2,367 research outputs found
Continuous breakdown of Purcell's scallop theorem with inertia
Purcell's scallop theorem defines the type of motions of a solid body -
reciprocal motions - which cannot propel the body in a viscous fluid with zero
Reynolds number. For example, the flapping of a wing is reciprocal and, as was
recently shown, can lead to directed motion only if its frequency Reynolds
number, Re_f, is above a critical value of order one. Using elementary
examples, we show the existence of oscillatory reciprocal motions which are
effective for all arbitrarily small values of the frequency Reynolds number and
induce net velocities scaling as (Re_f)^\alpha (alpha > 0). This demonstrates a
continuous breakdown of the scallop theorem with inertia.Comment: 6 pages, 1 figur
Investigation into Levels of Concentration, Seasonal Variations, and Source of Pesticide Toxicants in Some Species from Selected Bay Areas
Data presented in this paper indicate chronic pesticide levels present in oyster (Crassostrea virginica) tissue from all Texas bay areas sampled, including Galveston Bay. Random samples of fish and bird tissue show the scope of contamination is much greater than first suspected. These data indicate only the degree and sequence of pollution in specific areas. The need is shown for (1) an intensified investigation of persistent pesticides present in streams entering the areas sampled and (2) the effects of chronic levels of pesticide toxicants on the marine environment
Mortuary Vessels and Comparative Ceramic Analysis: An Example from the Chucalissa Site
This paper summarizes the results of descriptive, analytical, and comparative research conducted on an assemblage of 154 late prehistoric ceramic vessels from the site of Chucalissa
Physical Layer Security for Mimo Wireless Systems
A steadily growing portion of modern communication systems in use today is based on wireless technologies that make use of smaller and more portable electronic devices. As a result, the need to provide a light-weight security strategy for these systems is becoming a more important problem. This thesis focuses on two techniques that belong to an active research area known as Physical Layer Security (PLS). While the underlying techniques of PLS have been known for some time, the potential secrecy benefits of them need further investigation. These potential benefits have generated a rising interest with the development of Multiple Input Multiple Output (MIMO) multi-antenna systems. The first PLS technique considered in this thesis is that of beamforming which is made possible using MIMO. Here a sender can focus the information signal in the direction of the intended receiver while reducing the quality of the signal observed by a potential eavesdropper. In addition to beamforming, the technique of artificial noise (AN) is also investigated. AN requires the sender to generate a random noise signal in addition to the information signal to further degrade an eavesdropper’s ability to detect and decode the information signal being directed to the intended receiver. MATLAB simulations based on these PLS techniques are performed and the results presented
Ceramic Data: Chucalissa Site (40SY1), 1987-1995
This document contains illustrations made in 1988 and 1989 of engraved sherds discovered at the Chucalissa Site
Legal Malpractice in Virginia: Tort or Contract?
A client who attempts to recover from an attorney does so because the client feels that the attorney has acted negligently. The concepts of standard of care, negligence, and damages are usually associated with actions in tort. However, while an examination of applicable Virginia law reveals that concepts usually associated with tort apply to legal malpractice, the presence of elements of negligence does not always equal tort
Suppression of spin-torque in current perpendicular to the plane spin-valves by addition of Dy cap layers
We demonstrate that the addition of Dy capping layers in current
perpendicular to the plane giant magneto-resistive spin-valves can increase the
critical current density beyond which spin-torque induced instabilities are
observed by about a factor of three. Current densities as high as 5e7 A/cm2 are
measured provided that the electron current flows from the free to the
reference layer. While Dy capped samples exhibit nonmagnetic 1/f noise, it is
sufficiently small to be unimportant for read head operation at practical data
rates.Comment: 13 pages (manuscript form), with 5 figures. Submitted for publicatio
Ferromagnetic resonance force microscopy on microscopic cobalt single layer films
We report mechanical detection of ferromagnetic resonance signals from
microscopic Co single layer thin films using a magnetic resonance force
microscope (MRFM). Variations in the magnetic anisotropy field and the
inhomogeneity of were clearly observed in the FMR spectra of microscopic Co
thin films 500 and 1000 angstrom thick and 40 X 200 micron^2 in lateral extent.
This demonstrates the important potential that MRFM detection of FMR holds for
microscopic characterization of spatial distribution of magnetic properties in
magnetic layered materials and devices.Comment: 4 pages, 2 figures, RevTex. To be published in Applied Physics
Letters, October 5, 199
Thermal Effects on the Magnetic Field Dependence of Spin Transfer Induced Magnetization Reversal
We have developed a self-aligned, high-yield process to fabricate CPP
(current perpendicular to the plane) magnetic sensors of sub 100 nm dimensions.
A pinned synthetic antiferromagnet (SAF) is used as the reference layer which
minimizes dipole coupling to the free layer and field induced rotation of the
reference layer. We find that the critical currents for spin transfer induced
magnetization reversal of the free layer vary dramatically with relatively
small changes the in-plane magnetic field, in contrast to theoretical
predictions based on stability analysis of the Gilbert equations of
magnetization dynamics including Slonczewski-type spin-torque terms. The
discrepancy is believed due to thermal fluctuations over the time scale of the
measurements. Once thermal fluctuations are taken into account, we find good
quantitative agreement between our experimental results and numerical
simulations.Comment: 14 pages, 4 figures, Submitted to Appl. Phys. Lett., Comparison of
some of these results with a model described by N. Smith in cond-mat/040648
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