Search Results
Evolution of the charge carrier plasmon in the one-dimensional metal TTF-TCNQ as a function of temperature and momentum
2019, Kovbasa, N., Graf, L., Knupfer, M.
We have investigated the charge carrier plasmon in the quasi one-dimensional metal TTF-TCNQ using electron energy-loss spectroscopy. Our data reveal a negative plasmon dispersion with a slope that is independent of temperature, which is in agreement to predictions from model calculations and previous room temperature data. A plasmon energy shift upon temperature is observed, and we discuss possible contributions to this shift. The spectral width of the plasmon is rather temperature independent, but increases clearly above a momentum value of about 0.3 Ã…-1.
Interaction of a free burning arc with regenerative protective layers
2014, Uhrlandt, D., Gorchakov, S., Brueser, V., Franke, S., Khakpour, A., Lisnyak, M., Methling, R., Schoenemann, T.
The possible use of protective layers made of ceramic powders for walls in thermal plasma applications is studied. A stable free burning arc of currents up to 5 kA between copper- tungsten electrodes is used to analyse the arc interaction with samples coated by mixtures of CaCO3, MgCO3, and Mg(OH)2 with plaster. By means of optical emission spectroscopy the maximum arc temperature and the radiation impact on the surfaces are estimated to be around 15000 K and 20 MWm-2, respectively. Thermographic measurements confirm the efficient protection of substrates by all layer materials. Layers containing CaCO3 lead to the lowest heating of ceramic samples which may be caused by a strong evaporation of the layer material.
Photoelectron holography in strong optical and dc electric fields
2014, Stodolna, A., Huismans, Y., Rouzée, A., Lépine, F., Vrakking, M.J.J.
The application of velocity map imaging for the detection of photoelectrons resulting from atomic or molecular ionization allows the observation of interferometric, and in some cases holographic structures that contain detailed information on the target from which the photoelecrons are extracted. In this contribution we present three recent examples of the use of photoelectron velocity map imaging in experiments where atoms are exposed to strong optical and dc electric fields. We discuss (i) observations of the nodal structure of Stark states of hydrogen measured in a dc electric field, (ii) mid-infrared strong-field ionization of metastable Xe atoms and (iii) the reconstruction of helium electronic wavepackets in an attosecond pump-probe experiment. In each case, the interference between direct and indirect electron pathways, reminiscent of the reference and signal waves in holography, is seen to play an important role.
Influence of the arc plasma parameters on the weld pool profile in TIG welding
2014, Toropchin, A., Frolov, V., Pipa, A.V., Kozakov, R., Uhrlandt, D.
Magneto-hydrodynamic simulations of the arc and fluid simulations of the weld pool can be beneficial in the analysis and further development of arc welding processes and welding machines. However, the appropriate coupling of arc and weld pool simulations needs further improvement. The tungsten inert gas (TIG) welding process is investigated by simulations including the weld pool. Experiments with optical diagnostics are used for the validation. A coupled computational model of the arc and the weld pool is developed using the software ANSYS CFX. The weld pool model considers the forces acting on the motion of the melt inside and on the surface of the pool, such as Marangoni, drag, electromagnetic forces and buoyancy. The experimental work includes analysis of cross-sections of the workpieces, highspeed video images and spectroscopic measurements. Experiments and calculations have been performed for various currents, distances between electrode and workpiece and nozzle diameters. The studies show the significant impact of material properties like surface tension dependence on temperature as well as of the arc structure on the weld pool behaviour and finally the weld seam depth. The experimental weld pool profiles and plasma temperatures are in good agreement with computational results.
Inter-granular effects at high magnetic fields of cuprate and iron chalcogenide superconducting materials
2019, Buchkov, K., Valkovski, M., Gajda, D., Nenkov, K., Nazarova, E.
The weak links effects are one of the main challenges for effective power applications of high temperature superconducting materials. Studies of these effects help for their better understanding and subsequent improvement. An overview analysis of the intergranular properties of cuprate (Y0.8Ca0.2Ba2Cu3O7-δ) and iron-based chalcogenide (FeSe0.5Te0.5) polycrystalline samples was carried out, by means of series of electro-transport experiments at different magnetic fields. The temperature evolution of the Josephson coupling and intrinsic superconductivity effects for the both systems was constructed. The FeSe0.5Te0.5 compound shows very stable and superior behavior compared to Y0.8Ca0.2BCO up to the highest magnetic fields (14T) used. We have explored FeSe0.5Te0.5 Josephson weak links influence (as a non-linear process) over the resistive transition using different AC current amplitudes and applying the sensitive AC transport third harmonics technique.
Pulsed laser deposition of thick BaHfO3-doped YBa 2Cu307-δ films on highly alloyed textured Ni-W tapes
2014, Sieger, M., Hänisch, J., Iida, K., Gaitzsch, U., Rodig, C., Schultz, L., Holzapfel, B., Hühne, R.
YBa2Cu3O7-δ (YBCO) films with a thickness of up to 3 μm containing nano-sized BaHfO3 (BHO) have been grown on Y2O3/Y-stabilized ZrO2/CeO 2 buffered Ni-9at% W tapes by pulsed laser deposition (PLD). Structural characterization by means of X-ray diffraction confirmed that the YBCO layer grew epitaxial. A superconducting transition temperature T c of about 89 K with a transition width of 1 K was determined, decreasing with increasing BHO content. Critical current density in self-field and at 0.3 T increased with increasing dopant level.
Experimental strategies for optical pump - Soft x-ray probe experiments at the LCLS
2014, McFarland, B.K., Berrah, N., Bostedt, C., Bozek, J., Bucksbaum, P.H., Castagna, J.C., Coffee, R.N., Cryan, J.P., Fang, L., Farrell, J.P., Feifel, R., Gaffney, K.J., Glownia, J.M., Martinez, T.J., Miyabe, S., Mucke, M., Murphy, B., Natan, A., Osipov, T., Petrovic, V.S., Schorb, S., Schultz, T., Spector, L.S., Swiggers, M., Tarantelli, F., Tenney, I., Wang, S., White, J.L., White, W., Gühr, M.
Free electron laser (FEL) based x-ray sources show great promise for use in ultrafast molecular studies due to the short pulse durations and site/element sensitivity in this spectral range. However, the self amplified spontaneous emission (SASE) process mostly used in FELs is intrinsically noisy resulting in highly fluctuating beam parameters. Additionally timing synchronization of optical and FEL sources adds delay jitter in pump-probe experiments. We show how we mitigate the effects of source noise for the case of ultrafast molecular spectroscopy of the nucleobase thymine. Using binning and resorting techniques allows us to increase time and spectral resolution. In addition, choosing observables independent of noisy beam parameters enhances the signal fidelity.
Single-electron transitions in one-dimensional native nanostructures
2014, Reiche, M., Kittler, M., Schmelz, M., Stolz, R., Pippel, E., Uebensee, H., Kermann, M., Ortlepp, T.
Low-temperature measurements proved the existence of a two-dimensional electron gas at defined dislocation arrays in silicon. As a consequence, single-electron transitions (Coulomb blockades) are observed. It is shown that the high strain at dislocation cores modifies the band structure and results in the formation of quantum wells along dislocation lines. This causes quantization of energy levels inducing the formation of Coulomb blockades.
Phase-resolved measurement of electric charge deposited by an atmospheric pressure plasma jet on a dielectric surface
2014, Wild, R., Gerling, T., Bussiahn, R., Weltmann, K.-D., Stollenwerk, L.
The surface charge distribution deposited by the effluent of a dielectric barrier discharge driven atmospheric pressure plasma jet on a dielectric surface has been studied. For the first time, the deposition of charge was observed phase resolved. It takes place in either one or two events in each half cycle of the driving voltage. The charge transfer could also be detected in the electrode current of the jet. The periodic change of surface charge polarity has been found to correspond well with the appearance of ionized channels left behind by guided streamers (bullets) that have been identified in similar experimental situations. The distribution of negative surface charge turned out to be significantly broader than for positive charge. With increasing distance of the jet nozzle from the target surface, the charge transfer decreases until finally the effluent loses contact and the charge transfer stops.
Femtosecond spectroscopy in a nearly optimally doped Fe-based superconductors FeSe0.5Te0.5 and Ba(Fe 1-xCox)2As2/Fe thin film
2014, Bonavolontà , C., Parlato, L., De, Lisio, C., Valentino, M., Pepe, G.P., Kazumasa, I., Kurth, F., Bellingeri, E., Pallecchi, I., Putti, M., Ferdeghini, C., Ummarino, G.A., Laviano, F.
Femtosecond spectroscopy has been used to investigate the quasi-particle relaxation times in nearly optimally doped Fe-based superconductors FeSe 0.5Te0.5 and optimally doped Ba-122 thin films growth on a Fe buffer layer. Experimental results concerning the temperature dependence of the relaxation time of such pnictides both in the superconducting state are now presented and discussed. Modelling the T-dependence of relaxation times an estimation of both electron-phonon constant and superconducting energy gap in the excitation spectrum of both Fe(Se,Te) and Ba-122 compounds is obtained.