24,828 research outputs found
Polaron Excitations in Doped C60: Effects of Disorders
Effects on C by thermal fluctuations of phonons, misalignment of
C molecules in a crystal, and other intercalated impurities (remaining
C, oxygens, and so on) are simulated by disorder potentials. The
Su-Schrieffer-Heeger--type electron-phonon model for doped C is solved
with gaussian bond disorders and also with site disorders. Sample average is
performed over sufficient number of disorder configurations. The distributions
of bond lengths and electron densities are shown as functions of the disorder
strength and the additional electron number. Stability of polaron excitations
as well as dimerization patterns is studied. Polarons and dimerizations in
lightly doped cases (C) are relatively stable against disorders,
indicated by peak structures in distribution functions. In more heavily doped
cases, the several peaks merge into a single peak, showing the breakdown of
polaron structures as well as the decrease of the dimerization strength.
Possibility of the observation of polaronic lattice distortions and electron
structures in doped C is discussed.Comment: Note: This manusript was accepted for publication in Physical Review
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Structure and Response in the World Trade Network
We examine how the structure of the world trade network has been shaped by
globalization and recessions over the last 40 years. We show that by treating
the world trade network as an evolving system, theory predicts the trade
network is more sensitive to evolutionary shocks and recovers more slowly from
them now than it did 40 years ago, due to structural changes in the world trade
network induced by globalization. We also show that recession-induced change to
the world trade network leads to an \emph{increased} hierarchical structure of
the global trade network for a few years after the recession.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Measurement of Magnetization Dynamics in Single-Molecule Magnets Induced by Pulsed Millimeter-Wave Radiation
We describe an experiment aimed at measuring the spin dynamics of the Fe8
single-molecule magnet in the presence of pulsed microwave radiation. In
earlier work, heating was observed after a 0.2-ms pulse of intense radiation,
indicating that the spin system and the lattice were out of thermal equilibrium
at millisecond time scale [Bal et al., Europhys. Lett. 71, 110 (2005)]. In the
current work, an inductive pick-up loop is used to probe the photon-induced
magnetization dynamics between only two levels of the spin system at much
shorter time scales (from ns to us). The relaxation time for the magnetization,
induced by a pulse of radiation, is found to be on the order of 10 us.Comment: 3 RevTeX pages, including 3 eps figures. The paper will appear in the
Journal of Applied Physics as MMM'05 conference proceeding
Face Detection with Effective Feature Extraction
There is an abundant literature on face detection due to its important role
in many vision applications. Since Viola and Jones proposed the first real-time
AdaBoost based face detector, Haar-like features have been adopted as the
method of choice for frontal face detection. In this work, we show that simple
features other than Haar-like features can also be applied for training an
effective face detector. Since, single feature is not discriminative enough to
separate faces from difficult non-faces, we further improve the generalization
performance of our simple features by introducing feature co-occurrences. We
demonstrate that our proposed features yield a performance improvement compared
to Haar-like features. In addition, our findings indicate that features play a
crucial role in the ability of the system to generalize.Comment: 7 pages. Conference version published in Asian Conf. Comp. Vision
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Analysis of plasma instabilities and verification of the BOUT code for the Large Plasma Device
The properties of linear instabilities in the Large Plasma Device [W.
Gekelman et al., Rev. Sci. Inst., 62, 2875 (1991)] are studied both through
analytic calculations and solving numerically a system of linearized
collisional plasma fluid equations using the 3D fluid code BOUT [M. Umansky et
al., Contrib. Plasma Phys. 180, 887 (2009)], which has been successfully
modified to treat cylindrical geometry. Instability drive from plasma pressure
gradients and flows is considered, focusing on resistive drift waves, the
Kelvin-Helmholtz and rotational interchange instabilities. A general linear
dispersion relation for partially ionized collisional plasmas including these
modes is derived and analyzed. For LAPD relevant profiles including strongly
driven flows it is found that all three modes can have comparable growth rates
and frequencies. Detailed comparison with solutions of the analytic dispersion
relation demonstrates that BOUT accurately reproduces all characteristics of
linear modes in this system.Comment: Published in Physics of Plasmas, 17, 102107 (2010
Kernel density classification and boosting: an L2 sub analysis
Kernel density estimation is a commonly used approach to classification. However, most of the theoretical results for kernel methods apply to estimation per se and not necessarily to classification. In this paper we show that when estimating the difference between two densities, the optimal smoothing parameters are increasing functions of the sample size of the complementary group, and we provide a small simluation study which examines the relative performance of kernel density methods when the final goal is classification. A relative newcomer to the classification portfolio is “boosting”, and this paper proposes an algorithm for boosting kernel density classifiers. We note that boosting is closely linked to a previously proposed method of bias reduction in kernel density estimation and indicate how it will enjoy similar properties for classification. We show that boosting kernel classifiers reduces the bias whilst only slightly increasing the variance, with an overall reduction in error. Numerical examples and simulations are used to illustrate the findings, and we also suggest further areas of research
The Effects of Chronic Ethanol Self-Administration on Hippocampal Serotonin Transporter Density in Monkeys
Evidence for an interaction between alcohol consumption and the serotonin system has been observed repeatedly in both humans and animal models yet the specific relationship between the two remains unclear. Research has focused primarily on the serotonin transporter (SERT) due in part to its role in regulating extracellular levels of serotonin. The hippocampal formation is heavily innervated by ascending serotonin fibers and is a major component of the neurocircuitry involved in mediating the reinforcing effects of alcohol. The current study investigated the effects of chronic ethanol self-administration on hippocampal SERT in a layer and field specific manner using a monkey model of human alcohol consumption. [3H]Citalopram was used to measure hippocampal SERT density in male cynomolgus macaques that voluntarily self-administered ethanol for 18 months. Hippocampal [3H]citalopram binding was less dense in ethanol drinkers than in controls, with the greatest effect observed in the molecular layer of the dentate gyrus. SERT density was not correlated with measures of ethanol consumption or blood ethanol concentrations, suggesting the possibility that a threshold level of consumption had been met. The lower hippocampal SERT density observed suggests that chronic ethanol consumption is associated with altered serotonergic modulation of hippocampal neurotransmission
Abundances and Physical Conditions in the Interstellar Gas toward HD 192 639
We present a study of the abundances and physical conditions in the
interstellar gas toward the heavily reddened star HD 192639 [E_(B-V) = 0.64],
based on analysis of FUSE and HST/STIS spectra covering the range from 912 to
1361 A. This work constitutes a survey of the analyses that can be performed to
study the interstellar gas when combining data from different instruments.
Low-velocity (-18 to -8 km/s) components are seen primarily for various neutral
and singly ionized species such as C I, O I, S I, Mg II, Cl I, Cl II, Mn II, Fe
II and Cu II. Numerous lines of H2 are present in the FUSE spectra, with a
kinetic temperature for the lowest rotational levels T_(01) = (90 +/- 10) K.
Analysis of the C I fine-structure excitation implies an average local density
of hydrogen n_H = (16 +/- 3) cm^-3. The average electron density, derived from
five neutral/first ion pairs under the assumption of photoionization
equilibrium, is n_e = (0.11 +/- 0.02) cm^-3. The relatively complex component
structure seen in high-resolution spectra of K I and Na I, the relatively low
average density, and the measured depletions all suggest that the line of sight
contains a number of diffuse clouds, rather than a single dense, translucent
cloud. Comparisons of the fractions of Cl in Cl I and of hydrogen in molecular
form suggest a higher molecular fraction, in the region(s) where H2 is present,
than that derived considering the average line of sight. In general, such
comparisons may allow the identification and characterization of translucent
portions of such complex lines of sight. The combined data also show
high-velocity components near -80 km/s for various species which appear to be
predominantly ionized, and may be due to a radiative shock. A brief overview of
the conditions in this gas will be given.Comment: 37 pages, accepted for publication in Ap
Regge calculus in the canonical form
(3+1) (continuous time) Regge calculus is reduced to Hamiltonian form. The
constraints are classified, classical and quantum consequences are discussed.
As basic variables connection matrices and antisymmetric area tensors are used
supplemented with appropriate bilinear constraints. In these variables the
action can be made quasipolinomial with as the only deviation from
polinomiality. In comparison with analogous formalism in the continuum theory
classification of constraints changes: some of them disappear, the part of I
class constraints including Hamiltonian one become II class (and vice versa,
some new constraints arise and some II class constraints become I class). As a
result, the number of the degrees of freedom coincides with the number of links
in 3-dimensional leaf of foliation. Moreover, in empty space classical dynamics
is trivial: the scale of timelike links become zero and spacelike links are
constant.Comment: 24 pages,Plain LaTeX,BINP 93-4
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