619 research outputs found
AC field induced quantum rectification effect in tunnel junctions
We study the appearance of directed current in tunnel junctions, quantum
ratchet effect, in the presence of an external ac field f(t). The current is
established in a one-dimensional discrete inhomogeneous "tight-binding model".
By making use of a symmetry analysis we predict the right choice of f(t) and
obtain the directed current as a difference between electron transmission
coefficients in opposite directions, . Numerical
simulations confirm the predictions of the symmetry analysis and moreover, show
that the directed current can be drastically increased by a proper choice of
frequency and amplitudes of the ac field f(t).Comment: 4 pages, 3 figures, to be published in Physical Review
Multi-field approach in mechanics of structural solids
We overview the basic concepts, models, and methods related to the
multi-field continuum theory of solids with complex structures. The multi-field
theory is formulated for structural solids by introducing a macrocell
consisting of several primitive cells and, accordingly, by increasing the
number of vector fields describing the response of the body to external
factors. Using this approach, we obtain several continuum models and explore
their essential properties by comparison with the original structural models.
Static and dynamical problems as well as the stability problems for structural
solids are considered. We demonstrate that the multi-field approach gives a way
to obtain families of models that generalize classical ones and are valid not
only for long-, but also for short-wavelength deformations of the structural
solid. Some examples of application of the multi-field theory and directions
for its further development are also discussed.Comment: 25 pages, 18 figure
Incommensurate dynamics of resonant breathers in Josephson junction ladders
We present theoretical and experimental studies of resonant localized
resistive states in a Josephson junction ladder. These complex breather states
are obtained by tuning the breather frequency into the upper band of linear
electromagnetic oscillations of the ladder. Their prominent feature is the
appearance of resonant steps in the current-voltage (I-V) characteristics. We
have found the resonant breather-like states displaying incommensurate
dynamics. Numerical simulations show that these incommensurate resonant
breathers persist for very low values of damping. Qualitatively similar
incommensurate breather states are observed in experiments performed with
Nb-based Josephson ladders. We explain the appearance of these states with the
help of resonance-induced hysteresis features in the I-V dependence.Comment: 5 pages, 6 figure
Wave scattering by discrete breathers
We present a theoretical study of linear wave scattering in one-dimensional
nonlinear lattices by intrinsic spatially localized dynamic excitations or
discrete breathers. These states appear in various nonlinear systems and
present a time-periodic localized scattering potential for plane waves. We
consider the case of elastic one-channel scattering, when the frequencies of
incoming and transmitted waves coincide, but the breather provides with
additional spatially localized ac channels whose presence may lead to various
interference patterns. The dependence of the transmission coefficient on the
wave number q and the breather frequency Omega_b is studied for different types
of breathers: acoustic and optical breathers, and rotobreathers. We identify
several typical scattering setups where the internal time dependence of the
breather is of crucial importance for the observed transmission properties.Comment: 17 pages, 19 figures, submitted to CHAOS (Focus Issue
On Protostellar Disks in Herbig Ae/Be Stars
The spectral shape of IR emission from Herbig Ae/Be stars has been invoked as
evidence for accretion disks around high-mass protostars. Instead, we present
here models based on spherical envelopes with dust density profile
that successfully explain the observed spectral shapes. The spectral energy
distributions (SEDs) of eight primary candidates for protostellar disks are
fitted in detail for all wavelengths available, from visual to far IR. The only
envelope property adjusted in individual sources is the overall visual optical
depth, and it ranges from 0.3 to 3. In each case, our models properly reproduce
the data for both IR excess, visual extinction and reddening. The success of
our models shows that accretion disks cannot make a significant contribution to
the radiation observed in these pre-main sequence stars.Comment: 10 pages, 2 Postscript figures (included), uses aaspp4.sty. To appear
in Astrophysical Journal Letter
Development of the RF Ion Sources for Focused Ion Beam Accelerators
The paper presents the results of investigations of ion sources developed in the IAP of NAS of Ukraine for generation of high brightness ion beams with small energy spread. A series of RF ion sources operated at the frequency of 27.12 MHz were studied: the inductive RF ion source, the helicon ion source, the multi-cusp RF ion source, and the sputter type RF source of metal ions. A global model and transformer model were applied for calculation of RF source plasma parameters. Ion energy spread, ion mass, and ion current density of some sources were measured in the wide range of RF power, extraction voltage and gas pres-sure.
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