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

Fault-tolerant Quantum Communication with Minimal Physical Requirements

We describe a novel protocol for a quantum repeater which enables long distance quantum communication through realistic, lossy photonic channels. Contrary to previous proposals, our protocol incorporates active purification of arbitrary errors at each step of the protocol using only two qubits at each repeater station. Because of these minimal physical requirements, the present protocol can be realized in simple physical systems such as solid-state single photon emitters. As an example, we show how nitrogen vacancy color centers in diamond can be used to implement the protocol, using the nuclear and electronic spin to form the two qubits.Comment: 4 pages, 3 figures. V2: Minor modifications. V3: Major changes in the presentation and new titl

Teachers Engaging in Action Research to Increase Learner Agency

This study is designed to explore the perceptions of teachers in a Northwest Arkansas urban elementary school context as they engage in design-based action research to increase learner agency. It is based on the Carnegie Project on Education Doctorate (CPED) framework that includes identifying a problem of practice and engaging in research to address the problem. A problem of practice was identified in this school setting by an incoming school principal following a principal that had been in the school for many years. The new principal, also the researcher in this study, recognized that there was a low level of learner agency among both students and teachers. Students were not tracking their own learning or setting goals. Teachers had a low level of teacher efficacy and were frustrated with the low level of student achievement and the low level of student motivation. To address this problem of practice, a design-based action research study was developed by the new principal and the new leadership team. At the time that research data was collected for this qualitative study, the teachers had been engaging in action research for eight months. This study will include interview data collected from all fifteen teachers that were willing to participate including all first through six grade teachers, a self-contained special education teacher, and two interventionists that had engaged in the design-based action research to increase learner agency. This study helped determine the next steps that should be taken in the cycle of inquiry in this school and serves as a resource for educational leaders that wish to address similar problems of practice

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

Magnetic domain wall propagation in a submicron spin-valve stripe: influence of the pinned layer

The propagation of a domain wall in a submicron ferromagnetic spin-valve stripe is investigated using giant magnetoresistance. A notch in the stripe efficiently traps an injected wall stopping the domain propagation. The authors show that the magnetic field at which the wall is depinned displays a stochastic nature. Moreover, the depinning statistics are significantly different for head to head and tail-to-tail domain walls. This is attributed to the dipolar field generated in the vicinity of the notch by the pinned layer of the spin-valve

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

It\u27s not easy being a reef: Impacts of competition and corallivory in the Florida Keys

As coral cover in the Florida Keys continues to decline, understanding the factors driving this trend has become a priority for researchers. Previous studies have shown that macroalgae directly competes with corals for nutrients and space, and that parrotfish herbivory can reduce macroalgae density. However, parrotfish also graze on corals so the net impact of parrotfish on corals is unclear. To estimate the net impact of parrotfish abundance on macroalgae and corals, we measured substrate composition and parrotfish density on 14 reefs in the middle Florida Keys. Parrotfish abundance was not significantly related to macroalgal cover nor was macroalgal cover significantly related to coral cover. Parrotfish density, however, was sometimes positively related to coral cover. This suggests that the net impact of parrotfish grazing may have a positive impact on corals in the Florida Keys

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