1,463 research outputs found
Electrical and radiation characteristics of semilarge photoconductive terahertz emitters
We present experimental characterization of semilarge
photoconductive emitters, including their electrical/photoconductive
parameters and terahertz spectra. A range of emitters
were studied and fabricated on both LT-GaAs and SI-GaAs,
having a variety of electrode geometries. The spatial cone of terahertz
radiation was defined. The dependencies of the photocurrent
and the terahertz power on the bias voltage and the laser power
were determined. A Fourier-transform interferometer is used to
determine the terahertz spectra and to clarify the effects of the
substrate and electrode geometry
Generic theory of colloidal transport
We discuss the motion of colloidal particles relative to a two component
fluid consisting of solvent and solute. Particle motion can result from (i) net
body forces on the particle due to external fields such as gravity; (ii) slip
velocities on the particle surface due to surface dissipative phenomena. The
perturbations of the hydrodynamic flow field exhibits characteristic
differences in cases (i) and (ii) which reflect different patterns of momentum
flux corresponding to the existence of net forces, force dipoles or force
quadrupoles. In the absence of external fields, gradients of concentration or
pressure do not generate net forces on a colloidal particle. Such gradients can
nevertheless induce relative motion between particle and fluid. We present a
generic description of surface dissipative phenomena based on the linear
response of surface fluxes driven by conjugate surface forces. In this
framework we discuss different transport scenarios including self-propulsion
via surface slip that is induced by active processes on the particle surface.
We clarify the nature of force balances in such situations.Comment: 22 pages, 1 figur
Development of a microfluidic unit for sequencing fluid samples for composition analysis
A microfluidic sample-sequencing unit was developed as a part of a high-throughput catalyst screening facility. It may find applications wherever a fluid is to be selected
for analysis from any one of several sources, such as microreactors operating in parallel. The novel feature is that the key components are fluidic valves having no moving parts and operating at very low sample flow Reynolds numbers, typically below 100. The inertial
effects utilized in conventional no-moving-part fluidics are nearly absent; instead, the flows are pressure-driven. Switching between input channels is by high-Reynolds-number control flows, the jet pumping effect of which simultaneously cleans the downstream cavities to prevent crosscontamination between the samples. In the configuration discussed here, the integrated circuit
containing an array of 16 valves is etched into an 84mm diameter stainless steel foil. This is clamped into a massive assembly containing 16 mini-reactors operated at up to 400C and 4 MPa. This paper describes the design basis and experience with prototypes. Results of CFD
analysis, with scrutiny of some discrepancies when compared with flow visualization, is included
On the absence of appreciable half-life changes in alpha emitters cooled in metals to 1 Kelvin and below
The recent suggestion that dramatic changes may occur in the lifetime of
alpha and beta decay when the activity, in a pure metal host, is cooled to a
few Kelvin, is examined in the light of published low temperature nuclear
orientation (LTNO) experiments, with emphasis here on alpha decay. In LTNO
observations are made of the anisotropy of radioactive emissions with respect
to an axis of orientation. Correction of data for decay of metallic samples
held at temperatures at and below 1 Kelvin for periods of days and longer has
been a routine element of LTNO experiments for many years. No evidence for any
change of half life on cooling, with an upper level of less than 1%, has been
found, in striking contrast to the predicted changes, for alpha decay, of
several orders of magnitude. The proposal that such dramatic changes might
alleviate problems of disposal of long-lived radioactive waste is shown to be
unrealistic.Comment: 27 pages, 12 figures, accepted for publication in Nucl.Phys.A.
Revised version, including quantitative analysis of the sensitivity of
nuclear orientation experiments, discussed in this work, to changes of
alpha-decay lifetimes in metals at low temperatures. Conclusions remain
unchange
Nucleon-Meson Coupling Constants and Form Factors in the Quark Model
We demonstrate the calculation of the coupling constants and form factors
required by effective hadron lagrangians using the quark model. These relations
follow from equating expressions for strong transition amplitudes in the two
approaches. As examples we derive the NNm nucleon-meson coupling constants and
form factors for m = pi, eta, eta', sigma, a_0, omega and rho, using harmonic
oscillator quark model meson and baryon wavefunctions and the 3P0 decay model;
this is a first step towards deriving a quark-based model of the NN force at
all separations. This technique should be useful in the application of
effective lagrangians to processes in which the lack of data precludes the
direct determination of coupling constants and form factors from experiment.Comment: 7 pages, 4 figures, 1 tabl
Neutron stars from crust to core within the Quark-meson coupling model
Recent years continue to be an exciting time for the neutron star physics, providing many new observations and insights to these natural ‘laboratories’ of cold dense matter. To describe them, there are many models on the market but still none that would reproduce all observed and experimental data. The quark-meson coupling model stands out with its natural inclusion of hyperons as dense matter building blocks, and fewer parameters necessary to obtain the nuclear matter equation of state. The latest advances of the QMC model and its application to the neutron star physics will be presented, within which we build the neutron star’s outer crust from finite nuclei up to the neutron drip line. The appearance of different elements and their position in the crust of a neutron star is explored and compared to the predictions of various models, giving the same quality of the results for the QMC model as for the models when the nucleon structure is not taken into account.S. Antić, J.R. Stone, and A.W. Thoma
The dynamics of opinion in hierarchical organizations
We study the mutual influence of authority and persuasion in the flow of
opinion. Many social organizations are characterized by a hierarchical
structure where the propagation of opinion is asymmetric. In the normal flow of
opinion formation a high-rank agent uses its authority (or its persuasion when
necessary) to impose its opinion on others. However, agents with no authority
may only use the force of its persuasion to propagate their opinions. In this
contribution we describe a simple model with no social mobility, where each
agent belongs to a class in the hierarchy and has also a persuasion capability.
The model is studied numerically for a three levels case, and analytically
within a mean field approximation, with a very good agreement between the two
approaches. The stratum where the dominant opinion arises from is strongly
dependent on the percentage of agents in each hierarchy level, and we obtain a
phase diagram identifying the relative frequency of prevailing opinions. We
also find that the time evolution of the conflicting opinions polarizes after a
short transient.Comment: 6 pages, 5 figures, submitted to Phys. Rev.
Provenance analysis of Paleozoic strata in the Falkland/Malvinas Islands: implications for paleogeography and Gondwanan reconstructions
New U-Pb geochronological, Hf isotopic, heavy mineral, and sandstone petrographic results for Paleozoic clastic deposits of the Falkland/Malvinas Islands help address renewed debates on the plate tectonic history, regional paleogeography, and basin evolution of this geologic enigma prior to Mesozoic breakup of Gondwana. The Falkland/Malvinas Islands have been considered either an autochthonous part of the South American continent or part of an independent microplate displaced from the southeastern corner of Africa. We report detrital zircon U-Pb results (n = 1306 LA-ICPMS ages) for 11 sandstone samples from the Silurian-Devonian West Falkland Group (N = 7 samples, n = 837 grains) and Carboniferous-Permian Lafonia Group (N = 4 samples, n = 469 grains). Detrital zircon age distributions for the West Falkland Group point to consistent contributions from Neoproterozoic-Cambrian (650–520 Ma) and Mesoproterozoic (1100–1000 Ma) sources. Heavy mineral assemblages and sandstone petrographic data from these samples indicate significant input from recycled sediments. A potential shift in sediment sources during deposition of the Lafonia Group is indicated by the appearance of late Paleozoic (350–250 Ma) and Proterozoic (2000–1200 Ma) age populations, decreased proportions of stable heavy minerals, and a shift to juvenile Hf values for < 300 Ma zircons. The provenance change can be attributed to the onset of subduction-related arc magmatism and potential regional shortening and crustal thickening in southwestern Gondwana during the Permian transition of a passive margin into an active, retro-arc foreland basin. The detrital zircon age distributions identified here reflect potential source regions in southern Africa and/or the Transantarctic Mountains in Antarctica. These results are most readily accommodated within a Gondwana reconstruction that includes the Falkland/Malvinas Islands as a rotated microplate originating on the eastern side of southern Africa as part of the Gondwanide fold-thrust belt spanning from the Ventania region of Argentina through the Cape region of South Africa and into the Ellsworth and Pensacola mountains of Antarctica
Isovector part of nuclear energy density functional from chiral two- and three-nucleon forces
A recent calculation of the nuclear energy density functional from chiral
two- and three-nucleon forces is extended to the isovector terms pertaining to
different proton and neutron densities. An improved density-matrix expansion is
adapted to the situation of small isospin-asymmetries and used to calculate in
the Hartree-Fock approximation the density-dependent strength functions
associated with the isovector terms. The two-body interaction comprises of
long-range multi-pion exchange contributions and a set of contact terms
contributing up to fourth power in momenta. In addition, the leading order
chiral three-nucleon interaction is employed with its parameters fixed in
computations of nuclear few-body systems. With this input one finds for the
asymmetry energy of nuclear matter the value MeV,
compatible with existing semi-empirical determinations. The strength functions
of the isovector surface and spin-orbit coupling terms come out much smaller
than those of the analogous isoscalar coupling terms and in the relevant
density range one finds agreement with phenomenological Skyrme forces. The
specific isospin- and density-dependences arising from the chiral two- and
three-nucleon interactions can be explored and tested in neutron-rich systems.Comment: 14 pages, 7 figures, to be published in European Physical Journal
A Multiclassifier Approach for Drill Wear Prediction
Classification methods have been widely used during last years in order to predict patterns and trends of interest in data. In present paper, a multiclassifier approach that combines the output of some of the most popular data mining algorithms is shown. The approach is based on voting criteria, by estimating the confidence distributions of each algorithm individually and combining them according to three different methods: confidence voting, weighted voting and majority voting. To illustrate its applicability in a real problem, the drill wear detection in machine-tool sector is addressed. In this study, the accuracy obtained by each isolated classifier is compared with the performance of the multiclassifier when characterizing the patterns of interest involved in the drilling process and predicting the drill wear. Experimental results show that, in general, false positives obtained by the classifiers can be slightly reduced by using the multiclassifier approach
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