1,921 research outputs found
Ultraviolet Absorption and Luminescence Investigations
Ultraviolet photographic system for remote detection of luminescent minerals and rocks and discriminating among nonluminescent materials on basis of ultraviolet absorptio
Continuum description of finite-size particles advected by external flows. The effect of collisions
The equation of the density field of an assembly of macroscopic particles
advected by a hydrodynamic flow is derived from the microscopic description of
the system. This equation allows to recognize the role and the relative
importance of the different microscopic processes implicit in the model: the
driving of the external flow, the inertia of the particles, and the collisions
among them.
The validity of the density description is confirmed by comparisons of
numerical studies of the continuum equation with Direct Simulation Monte Carlo
(DSMC) simulations of hard disks advected by a chaotic flow. We show that the
collisions have two competing roles: a dispersing-like effect and a clustering
effect (even for elastic collisions). An unexpected feature is also observed in
the system: the presence of collisions can reverse the effect of inertia, so
that grains with lower inertia are more clusterized.Comment: Final (strongly modified) version accepted in PRE; 6 pages, 3 figure
A recurrent neural network with ever changing synapses
A recurrent neural network with noisy input is studied analytically, on the
basis of a Discrete Time Master Equation. The latter is derived from a
biologically realizable learning rule for the weights of the connections. In a
numerical study it is found that the fixed points of the dynamics of the net
are time dependent, implying that the representation in the brain of a fixed
piece of information (e.g., a word to be recognized) is not fixed in time.Comment: 17 pages, LaTeX, 4 figure
Phase separation in mixtures of colloids and long ideal polymer coils
Colloidal suspensions with free polymer coils which are larger than the
colloidal particles are considered. The polymer-colloid interaction is modeled
by an extension of the Asakura-Oosawa model. Phase separation occurs into
dilute and dense fluid phases of colloidal particles when polymer is added. The
critical density of this transition tends to zero as the size of the polymer
coils diverges.Comment: 5 pages, 3 figure
Concept for a Large Scalable Space Telescope: In-Space Assembly
We present a conceptual design for a scalable (10-50 meter segmented filled-aperture) space observatory operating at UV-optical-near infrared wavelengths. This telescope is designed for assembly in space by robots, astronauts or a combination of the two, as envisioned in NASA s Vision for Space Exploration. Our operations concept for this-space telescope provides for assembly and check-out in an Earth Moon L2 (EML2) orbit, and transport to a Sun-Earth L2 (SEL2) orbit for science operations and routine servicing, with return to EML2 for major servicing. We have developed and analyzed initial designs for the optical, structural, thermal and attitude control systems for a 30-m aperture space telescope. We further describe how the separate components are packaged for launch by heavy lift vehicle(s) and the approach for the robot assembly of the telescope from these components
Derivation of Hebb's rule
On the basis of the general form for the energy needed to adapt the
connection strengths of a network in which learning takes place, a local
learning rule is found for the changes of the weights. This biologically
realizable learning rule turns out to comply with Hebb's neuro-physiological
postulate, but is not of the form of any of the learning rules proposed in the
literature.
It is shown that, if a finite set of the same patterns is presented over and
over again to the network, the weights of the synapses converge to finite
values.
Furthermore, it is proved that the final values found in this biologically
realizable limit are the same as those found via a mathematical approach to the
problem of finding the weights of a partially connected neural network that can
store a collection of patterns. The mathematical solution is obtained via a
modified version of the so-called method of the pseudo-inverse, and has the
inverse of a reduced correlation matrix, rather than the usual correlation
matrix, as its basic ingredient. Thus, a biological network might realize the
final results of the mathematician by the energetically economic rule for the
adaption of the synapses found in this article.Comment: 29 pages, LaTeX, 3 figure
Parity forbidden excitations of Sr2CuO2Cl2 revealed by optical third-harmonic spectroscopy
We present the first study of nonlinear optical third harmonic generation in
the strongly correlated charge-transfer insulator Sr2CuO2Cl2. For fundamental
excitation in the near-infrared, the THG spectrum reveals a strongly resonant
response for photon energies near 0.7 eV. Polarization analysis reveals this
novel resonance to be only partially accounted for by three-photon excitation
to the optical charge-transfer exciton, and indicates that an even-parity
excitation at 2 eV, with a_1g symmetry, participates in the third harmonic
susceptibility.Comment: Requires RevTeX v4.0beta
Stream Microbial Communities Show Resistance to Pharmaceutical Exposure
Residues of pharmaceuticals are increasingly detected in surface waters throughout the world. In four streams in Baltimore, Maryland, USA, we detected analgesics, stimulants, antihistamines, and antibiotics using passive organic samplers. We exposed biofilm communities in these streams to the common drugs caffeine, cimetidine, ciprofloxacin, and diphenhydramine. Respiration rates in the least urban stream were suppressed when exposed to these drugs, but biofilm functioning in the most urban stream was resistant to drug exposure. Exposure to the antibiotic ciprofloxacin altered bacterial community composition at all sites, with the greatest change occurring in the most urban stream. These results indicated that continuous exposure to drugs in urban streams may select for sub‐populations of highly resistant bacteria that maintain community function in response to urban contaminants
Urban stream microbial communities show resistance to pharmaceutical exposure
Residues of pharmaceuticals are increasingly detected in surface waters throughout the world. In four streams in Baltimore, Maryland, USA, we detected analgesics, stimulants, antihistamines, and antibiotics using passive organic samplers. We exposed biofilm communities in these streams to the common drugs caffeine, cimetidine, ciprofloxacin, and diphenhydramine. Respiration rates in the least urban stream were suppressed when exposed to these drugs, but biofilm functioning in the most urban stream was resistant to drug exposure. Exposure to the antibiotic ciprofloxacin altered bacterial community composition at all sites, with the greatest change occurring in the most urban stream. These results indicated that continuous exposure to drugs in urban streams may select for sub-populations of highly resistant bacteria that maintain community function in response to urban contaminants
Mode-coupling theory for multiple-time correlation functions of tagged particle densities and dynamical filters designed for glassy systems
The theoretical framework for higher-order correlation functions involving
multiple times and multiple points in a classical, many-body system developed
by Van Zon and Schofield [Phys. Rev. E 65, 011106 (2002)] is extended here to
include tagged particle densities. Such densities have found an intriguing
application as proposed measures of dynamical heterogeneities in structural
glasses. The theoretical formalism is based upon projection operator techniques
which are used to isolate the slow time evolution of dynamical variables by
expanding the slowly-evolving component of arbitrary variables in an infinite
basis composed of the products of slow variables of the system. The resulting
formally exact mode-coupling expressions for multiple-point and multiple-time
correlation functions are made tractable by applying the so-called N-ordering
method. This theory is used to derive for moderate densities the leading mode
coupling expressions for indicators of relaxation type and domain relaxation,
which use dynamical filters that lead to multiple-time correlations of a tagged
particle density. The mode coupling expressions for higher order correlation
functions are also succesfully tested against simulations of a hard sphere
fluid at relatively low density.Comment: 15 pages, 2 figure
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