Mobility of Dislocations in Aluminum

The velocities of individual dislocations of edge and mixed types in pure aluminum single crystals were determined as a function of applied‐resolved shear stress and temperature. The dislocation velocities were determined from measurements of the displacements of individual dislocations produced by stress pulses of known duration. The Berg‐Barrett x‐ray technique was employed to observe the dislocations, and stress pulses of 15 to 108 μsec duration were applied by propagating torsional waves along the axes of [111]‐oriented cylindrical crystals. Resolved shear stresses up to 16×10^6 dynes∕cm^2 were applied at temperatures ranging from −150° to +70°C, and dislocation velocities were found to vary from 10 to 2800 cm∕sec over these ranges of stress and temperature. The experimental conditions were such that the dislocation velocities were not significantly influenced by impurities, dislocation curvature, dislocation‐dislocation interactions, or long‐range internal stress fields in the crystals. The velocity of dislocations is found to be linearly proportional to the applied‐resolved shear stress, and to decrease with increasing temperature. Qualitative comparison of these results with existing theories leads to the conclusion that the mobility of individual dislocations in pure aluminum is governed by dislocation‐phonon interactions. The phonon‐viscosity theory of dislocation mobility can be brought into agreement with the experimental results by reasonable choices of the values of certain constants appearing in the theory

Strain-Modified RKKY Interaction in Carbon Nanotubes

For low-dimensional metallic structures, such as nanotubes, the exchange coupling between localized magnetic dopants is predicted to decay slowly with separation. The long-range character of this interaction plays a significant role in determining the magnetic order of the system. It has previously been shown that the interaction range depends on the conformation of the magnetic dopants in both graphene and nanotubes. Here we examine the RKKY interaction in carbon nanotubes in the presence of uniaxial strain for a range of different impurity configurations. We show that strain is capable of amplifying or attenuating the RKKY interaction, significantly increasing certain interaction ranges, and acting as a switch: effectively turning on or off the interaction. We argue that uniaxial strain can be employed to significantly manipulate magnetic interactions in carbon nanotubes, allowing an interplay between mechanical and magnetic properties in future spintronic devices. We also examine the dimensional relationship between graphene and nanotubes with regards to the decay rate of the RKKY interaction.Comment: 7 pages, 6 figures, submitte

Implications of surface noise for the motional coherence of trapped ions

Electric noise from metallic surfaces is a major obstacle towards quantum applications with trapped ions due to motional heating of the ions. Here, we discuss how the same noise source can also lead to pure dephasing of motional quantum states. The mechanism is particularly relevant at small ion-surface distances, thus imposing a new constraint on trap miniaturization. By means of a free induction decay experiment, we measure the dephasing time of the motion of a single ion trapped 50~$\mu$m above a Cu-Al surface. From the dephasing times we extract the integrated noise below the secular frequency of the ion. We find that none of the most commonly discussed surface noise models for ion traps describes both, the observed heating as well as the measured dephasing, satisfactorily. Thus, our measurements provide a benchmark for future models for the electric noise emitted by metallic surfaces.Comment: (5 pages, 4 figures

Strain-induced modulation of magnetic interactions in graphene

The ease with which the physical properties of graphene can be tuned suggests a wide range of possible applications. Recently, strain engineering of these properties has been of particular interest. Possible spintronic applications of magnetically doped graphene systems have motivated recent theoretical investigations of the so-called Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between localized moments in graphene. In this work a combination of analytic and numerical techniques are used to examine the effects of uniaxial strain on such an interaction. A range of interesting features are uncovered depending on the separation and strain directions. Amplification, suppression, and oscillatory behavior are reported as a function of the strain and mathematically transparent expressions predicting these features are derived. Since a wide range of effects, including overall moment formation and magnetotransport response, are underpinned by such interactions we predict that the ability to manipulate the coupling by applying strain may lead to interesting spintronic applications.Comment: 6 pages, 3 figure

NREL Gearbox Reliability Collaborative Experimental Data Overview and Analysis: Preprint

Most turbines in the market today follow a modular configuration comprised of a main shaft, gearbox, high speed shaft, and generator. The gearbox has the important task of increasing the slow rotor speeds to meet the electromechanical requirements of the electromechanical. These gearboxes are commonly composed of a planetary stage and several parallel shaft stages. The planetary, or epicyclical, design of the gearbox is a feature of the design that has many advantages compared to the traditional parallel shaft arrangement. Among these are that higher gear ratios can be achieved in a single stage, they are capable to carrying higher loads, and they require less space than the traditional parallel shaft arrangement. For this reason, planetary gearboxes they are commonly used in the first stage of the wind turbine gearboxes. However, planetary stages are more complex than the typical parallel shaft arrangement, and can be affected by deflection in the planet carrier, annulus deformations and bearing clearances. Unanticipated levels of these motions can reduce their life expectancy. This paper gives a brief overview of a subset of the experimental efforts, data, and analysis of the GRC project focusing on the planet carrier deformation

Scenarios of domain pattern formation in a reaction-diffusion system

We performed an extensive numerical study of a two-dimensional reaction-diffusion system of the activator-inhibitor type in which domain patterns can form. We showed that both multidomain and labyrinthine patterns may form spontaneously as a result of Turing instability. In the stable homogeneous system with the fast inhibitor one can excite both localized and extended patterns by applying a localized stimulus. Depending on the parameters and the excitation level of the system stripes, spots, wriggled stripes, or labyrinthine patterns form. The labyrinthine patterns may be both connected and disconnected. In the the stable homogeneous system with the slow inhibitor one can excite self-replicating spots, breathing patterns, autowaves and turbulence. The parameter regions in which different types of patterns are realized are explained on the basis of the asymptotic theory of instabilities for patterns with sharp interfaces developed by us in Phys. Rev. E. 53, 3101 (1996). The dynamics of the patterns observed in our simulations is very similar to that of the patterns forming in the ferrocyanide-iodate-sulfite reaction.Comment: 15 pages (REVTeX), 15 figures (postscript and gif), submitted to Phys. Rev.