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