6,052 research outputs found
Ultrasonic transducer with Gaussian radial pressure distribution
An ultrasonic transducer that produces an output that is a symmetrical function comprises a piezoelectric crystal with several concentric ring electrodes on one side of the crystal. A resistor network applies different amplitudes of an ac source to each of the several electrodes. A plot of the different amplitudes from the outermost electrode to the innermost electrode is the first half of a Gaussian function. Consequently, the output of the crystal from the side opposite the electrodes has a Gaussian profile
Transformation, partitioning and flow–deposit interactions during the run-out of megaflows
Funded by BG Brasil E&P LtdaPeer reviewedPostprin
Erbium-doped fiber amplifier elements for structural analysis sensors
The use of erbium-doped fiber amplifiers (EDFA's) in optical fiber sensor systems for structural analysis is described. EDFA's were developed for primary applications as periodic regenerator amplifiers in long-distance fiber-based communication systems. Their in-line amplification performance also makes them attractive for optical fiber sensor systems which require long effective lengths or the synthesis of special length-dependent signal processing functions. Sensor geometries incorporating EDFA's in recirculating and multiple loop sensors are discussed. Noise and polarization birefringence are also considered, and the experimental development of system components is discussed
Superconductivity and Cobalt Oxidation State in Metastable Na(x)CoO(2-delta)*yH2O (x ~ 1/3; y ~ 4x)
We report the synthesis and superconducting properties of a metastable form
of the known superconductor NaxCoO2*yH2O (x ~ 1/3, y ~ 4x). Instead of using
the conventional bromine-acetonitrile mixture for sodium deintercalation, we
use an aqueous bromine solution. Using this method, we oxidize the sample to a
point that the sodium cobaltate becomes unstable, leading to formation of other
products if not controlled. This compound has the same structure as the
reported superconductor, yet it exhibits a systematic variation of the
superconducting transition temperature (Tc) as a function of time. Immediately
after synthesis, this compound is not a superconductor, even though it contains
appropriate amounts of sodium and water. The samples become superconducting
with low Tc values after ~ 90 h. Tc continually increases until it reaches a
maximum value (4.5 K) after about 260 h. Then Tc drops drastically, becoming
non-superconducting approximately 100 h later. Corresponding time-dependent
neutron powder diffraction data shows that the changes in superconductivity
exhibited by the metastable cobaltate correspond to slow formation of oxygen
vacancies in the CoO2 layers. In effect, the formation of these defects
continually reduces the cobalt oxidation state causing the sample to evolve
through its superconducting life cycle. Thus, the dome-shaped superconducting
phase diagram is mapped as a function of cobalt oxidation state using a single
sample. The width of this dome based on the formal oxidation state of cobalt is
very narrow - approximately 0.1 valence units wide. Interestingly, the maximum
Tc in NaxCoO2*yH2O occurs when the cobalt oxidation state is near 3.5. Thus, we
speculate that the maximum Tc occurs near the charge ordered insulating state
that correlates with the average cobalt oxidation state of 3.5.Comment: 22 pages, 9 figures, 1 tabl
Black Holes, Branes and Superconformal Symmetry
The main focus of this lecture is on extended objects in adS*S bosonic
backgrounds with unbroken supersymmetry. The backgrounds are argued to be
exact, special consideration are given to the non-maximal supersymmetry case.
The near horizon superspace construction is explained. The superconformal
symmetry appears in the worldvolume actions as the superisometry of the near
horizon superspace, like the superPoincare symmetry of GS superstring and BST
supermembrane in the flat superspace. The issues in gauge fixing of local
kappa-symmetry are reviewed. We describe the features of the gauge-fixed IIB
superstring in adS(5)*S(5) background with RR 5-form. From a truncated boundary
version of it we derive an analytic N=2 off shell harmonic superspace of
Yang-Mills theory. The reality condition of the analytic subspace, which
includes the antipodal map on the sphere, has a simple meaning of the symmetry
of the string action in the curved space. The relevant issues of black holes
and superconformal mechanics are addressed.Comment: 34 pages, Latex. To be published in the Proceedings of a conference
held in Corfu, Greece in September 1998. Improvement in Sec. 3.
Ultrafast dynamics of a magnetic antivortex - Micromagnetic simulations
The antivortex is a fundamental magnetization structure which is the
topological counterpart of the well-known magnetic vortex. We study here the
ultrafast dynamic behavior of an isolated antivortex in a patterned Permalloy
thin-film element. Using micromagnetic simulations we predict that the
antivortex response to an ultrashort external field pulse is characterized by
the production of a new antivortex as well as of a temporary vortex, followed
by an annihilation process. These processes are complementary to the recently
reported response of a vortex and, like for the vortex, lead to the reversal of
the orientation of the antivortex core region. In addition to its fundamental
interest, this dynamic magnetization process could be used for the generation
and propagation of spin waves for novel logical circuits.Comment: 4 pages, 4 figures. To be published in Physical Review B (R
Conformal Symmetry of Supergravities in AdS spaces
We show that the background field method applied to supergravity in adS
space-time provides the path integral for the theory in the bulk with conformal
symmetry associated with the isometry of the adS space. This in turn allows to
establish the rigid conformal invariance of the generating functional for the
supergravity correlators on the boundary.Comment: 14 pages, Late
An increase in under hydrostatic pressure in the superconducting doped topological insulator NbBiSe
We report an unexpected positive hydrostatic pressure derivative of the
superconducting transition temperature in the doped topological insulator \NBS
via SQUID magnetometry in pressures up to 0.6 GPa. This result is contrary
to reports on the homologues \CBS and \SBS where smooth suppression of is
observed. Our results are consistent with recent Ginzburg-Landau theory
predictions of a pressure-induced enhancement of in the nematic
multicomponent state proposed to explain observations of rotational
symmetry breaking in doped BiSe superconductors.Comment: 5 pages, 5 figure
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