6,233 research outputs found
Correlation-Strength Driven Anderson Metal-Insulator Transition
The possibility of driving an Anderson metal-insulator transition in the
presence of scale-free disorder by changing the correlation exponent is
numerically investigated. We calculate the localization length for
quasi-one-dimensional systems at fixed energy and fixed disorder strength using
a standard transfer matrix method. From a finite-size scaling analysis we
extract the critical correlation exponent and the critical exponent
characterizing the phase transition.Comment: 3 pages; 2 figure
Electronic properties of disordered corner-sharing tetrahedral lattices
We have examined the behaviour of noninteracting electrons moving on a
corner-sharing tetrahedral lattice into which we introduce a uniform (box)
distribution, of width W, of random on-site energies. We have used both the
relative localization length and the spectral rigidity to analyze the nature of
the eigenstates, and have determined both the mobility edge trajectories as a
function of W, and the critical disorder, Wc, beyond which all states are
localized. We find (i) that the mobility edge trajectories (energies Ec vs.
disorder W) are qualitatively different from those found for a simple cubic
lattice, and (ii) that the spectral rigidity is scale invariant at Wc and thus
provides a reliable method of estimating this quantity -- we find Wc/t=14.5. We
discuss our results in the context of the metal-to-insulator transition
undergone by LiAlyTi{2-y}O4 in a quantum site percolation model that also
includes the above-mentioned Anderson disorder, and show that the effects
produced by Anderson disorder are far less important than those produced by
quantum site percolation, at least in the determination of the doping
concentration at which the metal-to-insulator transition is predicted to occur
Fourier methods for quasi-periodic oscillations
Quasi-periodic oscillations and invariant tori play an important role in the study of forced or coupled oscillators. This paper presents two new numerical methods for the investigation of quasi-periodic oscillations. Both algorithms can be regarded as generalisations of the averaging and the harmonic (spectral) balance methods. The algorithms are easy to implement and require only minimal a-priory knowledge of the system. In particular, the methods do not depend on an a-priory coordinate transformation. The methods are applied to a number of illustrative examples from nonlinear electrical engineering and the results show that the methods are effcient and reliable. In addition, these examples show that the presented algorithms can also continue through regions of sub-harmonic (phase-locked) resonance even though they are designed only for the quasi-periodic case
Partitioning Schemes and Non-Integer Box Sizes for the Box-Counting Algorithm in Multifractal Analysis
We compare different partitioning schemes for the box-counting algorithm in
the multifractal analysis by computing the singularity spectrum and the
distribution of the box probabilities. As model system we use the Anderson
model of localization in two and three dimensions. We show that a partitioning
scheme which includes unrestricted values of the box size and an average over
all box origins leads to smaller error bounds than the standard method using
only integer ratios of the linear system size and the box size which was found
by Rodriguez et al. (Eur. Phys. J. B 67, 77-82 (2009)) to yield the most
reliable results.Comment: 10 pages, 13 figure
Switching the current through molecular wires
The influence of Gaussian laser pulses on the transport through molecular
wires is investigated within a tight-binding model for spinless electrons
including correlation. Motivated by the phenomenon of coherent destruction of
tunneling for monochromatic laser fields, situations are studied in which the
maximum amplitude of the electric field fulfills the conditions for the
destructive quantum effect. It is shown that, as for monochromatic laser
pulses, the average current through the wire can be suppressed. For parameters
of the model, which do not show a net current without any optical field, a
Gaussian laser pulse can establish a temporary current. In addition, the effect
of electron correlation on the current is investigated.Comment: 8 pages, 6 figure
Geant4 Monte Carlo simulation study of the secondary radiation fields at the laser-driven ion source LION
At the Center for Advanced Laser Applications (CALA), Garching, Germany, the LION (Laser-driven ION Acceleration) experiment is being commissioned, aiming at the production of laser-driven bunches of protons and light ions with multi-MeV energies and repetition frequency up to 1 Hz. A Geant4 Monte Carlo-based study of the secondary neutron and photon fields expected during LION’s different commissioning phases is presented. Goal of this study is the characterization of the secondary radiation environment present inside and outside the LION cave. Three different primary proton spectra, taken from experimental results reported in the literature and representative of three different future stages of the LION’s commissioning path are used. Together with protons, also electrons are emitted through laser-target interaction and are also responsible for the production of secondary radiation. For the electron component of the three source terms, a simplified exponential model is used. Moreover, in order to reduce the simulation complexity, a two-components simplified geometrical model of proton and electron sources is proposed. It has been found that the radiation environment inside the experimental cave is either dominated by photons or neutrons depending on the position in the room and the source term used. The higher the intensity of the source, the higher the neutron contribution to the total dose for all scored positions. Maximum neutron and photon ambient dose equivalent values normalized to 10(9) simulated incident primaries were calculated at the exit of the vacuum chamber, where values of about 85 nSv (10(9) primaries)(−1) and 1.0 μSv (10(9) primaries)(−1) were found
Multifractal analysis of the metal-insulator transition in anisotropic systems
We study the Anderson model of localization with anisotropic hopping in three
dimensions for weakly coupled chains and weakly coupled planes. The eigenstates
of the Hamiltonian, as computed by Lanczos diagonalization for systems of sizes
up to , show multifractal behavior at the metal-insulator transition even
for strong anisotropy. The critical disorder strength determined from the
system size dependence of the singularity spectra is in a reasonable agreement
with a recent study using transfer matrix methods. But the respective spectrum
at deviates from the ``characteristic spectrum'' determined for the
isotropic system. This indicates a quantitative difference of the multifractal
properties of states of the anisotropic as compared to the isotropic system.
Further, we calculate the Kubo conductivity for given anisotropies by exact
diagonalization. Already for small system sizes of only sites we observe
a rapidly decreasing conductivity in the directions with reduced hopping if the
coupling becomes weaker.Comment: 25 RevTeX pages with 10 PS-figures include
Parton Distributions for the Octet and Decuplet Baryons
We calculate the parton distributions for both polarized and unpolarized
octet and decuplet baryons, using the MIT bag, dressed by mesons. We show that
the hyperfine interaction responsible for the and splittings leads to large deviations from SU(3) and SU(6) predictions.
For the we find significant polarized, non-strange parton
distributions which lead to a sizable polarization in polarized,
semi-inclusive scattering. We also discuss the flavour symmetry violation
arising from the meson-cloud associated with the chiral structure of baryons.Comment: 29 pages, 15 figure
Distribution of fractal dimensions at the Anderson transition
We investigated numerically the distribution of participation numbers in the
3d Anderson tight-binding model at the localization-delocalization threshold.
These numbers in {\em one} disordered system experience strong level-to-level
fluctuations in a wide energy range. The fluctuations grow substantially with
increasing size of the system. We argue that the fluctuations of the
correlation dimension, of the wave functions are the main reason for
this. The distribution of these correlation dimensions at the transition is
calculated. In the thermodynamic limit () it does not depend on
the system size . An interesting feature of this limiting distribution is
that it vanishes exactly at , the highest possible value of
the correlation dimension at the Anderson threshold in this model
Processing and Transmission of Information
Contains research objectives and reports on three research projects.National Science Foundation (Grant G-16526)National Institutes of Health (Grant MH-04737-03)National Aeronautics and Space Administration (Grant NsG-496)Lincoln Laboratory, Purchase Order DDL BB-107U.S. Air Force under Contract AF 19(628)-50
- …