1,007 research outputs found
Spatio-Temporal Low Count Processes with Application to Violent Crime Events
There is significant interest in being able to predict where crimes will
happen, for example to aid in the efficient tasking of police and other
protective measures. We aim to model both the temporal and spatial dependencies
often exhibited by violent crimes in order to make such predictions. The
temporal variation of crimes typically follows patterns familiar in time series
analysis, but the spatial patterns are irregular and do not vary smoothly
across the area. Instead we find that spatially disjoint regions exhibit
correlated crime patterns. It is this indeterminate inter-region correlation
structure along with the low-count, discrete nature of counts of serious crimes
that motivates our proposed forecasting tool. In particular, we propose to
model the crime counts in each region using an integer-valued first order
autoregressive process. We take a Bayesian nonparametric approach to flexibly
discover a clustering of these region-specific time series. We then describe
how to account for covariates within this framework. Both approaches adjust for
seasonality. We demonstrate our approach through an analysis of weekly reported
violent crimes in Washington, D.C. between 2001-2008. Our forecasts outperform
standard methods while additionally providing useful tools such as prediction
intervals
Decay of Quasi-Particle in a Quantum Dot: the role of Energy Resolution
The disintegration of quasiparticle in a quantum dot due to the electron
interaction is considered. It was predicted recently that above the energy
\eps^{*} = \Delta(g/\ln g)^{1/2} each one particle peak in the spectrum is
split into many components ( and are the one particle level spacing
and conductance). We show that the observed value of \eps^{*} should depend
on the experimental resolution \delta \eps. In the broad region of variation
of \delta \eps the should be replaced by \ln(\Delta/ g\delta \eps).
We also give the arguments against the delocalization transition in the Fock
space. Most likely the number of satellite peaks grows continuously with
energy, being at \eps \sim \eps^{*}, but remains finite at \eps >
\eps^{*}. The predicted logarithmic distribution of inter-peak spacings may be
used for experimental confirmation of the below-Golden-Rule decay.Comment: 5 pages, REVTEX, 2 eps figures, version accepted for publication in
Phys. Rev. Let
Searching for additional heating - [OII] emission in the diffuse ionized gas of NGC891, NGC4631 and NGC3079
We present spectroscopic data of ionized gas in the disk--halo regions of
three edge-on galaxies, NGC 891, NGC 4631 and NGC 3079, covering a wavelength
range from [\ion{O}{2}] 3727\AA to [\ion{S}{2}] 6716.4\AA.
The inclusion of the [\ion{O}{2}] emission provides new constraints on the
properties of the diffuse ionized gas (DIG), in particular, the origin of the
observed spatial variations in the line intensity ratios. We used three
different methods to derive electron temperatures, abundances and ionization
fractions along the slit. The increase in the [\ion{O}{2}]/H line ratio
towards the halo in all three galaxies requires an increase either in electron
temperature or in oxygen abundance. Keeping the oxygen abundance constant
yields the most reasonable results for temperature, abundances, and ionization
fractions. Since a constant oxygen abundance seems to require an increase in
temperature towards the halo, we conclude that gradients in the electron
temperature play a significant role in the observed variations in the optical
line ratios from extraplanar DIG in these three spiral galaxies.Comment: 43 pages, 29 figure
Properties of low-lying states in a diffusive quantum dot and Fock-space localization
Motivated by an experiment by Sivan et al. (Europhys. Lett. 25, 605 (1994))
and by subsequent theoretical work on localization in Fock space, we study
numerically a hierarchical model for a finite many-body system of Fermions
moving in a disordered potential and coupled by a two-body interaction. We
focus attention on the low-lying states close to the Fermi energy. Both the
spreading width and the participation number depend smoothly on excitation
energy. This behavior is in keeping with naive expectations and does not
display Anderson localization. We show that the model reproduces essential
features of the experiment by Sivan et al.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. Let
Statistics of the Charging Spectrum of a Two-Dimensional Coulomb Glass Island
The fluctuations of capacitance of a two-dimensional island are studied in
the regime of low electron concentration and strong disorder, when electrons
can be considered classical particles. The universal capacitance distribution
is found, with the dispersion being of the order of the average. This
distribution is shown to be closely related to the shape of the Coulomb gap in
the one-electron density of states of the island. Behavior of the the
capacitance fluctuations near the metal - insulator transition is discussed.Comment: 4 pages, LaTex, 4 Postscript figures are included Discussion of the
situation with screening by metallic gate is adde
Energy Level Alignment at Molecule-Metal Interfaces from an Optimally-Tuned Range-Separated Hybrid Functional
The alignment of the frontier orbital energies of an adsorbed molecule with
the substrate Fermi level at metal-organic interfaces is a fundamental
observable of significant practical importance in nanoscience and beyond.
Typical density functional theory calculations, especially those using local
and semi-local functionals, often underestimate level alignment leading to
inaccurate electronic structure and charge transport properties. In this work,
we develop a new fully self-consistent predictive scheme to accurately compute
level alignment at certain classes of complex heterogeneous molecule-metal
interfaces based on optimally-tuned range-separated hybrid functionals.
Starting from a highly accurate description of the gas-phase electronic
structure, our method by construction captures important nonlocal surface
polarization effects via tuning of the long-range screened exchange in a
range-separated hybrid in a non-empirical and system-specific manner. We
implement this functional in a plane-wave code and apply it to several
physisorbed and chemisorbed molecule-metal interface systems. Our results are
in quantitative agreement with experiments, both the level alignment and work
function changes. Our approach constitutes a new practical scheme for accurate
and efficient calculations of the electronic structure of molecule-metal
interfaces.Comment: 15 pages, 8 figure
Regulation of Gonadotropin-Releasing Hormone (GnRH)-Receptor Gene Expression in Tilapia: Effect of GnRH and Dopamine
The present work was designed to study certain aspects of the endocrine regulation of gonadotropin-releasing hormone receptor (GnRH-R) in the pituitary of the teleost fish tilapia. A GnRH-R was cloned from the pituitary of hybrid tilapia (taGnRH-R) and was identified as a typical seven-transmembrane receptor. Northern blot analysis revealed a single GnRH-R transcript in the pituitary of approximately 2.3 kilobases. The taGnRH-R mRNA levels were significantly higher in females than in males. Injection of the salmon GnRH analog (sGnRHa; 5–50 μg/kg) increased the steady-state levels of taGnRH-R mRNA, with the highest response recorded at 25 μg/kg and at 36 h. At the higher dose of sGnRHa (50 μg/kg), taGnRH-R transcript appeared to be down-regulated. Exposure of tilapia pituitary cells in culture to graded doses (0.1–100 nM) of seabream (sbGnRH = GnRH I), chicken II (cGnRH II), or salmon GnRH (sGnRH = GnRH III) resulted in a significant increase in taGnRH-R mRNA levels. The highest levels of both LH release and taGnRH-R mRNA levels were recorded after exposure to cGnRH II and the lowest after exposure to sbGnRH. The dopamine-agonist quinpirole suppressed LH release and mRNA levels of taGnRH-R, indicating an inhibitory effect on GnRH-R synthesis. Collectively, these data provide evidence that GnRH in tilapia can up- regulate, whereas dopamine down-regulates, taGnRH-R mRNA levels
Calculation of dephasing times in closed quantum dots
Dephasing of one-particle states in closed quantum dots is analyzed within
the framework of random matrix theory and Master equation. Combination of this
analysis with recent experiments on the magnetoconductance allows for the first
time to evaluate the dephasing times of closed quantum dots. These dephasing
times turn out to depend on the mean level spacing and to be significantly
enhanced as compared with the case of open dots. Moreover, the experimental
data available are consistent with the prediction that the dephasing of
one-particle states in finite closed systems disappears at low enough energies
and temperatures.Comment: 4 pages, 3 figure
Two interacting quasiparticles above the Fermi sea
We study numerically the interaction and disorder effects for two
quasiparticles in two and three dimensions. The dependence of the
interaction-induced Breit-Wigner width on the excitation energy above the Fermi
level, the disorder strength and the system size is determined. A regime is
found where the width is practically independent of the excitation energy. The
results allow to estimate the two quasiparticle mobility edge.Comment: revtex, 4 pages, 4 figure
Chaos Thresholds in finite Fermi systems
The development of Quantum Chaos in finite interacting Fermi systems is
considered. At sufficiently high excitation energy the direct two-particle
interaction may mix into an eigen-state the exponentially large number of
simple Slater-determinant states. Nevertheless, the transition from Poisson to
Wigner-Dyson statistics of energy levels is governed by the effective high
order interaction between states very distant in the Fock space. The concrete
form of the transition depends on the way one chooses to work out the problem
of factorial divergency of the number of Feynman diagrams. In the proposed
scheme the change of statistics has a form of narrow phase transition and may
happen even below the direct interaction threshold.Comment: 9 pages, REVTEX, 2 eps figures. Enlarged versio
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