2,065 research outputs found
Improving the spatial resolution by effective subtraction technique at Irkutsk incoherent scatter radar: the theory and experiment
We describe a sounding technique that allows us to improve spatial resolution
of Irkutsk Incoherent Scatter Radar without loosing spectral resolution. The
technique is based on transmitting of rectangle pulses of different duration in
various sounding runs and subtracting correlation matrixes. Theoretically and
experimentally we have shown, that subtraction of the mean-square parameters of
the scattered signal for different kinds of the sounding signal one from
another allows us to solve the problem within the framework of quasi-static
ionospheric parameters approximation.Comment: 4 pages, 3 figures, to appear at URSI-2011 conferenc
Coulomb blockade and Bloch oscillations in superconducting Ti nanowires
Quantum fluctuations in quasi-one-dimensional superconducting channels
leading to spontaneous changes of the phase of the order parameter by ,
alternatively called quantum phase slips (QPS), manifest themselves as the
finite resistance well below the critical temperature of thin superconducting
nanowires and the suppression of persistent currents in tiny superconducting
nanorings. Here we report the experimental evidence that in a current-biased
superconducting nanowire the same QPS process is responsible for the insulating
state -- the Coulomb blockade. When exposed to RF radiation, the internal Bloch
oscillations can be synchronized with the external RF drive leading to
formation of quantized current steps on the I-V characteristic. The effects
originate from the fundamental quantum duality of a Josephson junction and a
superconducting nanowire governed by QPS -- the QPS junction (QPSJ).Comment: 5 pages, 4 figure
Superconducting MoSi nanowires
We have fabricated disordered superconducting nanowires of molybdenium
silicide. A molybdenium nanowire is first deposited on top of silicon, and the
alloy is formed by rapid thermal annealing. The method allows tuning of the
crystal growth to optimise, e.g., the resistivity of the alloy for potential
applications in quantum phase slip devices and superconducting nanowire
single-photon detectors. The wires have effective diameters from 42 to 79 nm,
enabling the observation of crossover from conventional superconductivity to
regimes affected by thermal and quantum fluctuations. In the smallest diameter
wire and at temperatures well below the superconducting critical temperature,
we observe residual resistance and negative magnetoresistance, which can be
considered as fingerprints of quantum phase slips
Effect of Multiple Scattering on the Critical Electric Field for Runaway Electrons in the Atmosphere
A simple method for taking into account the multiple Coulomb scattering in
construction of a separatrix (the line separating the regions of runaway and
decelerating electrons in an electric field) is described. The desired line is
obtained by solving a simple transcendental equation.Comment: 3 pages, 2 figure
The Spectral Energy Distribution of Self-gravitating Interstellar Clouds I. Spheres
We derive the spectral energy distribution (SED) of dusty, isothermal, self
gravitating, stable and spherical clouds externally heated by the ambient
interstellar radiation field. For a given radiation field and dust properties,
the radiative transfer problem is determined by the pressure of the surrounding
medium and the cloud mass expressed as a fraction of the maximum stable cloud
mass above which the clouds become gravitational unstable.
To solve the radiative transfer problem a ray-tracing code is used to
accurately derive the light distribution inside the cloud. This code considers
both non isotropic scattering on dust grains and multiple scattering events.
The dust properties inside the clouds are assumed to be the same as in the
diffuse interstellar medium in our galaxy. We analyse the effect of the
pressure, the critical mass fraction, and the ISRF on the SED and present
brightness profiles in the visible, the IR/FIR and the submm/mm regime with the
focus on the scattered emission and the thermal emission from PAH-molecules and
dust grains.Comment: accepted for publication in ApJS, May 2008, v176n1 issu
Co-Expression Network Models Suggest that Stress Increases Tolerance to Mutations
Network models are a well established tool for studying the robustness of complex systems, including modelling the effect of loss of function mutations in protein interaction networks. Past work has concentrated on average damage caused by random node removal, with little attention to the shape of the damage distribution. In this work, we use fission yeast co-expression networks before and after exposure to stress to model the effect of stress on mutational robustness. We find that exposure to stress decreases the average damage from node removal, suggesting stress induces greater tolerance to loss of function mutations. The shape of the damage distribution is also changed upon stress, with a greater incidence of extreme damage after exposure to stress. We demonstrate that the change in shape of the damage distribution can have considerable functional consequences, highlighting the need to consider the damage distribution in addition to average behaviour
A Corona Australis cloud filament seen in NIR scattered light II: Comparison with sub-millimeter data
We study a northern part of the Corona Australis molecular cloud that
consists of a filament and a dense sub-millimetre core inside the filament. Our
aim is to measure dust temperature and sub-mm emissivity within the region. We
also look for confirmation that near-infrared (NIR) surface brightness can be
used to study the structure of even very dense clouds. We extend our previous
NIR mapping south of the filament. The dust colour temperatures are estimated
using Spitzer 160um and APEX/Laboca 870um maps. The column densities derived
based on the reddening of background stars, NIR surface brightness, and thermal
sub-mm dust emission are compared. A three dimensional toy model of the
filament is used to study the effect of anisotropic illumination on
near-infrared surface brightness and the reliability of dust temperature
determination. Relative to visual extinction, the estimated emissivity at 870um
is kappa(870) = (1.3 +- 0.4) x 10^{-5} 1/mag. This is similar to the values
found in diffuse medium. A significant increase in the sub-millimetre
emissivity seems to be excluded. In spite of saturation, NIR surface brightness
was able to accurately pinpoint, and better than measurements of the colour
excesses of background stars, the exact location of the column density maximum.
Both near- and far-infrared data show that the intensity of the radiation field
is higher south of the filament.Comment: 9 pages, 9 figures, accepted to A&
Phase-coded pulse aperiodic transmitter coding
Both ionospheric and weather radar communities have already adopted the method of transmitting radar pulses in an aperiodic manner when measuring moderately overspread targets. Among the users of the ionospheric radars, this method is called Aperiodic Transmitter Coding (ATC), whereas the weather radar users have adopted the term Simultaneous Multiple Pulse-Repetition Frequency (SMPRF). When probing the ionosphere at the carrier frequencies of the EISCAT Incoherent Scatter Radar facilities, the range extent of the detectable target is typically of the order of one thousand kilometers – about seven milliseconds – whereas the characteristic correlation time of the scattered signal varies from a few milliseconds in the D-region to only tens of microseconds in the F-region. If one is interested in estimating the scattering autocorrelation function (ACF) at time lags shorter than the F-region correlation time, the D-region must be considered as a moderately overspread target, whereas the F-region is a severely overspread one. Given the technical restrictions of the radar hardware, a combination of ATC and phase-coded long pulses is advantageous for this kind of target. We evaluate such an experiment under infinitely low signal-to-noise ratio (SNR) conditions using lag profile inversion. In addition, a qualitative evaluation under high-SNR conditions is performed by analysing simulated data. The results show that an acceptable estimation accuracy and a very good lag resolution in the D-region can be achieved with a pulse length long enough for simultaneous E- and F-region measurements with a reasonable lag extent. The new experiment design is tested with the EISCAT Tromsø VHF (224 MHz) radar. An example of a full D/E/F-region ACF from the test run is shown at the end of the paper
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