Magnetization structure of a Bloch point singularity

Switching of magnetic vortex cores involves a topological transition characterized by the presence of a magnetization singularity, a point where the magnetization vanishes (Bloch point). We analytically derive the shape of the Bloch point that is an extremum of the free energy with exchange, dipole and the Landau terms for the determination of the local value of the magnetization modulus.Comment: 4 pages, 2 figure

Current-induced magnetic vortex core switching in a Permalloy nanodisk

We report on the switching of a magnetic vortex core in a sub-micron Permalloy disk, induced by a short current pulse applied in the film plane. Micromagnetic simulations including the adiabatic and non-adiabatic spin-torque terms are used to investigate the current-driven magnetization dynamics. We predict that a core reversal can be triggered by current bursts a tenth of a nanosecond long. The vortex core reversal process is found to be the same as when an external field pulse is applied. The control of a vortex core's orientation using current pulses introduces the technologically relevant possibility to address individual nanomagnets within dense arrays.Comment: 3 pages, 3 figure

Stereo electro-optical tracker study for the measurement of model deformations at the National Transonic Facility

The effects of model vibration, camera and window nonlinearities, and aerodynamic disturbances in the optical path on the measurement of target position is examined. Window distortion, temperature and pressure changes, laminar and turbulent boundary layers, shock waves, target intensity and, target vibration are also studied. A general computer program was developed to trace optical rays through these disturbances. The use of a charge injection device camera as an alternative to the image dissector camera was examined

Research in the development of an improved multiplier phototube

Performance and response characteristics of smoothing, image intensifier dissector for low light level astronomy and optical detectio

Inapproximability of the Standard Pebble Game and Hard to Pebble Graphs

Pebble games are single-player games on DAGs involving placing and moving pebbles on nodes of the graph according to a certain set of rules. The goal is to pebble a set of target nodes using a minimum number of pebbles. In this paper, we present a possibly simpler proof of the result in [CLNV15] and strengthen the result to show that it is PSPACE-hard to determine the minimum number of pebbles to an additive $n^{1/3-\epsilon}$ term for all $\epsilon > 0$, which improves upon the currently known additive constant hardness of approximation [CLNV15] in the standard pebble game. We also introduce a family of explicit, constant indegree graphs with $n$ nodes where there exists a graph in the family such that using constant $k$ pebbles requires $\Omega(n^k)$ moves to pebble in both the standard and black-white pebble games. This independently answers an open question summarized in [Nor15] of whether a family of DAGs exists that meets the upper bound of $O(n^k)$ moves using constant $k$ pebbles with a different construction than that presented in [AdRNV17].Comment: Preliminary version in WADS 201

Calculations of three-dimensional magnetic normal modes in mesoscopic permalloy prisms with vortex structure

Static flux-closure structures in three-dimensional (3D) mesoscopic ferromagnets are known to differ quite significantly from their 2D counterparts. How these differences reflect in the dynamic properties of the magnetization is, to date, an open question. Micromagnetic simulations are employed to study the normal modes of magnetic oscillations in thick (60-80 nm) rectangular Permalloy prisms with 3D Landau-type flux-closure domain structure. Various magnetic normal modes are excited by a short field pulse and extracted using methods based on Fourier analysis. In particular, well-defined modes in the range of a few GHz are identified as oscillations of vortices, domain walls, and as excitations localized in the corners. The asymmetric Bloch wall in the center of the 3D Landau structure wall is a genuinely three-dimensional feature and thus gives rise to effects which were not reported in previous studies on 2D systems. It is argued that experimental evidence of these findings can be obtained

Electronic structure and dynamics of optically excited single-wall carbon nanotubes

We have studied the electronic structure and charge-carrier dynamics of individual single-wall carbon nanotubes (SWNTs) and nanotube ropes using optical and electron-spectroscopic techniques. The electronic structure of semiconducting SWNTs in the band-gap region is analyzed using near-infrared absorption spectroscopy. A semi-empirical expression for $E_{11}^{\rm S}$ transition energies, based on tight-binding calculations is found to give striking agreement with experimental data. Time-resolved PL from dispersed SWNT-micelles shows a decay with a time constant of about 15 ps. Using time-resolved photoemission we also find that the electron-phonon ({\it e-ph}) coupling in metallic tubes is characterized by a very small {\it e-ph} mass-enhancement of 0.0004. Ultrafast electron-electron scattering of photo-excited carriers in nanotube ropes is finally found to lead to internal thermalization of the electronic system within about 200 fs.Comment: 10 pages, 10 figures, submitted to Applied Physics

The medial malleolar network: A constant vascular base of the distally based saphenous neurocutaneous island flap

Summary: Based on 30 fresh cadaver dissections a detailed anatomic study of the medial malleolar network is presented with particular attention to the anastomoses between the latter and the vascular axis that follows the saphenous nerve. The medial malleolar network is formed by the anterior medial malleolar artery, branches from the medial tarsal arteries, the posterior medial malleolar artery and branches from the medial plantar artery. A distinct anterior medial malleolar artery and posterior medial malleolar artery could be identified in 80 and 20%, respectively, as well as constant additional small branches arising from the anterior tibial or posterior tibial artery. A constant anastomosis was found between the arcade formed by the medial tarsal arteries and the medial plantar a. in 60%, and the medial branch of the medial plantar artery in 40%, respectively. This anastomosis always gave rise to branches to the medial malleolar network. In the perimalleolar area and with regard to the great saphenous v. a larger anterior and a smaller posterior branch of the saphenous nerve was found in 100 and 90%, respectively. In all dissections, for both branches of the saphenous nerve two to four small, but distinct anastomoses between the medial malleolar network and the perineural vascular axis were identified. These constant anastomoses represent a new and reliable vascular base for the distally-based saphenous neurocutaneous island flap. Thus, the pivotal point of the flap can be chosen in the area of the medial malleolus without respecting the most distal septocutaneous anastomosis between the perineural vascular axis and the posterior tibial artery. Additionally, an illustrative clinical case is presente