Spin-wave interference in three-dimensional rolled-up ferromagnetic microtubes

We have investigated spin-wave excitations in rolled-up Permalloy microtubes using microwave absorption spectroscopy. We find a series of quantized azimuthal modes which arise from the constructive interference of Damon-Eshbach type spin waves propagating around the circumference of the microtubes, forming a spin-wave resonator. The mode spectrum can be tailored by the tube's radius and number of rolled-up layers.Comment: 12 pages, 4 figure

Enhanced Transmission in Rolled-up Hyperlenses utilizing Fabry-Pe\'rot Resonances

We experimentally demonstrate that the transmission though rolled-up metal/semiconductor hyperlenses can be enhanced at desired frequencies utilizing Fabry-P\'erot resonances. By means of finite difference time domain simulations we prove that hyperlensing occurs at frequencies of high transmission.Comment: 3 pages, 3 figure

A Comparison of Low-Pressure and Supercharged Operation of Polymer Electrolyte Membrane Fuel Cell Systems for Aircraft Applications

Multifunctional fuel cell systems are competitive solutions aboard future generations of civil aircraft concerning energy consumption, environmental issues, and safety reasons. The present study compares low-pressure and supercharged operation of polymer electrolyte membrane fuel cells with respect to performance and efficiency criteria. This is motivated by the challenge of pressure-dependent fuel cell operation aboard aircraft with cabin pressure varying with operating altitude. Experimental investigations of low-pressure fuel cell operation use model-based design of experiments and are complemented by numerical investigations concerning supercharged fuel cell operation. It is demonstrated that a low-pressure operation is feasible with the fuel cell device under test, but that its range of stable operation changes between both operating modes. Including an external compressor, it can be shown that the power demand for supercharging the fuel cell is about the same as the loss in power output of the fuel cell due to low-pressure operation. Furthermore, the supercharged fuel cell operation appears to be more sensitive with respect to variations in the considered independent operating parameters load requirement, cathode stoichiometric ratio, and cooling temperature. The results indicate that a pressure-dependent self-humidification control might be able to exploit the potential of low-pressure fuel cell operation for aircraft applications to the best advantage

CHARACTERISTICS OF PEMFC OPERATION IN AMBIENT AND LOW PRESSURE ENVIRONMENT CONSIDERING THE FUEL CELL HUMIDIFICATION

This paper summarizes experimental results of an air-fed polymer electrolyte membrane fuel cell system HyPM XR 12 (Hydrogenics Corp.) considering fuel cell temperature, stoichiometry, and load requirement variations at ambient and low-pressure operation. The experimental work realized at a low-pressure test facility designed and assembled by the German Aerospace Center, Institute of Engineering Thermodynamics is based on an experimental design. The experimental results confirm reduced fields of fuel cell operation as well as a decreased gross stack performance and efficiency at low operating pressures (950 mbar C p C 600 mbar) for the defined fuel cell temperature, stoichiometry, and load requirement. In addition, indexes of the operating parameters are introduced, characterizing the fuel cell operation with regard to the gross stack performance and efficiency at ambient and low-pressure levels. The discussion of the results considers analyses of fuel cell humidification

Gain in Three-Dimensional Metamaterials utilizing Semiconductor Quantum Structures

We demonstrate gain in a three-dimensional metal/semiconductor metamaterial by the integration of optically active semiconductor quantum structures. The rolling-up of a metallic structure on top of strained semiconductor layers containing a quantum well allows us to achieve a three-dimensional superlattice consisting of alternating layers of lossy metallic and amplifying gain material. We show that the transmission through the superlattice can be enhanced by exciting the quantum well optically under both pulsed or continuous wave excitation. This points out that our structures can be used as a starting point for arbitrary three-dimensional metamaterials including gain

Testing Is More Desirable When It Is Adaptive and Still Desirable When Compared to Note-Taking

Testing is a well-established desirable difficulty. Yet there are still some open issues regarding the benefits of testing that need to be addressed. First, the possibility to increase its benefits by adapting the sequence of test questions to the learners’ level of knowledge has scarcely been explored. In view of theories that emphasize the benefits of adapting learning tasks to learner knowledge, it is reasonable to assume that the common practice of providing all learners with the same test questions is not optimal. Second, it is an open question as to whether the testing effect prevails if stronger control conditions than the typical restudy condition are used. We addressed these issues in an experiment with N = 200 university students who were randomly assigned to (a) adaptive testing, (b) non-adaptive testing, or note-taking (c) without or (d) with focus guidance. In an initial study phase, all participants watched an e-lecture. Afterward, they processed its content according to their assigned conditions. One week later, all learners took a posttest. As main results, we found that adaptive testing yielded higher learning outcomes than non-adaptive testing. These benefits were mediated by the adaptive learners’ higher testing performance and lower perceived cognitive demand during testing. Furthermore, we found that both testing groups outperformed the note-taking groups. Jointly, our results show that the benefits of testing can be enhanced by adapting the sequence of test questions to learners’ knowledge and that testing can be more effective than note-taking

Confinement effects on optical phonons in spherical, rod-, and tetrapod-shaped nanocrystals detected by Raman spectroscopy

Spherical, rod- and tetrapod shaped CdSe nanocrystals are investigated by Raman spectroscopy and the longitudinal-optical and surface optical phonons are observed. We find that the position of the longitudinal-optical phonon slightly red-shifts with decreasing diameter, whereas the position of the surface optical phonon depends significantly on diameter and length of the rods or the tetrapod arms

Impact of different S-cerevisiae yeast strains on gluten-free dough and bread quality parameters

Yeasts have been used for centuries for the leavening of bread. The main emphasis on the selection of yeast strains has been in relation to wheat products. This study is the first evaluation of different yeasts coming from the baking and brewing industry in a gluten-free system. Five different yeast strains (US-05, WB-06, T-58, S-23 and baker’s yeast) of the species Saccharomyces cerevisiae were evaluated for their suitability to leaven gluten-free dough. A wide range of dough quality characteristics such as the time and temperature-dependent rising behaviour, the chemical composition of the dough and the pH were determined. In addition to this, the bread quality attributes like, volume, texture, structure, aroma and flavour were evaluated. Obtained results indicated different activity levels between the selected yeast strains. Doughs prepared with US-05 showed a slower dough rise during proofing and a decreased height, in comparison to the baker’s yeast control. The application of WB-06 and T-58, however, resulted in a faster dough rise and increased dough height with greater gas cells (p < 0.05). These observations were also found in the baked breads, where these two yeasts reached a higher specific volume and a softer breadcrumb than the baker’s yeast bread (p < 0.05). Statistical analysis revealed strong correlations (p < 0.05) between activity level, dough properties and bread properties. Results obtained showed that the selected yeast strains reached different level of activity due to diverse preferences in temperature, time and sugars. Yeast strains which originated from the brewing industry were found to be suitable for gluten-free bread making