 177 ksec during November and
December 2000, a second observation motivated by the intriguing results from
the first 87 ksec observation in 1996. Analysis of the new dataset confirms
that thermal emission from isothermal gas does not provide a good fit to the
spectral distribution of the emission from the inner 1 degree radial region.
While the observed spectrum may be fit by emission from gas with a substantial
temperature gradient, it is more likely that the emission includes also a
secondary non-thermal component. If so, non-thermal emission comprises ~8% of
the total 4--20 keV flux. Interpreting this emission as due to Compton
scattering of relativistic electrons (which produce the known extended radio
emission) by the cosmic microwave background radiation, we determine that the
mean, volume-averaged magnetic field in the central region of Coma is B =
0.1-0.3 microgauss.Comment: 10 pages, 1 figure; APJ, in pres
Detecting THz current fluctuations in a quantum point contact using a nanowire quantum dot
We use a nanowire quantum dot to probe high-frequency current fluctuations in
a nearby quantum point contact. The fluctuations drive charge transitions in
the quantum dot, which are measured in real-time with single-electron detection
techniques. The quantum point contact (GaAs) and the quantum dot (InAs) are
fabricated in different material systems, which indicates that the interactions
are mediated by photons rather than phonons. The large energy scales of the
nanowire quantum dot allow radiation detection in the long-wavelength infrared
regime
Impact of tangled magnetic fields on AGN-blown bubbles
There is growing consensus that feedback from AGN is the main mechanism
responsible for stopping cooling flows in clusters of galaxies. AGN are known
to inflate buoyant bubbles that supply mechanical power to the intracluster gas
(ICM). High Reynolds number hydrodynamical simulations show that such bubbles
get entirely disrupted within 100 Myr, as they rise in cluster atmospheres,
which is contrary to observations. This artificial mixing has consequences for
models trying to quantify the amount of heating and star formation in cool core
clusters of galaxies. It has been suggested that magnetic fields can stabilize
bubbles against disruption. We perform MHD simulations of fossil bubbles in the
presence of tangled magnetic fields using the high order PENCIL code. We focus
on the physically-motivated case where thermal pressure dominates over magnetic
pressure and consider randomly oriented fields with and without maximum
helicity and a case where large scale external fields drape the bubble.We find
that helicity has some stabilizing effect. However, unless the coherence length
of magnetic fields exceeds the bubble size, the bubbles are quickly shredded.
As observations of Hydra A suggest that lengthscale of magnetic fields may be
smaller then typical bubble size, this may suggest that other mechanisms, such
as viscosity, may be responsible for stabilizing the bubbles. However, since
Faraday rotation observations of radio lobes do not constrain large scale ICM
fields well if they are aligned with the bubble surface, the draping case may
be a viable alternative solution to the problem. A generic feature found in our
simulations is the formation of magnetic wakes where fields are ordered and
amplified. We suggest that this effect could prevent evaporation by thermal
conduction of cold Halpha filaments observed in the Perseus cluster.Comment: accepted for publication in MNRAS, (downgraded resolution figures,
color printing recommended
In-plane gate single-electron transistor in Ga[Al]As fabricated by scanning probe lithography
A single-electron transistor has been realized in a Ga[Al]As heterostructure
by oxidizing lines in the GaAs cap layer with an atomic force microscope. The
oxide lines define the boundaries of the quantum dot, the in-plane gate
electrodes, and the contacts of the dot to source and drain. Both the number of
electrons in the dot as well as its coupling to the leads can be tuned with an
additional, homogeneous top gate electrode. Pronounced Coulomb blockade
oscillations are observed as a function of voltages applied to different gates.
We find that, for positive top-gate voltages, the lithographic pattern is
transferred with high accuracy to the electron gas. Furthermore, the dot shape
does not change significantly when in-plane voltages are tuned.Comment: 4 pages, 3 figure
Charge Detection in Graphene Quantum Dots
We report measurements on a graphene quantum dot with an integrated graphene
charge detector. The quantum dot device consists of a graphene island (diameter
approx. 200 nm) connected to source and drain contacts via two narrow graphene
constrictions. From Coulomb diamond measurements a charging energy of 4.3 meV
is extracted. The charge detector is based on a 45 nm wide graphene nanoribbon
placed approx. 60 nm from the island. We show that resonances in the nanoribbon
can be used to detect individual charging events on the quantum dot. The
charging induced potential change on the quantum dot causes a step-like change
of the current in the charge detector. The relative change of the current
ranges from 10% up to 60% for detecting individual charging events.Comment: 4 pages, 3 figure
High-frequency gate manipulation of a bilayer graphene quantum dot
We report transport data obtained for a double-gated bilayer graphene quantum
dot. In Coulomb blockade measurements, the gate dielectric Cytop(TM) is found
to provide remarkable electronic stability even at cryogenic temperatures.
Moreover, we demonstrate gate manipulation with square shaped voltage pulses at
frequencies up to 100 MHz and show that the signal amplitude is not affected by
the presence of the capacitively coupled back gate
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