93 research outputs found
Spatial chaos in weakly dispersive and viscous media: a nonperturbative theory of the driven KdV-Burgers equation
The asymptotic travelling wave solution of the KdV-Burgers equation driven by
the long scale periodic driver is constructed. The solution represents a
shock-train in which the quasi-periodic sequence of dispersive shocks or
soliton chains is interspersed by smoothly varying regions. It is shown that
the periodic solution which has the spatial driver period undergoes period
doublings as the governing parameter changes. Two types of chaotic behavior are
considered. The first type is a weak chaos, where only a small chaotic
deviation from the periodic solution occurs. The second type corresponds to the
developed chaos where the solution ``ignores'' the driver period and represents
a random sequence of uncorrelated shocks. In the case of weak chaos the shock
coordinate being repeatedly mapped over the driver period moves on a chaotic
attractor, while in the case of developed chaos it moves on a repellor. Both
solutions depend on a parameter indicating the reference shock position in the
shock-train. The structure of a one dimensional set to which this parameter
belongs is investigated. This set contains measure one intervals around the
fixed points in the case of periodic or weakly chaotic solutions and it becomes
a fractal in the case of strong chaos. The capacity dimension of this set is
calculated.Comment: 32 pages, 12 PostScript figures, useses elsart.sty and boxedeps.tex,
fig.11 is not included and can be requested from <[email protected]
Conductance of a Quantum Point Contact in the presence of a Scanning Probe Microscope Tip
Using the recursive Green's function technique, we study the coherent
electron conductance of a quantum point contact in the presence of a scanning
probe microscope tip. Images of the coherent fringe inside a quantum point
contact for different widths are obtained. It is found that the conductance of
a specific channel is reduced while other channels are not affected as long as
the tip is located at the positions correspending to that channel. Moreover,
the coherent fringe is smoothed out by increasing the temperature or the
voltage across the device. Our results are consistent with the experiments
reported by Topinka et al.[Science 289, 2323 (2000)].Comment: 5 page
The boson peak in structural and orientational glasses of simple alcohols: Specific heat at low temperatures
We review in this work specific-heat experiments, that we have conducted on
different hydrogen-bonded glasses during last years. Specifically, we have
measured the low-temperature specific heat Cp for a set of glassy alcohols:
normal and fully-deuterated ethanol, 1- and 2- propanol, and glycerol. Ethanol
exhibits a very interesting polymorphism presenting three different solid
phases at low temperature: a fully-ordered (monoclinic) crystal, an
orientationally-disordered (cubic) crystal or 'orientational glass', and the
ordinary structural glass. By measuring and comparing the low-temperature
specific heat of the three phases, in the 'boson peak' range 2-10 K as well as
in the tunneling-states range below 1K, we are able to provide a quantitative
confirmation that ''glassy behavior'' is not an exclusive property of amorphous
solids. On the other hand, propanol is the simplest monoalcohol with two
different stereoisomers (1- and 2-propanol), what allows us to study directly
the influence of the spatial rearrangement of atoms on the universal properties
of glasses. We have measured the specific heat of both isomers, finding a
noteworthy quantitative difference between them. Finally, low-temperature
specific-heat data of glassy glycerol have also been obtained. Here we propose
a simple method based upon the soft-potential model to analyze low-temperature
specific-heat measurements, and we use this method for a quantitative
comparison of all these data of glassy alcohols and as a stringent test of
several universal correlations and scaling laws suggested in the literature. In
particular, we find that the interstitialcy model for the boson peak [A. V.
Granato, Phys. Rev. Lett. 68 (1992) 974] gives a very good account of the
temperature at which the maximum in Cp/T^3 occurs.Comment: 16 pages, 2 figures, Proceedings of the 4th International Discussion
Meeting on Relaxations in Complex Systems, Hersonissos (Crete), June 2001.
Journal of Non-Crystalline Solids (accepted for publication
Interaction of quasilocal harmonic modes and boson peak in glasses
The direct proportionality relation between the boson peak maximum in
glasses, , and the Ioffe-Regel crossover frequency for phonons,
, is established. For several investigated materials . At the frequency the mean free path of the
phonons becomes equal to their wavelength because of strong resonant
scattering on quasilocal harmonic oscillators. Above this frequency phonons
cease to exist. We prove that the established correlation between
and holds in the general case and is a direct consequence of
bilinear coupling of quasilocal oscillators with the strain field.Comment: RevTex, 4 pages, 1 figur
Inter- and intragrain currents in bulk melt-grown YBaCuO rings
A simple contactless method suitable to discern between the intergrain
(circular) current, which flows in the thin superconducting ring, and the
intragrain current, which does not cross the weakest link, has been proposed.
At first, we show that the intergrain current may directly be estimated from
the magnetic flux density measured by the Hall sensor positioned
in the special points above/below the ring center. The experimental
and the numerical techniques to determine the value are discussed. Being
very promising for characterization of a current flowing across the joints in
welded YBaCuO rings (its dependencies on the temperature and the external
magnetic field as well as the time dissipation), the approach has been applied
to study corresponding properties of the intra- and intergrain currents flowing
across the -twisted grain boundaries which are frequent in bulk
melt-textured YBaCuO samples. We present experimental data related to the flux
penetration inside a bore of MT YBaCuO rings both in the non-magnetized, virgin
state and during the field reversal. The shielding properties and their
dependence on external magnetic fields are also studied. Besides, we consider
the flux creep effects and their influence on the current re-distribution
during a dwell.Comment: 13 pages, 16 figures (EPS), RevTeX4. In the revised version,
corrections to perturbing effects near the weak links are introduced, one
more figure is added. lin
COMPENSATING FOR TURBOFAN COMPONENTS DEGRADATION THROUGH AN ENGINE LIFECYCLE BY CONTROL METHODS
It describes an approach to improve performance of a deteriorated gas turbine engine, based on the use of thrust estimates in automatic control system algorithms
Nonlinear dynamics of soft boson collective excitations in hot QCD plasma III: bremsstrahlung and energy losses
Within of the framework of semiclassical approximation a general formalism
for deriving an effective current generating bremsstrahlung of arbitrary number
of soft gluons (longitudinal or transverse ones) in scattering of higher-energy
parton off thermal parton in hot quark-gluon plasma with subsequent extension
to two and more scatterers, is obtained. For the case of static color centers
an expression for energy loss induced by usual bremsstrahlung of lowest-order
with allowance for an effective temperature-induced gluon mass and finite mass
of the projectile (heavy quark), is derived. The detailed analysis of
contribution to radiation energy loss associated with existence of effective
three-gluon vertex induced by hot QCD medium, is performed. It is shown that in
general, the bremsstrahlung associated with this vertex have no sharp direction
(as in the case of usual bremsstrahlung) and therefore here, we can expect an
absence of suppression effect due to multiple scattering. For the case of two
color static scattering centers it was shown that the problem of calculation of
bremsstrahlung induced by four-gluon hard thermal loop (HTL) vertex correction
can be reduced to the problem of the calculation of bremsstrahlung induced by
three-gluon HTL correction. It was shown that for limiting value of soft gluon
occupation number all higher processes of
bremsstrahlung of arbitrary number of soft gluons become of the same order in
coupling, and the problem of resummation of all relevant contributions to
radiation energy loss of fast parton, arises. An explicit expression for matrix
element of two soft gluon bremsstrahlung in small angles approximation is
obtained.Comment: 68 pages, 9 EPS figures; added new sections 8, 10 and reference
Voronoi-Delaunay analysis of normal modes in a simple model glass
We combine a conventional harmonic analysis of vibrations in a one-atomic
model glass of soft spheres with a Voronoi-Delaunay geometrical analysis of the
structure. ``Structure potentials'' (tetragonality, sphericity or perfectness)
are introduced to describe the shape of the local atomic configurations
(Delaunay simplices) as function of the atomic coordinates. Apart from the
highest and lowest frequencies the amplitude weighted ``structure potential''
varies only little with frequency. The movement of atoms in soft modes causes
transitions between different ``perfect'' realizations of local structure. As
for the potential energy a dynamic matrix can be defined for the ``structure
potential''. Its expectation value with respect to the vibrational modes
increases nearly linearly with frequency and shows a clear indication of the
boson peak. The structure eigenvectors of this dynamical matrix are strongly
correlated to the vibrational ones. Four subgroups of modes can be
distinguished
Bosonic Excitations in Random Media
We consider classical normal modes and non-interacting bosonic excitations in
disordered systems. We emphasise generic aspects of such problems and parallels
with disordered, non-interacting systems of fermions, and discuss in particular
the relevance for bosonic excitations of symmetry classes known in the
fermionic context. We also stress important differences between bosonic and
fermionic problems. One of these follows from the fact that ground state
stability of a system requires all bosonic excitation energy levels to be
positive, while stability in systems of non-interacting fermions is ensured by
the exclusion principle, whatever the single-particle energies. As a
consequence, simple models of uncorrelated disorder are less useful for bosonic
systems than for fermionic ones, and it is generally important to study the
excitation spectrum in conjunction with the problem of constructing a
disorder-dependent ground state: we show how a mapping to an operator with
chiral symmetry provides a useful tool for doing this. A second difference
involves the distinction for bosonic systems between excitations which are
Goldstone modes and those which are not. In the case of Goldstone modes we
review established results illustrating the fact that disorder decouples from
excitations in the low frequency limit, above a critical dimension , which
in different circumstances takes the values and . For bosonic
excitations which are not Goldstone modes, we argue that an excitation density
varying with frequency as is a universal
feature in systems with ground states that depend on the disorder realisation.
We illustrate our conclusions with extensive analytical and some numerical
calculations for a variety of models in one dimension
Quantum anti-centrifugal force
In a two-dimensional world a free quantum particle of vanishing angular
momentum experiences an attractive force. This force originates from a
modification of the classical centrifugal force due to the wave nature of the
particle. For positive energies the quantum anti-centrifugal force manifests
itself in a bunching of the nodes of the energy wave functions towards the
origin. For negative energies this force is sufficient to create a bound state
in a two-dimensional delta function potential. In a counter-intuitive way the
attractive force pushes the particle away from the location of the delta
function potential. As a consequence, the particle is localized in a
band-shaped domain around the originComment: 8 pages, including three eps figures, submitted to Phys. Rev. A.
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