3,276 research outputs found
Development and field testing of a Light Aircraft Oil Surveillance System (LAOSS)
An experimental device consisting of a conventional TV camera with a low light level photo image tube and motor driven polarized filter arrangement was constructed to provide a remote means of discriminating the presence of oil on water surfaces. This polarized light filtering system permitted a series of successive, rapid changes between the vertical and horizontal components of reflected polarized skylight and caused the oil based substances to be more easily observed and identified as a flashing image against a relatively static water surface background. This instrument was flight tested, and the results, with targets of opportunity and more systematic test site data, indicate the potential usefulness of this airborne remote sensing instrument
Earthquake cycles and neural reverberations
Driven systems of interconnected blocks with stick-slip friction capture main features of earthquake processes. The microscopic dynamics closely resemble those of spiking nerve cells. We analyze the differences in the collective behavior and introduce a class of solvable models. We prove that the models exhibit rapid phase locking, a phenomenon of particular interest to both geophysics and neurobiology. We study the dependence upon initial conditions and system parameters, and discuss implications for earthquake modeling and neural computation
Statistical mechanics of temporal association in neural networks with transmission delays
We study the representation of static patterns and temporal sequences in neural networks with signal delays and a stochastic parallel dynamics. For a wide class of delay distributions, the asymptotic network behavior can be described by a generalized Gibbs distribution, generated by a novel Lyapunov functional for the determination dynamics. We extend techniques of equilibrium statistical mechanics so as to deal with time-dependent phenomena, derive analytic results for both retrieval quality and storage capacity, and compare them with numerical simulations
Simultaneous current-, force- and work function measurement with atomic resolution
The local work function of a surface determines the spatial decay of the
charge density at the Fermi level normal to the surface. Here, we present a
method that enables simultaneous measurements of local work function and
tip-sample forces. A combined dynamic scanning tunneling microscope and atomic
force microscope is used to measure the tunneling current between an
oscillating tip and the sample in real time as a function of the cantilever's
deflection. Atomically resolved work function measurements on a silicon
(111)-() surface are presented and related to concurrently recorded
tunneling current- and force- measurements.Comment: 8 pages, 4 figures, submitted to Applied Physics Letter
Charge trapping in polymer transistors probed by terahertz spectroscopy and scanning probe potentiometry
Terahertz time-domain spectroscopy and scanning probe potentiometry were used
to investigate charge trapping in polymer field-effect transistors fabricated
on a silicon gate. The hole density in the transistor channel was determined
from the reduction in the transmitted terahertz radiation under an applied gate
voltage. Prolonged device operation creates an exponential decay in the
differential terahertz transmission, compatible with an increase in the density
of trapped holes in the polymer channel. Taken in combination with scanning
probe potentionmetry measurements, these results indicate that device
degradation is largely a consequence of hole trapping, rather than of changes
to the mobility of free holes in the polymer.Comment: 4 pages, 3 figure
Optimal Population Codes for Space: Grid Cells Outperform Place Cells
Rodents use two distinct neuronal coordinate systems to estimate their position: place fields in the hippocampus and grid fields in the entorhinal cortex. Whereas place cells spike at only one particular spatial location, grid cells fire at multiple sites that correspond to the points of an imaginary hexagonal lattice. We study how to best construct place and grid codes, taking the probabilistic nature of neural spiking into account. Which spatial encoding properties of individual neurons confer the highest resolution when decoding the animal’s position from the neuronal population response? A priori, estimating a spatial position from a grid code could be ambiguous, as regular periodic lattices possess translational symmetry. The solution to this problem requires lattices for grid cells with different spacings; the spatial resolution crucially depends on choosing the right ratios of these spacings across the population. We compute the expected error in estimating the position in both the asymptotic limit, using Fisher information, and for low spike counts, using maximum likelihood estimation. Achieving high spatial resolution and covering a large range of space in a grid code leads to a trade-off: the best grid code for spatial resolution is built of nested modules with different spatial periods, one inside the other, whereas maximizing the spatial range requires distinct spatial periods that are pairwisely incommensurate. Optimizing the spatial resolution predicts two grid cell properties that have been experimentally observed. First, short lattice spacings should outnumber long lattice spacings. Second, the grid code should be self-similar across different lattice spacings, so that the grid field always covers a fixed fraction of the lattice period. If these conditions are satisfied and the spatial “tuning curves” for each neuron span the same range of firing rates, then the resolution of the grid code easily exceeds that of the best possible place code with the same number of neurons
Mesoscopic order and the dimentionality of long-range resonance energy transfer in supramolecular semiconductors
We present time-resolved photoluminescence measurements on two series of
oligo-p-phenylenevinylene materials that self-assemble into supramolecular
nanostructures with thermotropic reversibility in dodecane. One set of
derivatives form chiral, helical stacks while the second set form less
organised, frustrated stacks. Here we study the effects of supramolecular
organisation on the resonance energy transfer rates. We measure these rates in
nanoassemblies formed with mixed blends of oligomers and compare them with the
rates predicted by Foerster theory. Our results and analysis show that control
of supramolecular order in the nanometre lengthscale has a dominant effect on
the efficiency and dimentionality of resonance energy transfer.Comment: 17 Pages, 5 Figures, Submitted to J. Chem. Phy
Harmonic lattice behavior of two-dimensional colloidal crystals
Using positional data from video-microscopy and applying the equipartition
theorem for harmonic Hamiltonians, we determine the wave-vector-dependent
normal mode spring constants of a two-dimensional colloidal model crystal and
compare the measured band-structure to predictions of the harmonic lattice
theory. We find good agreement for both the transversal and the longitudinal
mode. For , the measured spring constants are consistent with the
elastic moduli of the crystal.Comment: 4 pages, 3 figures, submitte
Distances sets that are a shift of the integers and Fourier basis for planar convex sets
The aim of this paper is to prove that if a planar set has a difference
set satisfying for suitable than
has at most 3 elements. This result is motivated by the conjecture that the
disk has not more than 3 orthogonal exponentials. Further, we prove that if
is a set of exponentials mutually orthogonal with respect to any symmetric
convex set in the plane with a smooth boundary and everywhere non-vanishing
curvature, then # (A \cap {[-q,q]}^2) \leq C(K) q where is a constant
depending only on . This extends and clarifies in the plane the result of
Iosevich and Rudnev. As a corollary, we obtain the result from \cite{IKP01} and
\cite{IKT01} that if is a centrally symmetric convex body with a smooth
boundary and non-vanishing curvature, then does not possess an
orthogonal basis of exponentials
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