7,539 research outputs found
Performance of transducers with segmented piezoelectric stacks using materials with high electromechanical coupling coefficient
Underwater acoustic transducers often include a stack of thickness polarized
piezoelectric material pieces of alternating polarity interspersed with
electrodes, bonded together and electrically connected in parallel. The stack
is normally much shorter than a quarter wavelength at the fundamental resonance
frequency, so that the mechanical behavior of the transducer is not affected by
the segmentation. When the transducer bandwidth is less than a half octave, as
has conventionally been the case, stack segmentation has no significant effect
on the mechanical behavior of the device. However, when a high coupling
coefficient material such as PMN-PT is used to achieve a wider bandwidth, the
difference between a segmented stack and a similar piezoelectric section with
electrodes only at the two ends can be significant. This paper investigates the
effects of stack segmentation on the performance of wideband underwater
acoustic transducers, particularly tonpilz transducer elements. Included is
discussion of transducer designs using single crystal piezoelectric material
with high coupling coefficient compared with more traditional PZT ceramics.Comment: 26 pages including 14 figures, one table and one appendi
Conceptual design for the Space Station Freedom fluid physics/dynamics facility
A study team at NASA's Lewis Research Center has been working on a definition study and conceptual design for a fluid physics and dynamics science facility that will be located in the Space Station Freedom's baseline U.S. Laboratory module. This modular, user-friendly facility, called the Fluid Physics/Dynamics Facility, will be available for use by industry, academic, and government research communities in the late 1990's. The Facility will support research experiments dealing with the study of fluid physics and dynamics phenomena. Because of the lack of gravity-induced convection, research into the mechanisms of fluids in the absence of gravity will help to provide a better understanding of the fundamentals of fluid processes. This document has been prepared as a final version of the handout for reviewers at the Fluid Physics/Dynamics Facility Assessment Workshop held at Lewis on January 24 and 25, 1990. It covers the background, current status, and future activities of the Lewis Project Study Team effort. It is a revised and updated version of a document entitled 'Status Report on the Conceptual Design for the Space Station Fluid Physics/Dynamics Facility', dated January 1990
Trapped-ion quantum error-correcting protocols using only global operations
Quantum error-correcting codes are many-body entangled states that are
prepared and measured using complex sequences of entangling operations. Each
element of such an entangling sequence introduces noise to delicate quantum
information during the encoding or reading out of the code. It is important
therefore to find efficient entangling protocols to avoid the loss of
information. Here we propose an experiment that uses only global entangling
operations to encode an arbitrary logical qubit to either the five-qubit
repetition code or the five-qubit code, with a six-ion Coulomb crystal
architecture in a Penning trap. We show that the use of global operations
enables us to prepare and read out these codes using only six and ten global
entangling pulses, respectively. The proposed experiment also allows the
acquisition of syndrome information during readout. We provide a noise analysis
for the presented protocols, estimating that we can achieve a six-fold
improvement in coherence time with noise as high as on each
entangling operation.Comment: 7 pages, 4 figures, published version, comments are welcom
Accretion discs models with the "beta"-viscosity prescription derived from laboratory experiments
We examine under which conditions one may apply, to steady state keplerian
accretion discs, the "beta"-viscosity prescription which has been derived from
rotating shear flow experiments (Richard & Zahn 1999). Using a vertically
averaged model, we show that this law may be suitable for all three families of
known systems: in young stellar objects, evolved binary stars and Active
Galactic Nuclei discs (except in their outer gas pressure dominated regions
where turbulence becomes hypersonic). According to the standard criterion for
viscous stability, "beta"-discs are always stable throughout. Using realistic
opacities and equation of state, we demonstrate that these discs are thermally
unstable in the temperature domain where hydrogen recombines, when they are
optically thick, and this could lead to limit cycle behavior. Radiation
pressure dominated regions are thermally stable, in contrast with
"alpha"-discs. This results in a fully stable solution for the innermost parts
of AGN discs.Comment: 8 pages, PostScript. accepted in Astron. & Astrophy
Probing the 5th Dimension with the QCD String
A salient feature of String/Gauge duality is an extra 5th dimension. Here we
study the effect of confining deformations of AdS5 and compute the spectrum of
a string stretched between infinitely massive quarks and compare it with the
quantum states of the QCD flux as determined by Kuti, Juge and Morningstar in
lattice simulations. In the long flux tube limit the AdS string probes the
metric near the IR cutoff of the 5th dimension with a spectrum approximated by
a Nambu-Goto string in 4-d flat space, whereas at short distance the string
moves to the UV region with a discrete spectrum for pure AdS5. We also review
earlier results on glueballs states and the cross-over between hard and soft
diffractive scattering that support this picture.Comment: 12 pages, 4 figures, invited talk by Brower and Tan at the Eighth
Workshop on Non-Perturbative Quantum Chromodynamcis, June (2004
Virtual images and billiards
Students in introductory physics courses struggle to understand virtual image
formation by a plane mirror and the proper construction of ray diagrams. This
difficulty, if not sufficiently addressed, results in further problems
throughout the study of geometric optics. Specifically, students fail to apply
proper graphical representation of light rays during investigations of the
formation of real images by converging lenses and concave mirrors. We present a
guided-inquiry activity based on the research-verified Physics by Inquiry text
that incorporates a small and inexpensive billiards table, with billiard balls
acting as "light". In this way, we approach the abstract concept of virtual
images by relation to the concrete concept of physical reflection.Comment: 6 pages, 2 figures, submitted to Physics Educatio
LDA+DMFT Approach to Magnetocrystalline Anisotropy of Strong Magnets
The new challenges posed by the need of finding strong rare-earth free
magnets demand methods that can predict magnetization and magnetocrystalline
anisotropy energy (MAE). We argue that correlated electron effects, which are
normally underestimated in band structure calculations, play a crucial role in
the development of the orbital component of the magnetic moments. Because
magnetic anisotropy arises from this orbital component, the ability to include
correlation effects has profound consequences on our predictive power of the
MAE of strong magnets. Here we show that incorporating the local effects of
electronic correlations with dynamical mean-field theory provides reliable
estimates of the orbital moment, the mass enhancement and the MAE of YCo5.Comment: 7 pages, 4 figures, published versio
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