406 research outputs found
Synchronization properties of self-sustained mechanical oscillators
We study, both analytically and numerically, the dynamics of mechanical
oscillators kept in motion by a feedback force, which is generated
electronically from a signal produced by the oscillators themselves. This kind
of self-sustained systems may become standard in the design of
frequency-control devices at microscopic scales. Our analysis is thus focused
on their synchronization properties under the action of external forces, and on
the joint dynamics of two to many coupled oscillators. Existence and stability
of synchronized motion are assessed in terms of the mechanical properties of
individual oscillators --namely, their natural frequencies and damping
coefficients-- and synchronization frequencies are determined. Similarities and
differences with synchronization phenomena in other coupled oscillating systems
are emphasized.Comment: To appear in Phys. Rev.
Duffing revisited: Phase-shift control and internal resonance in self-sustained oscillators
We address two aspects of the dynamics of the forced Duffing oscillator which
are relevant to the technology of micromechanical devices and, at the same
time, have intrinsic significance to the field of nonlinear oscillating
systems. First, we study the stability of periodic motion when the phase shift
between the external force and the oscillation is controlled -contrary to the
standard case, where the control parameter is the frequency of the force.
Phase-shift control is the operational configuration under which self-sustained
oscillators -and, in particular, micromechanical oscillators- provide a
frequency reference useful for time keeping. We show that, contrary to the
standard forced Duffing oscillator, under phase-shift control oscillations are
stable over the whole resonance curve. Second, we analyze a model for the
internal resonance between the main Duffing oscillation mode and a
higher-harmonic mode of a vibrating solid bar clamped at its two ends. We focus
on the stabilization of the oscillation frequency when the resonance takes
place, and present preliminary experimental results that illustrate the
phenomenon. This synchronization process has been proposed to counteract the
undesirable frequency-amplitude interdependence in nonlinear time-keeping
micromechanical devices
Synchronization of a forced self-sustained Duffing oscillator
We study the dynamics of a mechanical oscillator with linear and cubic forces
-the Duffing oscillator- subject to a feedback mechanism that allows the system
to sustain autonomous periodic motion with well-defined amplitude and
frequency. First, we characterize the autonomous motion for both hardening and
softening nonlinearities. Then, we analyze the oscillator's synchronizability
by an external periodic force. We find a regime where, unexpectedly, the
frequency range where synchronized motion is possible becomes wider as the
amplitude of oscillations grows. This effect of nonlinearities may find
application in technological uses of mechanical Duffing oscillators -for
instance, in the design of time-keeping devices at the microscale- which we
briefly review.Comment: To appear in Eur. Phys. J. Special Topic
SU(3) phase states and finite Fourier transform
We describe the construction of SU(3) phase operators using Fourier-like
transform on a hexagonal lattice. The advantages and disadvantages of this
approach are contrasted with other results, in particular with the more
traditional approach based on polar decomposition of operators.Comment: to appear in Physica Script
Dependences of the Casimir-Polder interaction between an atom and a cavity wall on atomic and material properties
The Casimir-Polder and van der Waals interactions between an atom and a flat
cavity wall are investigated under the influence of real conditions including
the dynamic polarizability of the atom, actual conductivity of the wall
material and nonzero temperature of the wall. The cases of different atoms near
metal and dielectric walls are considered. It is shown that to obtain accurate
results for the atom-wall interaction at short separations, one should use the
complete tabulated optical data for the complex refractive index of the wall
material and the accurate dynamic polarizability of an atom. At relatively
large separations in the case of a metal wall, one may use the plasma model
dielectric function to describe the dielectric properties of wall material. The
obtained results are important for the theoretical interpretation of
experiments on quantum reflection and Bose-Einstein condensation.Comment: 5 pages, 1 figure, iopart.cls is used, to appear in J. Phys. A
(special issue: Proceedings of QFEXT05, Barcelona, Sept. 5-9, 2005
Measuring the evolution of contemporary western popular music
Popular music is a key cultural expression that has captured listeners'
attention for ages. Many of the structural regularities underlying musical
discourse are yet to be discovered and, accordingly, their historical evolution
remains formally unknown. Here we unveil a number of patterns and metrics
characterizing the generic usage of primary musical facets such as pitch,
timbre, and loudness in contemporary western popular music. Many of these
patterns and metrics have been consistently stable for a period of more than
fifty years, thus pointing towards a great degree of conventionalism.
Nonetheless, we prove important changes or trends related to the restriction of
pitch transitions, the homogenization of the timbral palette, and the growing
loudness levels. This suggests that our perception of the new would be rooted
on these changing characteristics. Hence, an old tune could perfectly sound
novel and fashionable, provided that it consisted of common harmonic
progressions, changed the instrumentation, and increased the average loudness.Comment: Supplementary materials not included. Please see the journal
reference or contact the author
Advanced Photodetectors for Hyperspectroscopy and Other Applications
Hyperspectroscopy is a new method of surface image taking, providing
simultaneously high position and spectral resolutions which allow one to make
some conclusions about chemical compositions of the surfaces. We are now
studying applications of the hyperspctroscopic technique to be used for
medicine. This may allow one to develop early diagnostics of some illnesses, as
for example, skin cancer. For image taking advanced MCPs are currently used,
sensitive in the spectral interval of 450-850 nm. One of the aims of this work
is to extend the hyperspectrocpic method to the UV region of spectra: 185-280
nm. For this we have developed and successfully tested innovative 1D and 2D UV
sealed photosensitive gaseous detectors with resistive electrodes. These
detectors are superior MCPs due to the very low rate of noise pulses and thus
due to the high signal to noise ratio. Other important features of these
detectors are that they have excellent position resolutions - 30 micron in
digital form, are vibration stable and are spark protected. The first results
from the application of these detectors for spectroscopy, hyperspectroscopy and
the flame detection are presented.Comment: Presented at the IEEE Nuclear Science Syposium, Puerto Rico, October
200
Dependences of the van der Waals atom-wall interaction on atomic and material properties
The 1%-accurate calculations of the van der Waals interaction between an atom
and a cavity wall are performed in the separation region from 3 nm to 150 nm.
The cases of metastable He and Na atoms near the metal,
semiconductor or dielectric walls are considered. Different approximations to
the description of wall material and atomic dynamic polarizability are
carefully compared. The smooth transition to the Casimir-Polder interaction is
verified. It is shown that to obtain accurate results for the atom-wall van der
Waals interaction at shortest separations with an error less than 1% one should
use the complete optical tabulated data for the complex refraction index of the
wall material and the accurate dynamic polarizability of an atom. The obtained
results may be useful for the theoretical interpretation of recent experiments
on quantum reflection and Bose-Einstein condensation of ultracold atoms on or
near surfaces of different nature.Comment: 14 pages, 5 figures, 3 tables, accepted for publication in Phys. Rev.
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