Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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33397 Publications

Dilepton analysis in the HADES spectrometer for C+C at 2 A GeV

Bielcik, J.; Agakichiev, G.; Agodi, C.; Alvarez-Pol, H.; Balanda, A.; Bellia, G.; Böhmer, M.; Boyard, J.; Braun-Munzinger, P.; Chernenko, S.; Christ, T.; Coniglione, R.; Djeridi, R.; Dohrmann, F.; Duran, I.; Eberl, T.; Fabbietti, L.; Fateev, O.; Finocchiaro, P.; Friese, J.; Fröhlich, I.; Garzon, J.; Gernhäuser, R.; Golubeva, M.; Gonzalez-Diaz, D.; Grosse, E.; Guber, F.; Hennino, T.; Hlavac, S.; Holzmann, R.; Ierusalimov, A.; Iori, I.; Jaskula, M.; Jurkovic, M.; Kämpfer, B.; Kanaki, K.; Karavicheva, T.; Koenig, I.; Koenig, W.; Kolb, B.; Kotte, R.; Kotulic-Bunta, J.; Krücken, R.; Kugler, A.; Kühn, W.; Kulessa, R.; Kurepin, A.; Lang, S.; Lehnert, J.; Maiolino, C.; Markert, J.; Metag, V.; Mousa, J.; Münch, M.; Müntz, C.; Naumann, L.; Novotny, R.; Novotny, J.; Otwinowski, J.; Pachmayer, Y.; Pant, L. M.; Pechenov, V.; Perez, T.; Pietraszko, J.; Pleskac, R.; Pospisil, V.; Pvrzygoda, W.; Rabin, N.; Ramstein, B.; Reshetin, A.; Ritman, J.; Roy-Stephan, M.; Rustamov, A.; Sadovsky, A.; Sailer, B.; Salabura, P.; Sanchez, M.; Sapienza, P.; Schmah, A.; Simon, R.; Smolyankin, V.; Smykov, L.; Spataro, S.; Spruck, B.; Stroebele, H.; Stroth, J.; Sturm, C.; Sudol, M.; Tlusty, P.; Toia, A.; Traxler, M.; Tsertos, H.; Wagner, V.; Wisniowski, M.; Wojcik, T.; Wüstenfeld, J.; Zanevsky, Y.; Zovinec, D.; Zumbruch, P.; Hades Collaboration
The high-acceptance dielectron spectrometer (HADES) has become operational at GSI Darmstadt. The primary physics programme is to study in-medium changes of light vector mesons via their e+e- decays. The methods of dilepton signal reconstruction in the HADES spectrometer and preliminary dilepton spectra for C+C reactions at 2 A GeV are presented. In the signal reconstruction, particularly important is the reduction of the huge combinatorial background arising from e+e- combinations of leptons from gamma conversion in materials with other leptons in the collision. The purity of the dilepton signal is determined by using GEANT simulations with the full HADES geometry and a realistic detector response.

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Permalink: https://www.hzdr.de/publications/Publ-7324
Publ.-Id: 7324


Doping magnetic materials - tunable properties due to Cr ion implantation

Fassbender, J.ORC; McCord, J.; Mattheis, R.; Potzger, K.; Mücklich, A.; von Borany, J.
Various magnetic properties like the saturation magnetization, the Curie temperature, the coercivity and the magnetic damping behavior can easily be modified by means of ion irradiation and implantation. In combination with focused ion beam techniques even a pure magnetic patterning without changing the surface topography becomes feasible. A magnetic domain pattern can be imprinted by using ion irradiation in an applied magnetic field. Thus doping of magnetic materials opens a route to a new class of artificial magnetic materials with adjustable magnetic properties.
Here, as an example, we report on the tailoring of the magnetic properties of Permalloy (20 nm Ni81Fe19) by means of 30 keV Cr implantation. Due to the doping the Curie temperature of the Permalloy film decreases with the implantation fluence and drops below room temperature at an averaged Cr concentration of about 7 at-%. Also the saturation magnetization and the uniaxial anisotropy decrease. However the magnetic damping behavior of Cr implanted Permalloy films is strongly enhanced which is due to a combination of structural changes and alloying effects in the thin film. In order to clarify the basic mechanism for the enhancement the chemical and structural contributions to the magnetic damping parameter are separated by a comparison to results of 30 keV Ni implantation.
Keywords: magnetism, ion implantation, magnetic anisotropy, Curie temperature, magnetization dynamics
  • Poster
    348. Heraeus-Seminar "Ions at Surfaces: Patterns and Processes", 19.-23.06.2005, Bad Honnef, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-7323
Publ.-Id: 7323


Magnetic anisotropy and domain patterning by means of ion irradiation

McCord, J.; Fassbender, J.ORC; Frommberger, M.; Liedke, M. O.; Quandt, E.; Schultz, L.
The control of magnetic properties and anisotropy in ferromagnetic thin films is of importance for many applications in spin electronics. Usually the uniaxial anisotropy in ferromagnetic alloys is set by applying a magnetic in-plane field during thin film deposition or adjusted by magnetic field annealing. In addition to temperature treatment, the alteration of magnetic properties in magnetic thin films by ion irradiation has gained increasing attention in recent years. However, most of the experiments focus on magnetic sandwiches or multilayers, where the magnetic properties depend strongly on the interface structure [1].
Here, we describe how to modify and tune the magnetic anisotropy axis and strength in single layer amorphous FeCoSiB soft magnetic films by ion irradiation with Co- and He-ions locally. Depending on the fluence and field angle during irradiation the uniaxial anisotropy can be realigned in the case of He-ion irradiation [2]. For Cobalt-implantation an increase of uniaxial anisotropy field with increasing fluence by more than a factor of two relative to the as-deposited anisotropy value (from Hk  15 Oe to Hk  40 Oe) is found. The ion treatment is used to “anneal” the films locally and thus to tailor the magnetic properties laterally. Periodic magnetic structures, consisting of regions of alternating anisotropy axis and strength, are generated using photo-lithographic processing. For that reason the films are partially masked with photo resist, patterned by optical lithography, and then irradiated with ions. Depending on the processing conditions and the size of the magnetic patterning, varying from mm in size down to sub-micrometer patterns, different kind of magnetic structures develop. The magnetization is modulated in accordance with the induced lateral anisotropy distribution. However, the domain walls do not align with the boundaries of the structures. Different types of periodic but rather complicated domain patterns are generated under different applied field conditions. The local change of magnetic properties due to the mixed anisotropy is limited by the micromagnetic feature size, not by the structural feature size. By structuring with a feature size smaller than the magnetic length scales, materials with novel magnetic properties are designed.

References
[1] J. Fassbender, D. Ravelosona, and Y. Samson, J. Phys. D: Appl. Phys. 37, R179 (2004).
[2] J. McCord, T. Gemming, L. Schultz, J. Fassbender, M.O. Liedke, M. Frommberger,
E. Quandt, Appl. Phys. Lett. 86, 162502 (2005).
Keywords: magnetism, ion irradiation, magnetic anisotropy, magnetic domains, amorphous films, Kerr microscopy
  • Poster
    348. Heraeus-Seminar "Ions at Surfaces: Patterns and Processes, 19.-23.06.2005, Bad Honnef, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-7322
Publ.-Id: 7322


Asymmetric polarity reversals, bimodal field distribution, and coherence resonance in a spherically symmetric mean-field dynamo model

Stefani, F.; Gerbeth, G.
Using a mean-field dynamo model with a spherically symmetric helical turbulence parameter alpha which is algebraically quenched and disturbed by additional noise, the basic features of geomagnetic polarity reversals are shown to be generic consequences of the dynamo action in the vicinity of exceptional points of the spectrum. This simple paradigmatic model yields long periods of constant polarity which are interrupted by self-accelerating field decays leading to asymmetric polarity reversals. It shows the recently discovered bimodal field distribution, and it gives a natural explanation of the correlation between polarity persistence time and field strength. The dependence of the persistence time on the noise shows typical features of coherence resonance.

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Permalink: https://www.hzdr.de/publications/Publ-7321
Publ.-Id: 7321


Field-tuned quantum critical point in CeCoIn5 near the superconducting upper critical field

Ronning, F.; Capan, C.; Bianchi, A.; Movshovich, R.; Lacerda, A.; Hundley, M.; Thompson, J.; Pagliuso, P.; Sarrao, J.
We report a systematic study of high-magnetic-field specific heat and resistivity in single crystals of CeCoIn5 for the field oriented in the basal plane (H parallel to ab)of this tetragonal heavy fermion superconductor. We observe a divergent electronic specific heat as well as an enhanced A coefficient of the T-2 law in resistivity at the lowest temperatures, as the field approaches the upper critical field of the superconducting transition. Together with the results for field along the tetragonal axis (H parallel to c), the emergent picture is that of a magnetic-field-tuned quantum critical point which exists in the vicinity of the superconducting H-c2(0) despite a variation of a factor of 2.4 in H-c2(0) for different field orientations. This suggests that an underlying physical reason exists for the superconducting H-c2(0) to coincide with the quantum critical field. Moreover, we show that the recovery of a Fermi-liquid ground state with increasing magnetic field is more!
gradual, meaning that the fluctuations responsible for the observed quantum critical phenomena are more robust with respect to magnetic field, when the magnetic field is applied in plane. Together with the close proximity of the quantum critical point and H-c2(0) in CeCoIn5 for both field orientations, the anisotropy in the recovery of the Fermi-liquid state might constitute an important piece of information in identifying the nature of the fluctuations that become critical.
Keywords: UNCONVENTIONAL SUPERCONDUCTIVITY; HIGH-PRESSURE
  • Physical Review B 71(2005), 104528

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Permalink: https://www.hzdr.de/publications/Publ-7320
Publ.-Id: 7320


RBS/Channeling-Messungen mit dem Mikrostrahl zur Untersuchung der Strahlenschaden-Akkumulation in Silizium durch Ionen-Implantation

Grambole, D.; Herrmann, F.; Dagkaldiran, Ü.; Meijer, J.
Um die bisherigen Untersuchungen zur Strahlenschaden-Akkumulation in Silizium durch Ionen-Implantation auf sehr hohe Ionenstromdichten zu erweitern, wurden (100) Si-Proben mit 600 keV Si+ -Ionen mit einer Stromdichte von 360 µA/cm² (2.2 x 1015 Si/s/cm²) bei Substrattemperaturen von 50°C bis 400°C bis zu einer Fluenz von 1018 Si/cm² implantiert. Die Implantation erfolgte mit dem Bochumer supraleitenden Ionenprojektor [J. Meijer, A. Stephan, Microelec. Eng. 41/42 (1998) 257]. Die Schädigung des Si-Kristalls in den implantierten Flächen von 185 µm Durchmesser wurde mit Hilfe der RBS/Channeling-Methode (3 MeV He-Ionen) an der Rossendorfer Mikrosonde untersucht. Für diese Messungen wurde eine zweite Messkammer mit einem 4-Achsen-Goniometer hinter der sonst zur Ionenstrahlanalytik genutzten Messkammer installiert. Die beobachtete deutliche Abhängigkeit der Strahlenschaden-Akkumulation bis hin zur kompletten Amorphisierung der bestrahlten Schicht von der Substrattemperatur und Fluenz wird diskutiert.
  • Lecture (Conference)
    Workshop "Ionenstrahlphysik und - technologie", 11.-12.04.2005, Leipzig, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-7319
Publ.-Id: 7319


Natural circulation and stability performance of BWRs (NACUSP)

Aguirre, C.; Caruge, D.; Castrillo, F.; Dominicus, G.; Geutjes, A. J.; Saldo, V.; van der Hagen, T. H. J. J.; Hennig, D.; Huggenberger, M.; Ketelaar, K. C. J.; Manera, A.; Munoz-Cobo, J. L.; Prasser, H.-M.; Rohde, U.; Royer, E.; Yadigaroglu, G.
From the beginning of BWR technology it was realized that a BWR can become unstable under particular circumstances caused by a feedback between the thermal-hydraulics and the neutronics. This instability can result in oscillations of the power and the flow rate, which is an unwanted phenomenon.

The NACUSP project addresses the stability issues in current and future BWRs by expanding the basic understanding through well structured testing and analyses of experimental data, by analyses of existing operational stability data from three different European reactors (Forsmark, Leibstadt, Cofrentes), by applying this knowledge via efficient models and validated computer codes to operating reactors and reactor designs, and by developing general guidelines for reactor operation and design on how to avoid BWR instabilities.
In order to cover the parameter range as efficiently as possible, four existing, sophisticated thermohydraulic test facilities (CLOTAIRE, DESIRE, CIRCUS and PANDA) have been selected. To extrapolate from small-scale separate-effect testing conditions to full-scale integral reactor conditions one needs to rely on the performance of computer codes (MONA, ATHLET , RAMONA-3(-5), LAPUR-V, DWOS, FLICA, SAPHYR, RELAP5/MOD3,TRAC-BF1). For specific items CFD codes are applied as well. Within NACUSP a linear stability analysis tool is developed.
Four of the three experimental facilities within the project have yielded a large, unique database.
Natural-circulation and stability characteristics at nominal pressure were collected from extensive experiments at the DESIRE facility. Low-pressure characteristics were measured at the PANDA (large scale) and the CIRCUS (small scale) facility.
The phenomena encountered (flow rate and stability trends, the occurrence of flashing induced oscillations) can be explained from basic physical models and are now well understood.
Several thermal-hydraulic codes have been benchmarked—some successfully, others with limited success—against these data.
The CLOTAIRE (nominal pressure, large scale) facility has been modified and is now ready for experiments.
Nuclear power plant data from three BWRs (Cofrentes, Forsmark and Leibstadt) have been collected. A very complete set of reactor data was obtained from measurements at the Leibstadt BWR during cycle-19. During this test, which was performed within the NACUSP project, safe reactor operation was demonstrated at extreme conditions, covering the entire power-flow map.
State-of-the-art, coupled thermal-hydraulic—neutronics codes are being benchmarked on these data.
The development of an easy to use, rapid and efficient analytical tool for parametric studies on BWR stability is in progress.
The last year of NACUSP will be devoted to finalising the before mentioned activities and to use the tools developed and the experience obtained for developing general guidelines for designing and operating BWRs.
Keywords: natural circulation, BWR, instability
  • Nuclear Engineering and Design (2005)235, 401-409

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Permalink: https://www.hzdr.de/publications/Publ-7318
Publ.-Id: 7318


Experiments on the magnetic damping of an inductively stirred liquid metal flow

Bojarevics, A.; Cramer, A.; Gelfgat, Y.; Gerbeth, G.
This work is concerned with the stabilisation of both the bulk liquid metal flow and the free surface shape in inductively heated melts. Steady magnetic fields of different orientation were used to damp fluctuations generated by an external alternating magnetic field as it is typical for an induction heater. This superposition of a driving alternating with a braking steady magnetic field was investigated experimentally at a low temperature isothermal model utilising a rectangular fluid volume in an induction--furnace--like setup fed by a current of intermediate frequency. Local velocity measurements in the liquid metal revealed quite different damping characteristics for steady fields aligned either normal or parallel to the melt surface.
  • Experiments in Fluids 40(2006)2, 257-266

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Permalink: https://www.hzdr.de/publications/Publ-7317
Publ.-Id: 7317


Annual Report 2004 - Institute of Bioinorganic and Radiopharmaceutical Chemistry

van den Hoff, J.; Seifert, S.; (Editors)
  • Open Access Logo Wissenschaftlich-Technische Berichte / Forschungszentrum Rossendorf; FZR-424 2005

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Permalink: https://www.hzdr.de/publications/Publ-7316
Publ.-Id: 7316


TRLFS with fs-Lasers – a tool to study interactions of actinides with organic ligands

Geipel, G.; Vulpius, D.; Bernhard, G.
Due to the sensitive detection of light fluorescence properties often are used to study the interaction of metal ions with various ligands. Several actinides as UO2++, Am3+ and Cm3+ show intense fluorescence properties and can be detected at very low concentrations. However, for neptunium and plutonium fluorescence properties are not known up to now. Nevertheless many organic ligands present in the environment as humic substances and wood degradation products fluorescence after excitation. To study the interaction of non-fluorescent actinides with these organic ligands fs-laser pulses were used for excitation. The light emitted from the ligand is observed by ICCD-cameras in a picosecond to nanosecond time scale. Using this TRLFS system the interaction of Np(V) with several small organic ligands was studied. As examples results will be presented for the interaction of Np(V) with 2,3-dihydroxybenzoic acid and 4-hydroxy-3-methoxybenzoic acid. For the interaction of uranyl with 2,3-dihydroxybenzoic acid a complex formation with the phenolic OH groups was found. In the case of Np(V) the interaction is very different. At pH values below 5.0 only the interaction with the carboxylic group can be observed and the interaction with the phenolic group starts at higher pH. This results in a 2:1 complex. Studies of the complex formation of Np(V) with 4-hydroxy-3-methoxybenzoic acid at pH 6 showed also a 2:1 complex, demonstrating the Np(V) interaction with the carboxylic group and the phenolic OH group.
Nevertheless the fluorescence spectra did not show in any case a single component spectrum. This may be connected to other reactions of the non-complexed ligand. Therefore the influence of excited state reactions of the ligand will be discussed.
Keywords: Fluorescence, complex formation, organic ligands
  • Contribution to proceedings
    Asia-Pacific Symposium on Radiochemistry, 17.-21.10.2005, Beijing, China
  • Lecture (Conference)
    APSORC 2005, 17.-21.10.2005, Beijing, China

Permalink: https://www.hzdr.de/publications/Publ-7315
Publ.-Id: 7315


Observation of negative-parity high-spin states of 68As

Stefanova, E. A.; Lieb, K. P.; Stefanescu, I.; de Angelis, G.; Curien, D.; Eberth, J.; Farnea, E.; Gadea, A.; Gersch, G.; Jungclaus, A.; Martinez, T.; Schwengner, R.; Steinhardt, T.; Warr, N.; Weisshaar, D.; Wyss, R.
The neutron-deficient nucleus 68AS was populated at high spin in two experiments using the reaction 40Ca(32S, 3pn) at beam energies of 105 and 95 MeV. A self-supporting and a gold backed, highly enriched 40Ca target were used. Gamma rays were detected with the EUROBALL array, combined with the charged-particle detector array EUCLIDES and the Neutron Wall. The 68As level scheme was considerably extended, especially at negative parity and many previous spin-parity assignments were confirmed or rejected. The total-Routhian-surface (TRS) calculation find shape coexistence and γ softness for the negative- and positive-parity states, respectively
  • European Physical Journal A 24(2005), 1-4

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Permalink: https://www.hzdr.de/publications/Publ-7314
Publ.-Id: 7314


Approaches for Silicon Based Light Emitters

Helm, M.
The realization of an efficient Si based light emitter or even a laser would be a breakthrough for micro- and optoelectronics, since it would allow for full integration of optical functionality on a chip. Being an indirect semiconductor, Si is an inherently bad light emitter, yet recently some remarkable progress has occurred. Light emitting devices based on Si pn junctions, Si MOS structures doped with rare earth elements, or containing Si nanoclusters have been demonstrated. I will present our work in some of these areas.

We have fabricated Si light emitting pn diodes (LED) by high-dose boron implantation into n-type Si. The free-exciton electroluminescence (EL) increases with temperature, reaching wall-plug efficiencies of more than 0.1% at room temperature. A model which is based on excitons localized near nanoscale boron doping spikes can explain the EL dependence on current and temperature as well as an electrical bistability occurring at low temperature. We have integrated such structures into a microcavity with a buried metallic CoSi2 bottom mirror and a Si/SiO2 Bragg mirror on top, representing the first electrically driven resonant-cavity LED based on silicon. This resonant-cavity LED exhibits significant spectral narrowing, consistent with the quality of the cavity.

A second class of devices are metal-oxide-semiconductor (MOS) structures, which are implanted with various rare-earth elements. Here the emission results from 4f intrashell transitions, impact excited by hot electrons in the oxide. The most widely studied example is the Er3+ ion emitting at the telecom wavelength of 1.54 microns. We have also fabricated a Gd doped MOS structure, which emits in the deep UV at 316 nm. This is, to our knowledge, the first Si based UV light emitter, with many potential applications in areas such as bio-sensing. Finally we demonstrate a bright-green MOS light emitter based on Tb ions implanted into the oxide, which could find applications in low-cost micro-displays.
Keywords: silicon light emitter, rare earth, electroluminescence
  • Invited lecture (Conferences)
    8th German-Vietnamese Seminar on Physics and Engineering (GVS-8), 03.-09.04.2005, Erlangen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-7313
Publ.-Id: 7313


Effects of Mg, Ca, Sr and Ba on Geochemical Uranyl Speciation

Geipel, G.; Bernhard, G.
The uranyl tricarbonato complex is one of the most important uranyl species under environmental conditions. The tendency to form stable metal-uranyl tricarbonato complexes was found particularly for the interaction with alkaline earth elements. However, under comparable chemical conditions the formation of these complexes is very different. While magnesium tends mainly to the formation of a MgUO2(CO3)32+ - complex, in the case of calcium the Ca2UO2(CO3)3(aq.) complex is the most stable and the formation of the CaUO2(CO3)32+ - complex is very limited. In the corresponding systems with strontium as well as for barium only the MeUO2(CO3)32+ - complex is formed. The stability constants of the MeUO2(CO3)32+ - complexes increase in the series Mg, Ca , Sr and Ba. The formation of Me2UO2(CO3)3 – complexes for Mg, Sr and Ba occurs only in a small concentration range and the formed complexes tends to precipitate. The Me2UO2(CO3)3 – complexes for Mg and Ca form stable minerals as bayleyite and liebigite. However several other mineral modifications as zellerite, fontanite, sharpite and rabbittite underline the geochemical importance of this class of compounds.
Analogous phenomena can be expected in the alkaline earth uranyl phosphate systems and first results will be presented.
Keywords: Uranium, Carbonate, Speciation
  • Contribution to proceedings
    PACIFICHEM 2005, 15.-20.12.2005, Honolulu, United States
  • Invited lecture (Conferences)
    PACIFICHEM 2005, 15.-20.12.2005, Honolulu, United States

Permalink: https://www.hzdr.de/publications/Publ-7312
Publ.-Id: 7312


Photothermal beam deflection (PTBD) spectroscopy using an FEL as a pump source

Foerstendorf, H.; Seidel, W.; Glotin, F.; Prazeres, R.; Ortega, J. M.
The PTBD technique is based on the theory of photothermal spectroscopy which describes the conversion of absorbed energy of a light beam incident on a sample into heat by nonradiative de-excitation processes [1]. The distribution of this induced, exponentially decaying, thermal field is given by the solution of the heat equation with a source term of a Gaussian beam [2]. In typical PTBD experiments the magnitude of the signal is proportional to the slope of the induced displacement of the sample surface. Additionally, it can be shown that there is a direct proportionality between the observed signal and the absorption coefficient of the material under investigation [2]. Therefore, a direct access to absorption spectra is provided [3]. In PTBD spectroscopy generating and detection of thermal waves occur generally in the sub-millimeter length scale. Therefore, PTBD provides spatial information about the surface of the sample and permits imaging and/or microspectrometry.
We investigated distinct patterns of O+-implanted and untreated regions at the surface of germanium substrates serving as model systems. This was achieved by special stainless steal masks in front of the substrate during the implantation processes. The different areas of the surfaces can be distinguished by optical absorption (i.e. the amplitude of the deflection signal) at λ = 11.6 µm of the germaniumoxide produced during ion implantation.
The dimensions of the masks were fully recovered by recording one dimensional absorption profiles of the substrate’s surfaces using a high resolution positioning system. Since the minimum dimensions of the implanted pattern was 30 µm the spatial resolution obtained is near the diffraction limit of the infrared pump beam.

References
[1] A. Rosencwaig and A. Gersho, J. Appl. Phys. 47 (1976) 64.
[2] M.A. Olmstead et al., Appl. Phys. A 32 (1983) 141.
[3] W. Seidel et al., Eur. Phys. J - Appl. Phys. 25 (2004) 39.
  • Poster
    WIRMS 2005 Int. Workshop on Infrared Microscopy and Spectroscopy with Accelerator Based Sources, 26.-30.06.2005, Rathen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-7311
Publ.-Id: 7311


Velocity measurements and concentration field visualizations in natural convection copper electrolysis under magnetic field influence

Weier, T.; Hüller, J.; Gerbeth, G.; Weiss, F.-P.
The effect of magnetic fields on momentum and mass transfer in electrochemical processes has been studied by means of Digital Particle Image Velocimetry (DPIV), shadowgraphy and mean current density measurements.

Chronoamperometric copper electrolysis was carried out in a small electrolytic cell (29x46x6 mm) made mainly from PMMA. The sidewalls forming the vertical electrodes consist of thin copper plates behind which permanent magnets could be fixed. The Lorentz force generated from the faradaic currents and the permanent magnets field has been always parallel to the electrodes. Depending on the orientation of the magnets, downwards or upwards directed Lorentz forces could be generated.

The moderate magnetic field of permanent magnets placed behind the electrodes, although its action is limited to the vicinity of the electrodes, is able to promote convection in the whole cell. Flow structures measured by DPIV compare very well with the patterns of the concentration field given by shadography. Steady state limiting current densities as well as initially instationary current density values can be explained by the corresponding velocity measurements. It will be shown that the interplay of Lorentz and buoyancy forces is substantial for the resulting flow structure.
  • Contribution to proceedings
    Joint 15th Riga and 6th PAMIR International Conference Fundamental and Applied MHD, 27.06.-01.07.2005, Rigas Jurmala, Latvia
    Proceedings 2(2005), 143-146

Permalink: https://www.hzdr.de/publications/Publ-7310
Publ.-Id: 7310


Reactive transport codes applied to gypsum dissolution in a laboratory column experiment with focus on sensitivity of model concepts and data uncertainty

Pfingsten, W.; Mibus, J.; Küchler, R.
Experimentally determined kinetic gypsum dissolution by pure water has been used as a data set to investigate different model concepts for mineral dissolution integrated into the four different couple codes. All concepts include a kinetic approach, but with different detail of parameter dependency. In the case of gypsum dissolution, only those concepts, which include an explicit dependency on the actual gypsum mineral surface area, gave good agreement with the measurements. Concepts which do not include such a dependency could not reproduce the measurements. More complex rate equations were necessary to describe the experiments. Nevertheless, the accessible mineral surface area, which decreases during the dissolution, was not directly measurable in the experiment and is so far a model parameter, albeit the one which reproduced measured Ca2+ in solution. Solution chemistry measurements forced the kinetic dissolution model to include a temporal evolving accessible mineral surface area to correctly describe the dissolution experiment.
Keywords: gypsum dissolution, coupled codes, column experiment, code comparison, mineral surface area, reaction rates
  • Contribution to proceedings
    Fifth International Conference on Calibration and Reliability in Groundwater Modelling, 06.-09.06.2005, Scheveningen, Netherlands
    Reactive transport codes applied to gypsum dissolution in a laboratory column experiment with focus
  • Lecture (Conference)
    Fifth International Conference on Calibration and Reliability in Groundwater Modelling, 06.-09.06.2005, Scheveningen, Netherlands

Permalink: https://www.hzdr.de/publications/Publ-7309
Publ.-Id: 7309


Reactor cell calculations with the codes HELIOS, MCNP and TransRay and comparison of the results

Beckert, C.; Koch, R.
The DYN3D code allowing to calculate the whole reactor core of light water reactors and its transient behaviour has been developed at the Forschungszentrum Rossendorf (FZR). It treats the three-dimensional (3d) neutron kinetics with a two-group diffusion approximation using the nodal method for assemblies with quadratic and hexagonal geometry. As input data DYN3D needs two-group cross sections for the neutrons, which are averaged over each node. These cross sections are generated using the 2d-code-system HELIOS. The necessity to calculate these two-group cross sections three-dimensionally was evaluated with the Monte-Carlo code MCNP and the 2d- and 3d-cell-code TransRay, which has been specially developed for that purpose at FZR. TransRay uses the same solution method as employed by HELIOS. The following reactor cells were investigated: A partially inserted control rod and void (or moderator with a lower density respectively) around a fuel rod as a model for a steam bubble in the moderator region. In general it could be concluded, that a three-dimensional data generation of averaged two-group cross sections is rather necessary for reactor cells with steam bubbles than for reactor cells with absorbers.
Keywords: cross section, neutron transport, HELIOS, MCNP, TransRay, ray-tracing
  • Contribution to proceedings
    Annual Meeting on Nuclear Technology 2005, 10.-12.05.2005, Nürnberg, Germany
    CD-ROM
  • Lecture (Conference)
    Annual Meeting on Nuclear Technology 2005, 10.-12.05.2005, Nürnberg, Germany

Permalink: https://www.hzdr.de/publications/Publ-7308
Publ.-Id: 7308


Module efficiencies and performance measurements: Summary of a 10 years measuring program

Rindelhardt, U.; Futterschneider, H.
Since 1994 a photovoltaic test field has been operated at Forschungszentrum Rossendorf near Dresden. It consists of about 15 modules (mono- and polycrystalline Si, a-Si:H and CIS) from different suppliers, which were mounted in a plane with an inclination of 30 degrees and directed to south. The module power and the module temperature were measured every minute. The 10-minute-mean-values were stored together with the corresponding irradiance data. The main results of this comprehensive program will be reported.
Keywords: photovoltaic, performance
  • Contribution to proceedings
    20th European Photovoltaic Solar Energy Conference and Exhibition, 06.-10.06.2005, Barcelona, Spain, 3-936338-19-1
  • Poster
    20th European Photovoltaic Solar Energy Conference and Exhibition, 06.-10.06.2005, Barcelona, Spain

Permalink: https://www.hzdr.de/publications/Publ-7307
Publ.-Id: 7307


Die Erholung der Talsperrenökosysteme von der atmosphärischen Versauerung 1. Trends ausgewählter chemischer Kriterien

Ulrich, K.-U.
Aufgrund des enormen Ausstoßes an Schwefeldioxid und Stickoxiden sowie des kalkarmen Grundgesteins von Erzgebirge und Vogtland litten zahlreiche sächsische Staugewässer spätestens seit Anfang der 1970er Jahre unter dem Phänomen der atmosphärischen Versauerung. Dies betraf auch einige Trinkwassertalsperren. Nach der Wiedervereinigung wurden weder Kosten noch Aufwand gescheut, um geeignete Technologien zur Aufbereitung des weichen, sauren Rohwassers zu entwickeln, die Wasserwerke zu modernisieren und auf die Einhaltung der geforderten Grenzwerte einzustellen. Innerhalb des letzten Jahrzehnts verringerten sich vor allem die Schwefeldioxid-Emissionen beträchtlich. Der Literatur zufolge gibt es große regionale Unterschiede bei der Erholung der Oberflächengewässer von der atmosphärischen Versauerung. Entsprechende Trends sind in einigen deutschen Mittelgebirgen oft nur schwach ausgeprägt oder überhaupt nicht nachweisbar. Dies begründet die Motivation, den Datenbestand der Landestalsperrenverwaltung Sachsen am Beispiel von sieben versauerten Talsperren und 22 Zuflüssen hinsichtlich der Trends ausgewählter wasserchemischer Parameter für 11 Jahre (1993-2003) statistisch mit dem Seasonal Kendall Test auszuwerten und zu vergleichen.
Der Vortrag stellt die wesentlichen Ergebnisse im Kontext mit der regionalen Entwicklung der Emissions- und Depositionssituation und vor dem Hintergrund der forstlichen Bodenschutzkalkung in Südsachsen dar. Als Hauptfaktor für den in 85% der untersuchten Gewässer signifikanten Konzentrationsrückgang von Protonen und Sulfat wird die Abnahme der Depositionsraten mindestens um Faktor 3 angesehen. In etwa 20% der Gewässer, bei denen die insgesamt applizierte Dolomitdosis im Einzugsgebiet 7 t ha-1 überschritt, fanden sich steigende Trends der Calcium- und/oder Magnesiumkonzentration, die jedoch nicht signifikant waren. Der direkte Beitrag der Bodenschutzkalkung zur Entsäuerung und nachhaltigen Erhöhung des Pufferungsvermögens der Oberflächengewässer wird als relativ gering eingeschätzt. Der starke Konzentrationsrückgang von Protonen und damit einhergehend von toxischen Aluminiumspezies bildet die Grundlage für eine Wiederbesiedelung der Gewässer durch säureempfindliche Organismen.
Keywords: acidification reversal, drinking-water reservoir, hydrochemistry, atmospheric emissions, acidic deposition, forest soil liming, sulfur, aluminum, recovery
  • Invited lecture (Conferences)
    Wassergüteberatung des Referats für Wassergütebewirtschaftung, 13.04.2005, Bad Gottleuba, Germany

Permalink: https://www.hzdr.de/publications/Publ-7306
Publ.-Id: 7306


Ion beam synthesis of 3C–(Si1–xC1–y)Gex+y solid solutions

Weih, P.; Stauden, T.; Ecke, G.; Shokhovets, S.; Zgheib, C.; Voelskow, M.; Skorupa, W.; Ambacher, O.; Pezoldt, J.
In the present study cubic 3C–(Si1–xC1–y)Gex+y solid solutions were created by using ion beam synthesis. 3C–SiC thin layers grown on on-axis Si (111) substrates by Molecular Beam Epitaxy were implanted with Ge in order to incorporate Ge atoms in the Silicon Carbide lattice. Two series of experiments were carried out. The implantation energy was chosen to be 140 keV and 200 keV and the implantation dose 1 × 1017 cm–2 and 4.7 × 1016 cm–2 respectively. The samples were annealed under rapid thermal annealing conditions in the temperature range between 800 °C and 1300 °C. X-ray diffraction measurements indicate an enlargement of the lattice constant. The observed higher absorption in the implanted layers could be a sign of a band gap reduction as a consequence of Ge incorporation.
  • Physica Status Solidi (A) 202(2005)4, 545-549

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Permalink: https://www.hzdr.de/publications/Publ-7305
Publ.-Id: 7305


Two-phase flow simulations in the FZ Rossendorf using CFX-5

Krepper, E.; Lucas, D.; Prasser, H.-M.; Shi, J.-M.; Rohde, U.
To qualify CFD codes for gas-liquid two-phase flows, they have to be equipped with constitutive models standing for the interaction between the gaseous and the liquid phases. In case of bubbly flow this particularly concerns the forces acting on the bubbles as well as bubble coalescence and break-up. Besides the drag forces describing the momentum exchange in flow direction, the non drag forces acting perpendicular to the flow direction play an important role for the development of the flow structure. The presentation describes the two phase flow test facilities in the Forschungszentrum Rossendorf, the applied measurement technique and the modelling efforts simulating the momentum exchange between the phases by means of a two-fluid model. The simulation of a rectangular bubble column and a hot channel in a nuclear reactor fuel assembly are presented as application examples.
Keywords: two-phase flow measurements, two-phase flow CFD calculations, Euler/Euler two fluid approach, momentum exchange non drag forces, wall boiling
  • Contribution to proceedings
    11th Workshop on Two Phase Flow Predictions, 05.-08.04.2005, Merseburg, Germany
  • Lecture (Conference)
    11th Workshop on Two Phase Flow Predictions, 05.-08.04.2005, Merseburg, Germany

Permalink: https://www.hzdr.de/publications/Publ-7304
Publ.-Id: 7304


Die Strahlungsquelle ELBE im Forschungszentrum Rossendorf Konzept und erste Betriebserfahrungen

Michel, P.
Der Elektronenbeschleuniger ELBE (Elektronenbeschleuniger mit hoher Brillanz und geringer Emittanz) [1] im Forschungszentrum Rossendorf wird nach seiner endgültigen Fertigstellung elektromagnetische Strahlung, Neutronen, Positronen sowie Elektronen für die Forschung liefern. Die Vielfalt und die hervorragenden Eigenschaften dieser Sekundärstrahlung machen ELBE zum zentralen und verbindenden Großgerät der Institute im Forschungszentrum Rossendorf. Ein 40 MeV, 1mA Elektronenbeschleuniger dient als Treiber für die verschiedenen Arten von Sekundärstrahlung.
  • Lecture (Conference)
    SEI - Herbsttagung (Studiengruppe für Elektronische Instrumentierung), 27.-29.09.2004, Forschungszentrum Rossendorf, Dresden, Germany
  • Contribution to proceedings
    Herbsttagung der SEI Studiengruppe für Elektronische Instrumentierung, 27.-29.09.2004, FZ Rossendorf, Dresden, Germany
    Bericht der Herbsttagung der Studiengruppe für Elektronische Instrumentierung, Berlin: HMI-B 600, ISSN 0936 - 0891, 2-8
  • Contribution to WWW
    http://www.hmi.de/bereiche/I/DS/sei/archiv/2004-09/vortrag/michel_text.pdf

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Permalink: https://www.hzdr.de/publications/Publ-7303
Publ.-Id: 7303


Die Strahlungsquelle ELBE im Forschungszentrum Rossendorf

Michel, P.
Die Strahlungsquelle ELBE (Elektronenbeschleuniger mit hoher Brillanz und niedriger Emittanz) im Forschungszentrum Rossendorf wird nach ihrer vollständigen Inbetriebnahme ein breites Spektrum von Sekundärstrahlung für die Forschung liefern. Im elektromagnetischen Bereich werden an ELBE intensive und polarisierte Bremsstrahlung (Maximalenergie 20 MeV), monochromatische Röntgenstrahlung (10-100 keV) und Infrarotstrahlung aus Freie-Elektronen-Lasern (6-400 meV) produziert. Darüber hinaus werden gepulste Neutronen und monoenergetische Positronen als Sekundärteilchenstrahlen und ein direkter Elektronenstrahl mit hoher Strahlleistung bis zu 40kW zur Verfügung stehen. Als Treiber für die Sekundärstrahlungserzeugung dient ein supraleitender Hochfrequenzelektronenbeschleuniger mit einer Maximalenergie von 40 MeV und einem mittleren Strahlstrom von 1 mA. ELBE wurde im Rahmen einer Projektgruppe des FZR konzipiert und aufgebaut. Die vollständige Inbetriebnahme soll im Jahre 2005 abgeschlossen sein. Mit ELBE steht dem Forschungszentrum Rossendorf und der Forschungsgemeinschaft insgesamt ein leistungsfähiges und modernes Instrument zu Verfügung. Das breite Spektrum von Strahlung und die exzellenten Strahleigenschaften eröffnen einer Vielzahl von Forschungsrichtungen, experimentellen Techniken und technologischen Anwendungen neue Möglichkeiten. Derzeit sind beispielsweise Experimente zur Untersuchung der Kernstruktur, der biologischen Wirksamkeit von Röntgenstrahlung, der Dynamik von metallischen Mehrphasenströmungen, von Halbleiter-, Festkörper- und Quantenstrukturen sowie von nuklearen Prozessen in Fusionsreaktormaterialien geplant. Der Vortrag soll die Parameter und die potentiellen Möglichkeiten der Strahlungsquelle ELBE aufzeigen und für weitere Anwendungen werben.
  • Contribution to WWW
    P. Michel et al., DACH-Jahrestagung, Zerstörungsfreie Materialprüfung, ZfP in Forschung, Entwicklung und Anwendung, Kurzfassungen der Vorträge und Plakatbeiträge, Vortrag 50, Sitzung E3, 17. – 19.05.2004, Salzburg, Austria: http://www.dgzfp.de/tagungen/Jahrestagung/Salzburg/KuFa2004.pdf
  • Lecture (Conference)
    DACH-Jahrestagung, Zerstörungsfreie Materialprüfung ZfP in Forschung, Entwicklung und Anwendung, 17.-19.05.2004, Salzburg, Austria

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Permalink: https://www.hzdr.de/publications/Publ-7302
Publ.-Id: 7302


Diagnostics at the ELBE Bremsstrahlung Facility

Michel, P.; Evtushenko, P.; Lehnert, U.; Schurig, R.; Teichert, J.; Schilling, K.-D.
The ELBE (Electron Linear accelerator with high Brilliance and low Emittance) machine is currently under transition from commissioning to regular user operation. The linac produces an up to 40 MeV, 1 mA (CW) electron beam which is used to generate various kinds of secondary radiation. While IR-FEL and X-ray facilities are still under construction, comprehensive nuclear physics experiments at the bremsstrahlung facility have been conducted. Both, to meet the experimentor’s demands on beam quality, stability and reproducibility and to ensure a safe operation of the machine, several diagnostic elements, such as backward OTR viewers, radiator temperature diagnostics, stripline detectors to monitor the beam incidence angle on the radiator and a four-quadrant loss monitor were developed, installed and tested. These elements are described and results are presented in this paper.
  • Poster
    11th Beam Instrumentation Workshop, 03.-06.05.2004, Knoxville, TN, USA
  • Contribution to proceedings
    Eleventh Beam Instrumentation Workshop 2004, 03.-06.05.2004, Knoxville, Tennessee, USA
    Beam Instrumentation Workshop 2004, Melville, N. Y., USA: American Institute of Physics, 0-7354-0214-0, 128-132

Permalink: https://www.hzdr.de/publications/Publ-7301
Publ.-Id: 7301


The ELBE accelerator and the SRF Gun project

Teichert, J.
leider nicht vorhanden
  • Lecture (Conference)
    EUROFEL Kick - Off Meeting 2005, 17.-18.01.2005, Hamburg, Germany

Permalink: https://www.hzdr.de/publications/Publ-7300
Publ.-Id: 7300


Speciation of Uranium in Environmental Relevant Compartments

Bernhard, G.
In the past, the chemistry of uranium was focused on its mining and milling for production of high pure uranium compounds as initial matter of reactor fuel elements for energy production and breeding of plutonium for weapons production. In this sense, the recovery of uranium and plutonium from the used reactor fuel elements was also technical realized. The increasing input of uranium into bio-sphere by mining and milling and industrial processes like production of cement, fossil fuels, and fertilizers has led to the realization of the importance of uranium environmental chemistry. For a better assessment of radiotoxicity and transport along the food chain knowledge about the chemistry of uranium is needed in all involved compartments.
  • Lecture (Conference)
    Asia-Pacific Symposium on Radiochemistry, 17.-21.10.2005, Beijing, China
  • Contribution to proceedings
    Asia-Pacific Symposium on Radiochemistry, 17.-21.10.2005, Beijing, China

Permalink: https://www.hzdr.de/publications/Publ-7299
Publ.-Id: 7299


Interfacial Reactions of Uranium Related to Geology and Biology

Bernhard, G.
The increasing input of uranium into bio-sphere by mining and milling an dindustrial processes like production of cement, using fossile fuels, and fertilizers has led to the realization of the importance of environmental chemistry of uranium. Detailed knowledge of the nature of uranium complexes formed on the interfaces of relevant geo- and bio-systems is an essential prerequisite to describe the migration behavior in the environment. Our investigations are focused on surface complexation of uranium on phyllite and its mineral constituents like chlorite, muscovite, albite and quartz. Weathering of iron containing chlorite forms ferrihydrite as a secondary phase on the mineral surface. Results showed that the formed ferrihydrite is dominating the surface complexation. Furthermmore, the changing of uranium speciation on the surface layer will be discussed in dependence on different weathering conditions, the presence of humic acids, and the formation of biofilms on the surface. Bacteria are ubiquitous in nature and can influence the uranium transport by mobilization or immobilization. Formation of biofilms can immobilize uranium by increasing the sorption. On the other hand the formation of bio-colloids can promote the uranium transport. In dependence on the living mechanism, bacteria have different structure of the outer cell membrane and can express a wide variety of complexing ligands with various functionalities. Results about binding of uranium on cell surfaces of selected bacterial strains will be presented. For uranium speciation determination various laser spectroscopic methods and X-ray absorption spectroscopy were used. For localization of uranium on the surfaces sensitive microscopic techniques were helpful.
  • Invited lecture (Conferences)
    Pacifichem 2005, 15.-20.12.2005, Honolulu, United States
  • Contribution to proceedings
    PACIFICHEM 2005, 15.-20.12.2005, Honolulu, United States

Permalink: https://www.hzdr.de/publications/Publ-7298
Publ.-Id: 7298


Oberflächenmodifizierung durch Ionenstrahlbehandlung – Charakterisierung mittels Elektronen

Reuther, H.
Oberflächenmodifizierung durch Ionenstrahlbehandlung – Charakterisierung mittels Elektronen
  • Lecture (others)
    Institutsseminar, 20.04.2005, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-7297
Publ.-Id: 7297


Electromagnetically driven convection applied during solidification of Pb-Sn alloys

Eckert, S.; Willers, B.; Nikritjuk, P. A.; Eckert, K.; Michel, U.; Zouhar, G.
Many references from the cast metal literature consider the application of sonic vibrations, mechanical or electromagnetic stirring as a tool to promote the formation of fine, equiaxed grains during solidification. In this paper experimental and numerical investigations will be presented concerning the influence of a flow driven by a rotating magnetic field (RMF) on the momentum, heat and mass transfer within binary Sn-Pb alloys solidified directionally.
Our results show that the forced convection influences significantly the concentration as well the temperature profile ahead of the solidification front. The convective transport of solute reduces the thickness of the solutal boundary layer and increases the constitutional supercooling. The RMF-application provokes a distinct grain refinement for all considered alloy compositions and equiaxed growth has shown to be encouraged. A flow effect can be supposed both on the presence of nuclei in the melt and suitable conditions allowing them to grow in competition with the columnar front.
Keywords: solidification, Pb-Sn alloys, convection, rotating magnetic field
  • Lecture (Conference)
    Material Science &Technology 2005, 25.-28.09.05, Pittsburgh, United States

Permalink: https://www.hzdr.de/publications/Publ-7296
Publ.-Id: 7296


Application of a rotating magnetic field during directional solidification of Pb-Sn alloys: Consequences on the CET

Willers, B.; Eckert, S.; Nikritjuk, P. A.; Eckert, K.; Michel, U.; Zouhar, G.
Many references from the cast metal literature are known discussing the effect of melt convection during the early stages of solidification on the grain structure. The application of mechanical or electromagnetic stirring, ultrasonic or sonic vibrations promotes the formation of fine, equiaxed grains. In this paper experimental and numerical investigations will be presented concerning the influence of a flow driven by a rotating magnetic field (RMF) on the momentum, heat and mass transfer within binary Sn-Pb alloys solidified directionally. The ultrasound Doppler velocimetry (UDV) was applied to measure the bulk flow during solidification.
The continuum formulation based model has been adopted for numerical simulations. The mushy region is modeled using a mixture viscosity formulation. The Lorentz force in the Navier-Stokes equation has been calculated by means of an analytical solution given by for a finite cylinder.
Our results show that the velocity field undergoes distinct modifications during solidification indicating the occurrence of more sophisticated flow patterns as known from the isothermal case. The forced convection causes distinct modifications of the temperature and concentration field such as a reduction of the temperature gradient ahead of the solidification front and a shift of the mixture concentration towards the eutectic concentration on the axis of the ingots. Without electromagnetic stirring the alloy solidifies solely in form of dendrites aligned parallel to the heat flow direction. In contrast, a transition from a columnar to an equiaxed growth (CET) is observed if the solidifying ingot is exposed to an RMF. The position of the CET is shifted downwards by increasing the field strength.
Keywords: solidification, Pb-Sn alloys, convection, rotating magnetic field
  • Lecture (Conference)
    International Conference on Advances in Solidification Processes, 07.-10.06.05, Stockholm, Sweden
  • Materials Science and Engineering A 413-414(2005), 211-216

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Publ.-Id: 7295


Technology Challenges for SRF Guns as ERL Sources in View of Rossendorf work

Janssen, D.; Büttig, H.; Evtushenko, P.; Lehnert, U.; Michel, P.; Möller, K.; Murcek, P.; Schneider, C.; Schurig, R.; Staufenbiel, F.; Teichert, J.; Xiang, R.; Stephan, J.; Lehmann, W.-D.; Kamps, T.; Lipka, D.; Volkov, V.; Will, I.
After discussing the demands on ERL injectors for different projects, we will discuss the technological challenges and the obtained solutions for different parts of the SRF gun. The first point is the shape of the gun cavity and the tuning system. An essential parameter is the length of the first cell, which is determined by the condition, that the field strength at the cathode has a maximal value at the moment of electron emission. If the cavity consists of different cells, as the 3 ½ cell cavity of the Rossendorf gun, two independent tuning systems are necessary. An essential point is the installation of a normal conducting cathode inside a superconducting cavity. The cathode must be isolated by a vacuum gap, it must be alignable and exchangeable from outside of the cryostat and on has to cool down it to LN2 temperature. The next point of discussion is the RF choke filter, which prevents the leakage of RF power, caused by the coaxial between the cathode and the cavity. Different solutions will be shown. In order to focus the beam and to avoid the increasing of transverse emittance a static magnetic field is applied immediately after the cathode in normal conducting RF guns. For a superconducting cavity three possibilities exist to reach the same effect: The first is to put a solenoid after the cavity and outside the cryostat. The second one is the so called RF focussing, where near the cathode a radial component of the RF field is created, which focus the beam. The most elegant way is to put an additional magnetic RF mode into the cavity. It is shown, that for emittance compensation the phase of this mode is not relevant and the amplitude is inside the limit, given by the maximal surface field strength. Especially for high current guns the input of RF power could be a problem. We will show the limits of the RF coupler in the current Rossendorf project and discuss the idea of a coaxial input of RF power from the cathode side of the gun, where the coaxial between the cathode and the cavity is the main part of the RF input coupler. In the last point we discuss the experimental results obtained in Rossendorf and present the status of the current project.
  • Nuclear Instruments and Methods in Physics Research A 557(2006), 80-86
    DOI: 10.1016/j.nima.2005.10.120
  • Invited lecture (Conferences)
    32nd Advanced ICFA Beam Dynamics Workshop on Energy Recovering Linacs ERL 2005, 19.-23.03.2005, Newport News, USA

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Permalink: https://www.hzdr.de/publications/Publ-7294
Publ.-Id: 7294


Status of superconducting module development suitable for cw operation: ELBE cryostats

Teichert, J.; Büttig, H.; Michel, P.; Lehnert, U.; Schneider, C.; Möller, K.; Büchner, A.; Gabriel, F.; Stephan, J.; Winter, A.
For the ELBE electron linear accelerator a superconducting accerating module was developed and is now in routine operation. The cryostat contains two TESLA cavities (1.3 GHz) and is designed for cw operation with an accelerating gradient of 10 MV/m and a maximum average beam current of 1 mA. For the power RF two 10 kW klystrons are used. Special tuners, power couplers, low level rf control, cryogenic control systems and safety systems were developed. We will discuss engineering design, operation parameters and our experience with the module.
  • Nuclear Instruments and Methods in Physics Research A 557(2006), 239-242
    DOI: PII: S0168-9002(5)02000-0
  • Invited lecture (Conferences)
    32nd Advanced ICFA Beam Dynamics Workshop on Energy Recovering Linacs ERL 2005, 19.-23.03.2005, Newport News, USA

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Publ.-Id: 7293


Tailoring magnetic properties of permalloy by means of Cr implantation

Fassbender, J.ORC; McCord, J.; Mattheis, R.; Potzger, K.; Mücklich, A.; von Borany, J.
As an example, we report on the tailoring of the magnetic properties of Permalloy (20 nm Ni81Fe19) by means of 30 keV Cr implantation. Due to the doping the Curie temperature of the Permalloy film decreases with the implantation fluence and drops below room temperature at an averaged Cr concentration of about 7 at-%. Also the saturation magnetization and the uniaxial anisotropy decrease. However the magnetic damping behavior of Cr implanted Permalloy films is strongly enhanced which is due to a combination of structural changes and alloying effects in the thin film. In order to clarify the basic mechanism for the enhancement the chemical and structural contributions to the magnetic damping parameter are separated by a comparison to results of 30 keV Ni implantation.
Keywords: magnetism, ion implantation, magnetization dynamics, tuning magnetic properties, Curie temperature, magnetic anisotropy
  • Lecture (Conference)
    Workshop "Ionenstrahlphysik und -technologie", 11.-12.04.2005, Leipzig, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-7292
Publ.-Id: 7292


Magnetic Anisotropy and Domain Pattering of Amorphous Films by He-ion Irraddiation

McCord, J.; Fassbender, J.ORC; Frommberger, M.; Liedke, M. O.; Quandt, E.; Schultz, L.
The alteration of magnetic properties in magnetic thin films by ion radiation has gained increasing attention in recent years. Here, we present data on the local alteration of the magnetic anisotropy axis in amorphous soft magnetic FeCoBSi films by He-ion irradiation in an applied magnetic in-plane field. Sputtered CoFeSiB (thickness 30 nm) were irradiated with 5 keV He-ions. A magnetic field of 600 Oe was applied during irradiation aligned orthogonal to the initial easy axis of anisotropy. Above a critical fluence an alignment of anisotropy in the applied field direction is observed by MOKE magnetometry and complementary domain observation by Kerr microscopy. Using irradiation together with photolithography the films were irradiated locally, thus resulting in anisotropypatterned
structures. Domain patterns in different elements with varying
angles of anisotropy and edge orientation, separating regions of different anisotropy alignment, are shown. The influence of the patterning on the (still) full film reversal is discussed in detail.
Keywords: magnetism, ion irradiation, magnetic anisotropy, domains, patterning
  • Lecture (Conference)
    Workshop "Ionenstrahlphysik und -technologie", 11.-12.04.2005, Leipzig, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-7291
Publ.-Id: 7291


Bildung von Goldpartikeln und Molekularbiologie von S-Layern

Pollmann, K.; Merroun, M.; Selenska-Pobell, S.
Zellen des Bakterienstammes Bacillus sphaericus JG-A12 sind in der Lage, Au(III) an ihre Zelloberfläche, vermutlich durch ihre S-layer, zu binden. Nach Zugabe eines Reduktionsmittels wird Au(III) zu Au(0) reduziert. Die Bildung von Goldnanopartikeln wurde mittels EXAFS, TEM und EDX-Analyse nachgewiesen.
Molekularbiologische Analysen der metallbindenden S-layer wiesen die Existenz von zwei unterschiedlichen S-layer-Genen nach. Sowohl das funktionale Gen als auch das stumme Gen wurde sequenziert. Zudem wurde ein neuartiges Insertionselement identifiziert, welches interessanterweise ein stummes S-layer-Gen enthält.
  • Lecture (others)
    SMWK-Projekttreffen, 31.03.2005, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-7290
Publ.-Id: 7290


First Lasing at the ELBE mid-IR FEL

Michel, P.; Evtushenko, P.; Lehnert, U.; Dekorsy, T.; Gabriel, F.; Grosse, E.; Helm, M.; Krenz, M.; Seidel, W.; Wohlfarth, D.; Wolf, A.; Wünsch, R.
First lasing of the mid infrared FEL at ELBE was achieved on May 7, 2004. The Radiation Source ELBE at the Forschungszentrum Rossendorf in Dresden is currently under transition from commissioning to regular user operation. Presently the electron linac produces an up to 18 MeV, 1 mA (cw) electron beam which is alotted to generate various kinds of secondary radiation. After the successful commissioning of the bremsstrahlung and channeling-X-ray facilities during 2003 stable lasing has now been observed in the IR range (15 to 22 μm). The oscillator FEL is equipped with two planar undulator units, both consisting of 34 hybrid permanent magnet periods of 27.3 mm (Krms = 0.3 - 0.8). The distance between the two parts is variable and the gaps can be adjusted and tapered independently. At 19.6 µm an optical power of 3W was out-coupled in a macro pulse of 0.6 ms duration using an electron beam energy of 16.1 MeV and an energy spread of less than 100 keV; the micropulse charge was 50 pC and its width slightly above 1ps. With the installation of a second acceleration module for additional 20 MeV smaller wavelengths will become available in the near future.
  • Lecture (Conference)
    26th International Free Electron Laser Conference FEL 2004, 29.08.-03.09.2004, Trieste, Italy
  • Contribution to proceedings
    26th International Free Electron Laser Conference and 11th FEL Users Workshop, 29.08.-03.09.2004, Trieste, Italy
    Proceedings of the 26th International Free Electron Laser Conference and 11th FEL Users Workshop, MOAIS04, Trieste: Graphart ts, 88-87992-02-9 (CD), 88-87992-02-9 (Book), 8-13

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Permalink: https://www.hzdr.de/publications/Publ-7288
Publ.-Id: 7288


Crystallization of amorphous-Si films by flash lamp annealing

Pecz, B.; Dobos, L.; Panknin, D.; Skorupa, W.; Lioutas, C.; Vouroutzis, N.
The crystallization of amorphous silicon films deposited on glass, using the flash lamp annealing process was realized and studied. The duration of the flash is 20 ms, about two orders of magnitude shorter than the standard rapid thermal annealing process. The a-Si films deposited on Coming glass were irradiated with different energy densities and crystallized exhibiting grains with a mean size up to 6 mum. In order to reduce the strain due to the thermal gradient, the samples were preheated from the backside. The ability of the FLA process to eliminate the ingrain defects in already crystallized poly-Si films at 600 degreesC is also demonstrated.
  • Applied Surface Science 242(2005)1-2, 185-191

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Permalink: https://www.hzdr.de/publications/Publ-7286
Publ.-Id: 7286


Programmes and Projects-examples of management in a German Research Institute

Joehnk, P.
- The Scientific System of Germany
. among: the Research Centre Rossendorf, Dresden
- The Current Change-Management Process
. Management of Programmes
. Management of Projects
- Tools for Management Assistance
- Controlling: Performance and Success Evaluation
- Summary
Keywords: -
  • Invited lecture (Conferences)
    Instituto nationale per la fizica della materia, 15.03.2005, Trieste, Italiy

Permalink: https://www.hzdr.de/publications/Publ-7285
Publ.-Id: 7285


Suppression of Modes in the Random Phase Approximation

Dönau, F.
A general but simple method is proposed to eliminate the quantum fluctuations generated by selected one-body operators in the excitation spectrum of a discrete random phase approximation (RPA) Hamiltonian. This method provides an outstanding tool for the removal of the contaminating spurious effects originated from symetry violations. It can be also applied as a mode filter for analyzing RPA response functions.
  • Physical Review Letters 94(2005), 092503

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Permalink: https://www.hzdr.de/publications/Publ-7284
Publ.-Id: 7284


Texture development during growth of Ti1-xAlxN thin films studied by in-situ x-ray diffraction

Beckers, M.; Schell, N.; Martins, R. M. S.; Möller, W.
Ti1-xAlxN coatings are of common use for a vast variety of applications. For each of them, controlling the microstructure is crucial because it determines usefulness, performance and lifetime. Thus, literature on the relationship between deposition parameters, microstructure, and performance of Ti1-xAlxN coatings is large. However, little is known regarding the atomistic mechanisms for these observed relationships. Our approach in understanding those mechanisms is in-situ x-ray diffraction during the growth of Ti1-xAlxN films using a deposition chamber installed at the Rossendorf beam line BM20 at the European Synchrotron Radiation Facility in Grenoble, France. All films were deposited by reactive co-sputtering from Ti and Al targets; one series at constant x = 0.06 varying substrate temperature, bias voltage, and nitrogen partial pressure and thus growth rate. In another series, x was systematically varied from 0 to 0.73 while keeping all other parameters constant. Values of x < 0.15 and high deposition rates lead to a typical cross-over behavior between initial (002) and final (111) preferred orientation. Reducing the deposition rate leads to (002) preferred orientation practically independent of film thickness and substrate temperature. Yet, suppressing collisionally-induced atomic N on the sample surface by applying a positive bias voltage, brings back a (111) preferred orientation. Those observations are consistent with proposed atomistic models. Keeping the deposition rates low, (111) preferred orientation can also be induced by increasing x above 0.15, which in the presence of atomic N can be explained by its higher adatom mobility. Increasing x towards the AlN segregation threshold at x = 0.60 leads to hard nano-composite TiAlN/AlN structures, and pushing x further to 0.73 leads to highly stressed AlN with an a-axis off-plane texture.
Keywords: Texture development, nucleation and growth, Ti-Al-N, sputter deposition, in-situ x-ray diffraction
  • Contribution to proceedings
    International Conference on Metallurgical Coatings and Thin Films, 02.-06.05.2005, San Diego, U.S.A.

Permalink: https://www.hzdr.de/publications/Publ-7283
Publ.-Id: 7283


In-situ x-ray diffraction studies concerning the influence of Al concentration on the texture development during sputter deposition of Ti-Al-N thin films

Beckers, M.; Schell, N.; Martins, R. M. S.; Mücklich, A.; Möller, W.
In-situ x-ray diffraction was employed during the growth of thin Ti1-xAlxN films, using a deposition chamber installed at a synchrotron radiation beamline. The films were deposited by reactive co-sputtering from Ti and Al targets. At constant x ~ 0.06, substrate temperature, bias voltage, and nitrogen partial pressure, and thus growth rate, was varied. Further, x was systematically varied from 0 to 0.73 while keeping all the other parameters constant. x < 0.15 and high deposition rates of ~ 1 Å/s lead to the typical crossover behavior between initial (001) and final (111) off-plane preferred orientation. Reducing the deposition rate to < 0.5 Å/s leads to a reversed behavior with a clear (001) preferred orientation above a film thickness of 600 Å which is essentially independent of the substrate temperature. Keeping the deposition rate low, the (111) preferred orientation can be recovered for x > 0.15, which can be explained by the higher adatom mobility of Al compared to Ti in the presence of atomic nitrogen. Increasing x towards the AlN segregation threshold at x ~ 0.60 leads to hard nano-composite nc-TiAlN/AlN structures, and x > 0.73 finally leads to dominant AlN with an a-axis off-plane texture.
Keywords: Texture development, nucleation and growth, Ti-Al-N, sputter deposition, in-situ x-ray diffraction
  • Journal of Vacuum Science & Technology A 23(2005)5, 1384-1391

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Publ.-Id: 7282


The influence of the growth rate on the preferred orientation of magnetron sputtered Ti-Al-N thin films studied by in-situ x-ray diffraction

Beckers, M.; Schell, N.; Martins, R. M. S.; Mücklich, A.; Möller, W.
In-situ x-ray diffraction has been used to characterize the growth and microstructure of wear protective Ti1-xAlxN thin films. The films were deposited onto oxidized Si(100) wafers in a sputter chamber mounted onto a six-circle goniometer located at a synchrotron radiation beamline. Off-plane and in-plane x-ray diffraction data were recorded in-situ during growth, in order to follow the development of microstructure and preferred orientation as a function of film thickness. The measurements were supplemented by ex-situ cross-sectional transmission electron microscopy analyses. The films were deposited by reactive co-sputtering from metallic Ti and Al targets in Ar/N2 gas mixtures at substrate temperatures of 150 °C and 300 °C, substrate bias voltages of -30 V and +10 V, and deposition rates between 0.9 Å/s and 0.3 Å/s. The film composition was changed between pure TiN and Ti0.91Al0.09N. Films deposited at higher deposition rates show columnar structure with competitive growth between (001) and (111) crystalline orientation, which slowly evolves into a (111) preferred orientation containing inter- and intracolumn porosities. Reducing the deposition rate to 0.3 Å/s leads to an almost complete (001) preferred orientation with reduced surface roughness, practically independent of the deposition temperature. As the stress state of the films remains low for both deposition rates, it is suggested that the ion-to-neutral arrival rate (JI/JTi+Al) determines the texture development rather than the stress. This is corroborated by applying a positive substrate bias, which, by suppressing ion impingement, leads back to an evolving (111) preferred orientation.
Keywords: Texture development, nucleation and growth, Ti-Al-N, sputter deposition, in-situ x-ray diffraction
  • Journal of Applied Physics 98(2005)4, 44901
  • ESRF Newsletter 42(2005), 20-21

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Publ.-Id: 7281


Identifikation elastoplastischer und schädigungsmechanischer Materialparameter aus dem Small Punch Test

Abendroth, M.
Der Small Punch Test (SPT) ist ein Kleinstprobenversuch, bei dem kleine scheibenförmige Proben in einem Miniaturtiefziehversuch bis zum Versagen belastet werden. Dabei wird eine charakteristische Kraft-Verschiebungs-Kurve für den Druckstempel ermittelt, welche Informationen über das Verformungs- und Versagensverhalten des verwendeten Probenmaterials enthält. Es wird eine den speziellen Anforderungen entsprechende Messapparatur vorgestellt, die es ermöglicht, die Kraft-Verschiebungs-Kurve zu ermitteln und auf optischem Wege den Versagenszeitpunkt der Probe zu bestimmen. Es wird ein Finite Elemente Modell des Small Punch Test benutzt, in dem ein schädigungsmechanisches Materialmodell implementiert ist, um für verschiedene Materialparametersätze die zu erwartenden Kraft-Verschiebungs-Kurven zu berechnen. Mit Hilfe der simulierten Kraft-Verschiebungs-Kurven und den dazugehörigen Materialparametern werden neuronale Netze trainiert, die in einem Lernprozess einen verallgemeinerten Zusammenhang zwischen Materialparametern und Kraft-Verschiebungs-Kurven herstellen. Dieser Zusammenhang kann entweder die approximierte Lösung des inversen Randwertproblems sein, die es ermöglicht aus einer experimentell ermittelten Kraft-Verschiebungs-Kurve des SPT direkt die gesuchten Materialparameter zu bestimmen. Es kann aber auch das Randwertproblem selbst approximiert werden, um in einem Optimierungsverfahren die Materialparameter zu bestimmen, für die sich eine optimale Übereinstimmung zwischen Simulation und Experiment ergeben würde. Beide Techniken werden an verschiedenen duktilen metallischen Werkstoffen getestet. Mit den ermittelten Materialparametern werden Zug- und Bruchmechanikproben simuliert, wobei nicht nur die Materialparameter verifiziert werden, sondern es auch gelingt, für einige Werkstoffe bruchmechanische Kennwerte vorherzusagen.
Keywords: small punch test, damage mechanics, fracture mechanics, parameter identification, neural networks
  • Book (Authorship)
    Freiberg: Freiberger Forschungshefte, Technische Universität Bergakademie Freiberg, 2005
    165 Seiten

Permalink: https://www.hzdr.de/publications/Publ-7280
Publ.-Id: 7280


Wavelet analysis of EXAFS data

Funke, H.; Chukalina, M.; Scheinost, A.
- wird nachgereicht
  • Lecture (others)
    Seminarvortrag, 17.03.2005, Zürich, Switzerland

Permalink: https://www.hzdr.de/publications/Publ-7278
Publ.-Id: 7278


Isotopically resolved residues frorn the fragmentation of projectiles with largely different N/Z - the isospin-thermometer method

Henzlova, D.; Audouin, L.; Benlliure, J.; Botvina, A.; Boudard, A.; Casarejos, E.; Ducret, J.; Enqvist, T.; Got, L.; Heinz, A.; Henzl, V.; Junghans, A.; Jurado, B.; Kelic, A.; Krasa, A.; Kurtukian, T.; Leray, S.; Napolitani, P.; Ordonez, M.; Pereira, J.; Pleskac, R.; Rejmund, F.; Ricciardi, M.; Schmidt, K.; Schmitt, C.; Stephan, C.; Villagrasa, C.; Volant, C.; Wagner, A.; Yordanov, O.
The mean N-over-Z (Z) ratio of residues produced in the fragmentation of Xe-136 (N/Z = 1.519), Xe-124 (N/Z = 1.296) in comparison with Fe-56 (N/Z = 1.154) is investigated over the full range of Z. The final residues from the Xe-136 projectile keep a memory on the initial N/Z. The idea to trace back the excitation energy of the fragment entering evaporation via direct exploration of the N/Z evolution in the evaporation process is investigated and applied to the data. The freeze-out temperature is deduced, which is consistent with the investigations of light and intermediate-mass fragments (IMF) in other experiments.
  • Nuclear Physics A 749(2005), 110c-113c

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Publ.-Id: 7277


Investigation of the creation of Fe nanoparticles in MgO by ion beam synthesis

Reuther, H.; Potzger, K.; Zhou, S.; Mücklich, A.; Eichhorn, F.
Recently, the investigation of metallic nanoparticles embedded in dielectric oxides has become a topic in magnetic data storage development. Especially the creation of high density data storage devices due to ion beam synthesis of magnetically, highly anisotropic FePt nanoparticles in sapphire yielded promising results. On the other hand, pure Fe-nanoparticles have been created in sapphire, Y-stabilized ZrO2 or MgO. The motivation for the latter includes the search for magneto-optically active surfaces as well as pronounced magnetoresistance.
For doping MgO single crystals with Fe, ion implantation is a promising technique. First extensive studies were performed by Perez et al. who investigated the annealing behaviour of samples implanted with different doses of 57Fe at different energies with conversion electron Mössbauer spectroscopy (CEMS) [1]. They obtained up to 20 % of the implanted iron atoms in a-Fe precipitates while the remaining Fe-atoms were found to be in different Fe2+ and Fe3+ charge states.

In this paper, we propose a method for the creation of large fractions of a-Fe nanoparticles (up to 60%). MgO(001) single crystals were implanted with of 6x1016 cm-2 ions of the Mössbauer isotope 57Fe at 100 keV. The implantation temperature was varied from room temperature up to 800 °C. Moreover, post annealing was investigated. Samples were characterized by CEMS, RBS, TEM and XRD. We found that the temperature during the implantation plays the important role for the formation of the Fe0 state while post annealing results in the preferential formation of Fe oxidation states, mainly Fe3+ states. The largest fraction of iron with Fe0 state was obtained at implantation temperatures between 600 and 800 °C.

[1] A. Perez, G. Marest, B.D. Sawicka, J.A. Sawicki, T. Tyliszczak, Phys. Rev. B, 28, 1227 (1983)
  • Lecture (Conference)
    International Conference on the Applications of the Mössbauer Effect, 05.-09.09.2005, Montpellier, France

Permalink: https://www.hzdr.de/publications/Publ-7276
Publ.-Id: 7276


Efficient silicon based light emitters

Helm, M.; Sun, J. M.; Potfajova, J.; Dekorsy, T.; Schmidt, B.; Skorupa, W.
Recent progress on electrically driven silicon based light emitters is reviewed, with emphasis on our work on light emitting pn diodes (LED) and MOS devices doped with rare-earth elements. The LEDs were fabricated by high-dose boron implantation, producing nanoscale modifications in the material. The electroluminescence (EL) efficiency increases with temperature, reaching 0.1% (wall plug efficiency) at room temperature for optimized conditions. Such devices were integrated into a microcavity. In the MOS devices the oxide was implanted with various rare-earth elements, resulting in strong EL in the visible (Tb) and ultraviolet (Gd). External quantum efficiencies in excess of 10% are reported.
Keywords: silicon, light emitting diode, electroluminescence, rare earth, microcavity
  • Microelectronics Journal 36(2005), 957-962

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Publ.-Id: 7275


Efficient silicon based light emitters

Helm, M.
Recently remarkable progress has occurred in the variety and efficiency of silicon based light emitters, based on Si pn junctions, Si MOS structures doped with rare earth elements, or containing Si nanoclusters. I will present our work in some of these areas.

We have fabricated Si light emitting diodes (LED) by high-dose boron implantation into n-type Si. The free-exciton electroluminescence (EL) increases with temperature, reaching wall-plug efficiencies of more than 0.1% at room temperature. A model which is based on excitons localized near nanoscale boron doping spikes can explain the EL dependence on current and temperature. We have integrated such structures into a microcavity with a buried metallic CoSi2 bottom mirror and a Si/SiO2 Bragg mirror on top. This resonant-cavity LED exhibits significant spectral narrowing, consistent with the quality of the cavity.

We also have fabricated light emitting Si MOS structures which were implanted with various rare-earth elements, from the well known Er3+ emitting at the telecom wavelength of 1.54 microns, to Gd3+, which emits in the deep UV at 316 nm. This is, to our knowledge, the first Si based UV light emitter, with many potential applications in areas such as bio-sensing.
Keywords: silicon, light emitting diode, rare earth, electroluminescence, microcavity
  • Invited lecture (Conferences)
    4th Workshop on Semiconductor Nanodevices and Nanostructured Materials (NANOSEMIMAT-4), 09.-13.03.2005, Sao Pedro, SP, Brazil

Permalink: https://www.hzdr.de/publications/Publ-7274
Publ.-Id: 7274


Krein space related physics: PT-symmetric Quantum Mechanics, MHD α²-dynamos, planar Couette flows, ...

Günther, U.; Stefani, F.; Znojil, M.
Operator theoretic structures are discussed which underlie PT-symmetric Quantum Mechanics (PTSQM), the spherically symmetric α²-dynamo of magnetohydrodynamics (MHD) and the plane Couette flow of hydrodynamics (described by the Squire equation). Mathematically, the three types of models are closely related as spectral problems in Krein spaces − Hilbert spaces with an indefinite metric structure.
In contrast to the purely real spectrum of self-adjoint operators in "usual" Hilbert spaces, the spectrum of self-adjoint operators in Krein spaces consists, in general, of two types of spectral sectors: sectors with purely real eigenvalues and other sectors with pairwise complex conjugate eigenvalues. Transitions between different sectors occur at exceptional points of square root branching type. Knowing the boundaries of the sectors in parameter space one would know, e.g., the boundaries of the physical sectors of PTSQM (with exact/unbroken PT-symmetry) or of the oscillatory regimes of α²-dynamos.
The underlying Krein space related structure of the different physical setups indicates a possible fruitful interplay of these models in handling their technical and conceptual aspects on a unified footing. We make some aspects of such an interplay explicit and demonstrate its usefulness on the example of a PT-symmetric interpolation model and the Squire equation of hydrodynamics. This allows us to gain a deeper insight into the specifics of the Herbst limit.

The talk is based on material from math-ph/0501069 (which is to appear in J. Math. Phys.) and extends it.
Keywords: MHD dynamos, PT-symmetric Quantum Mechanics, Squire equation, Krein spaces, exceptional points, Herbst limit
  • Invited lecture (Conferences)
    3rd International Workshop on Pseudo-Hermitian Hamiltonians in Quantum Physics, 20.-22.06.2005, Istanbul, Turkey

Permalink: https://www.hzdr.de/publications/Publ-7273
Publ.-Id: 7273


Diffusion von Huminstoffkolloiden im Laborsystem Kaolinit-Wasser

Sachs, S.; Mibus, J.
Abstract wird nachgereicht.
Keywords: Huminstoffe, Kolloide, Ton, Migration, Diffusion
  • Lecture (others)
    Workshop zum Forschungsvorhaben "Migration von Actiniden im System Ton, Huminstoff, Aquifer", 04.-05.04.2005, München, Germany

Permalink: https://www.hzdr.de/publications/Publ-7272
Publ.-Id: 7272


Inherent Dilution: Qualification of TRIO_U on a ROCOM experiment

Bieder, U.; Höhne, T.
In this report, a generic test case concerning the “heterogeneous inherent dilution during a small break LOCA” is analyzed. During the heat transfer phase of a small break in the primary circuit, vapour, which is formed in the core region, can condense in the steam generator tubes what yields pure, unborated water. Under certain conditions, in particular in the absence of natural circulation, this pure water can accumulate in the steam generator and/or the U form intermediate leg. When the natural circulation starts, the pure water can be transported into the reactor pressure vessel with the possible risk of a criticality accident.

For the validation of Trio_U to the application to such accidents, a generic ROCOM experiment with a constant flow rate in one loop in the magnitude of natural circulation and 10% density difference between ECC and loop water was analysed. The following results have been concluded:

The Trio_U calculation shows a good agreement with the experiment, provided that the exact ROCOM geometry is used.
The LES approach led not only to a good qualitative representation of the experiment, but also a good quantitative accordance. This is true for the two tested sub-grid models, the Smagorinsky model and the WALE model.
It seems that the WALE model treats more correctly the injection phase and the formation of the cold plume in the upper downcomer region. However, the detected azimuthal displacement of the cold plume with the WALE model can lead to errors in the prediction of the core region which is affected by the injection.
The Smagorinsky model seems to underestimate the mixing phenomena during the injection phase. However, the location of the cold plume at the lower end of the downcomer was better predicted than with the WALE model
The transient of the tracer concentrations in the downcomer has been calculated on one hand for the 1:5 KONVOI geometry and on the other hand for the ROCOM geometry. The comparison of these results has shown the importance of using very precise CAD data when applying CFD.
Keywords: ROCOM, TRIO_U, inherent dilution, boron mixing, turbulence, LES, CFD
  • Other report
    Grenoble: CEA/DEN/GRE/DER/SSTH/LMDL-2004-029, 2005
    29 Seiten

Permalink: https://www.hzdr.de/publications/Publ-7271
Publ.-Id: 7271


Quantification of iron minerals formed during aging of iron-containing colloids

Reuther, H.; Ulrich, K.-U.
Due to their large specific surface area and high sorption affinity, iron-containing colloids are capable to remove dissolved contaminants from solution and to retard their migration by the water path. In a pH range near to the point of zero charge, the low electrostatic stability favors agglomeration and sedimentation of such colloids, enabling their retention (e.g., by the formation of crusts). Freshly formed iron-rich colloids mainly consist of metastable iron phases such as schwertmannite or ferrihydrite. X-ray diffraction is not sensitive enough to quantify these phases because of small particle size (< 50 nm) and/or poor structural order. Aging of these metastable phases to more crystalline Fe minerals such as goethite or hematite may have consequences on the fate of the adsorbed contaminants. The modification of surface binding sites and of steric mineral properties by the recrystallization process may cause either desorption of the contaminants from the surface, or allow their incorporation into the lattice.

Our research focuses on the mobility of uranium during the flooding process of abandoned uranium mines in East Germany. The aim is to study the behavior of uranium during the aging process of iron-rich colloids which have scavenged uranium. Information on the speciation of uranium and its migration in the vicinity of such mines is crucial for hazard prognosis.
It is thus necessary to identify and to quantify precisely the transformation of iron phases into more crystalline Fe minerals by chemical analysis. Mössbauer spectroscopy is a powerful tool to discriminate the expected minerals goethite, hematite and ferrihydrite. In mixtures, fractions < 5 % can be separated without difficulties. We used Mössbauer spectroscopy at room temperature to identify the aging products. Transmission spectra of calibration standards of ferrihydrite, goethite, hematite and mixtures of defined ratios were compared with spectra of the iron-containing colloids. This enabled us to characterize the aging process.
  • Contribution to proceedings
    International Conference on Applications of Mössbauer Effect, 05.-09.09.2005, Montpellier, France
  • Poster
    International Conference on Applications of Mössbauer Effect, 05.-09.09.2005, Montpellier, France

Permalink: https://www.hzdr.de/publications/Publ-7269
Publ.-Id: 7269


Complex formation of neptunium(V) with 4-hydroxy-3-methoxybenzoic acid studied by time-resolved laser-induced fluorescence spectroscopy with ultra-short laser pulses

Vulpius, D.; Geipel, G.; Baraniak, L.; Bernhard, G.
The complex formation of neptunium(V) with 4-hydroxy-3-methoxybenzoic acid (vanillic acid) was studied by time-resolved laser-induced fluorescence spectroscopy with ultra-short laser pulses using the fluorescence properties of 4-hydroxy-3-methoxybenzoic acid. A 2:1 complex of neptunium(V) with 4-hydroxy-3-methoxybenzoic acid was found. The stability constant of this complex was determined to be log β210 = 7.33 ± 0.10 at an ionic strength of 0.1 mol/l (NaClO4) and at 21 °C. The determination of the stability constant required an investigation of the excited-state proton transfer of 4-hydroxy-3-methoxybenzoic acid over the whole pH range. It was realized that 4-hydroxy-3-methoxybenzoic acid undergoes excited-state reactions only at pH values below 5. At pH values above 5 stability constants can be determined without kinetic calculation of the proton transfer.
Keywords: Neptunium, 4-Hydroxy-3-methoxybenzoic acid, Complex formation, Time-resolved laser-induced fluorescence spectroscopy, Excited-state proton transfer

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Permalink: https://www.hzdr.de/publications/Publ-7268
Publ.-Id: 7268


Multiple implantations into Si: influence of the implantation sequence on ion range profiles

Posselt, M.; Mäder, M.; Lebedev, A.; Grötzschel, R.
In successive implantations of p- and/or n-dopants the implantation sequence may affect the ion range distributions. This is demonstrated for two consecutive implantations into the [001] channel direction: (i) 35 keV B followed by 50 keV As and (ii) 50 keV As followed by 35 keV B. The defects formed in the first implantation cause enhanced dechanneling of the subsequently implanted ions and, therefore, influence the shape of the range distributions in the second implantation step. The experimental range profiles can be reproduced very well by atomistic computer simulations which take into account damage accumulation or dynamic annealing during a single implantation step as well as the influence of the defects formed by the preceding implantation steps.
Keywords: ion implantation, channeling, radiation damage, computer simulation
  • Applied Physics Letters 87(2005), 043109

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Publ.-Id: 7267


5. Workshop "Measurement Techniques for Steady and Transient Multiphase Flows"

Prasser, H.-M.; (Editor)
The Forschungszentrum Rossendorf (FZR) carries on a series of workshops on Measurement Techniques for Steady and Transient Multiphase Flows with the support of the following organisations: DECHEMA / Gesellschaft für Chemische Technik und Biotechnologie e.V., Kerntechnische Gesellschaft e. V. (KTG), Technical Group on Thermal and Fluiddynamics and Local Section of Saxony, Institute of Process Technique, Process Automation, and Measuring Technique (IPM) at the University of Applied Sciences Zittau/Görlitz and the TELETRONIC GmbH in Rossendorf. The first workshop was held in 1997 and had the character of a national event. International participation started for the first time in the 2000 edition of the workshops series.

The main aim of this workshops series is to discuss recent developments and future tendencies in the field of high-resolution measuring techniques used to characterise two-phase or multi-phase flow fields. Examples are measurements of the gas-liquid interface, the characterisation of its structure and its evolution in the field of gas-liquid flows and measurements of size, shape and velocity of individual particles of the disperse phase (bubbles, droplets etc.) and their dynamics. The efforts in the measure-ments field correspond to the needs of the development of three-dimensional computational fluid-dynamics (CFD) models and computer codes. High-resolution data is necessary both for the development of geometry independent constitutive equations describing the interaction of the different phases taking part in the flow and for the validation of the CFD codes against experiments. Moreover, modern tendencies in instrumentation for industrial applications are discussed.
Keywords: multiphase flow, measurement instrumentation, workshop, transient flow
  • Book (Editorship)
    Dresden: Forschungszentrum Rossendorf, 2002
    99 Seiten

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Publ.-Id: 7266


EXAFS-Untersuchungen am System U(VI)-Huminsäure-Kaolinit und aktuelle Ergebnisse zur U(VI)-Sorption am Kaolinit

Krepelova, A.
Abstract wird nachgereicht
  • Lecture (others)
    Workshop des Verbundprojekts Migration von Actiniden im System Ton, Huminstoff, Aquifer, 04.-05.04.2005, München, Germany

Permalink: https://www.hzdr.de/publications/Publ-7265
Publ.-Id: 7265


Die Koordination von Actiniden in wässriger Lösung - Untersuchungen mit EXAFS-Spektroskopie

Hennig, C.
eingeladener Institutsvortrag
  • Invited lecture (Conferences)
    Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, 19.4.2005, Leipzig, Germany

Permalink: https://www.hzdr.de/publications/Publ-7264
Publ.-Id: 7264


Uncertainty and sensitivity analysis for the modeling of a start-up experiment in a VVER-1000 reactor

Kliem, S.; Mittag, S.; Weiß, F.-P.; Langenbuch, S.
The transition from the application of conservative models to the use of best-estimate models raises the question about the uncertainty of the obtained results. This question becomes especially important, if the best-estimate models should be used for safety analyses in the field of nuclear engineering. Different methodologies were developed to assess the uncertainty of the calculation results of computer simulation codes. One of them is the methodology developed by Gesellschaft fuer Anlagen- und Reaktorsicherheit (GRS) which uses the statistical code package SUSA.

In the frame of the recently finished EU FP5 funded research project VALCO, that methodology was extended and successfully applied to different coupled code systems, including the uncertainty analysis for neutronics. These code systems consist of a thermal hydraulic system code and a 3D neutron kinetic core model. One of the code systems applied was ATHLET coupled with the Rossendorf kinetics code DYN3D. Two real transients at NPPs with VVER-type reactors documented within the VALCO project were selected for analyses. One was a test with the switching-off of one of two main feed water pumps at the VVER-1000 Balakovo-4 NPP. Based on the relevant physical processes in the transients, a list of possible sources of uncertainties was compiled. Besides control parameters like control rod movement and thermal hydraulic parameters like secondary side pressure, mass flow rates, pressurizer sprayer and heater performance, different neutron kinetic parameters were included into the list of possible sources of uncertainties. Sets of input data with statistical variation of the relevant parameter values were generated for a large number of runs of the coupled code.

The SUSA package was used to make a statistical analysis of the result parameters from the output data of the calculations. Time-dependent rank correlation coefficients were calculated showing the influence of the varied parameters on the output parameter under investigation. The most interesting output parameters are the physical parameters for which experimental data are available. The calculation results allowed also the determination of time-dependent tolerance intervals for given coverage and confidence. The comparison of the experimental data, the (best-estimate) reference solution and the tolerance intervals showed how the agreement between experiment and calculation could be quantified. In most of the cases the tolerance intervals include the experimental curves. A compiled list of the most important input parameters based on the rank correlation coefficients shows, which input parameters and models are responsible for the deviations. This list gives indications for further model improvements and code developments.
  • Lecture (Conference)
    4th International Conference: Safety Assurance of Nuclear Power Plants with WWER", 23.-26.05.2005, Podolsk, Russia
  • Contribution to proceedings
    4th International Conference: Safety Assurance of Nuclear Power Plants with WWER", 23.-26.05.2005, Podolsk, Russia
    Proceedings CD-ROM

Permalink: https://www.hzdr.de/publications/Publ-7263
Publ.-Id: 7263


Fluid mixing and flow distribution in the reactor circuit (FLOMIX-R)

Rohde, U.; Höhne, T.; Kliem, S.; Hemström, B.; Scheuerer, M.; Dury, T.; Remis, J.; Muhlbauer, P.; Toth, I.; Elter, J.; Bezrukov, Y.; Toppila, T.; Lillington, J.
The coolant mixing in PWR is an issue relevant for reactor safety, but of importance also for the optimisation of normal reactor operation. In the case of boron dilution accidents, the coolant mixing is the only effective mitigative mechanism against severe consequences of the accident. The degree of mixing determines the thermal and, therefore, mechanical load of the reactor pressure vessel wall.

Within the European project FLOMIX-R, an unique experimental data base on coolant mixing in PWRs has been created. Experiments on coolant mixing have been performed at three European mixing test facilities representing different types of reactors. Additionally, slug mixing data from the FSUE EDO Gidropress VVER-1000 mock-up have been made available. Mixing under steady-state flow conditions, slug mixing during the start-up of the first main coolant pump and mixing of cold emergency core cooling (ECC) water have been investigated. Moreover, measurement data from a real VVER-440 reactor plant (NPP Paks in Hungary) have been made available.

The measurement data base was used for the validation of computational fluid dynamics (CFD) codes. Calculations were performed for selected tests from the data base using the CFD codes CFX and FLUENT. For quality assurance in the CFD code validation, so-called Best Practice Guidelines (BPG) have been applied. The BPG require a minimization of numerical errors and solution errors by systematic grid and time step refinement and sensitivity tests on the impact of uncertainties in the boundary conditions, before the effect of different physical models can be assessed. The applicability of various turbulence modeling techniques was studied for transient and steady state flow.

The paper gives on overview on the experimental data base and on first, preliminary conclusions from the CFD code validation.
Keywords: pressurised water reactor, boron dilution, turbulent mixing, measurement data, computational fluid dynamics, code validation, best practice guidelines
  • Lecture (Conference)
    The 4-th International Conference "Safety Assurance of NPP with WWER", 23.-25.05.2005, Podolsk, Russia
  • Contribution to proceedings
    4th Scientific and Technical Conference "Safety Assurance of NPP with WWER", 23.-26.05.2005, Podolsk, Russia

Permalink: https://www.hzdr.de/publications/Publ-7262
Publ.-Id: 7262


Measurement data base on fluid mixing and flow distribution in the reactor circuit

Rohde, U.; Kliem, S.; Höhne, T.; Prasser, H.-M.; Hemström, B.; Toppila, T.; Elter, J.; Bezrukov, Y.; Scheuerer, M.
Experimental investigations on coolant mixing in Pressurised Water Reactors (PWR) have been performed within the EC project FLOMIX-R. The project was aimed at describing the mixing phenomena relevant for both safety analysis, particularly in steam line break and boron dilution scenarios, and mixing phenomena of interest for economical operation and the structural integrity. Measurement data from a set of mixing experiments have been gained by using advanced measurement techniques with enhanced resolution in time and space. Slug mixing tests simulating the start-up of the first main circulation pump are performed with three 1:5 scaled facilities: the Rossendorf Coolant Mixing model ROCOM, the Vattenfall test facility and a metal mock-up of VVER-1000 type reactor at EDO Gidropress. Experimental results on buoyancy driven mixing of fluids with density differences have been obtained at ROCOM and the Fortum PTS test facility. In generic experiments with injection of water with higher density performed at ROCOM, transition between momentum driven mixing as it is typical for pump start-up scenarios, and buoyancy driven mixing was found. The Froude number was identified as a proper transition criterion. Measurement data available from NPP Paks VVER-440 type reactor commissioning tests together with data from the ROCOM facility are used as a basis for the flow distribution studies. Alltogether, a unique data base has been created to be used for the validation of Computational Fluid Dynamics (CFD) codes for the application to turbulent mixing in nuclear reactors.
Keywords: turbulent mixing, boron dilution, pre-stressed thermal shock, computational fluid dynamics, numerical simulation, measurement data base, pressurised water reactor
  • Lecture (Conference)
    The 11th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics (NURETH-11), 02.-06.10.2005, Avignon, France
  • Contribution to proceedings
    11th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, 02.-06.10.2005, Avignon, France, 2-9516195-0-2

Permalink: https://www.hzdr.de/publications/Publ-7261
Publ.-Id: 7261


Interaction of Actinides with the Predominant Indigenous Bacteria in Äspö Aquifer - Interactions of Selected Actinides U(VI), Cm(III), Np(V) and Pu(VI) with Desulfovibrio äspöensis

Moll, H.; Merroun, M.; Stumpf, T.; Rossberg, A.; Geipel, G.; Selenska-Pobell, S.; Bernhard, G.
Sulfate-reducing bacteria (SRB) frequently occur in the deep granitic rock aquifers at the Äspö Hard Rock Laboratory (Äspö HRL), Sweden. The new SRB strain Desulfovibrio äspöensis could be iso-lated. The objective of this project was to explore the basic interaction mechanisms of uranium, curium, neptunium and plutonium with cells of D. äspöensis DSM 10631T.
The cells of D. äspöensis were successfully cultivated under anaerobic conditions as well in an optimized bicarbonate-buffered mineral medium as on solid medium at 22 °C. To study the interaction of D. äspöensis with the actinides, the cells were grown to the mid-exponential phase (four days). The collected biomass was usually 1.0±0.2 gdry weight/L. The purity of the used bacterial cultures was verified using microscopic techniques and by applying the Amplified Ribosomal DNA Restriction Enzyme Analysis (ARDREA).
The interaction experiments with the actinides showed that the cells are able to remove all four actinides from the surrounding solution. The amount of removed actinide and the interaction mechanism varied among the different actinides.
The main U(VI) removal occurred after the first 24 h. The contact time, pH and [U(VI)]initial influence the U removal efficiency. The presence of uranium caused a damaging of the cell membranes. TEM revealed an accumulation of U inside the bacterial cell. D. äspöensis are able to form U(IV). A complex interaction mechanism takes place consisting of biosorption, bioreduction and bioaccumulation.
Neptunium interacts in a similar way. The experimental findings are indicating a stronger interaction with uranium compared to neptunium.
The results obtained with 242Pu indicate the ability of the cells of D. äspöensis to accumulate and to reduce Pu(VI) from a solution containing Pu(VI) and Pu(IV)-polymers.
In the case of curium at a much lower metal concentration of 3x10-7 M, a pure biosorption of Cm(III) on the cell envelope forming an inner-sphere surface complex most likely with organic phosphate groups was detected.
To summarize, the strength of the interaction of D. äspöensis with the selected actinides at pH 5 and actinide concentrations ≥ 10 mg/L ([Cm] 0.07 mg/L) follows the pattern: Cm > U > Pu >> Np.
Keywords: Uranium, Neptunium, Curium, Plutonium, Bacteria, Desulfovibrio äspöensis, TRLFS, LIPAS, XAS, XANES, EXAFS, TEM/EDX, Microbial Interactions, Surface Complexation, Biosorption, Bioreduction, Bioaccumulation, Microbiology, Cultivation, Liquid-liquid Extraction, Absorptionsspectroscopy
  • Open Access Logo Wissenschaftlich-Technische Berichte / Forschungszentrum Rossendorf; FZR-422 2005
  • Other report
    Dresden: Final Report, BMWA Project No. 02 E9491, 2005
    60 Seiten

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Permalink: https://www.hzdr.de/publications/Publ-7260
Publ.-Id: 7260


Annual Report 2004 - Institute of Safety Research

Weiss, F. P.; Rindelhardt, U.; (Editors)
  • Open Access Logo Wissenschaftlich-Technische Berichte / Forschungszentrum Rossendorf; FZR-420 2005

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Permalink: https://www.hzdr.de/publications/Publ-7258
Publ.-Id: 7258


Annual report 2004 - Institute of Radiochemistry

Bernhard, G.; Foerstendorf, H.; Mibus, J.; Richter, A.; Ulrich, K.-U.; (Editors)
  • Open Access Logo Wissenschaftlich-Technische Berichte / Forschungszentrum Rossendorf; FZR-419 2005

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Permalink: https://www.hzdr.de/publications/Publ-7257
Publ.-Id: 7257


A simple mean-field model of polarity reversals

Stefani, F.; Gerbeth, G.
The Earth magnetic field undergoes polarity reversals with a mean reversal rate that varies from zero during the supercrons to 4-5 per Myr in the present. Typically, these reversals have an asymmetric, saw-toothed shape. Recently, a bimodal distribution of the dipole moment has been observed with two peaks at about 4 x 10^22 (Am)^2 and at about twice that value. In an attempt to identify the basic mechanism of such reversals, we study a mean-field dynamo model with a spherically symmetric helical turbulence parameter alpha which is quenched by the magnetic energy and disturbed by additional noise. The basic features of geomagnetic polarity reversals are shown to be generic consequences of the dynamo action in the vicinity of branching points of the spectrum of the dynamo operator where two real eigenvalues coalesce and continue as complex conjugated pair of eigenvalues. The model yields long periods of constant polarity which are interrupted by asymmtric polarity reversals. In certain parameter regions, it exhibits a bimodal field distribution, and it gives a natural explanation of the correlation between polarity persistence time and field strength. Typical features of coherence resonance are identified in the dependence of the polarity persistence time on the noise.
  • Lecture (Conference)
    EGU General Assembly 2005, 24.-29.04.2005, Wien, Austria
  • Open Access Logo Abstract in refereed journal
    Geophysical Research Abstracts 7(2005), 07307

Permalink: https://www.hzdr.de/publications/Publ-7256
Publ.-Id: 7256


Modellierung von Luftblasen als Rotationsellipsoide anhand paarweiser ebener Projektionen

Hoppe, D.; Zaruba, A.; Prasser, H.-M.
Eine in ruhendem Wasser aufsteigende Luftblase wird parallel zur Wasseroberfläche in zwei zueinander senkrechte horizontale Richtungen parallel projiziert. Diese beiden Projektionen werden zunächst als Ellipsen approximiert. Anhand der Ellipsen wird die Blase dann als Rotationsellipsoid modelliert. Das Rotationsellipsoid eignet sich zur Analyse des Verhaltens von Blasen auf ihrem Weg durch das Wasser. Dies wird an einem Ensemble schraubenartig aufsteigender Blasen verdeutlicht.

An air bubble rising in resting water is imaged by a parallel projection in two perpendicular horizontal directions. Initially, these two projections are approximated by ellipses. In the next step, the bubble is modelled as a spheroid matching the ellipses. The resulting spheroid is suitable to analyse the orientation of the symmetry axis of the bubble during its propagation through the liquid phase. The capabilities of the method is illustrated on an ensemble of bubbles rising on a helical trajectory.
Keywords: Luftblase, Ellipsenfit, Rotationsellipsoid Bubble, Ellipsenfit, Spheroid
  • Technisches Messen 2(2006), 103-108

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Publ.-Id: 7255


Characterization of a SiC/SiC composite by X-ray diffraction, atomic force microscopy and positron spectroscopies

Brauer, G.; Anwand, W.; Eichhorn, F.; Skorupa, W.; Hofer, C.; Teichert, C.; Kuriplach, J.; Cizek, J.; Prochazka, I.; Coleman, P. G.; Nozawa, T.; Kohyama, A.
A SiC/SiC composite is characterized by X-ray diffraction, atomic force microscopy, and various positron spectroscopies (slow positron implantation, positron lifetime, re-emission). It is found that beside its main constituent 3C-SiC the composite still must contain some graphite. In order to better interpret the experimental findings of the composite, a pyrolytic graphite sample was also investigated by slow positron implantation and positron lifetime spectroscopies. In addition, theoretical calculations of positron properties of graphite are presented.
Keywords: SiC/SiC composite, graphite, X-ray diffraction, atomic force microscopy, slow-positron spectroscopy, positron lifetime, positron affinity, positron re-emission
  • Invited lecture (Conferences)
    10th International conference on slow positron beam techniques for solids and surfaces, 19.-25.03.2005, Doha, Qatar
  • Applied Surface Science 252(2006), 3342-3351

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Publ.-Id: 7254


Evidence of a low quartz structure at the SiO2/Si interface from coincidence Doppler broadening spectroscopy

Brauer, G.; Becvar, F.; Anwand, W.; Skorupa, W.
Results from coincidence Doppler broadening measurements on various Si samples and Brazilian quartz having low quartz structure are presented with the aim to give further strong evidence for the existence of a low quartz structure, but not Si divacancies, at the SiO2/Si interface.
Keywords: slow-positron spectroscopy, coincidence Doppler broadening, silicon, Brazilian quartz, silicon divacancy, SiO2/Si interface
  • Lecture (Conference)
    10th International conference on slow positron beam techniques for solids and surfaces, 19.-25.03.2005, Doha, Qatar
  • Applied Surface Science (2007)

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Publ.-Id: 7253


Three-dimensional modeling of EXAFS spectral mixtures by combining Monte Carlo Simulations and Target Transformation Factor Analysis

Rossberg, A.; Scheinost, A. C.
We developed a new method for the three-dimensional modeling of Extended X-ray Absorption Fine Structure (EXAFS) spectra, which is suited to extract the local structure of aqueous metal complexes from spectral mixtures of several components. The new method combines two techniques: Monte Carlo simulation and Target Transformation Factor Analysis (TFA). Monte Carlo simulation is used to create random arrangements between the X-ray absorbing metal ion and the ligand atoms, and to calculate the theoretical EXAFS spectrum of each arrangement. The theoretical EXAFS spectrum is then introduced as test spectrum in the TFA procedure, in order to test whether the test spectrum is likely a component of the spectral mixtures or not. This coupled procedure is repeated, until the error in the test spectrum is minimized. The new method is thus able to isolate and refine the structure of complexes from spectral mixtures and to determine their relative concentrations, based solely on an estimate of the ligand structure. The performance of the proposed method was validated using uranium LIII-edge EXAFS spectra of binary mixtures of two uranium(VI) 3,4-dihydoxy benzoic acid complexes.
Keywords: EXAFS, Complexation, Speciation, Factor Analysis, Monte Carlo

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Publ.-Id: 7252


Superconductivity at Ultralow Temperatures and its Interplay with Nuclear Magnetism

Herrmannsdörfer, T.; Pobell, F.
In this article we will describe experiments at microkelvin temperatures which were performed to give - at least partly - answers to the following questions:

a. will all nonmagnetic metals become superconducting if refrigerated to low enough temperatures,
b. what is the impact of the weakest version of magnetism, nuclear magnetism, on superconductivity.

a. The first question has intrigued low temperature physicists since the discovery of superconductivity by H. Kamerlingh-Onnes in 1911. He wrote already in 1913 “There is left little doubt that if Au and Pt could be obtained absolutely pure, they could also pass into the superconducting state at helium temperatures”. And in 1929 W. Meissner wrote “At present it does not seem unlikely that, opposite to former expectations, all metals will become superconducting at low enough temperature”. One can pose the question in more general terms: “Is the electron-phonon interaction or another possible pairing mechanism in all metals strong enough so that they will eventually become superconducting, if this transition is not hindered by other phenomena, like magnetic properties?” After all, somehow the conduction electrons have to get rid of their entropy when approaching absolute zero, and one way is a transition to a superconducting state.

Looking at the periodic system of the elements, one realizes that superconductivity is rather the rule than the exception. Most metallic elements become superconducting if they show no magnetic order like some 3d- and 4f-elements. Even most insulators, like S or O, if forced into a metallic state by high pressure, eventually enter the superconducting state. When we started our research, there were only two small “islands” in the periodic system of the elements where metals had shown neither a superconducting nor a magnetic transition: some alkali and alkaline-earth metals and the noble and platinum metals (Cu, Ag, Au, Rh, Pd, Pt).

b. Superconductivity in its simplest version - s-wave, singlet pairing of conduction electrons mediated by electron-phonon interaction - is counteracted by magnetic interactions. This has been well demonstrated by the depression of the superconducting transition temperature when magnetic impurities are introduced. The large variety of “magnetic interactions” has varying impacts on superconductivity. The clearest demonstration of its detrimental impact is the vanishing of the superconducting state when a superconducting metal enters a ferromagnetic state. This was demonstrated in 1977 by Matthias et al. for ErRh4B4 and by Ishikawa and Fischer for HoMo8S8.

The weakest known version of magnetism is caused by the interaction of nuclear magnetic moments. Hence, it was a natural question to investigate the impact of nuclear ferromagnetism on superconductivity. This investigation became possible when we had observed a nuclear ferromagnetic transition of the superconductor AuIn2 at 35 µK in 1994. The results of the investigation of the interplay between nuclear ferromagnetism and superconductivity in AuIn2 - some of it are not yet understood - will be described in Sect. 5. Further investigations of the impact of nuclear paramagnetism in AuAl2, Al, Sn, AuIn2, In, Rh, as well as TiH2+x on superconductivity are discussed in Sect. 6. In Sect. 7, a first study of the interplay of hyperfine enhanced nuclear magnetism and superconductivity is presented. Eventually, in Sect. 8, we will summarize our results.

The experiments described in this article have been performed at the Forschungszentrum (formerly: Kernforschungsanlage) Jülich and at the University of Bayreuth
  • Contribution to external collection
    in: Frontiers in Superconducting Materials, Heidelberg: Springer Verlag, 2005, 71

Permalink: https://www.hzdr.de/publications/Publ-7251
Publ.-Id: 7251


High-intensity terahertz radiation from a microstructured large-area photoconductor

Dreyhaupt, A.; Winnerl, S.; Dekorsy, T.; Helm, M.
We present a planar large-area photoconducting emitter for impulsive generation of terahertz (THz) radiation. The device consists of an interdigitated electrode metal-semiconductor-metal (MSM) structure which is masked by a second metallization layer isolated from the MSM electrodes. The second layer blocks optical excitation in every second period of the MSM finger structure. Hence charge carriers are excited only in those periods of the MSM structure which exhibit a unidirectional electric field. Constructive interference of the THz emission from accelerated carriers leads to THz electric field amplitudes up to 85 V/cm when excited with fs optical pulses from a Ti:sapphire oscillator with an average power of 100 mW at a bias voltage of 65 V applied to the MSM structure. The proposed device structure has a large potential for large-area high-power THz emitters.

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Permalink: https://www.hzdr.de/publications/Publ-7250
Publ.-Id: 7250


11B-NMR studies of weakly ferromagnetic BaB6

Mushkolaj, S.; Gavilano, J. L.; Rau, D.; Ott, H. R.; Bianchi, A.; Fisk, Z.
BaB6 is a weakly ferromagnetic material with a Curie temperature TC well above room temperature. From the results of d.c. magnetization measurements on single crystalline BaB6, the saturation magnetization at low temperatures is 8\times 10-4(\mu B/f.u.), in line with other weak ferromagnets of the hexaboride series. The 11B-NMR spectra measured on a collection of single crystals of BaB6 yield a quadrupolar frequency of 472 KHz, in good agreement with calculated field gradients for this type of materials. The central 11B-NMR transition consists of two partially resolved signals, where the frequency displacement between them is of the order of 10 KHz. One of the signals exhibits a positive, the other a negative frequency shift, both of the order of 50 ppm. Between 7 K and room temperature these shifts do not vary with temperature. The temperature dependence of the spin-lattice relaxation rate T1-1(T) at the B sites is similar to that of other alkaline-earth hexaborides.
  • Open Access Logo Acta Physica Polonica B 34(2003)22, 1537-1537

Permalink: https://www.hzdr.de/publications/Publ-7249
Publ.-Id: 7249


NMR studies of YbB6

Gavilano, J. L.; Mushkolaj, S.; Rau, D.; Ott, H. R.; Bianchi, A.; Fisk, Z.
We report results of DC-magnetization and 11B-NMR measurements on single crystalline YbB6. The magnetization data at temperatures between 4 and 300 K reveal weak ferromagnetic order with a TC>300 K. It involves very small ordered moments, of the order of 0.002 µB/f.u., representing only a small fraction of the effective paramagnetic moment per formula unit that is indicated by the magnetic susceptibility. The latter can be accounted for by assuming that 2% of all the Yb atoms adopt the Yb+3 configuration. Since almost all the Yb ions adopt the divalent configuration one expects YbB6 to be a poor metal.
Keywords: NMR; Magnetism; Hexaborides
  • Physica B 329-333(2003)Part II, 570-571

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Permalink: https://www.hzdr.de/publications/Publ-7248
Publ.-Id: 7248


Atomistic simulation of defects in Si: migration of di- and tri-interstitials

Posselt, M.
The state-of-the-art interpretation of physical processes during post-implantation annealing, such as defect evolution, transient-enhanced dopant diffusion and dopant activation, assumes that ion implantation produces only single vacancies and self-interstitials, and that these are the only mobile intrinsic defects. Theoretical investigations show that both assumptions may be not correct. The talk presents results of comprehensive atomistic simulations on the properties of di- and tri-interstitials. It is focused on the migration of these defects and on the atomic mechanisms of the defect diffusion. The results of the atomistic simulations are compared with experimental data. The fact that the simulations predict a high di-interstitial mobility may lead to a re-interpretation of some experimental results.
Keywords: intrinsic defects, silicon, computer simulation
  • Lecture (others)
    43. Arbeitskreis Punktdefekte, 02.-03.03.2005, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-7247
Publ.-Id: 7247


Low-temperature thermal conductivity of CaB6 and EuB6

Gianno, K.; Sologubenko, A. V.; Ott, H. R.; Bianchi, A. D.; Fisk, Z.
The thermal conductivities of stoichiometric CaB6, vacancy-doped Ca1-B6, and EuB6have been measured between 6 and 300 K. All our data may be rather well described across the entire temperature regime covered on the basis of a Debye-type relaxation-time approximation and by assuming the concurring influence of various scattering channels on the mean free path of the phonons. An unusual and strong resonance in the scattering rate of the phonons of all investigated materials is attributed to a strong interaction between acoustic itinerant and localized modes, the latter arising from oscillations of the metal cations around their equilibrium position.
  • Journal of Physics: Condensed Matter 15(2003)40, 6739-6748

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Publ.-Id: 7246


Possible Fulde-Ferrell-Larkin-Ovchinnikov superconducting state in CeCoIn5

Bianchi, A.; Movshovich, R.; Capan, C.; Pagliuso, P. G.; Sarrao, J. L.
We report observation of the specific heat anomaly within the superconducting state of the heavy fermion CeCoIn5. It appears in the vicinity of the superconducting critical field Hc2, where the superconducting transition changes from second to first order, above 10 T for H || [1 1 0] and H || [1 0 0], and above 4.7 T for H || [0 0 1], and at temperatures on the order of 0.1Tc. We interpret the anomaly within the superconducting state as a signature of a Fulde–Ferrell–Larkin–Ovchinnikov FFLO inhomogeneous superconducting state.
Keywords: FFLO; Heavy fermion; Superconductivity; Unconventional; Inhomogeneous superconducting state

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Permalink: https://www.hzdr.de/publications/Publ-7245
Publ.-Id: 7245


FFLO superconducting state in CeCoIn5

Bianchi, A.; Movshovich, R.; Capan, C.; Lacerda, A.; Pagliuso, P. G.; Sarrao, J. L.
We present specific heat data for the heavy fermion superconductor CeCoIn5 close to the upper critical field Hc2 for magnetic fields applied along the [1 0 0] crystallographic axis. For fields above 10 T, the superconducting phase transition becomes first-order. In the same field range, we observe a second specific heat anomaly within the superconducting state, which we attribute to a Fulde–Ferrell–Larkin–Ovchinnikov state.
Keywords: Heavy fermion superconductors; Superconductivity phase diagrams; Magnetic field-dependent specific heat
  • Journal of Magnetism and Magnetic Materials 272-76(2004), 140-141

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Permalink: https://www.hzdr.de/publications/Publ-7244
Publ.-Id: 7244


Electron-spin domains - Magnetic enhancement of superconductivity

Movshovich, R.; Bianchi, A.; Capan, C.; Jaime, M.; Goodrich, R. G.
Magnetic enhancement of superconductivity
  • Nature (2004)427, 802-802

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Publ.-Id: 7243


First-order transition from a Kondo insulator to a ferromagnetic metal in single crystalline FeSi1-xGex

Yeo, S.; Nakatsuji, S.; Bianchi, A. D.; Schlottmann, P.; Fisk, Z.; Balicas, L.; Stampe, P.; Kennedy, R. J.
The phase diagram of FeSi1-xGex, obtained from magnetic, thermal, and transport measurements on single crystals, shows a discontinuous transition from Kondo insulator to ferromagnetic metal with x at a critical concentration, xc ≈ 0.25. The gap of the insulating phase strongly decreases with x. The specific heat γ coefficient appears to track the density of states of a Kondo insulator. The phase diagram is consistent with an insulator-metal transition induced by a reduction of the hybridization with x in conjunction with disorder on the Si=Ge ligand site
  • Physical Review Letters 91(2003)046401, 1-4
    DOI: 10.1103

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Publ.-Id: 7242


Possible Fulde-Ferrell-Larkin-Ovchinnikov superconducting state in CeCoIn5

Bianchi, A.; Movshovich, R.; Capan, C.; Pagliuso, P. G.; Sarrao, J. L.
We report specific heat measurements of the heavy fermion superconductor CeCoIn5 in the vicinity of
the superconducting critical field Hc2, with magnetic fields in the [110], [100], and [001] directions, and at temperatures down to 50 mK. The superconducting phase transition changes from second to first
order for fields above 10 T for H ║ [110] and H ║ [100]. In the same range of magnetic fields, we observe
a second specific heat anomaly within the superconducting state. We interpret this anomaly as a
signature of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting state. We obtain similar results for H ║ [100], with the FFLO state occupying a smaller part of the phase
diagram.

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Publ.-Id: 7241


Avoided antiferromagnetic order and quantum critical point in CeCoIn5

Bianchi, A.; Movshovich, R.; Vekhter, I.; Pagliuso, P. G.; Sarrao, J. L.
We measured the specific heat and resistivity of heavy fermion CeCoIn5 between the superconducting critical field c2 = 5 T and 9 T, with the field in the [001] direction, and at temperatures down to 50 mK. At 5 T the data show a non-Fermi liquid (NFL) behavior down to the lowest temperatures. At the field above 8 T the data exhibit a crossover from the Fermi liquid to a non-Fermi liquid behavior. We analyzed the scaling properties of the specific heat and compared both the resistivity and the specific heat with the predictions of a spin-fluctuation theory. Our analysis leads us to suggest that the NFL behavior is due to incipient antiferromagnetism (AFM) in CeCoIn5 with the quantum critical point in the vicinity of Hc2. Below Hc2 the AFM phase which competes with the paramagnetic ground state is superseded by the superconducting transition.
  • Physical Review Letters 91(2003)257001, 1-4
    DOI: 10.1103

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Publ.-Id: 7240


Magneto-optical Kerr effect in Eu1-xCaxB6

Caimi, G.; Broderick, S.; Ott, H. R.; Degiorgi, L.; Bianchi, A. D.; Fisk, Z.
We have measured the magneto-optical Kerr rotation of ferromagnetic Eu1-xCaxB6 with x=0.2 and 0.4, as well as of YbB6 serving as the nonmagnetic reference material. As previously for EuB6, we could identify a feature at 1 eV in the Kerr response which is related with electronic transitions involving the localized 4 f electron states. The absence of this feature in the data for YbB6 confirms the relevance of the partially occupied 4 f states in shaping the magneto-optical features of Eu-based hexaborides. Disorder by Ca-doping broadens the itinerant charge carrier contribution to the magneto-optical spectra.
  • Physical Review B 69(2004)012406, 1-4
    DOI: 10.1103

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Publ.-Id: 7239


Optical evidence for a spin-filter effect in the charge transport of Eu0.6Ca0.4B6

Perucchi, A.; Caimi, G.; Ott, H. R.; Degiorgi, L.; Bianchi, A. D.; Fisk, Z.
We have measured the optical reflectivity R(ω) of Eu0.6Ca0.4B6 as a function of temperature (T) between 1.5 and 300 K and in external magnetic fields (H) up to 7 T. R(ω) increases with decreasing T and increasing H field, but the plasma edge feature does not exhibit the sharp onset and steep slope that is observed in EuB6. The analysis of the H-field dependence of the low-T optical conductivity confirms the previously observed exponential decrease of the electrical resistivity upon increasing bulk magnetization at constant T. The individual exponential magnetization dependences of the plasma frequency and scattering rate are also extracted from the optical data.
  • Physical Review Letters 92(2004)067401, 1-4
    DOI: 10.1103

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Publ.-Id: 7238


Weak ferromagnetism in CaB6

Bennett, M. C.; van Lierop, J.; Berkeley, E. M.; Mansfield, J. F.; Henderson, C.; Aronson, M. C.; Young, D. P.; Bianchi, A.; Fisk, Z.; Balakirev, F.; Lacerda, A.
We have combined the results of magnetization and Hall effect measurements to conclude that the ferromagnetic moments of lightly doped CaB6 samples display no systematic variation with electron doping level.
Removal of the surface with acid etching substantially reduces the measured moment, although the Hall constant and resistivity are unaffected, indicating that the ferromagnetism largely resides on and near the sample surface. Electron microprobe experiments reveal that Fe and Ni are found at the edges of facets and growth steps, and on other surface features introduced during growth. Our results indicate that the weak ferromagnetism previously reported in undoped CaB6 is most likely of extrinsic origin.
  • Physical Review B 69(2004)132407, 1-4
    DOI: 10.1103

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Publ.-Id: 7237


Transport and thermodynamic properties of Sr3Ru2O7 near the quantum critical point

Zhou, Z. X.; Mccall, S.; Alexander, C. S.; Crow, J. E.; Schlottmann, P.; Bianchi, A.; Capan, C.; Movshovich, R.; Kim, K. H.; Jaime, M.; Harrison, N.; Haas, M. K.; Cava, R. J.; Cao, G.
The specific heat and electrical resistivity of Sr3Ru2O7 single crystals are measured in several magnetic
fields applied along the c axis for temperatures below 2 K and at fields up to 17 T. Near the critical metamagnetic
field at B1*~7.8 T, the electronic specific heat divided by temperature increases logarithmically as the
temperature decreases, over a large range of T, before saturating below a certain T* (which is sample dependent),
indicating a crossover from a non-Fermi liquid (NFL) region dominated by quantum critical fluctuations
to a Fermi liquid (FL) region. This crossover from a NFL to a FL state is also observed in the resistivity data
near the critical metamagnetic field for Ic and Bc. The coefficient of electronic specific heat, γ, plotted as a
function of field shows two peaks, consistent with the two metamagnetic transitions observed in magnetization
and magnetic torque measurements. At the lowest temperatures, a Schottky-like upturn with decreasing temperature
is observed. The coefficient of the Schottky anomaly exhibits a field dependence similar to that of γ,
implying an influence by the electrons near the Fermi surface on the Schottky level splitting.
  • Physical Review B 69(2004)140409, 1-4
    DOI: 10.1103

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Publ.-Id: 7236


Anisotropy of Thermal Conductivity and Possible Signature of the Fulde-Ferrell-Larkin-Ovchinnikov state in CeCoIn5

Capan, C.; Bianchi, A.; Movshovich, R.; Christianson, A. D.; Malinowski, A.; Hundley, M. F.; Lacerda, A.; Pagliuso, P. G.; Sarrao, J. L.
We have measured the thermal conductivity of the heavy-fermion superconductor CeCoIn5 in the vicinity of the upper critical field, with the magnetic field perpendicular to the c axis. Thermal conductivity displays a discontinuous jump at the superconducting phase boundary below critical temperature T0≈1 K, indicating a change from a second- to first-order transition and confirming the recent results of specific heat measurements on CeCoIn5. In addition, the thermal conductivity data as a function of field display a kink at a field Hk below the superconducting critical field, which closely coincides with the recently discovered anomaly in specific heat, tentatively identified with the appearance of the spatially inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state. Our results indicate that the thermal conductivity is enhanced within the FFLO state, and call for further theoretical investigations of the order parameter’s real-space structure (and, in particular, the structure of vortices) and of the thermal transport within the inhomogeneous FFLO state.
  • Physical Review B 70(2004)134513, 1-7
    DOI: 10.1103

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Publ.-Id: 7235


Percolation and the Colossal Magnetoresistance of Eu-Based Hexaboride

Wigger, G. A.; Beeli, C.; Felder, E.; Ott, H. R.; Bianchi, A. D.; Fisk, Z.
Upon substituting Ca for Eu in the local-moment ferromagnet EuB6, the Curie temperature TC
decreases substantially with increasing dilution of the magnetic sublattice and is completely suppressed
for x ≤0:3. The Ca substitution leads to significant changes of the electronic properties across the
EuxCa1-xB6 series. Electron microscopy data for x ≈ 0.27 indicate a phase separation into Eu- and
Ca-rich clusters of 5 to 10 nm diameter, leading to percolation-type phenomena in the electrical
transport properties. The related critical concentration xp is approximately 0.3. For x ≈ 0.27, we
observe colossal negative magnetoresistance effects at low temperatures, similar in magnitude as those
reported for manganese oxides.
  • Physical Review Letters 93(2004)147203, 1-4
    DOI: 10.1103

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Publ.-Id: 7234


Non-Fermi Liquid behavior in CeIrI5 near a metamagnetic transition

Capan, C.; Bianchi, A.; Ronning, F.; Lacerda, A.; Thompson, J. D.; Pagliuso, P. G.; Sarrao, J. L.; Movshovich, R.
We present a specific heat and resistivity study of CeIrIn5 in magnetic fields up to 17 T and temperature down to 50 mK. Both quantities were measured with the magnetic field parallel to the c axis (H║[001]) and within the a-b plane (H┴[001]). Non-Fermi-liquid (NFL) behavior develops above 12 T for H║[001]. The Fermi-liquid state is much more robust for H║[001] and is suppressed only moderately at the highest applied field. Based on the observed trends and the proximity to a metamagnetic phase transition, which exists at fields above 25 T for H║[001], we suggest that the observed NFL behavior in CeIrIn5 is a consequence of a metamagnetic quantum critical point.
  • Physical Review B 70(2004)180502, 1-4
    DOI: 10.1103

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Publ.-Id: 7233


First Results from SHIP Experiment

Bagryansky, P. A.; Anikeev, A. V.; Ivanov, A. A.; Lizunov, A. A.; Maximov, V. V.; Murakhtin, S. V.; Stepanov, D. N.; Noack, K.; Prikhodko, V. V.; Solomakhin, A. L.
At present, the GDT facility of the Budker Institute of Nuclear Physics Novosibirsk is being upgraded. The first stage of the upgrade is the Synthesized Hot Ion Plasmoid (SHIP) experiment. It aims, on the one hand, at the investigation of plasmas the parameters of which are expected to appear in the region of high neutron production in a GDT based fusion neutron source as proposed by the Budker Institute and, on the other hand, at the investigation of plasmas the parameters of which have never been achieved before in magnetic mirrors.
The experiment is performed in a small mirror section which is installed at the end of one side of GDT. The magnetic field on axis is in the range of 0.5-20 Tesla and the mirror ratio is 1.2-1.4. The mirror is filled with background plasma streaming in from the central cell. This plasma component is maxwellized and has an electron temperature of about 100 eV. Two neutral beam injectors perpendicularly inject a total current of about 50 Atom Amperes of deuterium neutrals with an energy of 20 keV
as a pulse with a duration of about 1 ms. Ionization of the beams generates the high-energy ion component. The device has been equipped with several diagnostic methods which are successfully used in GDT experiments.
The paper presents first results of plasma parameter measurements in the SHIP experiment.
Keywords: gas dynamik trap, magnetic mirror, GDT facility, SHIP experiment, neutral beam injection, plasma diagnostic
  • Contribution to proceedings
    5th Int. Conf. on Open Magnetic Systems for Plasma Confinement,, 05.-09.07.2004, Novosibirsk, Russia
    Open Magnetic Systems for Plasma Confinement, Dexter, Michigan: Beljan Ltd.
  • Lecture (Conference)
    5th Int. Conference on Open Magnetic Systems for Plasma Confinement, 05.-09.07.2004, Novosibirsk, Russia
  • Fusion Science and Technology 47(2005)1T, 59-62

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Publ.-Id: 7229


Magnetic domain structure of micro-patterned PtMn/CoFe exchange bias bilayers

Liedke, M. O.; Potzger, K.; Hillebrands, B.; McCord, J.; Rickart, M.; Freitas, P.; Fassbender, J.ORC
Using magnetron sputter deposition a number of glass/Ta 7nm/PtMn 20nm/CoFe 4nm/Ta 4nm samples with large exchange bias field were prepared for magnetic pattering investigations. By means of optical lithography and physical etching several patterns with decreasing lateral sizes of either the elements or the spacing between the elements were prepared. The largest square is 50 x 50 µm2 and the smallest only 1 µm2. The separating lines range from 10 µm to 2 µm width. The magnetic characterization of the samples was done by VSM and MOKE. Kerr microscopy and MFM investigations in an applied magnetic field have been performed in order to get a deeper understanding of the domain pattern. All images show a monodomain magnetization state in zero magnetic field. The shape of the structure itself dose not influence the magnetization direction. The shape anisotropy contribution is thus smaller than the unidirectional anisotropy given by the exchange bias. In addition 5 keV He+ ion irradiation was used to decrease exchange bias field value and thereby modify the ratio between unidirectional and shape anisotropy. The magnetic domain structure is investigated as a function of this ratio.
Keywords: magnetism, exchange bias, magnetization reversal, magnetic force microscopy, Kerr microscopy, magnetic patterning
  • Poster
    The European Conference on Physics of Magnetism'05, 24.-27.06.2005, Poznan, Poland

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Publ.-Id: 7228


Velocity measurements and concentration field visualizations in natural convection copper electrolysis under magnetic field influence

Weier, T.; Hüller, J.; Gerbeth, G.; Weiss, F.-P.
The effect of magnetic fields on momentum and mass transfer in electrochemical processes has been studied by means of Digital Particle Image Velocimetry (DPIV), shadowgraphy and mean current
density measurements.

Chronoamperometric copper electrolysis was carried out in a small electrolytic cell (29x46x6 mm) made mainly from PMMA. The sidewalls forming the vertical electrodes consist of thin copper plates behind
which permanent magnets could be fixed. The Lorentz force generated from the faradaic currents and the permanent magnets field has been always parallel to the electrodes. Depending on the orientation of the magnets, downwards or upwards directed Lorentz forces could be generated.

The moderate magnetic field of permanent magnets placed behind the electrodes, although its action is limited to the vicinity of the electrodes, is able to promote convection in the whole cell. Flow
structures measured by DPIV compare very well with the patterns of the concentration field given by shadography. Steady state limiting current densities as well as initially instationary current density
values can be explained by the corresponding velocity measurements. It will be shown that the interplay of Lorentz and buoyancy forces is substantial for the resulting flow structure.
  • Lecture (Conference)
    Joint 15th Riga and 6th PAMIR International Conference on Fundamental and Applied MHD, 27.06.-01.07.2005, Riga, Latvia

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Publ.-Id: 7227


Status of the GDT Experiment and Future Plans

Abdrashitov, A.; Abdrashitov, G.; Anikeev, A.; Bagryansky, P.; Beklemishev, A.; Deichuli, P.; Ivanov, A.; Korepanov, S.; Maximov, V.; Murakhtin, S.; Lizunov, A.; Prikhodko, V.; Kapitonov, V.; Kolmogorov, V.; Khilchenko, A.; Mishagin, V.; Savkin, V.; Shoukajev, A.; Shulzhenko, G.; Solomakhin, A.; Sorokin, A.; Stepanov, D.; Stupishin, N.; Tsidulko, Y.; Zouev, A.; Noack, K.; Fiksel, G.; Den Hartog, D.
In recent experiments, on-axis transverse beta exceeding 0.4 in the fast ion turning points near the end mirrors has been achieved in the GDT experiment with 4 MW injection of 15-17 keV deuterium neutral beams at the center of the device. Neither enhanced transverse losses of the plasma nor anomalies in the fast ion scattering and slowing down were observed. The measured beta value is close to that needed in the versions of the GDT based 14 MeV neutron source. At the same time, the electron
temperature for given injection power and pulse duration is limited to 100-130 eV. Its further increase is planned after upgrading the injection system and increasing the magnetic field at the center of the device up to 0.3 T. The upgrade of the injection system assumes that the neutral beam power incident on the plasma will be increased up to 9-10 MW and the pulse duration is extended from 1.2 to 5 ms. According to the results
of numerical simulations, for the extended pulse duration
a plasma steady state will be achieved with electron temperature of 250-320 eV depending upon the assumptions on the transverse energy loss rate. Future experiments at the GDT upgrade are discussed in the paper.
Keywords: gasdynamic trap, GDT experiment, neutral beam injection, transverse plasma beta, fusion, neutron source
  • Lecture (Conference)
    5th Internationale Conference on Open Magnetic Systems for Plasma Confinement, 05.-09.07.2004, Novosibirsk, Russia
  • Contribution to proceedings
    5th Internationale Conference on Open Magnetic Systems for Plasma Confinement,, 05.-09.07.2004, Novosibirsk, Russia
    Status of the GDT Experiment and Future Plans, Dexter: Beljan Ltd.
  • Fusion Science and Technology 47(2005)1T, 27-34

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Publ.-Id: 7226


Irreversible magnetization deep in the vortex-liquid state of a 2D superconductor at high magnetic fields

Maniv, T.; Zhuravlev, V.; Wosnitza, J.; Hagel, J.
The remarkable phenomenon of weak magnetization hysteresis loops, observed recently deep in the vortex-liquid state of a nearly two-dimensional (:213) superconductor at low temperatures and high magnetic fields, is shown to reflect the existence of an unusual vortex-liquid state, consisting of collectively pinned crystallites of easily sliding vortex chains.
Keywords: Vortices, Lattices, Systems
  • Journal of Physics: Condensed Matter 16(2004)41, 429-435

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Publ.-Id: 7225


Fermi-surface reconstruction close to a pressure-induced metal-insulator transition

Wosnitza, J.; Hagel, J.; Stockert, O.; Pfleiderer, C.; Schlueter, J. A.; Mohtasham, J.; Gard, G. L.
We report on a detailed investigation of the pressure-dependent structural and electronic properties close to the pres-sure-induced metal-insulator transition of the quasi-two-dimensional organic superconductor beta"-(BEDT-TTF)2SF5CH2CF2SO3, where BEDT-TTF stand for bisethylenedithio-tetrathiafulvalene (or ET for short). Although the pressure-dependent hysteresis of the metal-insulator transition suggests a structural origin, no major crystallo-graphic modifications could be detected by neutron-scattering experiments. Shubnikov-de Haas (SdH) experiments, on the other hand, show that a reconstructed band structure precedes the phase transition to the insulating state. A new SdH frequency with a rather small effective mass appears.
Keywords: organic superconductors; band structure; metal-insulator transition
  • Journal de Physique IV 114(2004), 277-281

Permalink: https://www.hzdr.de/publications/Publ-7223
Publ.-Id: 7223


Transport properties of CeBiPt in magnetic fields up to 60 T

Wosnitza, J.; Hagel, J.; Kozlova, N.; Eckert, D.; Müller, K.-H.; Mielke, C. H.; Goll, G.; Yoshino, T.; Takabatake, T.
Electrical-transport measurements of the semimetal CeBiPt in magnetic fields up to 60 T reveal a drastic change of the electronic band structure. The oscillating Shubnikov-de Haas signal vanishes above about 25 T although the quantum limit is not yet reached. Above this field the magneto resistance rises strongly independent of angle and temperature. These unique features are caused by the Cc 4f electrons as evidenced by the absence of any unconventional behavior in the sister compound LaBiPt.
Keywords: Shubnikov-de Haas effect, Fermi-surface studies, magnetoresistance, CeBiPt, LaBiPt
  • Physica B 346-347(2004), 127-131

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Publ.-Id: 7222


Neutron-proton pairing in rotating N similar to Z nuclei: dominance of the isovector component

Afanasjev, A.; Frauendorf, S.
Theoretical calculations of rotating N approximate to Z nuclei with A = 58 - 80 within the cranked Nilsson+Strutinsky approach, cranked relativistic mean field and cranked relativistic Hartree+Bogoliubov theories show good agreement with experiment. They point on the presence of the isovector t = 1 np-pairing, but do not show any indications of the isoscalar t = 0 np-pairing.
  • Nuclear Physics A 746(2004), 575C-578C

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Publ.-Id: 7221


Shell stabilization and the survival of heavy compound nuclei

Heinz, A.; Schmidt, K.; Junghans, A.
Knowledge of the influence of nuclear shell structure on the survival probability of heavy compound nuclei against fission is important for a quantitative understanding of the production rates of spherical super-heavy elements (SHE). Fission probabilities of N = 126 isotones beyond astatine can be used as test cases for the production of spherical super-heavy elements, as those isotones possess a strong shell correction energy and are highly fissile. Here, we report on two new experimental approaches which probe the effect of the closed neutron shell at N = 126 on the competition between fission and particle evaporation using projectile fragmentation and electromagnetic-induced fission of radioactive beams. We conclude that these nuclei lose at least a great part of their stability against fission at low excitation energies and angular momenta-mostly due to the influence of collective contributions in the level density. Implications on the production of spherical SHE will be!
discussed.
  • Nuclear Physics A 746(2004), 483C-486C

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Publ.-Id: 7220


New determination of the Be-7(p,gamma)B-8 S-factor

Junghans, A.; Mohrmann, E.; Snover, K.; Et, A.
We present new measurements of the Be-7(p,gamma)B-8 cross section from (E) over bar (cm) = 116 to 2460 keV. Our new measurements lead to S-17(0) = 22.1 +/- 0.6(expt) +/- 0.6(theor) eV b based on data from (E) over bar (cm) = 116 to 362 keV, where the central value is based on the theory of Descouvemont and Baye. We compare our results to other S-17(0) values extracted from both direct (Be-7(p, gamma)B-8) and indirect (Coulomb dissociation and heavy-ion reaction) measurements, and show that the results of these 3 types of experiments are not mutually compatible. We recommend a "best" value, S-17(0) = 21.4 +/- 0.5(expt) +/- 0.6(theor) eV b, based on the mean of all modern direct measurements below the 1(+) resonance.
  • Nuclear Physics A 746(2004), 210C-214C

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Publ.-Id: 7219


Optical characterization of beta-FeSi2 layers formed by ion beam synthesis

Ayache, R.; Bouabellou, A.; Richter, E.
Thin beta-FeSi2 layers have been prepared by ion beam synthesis (IBS) on (111)Si substrates. The obtained samples have been characterized by means of infrared and Raman spectroscopies. The infrared (IR) transmittance spectra show the absorption at 310 cm(-1) as an indication of the initial nucleation of beta-FeSi2 precipitates during the implantation of iron into silicon substrate. The main feature of the photoluminescence (PL) measurements at 12 K in the beta-FeSi2/(111)Si samples annealed at 850 degreesC for 90 min is an intense peak localized at 0.811 eV. This peak is assigned to optical radiative transitions intrinsic to beta-FeSi2. (C) 2004 Elsevier Ltd. All rights reserved.
  • Materials Science in Semiconductor Processing 7(2004)4-6, 463-466

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Publ.-Id: 7218


Mesoscopic dots as collective terahertz oscillators

Metzner, C.; Stehr, D.
Electrons confined in a flat semiconductor quantum dot with a parabolic in-plane potential act like a collective many-particle oscillator under coherent intraband excitation. We investigate theoretically the properties of these oscillators under a simultaneous scale transformation of the lateral dimensions and the electron occupation number. As the lateral size increases from a few nm (typical for self-assembled dots) to the mesoscopic regime, the physics of the system is changing qualitatively: Quantization effects gradually lose importance against Coulomb interactions and eventually the electron lake in a mesoscopic dot resembles a classical Wigner liquid. This parabolically confined "Wigner lake" behaves to the outside like a form-elastic "superparticle" of high charge. It can be coherently controlled by THz dipole radiation just like a single electron, but with reduced Brownian diffusion in the phonon heat bath. We propose a flexible method to fabricate single mesoscopic!
dots of a controlled shape, Coulomb-coupled groups of dots, and almost arbitrary potential landscapes, using current semiconductor technology. As a first example, the collective modes of two Coulomb-coupled superparticles in neighboring dots are calculated. Also, we consider the possibility of steering a superparticle with shaped laser pulses to follow any complex two-dimensional orbit.
  • Physical Review B 7019(2004)19, 5433-5433

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Publ.-Id: 7217


Numerical investigation of transition control in low conductive fluids

Albrecht, T.; Grundmann, R.; Mutschke, G.; Gerbeth, G.
We investigate numerically the transition to turbulence in a flat-plate boundary layer controlled by electromagnetic forces. The fluid considered is incompressible, Newtonian and low conductive. Similar to boundary layer suction, when applying a steady, wall-parallel, and streamwise orientated Lorentz force, (as suggested by Gailitis and Lielausis [1] in the early 1960s) the Blasius velocity profile is transformed to an exponential one gaining a critical Reynolds number which is increased by two orders of magnitude.

Two and three dimensional direct numerical simulation (DNS) of both linear and nonlinear stages of the transition process were performed, as well as alinear stability analysis (LSA) of the calculated intermediate velocity profiles. DNS and preliminary LSA results confirm the expected increased stability of the controlled flow. Depending on Lorentz force strength transition to turbulence is delayed or even stopped. Suprisingly, both DNS and LSA results suggest interesting stability characteristics of the intermediate velocity profiles.

In DNS, to initiate transition, small amplitude disturbances are introduced by means of an oscillating body force within a small region near the inflow boundary, forming Tollmien-Schlichting waves (TSW) which grow and decay in uncontrolled case corresponging to linear stability theory. When applying Lorentz force, TSW of all investigated frequencies 0.4 <= F+ <= 3.75 are damped within the computational domain extending over 900 times the inflow displacement thickness d1i. Reynolds number, based on d1i, is 360. The decay rate based on the maximum rms value in wall-normal direction of the streamwise velocity component, is maximum in a region near the onset of control and decreases as the velocity profile approaches the exponential state. This observation could suggest that in the intermediate region there are profiles more stable than the exponential one, although we are aware that from these decay rates one cannot conclude directly the stability of a velocity profile, notably its critical reynolds number. However, our assumption is confirmed by preliminary LSA results where critical Reynolds numbers of intermediate profiles are found to be larger than for the exponential profile.

By three dimensional DNS we show that transition to turbulence can be stopped even in it's late stage. While the evolution of Lambda vorticies from former two dimensional TSW remains almost unchanged, the emerge of Omega vorticies is supressed with increasing Lorentz force strength, thus relaminarizing the flow.

[1] A. Gailitis, O. Lielausis: On a possibility to reduce the hydrodynamic resistance of a plate in an electrolyte. Applied Magnetohydrodynamics, Reports of the Physics Institute Riga, Vol. 12, pp. 143-146, 1961
Keywords: Fluid Mechanics, Magnetohydrodynamics, Stability, Transition, Control
  • Poster
    Joint 15th Riga and 6th PAMIR International Conference on Fundamental and Applied MHD, 27.06.-01.07.2005, Riga, Latvia
  • Contribution to proceedings
    Joint 15th Riga and 6th PAMIR International Conference on Fundamental and Applied MHD, 27.06.-01.07.2005, Riga, Latvia
    1(2005), 145-148

Permalink: https://www.hzdr.de/publications/Publ-7216
Publ.-Id: 7216


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