677 research outputs found
A construction of integer-valued polynomials with prescribed sets of lengths of factorizations
For an arbitrary finite set S of natural numbers greater 1, we construct an
integer-valued polynomial f, whose set of lengths in Int(Z) is S. The set of
lengths of f is the set of all natural numbers n, such that f has a
factorization as a product of n irreducibles in Int(Z)={g in Q[x] | g(Z)
contained in Z}.Comment: To appear in Monatshefte f\"ur Mathematik; 11 page
Interpretation of Nuclear Quadrupole Resonance Spectra in Doped LaCuO
The nuclear quadrupole resonance (NQR) spectrum of strontium doped
LaCuO surprisingly resembles the NQR spectrum of LaCuO doped
with excess oxygen, both spectra being dominated by a main peak and one
principal satellite peak at similar frequencies. Using first-principles cluster
calculations this is investigated here by calculating the electric field
gradient (EFG) at the central copper site of the cluster after replacing a
lanthanum atom in the cluster with a strontium atom or adding an interstitial
oxygen to the cluster. In each case the EFG was increased by approximately 10 %
leading unexpectedly to the explanation that the NQR spectra are only
accidentally similar and the origins are quite different. Additionally the
widths of the peaks in the NQR spectra are explained by the different EFG of
copper centres remote from the impurity. A model, based on holes moving rapidly
across the planar oxygen atoms, is proposed to explain the observed increase in
frequency of both the main and satellite peaks in the NQR spectrum as the
doping concentration is increased
On the universality of small scale turbulence
The proposed universality of small scale turbulence is investigated for a set
of measurements in a cryogenic free jet with a variation of the Reynolds number
(Re) from 8500 to 10^6. The traditional analysis of the statistics of velocity
increments by means of structure functions or probability density functions is
replaced by a new method which is based on the theory of stochastic Markovian
processes. It gives access to a more complete characterization by means of
joint probabilities of finding velocity increments at several scales. Based on
this more precise method our results call in question the concept of
universality.Comment: 4 pages, 4 figure
Velocity Correlations, Diffusion and Stochasticity in a One-Dimensional System
We consider the motion of a test particle in a one-dimensional system of
equal-mass point particles. The test particle plays the role of a microscopic
"piston" that separates two hard-point gases with different concentrations and
arbitrary initial velocity distributions. In the homogeneous case when the
gases on either side of the piston are in the same macroscopic state, we
compute and analyze the stationary velocity autocorrelation function C(t).
Explicit expressions are obtained for certain typical velocity distributions,
serving to elucidate in particular the asymptotic behavior of C(t). It is shown
that the occurrence of a non-vanishing probability mass at zero velocity is
necessary for the occurrence of a long-time tail in C(t). The conditions under
which this is a tail are determined. Turning to the inhomogeneous
system with different macroscopic states on either side of the piston, we
determine its effective diffusion coefficient from the asymptotic behavior of
the variance of its position, as well as the leading behavior of the other
moments about the mean. Finally, we present an interpretation of the effective
noise arising from the dynamics of the two gases, and thence that of the
stochastic process to which the position of any particle in the system reduces
in the thermodynamic limit.Comment: 22 files, 2 eps figures. Submitted to PR
How to quantify deterministic and random influences on the statistics of the foreign exchange market
It is shown that prize changes of the US dollar - German Mark exchange rates
upon different delay times can be regarded as a stochastic Marcovian process.
Furthermore we show that from the empirical data the Kramers-Moyal coefficients
can be estimated.
Finally, we present an explicite Fokker-Planck equation which models very
precisely the empirical probabilitiy distributions.Comment: 3 figure
Developed turbulence: From full simulations to full mode reductions
Developed Navier-Stokes turbulence is simulated with varying wavevector mode
reductions. The flatness and the skewness of the velocity derivative depend on
the degree of mode reduction. They show a crossover towards the value of the
full numerical simulation when the viscous subrange starts to be resolved. The
intermittency corrections of the scaling exponents of the pth order velocity
structure functions seem to depend mainly on the proper resolution of the
inertial subrange. Universal scaling properties (i.e., independent of the
degree of mode reduction) are found for the relative scaling exponents rho
which were recently defined by Benzi et al.Comment: 4 pages, 5 eps-figures, replaces version from August 5th, 199
Are galactic disks dynamically influenced by dust?
Dynamically cold components are well known to destabilize hotter, even much
more massive components. In this paper we studied the dynamical influence of a
cold dust component on the gaseous phase in the central regions of galactic
disks. We performed two-dimensional hydrodynamical simulations for flat
multi-component disks embedded in a combined static stellar and dark matter
potential. The pressure-free dust component is coupled to the gas by a drag
force depending on their velocity difference.
It turned out that the most unstable regions are those with either a low or
near to minimum Toomre parameter or with rigid rotation, i.e. the central area.
In that regions the dust-free disks become most unstable for high azimuthal
modes (m~8), whereas in dusty disks all modes have a similar amplitude
resulting in a patchy appearance. The structures in the dust have a larger
contrast between arm and inter-arm regions than those of the gas. The dust
peaks are frequently correlated with peaks of the gas distribution, but they do
not necessarily coincide with them. Therefore, a large scatter in the
dust-to-gas ratios is expected. The appearance of the dust is more cellular
(i.e. sometimes connecting different spiral features), whereas the gas is
organized in a multi-armed spiral structure.
An admixture of 2% dust destabilizes gaseous disks substantially, whereas
dust-to-gas ratios below 1% have no influence on the evolution of the gaseous
disk. For a high dust-to-gas ratio of 10% the instabilities reach a saturation
level already after 30 Myr.Comment: 21 pages including 24 figures (some figures degraded in quality), in
press in Astronomy & Astrophysics 418, 959(2004), A&A version available at
http://www.edpsciences.org/articles/aa/full/2004/18/aa0047/aa0047.htm
Angle-resolved photoemission in doped charge-transfer Mott insulators
A theory of angle-resolved photoemission (ARPES) in doped cuprates and other
charge-transfer Mott insulators is developed taking into account the realistic
(LDA+U) band structure, (bi)polaron formation due to the strong electron-phonon
interaction, and a random field potential. In most of these materials the first
band to be doped is the oxygen band inside the Mott-Hubbard gap. We derive the
coherent part of the ARPES spectra with the oxygen hole spectral function
calculated in the non-crossing (ladder) approximation and with the exact
spectral function of a one-dimensional hole in a random potential. Some unusual
features of ARPES including the polarisation dependence and spectral shape in
YBa2Cu3O7 and YBa2Cu4O8 are described without any Fermi-surface, large or
small. The theory is compatible with the doping dependence of kinetic and
thermodynamic properties of cuprates as well as with the d-wave symmetry of the
superconducting order parameter.Comment: 8 pages (RevTeX), 10 figures, submitted to Phys. Rev.
Flux-Induced Vortex in Mesoscopic Superconducting Loops
We predict the existence of a quantum vortex for an unusual situation. We
study the order parameter in doubly connected superconducting samples embedded
in a uniform magnetic field. For samples with perfect cylindrical symmetry, the
order parameter has been known for long and no vortices are present in the
linear regime. However, if the sample is not symmetric, there exist ranges of
the field for which the order parameter vanishes along a line, parallel to the
field. In many respects, the behavior of this line is qualitatively different
from that of the vortices encountered in type II superconductivity. For samples
with mirror symmetry, this flux-induced vortex appears at the thin side for
small fluxes and at the opposite side for large fluxes. We propose direct and
indirect experimental methods which could test our predictions.Comment: 6 pages, Latex, 4 figs., uses RevTex, extended to situations far from
cylindrical symmetr
Ab initio Quantum and ab initio Molecular Dynamics of the Dissociative Adsorption of Hydrogen on Pd(100)
The dissociative adsorption of hydrogen on Pd(100) has been studied by ab
initio quantum dynamics and ab initio molecular dynamics calculations. Treating
all hydrogen degrees of freedom as dynamical coordinates implies a high
dimensionality and requires statistical averages over thousands of
trajectories. An efficient and accurate treatment of such extensive statistics
is achieved in two steps: In a first step we evaluate the ab initio potential
energy surface (PES) and determine an analytical representation. Then, in an
independent second step dynamical calculations are performed on the analytical
representation of the PES. Thus the dissociation dynamics is investigated
without any crucial assumption except for the Born-Oppenheimer approximation
which is anyhow employed when density-functional theory calculations are
performed. The ab initio molecular dynamics is compared to detailed quantum
dynamical calculations on exactly the same ab initio PES. The occurence of
quantum oscillations in the sticking probability as a function of kinetic
energy is addressed. They turn out to be very sensitive to the symmetry of the
initial conditions. At low kinetic energies sticking is dominated by the
steering effect which is illustrated using classical trajectories. The steering
effects depends on the kinetic energy, but not on the mass of the molecules.
Zero-point effects lead to strong differences between quantum and classical
calculations of the sticking probability. The dependence of the sticking
probability on the angle of incidence is analysed; it is found to be in good
agreement with experimental data. The results show that the determination of
the potential energy surface combined with high-dimensional dynamical
calculations, in which all relevant degrees of freedon are taken into account,
leads to a detailed understanding of the dissociation dynamics of hydrogen at a
transition metal surface.Comment: 15 pages, 9 figures, subm. to Phys. Rev.
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