Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

"Online First" included
Without submitted and only approved publications
Only approved publications

31828 Publications
Molecular Hydrogen in Amorphous Silicon with High Internal Stress
Danesh, P.; Pantchev, B.; Schmidt, B.; Grambole, D.;
Hydrogenated amorphous silicon with high compressive stress and hydrogen concentration as high as the hydrogen solubility limit has been studied. The concentrations of total hydrogen and bonded hydrogen have been determined by nuclear reaction analysis and infrared transmission spectroscopy, respectively. The amount of molecular hydrogen has been estimated as a difference between these two concentrations. Silicon ion implantation has been used to transform the hydrogen molecules into silicon-bonded hydrogen. The sensitivity of the Fourier transform infrared spectrometer to the expected variations in the bonded hydrogen concentration has been proved using hydrogen implantation. It has been concluded that in spite of high hydrogen content in the material the concentration of molecular hydrogen in the studied films is below 1 at.% and is not the reason for the high internal stress.
Keywords: ion implantation, nuclear reaction analysis, infrared spectroscopy, amorphous silicon, molecular hydrogen

Publ.-Id: 10321 - Permalink


Fluorescence-Infrared-Cross-Correlation Spectroscopy: A Novel Approach to the Study of Domain Coupling in Proteins
Fahmy, K.;
Fluorescence-Infrared-Cross-Correlation Spectroscopy: A Novel Approach to the Study of Domain Coupling in Proteins
  • Invited lecture (Conferences)
    XIIth European Conference on the Spectroscopy of Biological Molecules 2007, 01.-06.09.2007, Paris, France

Publ.-Id: 10320 - Permalink


Proton-regulated side chain lipid interactions in conformational switching by transmembrane G-protein-coupled receptors
Madathil, S.;
Proton-regulated side chain lipid interactions in conformational switching by transmembrane G-protein-coupled receptors
  • Invited lecture (Conferences)
    51st Annual Meeting of the Biophysical Society 2007, 06.03.2007, Baltimore, United States
  • Invited lecture (Conferences)
    Rockefeller University, 08.03.2007, New York, United States

Publ.-Id: 10319 - Permalink


Molecular switching in GPCRs: FTIR- and fluorescence-Spectroscopic Identification of Functional Modules in Rhodopsin
Fahmy, K.;
Molecular switching in GPCRs: FTIR- and fluorescence-Spectroscopic Identification of Functional Modules in Rhodopsin
  • Lecture (Conference)
    Max-Planck-Institut für Molekulare Physiologie, 29.03.2007, Dortmund, Germany
  • Invited lecture (Conferences)
    Rockefeller University, 09.03.2007, New York, United States

Publ.-Id: 10318 - Permalink


Festkörperspektroskopie bei Terahertz-Frequenzen mit dem Freie-Elektronenlaser
Helm, M.;
Am Forschungszentrum Dresden-Rossendorf ist seit einigen Jahren ein Freie-Elektronenlaser (FEL) in Betrieb, der mittlerweile einen Wellenlängenbereich zwischen 3 und 200 Mikrometer (entsprechend Frequenzen zwischen 1.5 und 100 THz) überstreicht. Nach einer Beschreibung der Maschine und ihrer wesentlichen Eigenschaften und Parameter sollen einige der ersten Experimente diskutiert werden:
(1) Untersuchung der zeitaufgelösten Dynamik von Elektronen in Halbleiter-Übergittern mittels Pikosekunden pump-probe Spektroskopie.
(2) Nahfeldmikroskopie von ferroelektrischen Domänen in der Nähe von Phonon-Resonanzen.
Keywords: free electorn laser, terahertz, semiconductor superlattices, near field microscopy
  • Lecture (others)
    Phyikalisches Kolloquium an der Universität Konstanz (invited), 26.06.2007, Konstanz, Germany
  • Lecture (others)
    Physikalisches Kolloquium der Universität Linz (invited), 28.06.2007, Linz, Austria
  • Lecture (others)
    Physikalisches Kolloquium der Universität Leipzig (invited), 10.07.2007, Leipzig, Germany

Publ.-Id: 10317 - Permalink


Fundamentals and applications of ellipsometry
Vinnichenko, M.;
The short tutorial on fundamentals and applications of spectroscopic ellipsometry was given. The sensitivity of the technique in different configurations, problems of data acquisition and analysis have been addressed.
Keywords: Spectroscopic elipsometry
  • Lecture (others)
    Informal seminar at Centro de Micro-Analisis de Materiales (CMAM), 29.06.2007, Madrid, Spain

Publ.-Id: 10316 - Permalink


Implantation-caused open volume defects in Ge after Flash Lamp Annealing (FLA) probed by Slow Positron Implantation Spectroscopy (SPIS)
Anwand, W.; Skorupa, W.; Schumann, T.; Posselt, M.; Schmidt, B.; Grötzschel, R.; Brauer, G.;
B+ and P+ ions were implanted into Ge wafers covered with an amorphous surface layer of 150 nm. After this, FLA in Ar atmosphere was used as post-implantation heat treatment. Flash lamp radiation with Xenon flash lamps having a spectrum in the visible range of light and a pulse length of 3 or 20 ms allowed an ultra-short heating up of the near surface region. In this way, a modification of the structure of the amorphous layer containing the implanted range was possible. Depth profiles of defects, especially of vacancy-type, were investigated by SPIS before and after FLA.
It could be shown that the remaining vacancy-type defect structure depends on the parameters of the process of heat treatment, and that these defects could not be completely removed by FLA.
These results will be compared with such from SRIM 2006 calculations (Stopping and Range of Ions in Matter) and Rutherford Backscattering Spectrometry (RBS).
Keywords: Ge, ion implantation, flash lamp annealing, defects, slow positron implantation spectroscopy
  • Applied Surface Science 255(2008)1, 81-83
  • Poster
    11th Workshop on Positron Beam Techniques for Solids and Surfaces, 09.-13.07.2007, Orleans, France

Publ.-Id: 10315 - Permalink


Nuclear astrophysics with real photons
Nair, C.; Junghans, A. R.; Erhard, M.; Bemmerer, D.; Beyer, R.; Crespo, P.; Grosse, E.; Fauth, M.; Kosev, K.; Rusev, G.; Schilling, K.-D.; Schwengner, R.; Wagner, A.;
Majority of the light elements up to iron are formed by successive rounds of thermonuclear fusion burning in the stellar interiors. The nuclei heavier than iron (Z>26) are being synthesized mainly by neutron-capture reactions – the astrophysical r-and s-processes. But the 35 neutron deficient stable isotopes between Se and Hg, the so-called p-nuclei, are shielded from the rapid neutron capture by stable isobars [1]. The possible site for the formation of p-nuclei are astrophysical scenarios like supernova explosions and their production mechanism is understood as chains of photodisintegrations of the type (γ, n), (γ, p) or (γ, α) on heavy seed nuclei. The natural abundances of the p-nuclei are in the order of 0.01 – 1%. For the network calculations that yield the p-nuclei abundances, precise values of the relevant astrophysical reaction rates are necessary. The data used presently for p-process nucleosynthesis modelling are mainly based on theoretical cross sections obtained from Hauser-Feshbach statistical model calculations. In this context, the knowledge of the experimental cross sections of the p-nuclei is of crucial importance. We are focussing on the photo-dissociation studies of various medium-mass nuclei and our aim is to get more accurate experimental cross sections which will surely be able to improve the present astrophysical network calculations. To study the p-nuclei by the method of bremsstrahlung induced activation, the superconducting electron accelerator ELBE delivers beams up to 40 MeV energy with average currents up to 1mA. An overview on the experimental facilities for nuclear physics at the superconducting electron accelerator ELBE of Forschungszentrum Dresden-Rossendorf, Germany will be given . In particular, the photodissociation studies on the p-nuclei 92Mo and 144Sm will be discussed.
Keywords: thermonuclear fusion, neutron-deficient nucei, photodisintegration
  • Lecture (Conference)
    Frontiers and Perspectives of Nuclear and Hadron Physics (FPNH07), 11.-12.06.07, Tokyo, O-okayama, Japan

Publ.-Id: 10314 - Permalink


Photodisintegration studies on the p-nuclei 92Mo and 144Sm for the astrophysical p-process
Nair, C.; Junghans, A. R.; Erhard, M.; Bemmerer, D.; Beyer, R.; Crespo, P.; Fauth, M.; Grosse, E.; Klug, J.; Kosev, K.; Rusev, G.; Schilling, K. D.; Schwengner, R.; Wagner, A.;
In burning stellar environments like supernova explosions, the temperatures are high enough for the production of heavy neutron deficient nuclei, the so-called p-nuclei. These are thought to be produced in such explosive scenarios either through chains of photodisintegration reactions on heavy seed nuclei. The modelling of the nucleosynthesis for the p-nuclei is mainly based on statistical model calculations. In this context, the knowledge of the experimental cross sections for the prediction of the p-nuclei abundances is of crucial importance and to forward in this direction we have started and experimental program at the bremsstrahlung facility of the superconducting electron accelerator ELBE of FZ Dresden-Rossendorf . Photodisintegration measurements on the astrophysically relevant p-nuclei 92Mo and 144Sm have been performed via photoactivation technique with bremsstrahlung end-point energies from 10.0 to 16.5 MeV. In particular the (gamma,alpha) reactions of the mentioned nuclei were studied for the first time at different endpoint energies above and close to the threshold. The bremsstrahlung facility and the experimental area are deigned so as to facilitate the studies under optimized background conditions. To probe the fascinating investigations on short-lived nuclei a new pneumatic delivery system has been built recently. First experiments on the short-lived decays following the reaction 144Sm(gamma,n) is discussed. The activation yields from all measurements are compared with calculations using cross sections from recent Hauser-Feshbach models .
Keywords: p-nuclei , photodisintegration, pneumatic delivery system
  • Poster
    International Nuclear Physics Conference (INPC 2007), 03.-08.06.07, Tokyo, Japan

Publ.-Id: 10313 - Permalink


Palladium-mediated cross-coupling reactions with [11C]methyl iodide and 4-[18F]fluorohalobenzenes for the synthesis of positron emission tomography (PET) radiotracers
Wüst, F.; Berndt, M.; Knieß, T.;
Radiochemistry with the short-lived positron emitters 11C (11C, t1/2 =20.38 min) and 18F (18F, t1/2 = 109.6 min) represents special challenges in terms of synthesis time and applicable labelling techniques. The implementation of modern organic chemistry methods into 11C and 18F radiochemistry has steadily expanded the number of 11C- and 18F- labelled PET radiotracers. The review wants to address the progress in the development of palladium-mediated carbon-carbon and carbon-heteroatom bond formations based on the readily available labelling precursors [11C]methyl iodide and 4-[18F]fluorohalobenzenes (4-[18F]bromofluorobenzene and 4-[18F]fluoroiodobenzene).
  • Contribution to external collection
    Xiaoyuan Chen: Recent Advances of Bioconjugation Chemistry in Molecular Imaging, Trivandrum, Kerala, India: Research Signpost, 2008, 978-81-308-0210-7, 155-173

Publ.-Id: 10312 - Permalink


Bericht der Sitzung Energiesysteme – Energiewirtschaft, Jahrestagung Kerntechnik 2007, Karlsruhe
Willschütz, H.-G.;
Die Sitzung Energiesysteme – Energiewirtschaft (Energy Systems - Energy Economics) unter der Leitung von Herrn Dr.-Ing. Hans-Georg Willschütz (Forschungszentrum Dresden-Rossendorf e.V., Dresden, D) umfasste vier Vorträge, die sich mit den Perspektiven für neue Kernkraftwerke in Litauen und den Niederlanden, Strompreisentwicklungen und -angeboten in Deutschland und Aspekten der zukünftigen nuklearen Entsorgung befassten. Die Sitzung stieß auf reges Interesse und wurde zeitweilig von über 70 Teilnehmern besucht.
Keywords: Session report, Energy Systems - Energy Economics, new nuclear power plants in Lithuania and The Netherlands, prices in the electricity market, future development of nuclear waste disposal
  • atw - International Journal for Nuclear Power 52(2007)10, 663-664

Publ.-Id: 10311 - Permalink


Photodisintegration studies on p-nuclei : The case of Mo and Sm isotopes
Nair, C.; Junghans, A. R.; Erhard, M.; Bemmerer, D.; Beyer, R.; Crespo, P.; Grosse, E.; Fauth, M.; Kosev, K.; Rusev, G.; Schilling, K.-D.; Schwengner, R.; Wagner, A.;
In explosive stellar environments like supernovae, the temperatures are high enough for the production of heavy neutron-deficient nuclei, the so-called p-nuclei. Up to now, the knowledge of the reaction rates of p-nuclei is based on theoretical parameterizations using statistical model calculations. At the bremsstrahlung facility of the superconducting electron accelerator ELBE of FZ Dresden-Rossendorf, we aim to measure the photodisintegration rates of heavy nuclei experimentally. Photoactivation measurements on the astrophysically relevant p-nuclei 92Mo and 144Sm have been performed with bremsstrahlung end-point energies from 10.0 to 16.5 MeV. First experiments on the short-lived decays following the reaction 144Sm(gamma,n) are carried out using a pneumatic delivery system for rapid transport of activated samples. The activation yields are compared with calculations using cross sections from recent Hauser-Feshbach models.
  • Poster
    Nuclear Physics in Astrophysics III, 26.-31.03.07, Dresden, Germany
  • Journal of Physics G 35(2008)1, 014036
  • Contribution to WWW
    http://arxiv.org/abs/0710.5086

Publ.-Id: 10310 - Permalink


Experimente und CFD Simulationen zu geschichteten Strömungen in horizontalen Kanälen
Höhne, T.; Vallee, C.;
Slug flow is potentially hazardous to the structure of the system due to the strong oscillating pressure levels formed behind the liquid slugs. It is usually characterized by an acceleration of the gaseous phase and by the transition of fast liquid slugs, which carry out a significant amount of liquid with high kinetic energy. For the experimental investigation of air/water flows, a horizontal channel with rectangular cross-section was build at Forschungszentrum Dresden-Rossendorf (FZD). Experimental data were used to check the feasibility to predict the slugging phenomenon with the existing multiphase flow models build in ANSYS CFX. Further it is of interest to prove the understanding of the general fluid dynamic mechanism leading to slug flow and to identify the critical parameters affecting the main slug flow parameters (like e.g. slug length, frequency and propagation velocity; pressure drop).
For free surface simulations, the inhomogeneous multiphase model was used, where the gaseous and liquid phases can be partially mixed in certain areas of the flow domain. In this case the local phase demixing after a gas entrainment is controlled by buoyancy and interphase drag and is not hindered by the phase interface separating the two fluids. A further decision has to be made regarding the applied fluid morphology and interphase drag law for the multiphase flow. The fluid-dependent shear stress transport (SST) turbulence models were selected for each phase. Damping of turbulent diffusion at the interface has been considered. The k-ω based SST model accounts for the transport of the turbulent shear stress and gives highly accurate predictions of the onset and the amount of flow separation under adverse pressure gradients. The tail of the calculated slug and the flow behind it is in good agreement with the experiment. The entrainment of small bubbles in front of the slug could not be observed in the calculation. However, the front wave rolls over and breaks. It is created due to the high air velocity. In contrast to the measurement, the slug period is increasing with the time in the calculation. This could be a result of different amount of water in the channel at the beginning of experiment and CFD simulation. While in the simulation, the liquid phase covers 78% of the channel, it represents about 70% at the beginning of the experiment. Furthermore, in the experiment, this value is also reduced by a first slug which carries a significant amount of water out of the channel. This first slug could not be simulated. The behavior of slug propagation at the experimental setup was qualitatively reproduced, while quantitative deviations require a continuation of the work.
As some uncertainties were noticed at the inlet of this channel, the HAWAC (Horizontal Air/Water Channel) with well defined inlet boundary conditions dedicated to co-current flows was built. A picture sequence recorded during slug flow was compared with the equivalent CFD simulation made with the code ANSYS CFX. The two-fluid model was applied with a special free surface treatment. Due to an interfacial momentum transfer, it was possible to generate slugs based on instabilities. The behaviour of slug generation and propagation at the experimental setup was qualitatively reproduced, while deviations require a continuation of the work. The creation of small instabilities due to pressure surge or an increase of interfacial momentum should be analysed in the future. Furthermore, experiments like pressure and velocity measurements are planned and will allow quantitative comparisons, also at other superficial velocities.
Keywords: Slug Flow, Horizontal flow, CFD
  • Invited lecture (Conferences)
    Fachsitzung Anwendung von CFD-Methoden in der Reaktorsicherheit, Jahrestagung Kerntechnik, 22.-24.05.2007, Karlsruhe, Germany

Publ.-Id: 10309 - Permalink


Investigation of single-crystals of chromium implanted with 119Sn-ions of various energies
Cieslak, J.; Dubiel, S. M.; Eichhorn, F.; Menzel, M.; Reuther, H.;
Single-crystals of chromium doped with 1016 119Sn ions per cm2 and energies ranging between 45 and 190 keV were investigated with conversion electron M¨ossbauer spectroscopy (CEMS), Auger electron spectroscopy (AES), grazing angle X-ray diffraction (GAXRD), and scanning electron microscopy (SEM). The M¨ossbauer spectra were found to be significantly different than those of the bulk. Both the maximum and the average hf. field values were higher by a factor of ∼3 than the corresponding quantities for the bulk. The overall shape of the spectra is also different. GAXRD study has revealed that the near-surface zone of the investigated samples has a nanocrystalline structure with the average size of grains ranging between ∼20 and ∼40 nm. The latter seems to be the main reason for the observed enhancement of the spin-density and its distortion.
  • Journal of Alloys and Compounds 442(2007), 235-238

Publ.-Id: 10308 - Permalink


New aspects about the growth of metal-filled CNT on structured substrates and the tuning of their magnetic properties
Müller, C.; Leonhardt, A.; Elefant, D.; Reuther, H.; Büchner, B.;
Nanostructured magnetic materials such as arrays of nanowires and filled nanotubes exhibit unique and tuneable magnetic properties e.g. the anomalous high coercivity, which make them favourable for applications in high density magnetic recording media.
We show effective CVD-routes for the defined growth of ensembles as well as individuals of metall-filled carbon nanotubes (Fe, Co, Ni) on structured silicon substrates and alumina substrates. The filled nanotubes were obtained due to continuous decomposition (thermic or plasma) of metal-containing precursors or hydrocarbons at 750-850°C. The nanotube dimensions can be controlled by the size of the catalyst on the substrate and with the reaction conditions. Further we report about a homogeneous embedding procedure of the CNT ensembles in a solid matrix. After the mechanical treatment the remaining nanotube stubs are analyzed with the magnetometer. In addition detailed investigations with transmission electron microscopy (TEM), atomic force microscopy (AFM) and Moessbauer spectroscopy give new insightes in the growth of filled carbon nanotubes.
  • Poster
    Eighth International Conference on the Science and Application of Nanotubes, 24.-29.06.2007, Ouro Preto, Brasil

Publ.-Id: 10307 - Permalink


Streustrahlungskorrektur bei Gammatomographie auf der Grundlage einer Faltungsoperation
Hoppe, D.;
Entwickelt wird eine einfache und leicht handhabbare Methode, die sich vorzugsweise bei der Gammatomographie mit hoch- und monoenergetischer Strahlung zur Korrektur von Streustrahlungsartefakten eignet. Der Grundgedanke besteht darin, dass zwischen der im Untersuchungsobjekt bestehenden Schwächungsverteilung der Primärstrahlung und der Verteilung der vom einzelnen Objektpunkt ausgehenden Streustrahlung eine Faltungsoperation vorgenommen wird.

We develop a straightforward and easy to be used method that lends itself preferably to tomography with high and mono energetic Gamma radiation for correction of scattered radiation artifacts. It is the basic idea of the method to convolve the attenuation distribution of the impinging radiation with the scattered radiation that is emitted from the individual points of the object under investigation.
Keywords: Gammatomography, Compton scattering, scatter distribution, scatter correction, convolution
  • Technisches Messen (2008)6, 413

Publ.-Id: 10306 - Permalink


Suppression of secondary phase formation in Fe implanted ZnO single crystals
Potzger, K.; Zhou, S.; Reuther, H.; Kuepper, K.; Talut, G.; Helm, M.; Fassbender, J.ORC; Denlinger, J. D.
Unwanted secondary phases are one of the major problems in diluted magnetic semiconductor (DMS) creation. Here we show possibilities to avoid such phases in Fe implanted and post-annealed ZnO(0001) single crystals. While -Fe nanoparticles are formed after such doping in as-polished crystals, high temperature (1273 K) annealing in O2 or high vacuum before implantation suppresses these phases. Thus, the residual saturation magnetization in the pre-annealed ZnO single crystals is about 20 times lower than for the as-polished ones and assigned to indirect coupling between isolated Fe ions rather than to clusters.
Keywords: ZnO, Diluted Magnetic Semiconductors, Ion implantation
  • Applied Physics Letters 91(2007), 062107

Publ.-Id: 10305 - Permalink


Projective Hilbert space structures near exceptional points and the quantum brachistochrone
Günther, U.; Samsonov, B.;
The talk consists of two parts. In the first part, a brief overview of projective Hilbert space structures related to exceptional points (EPs) is presented. The apparent contradiction between operator (matrix) perturbation schemes related to root-vector expansions and expansions in terms of eigenvectors for diagonal spectral decompositions is projectively resolved. In the second part of the talk, the gained insight is used for a geometric analysis of the brachistochrone problem for non-Hermitian as well as for PT-ssymmetric/pseudo-Hermitian quantum mechanical systems. The passage time contraction for non-Hermitian Hamiltonians compared to Hermitian ones is attributed to a distance contraction in projective Hilbert space due to non-unitary evolution. In the limiting case when a parameter dependent Hamiltonian approaches an EP in its spectral decomposition the distance between the coalescing eigenvectors vanishes and with it the passage time of the brachistochrone.
Keywords: Quantum Mechanics, exceptional point, Hilbert space, projective space, PT-symmetry, non-Hermitian operator, quantum brachistochrone, passage time
  • Lecture (Conference)
    6th International Workshop on Pseudo-Hermitian Hamiltonians in Quantum Physics, 16.-18.07.2007, London, United Kingdom

Publ.-Id: 10304 - Permalink


Non-Hermitian dynamics and a Hilbert space "relativity principle"
Samsonov, B.; Günther, U.;
Quantum mechanics (QM) is reexamined from the view-point of operator equivalence classes. Two assumptions are used as starting points:
1.) Any operator with a diagonal spectral decomposition, purely real spectrum and which is densely defined in a suitable Hilbert space may describe a physical observable.
2.) Two sets of observables with operators which are related by a similarity transformation (i.e. which are elements from operator equivalence classes and lie on corresponding conjugacy orbits) are physically indistinguishable. In other words, the physical properties related to the observables X and AXA-1 are exactly the same when A is a non-singular bounded operator (Hilbert space "relativity principle").
This implies that a given set of observables which is equivalent to a set with all operators Hermitian in a corresponding Hilbert space will not lead to the appearance of new properties compared to conventional QM. In contrast, if a set of observables consists of Hermitian and non-Hermitian operators in a certain Hilbert space then new effects can be expected which will go beyond those of conventional QM.
In this respect we analyze the evolution of a Hermitian observable governed by a non-Hermitian Hamiltonian.
The general approach is illustrated by a toy model 2×2 matrix Hamiltonian as it was recently used by Bender et al [Phys. Rev. Lett. 98 (2007) 040403] for considerations of a PT-symmetric quantum brachistochrone problem. We calculate the corresponding spin flip probabilities and show that the spin flips in non-Hermitian models may have flip (passage) times which are shorter or longer compared to those of Hermitian systems. The effect depends on the concrete exceptional point which is approached in the parameter space of the model.
Keywords: Quantum Mechanics, PT-symmetry, pseudo-Hermiticity, Krein space, quantum brachistochrone, passage time, equivalence class, conjugacy orbit
  • Lecture (Conference)
    6th International Workshop on Pseudo-Hermitian Hamiltonians in Quantum Physics, 16.-18.07.2007, London, United Kingdom

Publ.-Id: 10303 - Permalink


Numerische Simulationen zur elektrolytischen Kupferabscheidung in externen Magnetfeldern
Mutschke, G.;
Nach einem einleitenden Überblick über die CFD-Aktivitäten
der Abteilung Magnetohydrodynamik im FZD werden aktuelle
numerische Ergebnisse zum genannten Thema vorgestellt, welches
Bestandteil des Sonderforschungsbereiches 609 ist.
  • Lecture (Conference)
    Workshop Strömungssimulation, 29.06.2007, Dresden, Germany

Publ.-Id: 10302 - Permalink


Aneuploidy and DNA Replication in the Normal Human Brain and Alzheimer’s Disease
Mosch, B.; Morawski, M.; Mittag, A.; Lenz, D.; Tarnok, A.; Arendt, T.;
Reactivation of the cell cycle, including DNA replication, might play a major role in Alzheimer’s disease (AD). A more than diploid DNA content in differentiated neurons might alternatively result from chromosome mis-segregation during mitosis in neuronal progenitor cells. It was our objective to distinguish between these two mechanisms for aneuploidy and to provide evidence for a functional cell cycle in AD. Using slide-based cytometry, chromogenic in situ hybridization, and PCR amplification of alu-repeats, we quantified the DNA amount of identified cortical neurons in normal human brain and AD and analyzed the link between a tetraploid DNA content and expression of the early mitotic marker cyclin B1. In the normal brain, the number of neurons with a more than diploid content amounts to ~10%. Less than 1% of neurons contains a tetraploid DNA content. These neurons do not express cyclin B1, most likely representing constitutional tetraploidy. This population of cyclin B1-negative tetraploid neurons, at a reduced number, is also present in AD. In addition, a population of cyclin B1-positive tetraploid neurons of ~2%of all neurons was observed in AD. Our results indicate that at least two different mechanisms need to be distinguished giving rise to a tetraploid DNA content in the adult brain. Constitutional aneuploidy in differentiated neurons might be more frequent than previously thought. It is, however, not elevated in AD. In addition, in AD some neurons have re-entered the cell cycle and entirely passed through a functional interphase with a complete DNA replication.
Keywords: cell cycle; cyclin B1; neurodegeneration; chromogenic in situ hybridization; CISH; slide-based cytometry; SBC; quantitative real-time PCR

Publ.-Id: 10301 - Permalink


X-ray radioscopic visualization of the solutal convection during solidification of a Ga-30wt%In alloy
Boden, S.; Eckert, S.; Willers, B.; Gerbeth, G.;
Capabilities of the X-ray attenuation contrast radioscopy were utilized to provide a real-time diagnostic technique of the melt flow during unidirectional solidification of a Ga-30wt%In alloy from the bottom which is significantly affected by solutal convection. The solidification process was visualised using a microfocus X-ray tube. The X-ray facility provided shadow radiographs at spatial resolutions of about 10 µm at frame repetition rates of 25 Hz. We used the optical flow approach to derive information about the velocity field ahead of the solidification front and in the mushy zone from the observed displacement of the brightness patterns appearing during the X-ray image sequence. Buoyancy-driven flow patterns were found at length scales much larger than the length scales of the solidifying microstructure. A strong coupling between convection and dendritic growth became apparent via flow-induced modifications of the concentration profile in the liquid.
Keywords: Directional solidification, Binary alloy, Dendritic growth, Solutal convection, X-ray radioscopy
  • Metallurgical and Materials Transactions A 39A(2008), 613-623

Publ.-Id: 10300 - Permalink


Modellentwicklung und Validierung von CFD-Codes für Mehrphasenströmungen
Lucas, D.;
Der Vortrag gibt einen Überblick über die am FZD laufenden Arbeiten zur Qualifikation von CFD Codes für Mehphasenströmungen. Neben einer Darstellung der Motivation für die Arbeiten wird die enge Verknüpfung zwischen Experimenten, Modellentwicklung, Implementierung der Modelle in den CFD-Code CFX und die anschließende Validierung eingegangen. Exemplarisch wird das an der Modellentwicklung und Validierung für polydiperse Blasenströmungen dargestellt. Ein Ausblick auf zukünftige Arbeiten wird gegeben.
Keywords: CFD, two-phase flow, bubbly flow
  • Invited lecture (Conferences)
    Workshop Strömungssimulation, 29.06.2007, Dresden, Germany

Publ.-Id: 10299 - Permalink


Numerical and experimental investigations for insulation particle transport phenomena in water flow
Krepper, E.; Cartland-Glover, G.; Grahn, A.; Weiss, F.-P.; Alt, S.; Hampel, R.; Kästner, W.; Kratzsch, A.; Seeliger, A.;
The investigation of insulation debris generation, transport and sedimentation becomes more important with regard to reactor safety research for Pressurized and Boiling Water Reactors, when considering the long-term behaviour of emergency core coolant systems during all types of loss of coolant accidents (LOCA). The insulation debris released near the break during a LOCA incident consists of a mixture of a disparate particle population that varies with size, shape, consistency and other properties. Some fractions of the released insulation debris can be transported into the reactor sump, where it may perturb or impinge on the emergency core cooling systems.
Open questions of generic interest are for example the particle load on strainers and corresponding pressure drop, the sedimentation of the insulation debris in a water pool, its possible re-suspension and transport in the sump water flow. A joint research project on such questions is being performed in cooperation with the University of Applied Science Zittau/Görlitz and the Forschungszentrum Dresden-Rossendorf. The project deals with the experimental investigation and the development of Computational Fluid Dynamic (CFD) models for the description of particle transport phenomena in coolant flow. While the experiments are performed at the University Zittau/Görlitz, the theoretical work is concentrated at Forschungszentrum Dresden-Rossendorf.
In the present paper, the basic concepts for CFD modelling are described and experimental results are presented. Further experiments are designed and feasibility studies were performed.
Keywords: CFD, two-fluid approach, fibre flow
  • Annals of Nuclear Energy 35(2008), 1564-1579

Publ.-Id: 10298 - Permalink


Anwendung des Master Curve-Konzeptes zur Charakterisierung der Zähigkeit neutronenbestrahlter Reaktordruckbehälterstähle
Viehrig, H.-W.; Zurbuchen, C.;
Die Anwendbarkeit des Master Curve-(MC-)Konzepts zur Charakterisierung des Zähigkeitszustandes bestrahlter Reaktordruck¬behälter-(RDB-)Stähle wurde an drei RDB-Stählen überprüft: IAEA-Referenzstahl 3JRQ57, 1JFL11 (vergleichbar mit 22NiMoCr3-7) sowie russischer WWER-440 Grundwerkstoff KAB-B. In Zugversuchen, Charpy-V-Tests, Risswiderstandskurven nach ASTM E1820 und Master Curve Tests zur Bestimmung der Referenztemperatur T0 nach ASTM E1921 wurden der unbestrahlte Ausgangszustand, je drei Bestrahlungszustände bis hin zu Neutronenfluenzen von 100∙1018 n/cm² (E>1MeV) sowie bei 475°C/100h thermisch ausgeheilte Zustände untersucht. Mit zusätzlichen auf dem MC-Konzept basierenden Auswerteverfahren nach SINTAP, multimodalem MC-Ansatz (MML) sowie der Unified Curve erfolgte die Bewertung des Einflusses von Materialinhomogenitäten und möglicher MC-Formänderung bei hohen Fluenzen.
Wie erwartet geht Neutronenbestrahlung mit Verfestigung und Duktilitätsabnahme einher, d.h. Härte, Festigkeitskennwerte, Charpy-V-Übergangstemperaturen T28J und T41J sowie T0 steigen mit der Neutronenfluenz, während die Bruchdehnung und Hochlagenzähigkeit abnehmen. Am bestrahlungsempfindlichsten reagiert der Stahl 3JRQ57, gefolgt von KAB-B und 1JFL11. Durch die Ausheilbehandlung von 475°C/100h erholen sich die Werkstoffkennwerte der Zugversuche, Charpy-V-Tests und MC-Versuche auf den jeweiligen unbestrahlten Ausgangszustand. Die technischen Ersatzkennwerte für duktile Rissinitiierung bleiben relativ unbeeinflusst von der Neutronenbestrahlung. Die MC nach ASTM E1921 beschreibt die Bruchzähigkeits-Temperaturverläufe für alle drei RDB-Stähle in allen Bestrahlungs- und Ausheilzuständen gut. Bei den niedrig und mittel bestrahlten Zuständen liegen meist mehr als 5% der KJc(1T)-Werte unterhalb der MC-Kurve für 5% Versagenswahrscheinlichkeit. Die MC beschreibt den hoch bestrahlte Zustand (ca. 100∙1018 n/cm², E>1MeV) aller drei RDB-Stähle sehr gut, auch für Daten außerhalb des Gültigkeitsbereiches T0±50K, und auch für den bestrahlungsempfindlichen 3JRQ57 mit inhomogenem Gefüge. Die Unified Curve überbewertet den Einfluss der Neutronenbestrahlung auf die MC-Kurvenform. Eine mögliche Formänderung der MC durch Neutronenbestrahlung konnte bei keinem der drei untersuchten RDB-Stähle nachgewiesen werden.
Keywords: Master Curve, neutron irradiation, embrittlement, Unified Curve, RPV steels
  • Open Access LogoWissenschaftlich-Technische Berichte / Forschungszentrum Dresden-Rossendorf; FZD-476 2007

Downloads:

Publ.-Id: 10297 - Permalink


Coherent terahertz detection with a large-area photoconductive antenna
Peter, F.; Winnerl, S.; Nitsche, S.; Dreyhaupt, A.; Schneider, H.; Helm, M.;
We present a nonresonant photoconductive terahertz detection antenna suitable for detection of both focused and unfocused terahertz radiations. Our system consists of a scalable terahertz emitter based on an interdigitated electrode structure and a detection antenna with similar electrode geometry. While the emitter is fabricated on semi-insulating GaAs we compare different ion-implanted GaAs-based detection antennas. We studied the dependence of the measured terahertz signal on the power and spot size of the gating laser pulse. In addition we compare the performance of our antenna with that of electro-optical sampling.

Publ.-Id: 10296 - Permalink


Kompetenzen und Wahrnehmung der Unternehmensführung am Beispiel slowakischer Unternehmen
Kozikova, O.; Joehnk, P.;
  • Invited lecture (Conferences)
    CO-MAT-TECH 2006, 19.-20.10.2006, Trnava, Slovakia

Publ.-Id: 10293 - Permalink


Evaluierung wissenschaftlicher Einrichtungen in Deutschland : Grundlagen, Verfahren, Ergebnisse
Joehnk, P.; Wahl, B.;
  • Invited lecture (Conferences)
    Workshop, 12.-14.04.2006, Tschernogolovka, Russia

Publ.-Id: 10292 - Permalink


Der Masterplan des Forschungszentrums Rossendorf
Joehnk, P.;
  • Lecture (others)
    Workshop, 12.10.2005, Dresden-Rossendorf, Germany

Publ.-Id: 10291 - Permalink


The conversion process in East Germany after the reunification in 1990
Joehnk, P.;
  • Lecture (others)
    Workshop, 06.09.2005, Tschernogolovka, Russia

Publ.-Id: 10290 - Permalink


Kosten- und Finanzmanagement/Controlling
Joehnk, P.;
  • Lecture (others)
    Workshop, 22.06.2005, Dresden, Germany

Publ.-Id: 10289 - Permalink


[Re6Q7O(3,5-Me2PzH)6]Br2·3,5-Me2PzH (Q = S, Se) – new octahedral rhenium cluster complexes with organic ligands: original synthetic approach and unexpected ligand exchange in cluster core
Mironov, Y. V.; Shestopalov, M. A.; Brylev, K. A.; Yarovoi, A. S.; Romanenko, G. V.; Fedorov, V. E.; Spies, H.; Pietzsch, H.-J.; Stephan, H.; Geipel, G.; Bernhard, G.; Kraus, W.;
  • European Journal of Inorganic Chemistry (2005), 657-661

Publ.-Id: 10288 - Permalink


Weiterentwicklung und Realisierung der Diagnostik der IR-Strahlung am Frei-Elektronen-Laser
Weigelt, H.;
  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2006
    Diploma Thesis, Hochschule Mittweida (FH), März 2006

Publ.-Id: 10287 - Permalink


Energy Loss of Heavy Quarks Passing Hot Deconfined Matter
Schade, H.;
  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2006
    Diploma Thesis, Technische Universität Dresden, Oktober 2006

Publ.-Id: 10286 - Permalink


Investigation of K+ meson production in C + C collisions at 2 A GeV with HADES
Sadovsky, A.;
  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2006
    Ph.D. Thesis, Technische Universität Dresden, Dezember 2006

Publ.-Id: 10285 - Permalink


Study of Λ hyperon production in C+C collisions at 2 AGeV beam energy with the HADES spectrometer
Kanaki, K.;
  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2006
    Ph.D. Thesis, Technische Universität Dresden, Dezember 2006
  • Open Access LogoWissenschaftlich-Technische Berichte / Forschungszentrum Dresden-Rossendorf; FZD-467 2007

Downloads:

Publ.-Id: 10284 - Permalink


Simulations of Dielectron Detection Capabilities of HADES at SIS100
Bannier, B.;
  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2006
    Diploma Thesis, Technische Universität Dresden, November 2006

Publ.-Id: 10283 - Permalink


Photodissociation experiments for p-process nuclei
Wagner, A.; Erhard, M.; Grosse, E.; Junghans, A. R.; Klug, J.; Kosev, K.; Nair, C.; Nankov, N.; Rusev, G.; Schilling, K. D.; Schwengner, R.;
  • Contribution to proceedings
    International Conference on Frontiers in Nuclear Structure, Astrophysics and Reactions, 12.-17.09.2006, Kos, Greece
    Proceedings of the International Conference on Frontiers in Nuclear Structure, Astrophysics and Reactions CP 831, 16

Publ.-Id: 10282 - Permalink


Cross section measurements of the Big Bang nucleosynthesis reaction D(α,γ)6Li by Coulomb dissociation of 6Li
Hammache, F.; Galaviz, D.; Sümmerer, K.; Typel, S.; Attallah, F.; Caamano, M.; Coc, A.; Cortina, D.; Geissel, H.; Hellström, M.; Iwasa, N.; Kiener, J.; Coczon, P.; Kohlmeyer, B.; Schwab, E.; Schwarz, K.; Schümann, F.; Senger, P.; Sorlin, O.; Tatischeff, V.; Thibaud, J. P.; Uhlig, F.; Wagner, A.; Walus, A.;
  • Contribution to proceedings
    International Conference on Frontiers in Nuclear Structure, Astrophysics and Reactions, 12.-17.09.2005, Kos, Greece
    Congress Proceedings 831, 21

Publ.-Id: 10281 - Permalink


g-factor measurements at RISING: the case of 127Sn
Atanasova, L.; Balabanski, D. L.; Hass, M.; Bazzacco, D.; Becker, F.; Bednarczyk, P.; Benzoni, G.; Blasi, N.; Blazhev, A.; Bracco, A.; Brandau, C.; Caceres, L.; Camera, F.; Chamoli, S. K.; Crespi, F. C. L.; Detistov, P.; Doornenbal, P.; Fahlander, C.; Farnea, E.; Georgiev, G.; Gerl, J.; Gladnishki, K.; Gorska, M.; Grawe, H.; Grebosz, J.; Hoischen, R.; Ilie, G.; Ionescu-Bujor, M.; Iordachescu, A.; Jungclaus, A.; Lo Bianco, G.; Kmiecik, M.; Kojouharov, I.; Kurz, N.; Lakshmi, S.; Lozeva, R.; Maj, A.; Montanari, D.; Neyens, G.; Pfützner, M.; Pietri, S.; Podolyak, Z.; Prokopowicz, W.; Rudolph, D.; Rusev, G.; Saito, T. R.; Saltarelli, A.; Schaffner, H.; Schwengner, R.; Simpson, G.; Tashenov, S.; Valiente-Dobon, J. J.; Vermeulen, N.; Walker, J.; Werner-Malento, E.; Wieland, O.; Wollersheim, H. J.;
  • Contribution to proceedings
    Proceedings of the XXV International Workshop on Nuclear Theory, 26.06.-01.07.2006, Rila Mountains, Bulgaria, 161-171

Publ.-Id: 10280 - Permalink


Impact of nucleon mass shift on the freeze-out process
Zschocke, S.; Csernai, L. P.; Molnár, E.; Nyíri, Á.; Manninen, J.;
  • Physical Review C 72(2005), 064909

Publ.-Id: 10279 - Permalink


Collective Rotation of Nuclei with Tetrahedral Symmetry
Schunck, N.; Dudek, J.; Frauendorf, S.;
  • Open Access LogoActa Physica Polonica B 36(2005), 1071

Publ.-Id: 10278 - Permalink


Validierung des FLUKA-Codes für Anwendungen in der Ionentherapie
Sommerer, F.;
  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2004
    Diploma Thesis, Atominstitut der österreichischen Universitäten, Wien, August 2004

Publ.-Id: 10277 - Permalink


Schwellennahe Erzeugung von Kaonen und Antikaonen in Proton-Kern-Stößen
Scheinast, W.;
  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2004
    Ph.D. Thesis, TU Dresden, August 2004

Publ.-Id: 10276 - Permalink


Dosimetrische Charakterisierung einer für strahlenbiologische Experimente als Referenzstrahlungsquelle dienenden Röntgenanlage
Mikuletz, T.;
  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2004
    Diploma Thesis, Hochschule Mittweida (FH), April 2004

Publ.-Id: 10275 - Permalink


Investigation of receptor G protein coupling in vision
Lehmann, N.;
  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2004
    Ph.D. Thesis, Univ. Freiburg, Juli 2004

Publ.-Id: 10274 - Permalink


Experimentelle Verifikation eines Verfahrens zur Unterdrückung zufälliger Koinzidenzen bei der in-beam Positronen-Emissions-Tomographie
Barthel, T.;
  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2004
    Diploma Thesis, Hochschule Mittweida (FH), 2004

Publ.-Id: 10273 - Permalink


First nuclear-resonance-fluorescence experiments using enriched high-pressure xenon targets
von Garrel, H.; Käppeler, F.; Käubler, L.; Kneissl, U.; Pitz, H. H.; Rusev, G.; Scheck, M.; Schilling, K. D.; Schwengner, R.; Stedile, F.; Walter, S.; Werner, V.; Wisshak, K.;
  • Open Access LogoBgNS Transactions 9(2004), 178

Publ.-Id: 10272 - Permalink


Search for Λ in CC at 2 AGeV with the HADES spectrometer
Kanaki, K.;
  • Lecture (Conference)
    XIV HADES Collaboration Meeting, 29.09.-02.10.2004, Darmstadt, Deutschland

Publ.-Id: 10271 - Permalink


K+ mesons in C+C at 2 AGeV
Sadovsky, A.;
  • Lecture (Conference)
    XIV HADES Collaboration Meeting, 29.09.-02.10.2004, Darmstadt, Deutschland

Publ.-Id: 10270 - Permalink


Searching Λ hyperons in C+C at 2 AGeV
Kanaki, K.;
  • Lecture (Conference)
    XV HADES Collaboration Meeting, 07.-12.06.2005, Dubna, Russia

Publ.-Id: 10269 - Permalink


Analysis of K+ production in the reaction C(2 AGeV)+C with HADES
Sadovsky, A.;
  • Lecture (Conference)
    XVI HADES Collaboration Meeting, 04.-08.04.2006, Dresden, Deutschland

Publ.-Id: 10268 - Permalink


Study of Λ production in C+C collisions at 2 AGeV with HADES
Kanaki, K.;
  • Lecture (Conference)
    XVI HADES Collaboration Meeting, 04.-08.04.2006, Dresden, Deutschland

Publ.-Id: 10267 - Permalink


HADES MDC I at FZ Dresden-Rossendorf
Naumann, L.;
  • Lecture (Conference)
    XVI HADES Collaboration Meeting, 04.-08.04.2006, Dresden, Deutschland

Publ.-Id: 10266 - Permalink


Simulation of dielectron detection capabilities of HADES at T=8 AGeV and T=25 AGEV
Bannier, B.;
  • Lecture (Conference)
    XVII HADES Collaboration Meeting, 25.-28.10.2006, Darmstadt, Deutschland

Publ.-Id: 10265 - Permalink


Study of Λ production in C+C collisions at 2 AGeV
Kanaki, K.;
  • Lecture (Conference)
    XVII HADES Collaboration Meeting, 25.-28.10.2006, Darmstadt, Deutschland

Publ.-Id: 10264 - Permalink


Multigroup Diffusion and SP3 Solutions for a PWR MOX/UO2 Benchmark with the Code DYN3D
Beckert, C.; Grundmann, U.; Mittag, S.;
The core neutronic and thermal-hydraulic simulator code DYN3D, which has been developed for three-dimensional analyses of steady states and transients in thermal reactors with quadratic or hexagonal fuel assemblies, is based on nodal methods for the solution of the two group neutron diffusion equation. Loading cores with higher content of MOX fuel, the increase of the fuel cycle length, and the consideration of new reactor types require improvements of these standard methods. Therefore, DYN3D has been extended to use more than two energy groups. Furthermore, a nodal expansion method for solving the equations of the simplified P3 approximation (SP3) of the multigroup transport equation was developed for quadratic fuel assembly geometry. The new extensions of DYN3D are verified with calculations of the OECD/NEA and U.S. NRC PWR MOX/UO2 Core Transient Benchmark. The nodal SP3 method in DYN3D is outlined and results for a steady state are shown, which are compared with the solutions of the diffusion and transport codes used by the participants of the benchmark. Improved DYN3D-results are obtained with the new extensions.
  • Transactions of the American Nuclear Society and the European Nuclear Society 97(2007)
  • Lecture (Conference)
    2007 ANS/ENS International Meeting, 11.-15.11.2007, Washington D.C., United States

Publ.-Id: 10263 - Permalink


The normal and vortex-states of quasi-two-dimensional organic superconductors
Wosnitza, J.;
  • Lecture (Conference)
    Research Workshop of the Israel Science Foundation on Correlated Electrons at High Magnetic Fields, 19.-23.12.2004, Ein-Gedi/Holon, Israel

Publ.-Id: 10262 - Permalink


Vom Plastik zum organischen Supraleiter
Wosnitza, J.;
  • Lecture (Conference)
    Physikalisches Kolloqium der TU Braunschweig, 07.12.2004, Braunschweig, Deutschland

Publ.-Id: 10261 - Permalink


Morphology and bonding structure of fullerene-like nanocomposite C:Ni (~30 at%.) thin films grown by ion beam sputtering
Abrasonis, G.; Mücklich, A.; Kuepper, K.; Krause, M.; Kreissig, U.; Kolitsch, A.; Möller, W.; Sedlackova, K.; Radnoczi, G.; Torres, R.; Jimenez, I.; Gago, R.;
The morphology and bonding structure of nanocomposite C:Ni (~30 at.%) thin films grown by ion beam sputtering at different temperatures (RT-500°C) is investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray absorption near edge structure (XANES). XRD and XANES of the LII–LIII absorption edges of Ni reveal that at lower temperatures nickel is in the carbidic state whose signatures become more pronounced when the growth temperature increases up to ~300°C. Further increase in the growth temperature results in XRD and XANES features resembling to those of metallic nickel. According to the TEM observations, nickel or nickel carbide nanoparticles embedded in a carbon matrix have a columnar structure with the diameter of ~2-5 nm, which is typical for the films grown under surface diffusion dominated conditions for the structuring processes. At lower temperatures carbon is mainly in an amorphous state, while the transition into a fullerene-like structure, which consists of nickel nanoparticles wrapped in curved graphite sheets, can be clearly identified at ~200°C. The present results show that the nickel nanoparticles act as nucleation centres for the formation of the fullerene-like structures in both carbidic and metallic states, provided that the temperature for surface diffusion during the deposition process is sufficiently high.
  • Poster
    XXIst International Winterschool on Electronic Properties of Novel Materials 2007, 10.-17.03.2007, Kirchberg, Austria

Publ.-Id: 10259 - Permalink


Luminescence of isoelectronically ion-implanted SiO2 layers
Salh, R.; Fitting Kourkoutis, L.; Schmidt, B.; Fitting, H.-J.;
Scanning electron microscopy (SEM) and cathodoluminescence (CL) in combination with scanning transmission electron microscopy (STEM) have been used to investigate thermally grown amorphous silicon dioxide layers implanted isoelectronically with group IV ions (C+, Si+, Ge+, Sn+, Pb+) as well as with group VI ions (O+, S+, Se+). Besides the main luminescent centers in a-SiO2 layers: the red R luminescence (650 nm; 1.9 eV) of the non-bridging oxygen hole centers (NBOHC), the blue B (460 nm; 2.7 eV) and the UV (290 nm; 4.3 eV) band of the oxygen deficient centers (ODC), in ion implanted silica additional emission bands are observed. E.g. in Ge+ implanted layers a huge violet band appears at 410 nm (3.1 eV) increasing with the thermal annealing process due to formation of Ge dimers, trimers or higher aggregates, finally leading to destruction of the luminescence centers by further growing to Ge nanoclusters. The Ge cluster size is shown by STEM cross section micrographs.
Generally, group IV element implantation and partial substitution of silicon increases the luminescence in the blue/violet region whereas group VI elements and additional oxygen increase the intensity in the red region, confirming the association of the blue and the red luminescence with oxygen deficient centers and oxygen excess centers, respectively. Thus, the cathodoluminescence spectra of sulfur and oxygen implanted SiO2 layers under special conditions show besides the characteristic luminescence bands a multimodal structure beginning in the green region at 500 nm and extending up to the near infrared region at 820 nm. The energy step differences of the sublevels amount in the average 120 meV and indicate vibronic-electronic transitions, probably of O − interstitial molecules.
Keywords: ion implantation, silicon dioxide, nanocluster, TEM, cathodoluminesence

Publ.-Id: 10258 - Permalink


Master Curve and Unified Curve applicability to highly neutron irradiated Western type Reactor Pressure Vessel steels
Zurbuchen, C.; Viehrig, H.-W.; Weiß, F.-P.;
While the Master Curve (MC) method is gradually entering brittle fracture safety assessment procedures world-wide, knowledge is still lacking about its limits of applicability to highly neutron irradiated material. In this paper two reactor pressure vessel (RPV) steels A533B Cl. 1 (IAEA reference material code JRQ) and A508 Cl.3 (code JFL) were scrutinized for possible deviations of the postulated invariant MC shape and the MC validity for macroscopically inhomogeneous microstructure. Besides tensile and Charpy-V tests, MC tests were performed on Charpy size three-point bend specimens in the unirradiated, neutron irradiated with fluences up to nearly 1020 n/cm² (E>1MeV) and recovery heat treated condition. Evaluation procedures include Master Curve reference temperature T0 determination according to ASTM E1921-05 as well as additional analysis methods such as SINTAP, multimodal MC method (MML) and the Unified Curve (UC). Integrity assessment according to ASME Code Cases N-629 and N-631has been applied. It is shown that the standard MC concept provides a precise description of the fracture toughness for all conditions, even exceptionally well for the highly irradiated state. No MC shape change could be observed, whereas the UC concept indicates a significant influence of irradiation on the fracture toughness curves for the highly irradiated JRQ.
Keywords: Fracture Toughness, RPV steels, neutron irradiation, Master Curve, Unified Curve, multi-modal method

Publ.-Id: 10257 - Permalink


Si and Ge nanocluster formation in Silica matrix
Salh, R.; Fitting, L.; Kolesnikova, E. V.; Sitnikova, A. A.; Zamoryanskaya, M. V.; Schmidt, B.; Fitting, H.-J.;
High resolution transmission electron microscopy, scanning transmission electron microscopy and cathodoluminescence have been used to investigate Si and Ge cluster formation in amorphous silicon dioxide layers. Commonly, cathodoluminescence emission spectra of pure SiO2 are identified with particular defect centers within the atomic network of silica including the nonbridging oxygen hole center associated with the red luminescence at 650nm (1.9 eV) and the oxygen deficient centers with the blue (460 nm; 2.7 eV) and ultraviolet band (295 nm; 4.2 eV). In Ge+ ion implanted SiO2 an additional violet emission band appears at 410 nm (3.1 eV). The strong increase of this violet luminescence after thermal annealing is associated with formation of low-dimension Ge aggregates like dimers, trimers and higher formations, further growing to Ge nanoclusters. On the other hand, pure silica layers were modified by heavy electron beam irradiation (5 keV; 2.7 A/cm2) leading to electronic as well as thermal dissociation of oxygen and appearance of under-stoichiometric SiOx. This SiOx will undergo a phase separation and we observe Si cluster formation with a most probable cluster diameter of 4 nm. Such largely extended Si clusters will diminish the SiO2 related luminescence and Si crystal related luminescence in the near IR appears.
Keywords: ion implantation, nanocluster, cathodoluminesence, TEM
  • Semiconductors 41(2007)4, 397-402

Publ.-Id: 10256 - Permalink


Master Curve applicability to highly neutron irradiated Reactor Pressure Vessel steels - Results of a BMWi Grant Project
Zurbuchen, C.; Viehrig, H.-W.;
Currently, several countries already have adopted or are in the process of adopting the Master Curve (MC) approach into their reactor pressure vessel (RPV) integrity assessment. Yet, for highly irradiated material some open technical issues like the postulated invariance of the MC shape and the behaviour of inhomogeneous material need resolving to allow the MC approach to be incorporated into the German Nuclear Regulatory Guidelines (KTA). Within the framework of the reactor safety research of BMWi specimens from 3 different RPV steels were scrutinized (IAEA reference material 3JRQ57, 1JFL11 comparable to German RPV steel 22NiMoCr3-7 and Russian WWER-440 type base metal KAB-B), which were neutron irradiated up to 1020 n/cm2 (E>1MeV). Additionally, the effects of a recovery heat treatment at 475°C/100h are investigated. Besides microstructure analysis, hardness, tensile and Charpy-V tests, the experimental part focuses on fracture mechanical testing, i.e. JR curves according to ASTM E1820-06 and MC reference temperature T0 determination according to ASTM E1921-05 as well as additional analysis methods like SINTAP, multimodal MC method and the Unified Curve. Resistance against ductile crack initiation (JR curves) remains relatively unaffected by irradiation (JQ/J0,2BL according to ASTM E1820 and J0,2 according to GKSS). The MC provides a precise description of the fracture toughness vs. temperature of unirradiated, irradiated, and recovery annealed material. Especially for the highly irradiated states, the MC describes the KJc(1T) values exceptionally well, including data points outside the temperature limit T0±50K according to Standard ASTM E1921 and even for highly irradiation sensitive and inhomogeneous RPV steel 3JRQ57. Some scatter occurs for low and medium irradiated states, where more points than expected lie below the MC for 5% failure probability. The Unified Curve, which allows for MC shape changes at high irradiation levels, overpredicts the influence of irradiation with 3JRQ57. For none of the 3 investigated RPV steels a shape change of the Master Curve due to irradiation could be observed. Embrittlement prediction formulae according to Reg. Guide 1.99, Rev. 2 and WWER specific VERLIFE procedure tend to underpredict the irradiation-induced changes in material properties.
Keywords: RPV steels, Master Curve, embrittlement, neutron irradiation
  • Lecture (Conference)
    33. MPA-Seminar, 11.-12.10.2007, Stuttgart, Germany
  • Contribution to proceedings
    33. MPA-Seminar, 11.-12.10.2007, Stuttgart, Germany
    Proceedings 33. MPA-Seminar "Werkstoff- & Bauteilverhalten in Energie- & Anlagentechnik", Stuttgart: Materialprüfungsanstalt Universität Stuttgart, 1861-5414

Publ.-Id: 10255 - Permalink


Defect engineering for SIMOX processing
Kögler, R.; Mücklich, A.; Anwand, W.; Eichhorn, F.; Skorupa, W.;
SIMOX (Separation-by-Implantation-of-Oxygen) is an established techniques to fabricate silicon-on-insulator (SOI) structures by oxygen ion implantation into silicon. The main problem of SIMOX is the very high oxygen ion fluence and the related defects. It is demonstrated that vacancy defects promote and localize the oxide growth. The crucial point is to control the distribution of vacancies. Oxygen implantation generates excess vacancies around RP/2 which act as trapping sites for oxide growth outside the region at the maximum concentration of oxygen at RP. The introduction of a narrow cavity layer by He implantation and subsequent annealing is shown to be a promising technique of defect engineering. The additional He implant does not initiate oxide growth in the top-Si layer of SOI.
Keywords: Ion Implantation, Defect engineering, SOI, Excess Vacancies, Cavities
  • Contribution to proceedings
    GADEST'07 - Gettering and Defect Engineering in Semiconductor Technology, 14.-19.10.2007, Erice, Italy
    Solid State Phenomena Vol.131-133, Zürich: Trans Tech Publications, 339-344
  • Lecture (Conference)
    GADEST'07 - Gettering and Defect Engineering in Semiconductor Technology, 14.-19.10.2007, Erice, Italy

Publ.-Id: 10254 - Permalink


Influence of local plasma flow on optical properties and thickness of ITO-films observed with spectroscopic imaging ellipsometry
Vaupel, M.; Vinnichenko, M.;
Tin-doped-indium oxide In2O3:Sn (ITO) is a degenerate n-type semiconductor with high transparency and nearly metallic conductivity. Thin films of ITO find applications in optoelectronics, photovoltaics, and in the liquid crystal display and OLED industry as transparent electrodes. The interaction of the plasma with the growing film surface may be employed to manipulate adatom mobilities and nucleation rates [1, 2], and thus contribute to the definition of the film structure and the evolution of its properties during deposition. The aim of the present work is to investigate the influence of the intentionally inhomogeneous plasma flow on the distribution of the ITO film thickness and optical properties along the substrate. For large scale inspection spectra of Delta/Psi were recorded scanning across the sample. For high resolution (small scale inspection) a spectrum of Delta micromaps with wavelength from 420 nm to 830 nm with 4 µm lateral resolution was recorded using Nanofilm’s imaging ellipsometer EP3 within 3 min. From fitting of the spectrum one obtains maps for each of the fit parameters. It is observed, that the damping of the oscillator is rather independent of location. A negative oscillator frequency shift with thickness increasing from the low towards the high plasma flow area is observed. Such a correlation can be explained by the change of the film microsctructure and stoichiometry [3] which, in turn, can affect electronic structure of the film.
References
1. I. Petrov, P.B. Barna, L. Hultman, and J.E. Greene, J. Vac. Sci. Technol. A 21, 1 (2003).
2. W. Ensinger, Nucl. Instr. Meth. B 127/128, 796 (1997).
3. A. Rogozin, M. Vinnichenko, N. Shevchenko, A. Kolitsch, and W. Moeller, Thin Solid Films 496, 197 (2006) .
Keywords: Imaging ellipsometry, tin-doped indium oxide, reactive magnetron sputtering
  • Poster
    4th International Conference on Spectroscopic Ellipsometry, 11.-15.06.2007, Stockholm, Sweden
  • Physica Status Solidi (C) 5(2008), 1137-1140
    DOI: 10.1002/pssc.200777772

Publ.-Id: 10253 - Permalink


Real-time evolution of tin-doped indium oxide film properties during growth and crystallization studied by spectroscopic ellipsometry
Vinnichenko, M.; Rogozin, A.; Shevchenko, N.; Kolitsch, A.; Möller, W.;
Understanding of the free electron generation mechanisms in tin-doped indium oxide (ITO) films grown at elevated temperatures is important to decrease their electrical resistivity and keep high optical transmittance. Therefore, this study is focused on a real-time determination of the free electron optical absorption of ITO layers by in situ spectroscopic ellipsometry (SE). The experiments were carried out using rotating compensator ellipsometer M-2000 (J.A. Woollam Inc., U.S.A.) during: (i) growth by reactive magnetron sputtering at elevated temperatures (Ts=RT-500 °C); and (ii) post deposition annealing (Ta=200-320 °C) of the amorphous films. In situ four-point probe resistivity measurements during annealing and the data of separate in situ X-ray diffraction (XRD) experiments at the European Synchrotron Radiation Facility were used to contrast the SE results.
Free electron density, Ne, and mobility, µe, are estimated for the growing film from parameterization of the complex dielectric function in Drude-Lorentz approach. The Drude term accounts for the free electron optical absorption. The Ne values range from ~3x10^20 to 10^21 cm^-3 depending on the growth temperature. It is in good agreement with results of ex situ Hall effect measurements, while µe values are usually overestimated by SE. The SE applicability as a non-contact and in situ tool for monitoring of the film resistivity during growth is demonstrated. The method indicates decrease of the resistivity with increasing film thickness at Ts<270 °C mainly due to enhancement of the Ne while there is no such variation of resistivity observed at Ts>400 °C. This result is discussed in terms of thickness-dependent film morphology.
It is shown that postdeposition annealing modifies the film properties in two stages. During the first stage film remains amorphous according to in situ XRD; SE shows slight enhancement of Ne, while film resistivity strongly decreases. It is attributed to relaxation of In-O bonds in the amorphous phase and subsequent rearrangement of defect structure mainly improving the electron mobility. During the second stage, time dependence of the resistivity changes slope and Ne increases by a factor of 2. It coincides in time with the film crystallization outset, which enables Sn-donor activation by removal of interstitial oxygen. Therefore, this study provides experimental evidence of Sn donor activation during crystallization of the films and shows its kinetics depending on the annealing conditions.
Keywords: In situ spectroscopic ellipsometry, tin-doped indium oxide, reactive magnetron sputtering
  • Lecture (Conference)
    4th International Conference on Spectroscopic Ellipsometry, 11.-15.06.2007, Stockholm, Sweden

Publ.-Id: 10252 - Permalink


Reactions of light nuclei studied deep underground at Gran Sasso, Italy
Bemmerer, D.;
New and precise astronomical observations call for nuclear data of equal precision for their interpretation. This will permit a better understanding of big-bang nucleosynthesis and of asymptotic giant branch stars.

Based on the recent experimental study of the 3He(alpha,gamma)7Be nuclear reaction directly in the energy range of big-bang nucleosynthesis, a new experiment to study the 2H(alpha,gamma)6Li reaction at big-bang energies is proposed. The new data will allow to test a possible nuclear solution to the big-bang Li-6 posed by new observations of Li-6 in very old stars.

The 14N(p,gamma)15O reaction determines the rate of the CNO cycle. A new study of this reaction is currently underway at the LUNA accelerator deep underground in Gran Sasso, Italy. The new data can be coupled with observations of low-energy solar neutrinos (for example in the Borexino and SNO+ detectors) in order to determine the solar metallicity.

Satellite-based observations of the decay of radioactive Al-26 allow to determine the rate of supernovae in our galaxy, provided the production process is sufficiently well known. The 25Mg(p,gamma)26Al reaction influences Al-26 production; its rate is under experimental study at LUNA.
Keywords: nuclear astrophysics supernova rate solar neutrinos big-bang nucleosynthesis
  • Lecture (others)
    Seminar, 18.01.2007, Dresden, Deutschland

Publ.-Id: 10251 - Permalink


14N(p,gamma)15O ground state capture studied above the 259 keV resonance at LUNA
Marta, M.; Bemmerer, D.; Kunz, R.; Rolfs, C.; Strieder, F.; Trautvetter, H.-P.;
The cross section of the 14N(p,gamma)15O reaction directly influences the rate of the CNO cycle of hydrogen burning. In order to reliably extrapolate the cross section to the solar Gamow peak, in a previous LUNA experiment capture to the ground state and several excited states in O-15 has been measured and used in an R-matrix fit [1,2].
The data on the ground state capture had been affected by the true coincidence summing effect in a large volume HPGe detector placed in close geometry [1], limiting the precision of the extrapolation. A new measurement of the cross section for capture to the ground state in O-15 is running at LUNA in Gran Sasso (Italy). A clover HPGe detector is used to reduce the summing correction and its consequent uncertainty. We concentrate on energies above the 259 keV resonance, where the R-matrix fit can be constrained by precision data.

[1] A. Formicola et al., Phys. Lett B 591, 61 (2004)
[2] G. Imbriani et al., Eur. Phys. J. A 25, 455 (2005)
Keywords: nuclear astrophysics solar neutrinos age of the universe globular clusters
  • Lecture (Conference)
    Frühjahrstagung Hadronen und Kerne, 13.03.2007, Gießen, Deutschland

Publ.-Id: 10250 - Permalink


The 3He(alpha,gamma)7Be reaction studied at LUNA
Bemmerer, D.; Kunz, R.; Marta, M.; Rolfs, C.; Strieder, F.; Trautvetter, H.-P.;
The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a major uncertainty in the fluxes of Be-7 and B-8 neutrinos from the Sun predicted by solar models and in the Li-7 abundance obtained in big-bang nucleosynthesis calculations.
Here we report on a precision cross section measurement performed by the LUNA collaboration at Gran Sasso (Italy). At energies directly relevant to big-bang nucleosynthesis, the cross section has been studied by both the activation [1] and the prompt-gamma technique.
Using a windowless gas target, high beam intensity, a low background in beam gamma-detector and low background gamma-counting facilities, the 3He(alpha,gamma)7Bee cross section has been determined at 90-170 keV center-of-mass energy with a total uncertainty as low as 4%.

The new data can be used in big-bang nucleosynthesis
calculations and to constrain the extrapolation of the
3He(alpha,gamma)7Be astrophysical S-factor to solar
energies.

[1] D. Bemmerer et al., Phys. Rev. Lett. 97, 122502 (2006)
Keywords: nuclear astrophysics big-bang solar neutrinos
  • Lecture (Conference)
    Frühjahrstagung Hadronen und Kerne, 13.03.2007, Gießen, Deutschland

Publ.-Id: 10249 - Permalink


Experimentelle Nukleare Astrophysik über Tage und tief unter Tage im Gran-Sasso-Labor (Italien)
Bemmerer, D.;
New and precise astronomical observations call for nuclear data of equal precision for their interpretation. This will permit a better understanding of big-bang nucleosynthesis and of asymptotic giant branch stars.

Based on the recent experimental study of the 3He(alpha,gamma)7Be nuclear reaction directly in the energy range of big-bang nucleosynthesis, a new experiment to study the 2H(alpha,gamma)6Li reaction at big-bang energies is proposed. The new data will allow to test a possible nuclear solution to the big-bang Li-6 posed by new observations of Li-6 in very old stars.

The 15N(p,alpha)12C reaction is an important source of uncertainty in the prediction of the F-19 yield observed in asymptotic giant branch stars. Based on angular distribution measurements performed at TU Berlin, the potential of a new precision cross section measurement is evaluated.
Keywords: nuclear astrophysics
  • Invited lecture (Conferences)
    Kolloquium Universität Potsdam, 21.02.2007, Potsdam, Deutschland

Publ.-Id: 10248 - Permalink


The 3He(alpha,gamma)7Be reaction measured by activation at LUNA
Bemmerer, D.;
The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis calculations. In the seminar I will report on a new precision experiment on this reaction performed by the LUNA collaboration [1].

Using a windowless gas target, the high beam intensity of the LUNA2 accelerator, and the Gran Sasso low background gamma-counting facilities, the 3He(alpha,gamma)7Be cross section has been determined by the activation method at 90 -- 170 keV center-of-mass energy with a total uncertainty as low as 4%. The present low energies are directly relevant to big-bang nucleosynthesis and had previously been reached experimentally only by the prompt-gamma technique and with inferior precision.

The new LUNA data can be used in big-bang nucleosynthesis calculations and to constrain the extrapolation of the 3He(alpha,gamma)7Be astrophysical S-factor to solar energies.

[1] D. Bemmerer et al. (LUNA Collaboration), Phys. Rev. Lett. 97, 122502 (2006)
Keywords: big-bang nuclear astrophysics activation
  • Invited lecture (Conferences)
    Physics Seminar, 31.01.2007, Assergi (AQ), Italy

Publ.-Id: 10247 - Permalink


Anisotropy contrast in phonon-enhanced apertureless near-field microscopy using a free-electron laser
Kehr, S. C.; Cebula, M.; Mieth, O.; Härtling, T.; Seidel, J.; Grafström, S.; Eng, L. M.; Winnerl, S.; Stehr, D.; Helm, M.;
We demonstrate the imaging of ferroelectric domains in BaTiO3, using an infrared-emitting free-electron laser as a tunable optical source for scattering scanning near-field optical microscopy and spectroscopy. When the laser is tuned into the spectral vicinity of a phonon resonance, ferroelectric domains can be resolved due to the anisotropy of the dielectric properties of the material. Slight detuning of the wavelength gives rise to a contrast reversal clearly evidencing the resonant character of the excitation. The near-field domain contrast shows that the orientation of the dielectric tensor with respect to the sample surface has a clear influence on the near-field signal.
  • Physical Review Letters 100(2008), 256403

Publ.-Id: 10246 - Permalink


EXAFS studies of Zn/Al and Ni/Al Layered Double Hydroxides
Funke, H.; Chukalina, M.; Scheinost, A. C.; Voegelin, A.;
- wird nachgereicht
  • Poster
    ESRF User's Meeting 2007, 07.-09.02.2007, Grenoble, Frankreich

Publ.-Id: 10245 - Permalink


Generation of Ultrahigh Light Intensities and Relativistic Laser-Matter Interaction
Sauerbrey, R.; Hein, J.;
Modern technologies allow the amplification of short laser pulses to energies of some tens of kJ. Additionally, ultrashort pulses containing only some optical cycles can be generated. By merging these techniques nowadays focused laser beams can reach unprecedented intensities in the range of 1021 W/cm2 and will reach even higher ones in the near future. At these intensities the electric and magnetic field strength is many orders of magnitude higher than those that will ever possible in a static generation scheme. By applying these fields on a target it becomes possible to get access to a new interaction regime of light and matter: relativistic optics. This opens a new wide area in experimental science where classical optics meets plasma dynamics, relativistic quantum mechanics, and high energy physics.
  • Contribution to external collection
    Frank Träger: Springer Handbook of Lasers and Optics, LLC New York: Springer, 2007, 978-0-387-95579-7, 827-841

Publ.-Id: 10244 - Permalink


Interactions between Metals and Bacteria: Fundamental and Applied Research
Merroun, M.;
Soils, sediments, and waters heavily polluted with radionuclides and other toxic metals, are a reservoir of unusual bacteria well adapted to these toxic environments. These bacteria possess fascinating mechanisms for interaction with and bio-transformation of radionuclides and other heavy metals, thus regulating the mobility of the metals in the environment. This paper will give an overview on the different mechanisms of interaction between radionuclides/metals and bacterial strains isolated from different extreme habitats including uranium mining waste piles as well as groundwater of a radioactive repository. For this purpose, a combination of spectroscopic (EXAFS, XANES, TRLFS), microscopic (TEM), microbiological and wet chemistry techniques is used. Elucidating the interaction mechanisms microbe/metals is helpful for understanding the role which bacteria play in the transport and mobility of toxic metals in the environment as well as their biotechnological application in the bioremediation of heavy metal contaminated waters. Another application of the isolated bacterial cells and their biocomponents is in the field of nanotechnology. Thus, the surface layer (S-layer) protein of Bacillus sphaericus JG-A12, a bacterium isolated from a uranium mining waste pile near the town of Johanngeorgenstadt in Germany, is used as template for the formation of noble metal (Pd, Pt, Au, etc.) nanoparticles for industrial application (e.g. catalysis). The structure and the size of these metallic nanoparticles were characterized using synchrotron radiation-based methods such as X-ray absorption spectroscopy.
  • Contribution to external collection
    Mendez-Vilas, A.: Communicating Current Research and Educational Topics and Trends in Applied Microbiology, Badajoz, Spain: Formatex, 2007, 978-84-611-9422-3, 108-119

Publ.-Id: 10241 - Permalink


Physik im Kraftfeld von Wissenschaft, Politik und Kultur
Sauerbrey, R.;
Was ist Physik für eine Wissenschaft? Eine populärwissenschaftliche Betrachtung der Wissenschaft Physik, erläutert anhand einer Liste ungelöster physikalischer Aufgaben.
  • Contribution to external collection
    Hans-Peter klöcking: Klassensitzungsvorträge 2000-2004, Mathematisch-Naturwissenschaftliche Klasse, Sitzungsberichte 13/2006, Erfurt: Verlag der Akademie gemeinnütziger Wissenschaften, 2006, 3-932295-69-2, 151-161

Publ.-Id: 10239 - Permalink


Crystal structure and its correlation to intrinsic and extrinsic magnetic properties of epitaxial hard magnetic Pr-Co films
Patra, A. K.; Neu, V.; Fähler, S.; Groetzschel, R.; Bedanta, S.; Kleemann, W.; Schultz, L.;
Pulsed laser deposited epitaxial PrxCo100−x (x=8.7–27.6 at. %) thin films were systematically studied as a function of Pr content. Structural and magnetic measurements reveal different phases for specific composition ranges. In some cases, the phases observed are in contrast to their bulk counterparts. Uniaxial anisotropy at room temperature is observed in all the films enabling excellent hard magnetic properties. Polarization decreases monotonically with the increase of Pr content, whereas coercivity exhibits a broad maximum near the highly anisotropic PrCo5 composition. For the optimum combination of coercivity and polarization, the measured (BH)max reaches values of 310 kJ/m3, which exceeds the highest-energy product value reported for RE-Co (RE=rare-earth) systems. Temperature-dependent ac susceptibility measurements reveal that films with x=8.7–20.4 undergo a spin reorientation from easy axis to easy cone, but films with x=22.9–27.6 maintain their uniaxial anisotropy throughout the temperature range of investigation. From the comparison of the structural investigations and the spin reorientation temperature measurements, it is concluded that the spin reorientation temperature is insensitive to the change in the lattice parameter of the PrCo5±δ phase.
  • Physical Review B 75(2007), 184417-1-184417-8

Publ.-Id: 10238 - Permalink


Intersubband relaxation dynamics in InGaAs/AlAsSb multiple quantum wells
Tribuzy, C. V.-B.; Schneider, H.; Ohser, S.; Sellesk, M.; Winnerl, S.; Grenzer, J.; Helm, M.; Neuhaus, J.; Dekorsy, T.; Biermann, K.; Künzel, H.;
Intersubband transitions in semiconductor quantum wells (QW) are crucial for mid-infrared lasers, detectors, and modulators. New compound materials such as lattice matched InGaAs/AlAsSb and strain compensated InGaAs/AlAs, both grown on InP, feature large conduction band discontinuities (>1eV) and allow the extension of the available wavelength range into the near infrared. Such short wavelengths require narrow QWs (<3 nm) where the first excited state inside the QW may be raised above indirect (X or L) valleys within the Brillouin zone. Quantum cascade lasers involving subbands above the indirect minimum have recently been reported [1].
We have studied intersubband relaxation dynamics in In0.53Ga0.47As/AlAs0.56Sb0.44 multiple QWs with thicknesses from 2.9 to 4 nm (corresponding to absorption wavelengths of 2.4 to 3.2 µm) by femtosecond pump-probe experiments [2]. At early delay times, all samples show an exponential decay of the transient transmission occurring with time constants of 0.8 to 1.5 ps. The relaxation dynamics at later delay times strongly depends on the QW thickness and doping. For very narrow QWs the observed bi-exponential decay indicates several competing relaxation channels. Here transfer of electrons to X- and L-states in the barriers, which exist in the case of n-type modulation doping, or in the wells is energetically possible. The data are analyzed in terms of an effective three-level configuration. Our results indicate that intervalley scattering in QWs is in the ps regime, much slower than in a bulk semiconductor. This observation suggests that intersubband lasing involving states above indirect minima of the well material should be possible, as also confirmed by the results of [1].
[ ] D. G. Revin et al., Appl. Phys. Lett. 90, 021108 (2007).
[2] C. V.-B. Tribuzy et al., Appl. Phys. Lett. 89, 171104 (2006).
Keywords: lattice matched InGaAs/AlAsSb, multiple quantum wells, intersubband relaxation, indirect valleys
  • Poster
    ITQW07 - The Ninth International Conference on Intersubband Transitions in Quantum Wells, 14.09.2007, Ambleside, Cumbria, United Kingdom

Publ.-Id: 10237 - Permalink


Design of an optical tomograph for the investigation of single and two phase pipe flows
Schleicher, E.; Hampel, U.; Da Silva, M. J.; Thiele, S.; Li, A.; Wollrab, E.;
We describe a fast optical tomography sensor which has been designed for the investigation of single and two phase flows in pipes and bubble columns. It enables image acquisition at frame rates of up to 4.5 kHz at roughly 2 mm image resolution. The sensor works similar to a conventional CT with 256 light emitters and 32 light receivers arranged about the object’s cross-section. The light emitters are sequentially flashed while the light receiver intensities are recorded synchronously. Primary area of application is single phase flows with dye tracers. Another potential application is the investigation of bubbly two phase flows at low gas fractions. Principle tests have been made for both problems.
Keywords: optical tomography, multiphase flow sensors, flow visualization
  • Measurement Science and Technology 19(2008)9, 094006

Publ.-Id: 10236 - Permalink


Beam parameter simulation of the Rossendorfer SRF gun and comparison with other RF photo injectors
Staufenbiel, F.; Arnold, A.; Büttig, H.; Janssen, D.; Kamps, T.; Lehnert, U.; Lipka, D.; Michel, P.; Möller, K.; Schneider, C.; Schurig, R.; Staats, G.; Teichert, J.; Will, I.; Xiang, R.;
For each accelerator the choice of the injector is a crucial topic and the best gun concept for the different demands must be determined. This paper presents simulated data of beam parameters from the Rossendorfer SRF gun and other RF photo injectors. The differences between normal and superconducting guns lead to different possibilities in the operation modes. Due to the high RF power losses in normal conducting guns the duty cycle is low and cw-mode operation impossible. For the SRF gun an exterior magnetic field for emittance compensation close to the photo cathode is prohibited due to the Meißner-Ochsenfeld effect. Therefore, other mechanisms for emittance compensation must be used. In the simulations RF focusing with a proper cathode visor and a solenoid focusing after the gun are considered.
  • Lecture (Conference)
    41st Advanced ICFA Beam Dynamics Workshop on Energy Recovery Linacs ERL 2007, 21.-25.05.2007, Warrington, UK

Publ.-Id: 10235 - Permalink


Inefficiency of intervalley transfer in narrow InGaAs/AlAsSb quantum wells
Tribuzy, C. V.-B.; Ohser, S.; Sellesk, M.; Winnerl, S.; Schneider, H.; Helm, M.; Neuhaus, J.; Dekorsy, T.; Biermann, K.; Künzel, H.;
It has been shown that the performance of GaAs/AlGaAs quantum cascade lasers (QCL) can be severely hampered by the influence of indirect states [1]. Yet very recent work has demonstrated that lasing can be achieved in InGaAs/AlAsSb [2], InGaAs/AlAs [3] and InAs/AlSb [4] at around 3 um, where the upper laser level lies above some indirect minima.
We have performed a pump-probe investigation of intersubband relaxation in doped narrow In0.53Ga0.47As/AlAs0.56Sb0.44 multi quantum wells (QW) with different thicknesses from 2.9 to 4 nm, grown by MBE latticed matched to an InP substrate. The measurements were performed with a high-repetition-rate (78 MHz) optical parametric oscillator tunable between 1.1 and 3.3 um with a pulse length of 280 fs [5]. The extremely high signal-to-noise ratio allows us to analyze the decay dynamics in detail.
In a well-doped 3 nm QW, where the second subband lies above the InGaAs X-minimum, we observe a non-exponential decay, which can be very well reproduced with two exponentials of 1.5 and 6.2 ps (Fig. 1). Analyzing this behavior with three-level rate equations, the first time constant represents the combined decay rate from the upper subband to the lower one and to the X-state. The second time constant correspond to the return time from the X-level to the ground state. This means that the intervalley transfer time (≥ 2 ps) is much longer than known from bulk systems and implies that population inversion in a QCL can persist, thus explaining the functioning of QCLs at wavelength as short as 3 um.
As a cross check, we also investigated wider-QW samples, where the second subband lies below the X-level. As expected, these were found to exhibit a simple mono-exponential behavior. We are presently extending this investigation to coupled QWs, which bear closer similarity to actual QCL structures.

References
[1] L. R. Wilson et al., Appl. Phys. Lett. 81, 1378 (2002).
[2] D. G. Revin et al., Appl. Phys. Lett. 90, 021108 (2007).
[3] M. P. Semtsiv et al., Appl. Phys. Lett. 90, 051111 (2007).
[4] K J. Devenson et al., Appl. Phys. Lett. 90, 111118 (2007).
[5] C. V.-B. Tribuzy et al., Appl. Phys. Lett. 89, 171104 (2006).
Keywords: multi quantum wells, intersubband relaxation, InGaAs/AlAsSb
  • Lecture (Conference)
    15th International Conference on Nonequilibrium Carrier Dynamics in Semiconductors HCIS15, 23.-27.07.2007, Tokyo, Japan
  • Physica Status Solidi (C) 5(2008), 229

Publ.-Id: 10234 - Permalink


Detailed Analysis of Amorphous Silicon Passivation Layers Deposited in Industrial In-Line and Laboratory-type PECVD Reactors
Hofmann, M.; Schmidt, C.; Kohn, N.; Grambole, D.; Rentsch, J.; Glunz, S.; Preu, R.;
Amorphous silicon (a-Si) layers for the passivation of p-type silicon wafer surfaces are investigated. The first main topic is the thermal stability of a-Si passivation. Here, an improved thermal stability in annealing (400 °C) and firing processes (wafer temp. 550 °C) could be achieved by stacking a-Si layers with PECVD SiOx layers of different thickness. Hydrogen depth profiling using nuclear reaction analysis shows a hydrogen concentration of 11 at% in the bulk of the a-Si. After firing of single layer a-Si samples a hydrogen concentration peak at the a-Si / c-Si interface could be observed. The second major topic of this paper is the deposition of a-Si layers using an industrial-type inline PECVD reactor. Excellent surface passivation (>1 ms on 1 Ohm cm FZ wafers) can be reported. These a-Si layers are further characterised using FTIR and spectroscopic ellipsometry.
Keywords: a-Si, annealing, CVD based deposition, lifetime, PECVD
  • Lecture (Conference)
    22nd European Photovoltaic Solar Energy Conference and Exhibition, 03.-07.09.2007, Milano, Italy
  • Contribution to proceedings
    22nd European Photovoltaic Solar Energy Conference and Exhibition, 03.-07.09.2007, Milano, Italy
    Proceedings of 22nd European Photovoltaic Solar Energy Conference and Exhibition, 1528-1531

Publ.-Id: 10233 - Permalink


Status of the Superconducting RF Photo-Injector Development
Teichert, J.; Arnold, A.; Buettig, H.; Janssen, D.; Justus, M.; Lehnert, U.; Michel, P.; Moeller, K.; Murcek, P.; Schneider, C.; Schurig, R.; Staufenbiel, F.; Xiang, R.; Stephan, J.; Lehmann, W.-D.; Kamps, T.; Klemz, G.; Will, I.; Matheisen, A.; Horst, B. V. D.; vom Stein, P.; Volkov, V.; Lipka, D.;
A status report of the superconducting RF photo electron injector development at Forschungszentrum Dresden-Rossendorf (FZD) will be given. The SRF gun project is a collaboration of BESSY, DESY, MBI and FZD and aims at the installation of a high average current CW photo injector at the ELBE superconducting electron linac. Main design parameters of the SRF gun are an electron energy of 9.5 MeV, a maximum average current of 1 mA, transverse normalized emittances (rms) of 1 mm mrad at 77 pC and 2.5 mm mrad at 1 nC bunch charge. The 1.3 GHz niobium cavity consists of three full cells with TESLA geometry, a specially designed half-cell in which the photo cathode is placed, and a choke filter in order to prevent rf losses at the cathode side of the cavity. Presently, the helium tank welding and cavity treatment have been finished. The cavity is now in the FZD and the cryomodule assembly has been started. Various subsystems like cathode cooler, cavity tuners, cryostat components, and the niobium cavity are still being tested and measured. A photo cathode preparation system was developed and installed. The equipment is now in operation and the first series of Cs2Te photo cathodes have been produced. The development of the 262 nm driver laser system for the high charge mode (500 kHz, 1 nC) is finished. A diagnostic beamline, which is especially designed for the SRF gun parameter measurement, is being build up.
  • Lecture (Conference)
    ERL07 - 41st Advanced ICFA Beam Dynamics Workshop on Energy Recovery Linacs, 21.-25.05.2007, Warrington, United Kingdom
  • Poster
    13th International Workshop on RF Superconductivity, 14.-19.10.2007, Peking, China

Publ.-Id: 10232 - Permalink


Current status of Riga dynamo experiment
Gailitis, A.; Lielausis, O.; Platacis, E.; Gerbeth, G.; Stefani, F.;
Almost eight years ago the first magnetic field was created by an intense flow of molten sodium in the Riga dynamo experiment. Since then a lot of data has been collected. In our geometry dominates the vertical field component which is remarkably strong with a maximum value exceeding 0.1 T. The principal symmetry of field with respect to the vertical axis is m=1. As such a strong field considerably deforms the flow some additional modes with m=3, 5 and even 7 are observed. The field pattern rotates around the axis with a frequency of 1.2 to 1.8 Hz. This rotation is not completely constant as turbulence in sodium flow creates some turbulence in magnetic field, too. Nevertheless the field turbulence is low and causes no considerable secondary electromagnetic effects. Hence the field generation itself is based on the mean-flow only. For the last two years the experiment was in repair. Among other things a worn out sodium seal was replaced by a magnetic coupler. This will widen the measuring possibilities particularly in the vicinity of the threshold. We plan to restart experiments this summer and hope to report new results at the meeting.
  • Lecture (Conference)
    MHD Laboratory Experiments for Geophysics and Astrophysics, 01.-03.10.2007, Catania, Italy

Publ.-Id: 10231 - Permalink


Micro channeling study of crystal damage in ZnO by ion implantation
Munnik, F.; Grambole, D.; Bischoff, L.; Grötzschel, R.;
The study of crystal damage by ion implantation with high current densities is important to obtain better insights in the processes taking place during ion implantation (under ex-treme conditions). The combination of a Focussed Ion Beam (FIB) and an ion microprobe with channeling capability within the same institute facilitates this type of research by enabling small implanted areas to be analysed. In this study Ga was implanted by FIB into ZnO at different ion fluxes and fluences. The fluences were varied from 1013 to 1017 at/cm² and fluxes of 5•1013 at. cm-²s-1 and 5•1018 at. cm-²s-1 were used. The implanted areas were analysed with a 1 MeV He+ ion beam focussed to well below the size of the implanted area (about 100 x 100 µm²). The damage of the sample caused by the ion microprobe was also studied and the fluence for the analysis has been chosen so low that no significant damage occurs.
The results of the channeling measurements are presented and the effects of flux and flu-ence on the crystal damage are discussed. It can be noted that the effects of the flux are relatively minor and crystal damage in ZnO occurs only at high fluences
Keywords: FIB, Ion microprobe, damage
  • Lecture (Conference)
    18th International Conference on Ion Beam Analysis, 23.-28.09.2007, Hyderabad, India

Publ.-Id: 10230 - Permalink


Firing Stable Surface Passivation Using All-PECVD Stacks of SiOx:H and SiNx:H
Hofmann, M.; Kambor, S.; Schmidt, C.; Grambole, D.; Rentsch, J.; Glunz, S.; Preu, R.;
Surface passivation stack systems, all deposited using PECVD, are investigated. Stacks of SiOx, SiNx and SiOx (PECVD-ONO) are shown to be a suitable passivation layer system for the rear of silicon solar cells (p type bulk). The thermal stability during annealing at 425 °C and firing of screen printed front contacts could be shown with surface recombination velocities below 60 cm/s after firing. Solar cell precursors without metallisation showing implied Voc values above 680 mV are presented. Hydrogen depth profiling using nuclear reaction analysis (NRA) shows the hydrogen distribution after deposition and different thermal treatments. Finally, solar cells using the new stack system as rear passivation and laser-fired rear contacts are presented with a peak efficiency of 19.4 %.
Keywords: CVD Based Deposition, Deposition, Lifetime, Passivation, PECVD, Silicon-Nitride, SiO2
  • Lecture (Conference)
    22nd European Photovoltaic Solar Energy Conference and Exhibition, 03.-07.09.2007, Milano, Italy
  • Contribution to proceedings
    22nd European Photovoltaic Solar Energy Conference and Exhibition, 03.-07.09.2007, Milano, Italy
    Proceedings of the 22nd European Photovoltaic Solar Energy Conference and Exhibition, 1030-1033

Publ.-Id: 10229 - Permalink


Intersubband-dephasing in an undoped multi-quantum well
Wagner, M.; Stehr, D.; Schneider, H.; Winnerl, S.; Helm, M.; Andrews, M.; Roch, T.; Strasser, G.;
We have investigated the dephasing time associated with intersubband transitions of photocarriers in an undoped GaAs/AlGaAs multi quantum-well heterostructure. Our measurements were performed directly in the time-domain. After optical generation of electron-hole pairs across the band-gap, a resonant THz-pulse excited the electrons to the second subband and generated a coherent polarization involving the ground and the first excited subbands. The re-radiation from this polarization was detected by a cross-correlation technique with a second THz-pulse. The polarization was observed to decay with short decay-times between 50 fs and approx. 200 fs. They depend on the carrier concentration which was adjusted by the optical excitation power. These time constants determine directly the linewidth of this intersubband transition. At low temperatures, the dephasing signals show a pronounced beating at all optical excitation powers which we attribute to excitonic effects such that more than two energy levels are involved in the interaction with the THz-pulse.
By varying the excitation power, we also found a strong depolarization shift of the absorption line.
Keywords: GaAs/AlGaAs multi quantum-well heterostructure, intersubband dephasing, depolarization shift
  • Lecture (Conference)
    71. Jahrestagung der Deutschen Physikalischen Gesellschaft und DPG Frühjahrstagung des Arbeitskreises Festkörperphysik, 26.-30.03.2007, Regensburg, Deutschland

Publ.-Id: 10226 - Permalink


Cryogenic experiences at the ELBE accelerator
Schneider, C.; Haberstroh, C.;
The superconducting linear accelerator ELBE (Electron Linac for beams with high Brilliance and low Emittance) at the Forschungszentrum Rossendorf has a wide variety of experimental applications ranging form nuclear spectroscopy over x-ray to infrared FEL experiments. The two superconducting accelerator modules of ELBE and in the future a superconducting photo injector are connected to a 200 W at 1.8 K cryogenics plant. The presentation will give an overview of the ELBE cryogenics system and will focus on operation experience with the superconducting cryogenics modules and of the cryogenic plants. Specific problems of fill level stability, helium pressure stability and cool down procedures of the cryomodules will be presented. Specific problems of the ELBE helium plant concerning purity, maintenance and reliability will be discussed, too.
  • Lecture (Conference)
    CryoPraque 2006, 17.-21.07.2006, Prag, Czech Republic

Publ.-Id: 10225 - Permalink


Phase inhomogeneities in the charge-orbital-ordered manganite Nd0.5Sr0.5MnO3 revealed through polaron dynamics
Prasankumar, R. P.; Zvyagin, S. A.; Kamenev, K. V.; Balakrishnan, G.; Paul, D. M.; Taylor, A. J.; Averitt, R. D.;
Ultrafast midinfrared spectroscopy is used to probe dynamics in the intermediate bandwidth manganite Nd0.5Sr0.5MnO3. In the majority paramagnetic and ferromagnetic phases, the early time dynamics are consistent with the excitation and subsequent redressing of uncorrelated lattice polarons, with longer time dynamics related to spin-lattice thermalization. These polaron excitations reveal the intrinsically inhomogeneous nature of these phases. At lower temperatures we observe ultrafast melting of charge-orbital order, liberating quasiparticles that subsequently relax into bound polaronic states on a subpicosecond time scale. The temperaturedependent amplitude of the polaron excitations scales with the volume fraction of the CE phase. Thus, polaron dynamics, as measured using ultrafast spectroscopy, serve as a sensitive probe of phase inhomogeneity.
  • Physical Review B 76(2007), 020402(R)

Publ.-Id: 10224 - Permalink


The effect of metal coated carbon black filling on the physical and magnetic behaviour of natural and synthetic rubbers
Ürögiova, E.; Piest, J. A.; Heinrich, G.; Probst, N.; Pronin, A.; Wosnitza, J.;
Elastomer materials filled with magnetically and/or electrically susceptible particles promise to have different functionality than conventional elastomers, and therefore, could likely be applied in state-of-the-art control technologies [1]. Of particular interest are elastomers, which are filled with metal coated carbon black (MCCB). These filler fulfils its reinforcing function to the rubber and also chang the elastomer´s electro-magnetic properties. Thus, the rheological and viscoelastic properties of rubber can be changed and controlled by subjecting the compound to a magnetic field.
  • Poster
    PMA 2007 & SRC 2007 - 2nd International Conference on Polymeric Materials in Automotive 2007 & 19th Rubber Conference 2007, 15.-17.05.2007, Bratislava, Slovakia

Publ.-Id: 10223 - Permalink


High-field magnetization measurements on Er2Fe17 single crystals
Kuz´Min, M. D.; Skourski, Y.; Skokov, K. P.; Müller, K.-H.;
If the easy magnetization direction of a 3d-4f ferrimagnet is perpendicular to a high-symmetry axis, a magnetic field applied in the easy direction may induce a number of first-order transitions, the first (lowestfield ) one of which carries the information sufficient for an unambiguous determination of the intersublattice molecular field. This idea has been used to find the molecular field in Er2Fe17. To this end, magnetization curves have been measured in pulsed magnetic fields of up to 50 T applied along [100] or [001]. In order to obtain a reference value of the molecular field by a conventional method, high-field measurements have also been performed on crystals free to rotate. The molecular fields determined by both techniques are in good agreement with each other as well as with the values deduced from literature data.
  • Physical Review B 75(2007), 184439

Publ.-Id: 10222 - Permalink


Coherent detection of terahertz radiation with scalable antennas
Winnerl, S.; Peter, F.; Nitsche, S.; Dreyhaupt, A.; Drachenko, O.; Schneider, H.; Helm, M.; Köhler, K.;
We present a novel non-resonant photoconductive THz detection antenna. Compared to electro-optic sampling, photoconductive antennas can be integrated more easily into compact THz setups, e.g. by using substrate lenses and by coupling to optical fibers. However, since the antenna gap of typical THz detection antennas is usually only a few µm wide, the alignment of photoconductive antennas is not simple and the possibility to move the antenna is limited. Here we report on a THz system consisting of a scalable THz emitter based on an interdigitated electrode structure [1] and a detection antenna with similar electrode geometry. While the THz emitter is fabricated on SI-GaAs substrate, various materials with short carrier lifetimes are used for the detection antenna. Detection antennas based on LT-GaAs and GaAs implanted with As+ (dual-energy implants, 1MeV and 2.4 MeV, doses in the range from 10e13 cm-2 to 10e16 cm-2) and N+ (dual-energy implants, 0.4 MeV and 0.9 MeV, doses in the range from 10e12 cm-2 to 10e14 cm-2) are compared. The strongest detected signals are found for As+ implantations with a dose in the 10e14 cm-2 range. Antenna currents of up to 7 nA are observed, the corresponding spectrum extends up to 4 THz. The THz radiation was modulated with frequencies up to 100 kHz. In this range the detected signal was independent of the modulation frequency. From the RC time constant of the detector antenna a cut-off frequency of 500 kHz is expected. Furthermore the dependence of the detected signal on the gating laser power and the spot size of the gating beam were studied and optimum conditions are discussed. In conclusion, the scalable antennas constitute an efficient, easy-to-use, symmetric emitter-detector pair.
[1] A. Dreyhaupt, S. Winnerl, T. Dekorsy, and M. Helm, Appl. Phys. Lett. 86, 121114 (2005).
  • Poster
    The Joint 32nd International Conference on Infrared and Millimeter Waves and the 15th International Conference on Terahertz Electronics, 02.-09.09.2007, Cardiff, UK
  • Contribution to proceedings
    The Joint 32nd International Conference on Infrared and Millimeter Waves and the 15th International Conference on Terahertz Electronics, 02.-09.09.2007, Cardiff, UK
    Conference Digest of the Joint 32nd International Conference on Infrared and Millimeter Waves and the 15th International Conference on Terahertz Electronics, 1-4244-1439-3, 740-741

Publ.-Id: 10221 - Permalink


Easy-to-use scalable antennas for coherent detection of THz radiation
Winnerl, S.; Nitsche, S.; Peter, F.; Drachenko, O.; Schneider, H.; Helm, M.; Köhler, K.;
We present a terahertz transceiver consisting of a photoconductive emitter and detection antenna with similar electrode geometry. Here, we focus on the detection antenna. Compared to electro-optic sampling, photoconductive antennas can be integrated more easily into compact THz setups, e.g. by using substrate lenses and by coupling to optical fibers. However, since the antenna gap of typical THz detection antennas is usually only a few µm wide, the alignment of photoconductive antennas is not simple and the possibility to move the antenna is limited. The transceiver consists of a scalable THz emitter based on an interdigitated electrode structure [1] and a detection antenna with similar electrode geometry. While the THz emitter is fabricated on SI-GaAs substrate, various materials with short carrier lifetimes are used for the detection antenna. Detection antennas based on LT-GaAs and GaAs implanted with As+ (dual-energy implants, 1 MeV and 2.4 MeV, doses in the range from 1013 cm-2 to 1016 cm-2) and N+ (dual-energy implants, 0.4 MeV and 0.9 MeV, doses in the range from 1012 cm-2 to 1014 cm-2) are compared. The strongest detected signals are found for As+ implantations with a dose in the 1014 cm-2 range. In Fig. 1 THz transients detected with an LT-GaAs antenna are shown together with the corresponding power spectra. The spectra extend up to 4 THz. The dependence of the detected signal on the gating laser power and the spot size of the gating beam were studied. For 70 mW power of the gating beam, a spot size in the range from 200 to 700 µm yields the strongest signals. The lower limit is determined by a nonlinear dependence of the signal on the excitation density. The upper limit for the spot size is simply given by the size of the antenna, which was 1 mm x 1 mm. Furthermore the detector was used to study the spatial profile of the THz beam. In this experiment, the detector was placed 27 mm behind the emitter and scanned across the THz beam. The beam had Gaussian shape with a full width at half maximum of 8 mm and 18 mm for frequency components of 0.7 THz and 0.2 THz, respectively. This experiment demonstrates the potential of the detector to map out unfocussed THz fields with a good signal-to-noise ratio. In conclusion, the scalable antennas constitute an efficient, easy-to-use, symmetric emitter-detector pair.
[1] A. Dreyhaupt, S. Winnerl, T. Dekorsy, and M. Helm, Appl. Phys. Lett. 86, 121114 (2005).
  • Lecture (Conference)
    13th International Conference on Narrow Gap Semiconductors, 08.-12.07.2007, Guildford, UK
  • Springer Proceedings in Physics 119(2008), 167-169

Publ.-Id: 10220 - Permalink


Forschung in hohen Magnetfeldern
Wosnitza, J.;
hat nicht vorgelegen
  • Invited lecture (Conferences)
    Physikalisches Festkolloquium der Universität Karlsruhe (TH), 27.10.2006, Karlsruhe, Deutschland

Publ.-Id: 10219 - Permalink


Intervally transfer in narrow InGaAS/AlAsSb quantum wells studied by pump-probe spectroscopy
Tribuzy, C. V.-B.; Ohser, S.; Sellesk, M.; Winnerl, S.; Schneider, H.; Helm, M.; Neuhaus, J.; Dekorsy, T.; Biermann, K.; Künzel, K.;
The development of quantum cascade lasers (QCL) for frequencies as low as 3 µm has gained a lot of attention recently. While the performance of GaAs/AlGaAs quantum cascade lasers (QCL) can be severely hampered by the influence of indirect states [1], very recent work has demonstrated that lasing can be achieved in InGaAs/AlAsSb [2], InGaAs/AlAs [3] and InAs/AlSb [4] at around 3 m, where the upper laser level lies above some indirect minima.
We have performed a pump-probe investigation of intersubband relaxation in doped narrow In0.53Ga0.47As/AlAs0.56Sb0.44 multi quantum wells (QW) with different thicknesses from 2.9 to 4 nm, grown by MBE latticed matched to an InP substrate. The measurements were performed with a high-repetition-rate (78 MHz) optical parametric oscillator tunable between 1.1 and 3.3 µm with a pulse length of 280 fs [5]. The extremely high signal-to-noise ratio allows us to analyze the decay dynamics in detail.
The relaxation dynamics of a well-doped 3 nm QW was studied with 130 pJ pulses at a wavelength of 2.4 µm. In this sample, where the second subband lies above the InGaAs X-minimum, we observe a non-exponential decay, which can be very well reproduced with two exponentials of 1.5 and 6.2 ps (Fig. 1). Analyzing this behavior with three-level rate equations, tau =1.5 ps represents the combined decay rate from the upper subband to the lower one and to the X-state. 6.2 ps correspond to the return time from the X-level to the ground state (see inset). This means that the intervalley transfer time (tauX2 ≥ 2 ps) is much longer than known from bulk systems and implies that population inversion in a QCL can persist, thus explaining the functioning of QCLs at wavelength as short as 3 µm.
As a cross check, we also investigated wider-QW samples, where the second subband lies below the X-level. As expected, these were found to exhibit a simple mono-exponential behavior. We are presently extending this investigation to coupled QWs, which bear closer similarity to actual QCL structures.

References
[1] L. R. Wilson et al., Appl. Phys. Lett. 81, 1378 (2002).
[2] D. G. Revin et al., Appl. Phys. Lett. 90, 021108 (2007).
[3] M. P. Semtsiv et al., Appl. Phys. Lett. 90, 051111 (2007).
[4] K J. Devenson et al., Appl. Phys. Lett. 90, 111118 (2007).
[5] C. V.-B. Tribuzy et al., Appl. Phys. Lett. 89, 171104 (2006).
  • Lecture (Conference)
    13th International Conference on Narrow Gap Semiconductors, 08.-12.07.2007, Guildford, UK

Publ.-Id: 10218 - Permalink


Magnetic-field and temperature-dependent Fermi surface of CeBiPt
Wosnitza, J.;
hat nicht vorgelegen
  • Invited lecture (Conferences)
    9th Japanese-German Symposium, 26.-29.08.2006, Hikone, Japan

Publ.-Id: 10216 - Permalink


THz Strahlung von einem skalierbaren photoleitenden Emitter
Peter, F.; Nitsche, S.; Winnerl, S.; Dreyhaupt, A.; Schneider, H.; Helm, M.;
Terahertz radiation from a large-area Photoconductive device
  • Lecture (others)
    THz – Frischlinge - Meeting, 01.-04.04.2007, Freiburg, Germany

Publ.-Id: 10215 - Permalink


The High Magnetic Field Laboratory at Dresden
Wosnitza, J.;
hat nicht vorgelegen
  • Invited lecture (Conferences)
    Sino-German Workshop, 17.-19.10.2005, Dresden, Deutschland

Publ.-Id: 10214 - Permalink


Forschung in hohen Magentfeldern
Wosnitza, J.;
Abstrakt hat nicht vorgelegen!
  • Invited lecture (Conferences)
    Physikalisches Kolloquium der Universität Leipzig, 08.05.2007, Leipzig, Germany

Publ.-Id: 10212 - Permalink


Pages: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116] [117] [118] [119] [120] [121] [122] [123] [124] [125] [126] [127] [128] [129] [130] [131] [132] [133] [134] [135] [136] [137] [138] [139] [140] [141] [142] [143] [144] [145] [146] [147] [148] [149] [150] [151] [152] [153] [154] [155] [156] [157] [158] [159] [160] [161] [162] [163] [164] [165] [166] [167] [168] [169] [170] [171] [172] [173] [174] [175] [176] [177] [178] [179] [180] [181] [182] [183] [184] [185] [186] [187] [188] [189] [190] [191] [192] [193] [194] [195] [196] [197] [198] [199] [200] [201] [202] [203] [204] [205] [206] [207] [208] [209] [210] [211] [212] [213] [214] [215] [216] [217] [218] [219] [220] [221] [222] [223] [224] [225] [226] [227] [228] [229] [230] [231] [232] [233] [234] [235] [236] [237] [238] [239] [240] [241] [242] [243] [244] [245] [246] [247] [248] [249] [250] [251] [252] [253] [254] [255] [256] [257] [258] [259] [260] [261] [262] [263]