5,442 research outputs found
Detecting periodicity in experimental data using linear modeling techniques
Fourier spectral estimates and, to a lesser extent, the autocorrelation
function are the primary tools to detect periodicities in experimental data in
the physical and biological sciences. We propose a new method which is more
reliable than traditional techniques, and is able to make clear identification
of periodic behavior when traditional techniques do not. This technique is
based on an information theoretic reduction of linear (autoregressive) models
so that only the essential features of an autoregressive model are retained.
These models we call reduced autoregressive models (RARM). The essential
features of reduced autoregressive models include any periodicity present in
the data. We provide theoretical and numerical evidence from both experimental
and artificial data, to demonstrate that this technique will reliably detect
periodicities if and only if they are present in the data. There are strong
information theoretic arguments to support the statement that RARM detects
periodicities if they are present. Surrogate data techniques are used to ensure
the converse. Furthermore, our calculations demonstrate that RARM is more
robust, more accurate, and more sensitive, than traditional spectral
techniques.Comment: 10 pages (revtex) and 6 figures. To appear in Phys Rev E. Modified
styl
Model for the structure function constant for index of refraction fluctuations in Rayleigh-Benard turbulence
A model for the structure function constant associated with index of
refraction fluctuations in Rayleigh-Benard turbulence is developed. The model
is based upon the following assumptions: (1) the turbulence is homogeneous and
isotropic at or near the mid-plane, (2) the rate of production is in balance
with the rate of dissipation, (3) an inertial region exists, and (4) estimates
for the rate of dissipation of temperature fluctuations and of turbulent
kinetic energy can be made by assuming that the large-scale turbulence is
dissipated in one eddy turnover time. From these assumptions, the dependence of
the structure function on the geometry, heat flux, and the properties of the
fluid is obtained. The model predicts that the normalized structure function
constant is independent of the Rayleigh number. To verify the model, numerical
simulations of Rayleigh-Benard turbulence were performed using two different
approaches: an in-house code based on a pseudo-spectral method, and a finite
volume code which employs a model for the smallest scales of the turbulence.
The model was found to agree with the results of the simulations, thereby
lending support for the assumptions underlying the theory.Comment: 25 pages, 10 figures, 1 tabl
Dynamical modeling of collective behavior from pigeon flight data: flock cohesion and dispersion
Several models of flocking have been promoted based on simulations with
qualitatively naturalistic behavior. In this paper we provide the first direct
application of computational modeling methods to infer flocking behavior from
experimental field data. We show that this approach is able to infer general
rules for interaction, or lack of interaction, among members of a flock or,
more generally, any community. Using experimental field measurements of homing
pigeons in flight we demonstrate the existence of a basic distance dependent
attraction/repulsion relationship and show that this rule is sufficient to
explain collective behavior observed in nature. Positional data of individuals
over time are used as input data to a computational algorithm capable of
building complex nonlinear functions that can represent the system behavior.
Topological nearest neighbor interactions are considered to characterize the
components within this model. The efficacy of this method is demonstrated with
simulated noisy data generated from the classical (two dimensional) Vicsek
model. When applied to experimental data from homing pigeon flights we show
that the more complex three dimensional models are capable of predicting and
simulating trajectories, as well as exhibiting realistic collective dynamics.
The simulations of the reconstructed models are used to extract properties of
the collective behavior in pigeons, and how it is affected by changing the
initial conditions of the system. Our results demonstrate that this approach
may be applied to construct models capable of simulating trajectories and
collective dynamics using experimental field measurements of herd movement.
From these models, the behavior of the individual agents (animals) may be
inferred
Geometric scaling in the spectrum of an electron captured by a stationary finite dipole
We examine the energy spectrum of a charged particle in the presence of a
{\it non-rotating} finite electric dipole. For {\emph{any}} value of the dipole
moment above a certain critical value p_{\mathrm{c}}$ an infinite series of
bound states arises of which the energy eigenvalues obey an Efimov-like
geometric scaling law with an accumulation point at zero energy. These
properties are largely destroyed in a realistic situation when rotations are
included. Nevertheless, our analysis of the idealised case is of interest
because it may possibly be realised using quantum dots as artificial atoms.Comment: 5 figures; references added, outlook section reduce
Chaos and localization in the wavefunctions of complex atoms NdI, PmI and SmI
Wavefunctions of complex lanthanide atoms NdI, PmI and SmI, obtained via
multi-configuration Dirac-Fock method, are analyzed for density of states in
terms of partial densities, strength functions (), number of principal
components () and occupancies (\lan n_\alpha \ran^E) of single
particle orbits using embedded Gaussian orthogonal ensemble of one plus
two-body random matrix ensembles [EGOE(1+2)]. It is seen that density of states
are in general multi-modal, 's exhibit variations as function of the
basis states energy and 's show structures arising from localized
states. The sources of these departures from EGOE(1+2) are investigated by
examining the partial densities, correlations between , and
\lan n_\alpha \ran^E and also by studying the structure of the Hamiltonian
matrices. These studies point out the operation of EGOE(1+2) but at the same
time suggest that weak admixing between well separated configurations should be
incorporated into EGOE(1+2) for more quantitative description of chaos and
localization in NdI, PmI and SmI.Comment: There are 9 figure
Zone-plate focusing of Bose-Einstein condensates for atom optics and erasable high-speed lithography of quantum electronic components
We show that Fresnel zone plates, fabricated in a solid surface, can sharply
focus atomic Bose-Einstein condensates that quantum reflect from the surface or
pass through the etched holes. The focusing process compresses the condensate
by orders of magnitude despite inter-atomic repulsion. Crucially, the focusing
dynamics are insensitive to quantum fluctuations of the atom cloud and largely
preserve the condensates' coherence, suggesting applications in passive
atom-optical elements, for example zone plate lenses that focus atomic matter
waves and light at the same point to strengthen their interaction. We explore
transmission zone-plate focusing of alkali atoms as a route to erasable and
scalable lithography of quantum electronic components in two-dimensional
electron gases embedded in semiconductor nanostructures. To do this, we
calculate the density profile of a two-dimensional electron gas immediately
below a patch of alkali atoms deposited on the surface of the nanostructure by
zone-plate focusing. Our results reveal that surface-induced polarization of
only a few thousand adsorbed atoms can locally deplete the electron gas. We
show that, as a result, the focused deposition of alkali atoms by existing zone
plates can create quantum electronic components on the 50 nm scale, comparable
to that attainable by ion beam implantation but with minimal damage to either
the nanostructure or electron gas.Comment: 13 pages, 7 figure
Measurements of one-point statistics in 21 cm intensity maps via foreground avoidance strategy
Measurements of the one-point probability distribution function and
higher-order moments (variance, skewness, and kurtosis) of the high-redshift 21
cm fluctuations are among the most direct statistical probes of the
non-Gaussian nature of structure formation and evolution during reionization.
However, contamination from astrophysical foregrounds and instrument
systematics pose significant challenges in measuring these statistics in real
observations. In this work, we use forward modelling to investigate the
feasibility of measuring 21 cm one-point statistics through a foreground
avoidance strategy. Leveraging the well-known characteristic of foreground
contamination in which it occupies a wedge-shape region in k-space, we apply a
foreground wedge-cut filter that removes the contaminated modes from a mock
data set based on the Hydrogen Epoch of Reionization Array (HERA) instrument,
and measure the one-point statistics from the image-space representation of the
remaining non-contaminated modes. We experiment with wedge-cutting over
different frequency bandwidths and varying degrees of removal that correspond
to different assumptions on the extent of the foreground sources on the sky and
leakage from the Fourier Transform window function. We find that the centre of
the band is the least biased from wedge-cutting while the edges of the band are
unusable due to being highly down-weighted by the window function. Based on
this finding, we introduce a rolling filter method that allows reconstruction
of an optimal wedge-cut 21~cm intensity map over the full bandwidth using
outputs from wedge-cutting over multiple sub-bands. We perform Monte Carlo
simulations to show that HERA should be able to measure the rise in skewness
and kurtosis near the end of reionization with the rolling wedge-cut method if
foreground leakage from the Fourier transform window function can be
controlled.Comment: 12 pages, 8 figures, submitted to MNRA
Resonant Inelastic X-ray Scattering Studies of Elementary Excitations
In the past decade, Resonant Inelastic X-ray Scattering (RIXS) has made
remarkable progress as a spectroscopic technique. This is a direct result of
the availability of high-brilliance synchrotron X-ray radiation sources and of
advanced photon detection instrumentation. The technique's unique capability to
probe elementary excitations in complex materials by measuring their energy-,
momentum-, and polarization-dependence has brought RIXS to the forefront of
experimental photon science. We review both the experimental and theoretical
RIXS investigations of the past decade, focusing on those determining the
low-energy charge, spin, orbital and lattice excitations of solids. We present
the fundamentals of RIXS as an experimental method and then review the
theoretical state of affairs, its recent developments and discuss the different
(approximate) methods to compute the dynamical RIXS response. The last decade's
body of experimental RIXS data and its interpretation is surveyed, with an
emphasis on RIXS studies of correlated electron systems, especially transition
metal compounds. Finally, we discuss the promise that RIXS holds for the near
future, particularly in view of the advent of x-ray laser photon sources.Comment: Review, 67 pages, 44 figure
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Shaping public opinion about regional integration: the rhetoric of justification and party cues
The article investigates how justifications used by politicians to explain their positions on policies of regional integration shape public opinion about these policies. I argue that support for a policy position increases when politicians tailor their justifications to the expectations of their audience; and I suggest that this happens even when party cues offer a less effortful way of forming opinions. I test my theoretical expectations in laboratory experiments with diverse samples, which manipulate party cues and justifications for a policy of European integration. I find that citizens use justifications and cues to form opinions. The relative importance of the two factors depends on individual dispositions and political context. In a non-competitive context (study 1), politically invested citizens use cues, while uninvested citizens use justifications. In a competitive context (study 2), the opinions of politically invested citizens are shaped by both factors, while the opinions of uninvested citizens become erratic
Theory of Crystalline Electric Field and Kondo Effect in Pr Skutterudites
Possible Kondo effect in Pr skutterudite is studied with attention to
characteristic features of low-lying crystalline electric field (CEF) levels
and the conduction band. A mechanism for the small CEF splitting between a
singlet and a triplet is proposed as combination of the point-charge
interaction and hybridization of 4f with ligand p states. Provided 4f^3
configurations dominate over 4f^1 as intermediate states, p-f hybridization
favors the triplet, while point-charge interaction favors the singlet. For
realistic parameters for hybridization as well as 4f^1 and 4f^3 levels, these
singlet and triplet can form a nearly degenerate pseudo-quartet. It is found
that one of two spin 1/2 objects composing the pseudo-quartet has a
ferromagnetic exchange, while the other has an antiferromagnetic exchange with
conduction electrons. The magnitude of each effective exchange depends strongly
on a parameter characterizing the triplet wave function under the T_h symmetry.
It is argued that differences of this parameter among Pr skutterdudites are
responsible for the apparent diversity of their physical properties. Numerical
renormalization group is used to derive the renormalization flows going toward
singlet, doublet, triplet or quaret according to the CEF splitting and exchange
interactions.Comment: 19 pages, 6 figures, to be published in Special Invited Section
(Kondo Effect) of Journal of Physical Society of Japa
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