16,302 research outputs found
Crossover of Level Statistics between Strong and Weak Localization in Two Dimensions
We investigate numerically the statistical properties of spectra of
two-dimensional disordered systems by using the exact diagonalization and
decimation method applied to the Anderson model. Statistics of spacings
calculated for system sizes up to 1024 1024 lattice sites exhibits a
crossover between Wigner and Poisson distributions. We perform a self-contained
finite-size scaling analysis to find a single-valued one-parameter function
which governs the crossover. The scaling parameter is
deduced and compared with the localization length. does {\em
not} show critical behavior and has two asymptotic regimes corresponding to
weakly and strongly localized states.Comment: 4 pages in revtex, 3 postscript figure
Shock-resolved Navier–Stokes simulation of the Richtmyer–Meshkov instability start-up at a light–heavy interface
The single-mode Richtmyer–Meshkov instability is investigated using a first-order perturbation of the two-dimensional Navier–Stokes equations about a one-dimensional unsteady shock-resolved base flow. A feature-tracking local refinement scheme is used to fully resolve the viscous internal structure of the shock. This method captures perturbations on the shocks and their influence on the interface growth throughout the simulation, to accurately examine the start-up and early linear growth phases of the instability. Results are compared to analytic models of the instability, showing some agreement with predicted asymptotic growth rates towards the inviscid limit, but significant discrepancies are noted in the transient growth phase. Viscous effects are found to be inadequately predicted by existing models
Maxwell Fields in Spacetimes Admitting Non-Null Killing Vectors
We consider source-free electromagnetic fields in spacetimes possessing a
non-null Killing vector field, . We assume further that the
electromagnetic field tensor, , is invariant under the action of the
isometry group induced by . It is proved that whenever the two
potentials associated with the electromagnetic field are functionally
independent the entire content of Maxwell's equations is equivalent to the
relation \n^aT_{ab}=0. Since this relation is implied by Einstein's equation
we argue that it is enough to solve merely Einstein's equation for these
electrovac spacetimes because the relevant equations of motion will be
satisfied automatically. It is also shown that for the exceptional case of
functionally related potentials \n^aT_{ab}=0 implies along with one of the
relevant equations of motion that the complementary equation concerning the
electromagnetic field is satisfied.Comment: 7 pages,PACS numbers: 04.20.Cv, 04.20.Me, 04.40.+
Quantum theory of an atom laser originating from a Bose-Einstein condensate or a Fermi gas in the presence of gravity
We present a 3D quantum mechanical theory of radio-frequency outcoupled atom
lasers from trapped atomic gases in the presence of the gravitational force.
Predictions for the total outcoupling rate as a function of the radio-frequency
and for the beam wave function are given. We establish a sum rule for the
energy integrated outcoupling, which leads to a separate determination of the
coupling strength between the atoms and the radiation field.
For a non-interacting Bose-Einstein condensate analytic solutions are derived
which are subsequently extended to include the effects of atomic interactions.
The interactions enhance interference effects in the beam profile and modify
the outcoupling rate of the atom laser. We provide a complete quantum
mechanical solution which is in line with experimental findings and allows to
determine the validity of commonly used approximative methods.
We also extend the formalism to a fermionic atom laser and analyze the effect
of superfluidity on the outcoupling of atoms.Comment: 13 pages, 8 figures, slightly expanded versio
Discussion quality diffuses in the digital public square
Studies of online social influence have demonstrated that friends have
important effects on many types of behavior in a wide variety of settings.
However, we know much less about how influence works among relative strangers
in digital public squares, despite important conversations happening in such
spaces. We present the results of a study on large public Facebook pages where
we randomly used two different methods--most recent and social feedback--to
order comments on posts. We find that the social feedback condition results in
higher quality viewed comments and response comments. After measuring the
average quality of comments written by users before the study, we find that
social feedback has a positive effect on response quality for both low and high
quality commenters. We draw on a theoretical framework of social norms to
explain this empirical result. In order to examine the influence mechanism
further, we measure the similarity between comments viewed and written during
the study, finding that similarity increases for the highest quality
contributors under the social feedback condition. This suggests that, in
addition to norms, some individuals may respond with increased relevance to
high-quality comments.Comment: 10 pages, 6 figures, 2 table
NMR evidence for a strong modulation of the Bose-Einstein Condensate in BaCuSiO
We present a Cu and Si NMR study of the quasi-2D coupled
spin 1/2 dimer compound BaCuSiO in the magnetic field range 13-26 T and
at temperatures as low as 50 mK. NMR data in the gapped phase reveal that below
90 K different intra-dimer exchange couplings and different gaps
( = 1.16) exist in every second plane along
the c-axis, in addition to a planar incommensurate (IC) modulation. Si
spectra in the field induced magnetic ordered phase reveal that close to the
quantum critical point at = 23.35 T the average boson density
of the Bose-Einstein condensate is strongly modulated along the
c-axis with a density ratio for every second plane
. An IC modulation of the local
density is also present in each plane. This adds new constraints for the
understanding of the 2D value = 1 of the critical exponent describing
the phase boundary
Information Content in Decays and the Angular Moments Method
The time-dependent angular distributions of decays of neutral mesons into
two vector mesons contain information about the lifetimes, mass differences,
strong and weak phases, form factors, and CP violating quantities. A
statistical analysis of the information content is performed by giving the
``information'' a quantitative meaning. It is shown that for some parameters of
interest, the information content in time and angular measurements combined may
be orders of magnitude more than the information from time measurements alone
and hence the angular measurements are highly recommended. The method of
angular moments is compared with the (maximum) likelihood method to find that
it works almost as well in the region of interest for the one-angle
distribution. For the complete three-angle distribution, an estimate of
possible statistical errors expected on the observables of interest is
obtained. It indicates that the three-angle distribution, unraveled by the
method of angular moments, would be able to nail down many quantities of
interest and will help in pointing unambiguously to new physics.Comment: LaTeX, 34 pages with 9 figure
Changes in Polarization Position Angle across the Eclipse in the Double Pulsar System
We investigate the changes in polarization position angle in radiation from
pulsar A around the eclipse in the Double Pulsar system PSR J0737-3039A/B at
the 20 cm and 50 cm wavelengths using the Parkes 64-m telescope. The changes
are ~2\sigma\ during and shortly after the eclipse at 20 cm but less
significant at 50 cm. We show that the changes in position angle during the
eclipse can be modelled by differential synchrotron absorption in the eclipse
regions. Position angle changes after the eclipse are interpreted as Faraday
rotation in the magnetotail of pulsar B. Implied charge densities are
consistent with the Goldreich-Julian density, suggesting that the particle
energies in the magnetotail are mildly relativistic.Comment: Accepted for publication in The Astrophysical Journal Letter
Tunneling out of a time-dependent well
Solutions to explicit time-dependent problems in quantum mechanics are rare.
In fact, all known solutions are coupled to specific properties of the
Hamiltonian and may be divided into two categories: One class consists of
time-dependent Hamiltonians which are not higher than quadratic in the position
operator, like i.e the driven harmonic oscillator with time-dependent
frequency. The second class is related to the existence of additional
invariants in the Hamiltonian, which can be used to map the solution of the
time-dependent problem to that of a related time-independent one.
In this article we discuss and develop analytic methods for solving
time-dependent tunneling problems, which cannot be addressed by using quadratic
Hamiltonians. Specifically, we give an analytic solution to the problem of
tunneling from an attractive time-dependent potential which is embedded in a
long-range repulsive potential.
Recent progress in atomic physics makes it possible to observe experimentally
time-dependent phenomena and record the probability distribution over a long
range of time. Of special interest is the observation of macroscopical
quantum-tunneling phenomena in Bose-Einstein condensates with time-dependent
trapping potentials. We apply our model to such a case in the last section.Comment: 11 pages, 3 figure
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