2,431 research outputs found
The Problem of Adhesion Methods and Locomotion Mechanism Development for Wall-Climbing Robots
This review considers a problem in the development of mobile robot adhesion
methods with vertical surfaces and the appropriate locomotion mechanism design.
The evolution of adhesion methods for wall-climbing robots (based on friction,
magnetic forces, air pressure, electrostatic adhesion, molecular forces,
rheological properties of fluids and their combinations) and their locomotion
principles (wheeled, tracked, walking, sliding framed and hybrid) is studied.
Wall-climbing robots are classified according to the applications, adhesion
methods and locomotion mechanisms. The advantages and disadvantages of various
adhesion methods and locomotion mechanisms are analyzed in terms of mobility,
noiselessness, autonomy and energy efficiency. Focus is placed on the physical
and technical aspects of the adhesion methods and the possibility of combining
adhesion and locomotion methods
Giant Magnetoresistance Oscillations Induced by Microwave Radiation and a Zero-Resistance State in a 2D Electron System with a Moderate Mobility
The effect of a microwave field in the frequency range from 54 to 140
on the magnetotransport in a GaAs quantum well with AlAs/GaAs
superlattice barriers and with an electron mobility no higher than
is investigated. In the given two-dimensional system under
the effect of microwave radiation, giant resistance oscillations are observed
with their positions in magnetic field being determined by the ratio of the
radiation frequency to the cyclotron frequency. Earlier, such oscillations had
only been observed in GaAs/AlGaAs heterostructures with much higher mobilities.
When the samples under study are irradiated with a 140- microwave
field, the resistance corresponding to the main oscillation minimum, which
occurs near the cyclotron resonance, appears to be close to zero. The results
of the study suggest that a mobility value lower than
does not prevent the formation of zero-resistance states in magnetic field in a
two-dimensional system under the effect of microwave radiation.Comment: 4 pages, 2 figur
Anisotropic positive magnetoresistance of a nonplanar 2D electron gas in a parallel magnetic field
We study the transport properties of a 2D electron gas in narrow GaAs quantum
wells with AlAs/GaAs superlattice barriers. It is shown that the anisotropic
positive magnetoresistance observed in selectively doped semiconductor
structures in a parallel magnetic field is caused by the spatial modulation of
the 2D electron gas.Comment: 4 pages, 3 figure
Ultrahard spectra of PeV neutrinos from supernovae in compact star clusters
Starburst regions with multiple powerful winds of young massive stars and
supernova remnants are favorable sites for high-energy cosmic ray acceleration.
A supernova shock colliding with a fast wind from a compact cluster of young
stars allows the acceleration of protons to energies well above the standard
limits of diffusive shock acceleration in an isolated SN. The proton spectrum
in such a wind-supernova PeV accelerator is hard with a large flux in the
high-energy-end of the spectrum producing copious gamma-rays and neutrinos in
inelastic nuclear collisions. We argue that SN shocks in the Westerlund 1
cluster in the Milky Way may accelerate protons to about 40 PeV. Once
accelerated, these CRs will diffuse into surrounding dense clouds and produce
neutrinos with fluxes sufficient to explain a fraction of the events detected
by IceCube Observatory from the inner Galaxy.Comment: 10 pages, 7 figures, MNRAS v.453, p.113-121, 201
Effect of parallel magnetic field on the Zero Differential Resistance State
The non-linear zero-differential resistance state (ZDRS) that occurs for
highly mobile two-dimensional electron systems in response to a dc bias in the
presence of a strong magnetic field applied perpendicular to the electron plane
is suppressed and disappears gradually as the magnetic field is tilted away
from the perpendicular at fixed filling factor . Good agreement is found
with a model that considers the effect of the Zeeman splitting of Landau levels
enhanced by the in-plane component of the magnetic field.Comment: 4 pages, 4 figure
Non-Maxwellian electron distributions in clusters of galaxies
Thermal X-ray spectra of clusters of galaxies and other sources are commonly
calculated assuming Maxwellian electron distributions. There are situations
where this approximation is not valid, for instance near interfaces of hot and
cold gas and near shocks. The presence of non-thermal electrons affects the
X-ray spectrum. To study the role of these electrons in clusters and other
environments, an efficient algorithm to calculate the X-ray spectra is needed.
We approximate an arbitrary electron distribution by the sum of Maxwellian
components. The decomposition is done using either a genetic algorithm or an
analytical approximation. The X-ray spectrum is then evaluated using a linear
combination of those Maxwellian components. Our method is fast and leads to an
accurate evaluation of the spectrum. The use of Maxwellian components allows to
use the standard collisional rates that are available in plasma codes such as
SPEX. We give an example of a spectrum for the supra-thermal electron
distribution behind a shock in a cluster of galaxies. The relative intensities
of the satellite lines in such a spectrum are sensitive to the presence of the
supra-thermal electrons. These lines can only be investigated with high
spectral resolution. We show that the instruments on future missions like
Astro-H and IXO will be able to demonstrate the presence or absence of these
supra-thermal electrons.Comment: 6 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics, main journa
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