Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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31828 Publications
RBE of soft X-rays for chromosomal damage determined for two human mammary epithelial cell lines
Lehnert, A.; Beyreuther, E.; Enghardt, W.; Lessmann, E.; Pawelke, J.; Wagner, W.; Dörr, W.;
There is an ongoing discussion on the relative biological effectiveness (RBE) of low-energy X-rays below about 50 keV. Considering their wide application in diagnostic radiology and particularly in mammography, RBE has to be determined for an appropriate biological object. Further, the dependence on the photon energy distribution of the radiation sources has to be considered.
We have determined the effectiveness of low-energy X-rays by X-ray tube irradiation for two human mammary epithelial cell lines, MCF-12A and 184A1. For the soft X-ray irradiation, a tungsten-anode X-ray tube operated at either 10 kV (no filtration) or 25 kV (0.3 mm Al filter) was used. The reference radiation source was a 200 kV X-ray tube filtered with 0.5 mm Cu. The dose rate for all irradiations was 0.3 -1.9 Gy/min, and doses in the range 0.5 - 5 Gy were applied. The cytokinesis-block micronucleus (MN) test was used to assess chromosomal damage in the cell line MCF-12A. The fraction of binucleated cells (BNC) with MN, the number of MN per BNC and the number of MN per BNC with MN were determined for all radiation qualities. Additionally, chromosomal aberrations were studied by conventional Giemsa-staining in both cell lines. Dicentrics, rings and excess acentric fragments were assessed. In addition, first results will be presented on stable chromosome aberrations studied by whole-chromosome FISH painting of chromosomes 1, 8 and 17 for the cell line 184A1.
All data has been fitted to the linear-quadratic model. The soft X-rays were observed to be more effective in inducing radiation damage, RBE increasing with decreasing photon energy. Whereas the RBE for 25 kV X-rays determined by the MN test resulted in a value of 1.1 - 1.4, values of up to 3 were obtained for the 10 kV X-rays for the cell line MCF-12A. The chromosomal aberration data for both cell lines resulted in an RBE value of up to 2 for both soft X-ray radiation qualities.
The data obtained in the present work confirm the observations of other authors for the photon energy dependence of RBE in this energy range. However, the study of the detailed RBE dependence on photon energy is possible only at a monochromatic X-ray source. The progress in the installation of a quasi-monochromatic channelling radiation source at the ELBE accelerator at Forschungszentrum Rossendorf will be presented.
Keywords: RBE, soft X-rays, micronuclei, chromosomal aberrations, MCF-12A, 184A1, human mammary epithelial cells, ELBE
  • Lecture (Conference)
    The 35th Annual Meeting of the European Radiatiton Research Society, 22.-25.08.06, Kiev, Ukraina
  • Contribution to proceedings
    The 35th Annual Meeting of the European Radiatiton Research Society, 22.-25.08.2006, Kiev, Ukraine
    Book of Abstracts, 32

Publ.-Id: 8808 - Permalink

NMR-study on the structure and stability of 4-substituted aromatic iodosyl compounds
Hiller, A.; Patt, J. T.; Steinbach, J.;
Two 4-substituted aromatic iodosyl compounds were investigated with regard to their solubility, stability and chromatographic behaviour. 1-Iodosyl-4-methoxy- and 1-iodosyl-4-nitro-benzene are soluble in methanol and provide acceptable 1H and 13C NMR spectra; however, gradual oxidation of the solvent was observed. LC-MS analyses suggest that unlike the parent substance, iodosylbenzene, which has a polymeric structure, both compounds rather exist in the monomeric form.
Keywords: NMR; 1H NMR; 13C NMR; aromatic iodosyl compounds; oxidation; HPLC-MS

Publ.-Id: 8806 - Permalink

The use of the GDT based neutron source as driver in a sub-critical burner of minor actinides
Noack, K.;
The lecture is divided in two parts. First, as introduction on the subject, in part I a brief overview is given on the actual state in the area of transmutation of radioactive nuclear waste by means of reactors and driven sub-critical systems. In part II the application of the GDT-based neutron source as driver in a sub-critical system dedicated to the transmutation of minor actinides is discussed. This GDT driven minor actinide burner is compared with the corresponding accelerator driven system, which is presently considered as the favored option.
  • Lecture (others)
    Institutskolloquium des Budker Instituts Novosibirsk, 26.09.2006, Novosibirsk, Russia

Publ.-Id: 8793 - Permalink

A Comparative Study of U(V)O2+- and U(VI)O22+-Carbonato Complexes in Aqueous Solution
Ikeda, A.; Hennig, C.; Rossberg, A.; Scheinost, A.; Bernhard, G.; Mizuoka, K.; Ikeda, Y.;
The properties of pentavalent uranium have not been well understood yet, although a large number of studies have been performed and reported concerning the uranium chemistry up to now, because it is very unstable due to its disproportionation reaction to U(IV) and U(VI). At the moment, the uranyl tricarbonate complex, [UO2(CO3)3]n-, is the only known species that can be stabilized as U(V) in aqueous solution. In order to elucidate the properties of this complex in detail, we performed electrochemical (cyclic voltammetry) and spectroscopic (UV-visible absorption and XAFS) measurement.
From the electrochemical study, it was found that the UVO2+-carbonate complex was stable only in the limited concentration range of 0.8 M < [Na2CO3] < 1.4 M, and a Pt- or Au working electrode is more efficient to reduce UVIO22+ to UVO2+ than a glassy carbon electrode. Therefore, the UVO2+ samples for XAFS measurements were prepared in 1.4 M-Na2CO3 solution by using a Pt-mesh working electrode. The UVO2+ solution was transferred to a glass cuvette that was completely sealed by hot melting to avoid the possibility of any O2-penetration in the cuvette during sample transportation and measurement. The results of XAFS measurement indicated that both uranyl(V) and uranyl(VI) ions form the tricarbonate complex, [UO2(CO3)3]n- (n = 5 for uranyl(V) and 4 for uranyl(VI)). The coordination occurs in a bidentate manner and, in general, all bond lengths for the uranyl(V) complex were longer than those for the uranyl(VI) complex. Additionally, a chemical shift was also observed in the U LI- and LIII-XANES spectra.
  • Poster
    Actinide XAS 2006, 18.-20.09.2006, Karlsruhe, Germany
  • Contribution to proceedings
    Actinide XAS 2006, 18.-20.09.2006, Karlsruhe, Germany
    Speciation Techniques and Facilities for Radioactive Materials at Synchrotron Light Sources: OECD, ISBN 978-92-64-99006-7, 225-231

Publ.-Id: 8792 - Permalink

Investigation of Element Specific Hysteresis Loops of ion-induced Magnetically Patterned Ni81Fe19/Ru/Co90Fe10 Films with Magnetic Soft X-ray Microscopy.
Küpper, K.; Bischoff, L.; Mattheis, R.; Fischer, P.; Kim, D.-H.; Faßbender, J.ORC
We present a layer resolved magnetic soft x-ray microscopy study of a RKKY coupled Ni81Fe19/Ru/Co90Fe10 layered system where magnetic patterns were imprinted by using a 60 keV fine focused Co ion beam so as to change the coupling from antiferromagnetic to ferromagnetic on a micron scale. Thereby artificial structures in form of stripes with locally varying interlayer exchange coupling are generated. Utilizing the elemental specificity of high-resolution full field soft x-ray microscopy at the Co L3 and the Ni L3 edges we determined the magnetic domain configuration during full magnetization reversal processes locally and layer resolved. In addition to the locally varying interlayer exchange coupling across the Ru layer our data verify a direct exchange coupling within each ferromagnetic layer in the magnetically patterned structure. We conclude that the magnetization reversal behaviour of the irradiated
stripes is largely influenced by the surrounding magnetic film for both, the Permalloy and the Co90Fe10 layer.
Keywords: magnetism, RKKY, exchange coupling, magnetic patterning, x-ray microscopy, XMCD
  • Poster
    MRS Fall Meeting 2006, 27.11.-01.12.2006, Boston, USA

Publ.-Id: 8791 - Permalink

Ripple induced modifications of magnetic properties
Liedke, M. O.; Keller, A.; Facsko, S.; Fassbender, J.ORC
Self-organized ripple formation during ion erosion of a Si wafer is used to create a template system with a well defined roughness of uniaxial symmetry. By using special buffer layers subsequent thin magnetic film deposition by molecular beam epitaxy leads to a periodically modulated magnetic thin film with drastically modified magnetic properties with respect to a nominally “flat” film of the same thickness. In the case of Permalloy thin films, an enhancement of the uniaxial in-plane anisotropy by approximately a factor of 20 is observed (see Fig. 2). The enhancement can be explained by a combination of step induced dipolar and magnetocrystalline surface anisotropy contributions.
If a ferromagnet/antiferromagnet-bilayer is deposited a superposition of ripple-induced uniaxial anisotropy and exchange coupling induced unidirectional anisotropy is observed. Since the direction of the unidirectional anisotropy depends only on the magnetic field direction during a field cooling procedure any angle between both anisotropy contributions can be set. The observed angular dependence of the magnetization reversal behavior is in perfect agreement with simulations based on the Stoner-Wolfarth coherent rotation model.
Keywords: magnetism, ripples, ion errosion, magnetic anisotropy, exchange bias
  • Poster
    International Workshop on Nanomagnets by Self-Organization, 08.-11.10.2006, Eisenerz, Österreich
  • Lecture (Conference)
    MRS Fall Meeting 2006, 27.11.-01.12.2006, Boston, USA

Publ.-Id: 8790 - Permalink

Magnetic properties of thin magnetic films on rippled and smooth substrates
Fassbender, J.ORC
A review about the influence of the substrate, buffer layers and the substrate morphology on the magnetic properties of ultrathin films is presented.
Keywords: magnetism, ripple, ion erosion, exchange bias, magnetic anisotropies
  • Invited lecture (Conferences)
    Workshop on Properties of ultrathin magnetic films, 07.-09.09.2006, Bialowieza, Poland

Publ.-Id: 8789 - Permalink

Growth of carbon and carbon nitride thin films by low energy (~150 eV) N2/Ar ion assisted evaporation of C60
Berndt, M.; Vinnichenko, M.; Kreissig, U.; Abrasonis, G.; Kolitsch, A.; Möller, W.;
The influence of assisting low energy (~150 eV) ion bombardment during the deposition of evaporated C60 on the composition and bonding structure is investigated. Thin films were grown on Si (100) substrates by evaporation of C60 with assisting low-energy (~150 eV) N or Ar ion bombardment at different ion to molecule arrival ratios Iion/Imolecule (4-10) and substrate temperatures (RT-300°C). The film composition and areal density was analysed using elastic recoil detection analysis, while the obtained chemical bond structure was analysed by Raman spectroscopy. Spectroscopic ellipsometry was used to determine the film thickness. Raman spectra of the films grown without ion assistance show similar features as the Raman spectrum of initial C60 powder. The additional N or Ar assistance results in a decomposition of the C60 molecules independent of the ion type and Iion/Imolecule, resulting in a bonding structure similar to that observed in a-C or a-CNx. The N and Ar atomic ratios in the films (16-27 at. % for N and 2-4 at.% for Ar) are proportional to Iion/Imolecule and no saturation limit was reached under the present working conditions. The amount of deposited material decreases with increasing Iion/Imolecule for a given substrate temperature for both type of ions. Besides, it is observed that ion bombardment results in a densification of the films compared to the films obtained without ion assistance. The results are discussed on the basis of ion induced displacements of C atoms in the fullerene molecules resulting in a decomposition of the initial fullerene structure.
  • Poster
    Tenth International Conference on Plasma Surface Engineering - PSE 2006, 10.-15.09.2006, Garmisch-Partenkirchen, Germany
  • Contribution to proceedings
    International Doctoral Seminar 2006, 23.-26.04.2006, Smolenice, Slovak
    Proceedings, 14-18

Publ.-Id: 8788 - Permalink

Effect of the growth rate on the morphology and structural properties ofhut-shaped Ge islands in Si(001)
Yakimov, A. I.; Nikiforov, A. I.; Dvurechenskii, A. V.; Ulyanov, V. V.; Volodin, V. A.; Groetzschel, R.;
The effect of Ge deposition rate on the morphology and structural properties of self-assembled Ge/Si(001) islands was studied. Ge/Si(001) layers were grown by solid-source molecular-beam epitaxy at 500 ◦C. We adjusted the Ge coverage, 6 monolayers (ML), and varied the Ge growth rate by a factor of 100, R = 0.02–2 ML s−1, to produce films consisting of hut-shaped Ge islands. The samples were characterized by scanning tunnelling microscopy, Raman spectroscopy, and Rutherford backscattering measurements. The mean lateral size of Ge nanoclusters decreases from 14.1 nm at R = 0.02 ML s−1 to 9.8 nm at R = 2 ML s−1. The normalized width of the size distribution shows non-monotonic behaviour as a function of R and has a
minimum value of 19% at R = 2 ML s−1. Ge nanoclusters fabricated at the highest deposition rate demonstrate the best structural quality and the highest Ge content (∼0.9).
Keywords: Epitaxy; Germanium; Nanoclusters; Quantum dots
  • Nanotechnology 17(2006), 4743-4747

Publ.-Id: 8787 - Permalink

RBS-channeling, ERDA and XRD study of lithium tantalate modified by annealed proton echange (APE)
Mackova, A.; Salavkova, L.; Spirkova, J.; Grötzschel, R.; Eichhorn, F.;
Lithium tantalate (LT) single crystals are very promising substrates for constructing highly sophisticated photonics devices. The used
procedure for fabricating optical waveguides in them was annealed proton exchange (APE). Here we have used Rutherford backscattering
spectrometry (RBS) – channeling analysis, X-ray diffraction (XRD) and elastic recoil detection analysis (ERDA) to study changes in
the surface of the LT wafers caused by the APE treatment and to determine the concentration depth profiles of hydrogen atoms. The
measurements were taken for both virgin (untreated) and APE:LT samples fabricated using various experimental conditions. We have
found that the incorporation of hydrogen during the first step, i.e. the PE procedure (proton exchange), leads to modifications of the
structure of the LT single crystal, which gradually relaxed towards the structure of the original LT during the post-proton-exchange
annealing (A). This study also revealed a substantial dependence of sample behaviour on the crystallographic orientation of the pertinent
substrate wafers (commonly used X-(h1120i) and Z-(h0001i) cuts).
Keywords: RBS-channeling; ERDA; XRD;APE: LiTaO3
  • Nuclear Instruments and Methods in Physics Research B 249(2006), 339-342

Publ.-Id: 8786 - Permalink

How do high energy heavy ions shape Ge nanoparticles embedded in SiO2?
Schmidt, B.; Mücklich, A.; Röntzsch, L.; Heinig, K.-H.;
Crystalline Ge nanospheres embedded in SiO2 where irradiated at room temperature with fluences from 1x1015 to 4x1015 cm-2 of energetic J7+ ions. The spheres transform into amorphous oblate-like Ge nanoparticles, with the short axis aligned to the ion beam direction. This ion beam shaping is similar to that of free silica spheres, but differs from the prolate-like ion beam shaping of Au spheres embedded in SiO2.
Keywords: heavy ion irradiation, ion beam shaping, nanoparticles
  • Nuclear Instruments and Methods in Physics Research B 257(2007), 30-32
    DOI: 10.1016/j.nimb.2006.12.152
  • Lecture (Conference)
    15th International Conference on Ion Beam Modification of Materials, 18.-22.09.2006, Taormina, Italy

Publ.-Id: 8785 - Permalink

X-ray diffraction spot mapping – a tool to study structural properties of semiconductor disk laser devices
Zeimer, U.; Grenzer, J.; Korn, D.; Döring, S.; Zorn, M.; Pittroff, W.; Pietsch, U.; Saas, F.; Weyers, M.;
The local lattice plane curvature of semiconductor disk laser devices were determined by a newly devel-oped X-ray spot mapping technique using white beam synchrotron radiation. This method is suited for in-situ studies of the dependence of both parameters on device temperature and on local heating by optical pumping. The influence of different device mounting procedures on the structural and optical device pa-rameters was investigated as well. This knowledge was used for device optimisation.
Keywords: SCDL laser, white beam synchrotron radiation, wafer mapping
  • Poster
    8th Biennial Conference on High Resolution X-Ray Diffraction and Imaging, 19.-22.09.2006, Baden-Baden/Karlsruhe, Germany
  • Physica Status Solidi (A) 204(2007)8, 2753-2759
    DOI: DOI 10.1002/pssa.200675655

Publ.-Id: 8784 - Permalink

Defect induced nanowire growth by FIB implantation
Bischoff, L.; Akhmadaliev, C.; Schmidt, B.;
In this work a defect induced formation of CoSi2 nanoparticles and nanowires is investigated and discussed. The results show that the defects created in silicon by FIB irradiation can getter Co atoms, solved in the sample before the ion beam treatment. Nanowires and nanoparticles are formed in the defect regions and are stabilized by following Ostwald ripening and can grow due to diffusion of the Co atoms during annealing. The nanowires are oriented in the <110> crystalline directions energetically favored for CoSi2 growth. Presumably this process is initiated by the formation of the well known {311}-extended rod-like defects elongated in those <110> directions.
Keywords: mass separated FIB; implantation, defects, annealing, nanowire growth.
  • Lecture (Conference)
    32nd International Conference on Micro- and Nano-Engineering 2006, 17.-21.09.06, Barcelona, Spain
  • Microelectronic Engineering 84(2007), 1459-1462

Publ.-Id: 8783 - Permalink

A statistical analysis of polarity reversals of the geomagnetic field
Sorriso-Valvo, L.; Carbone, V.; Stefani, F.; Nigro, G.;
We investigate the temporal distribution of polarity reversals of the geomagnetic field. In spite of the commonly assumption that the reversal sequence can be modeled as a realization of a renewal Poisson process with a variable rate, we show that the polarity reversals strongly depart from a local Poisson statistics. The origin of this failure can be attributed to the presence of temporal clustering. We also show that a Lévy function is able to reproduce the distribution of polarity persistence times, thus suggesting the presence of long-range correlations in the underlying dynamo process. In this framework we compare our results with the behavior of some toy models that describe the time evolution of the reversals and with MHD geodynamo numerical simulations.
  • Lecture (Conference)
    AGU 2006 Fall Meeting, 11.-15.12.2006, San Francisco, United States
  • Physics of the Earth and Planetary Interiors 164(2007), 197-207
    DOI: 10.1016/j.pepi.2007.07.001
  • Abstract in refereed journal
    Eos, Transactions, American Geophysical Union 87(2006)52, GP21B-1308

Publ.-Id: 8781 - Permalink

Defect induced formation of CoSi2 nanowires by focused ion beam synthesis
Akhmadaliev, C.; Schmidt, B.; Bischoff, L.;
Cobalt implantation with a focused ion beam (FIB) was applied to study ion beam synthesis of cobalt disilicide nanowires in silicon. Two mechanisms of CoSi2 nanowire formation were investigated: (a) conventional synthesis by Co++ FIB implantation at elevated temperatures into silicon along in-plane <110> Si crystal direction and subsequent annealing, and (b) self-aligned CoSi2 nanowire growth in Co supersaturated silicon on FIB-induced defects at RT during subsequent annealing. The obtained CoSi2 nanowires are 20-100 nm in diameter and several micrometers long.
Keywords: nanowire, cobalt disilicide, focused ion beam, defects

Publ.-Id: 8780 - Permalink

Untersuchungen zur Aufnahme von Organika aus natürlichen Wässern durch Ionenaustauscher mit C-14-markierten Modellsubstanzen
Ender, V.; Schumann, T.; Sachs, S.; Bernhard, G.;
Untersucht wurde die Aufnahme C-14-markierter Modellsubstanzen (beta-Alanin, Stärke und synthetische Huminsäure M42) durch Anionenaustauscher und ihre korrespondierenden nichtfunktionalisierten Copolymere. Die Modellsubstanzen stehen für die TOC-Fraktionen der Neutralstoffe / Amphiphile, Polysaccharide und Huminstoffe enstprechender der LC-OCD-Methode von HUBER. Variiert wurden der Salzgehalt resp. der pH-Wert und die Temperatur bei der Beladung...
  • Contribution to proceedings
    VGB-Konferenz "Chemie im Kraftwerk 2006" mit Fachausstellung, 11.-12.10.2006, Bad Neuenahr, Germany
    Proceedings VGB-Konferenz "Chemie im Kraftwerk 2006", TB 442/P11, 13
  • Lecture (Conference)
    VGB-Konferenz "Chemie im Kraftwerk 2006" mit Fachausstellung, 11.-12.10.2006, Bad Neuenahr, Germany

Publ.-Id: 8779 - Permalink

GaBi liquid metal alloy ion source for the production of ions of interest in microelectronics research
Bischoff, L.; Pilz, W.; Ganetsos, T.; Forbes, R.; Akhmadaliev, C.;
Focused Ion beam (FIB) systems employing liquid metal ion sources (LMISs) have become of increasing importance in the microelectronics research and industry [1]. Applications of LMISs include IC analysis and modification, maskless ion implantation, secondary ion mass spectroscopy, scanning ion microscopy and ion beam lithography [2-3].
Alloy LMISs are the basis of mass separated FIB systems applying a variety of ion species which can be focused into diameters smaller than 10nm with current densities of several A/cm2 [4].
In this work a Ga38Bi62 alloy liquid metal ion source (Tmelt = 222°C), prepared on a mechanical treated Ta - emitter has been studied, which allows to implant in the case of a silicon substrate shallow donor ions (Bi) as well as acceptors (Ga) in the sub micron range without changing the source. A detailed analysis of the mass spectra as a function of emission current, obtained from this source, shown in Fig. 1, was used to investigate the mechanism for the production of single and double-charged ions. The transition metals in the mass spectra result from the re-sputtered extractor stainless steal material. Moreover, we represent the intensity of cluster ions extracted by the source, as function of emission current. Theoretical modeling support the experimental results.
[1]P.D. Prewett, G.L.R. Mair, Focused Ion Beams from Liquid Metal Ion Sources,
Taunton, Somerset, UK: Research Studies Press, 1991, Chap. 2.
[2]J. Orloff et al, High Resolution Focused Ion Beams – FIB and Its Applications,
Kluwer Academic / Plenum Publishers, 2002, Chap. 6.
[3]L. Bischoff, J. Teichert, S. Hausmann, Th. Ganetsos and GLR. Mair, Microelectronic Engineering, 53 (2000) 613.
[4]L. Bischoff, Ultramicroscopy, 103 (2005) 59.
Keywords: Alloy liquid metal ion source, GaBi, mass spectra
  • Lecture (Conference)
    The Joint 19th International Vacuum Nanoelectronics Conference and 50th International Field Emission Symposium, 17.-20.07.2006, Guilin, China
  • Ultramicroscopy 107(2007)9, 865-868

Publ.-Id: 8778 - Permalink

Planar channeling radiation from electrons in quartz
Azadegan, B.; Wagner, W.; Pawelke, J.; Grigoryan, L. S.;
Planar channeling radiation from 32 MeV electrons channeled in quartz has for the first time been measured at the radiation source ELBE. The theory of channeling radiation was applied for the assignment of the observed spectra to the crystal planes. Calculations of the continuum potentials, transverse electron states, transition energies, and radiation intensities have been performed for selected planes of quartz. Specific properties of the hexagonal binary quartz crystal influencing the generation of channeling radiation are discussed.
Keywords: Channeling radiation, X-rays, quartz
  • Contribution to proceedings
    "Channeling 2006", International Conference on Charged and Neutral Particles Channeling Phenomena II, 03.-07.07.2006, Frascati (Rome), Italy
    Proceedings of the International Conference on Charged and Neutral Particles Channeling Phenomena II, Bellingham (USA): SPIE, 9780819467782, 66340S
  • Lecture (Conference)
    "Channeling 2006", International Conference on Charged and Neutral Particles Channeling Phenomena II, 03.-07.07.2006, Rom, Italy

Publ.-Id: 8777 - Permalink

Deposition and Re-emission of Potential Energy - extended dynamical COB simulation -
Kost, D.; Röder, F.; Möller, W.;
We present results of a numerical computer simulation based on the extended dynamical classical over the barrier model (EDCOB). To get an energy balance for the transferred potential energy of the highly charged ions (HCIs) to the solid surface, we apply an additional extension of the model evaluating the energy loss of Auger electrons which are generated during the relaxation of the HCI. From the results of the simulation we can distinguish between the potential energy deposited in the nuclear system due to image charge acceleration of the incoming HCI and the potential energy which is released over Auger processes. Further, we break down this energy into that which is deposited in the solid and that which is re-emitted due to escaping secondary electrons.
  • Open Access LogoJournal of Physics: Conference Series 58(2007), 343
  • Poster
    13th International Conference on the Physics of Highly Charged Ions, 27.08.-01.09.2006, Belfast, Northern Ireland, UK

Publ.-Id: 8776 - Permalink

Subthreshold production of Sigma(1385) baryons in Al+Al collisions at 1.9A GeV
Lopez, X.; Herrmann, N.; Crochet, P.; Andronic, A.; Basrak, Z.; Bastid, N.; Benabderrahmane, M. L.; Bühler, P.; Cargnelli, M.; Caplar, R.; Cordier, E.; Dupieux, P.; Dzelalija, M.; Fabietti, L.; Fodor, Z.; Gasparic, I.; Grishkin, Y.; Hartmann, O. N.; Hildenbrand, K. D.; Hong, B.; Kang, T. I.; Kecskemeti, J.; Kirejczyk, M.; Kim, Y. J.; Kis, M.; Koczon, P.; Korolija, M.; Kotte, R.; Lebedev, A.; Leifels, Y.; Marton, J.; Merschmeyer, M.; Neubert, W.; Pelte, D.; Petrovici, M.; Rami, F.; Reisdorf, W.; Ryu, M. S.; Schüttauf, A.; Seres, Z.; Sikora, B.; Sim, K. S.; Simion, V.; Siwek-Wilczynska, K.; Smolyankin, V.; Stoicea, G.; Suzuki, K.; Tyminski, Z.; Wagner, P.; Wisniewski, K.; Wohlfarth, D.; Xiao, Z.; Yushmanov, I.; Zhang, X. Y.; Zhilin, A.; Zmeskal, J.; Kienle, P.; Yamazaki, T.;
First deep sub-threshold production of the Sigma(1385) hyperon resonance is presented. The measurement was performed in Al+Al collisions at a beam energy of 1.9A GeV with the FOPI detector at GSI-Darmstadt. The result is found to be in relatively good agreement with the statistical model prediction. It allows to better understand strangeness sub-threshold production by leading information on the strangeness exchange reaction which is the dominant process for K- production below and close-to threshold.

Publ.-Id: 8775 - Permalink

Structure and conductivity of self-assembled films of gold nanoparticles
Pelka, J. B.; Brust, M.; Gierlowski, P.; Paszkowicz, W.; Schell, N.;
Self-assembled thin films of gold nanoparticles of 4-5 nm, prepared on glass using aliphatic dithiols of different hydrocarbon chain lengths as interparticle linker molecules, have been studied by x-ray methods and dc conductivity. X-ray data revealed small spacer-dependent cluster size variations in the films. Conductivity, in the temperature range of 4.2-300 K, showed a strong relation with spacer length, indicating that different mechanisms of conduction from metal-like through hopping to tunneling dominate, depending on temperature range and particle spacing. The results demonstrate that the electronic and optical properties of such films can be precisely controlled by interparticle distance.
  • Applied Physics Letters 89(2006), 063110

Publ.-Id: 8774 - Permalink

Nitrogen Plasma Íon Implantation of Al Alloy in the High Voltage Glow Discharge Mode
Oliveira, R. M.; Ueda, M.; Rossi, J. O.; Reuther, H.; Lepienski, C. M.; Beloto, A. F.;
Nitrogen Plasma Íon Implantation of Al Alloy in the High Voltage Glow Discharge Mode
  • Lecture (Conference)
    International Conference on Íon Implantation Technology, 11.-16.06.2006, Marseille, France

Publ.-Id: 8773 - Permalink

Plasma Immersion Ion Implantation with a 4 kV/10kHz Compact High Voltage Pulser
Ueda, M.; Oliveira, R. M.; Rossi, J. O.; Reuther, H.; Silva, G.;
Plasma Immersion Ion Implantation with a 4 kV/10kHz Compact High Voltage Pulser
  • Lecture (Conference)
    6th International Conference on Íon Implantation Technology, 11.-16.06.2006, Marseille, France

Publ.-Id: 8772 - Permalink

Implantação Iônica por Imersão em Plasma de Nitrogênio Usando Baixa Energia
Silva, G.; Ueda, M.; Rossi, J. O.; Reuther, H.;
Implantação Iônica por Imersão em Plasma de Nitrogênio Usando Baixa Energia
  • Contribution to proceedings
    XII Encontro e Exposição Brasileira de Tratamentos de Superfície & II INTERFINISH Latino- Americano, 09.-11.05.2006, Sao Paulo, Brazil
    Proceedings of XII Encontro e Exposição Brasileira de Tratamentos de Superfície & II Interfinish Lat, paper 33

Publ.-Id: 8771 - Permalink

Hybrid Processing of Ti-6Al-4V Using Plasma Immersion Ion Implantation Combined with Plasma Nitriding
Silva, M. M.; Ueda, M.; Otani, C.; Reuther, H.; Lepienski, C. M.; Soares Jr, P. C.; Otubo, J.;
Hybrid Processing of Ti-6Al-4V Using Plasma Immersion Ion
Implantation Combined with Plasma Nitriding
  • Open Access LogoMaterials Research 9(2006)1, 97-100

Publ.-Id: 8770 - Permalink

Numerical and experimental modeling of the melt flow in a traveling magnetic field for Vertical Gradient Freeze crystal growth
Galindo, V.; Grants, I.; Lantzsch, R.; Pätzold, O.; Gerbeth, G.;
A Traveling Magnetic Field (TMF) is attractive for melt crystal growth as it offers a direct flow driving mechanism. We present numerical and experimental results on the TMF driven flow in an isothermal model fluid. The TMF is generated by a system of six equidistant coils, which are fed by a three phase current power supply to create an up- or downward directed traveling field. Special attention is focused on the description of the electromagnetic fields for varying field frequency, and its influence on the resulting melt velocity.
Keywords: Fluid Flows, Magnetic Fields, Stirring, Crystal Growth
  • Journal of Crystal Growth 303(2007), 258-261
    DOI: 10.1016/j.jcrysgro.2006.11.194
  • Lecture (Conference)
    5th International Workshop on Modeling in Crystal Growth, 10.-13.09.2006, Bamberg, Germany

Publ.-Id: 8769 - Permalink

Thin film patterning by surface-plasmon-induced thermocapillarity
Röntzsch, L.; Heinig, K.-H.; Schuller, J. A.; Brongersma, M. L.;
It is reported that standing surface-plasmon-polariton (SPP) waves can cause regular thickness undulations of thin polymethyl methacrylate (PMMA) films above a metallic substrate. Ripples and hillock arrays with long-range order were found. Numerical calculations reveal that periodic in-plane temperature profiles are generated in the PMMA due to the non-radiative damping of SPP interference patterns. Atomistic computer simulations on the temperature-gradient-driven mass transport confirm that thermocapillarity is the dominating mechanism of the observed surface patterning.
Keywords: Thermocapillarity, surface plasmon lithography, ploymer, PMMA, laser irradiation, thin films

Publ.-Id: 8768 - Permalink

The structure of soddyite-exafs shell fitting and wavelet analysis
Funke, H.; Hennig, C.; Scheinost, A.;
Many uranyl (U(VI)) minerals have a characteristic layer structure in the equatorial plane due to the strong electrovalence of the two axial oxygen atoms. For structures containing heavy atoms like U, the XRD patterns are dominated by the backscattering from the heavy atoms, while the positions of light atoms like O or Si may be occasionally inaccurate. The EXAFS spectrum were measured of a soddyite sample of Renaud Vochten’s collection of uranium minerals at 30 K. The local structure determined by shell fitting is fairly consistent with crystallographic data. Wavelet analysis clearly resolved the Si and U atoms at nearly the same distance between 3.6 and 3.9 Å.
  • Poster
    4th Workshop on Speciation, Techniques, and Facilities for Radioactive Materials at Synchrotron Light Sources (Actinide XAS 2006), 18.-20.09.2006, Karlsruhe, Germany
  • Contribution to proceedings
    Actinide XAS 2006, 18.-20.09.2006, Karlsruhe, Germany
    Speciation Techniques and Facilities for Radioactive Materials at Synchrotron Light Sources: OECD, 978-92-64-99006-7, 211

Publ.-Id: 8767 - Permalink

Radioimmunotherapy of solid tumors by targeting extra domain B fibronectin: identification of the best-suited radioimmunoconjugate.
Berndorff, D.; Borkowski, S.; Sieger, S.; Rother, A.; Friebe, M.; Viti, F.; Friebe, M.; Hilger, C. S.; Cyr, J. E.; Dinckelborg, L. M.;
PURPOSE: The expression of extra domain B (ED-B) fibronectin is always associated with angiogenic processes and can be exclusively observed in tissues undergoing growth and/or extensive remodeling. Due to this selective expression, ED-B fibronectin is an interesting target for radioimmunotherapy of malignant diseases. The aim of this study was to identify the most appropriate ED-B-targeting radioimmunoconjugate for the therapy of solid tumors. EXPERIMENTAL DESIGN: Three ED-B fibronectin-binding human antibody formats of L19 were investigated: dimeric single-chain Fv (approximately 50 kDa), "small immunoprotein" (SIP, approximately 80 kDa), and immunoglobulin G1 (IgG1, approximately 150 kDa). These L19 derivatives were either labeled with I-125 or with In-111 (using MX-diethylenetriaminepentaacetic acid, MX-DTPA). Pharmacokinetics and tumor accumulation of the radiolabeled immunoconjugates were investigated in F9 (murine teratocarcinoma) tumor-bearing mice. Subsequently, dosimetry for the corresponding therapeutic isotopes I-13-1 and Y-90 was done. After testing the myelotoxicity of I-131-L19-SIP and I-131-L19-IgG1 in non-tumor-bearing mice, the therapeutic efficacy of these iodinated antibody formats was finally investigated in F9 tumor-bearing mice. RESULTS: The most favorable therapeutic index was found for I-131-L19-SIP followed by I-131-L19-IgG1. The therapeutic index of all In-111-labeled derivatives was significantly inferior. Considering the bone marrow as the dose-limiting organ, it was calculated that activities of 74 MBq I-131-L19-SIP and 25 MBq I-131-L19-IgG1 could be injected per mouse without causing severe myelotoxicity. The best therapeutic efficacy was observed using I-131-L19-SIP, resulting in significant tumor growth delay and prolonged survival after a single injection. CONCLUSION: Compared with other L19-based radioimmunoconjugates, I-131-L19-SIP is characterized by superior antitumor efficacy and toxicity profile in the F9 teratocarcinoma animal model. These results indicate that ED-B fibronectin-targeted radioimmunotherapy using I-131-L19-SIP has potential to be applied to treatment of solid cancers.
  • Clinical Cancer Research 11(2005), 7053s-7063s

Publ.-Id: 8766 - Permalink

In vivo magnetic resonance imaging and spectroscopy of various adipose tissue deposits in mice
Strobel, K.; van den Hoff, J.; Pietzsch, J.;
An emerging paradigm supports the view that adipose tissue (AT) dysregulation might play a crucial role in the pathogenesis of insulin-resistance, atherosclerosis, and other disease states. In the last years, rodent models have played an important role in the investigation of AT biology and disorder. In this line, non-invasive differentiation and characterization of various AT deposits in the living animal is a current challenge. In the present paper, magnetic resonance imaging (MRI) and spectroscopy (MRS) techniques are applied for quantitative in vivo evaluation of superficial and mediastinal brown adipose tissue (BAT), as well as subcutaneous and intra-abdominal white adipose tissue (WAT) deposits in mice.
All experiments were carried out in NMRI mice and nude mice aging from 8 to 24 weeks. Morphological differentiation between various BAT and WAT deposits was obtained by 1H-MRI at 7 Tesla using a Biospec 70/30 (Bruker, Germany). Images were obtained with high spatial resolution of 156 microns (field of view 4 x 4 cm, matrix size 256 x 256; slice thickness 1.5 to 3 mm). Furthermore, 1H-MRS has been performed to quantify in vivo the different lipid patterns in BAT and WAT deposits using a volume selective PRESS sequence on 3 to 8 mm^3 voxels.
In both nude mice and NMRI mice the various BAT and WAT deposits were clearly distinguished from the non-AT tissue with excellent contrast by T1-weighted MSME MRI sequences. Comparison of MR images with corresponding histological whole-animal sections indicated high specificity and sensitivity of the MRI sequences applied. The high resolution spectra obtained at 7 T allow identification of at least 9 different proton resonances specific for lipids, and thus, for calculation of mono- to polyunsaturated fatty acid ratio in vivo. In this study, nude mice showed a 1.5- to 3-fold higher degree of unsaturation and polyunsaturation of triglyceride fatty acid acyl chains in BAT when compared to NMRI mice. No differences were observed in WAT deposits.
High-resolution MRI and MRS are potentially useful tools for studying the biology of different BAT and WAT deposits non-invasively in small experimental animals in vivo.
  • Poster
    ESMRMB 23. Annual Scientific Meeting, European Society for Magnetic Resonance in Medicine and Biology, 21.-23.09.2006, Warsaw, Poland
  • Abstract in refereed journal
    Magnetic Resonance Materials in Physics, Biology and Medicine 19(2006)Suppl. 1, 236

Publ.-Id: 8765 - Permalink

β´´-(ET)2SF5CH2CF2SO3 - a layered 2D metal with vanishing interlayer coupling
Wosnitza, J.;
  • Invited lecture (Conferences)
    International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets (ISCOM2005), 11.-16.09.2005, Key West, USA

Publ.-Id: 8763 - Permalink

Recent developments at the Dresden High Magnetic Field Laboratory
Wosnitza, J.;
  • Invited lecture (Conferences)
    Physical Phenomena at High Magnetic Fields - V, 05.-09.08.2005, Tallahassee, USA

Publ.-Id: 8762 - Permalink

Metalle in hohen Magnetfeldern
Wosnitza, J.;
  • Invited lecture (Conferences)
    Physikalisches Kolloquium der Universität Konstanz, 05.07.2005, Konstanz, Deutschland

Publ.-Id: 8761 - Permalink

Coil design for non-destructive pulsed-field magnets targeting 100 T
Zherlitsyn, S.; Bianchi, A. D.; Hermannsdörfer, T.; Pobell, F.; Skourski, Y.; Sytcheva, A.; Zvyagin, S. A.; Wosnitza, J.;
Progress and recent achievements in coil design are presented for the new Dresden High Magnetic Field Laboratory (HLD), which is under construction at the Forschungszentrum Rossendorf. This laboratory is planned to be open for external users in 2007. The facility is placed near a free electron laser which will offer the opportunity to perform infrared spectroscopy in pulsed magnetic fields. Implementation of various experimental techniques, such as transport, magnetization, specific heat, ultrasound, and magnetic resonance in pulsed magnetic fields up to 100 T are planned. Typical pulse durations will be in the range between 10 and 1000 ms with magnet bores ranging from 20 to 40 mm. The pulsed magnets will be energized by a 50 MJ/24 kV modular capacitive pulsed-power supply. With our newly designed coils, so far we were able to reach 65 T in a non-destructive manner. These coils are built using regular copper wire reinforced with an organic fiber (Zylon). Pulse durations for the various coils are between 20 and 50 ms. Different magnet failure modes have been analysed and possible improvements of the magnets are discussed.
We also present numerical simulations of our pulsed magnets.
  • IEEE Transactions on Applied Superconductivity 16(2006), 1660

Publ.-Id: 8760 - Permalink

Metals at high magnetic fields
Wosnitza, J.;
es hat kein Abstract vorgelegen.
  • Invited lecture (Conferences)
    Seminar at the Laboratoire National des Champes Megnétiques Pulsés (CNRS-LNCMP), 15.03.2005, Toulouse, France
  • Invited lecture (Conferences)
    Seminar at the International Max-Planck Research School, Dresden, 13.07.2005, Dresden, Deutschland

Publ.-Id: 8759 - Permalink

Increase of blue electroluminescence from Ce-doped SiO2 layers through sensitization by Gd3+ ions
Sun, J. M.; Prucnal, S.; Skorupa, W.; Helm, M.; Rebohle, L.; Gebel, T.;
Efficient blue electroluminescence peak at around 440 nm with a maximum output power density of 34 mW/cm2 was obtained from Ce and Gd coimplanted metal-oxide-semiconductor light emitting devices. Energy transfer from Gd3+ to Ce3+ ions was observed during the excitation process, leading to a more than threefold increase of the external quantum efficiency of the blue Ce3+ luminescence up to 1.8%. This is evidenced by the increase of the excitation cross section of Ce3+ ions from 4.8x10−13 to 3.5x10−12 cm2 and the simultaneous reduction of the decay time and the impact
cross section of Gd3+ ions.
Keywords: Si light emitter, electroluminescence, rare earth

Publ.-Id: 8758 - Permalink

Extraction of radio-labelled xanthine derivatives by artificial receptors – deep insight into the association behaviour
Bomkamp, M.; Siering, C.; Landrock, K.; Stephan, H.; Fröhlich, R.; Waldvogel, S. R.;
Association constants for the interaction of almost insoluble substrates with triphenylene ketal-based receptors in toluene have been determined by means of an extraction method employing the corresponding radio-labelled substrates. Flexible and more polar receptors tend to aggregate and exhibit inferior extraction qualities. Binding constants in toluene were found to be in the range 105–107M-1 which is significantly higher than in dichloromethane.
X-ray analyses indicate the direct participation of a water molecule in the binding process, which may account for the surprisingly small effect of moisture in the solvent on the stability of the complexes.
Keywords: caffeine · extraction · isotopic labelling · receptors · supramolecular chemistry

Publ.-Id: 8757 - Permalink

The new Seattle-TRIUMF 7Be(p,γ)8B S-factor determination
Junghans, A. R.; Mohrmann, E. C.; Snover, K. A.; Steiger, T. D.; Adelberger, E. G.; Casandjian, J. M.; Swanson, H. E.; Buchmann, L. R.; Laird, A.; Zyuzin, S. P. A. Y.;
We present new measurements of the 7Be(p,γ)8B cross section from ‏‾Ecm = 116 to 2460 keV. Our new measurements lead to S17(0) = 22.1 ± 0.6(expt) ± 0.6(theor) eV b, where the central value is based on the theory of Descouvemont and Baye. We recommend a “best” value, S17(0) = 21.4 ± 0.5(expt) ± 0.6(theor) eV b, based on the mean of all modern direct measurements below the 1+ resonance.

Publ.-Id: 8756 - Permalink

Structure analysis of actinide compounds in solution by X-ray absorption spectroscopy
Hennig, C.;
Actinides with their large number of oxidation states are susceptible to redox conditions, forming different complexes in aqueous solution which may greatly differ by solubility and mobility. These complexes are often difficult to investigate due to their complex species distribution and thermodynamic metastability. Thermodynamic estimation of species distribution may fail for higher actinide concentrations. In such cases X-ray absorption spectroscopy is able to reveal structural parameters that provide information on the solution species. A spectroelectrochemical cell was developed for the study of structure and speciation of actinide complexes in situ by X-ray absorption spectroscopy, while applying and maintaining a constant potential. This equipment allows the investigation of actinide complexes under different redox conditions. An overview about the actual knowledge of actinides coordination in valence states III, IV, V, VI and VII in aqueous solution will be given.
Keywords: EXAFS, coordination, An(III), An(IV), An(V), An(VI), An(VII)
  • Invited lecture (Conferences)
    23nd European Crystallographic Meeting, 06.-11.08.2006, Leuven, Belgium
  • Abstract in refereed journal
    Acta Crystallographica Section A 62(2006), 61

Publ.-Id: 8749 - Permalink

Mo K-edge EXAFS investigation of the [Mo7O24]6- promoted phosphate ester hydrolysis
van Deun, R.; Hennig, C.; Cartuyvels, E.; Nikitenko, S.; Parac-Vogt, T. N.;
The early transition metals (V, Nb, Ta, Mo, W) in their high oxidation states are able to form metal-oxygen clusters, commonly referred to as polyoxometallates (POMs). The diverse structures and compositions of polyoxometallates offer a wide versatility in terms of shape, polarity, redox potentials, surface charge distribution and acidity, and allow for numerous potential applications of POMs. The first reports describing the antitumoral activity of POMs appeared about ten years ago, and they have revealed that the antitumoral activity of [Mo7O24]6- is even better than that of commercial drugs. We have investigated the potential towards phosphate ester bond cleavage of the [Mo7O24]6- cluster.
Keywords: EXAFS, polyoxometallate, anti-tumor activity
  • Poster
    23nd European Crystallographic Meeting, 06.-11.08.2006, Leuven, Belgium
  • Abstract in refereed journal
    Acta Crystallographica Section A 62(2006), s206

Publ.-Id: 8748 - Permalink

A comparative UV-Vis and EXAFS investigation of the coordination of the uranyl ion UO22+ in solution
Servaes, K.; Hennig, C.; van Deun, R.; Görller-Walrand, C.;
The identification of uranyl complexes in solution is not always straightforward. Unlike crystal structures, which can be revealed by X-ray diffraction, the structure of solution species could only be indicated in the past by means of spectroscopic techniques (UV-Vis absorption spectroscopy, luminescence, magnetic circular dichroism). These methods demonstrated the existence of a uranyl tetrachloro complex [UO2Cl4]2- with D4h coordination symmetry and a uranyl trinitrato complex [UO2(NO3)3]- with D3h symmetry in non-aqueous solution, based on their characteristic absorption spectra.Nowadays, limited structural parameters of solution species can be determined using Extended X-ray Absorption Fine Structure (EXAFS). The complex formation of the uranyl ion UO22+ with small inorganic ligands like Cl- and NO3- in non-aqueous solution was investigated using U LIII EXAFS spectroscopy and UV-Vis absorption spectroscopy. Both techniques confirm the existence of a [UO2Cl4]2- limiting species with D4h symmetry as well as a [UO2(NO3)3]- limiting species with D3h symmetry in acetonitrile solution. The distances in the U(VI) coordination sphere of [UO2Cl4]2- are U-Oax = 1.77 ± 0.01 Ǻ and U-Cl = 2.68 ± 0.01 Ǻ. For [UO2(NO3)3]- the distances in the U(VI) coordination sphere are U-Oax = 1.77 ± 0.01 Ǻ and U-Oeq = 2.49 ± 0.01 Ǻ. The U-N distance of 2.94 ± 0.01 Ǻ points at a bidentate coordination mode of the nitrate group.
Keywords: UV-vis, EXAFS, uranium complexes, nonaqueous solution
  • Poster
    23rd European Crystallographic Meeting, 06.-11.08.2006, Leuven, Belgium
  • Abstract in refereed journal
    Acta Crystallographica Section A 62(2006), 205

Publ.-Id: 8747 - Permalink

Comparison of in vitro and in vivo properties of 99mTc-cRGD peptides labelled using different novel Tc-cores
Decristoforo, C.; Santos, I.; Pietzsch, H.-J.; Duatti, A.; Smith, C. J.; Rey, A.; Alberto, R.; von Guggenberg, E.; Haubner, R.;
The alphavbeta3 integrin is involved in angiogenesis and tumor metastasis. RGD-peptides bind with high affinity to this integrin. This study compares the influence of 99mTc-labelling applying novel Technetium-cores on imaging characteristics of the radiolabelled peptide. Different peptide conjugates based on the cyclic pentapeptide c(RGDyK) (cRGD) were prepared and characterised (HYNIC-, Cys-, L2- and Pz1-cRGD). Radiolabelling experiments using different coligands for HYNIC-cRGD, the 99mTc(CO)3 metal fragment for PZ-1-cRGD (pyrazolyl-derivative), the Tc-nitrido-core using a phosphine-coligand (PNP) for Cys-cRGD and an isonitrile–conjugate (L2-cRGD) together with a NS3-coligand (4+1 concept) were performed and showed labelling yields >90% at high specific activities. A high in vitro stability was observed, plasma protein binding and lipophilicity varied considerably between different radiolabelled cRGD conjugates. Experiments on biological activity of the radiolabelled peptides using alphavbeta3 positive (M21) and negative (M21L) tumour cells did show specific uptake of various conjugates. Studies in tumour bearing animals revealed significant differences between different conjugates concerning pharmacokinetic behaviour (predominant renal excretion to considerable hepatobiliary clearance) as well as tumour uptake (0.2-2.7%ID/g). Highest specific tumour uptake and tumour/background values were found for 99mTc-EDDA/HYNIC-c(RGDyK), 99mTc-Nitrido-PNP-Cys-c(RGDyK) and 99mTc(CO)3-Pz1-c(RGDyK). In conclusion, using novel Tc-cores such as the 99mTc(CO)3 metal fragment, Tc-nitrido- and the 4+1 concept peptides could be labelled with 99mTc-technetium at high specific activities resulting in complexes with high stability, but binding moieties have to be optimized especially concerning hydrophilicity resulting in renal rather than hepatobiliary excretion. This comparative study underlines that peptide labelling strategies using 99mTc have to be properly selected and optimized. Different in vitro assays are necessary to predict targeting properties in vivo.
  • Quarterly Journal of Nuclear Medicine and Molecular Imaging 51(2007), 33-41

Publ.-Id: 8746 - Permalink

Röntgenographische Elektronendichtebestimmung von unterschiedlich synthetisierten YNi2B2C-Einkristallen
Leisegang, T.; Meyer, D. C.; Paufler, P.; Souptel, D.; Behr, G.; Bergk, B.; Ignatchik, O.; Wosnitza, J.; Ormeci, A.; Rosner, H.;
Das Übergangsmetall-Borkarbid YNi2B2C (Raumgruppe 8139) I4/mmm) geht bei Temperaturen unterhalb von ca. 15 K vom normalleitenden in den supraleitenden Zustand über [1]. Das supraleitende Verhalten ist stark von der chemischen Zusammensetzung im schmalen Homogenitätsbereich und von Feinheiten der Struktur abhängig.
Qualitativ hochwertige Einkristalle lassen sich mit dem Zonenschmelzverfahren [2] oder der Flusszüchgung [3] synthetisieren. Zur Bestimmung der elektronischen Bandstruktur sowie zur Charakterisierung der supraleitenden Energielücke wurden von Ignatchik et al. [4] de Haas-van Alphen (dHvA) Messungen an Einkristallen beider Züchungsmethoden durchgeführt. Die dHvA-Oszillationen weisen eine ungewöhnliche Dämpfung in der supraleitenden Phase für die zonengeschnmolzenen Proben auf. Im Unterschied dazu lassen sich für flussgezogene Proben Oszillationen deutlich bis zu tiefen Temperaturen beobachten.
Zur Aufklärung dieser Unterschiede wurden nun an denselben Proben röntgeneinkristalldiffraktometrische Messungen bei Raumtemperatur durchgeführt. Neben der Strukturverfeinerung zur Bestimmung der exakten Kristallstruktur werden verschiedene Fehlordnungsmodelle diskutiert. Die aus den verfeinerten Parametern berechneten Eletronendichteverteilungen werden mit theoretischen Berechnungen verglichen. Erst Ergebnisse weisen auf unterschiedliche Besetzungswahrscheinlichkeiten der C-Lage hin.
Diese Arbeit wurde von der Deutschen Forschungsgemeinschaft im Rahmen des SFB 463 unterstützt.
[1] R. Nagarajan et al. Phys. Rev. Lett. 72, 274-277 (1994).
[2] D. Souptel et al. J. Cryst. Growth 276, 652-662 (2005).
[3] P. Canfield et al. Phys. Today 51 (10), 40-46 (1998).
[4] O. Ignatchik et al. J. Magn. Magn. Mat. 290-291, 424-427 (2005).
  • Poster
    14. Jahrestagung der Deutschen Gesellschaft für Kristallographie 2006, 03.-06.04.2006, Freiburg, Deutschland

Publ.-Id: 8745 - Permalink

Magnetic quantum oscillations in the normal and superconducting state of nonmagnetic borocarbides
Ignatchik, O.; Bergk, B.; Jäckel, M.; Wosnitza, J.; Souptel, D.; Behr, G.; Canfield, P. C.;
We report on de Haas-van Alphen (dHvA) investigations of the non-magnetic borocarbides YNi2B2C and LuNi2B2C. The measurements were carried out on high-quality single crystals by the torque method in magnetic fields up to 30 T. In the normal state the Fermi-surface topology has been analysed. In comparison with band-structure calculations several deviations are determined. Below the upper critical field, Bc2, in the vortex state an additional damping of the dHvA amplitudes appears. This damping is caused by the evolution from of the superconducting gap parameter. Concerning these topics we observe strong differences between differently grown crystals. The crystals produced by a zone-melting method show an abrupt vanishing of the dHvA signal at the upper critical field Bc2. In contrast, the flux grown crystals display a broader phase transition combined with a continuous decrease of the dHvA amplitude.
  • Poster
    DPG-Frühjahrstagung, 27.-31.03.2006, Dresden, Deutschland

Publ.-Id: 8744 - Permalink

Granularity and Spontaneous Vortex State for the Weakly Ferromagnetic Superconductor RuSr2GdCu2O8
Papageorgiou, T.; Casini, E.; Braun, H. F.; Herrmannsdörfer, T.; Bianchi, A. D.; Wosnitza, J.;
In the high-Tc cuprate RuSr2GdCu2O8 (Ru1212) weak ferromagnetism (T Ru/N ? 130 K) coexists with superconductivity (Tc,conset ? 50 K). This rises the interesting question concerning the formation of a spontaneous vortex state (SVS) in the case that the internal magnetic field is greater than the first critical field Hc1. Recently, the formation of a SVS has been proposed for Ru1212 after the phase diagram for this compound was constructed from dc-magnetization and resistance measurements [1]. We show, by a comparison of resistance with ac-susceptibility and dc-magnetization measurements, where both the intra- and inter-granular superconducting transition are obvious, that the granular nature of the investigated samples has to be carefully considered in the investigations of possible SVS formation. A particular SVS with vortices pinned in the intergrain area is much more likely. Single crystals would be required to unambiguously demonstrate the formation or non-formation of a spontaneaous vortex state in bulk Ru1212.
[1] C. Y. Yang, B. C. Chang, H. C. Ku, Y. Y. Hsu, cond-mat/0507014
  • Poster
    DPG-Frühjahrstagung, 27.-31.03.2006, Dresden, Deutschland

Publ.-Id: 8743 - Permalink

Single crystal X-ray diffraction analysis and electron density calculation of YNi2B2C
Leisegang, T.; Meyer, D. C.; Paufler, P.; Souptel, D.; Behr, G.; Ignatchik, O.; Ormeci, A.; Rosner, H.; Wosnitza, J.;
The quarternary borocarbide YNi2B2C, space group (139) I4/mmm, exhibits superconductivity (Tc ≈ 15 K) as was first reported in [1]. This superconducting behaviour depends strongly on the crystal composition within the samll homogeneity range and on the crystal growth conditions. Here we report on investigations of two different samples, namely bulk samples grown by a floating zone technique [2] and plate samples grown by a flux-growth method [3]. De Haas-van Alphen (dHvA) measurements were performed to determine the electronic band structure as well as the evolution of a superconducting energy gap at the Fermi surface [4]. To evaluate the exact crystal structure, single-crystal X-ray diffaction measurements at room temperature were performed. Different models of structural disorder were refined and a difference-Fourier analysis was carried out. The experimental electron density will be compared with theoretical calculations. The work was supported by the Deutsche Forschungsgemeinschaft (SFB 463).
[1] Nagarajan et al., Phys. Rev. Lett. 72, 274 (1994).
[2] Souptel et al., J. Cryst. Growth 276, 652 (2005).
[3] Canfield et al., Phys. Today 51, 40 (1998).
[4] Ignatchik et al., J. Magn. Magn. Mat. 290-291, 424 (2005).
  • Lecture (Conference)
    DPG-Frühjahrstagung, 27.-31.03.2006, Dresden, Deutschland

Publ.-Id: 8742 - Permalink

Uranium(VI) complexes with phospholipid model compounds – a laser spectroscopic study
Koban, A.; Bernhard, G.;
We present the complex formation of the uranyl ion (UO22+) in the aqueous system with phosphocholine, O-phosphoethanolamine and O-phosphoserine. These phosphonates (R-O-PO32-) represent the hydrophilic head groups of phospholipids. The complexation was investi-gated by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pH 2 to 6. An increase of the fluorescence intensity, connected with a strong red shift of about 8 nm com-pared to the free uranyl ion, indicates a complex formation between UO22+ and the phospho-nates already at pH 2. Even at pH 6 these complexes prevail over the uranyl hydroxide and carbonate species, which are generated naturally at this pH. At pH 4 and higher a 1 : 2 com-plex between uranyl and O-phosphoserine was found. Complexes with a metal-to-ligand ratio of 1 : 1 were observed for all other ligands. Fluorescence lifetimes, emission maxima and complex stability constants at T = 22 ± 1°C are reported. The TRLFS spectra of uranyl com-plexes with two phosphatidic acids (1,2-dimyristoyl-sn-glycero-3-phosphate and 1,2-dipalmitoyl-sn-glycero-3-phosphate), which represent the apolaric site of phospholipids, show in each case two different species.
Keywords: TRLFS, uranyl, phospholipid fragments, phosphonates, complexation

Publ.-Id: 8741 - Permalink

Spin-zero anomaly in the magnetic quantum oscillatins of a two-dimensional metal
Bergk, B.; Ignatchik, O.; Jäckel, M.; Wosnitza, J.; Gvozdikov, V. M.; Schlueter, J. A.; Mohtasham, J.; Winter, R. W.; Gard, G. L.;
We present de Haas-van Alplhen (dHvA) and Shubnikov-de Haas (SdH) measurements on the quasi-two-dimensional organic superconductor β"-(BEDT-TTF)2SF5CH2CF2SO3. The measurements were carried out by the torque method and by four-point low-frequency ac-resistance measurements in magnetic fields up to 15 T. Unlike theoretical expectations for two-dimensional metals the dHvA signal shows an unconventional sawtooth wave-form, e. e., an 'inversed sawtooth' wave form is observed.In order to investigate the behaviour in more detail we performed angle-dependent dHvA and SdH measurements. The SdH effect displays the behaviour predicted by the grand-canonical Lifshitz-Kosevich theorie. In contrast, the dHvA signal can be explained by a theory, which includes a slightly oscillating chemical potential. Even for simultaneous measurements of both effects the behaviour does not change. This means that the dHvA signal is not affected by an external charge carrier reservoir.
  • Poster
    DPG-Frühjahrstagung, 27.-31.03.2006, Dresden, Deutschland

Publ.-Id: 8740 - Permalink

Electron Spin Resonance in sine-Gordon Spin Chains in the Perturbative Spinon Regime
Zvyagin, S. A.; Wosnitza, J.; Kolezhuk, A.; Krzystek, J.; Feyerherm, R.;
We report on low-temperature multi-frequency ESR studies of coopper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating g-tensor and the Dzyaloshinskii-Moriya interaction, allowing us to test a new theoretical concept proposed recently by Oshikawa and Affleck [Phys. Rev. Lett. 82, 5136 (1999)]. Their theory, based on bosonization and the self-energy formalism, can be applied for precise calculation of ESR parameters of S = 1/2 antiferromagnetic chains in the perturbative spinon regime. Excellent quantitative agreement between the theoretical predictions and experiment is obtained. Results of the presentation are published in: S.A. Zvyagin et al., Phys. Rev. Lett. 95, 017207, 2005.
  • Poster
    DPG-Frühjahrstagung, 27.-31.03.2006, Dresden, Deutschland

Publ.-Id: 8739 - Permalink

Sample-dependent resistivity profiles in K-(ET)2Cu[N(CN)2]Br organic superconductors - interrelations with other normal- and superconducting-state properties
Strack, C.; Akinci, C.; Paschenko, V.; Wolf, B.; Uhrig, E.; Assmus, W.; Lang, M.; Schreuer, J.; Wiehl, L.; Schlueter, J.; Wosnitza, J.; Schweitzer, D.; Wyckhoff, J.;
The organic superconductor κ-(ET)2Cu[N(CN)2Br shows striking sample-to-sample variations in the electrical resistivity [1]: While most of the crystals reveal a ρ(T) maximum around 90 K with a semiconducting behaviour above, some remain metallic for T < 300 K. In the absence of significant differences in the crystals´structural parameters, chemical composition and ESR spectra, these results indicate that real structure phenomena, i.e. disorder and/or defects, strongly affect the inclastic scattering. [1]. Here we report on a comparative resistivity study on a variety of crystals with quite different ρ(T) profiles. The work aims at seeking out interrelations between the anomalous scattering contributions at intermediate temperatures and features of the normal- and superconducting-state, susch as the glass transition at T9 ≈ 77 K, the temperature T* ≈ 40 K [1], as well as the superconducting transition temperature. [1] C. Strack et al., Phys. Rev. B 72 054511 (2005)
  • Lecture (Conference)
    DPG-Frühjahrstagung, 27.-31.03.2006, Dresden, Deutschland

Publ.-Id: 8737 - Permalink

Untersuchung des Verhaltens von freigesetztem Isolationsmaterial in einer Kühlmittelströmung (Kühlmittelströmung mit Feststoffpartikeln) - Modellentwicklung
Krepper, E.; Grahn, A.;
Die CFD-Modellierung des Fasertransports wurde auf der Basis des Euler/Euler-Ansatzes untersucht. Die Nachbildung des Transportverhaltens der Partikel erfordert die korrekte Beschreibung des Impulsaustausches. Hierfür liefert die Auswertung der Sinkgeschwindigkeiten aus den Experimenten am Versuchsstand „Säule“ die notwendigen Informationen. Über die Vorgabe von Partikeldichte und Partikeldurchmesser kann die Sinkgeschwindigkeit nachgebildet und der Partikeltransport adäquat modelliert werden. Beispielsimulationen einer partikelbeladenen Strömung im Versuchsstand „Ringkanal“ lieferten plausible Resultate.
Das Anlagerungs- bzw. Abtragungsverhalten der Partikel an Hindernissen (z. B. Wehren) wurde über die Modellierung einer vom Feststoffanteil abhängigen Viskosität simuliert. Dieser Zusammenhang wurde an die Ergebnisse der Experimente im Ringkanal angepasst. Erste Demonstrationsrechnungen zur Beschreibung des Partikeltransportes an Hindernissen ergaben plausible Resultate. In anderen Beispielen zeigte die Parametervariation einer feststoffabhängigen Viskosität jedoch einen zu geringen Einfluss auf die berechneten Ablagerungsmengen, um die experimentellen Befunde zu reproduzieren.
Es wurde ein Modell des Differenzdruckaufbaus aus den Kompaktierungseigenschaften der auf dem Sieb abgelagerten Fasern entwickelt und an Experimenten justiert. Damit steht neben den in der Literatur veröffentlichten Korrelationen ein weiteres Verfahren zur Berechnung des Differenzdrucks zur Verfügung. In einer Machbarkeitsstudie wurde die Wirkung eines Siebes auf das Strömungsfeld unter Nutzung des Modellansatzes des porösen Körpers in CFX-4.4 implementiert. Das implementierte Modell liefert qualitativ plausible Resultate.
Keywords: CFD, fibre transport, sedimentation, resuspension, difference pressure at sieves
  • Open Access LogoWissenschaftlich-Technische Berichte / Forschungszentrum Rossendorf; FZR-458 2006


Publ.-Id: 8736 - Permalink

Sound wave anomalies in superconducting compounds
Lüthi, B.; Zherlitsyn, S.; Wolf, B.;
Temperature dependence of the elastic constants in the superconducting state is described on the basis of the Ginsburg-Landau theory. This approach shows very general character and allows to describe quantitative renormalization of the elastic constants in the superconducting state for different materials including non-conventional superconductors. Various examples (A-15 compounds, Chevrel compounds, CeRu2, Ba0.63K0.37BiO3, Yb3Rh4Sn13, Ca3Rh4Sn13, HfV2, some heavy fermion compounds and Sr2RuO4) are given.
  • European Physical Journal B 46(2005), 169-176

Publ.-Id: 8734 - Permalink

Bindungsformen des Urans in umweltrelevanten Kompartimenten
Bernhard, G.;
Ausgehend vom Vorkommen, der physikalisch-chemischen Eigenschaften und der Verwendung werden die Eintragspfade für Uran in die Umwelt behandelt. Hintergrundwissen bilden dabei die zahlreichen Daten zu den Gehalten in den Kompartimenten Boden, Gestein, Mineral, unterschiedlichen Wässern, der Luft und in den Biosystemen.
Nach der Besprechung der Methoden zur Bestimmung der Bindungsform werden die Ergebnisse zur Uranbindungsform in unterschiedlichen Wässern und in Bakterien vorgestellt.
  • Lecture (others)
    Vortrag im Forschungszentrum Jülich GmbH, 13.09.2006, Jülich, Germany

Publ.-Id: 8733 - Permalink

Solitary waves in S = 1/2 quantum spin chains: ESR study in magnetic fields up to 25 T
Zvyagin, S. A.; Kolezhuk, A. K.; Krzystek, J.; Feyerherm, R.;
We report the low-temperature multifrequency ESR studies of copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating g tensor and the Dzyaloshinskii-Moriya interaction, allowing us to test a new theoretical concept proposed recently by Oshikawa and Affleck [Phys. Rev. Lett. 82, 5136 (1999)]. Their theory, based on bosonization and self-energy formalism, can be applied for precise calculation of ESR parameters of S = 1/2 antiferromagnetic chains in the perturbative spinon regime. Excellent quantitative agreement between the theoretical predictions and experiment is obtained.
  • Invited lecture (Conferences)
    Physical Phenomena at High Magnetic Fields-V, 05.-09.08.2005, Tallahassee, USA

Publ.-Id: 8732 - Permalink

Tunable-frequency Electron Paramagnetic Resonance - a Novel Tool to Investigate High-Spin Transition Metal Complexes
Krzystek, J.; Ozarowski, A.; Trofimenko, S.; Zvyagin, S. A.; Telser, J.;
Although Electron Paramagnetic Resonance (EPR) has been a very successful method to investigate transition metal ions in coordination complexes and biomolecules, not all such ions can be subject to conventional EPR investigations even if they are paramagnetic. In particular, high-spin (S >1/2) species pose serious challenges to spectroscopists, and of these, the non-Kramers (integer-spin) ions have been long considered ‘EPR-silent’ at conventional frequencies and fields. We propose a novel EPR-related technique that takes an advantage of tunable sources operating in the sub-THz range of frequencies in conjunction with very high magnetic fields (up to 25 Tesla) to determine accurate intrinsic spin Hamiltonian parameters not only for a variety of non-Kramers transition metal ions (such as Ni2+, Mn3+, and Fe2+) that have traditionally been termed ‘EPR-silent’, but also for those Kramers (half-integer) species that are poorly characterized in their high-spin states, such as Co2+. The obtained parameters can serve to better characterize the electronic structure of the ions in question, in combination with other experimental methods.
  • Advances in Coordination, Bioinorganic and Inorganic Chemistry (2005), 143-158

Publ.-Id: 8730 - Permalink

Magnetic excitations in sine-Gordon spin chains: ESR studies
Zvyagin, S. A.;
  • Invited lecture (Conferences)
    Workshop, 15.08.2005, Kharkov, Ukrainian

Publ.-Id: 8729 - Permalink

High-Field Electron Spin Resonance in Quasi-2D Frustrated Spin System SrCu2(BO3)2
Zvyagin, S. A.; Wosnitza, J.; Krzysted, J.; Stern, R.; Dabkowska, H.; Gaulin, B.;
We present results of high-frequency/field Electron Spin Resonance (ESR) studies of the highly frustrated spin system SrCu2(BO3)2,which can be regarded as to the best known realization of the Shastry-Sutherland model for two-dimensional Heisenberg antiferromagnets. The frequency-field diagram of magnetic excitations has been studied in a frequency range of 150-700 GHz, employing unable-frequency ESR technique. Two gapped odes were observed in the excitation spectrum. The size of the quantum spin gaps determined directly is: 708 GHz and 764 GHz (34 K and 36.7 K). The observation of excitations from the ground state (which are normally forbidden) and a double-gap structure of the ESR excitation spectrum in SrCu2(BO3)2 suggest and essential role of the Dzyaloshinski-Moriya interaction. The results are compared with those obtained by Nojiri et al. [J. Phys. Soc. Jpn. 68, 2906 (1999); J.Phys. Soc. Jpn. 72, 3243 (2003)]. The temperature behavior of excited-state transitions in SrCu2(BO3)2 (ESR linewidth and intensity) is presented and discussed.
  • Poster
    Workshop Highly Frustrated Magnetism (HFM), 07.-09.11.2005, Agelonde, La Londe Les Maures, Frankreich

Publ.-Id: 8728 - Permalink

Adhesion Issues in PBO/Epoxy Composites
Liu, J.; Gao, S. L.; Mäder, E.; Bianchi, A. D.; Herrmannsdörfer, T.; Zherlitsyn, S.; Zvyagin, S. A.; Wosnitza, J.;
This study aims to address interfacial adhesion requirements for composites with high electrical insulation properties for extreme high magnetic field and pressure environments. The magnetic field is one fundamental parameter influencing the physics of many systems. Over the recent years, research with high magnetic fields has led to a range of enhanced modern technologies. These efforts were triggered by experiments such as the integer and fractional quantum Hall effect and as a result several high magnetic field facilities were built in the US, Japan, and Europe. The current technology limits the generation of static magnetic fields to 45 T. Magnetic fields of up to about 70 T are only available for experiments with pulse duration in the ms range. The Hochfeld-Magnetlabor Dresden, as well as the Los Alamos branch of the National High Magnetic Field Laboratory, are aiming at generating fields up to 100 T. The access to this magnetic field range possesses a large number of technological issues, particularly abnormal heating and high stress, generated by the Lorentz forces from the electrical current and the magnetic field of the magnet. In a 100 T magnet these forces amount to a pressure of the order of 4 GPa and ordinary steel would burst under the stresses involved in confining the high magnetic field inside the magnet. This problem is circumvented by reinforcing all conductor layers of the coil with high modulus fibre reinforced composites. Because the pulsed field magnets are operated at high voltages and large currents, this demands high electrical insulation properties of the composite and makes the use of electrically conducting carbon fibres problematic. Today, the high ultimate tensile strength of 5 GPa, high heat resistance/service temperature and the good insulation properties make poly p-phenylene-2,6-benzobisoxazole (PBO) fibre of good choice. However, it is challenging to get adequate interfacial adhesion of PBO fibre due to the nonpolar surface nature of PBO preventing any chemical bonding the smooth PBO surface with polymer matrix.
We have characterised the interfacial adhesion strength and critical interphase energy release rate by single fibre pull-out tests. The PBO fibre surfaces were de-sized by extraction of the commercial finish, and sized with different sizings. In addition, both O2 and NH3 gas plasma treatments and plasma modification with Maleic anhydride were used. It was found that the rather low surface free energy of the as-received PBO fibres (34 mJ/m²) could be increased by appropriate surface modification, particularly the oxygen-plasma and Maleic anhydride graft. However, the adhesion strength increased marginally with increasing surface free energy due to induced limited hydrogen and covalent bonds at their interface, which is in the same level like aramid, i.e. below the adhesion strength of glass fibre / epoxy systems. A short time plasma treatment (30s) with NH3 gas results in both reduction of surface energy and interfacial adhesion strength. AFM was used to characterize the surface topography varied by different fibre surface modifications and to evaluate the fracture surfaces. The fractographs are dominated by adhesive failure, that is to say, the interfacial crack propagation occurs primarily in the fibre/matrix interface plane, suggesting a further improvement of chemical bonding/mechanical interlocking is required. Moreover, for the application in a high magnetic field also the temperature change in the coil during operation has to be taken into account. Epoxy resins with different temperature resistance have not shown significant differences in the interfacial shear strength. The micro-mechanical results agree well with those of compression shear tests performed with real composite samples.
  • Lecture (Conference)
    Polymer Fibres 2006 Conference, 12.-14.07.2006, Manchester, UK

Publ.-Id: 8727 - Permalink

Spin excitations in S = 1/2 antiferromagnetic chains with alternating g-tensor and the Dzyaloshinskii-Moriya interaction
Zvyagin, S. A.; Wosnitza, J.; Kolezhuk, A. K.; Krzystek, J.; Feyerherm, R.;
The magnetic excitation spectrum of copper pyrimidine dinitrate, a material containing s = 1/2 antiferromagnetic chains with alternating g-tensor and the Dzyaloshinskii-Moriya interaction, and exhibiting a field-induced spin gap, is probed using multi-frequency electron spin resonance spectroscopy in magnetic fields up to 25 T. Signatures of three breather branches an a soliton, as well as those of several multi-particle excitation modes are identified, and their frequency-field dependences are described in frame of the quantum sine-Gordon field theory. In addition, a new theoretical concept proposed recently by Oshikawa and Affleck [Phys. Rev. Lett. 82, 5136 (1999)] ist tested. Their theory, based on bosonization and the self-energy formalism, can be applied for precise calculation of ESR parameters of s = 1/2 antiferromagnetic chains in the perturbative spinon regime. Excellent quantitative agreement between the theoretical predictions and experiment is obtained. Results are published in S.A. Zvyagin et al., Phys. Rev. Lett. 93, 027201 (2004);ibid. Phys. Rev. Lett. 95, 012707 (2005).
  • Lecture (Conference)
    APS March Meeting, 13.-17.03.2006, Baltimore, USA

Publ.-Id: 8725 - Permalink

Phase control of Al2O3 thin films grown at low temperatures
Andersson, J. M.; Wallin, E.; Helmersson, U.; Kreissig, U.; Münger, E. P.;
Low-temperature growth (500°C) of alpha-Al2O3 thin films by reactive magnetron sputtering was achieved for the first time. The films were grown onto Cr2O3 nucleation layers and the effects of the total and O2 partial pressures were investigated. At 0.33 Pa total pressure and >16 mPa O2 partial pressure alpha-Al2O3 films formed, while at lower O2 pressure or higher total pressure (0.67 Pa), only gamma phase was detected in the films (which were all stoichiometric). Based on these results we suggest that alpha phase formation was promoted by a high energetic bombardment of the growth surface. This implies that the phase content of Al2O3 films can be controlled by controlling the energy of the depositing species. The effect of residual H2O (approx. 10(-4) Pa) on the films was also studied, showing no change in phase content and no incorporated H (< 0.1%). Overall, these results are of fundamental importance in the further development of low-temperature Al2O3 growth processes.
Keywords: Aluminum oxide; Chromium oxide; Sputtering; Ion bombardment; X-ray diffraction; Phase formation
  • Thin Solid Films 513(2006), 57-59

Publ.-Id: 8724 - Permalink

Dielectron production in 12C+12C collisions at 2 AGeV with HADES
Agakichiev, G.; Agodi, C.; Alvarez-Pol, H.; Balanda, A.; Bertini, D.; Bielcik, J.; Bellia, G.; Böhmer, M.; Bokemeyer, H.; Boyard, J. L.; Braun-Munziger, P.; Cabanas, P.; Chernenko, S.; Christ, T.; Coniglione, R.; Cosentino, L.; Diaz, J.; Dohrmann, F.; Duran, I.; Eberl, T.; Enghardt, W.; Fabbietti, L.; Fateev, O.; Fernandez, G.; Finocchiaro, P.; Friese, J.; Fröhlich, I.; Fuentes, B.; Garabatos, C.; Garzon, J. A.; Gernhäuser, R.; Golubeva, M.; Gonzalez-Diaz, D.; Grosse, E.; Guber, F.; Hennino, T.; Hlavac, S.; Holzmann, R.; Homolka, J.; Ierusalimov, A.; Iori, I.; Ivashkin, A.; Jaskula, M.; Jurkovic, M.; Kargalis, M.; Kajetanowicz, M.; Kämpfer, B.; Kanaki, K.; Karavicheva, T.; Kastenmüller, A.; Kidon, L.; Kienle, P.; Koenig, I.; Koenig, W.; Körner, H. J.; Kolb, B. W.; Kotte, R.; Krücken, R.; Kugler, A.; Kühn, W.; Kulessa, R.; Kurepin, A.; Lang, S.; Lange, S.; Lehnert, J.; Lins, E.; Magestro, D.; Maiolino, C.; Malarz, A.; Markert, J.; Metag, V.; Mousa, J.; Münch, M.; Müntz, C.; Naumann, L.; Nekhaev, A.; Novotny, J.; Otwinowski, J.; Pachmayer, Y.; Pechenov, V.; Perez, T.; Piattelli, P.; Pietraszko, J.; Pleskac, R.; Ploskon, M.; Pospisil, V.; Prokopovicz, W.; Przygoda, W.; Ramstein, B.; Reshetin, A.; Ritman, J.; Roy-Stephan, M.; Rustamov, A.; Sadovsky, A.; Sailer, B.; Salabura, P.; Sanchez, M.; Sapienza, P.; Schmah, A.; Schön, H.; Schön, W.; Simon, R. S.; Smolyankin, V.; Smykov, L.; Spataro, S.; Spruck, B.; Ströbele, H.; Stroth, J.; Strurm, C.; Sudol, M.; Suk, M.; Taranenko, A.; Tlusty, P.; Toia, A.; Traxler, M.; Tsertos, H.; Vassiliev, D.; Vazquez, A.; Wagner, V.; Walus, W.; Wisniovski, M.; Wojcik, T.; Wüstenfeld, J.; Zanevsky, Y.; Zeitelhack, K.; Zoviec, D.; Zumbruch, P.;
The invariant mass spectrum of e^+ e^- pairs produced in ^{12}C+^{12}C collisions at 2 GeV per nucleon incident energy has been measured for the first time with the HADES spectrometer at GSI. The measured pair production probabilities span over five orders of magnitude within the \pi^0\to e^+e^-\gamma to \rho/\omega\to e^+e^- invariant mass region. A thermal model of the dielectron sources underestimates the measured probabilities by a factor of roughly two at intermediate masses (0.2 -- 0.6 GeV/c^2) whereas it overshoots data at the \rho/\omega pole. Transport model calculations assuming vacuum spectral functions of the vector mesons fail to describe the entire invariant mass spectrum, as well.

Publ.-Id: 8723 - Permalink

Fermi Surface of the Half Heusler Compounds Ce1-xLaxBiPt
Bianchi, A. D.; Wosnitza, J.; Kozlova, N.; Eckert, D.; Schultz, L.; Opahle, I.; Elgazzar, S.; Richter, M.; Doerr, M.; Hagel, J.; Goll, G.; von Löhneysen, H.; Zwicknagel, G.; Yoshino, T.; Takabatake, T.;
We report on the Fermi surface in the correlated half-Heusler compound Ce1-xLaxBiPt. In CeBiPt we find a field-induced change of the electronic band structure as discovered by electrical-transport measurements in pulsed magnetic fields. For magnetic fields above ca 25 T, the charge-carrier concentration determined from Hall-effect measurements increases nearly 30%, whereas the Shubnikov-de Haas (SdH) signal disappears at the same field. In the non-4f compound LaBiPt the Fermi surface remains unaffected, suggesting that these features are intimately related to the Ce 4F electrons. Electronic band-structure calculations point to a 4f-polarization-induced change of the Fermi-surfache topology. In order to test this hypothesis, we have measured the (SdH) signal in a Ce0.95La0.05BiPt sample with a low La concentration.
  • Lecture (Conference)
    APS March Meeting, 13.-17.03.2006, Baltimore, USA
  • Poster
    DPG-Frühjahrstagung, 27.-31.03.2006, Dresden, Deutschland

Publ.-Id: 8722 - Permalink

Elementary excitations in the S=1/2 quantum sine-gordon spin chain
Zvyagin, S. A.; Wosnitza, J.; Kolezhuk, A. K.; Krzystek, J.; Feyerherm, R.;
An isotropic S = 1/2 Heisenberg antiferromagnetic (AFM) chain with uniform nearest-neighbor exchange coupling represents one of the paradigm models of quantum magnetism. Its ground state is a spin singlet and the dynamics are determined by a gapless two-particle continuum of spin- 1/2 excitations, commonly referred to as spinons. Since the S = 1/2 AFM chain is critical, even small perturbations can considerably change fundamental properties of the system. One of the most prominent examples is the S = 1/2 AFM chain perturbed by an alternating g-tensor and the Dzyaloshinskii-Moriya interaction; this situation is realized experimentally in the copper pyrimidine dinitrate, Cu-PM. In the presence of such interactions, application of a uniform external field H induces an effective transverse staggered field h ∝ H, which leads to the opening of an energy gap Δ ∝ H2/3. Here we report on the excitation spectrum in Cu-PM measured using submillimeter wave electron spin resonance (ESR) spectroscopy in fields up to 25 T [1]. Ten excitation modes are resolved in the low-temperature spectrum. The field-induced gap is measured directly. Signatures of three breather branches and a soliton, as well as those of several multi-particle excitation modes are identified. The experimental data are sufficiently detailed to make a very accurate comparison with predictions based on the quantum sine-Gordon field theory [2]. In addition, a new theoretical concept proposed recently by Oshikawa and Affleck [3] has been tested. Their theory, based on bosonization and the self-energy formalism, can be applied for precise calculation of ESR parameters of S = 1/2 AFM chains in the perturbative spinon regime. Excellent quantitative agreement between the theoretical predictions and experiment is obtained [4].
[1] S.A. Zvyagin et al., Phys. Rev. Lett. 93 (2004) 027201.
[2] M. Oshikawa and I. Affleck, Phys. Rev. Lett. 79 (1997) 2883; I. Affleck and
M. Oshikawa, Phys. Rev. B 60 (1999) 1038; ibid 62 (2000) 9200.
[3] M. Oshikawa and I. Affleck, Phys. Rev. Lett. 82 (1999) 5136.
[4] S.A. Zvyagin et al., Phys. Rev. Lett. 95 (2005) 017207.
  • Poster
    International Conference on Magnetism (ICM), 20.-25.08.2006, Kyoto, Japan

Publ.-Id: 8720 - Permalink

Anisotropy and spin-triplet excitions in theS = 1/2 gapped antiferromagnet BaCuSi2O6.
Zvyagin, S. A.; Wosnitza, J.; Krzystek, J.; Stern, R.; Jaime, M.; Sasago, Y.; Uchinokura, K.;
BaCuSi2O6 (also known as Han Purple Pigment) can be regarded as an almost ideal realization of the S = 1/2 system of weakly-interacting spin dimers with the spin-singlet ground state and gapped excitation spectrum [1]. By application of an external magnetic field the gap can be closed, creating a gas of interacting bosonic spin-triplet excitations (triplons). In BaCuSi2O6 this phenomenon can be effectively described in terms of the field-induced Bose-Einstein condensation of triplons [2]. Here, we focus on another interesting phenomenon associated with interacting excited triplets but in the low-field quantum-disordered state. We argue that a fine structure observed in low-temperature EPR spectra of BaCuSi2O6 is a fingerprint of triplet excitations (excitons), which are mobile at low temperatures and getting localized when the temperature is increased. Analyzing the angular dependence of the exciton modes allows us to precisely calculate the zero-field splitting with the triplet state and,correspondingly, the anisotropy parameter, D = 0.07 cm-1. The proposed procedure can be applied for a large number of S = 1/2 gapped quantum antiferromagnets with dimerized or alternating spin structure and might be of particular importance for studying anisotropy effects in S = 1/2 quantum chains (see for instance [3]). In addition, the temperature dependence of the EPR intensity and linewidth has been measured and discussed. The magnitude of the energy spin gap determined by analyzing the temperature dependence of the integrated signal intensity (¢ = 53 K) is in excellent agreement with data obtained from neutron-scattering measurements [1].
[1] Y. Sasago et al., Phys. Rev. B 55 (1997) 8357.
[2] M. Jaime et al., Phys. Rev. Lett. 93 (2004) 087203.
[3] R.D. Somma and A.A. Aligia, Phys. Rev. B 64 (2001) 024410.
  • Poster
    International Conference on Magnetism (ICM), 20.-25.08.2006, Kyoto, Japan

Publ.-Id: 8719 - Permalink

Untersuchungen zum Flüssigkeits-Holdup und zur Flüssigkeitsverweilzeit in Schüttungen mittels Röntgenradiographie und Gammatomographie
Hampel, U.; Bieberle, A.; Hessel, G.; Zimmermann, W.; Zippe, C.;
Es wurden Verweilzeituntersuchungen und Gasverteilungsmessungen an einer senkrecht mit Wasser durchströmten kiesbefüllten Säule mit vier intermediären Eisengranulatschichten durchgeführt. Zur Messung der Verweilzeitspektren wurde in die Zuleitung der Säule ein 60 ml Bolus BaCl2-Lösung (1,4 mol/l) eingespeist. Der Eintrittszeitpunkt des Bolus an der Säule wurde mittels einer direkt am Eintrittstutzen angebrachten Leitfähigkeitsnadelsonde erfasst. Der Durchgang des Tracerbolus durch verschiedene Messebenen wurde mittels Gammadurchstrahlung aufgezeichnet. Aus dem Schwächungssignal lassen sich Verweilzeitspektren direkt ableiten. Zur Bestimmung der Verweilzeit bzw. der mittleren Strömungsgeschwindigkeit in der Säule wurden zwei charakteristische Zeitpunkte des Verweilzeitspektrums verrechnet. Einmal der Frontzeitpunkt, der den erstmaligen Nachweis von Tracerflüssigkeit in der Messebene bezeichnet, sowie der Boluszeitpunkt, der durch die maximale Tracerkonzentration in der Messebene gegeben ist. Der gammadensitometrische Nachweis des Tracerbolus war aufgrund der starken axialen Dispersion nur in der unteren Hälfte der Säule möglich. Die Ergebnisse gaben keinen Hinweis auf größere gasbedingte hydraulische Obstruktionen in der Säule.
Der Nachweis von Gaseinschlüssen bzw. die Darstellung der Gasverteilung in vier ausgewählten Messebenen wurde mit dem Verfahren der Gammastrahlungstomographie realisiert. Die Messebenen wurden jeweils mittig zwischen den Eisengranulatschichten gewählt. Bezüglich der Verteilung des Bariumtracers wurden keine nennenswerten Konzentrationsgradienten im Messquerschnitt gefunden, so dass von einer homogenen Durchströmung der Säule ausgegangen werden kann. Erkannt wurden Gaseinschlüsse besonders im peripheren Bereich des Säulenquerschnitts. Der globale Gasgehalt ist dabei kleiner als 5%.
Keywords: Gammatomographie, Gammaradiographie, Röntgenradiographie, Flüssigkeitsverweilzeit, Schüttbett
  • Open Access LogoWissenschaftlich-Technische Berichte / Forschungszentrum Rossendorf; FZR-456 2006


Publ.-Id: 8717 - Permalink

Magnetic Excitations in S = 1/2 Spin Chains with Alternating g-tensor and theDzyaloshinskii-Moriya Interaction
Zvyagin, S. A.; Wosnitza, J.; Kolezhuk, A. K.; Krzystek, J.; Feyerherm, R.;
The magnetic excitation spectrum in copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic (AFM) chain system with an alternating g-tensor and the Dzyaloshinskii-Moriya interaction, has been studied using electron-spin resonance (ESR) spectroscopy in magnetic fields up to 25 T. Ten modes were resolved in the spectrum. The data were analyzed in terms of the sine-Gordon quantum field theory [Phys. Rev. Lett. 79, 2883 (1997)]; signatures of three breather branches and a soliton were identified. The field-induced gap was measured directly. In addition, a new theoretical concept proposed recently by Oshikawa and Affleck [Phys. Rev. Lett. 82, 5136 (1999)] has been tested.
Their theory, based on bosonization and the self-energy formalism, can be applied for the precise calculation of ESR parameters of spin-1/2 AFM chains in the perturbative spinon regime. Excellent quantitative agreement between the theoretical predictions and experiment is obtained.
  • Lecture (Conference)
    Yamada Conference LX on Research in High magnetic Fields (RHMF), 16.-19.08.2006, Sendai, Japan

Publ.-Id: 8716 - Permalink

Superconducting Properties of RuSr2RECu2O8
Papageorgiou, T. P.; Casini, E.; Braun, H. F.; Herrmannsdörfer, T.; Bianchi, A. D.; Wosnitza, J.;
The resistance R of the (weakly) ferromagnetic (TM ≈ 133K) superconductor RuSr2EuCu2O8 (Ru1212Eu) shows a maximum at Tmax ≈ 30 K while Tc (R = 0) ≈ 10 K. Using dc-magnetization measurements we were able to identify Tmax as the temperature where the intragrain superconducting transition takes place,whereas Tc fits well with the temperature where intergrain (Josephson)coupling between the grains is established. Our findings suggest that the wide(≈ 20 K) superconducting transition of Ru1212Eu in zero applied magnetic field is most probably due to resistive grain contacts. A similar situation is realised for RuSr2GdCu2O8. These results contradict reports which attribute the wide superconducting transition of these compounds to the movement of spontaneously induced vortices [1]. Spontaneous vortex-state formation at T < Tc cannot be excluded, nevertheless we show that it is more likely that the vortices are pinned in the intergrain area (area between the grains). Single crystals would be required to unambiguously clarify the state of the individual grains. Furthermore, Tmax for Ru1212Eu is shifted to lower temperatures when an external magnetic field is applied. This challenges reports [2], where an increase of the critical temperature with the application of an external magnetic field in specific heat measurements was interpreted as possible evidence for triplet pairing in the ruthenium cuprates.
[1] C. Y. Yang et al., Phys. Rev. B 72, 174508 (2005)
[2] J. L.Tallon et al., Phys. Rev. B 61, R6471 (2000)
  • Poster
    International Conference on Magnetism (ICM), 20.-25.08.2006, Kyoto, Japan

Publ.-Id: 8715 - Permalink

Polyoxometalates – a new class of potent ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) inhibitors
Müller, C. E.; Iqbal, J.; Baqi, Y.; Zimmermann, H.; Röllich, A.; Stephan, H.;
Polyoxotungstates were identified as potent inhibitors of NTPDases1, 2, and 3. The most potent compound was K6H2[TiW11CoO40], exhibiting Ki values of 0.140 µM (NTPDase1), 0.910 µM (NTPDase2), and 0.563 µM (NTPDase3). One of the compounds, (NH4)18[NaSb9W21O86], was selective for NTPDases2 and 3 versus NTPDase1. NTPDase inhibition might contribute to the described biological effects of polyoxometalates, including their anti-cancer activity.

Publ.-Id: 8714 - Permalink

Complexation of curium(III) with L2-aminobutyric acid investigated by time-resolved laser-induced fluorescence spectroscopy (TRLFS).
Moll, H.; Bernhard, G.;
The complex formation of curium(III) with L2-Aminobutyric acid was characterized by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at trace Cm(III) concentrations (3x10-7 M).
The different curium(III) species, MpHqLr, identified are characterized by their individual luminescence spectra and luminescence lifetimes. The following formation constants were determined log β101 = 5.17 ± 0.07, log β102 = 9.00 ± 0.07, and log β103 = 11.30 ± 0.09 at ionic strength I = 0.5 M. Possible structures of the curium aminobutyrate species will be discussed on the basis of the luminescence lifetime measurements and the magnitude of the formation constants.
Keywords: Amino acids; L2-Aminobutyric acid; TRLFS; Curium; Complexation

Publ.-Id: 8713 - Permalink

Pulsed-magnet design at the Dresden High Magnetic Field Laboratory
Zherlitsyn, S.; Herrmannsdoerfer, T.; Skourski, Y.; Sytcheva, A.; Wosnitza, J.;
The current status of the pulsed-magnet program of the High Magnetic Field Laboratory at Dresden (HLD) is reported. The non-destructive pulsed magnets for the HLD include a wide spectrum of coils designed for energies between 1 and 46 MJ, magnetic fields of 60 - 100 T, and pulse durations of 10 - 1000 ms. Magnet bores from 20 to 40 mm will accommodate different experimental setups. A universal core design has been developed at the HLD for these pulsed magnets. In spite of a strong external mechanical structure, our magnets are designed in a compact way. That allows to use this design both for inner and outer coils in multi-coil configurations. The core design has been successfully examined for a number of magnets for fields between 60 and 65 T. Already some user magnets for first scientific experiments have been installed and tested. The construction of a 9 MJ / 70 T mono-coil magnet has been accomplished. First test results are presented. The design of a two-coil 46 MJ magnet for magnetic fields above 85 T has been completed. Different aspects of the pulsed-magnet design and coil construction are discussed. Important issues of the coil design are numerical simulations of the pulsed-magnet behavior. Both analytical approximations and finite element analysis are used for these simulations at the HLD. Some simulation results are presented.
  • Poster
    Yamada Conference LX on Research in High magnetic Fields (RHMF), 16.-19.08.2006, Sendai, Japan

Publ.-Id: 8712 - Permalink

Fast THz time-domain spectrometer for pulsed magnets
Drachenko, O.; Winnerl, S.; Schneider, H.; Helm, M.; Wosnitza, J.;
  • Poster
    17th International Conference on High Magnetic Fields in Semiconductor Physics (HMF), 20.07.-04.08.2006, Würzburg, Deutschland

Publ.-Id: 8710 - Permalink

Characterization of ZnO single crystals by positron annihilation
Brauer, G.;
  • Lecture (others)
    Seminar an der University of Western Ontario, 31.07.2006, London/Ontario, Canada
  • Lecture (others)
    Seminar an der McMaster University, 03.08.2006, Hamilton/Ontario, Canada
  • Lecture (others)
    Seminar an der University of Texas at Arlington, 31.08.2006, Arlington/Texas, USA
  • Lecture (others)
    Seminar an der Texas Christian University, 01.09.2006, Ft. Worth/Texas, USA
  • Lecture (others)
    Seminar an der University of Texas at Austin, 30.08.2006, Austin/TX, USA
  • Lecture (others)
    Seminar an der University of North Texas, 28.08.2006, Denton/Texas, USA

Publ.-Id: 8708 - Permalink

Positron annihilation-induced Auger electron emission from 6H-SiC surfaces
Mukherjee, S.; Nadesalingam, M.; Davis, B.; Brauer, G.; Kohyama, A.; Nozawa, T.; Weiss, A.;
  • Lecture (Conference)
    14th International Conference on Positron Annihilation (ICPA-14), 23.-28.07.2006, Hamilton/Ontario, Canada

Publ.-Id: 8707 - Permalink

Positron annihilation in three zirconia polymorphs
Melikhova, O.; Kuriplach, J.; Cizek, J.; Prochazka, I.; Anwand, W.; Brauer, G.; Konstantinova, T. E.; Danilenko, I. A.;
Positron lifetimes and high momentum profiles both for the perfect lattice and selected defects are calculated in three (cubic, tetragonal and monoclinic) zirconia polymorphs using the atomic superposition method. Theoretical data are compared with the measured positron lifetime for cubic and tetragonal monocrystals of yttria-stabilized zirconia (YSZ) and coincidence Doppler broadening measurements on tetragonal monocrystals of YSZ. Positron lifetime spectra of YSZ monocrystals exhibit a single component spectrum with lifetimes 178 ps and 174 ps for cubic and tetragonal phases, respectively. Possible interpretations of measured lifetime and Doppler data are discussed.
  • Lecture (Conference)
    14th International Conference on Positron Annihilation (ICPA-14), 23.-28.07.2006, Hamilton/Ontario, Canada
  • Physica Status Solidi (C) 4(2007), 3831-3834

Publ.-Id: 8706 - Permalink

Positron annihilation spectroscopic study of hydrothermal grown n-type zinc oxide single crystal
Hui, C. W.; Zhang, Z. D.; Taojun, Z.; Ling, C. C.; Beling, C. D.; Fung, S.; Brauer, G.; Anwand, W.; Skorupa, W.;
Positron lifetime and coincidence Doppler broadening spectroscopic (CDBS) measurements were carried out to study the defects in two hydrothermal (HT) grown ZnO single crystal samples (HT1 and HT2) obtained from two companies. Single component model could offer good fittings to the room temperature spectra of HT1 and HT2, with the positron lifetimes equal to 199 ps and 181 ps respectively. These two lifetime components were associated with saturated positron trapping into two VZn-related defects with different microstructures. The positron lifetimes of HT1 was found to be temperature independent. For the HT2 sample, the positron lifetime remained unchanged with T > 200 K and decreased with decreasing temperature as T<200K. This could be explained by the presence of an additional positron trap having similar electronic environment to that of the delocalized state and competing in trapping positrons with the 181 ps component at low temperatures. Positron-electron autocorrelation function, which was the fingerprint of the annihilation site, was extracted from the CDBS spectrum. The obtained autocorrelation functions of HT1 and HT2 at room temperature, and HT2 at 50 K had features consistent with the above postulates that the 181 ps and the 199 ps components had distinct microstructures and the low temperature positron trap existed in HT2.
  • Lecture (Conference)
    14th International Conference on Positron Annihilation (ICPA-14), 23.-28.07.2006, Hamilton/Ontario, Canada
  • Physica Status Solidi (C) 4(2007), 3672-3675

Publ.-Id: 8704 - Permalink

Influence of hydrogen peroxide treatment on Au/n-ZnO contact
Chen, X. D.; Ling, C. C.; Djurisic, A. B.; Brauer, G.; Anwand, W.; Skorupa, W.; Reuther, H.;
The Au/n-ZnO contact behavior was investigated as a function of hydrogen peroxide pre-treatment of the ZnO sample by current-voltage (IV) measurement. The change from non-rectifying to rectifying performance upon etching was discussed in connection with sample characterization by positron annihilation spectroscopy (PAS), Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). It was concluded that the gain of the rectifying property was properly associated with the formation of a defective interfacial region at the Au/n-ZnO contact.
  • Lecture (Conference)
    14th International Conference on Positron Annihilation (ICPA-14), 23.-28.07.2006, Hamilton/Ontario, Canada
  • Physica Status Solidi (C) 4(2007), 3633-3636

Publ.-Id: 8703 - Permalink

Defect studies of hydrogen loaded Nb: bulk metals and thin films
Cizek, J.; Prochazka, I.; Danis, S.; Vlach, M.; Zaludova, N.; Brauer, G.; Anwand, W.; Mücklich, A.; Gemma, R.; Nikitin, E.; Kirchheim, R.; Pundt, A.;
Microstructure investigations of Nb loaded with H are presented in this work. The microstructure was examined by positron annihilation spectroscopy (PAS), combined with X-ray diffraction (XRD) and transmission electron microscopy (TEM). The behaviour of H-loaded bulk samples and thin films was compared. First, the microstructure of the virgin (H-free) specimens was characterized. Subsequently, the development of the microstructure during step-by-step electrochemical H charging was studied. The investigations were performed mainly in the low H concentration region (α-phase), where the Nb-H system represents a single phase interstitial solid solution. In bulk samples it was found that new vacancy-like defects are introduced by H loading. Vacancies surrounded by H were detected also in the electron irradiated bulk samples. Nanocrystalline thin films were produced by sputtering at room temperature. They exhibit a significant volume fraction of grain boundaries with open volume defects which trap H.
  • Lecture (Conference)
    14th International Conference on Positron Annihilation (ICPA-14), 23.-28.07.2006, Hamilton/Ontario, Canada
  • Physica Status Solidi (C) 4(2007), 3485-3488

Publ.-Id: 8702 - Permalink

The pulsed high-brightness positron source EPOS
Krause-Rehberg, R.; Brauer, G.; Krille, A.; Jungmann, M.; Sachert, S.; Rogov, A.; Noack, K.;
  • Lecture (Conference)
    14th International Conference on Positron Annihilation (ICPA-14), 23.-28.07.2006, Hamilton/Ontario, Canada
  • Lecture (Conference)
    19th International Conference on the Application of Accelerators in Research and Industry (CAARI2006), 20.-25.08.2006, Ft. Worth/Texas, USA

Publ.-Id: 8701 - Permalink

Copper deposition and dissolution in seemingly parallel electric and magnetic fields: Lorentz force distributions and flow configurations
Cierpka, C.; Weier, T.; Gerbeth, G.; Uhlemann, M.; Eckert, K.;
In two different cylindrical electrochemical cells, copper deposition and dissolution in the presence of a magnetic field mostly parallel to the electric field has been investigated. Particle image velocimetry measurements show that even under such a field configuration where Lorentz forces are often a priori neglected in fact they may dominate the flow. Further on, depending on the electrode radius a reversal of the secondary flow has been found. This feature can be explained by the different Lorentz force configurations calculated from the magnetic field and the primary current distributions. The components of the magnetic field have been determined by means of a traversed Hall probe as well as calculated by a commercial finite element package. Experimentally and numerically determined field distributions match very well. A good agreement between the measured and calculated velocity distributions has also been found, suggesting that in the present case the effect of the Lorentz force alone is sufficient to explain the magnetic field influence on the flow.

Publ.-Id: 8700 - Permalink

Electron Imaging Based on Charge Density Potentials and Frozen Lattices
Scheerschmidt, K.; Kuhlmann, V.; Rother, A.; Gemming, S.;
Standard potential for the simulation of electron holographic images are based on the superposition of atomic scattering potentials, and thermal effects are included by a Debye-Waller factor. In the present theoretical treatment these limitations have been overcome by the use of MD simulations with atomistic pair potentials, which yield an atom-resolved description of the thermal motion. A further refinement of the scattering potential is possible, if the self-consistently calculated electron distribution from a first-principles calculation on selected MD frames is employed instead of the spherical electronic part of the superposed atomic potentials.
Keywords: Electron scattering, thermal effects, Debye-Waller factor
  • Poster
    IMC 16, 02.-07.09.2006, Sapporo, Japan

Publ.-Id: 8699 - Permalink

Modelling ferroic functional elements
Gemming, S.;
The modelling concepts of a nanoscale organic field effect transistor on a ferroelectric substrate are presented. The device consists of a ferroelectric substrate with a switchable, macroscopic electric polarisation, an electric-field-sensitive organic molecule, and suitable connectors. The structural and electronic properties of the single components and the major interactions between them are modelled by first-priciples electronic structure calculations. For the influence of the electric field on the optical and the conductivity properties of the larger organic molecules, a related density-functional-based tight-binding scheme was employed, which focusses on the salient features of the electronic system. For the self-assembly of the transistor building blocks, mesoscale approaches based on atomistic pair potentials and on a supramolecular Ising-type modelling have been developped and successfully applied.
Keywords: multi-scale modelling, functional materials, organic field effect transistor, optical properties, conductivity
  • Invited lecture (Conferences)
    MMM 2006 - International Conference on Multiscale Materials Modelling, 17.-22.09.2006, Freiburg, Brsg., Deutschland
  • Contribution to proceedings
    MMM 2006 - Third International Conference on Multiscale Materials Modeling, 18.-22.09.2006, Freiburg, Brsg., Deutschland
    Proceedings of MMM III, Stuttgart: Fraunhofer IRB Verlag, 3-8167-7206-4, 835-842
  • Invited lecture (Conferences)
    SFB 484 - Kooperative Phänomene im Festkörper, 16.10.2007, Augsburg, Germany

Publ.-Id: 8698 - Permalink

Simulation and AFM-Measurement Of PTCDA On Ag-supported KBr Films
Gemming, S.; Olbrich, C.; Morawetz, K.; Schreiber, M.; Loppacher, C.; Seifert, G.; Eng, L. M.;
On a partially KBr-covered Ag(111) substrate thin films of PTCDA molecules exhibit different growth modes, which depend on the thickness of the KBr layer. If no KBr is present, PTCDA form large islands with a herringbone-type substructure; on 1 ML KBr many small, not well aligned PTCDA islands with a large size distribution are formed. On 2ML KBr, again large flat PTCDA clusters are observed, which exhibit a pile-up of PTCDA molecules at the edges of the KBr islands. These growth modes have been analysed with an anisotropic Ising model for a square lattice including next-nearest neighbour interactions. The coupling constants and the adsorbate-substrate interaction strength were derived from electronic-strcuture calculations. The simulations give evidence that large clusters are formed if either the adsorbate-substrate interactions (no KBr) or the lateral inter-molecular interactions (2 ML KBr) dominate. In the intermediate case of 1 ML KBr on Ag(111), both mechanisms compete and a high number of cluster seeds is generated. The presence of the surface steps between regions of different KBr layer thickness enhances this effect.
Keywords: Monte-Carlo, Ising, 2D square lattice, surface step, pattern formation
  • Lecture (Conference)
    IMC16, 02.-07.09.2006, Sapporo, Japan

Publ.-Id: 8697 - Permalink

Structure and stability of Molybdenum Sulfide Fullerenes
Enyashin, A. N.; Gemming, S.; Bar-Sadan, M.; Popovits-Biro, R.; Hong, S. Y.; Prior, Y.; Tenne, R.; Seifert, G.;
Atomic models of MoS-based nanoparticles with fullerene-like structures were constructed. Their stability and electronic properties with respect to the particle size and composition have been investigated for the first time using the DF-TB method. The present calculations and MD simulations show that stoichiometric single-walled moS2 fullerenes with octahedral shape are unstable, at least up to sizes of several hundreds of atoms. The instability is attributed to the high strain energies at the corners of the MoS2 nanooctahedra. However, the particles are considerably stabilized if local deviations from the stoichiometry is allowed at the corners. An additional attractive inter-layer interaction enhances this trend, such that multi-wall nanooctahedra with sizes between thousand and ten thousand atoms are stable species, which can be observed experimentally. Larger nanooctahedra are transformed into multiwall particles with a spherical shell.
Keywords: DFTB calculations, MoS2, inorganic fullerenes

Publ.-Id: 8696 - Permalink

Reduction of surface coverage of finite systems due to geometrical steps
Morawetz, K.; Olbrich, C.; Gemming, S.;
The coverage of vicinal, stepped surfaces with molecules is simulated with the help of a two-dimensional Ising model including local distortions and a Schwoebel barrie term at the steps. An effective two-spin model is capable to describe the main properties of this distorted Ising model. It is employed to analyze the behavior of the system close to the critical points. Within a well-defined regime of bonding strengths and Schwoebel barriers we find a reduction of coverage (magnetization) due to the presence of the surface step. This results in a second, low-temperature transition besides the standard Ising order-disorder transition. The additional transition is characterized by a divergence of the susceptibility and the magnetization as finite-size effects. Due to the surface step the specific heat diverges with a power law.
Keywords: Monte-Carlo simulation, Ising model, finite-size effects, mean-field model, 2D square lattice, Schwoebel barrier

Publ.-Id: 8695 - Permalink

SrTiO3(001)|LaAlO3(001) multilayers: A density-functional investigation
Gemming, S.; Seifert, G.;
SrTiO3(0 0 1)|LaAlO3(0 0 1) (STO|LAO) multilayers were investigated by density-functional band structure calculations. The lattice constant parallel to the interface equals roughly the average of the two constituents (3.81 Angstrom). The system contains two interface terminations: (I) SrO(0 0 1)|AlO2(0 0 1) with a spacing of 1.92 Angstrom as in STO and (II) TiO2(0 0 1)|LaO(0 0 1) with a spacing of 1.89 Angstrom as in LAO.
Additional displacements of the metal atoms close to the boundaries lead to rumplings of up to 0.06 Angstrom. A stoichiometric model is electronically neutral and insulating. Model systems with only termination I exhibit spatially localised holes in the O-based valence band; in systems with termination II the additional electron occupies the Ti-based conduction band. The strongly anisotropic dielectric constants obtained from linear response calculations show that the interface chemistry allows a fine tuning of perovskite multilayers.
Keywords: electronic structure, DFT, interfaces, non-linear dielectric properties, ferroic oxides

Publ.-Id: 8694 - Permalink

Structure and dynamics in the complex ion (UO2)2(CO3)(OH)3–
Szabó, Z.; Moll, H.; Grenthe, I.;
The structure and ligand exchange dynamics of the ternary complex (UO2)2(CO3)(OH)3- have been investigatedd by EXAFS and NMR spectroscopy. Very broad signals can be observed in both the 13C and the 17O NMR spectra.
The EXAFS data show the presence of 1.3 ± 0.3 short uranium–oxygen distances at 2.26 Å, consistent with single bonded hydroxide and 3.9 ± 0.6 distances at 2.47 Å for the other ligands in the first co-ordination shell. There is also evidence for a U U interaction at 3.90 Å. Based on the EXAFS and NMR data we suggest the presence of three isomers with different bridge arrangements, the dominant one, C, contains 80% of the uranium and the minor ones A and B, 5 and 15%, respectively. The ligand exchange reactions between these isomers are slow.
The NMR data indicate that the main reactions involve intramolecular exchanges between isomers with different positions of the non-bridging ligands in A, B and C. We suggest that these take place through water exchange as discussed earlier for other ternary uranium() complexes.

Publ.-Id: 8693 - Permalink

Characterization of Nickel induced crystallized Silicon by spectroscopic ellipsometry
Pereira, L.; Águas, H.; Beckers, M.; Martins, R. M. S.; Fortunato, E.; Martins, R.;
In this work Spectroscopic Ellipsometry (SE) was used to study metal induced crystallization (MIC) on amorphous silicon films in order to analyze the influence of different annealing conditions on their structural properties. The variation of the metal thickness has shown to be determinant on the time needed to full crystallize silicon films. Films of 100 nm thickness crystallize after 2h at 500ºC using 1 nm of Ni deposited on it. When reducing the average metal thickness down to 0.05 nm the same silicon film will need almost 10 hours to be totally crystallized. Using a new approach on the modelling procedure of the SE data we show to be possible to determine the Ni remaining inside the crystallized films. The method consists in using Ni as reference on the Bruggeman Effective Medium Approximation (BEMA) layer that will simulated the optical response of the crystallized silicon. Silicon samples and metal layers with different thicknesses were analyzed and this new method has shown to be sensible to changes on the initial metal/silicon ratio. The nickel distribution inside the silicon layers was independently measured by Rutherford Backscattering Spectroscopy (RBS) to check the data
obtained from the proposed approach.
  • Materials Research Society Symposium Proceedings 910(2006), 0910-A21-06

Publ.-Id: 8692 - Permalink

The geomagnetic dynamo - laboratory experiments
Stefani, F.; Gerbeth, G.; Gailitis, A.;
After seven years of successful experimental dynamo work, and after six years work of the DFG Priority Programme 1097 "Geomagnetic field variations", we summarize what has been achieved in the liquid metal laboratories, and what part of this could be of interest for geodynamo researchers. We try to make a compromise between a general survey of the worldwide dynamo scene, and a stock-taking of the activities in the framework of the Priority Programme. Since one project was directly devoted to the Riga dynamo experiment, we discuss this experiment a bit more in detail.
  • Invited lecture (Conferences)
    International Final Colloquium of the German Science Foundation Priority Programme 1097 "Geomagnetic Field Variations: Space-Time Structure, Processes, and Effects on System Earth", 04.-05.10.2006, Braunschweig, Germany
  • Contribution to proceedings
    International Final Colloquium of the German Science Foundation Priority Programme 1097 "Geomagnetic Field Variations: Space-Time Structure, Processes, and Effects on System Earth", 04.-05.10.2006, Braunschweig, Germany
    Terra Nostra 3(2006), Berlin: Selbstverlag der Alfred-Wegener-Stiftung, 0946-8978, 83-93

Publ.-Id: 8691 - Permalink

Electron intraband capture and relaxation in self-assembled InAs/GaAsquantum dots
Menzel, S.; Zibik, E. A.; Aivaliotis, P.; Carpenter, B. A.; Stehr, D.; Winnerl, S.; Helm, M.; Steer, M. J.; Hopkinson, M.; Cockburn, J. W.; Wilson, L. R.;
We will present a detailed investigation of intraband electron capture and relaxation processes in InAs quantum dots (QDs) using an energy and temperature dependent, mid-infrared pump-probe (PP) measurement. We find that the electron relaxation time is strongly dependent on the photo-excitation energy and varies from ~4 ps for relaxation from the higher energy excited states inside the QDs up to ~10 ps for relaxation from the wetting layer (WL)/barrier states. Although the ‘phonon bottleneck’ effect predicts the capture time to be much longer, the measured times still exceed significantly the typical ~1ps relaxation time for quantum wells. Also our intraband measurements clearly indicate that the electron capture/relaxation occurs directly to the QD ground state, not step-wise between the QD excited states as it was suggested previously. An advantage of the intraband PP measurement is that the electron population is determined by the doping, varying from 0.5 up to >2 electrons per dot, giving a more precise control of the QD population compared with interband excitation.
Our previous studies have shown that the electron relaxation in QDs from the p-like first excited state to s-like ground state (low energy peak in Fig.1) is typically >30ps [1]. Surprisingly, in the present work, the PP signal corresponding to the relaxation of electrons from higher energy excited states (dashed line in Fig.2) decays with time constant τ ~4ps. Such a short decay time allows us to conclude, that electron relaxation occurs directly to the ground state, avoiding the p-state. When we excite into the WL/barrier states the intraband relaxation time increases to ~10ps for the PP energy of ~210meV. Furthermore, at low temperatures a bi-exponential decay of the PP signal corresponding to electron relaxation from WL/barrier states to QD ground state with τ1=(8 ± 2) ps and τ2=(300 ± 100) ps is observed (solid line in Fig.2 and inset). The short decay time is related to electron capture in the same QD from which it was originally excited, whereas the long decay time is strongly dependent on the temperature and originates from the electron capture and thermal re-emission in adjacent QDs.
Additionally, interband pump/intraband probe measurements were performed from which the electron capture times from the GaAs barrier and WL states into the QDs of ~6ps and ~4ps, respectively, has been determined. Since interband excitation creates photo-generated electron-hole pairs, we explain the shortening of the electron capture time by Auger-type carrier-carrier scattering.
Using a combination of these two techniques we have been able to gain detailed insight into the electron relaxation processes in QDs, which are significant for quantum dot based lasers and infrared photodetectors. Our results show that due to longer WL states lifetime (~10ps) QD infrared photodetectors have the potential to be more efficient compared to quantum well analogues.
  • Lecture (Conference)
    International Conference on Superlattices, Nanostructures and Nanodevices 2006, 30.07.-04.08.2006, Instanbul, Turkei

Publ.-Id: 8690 - Permalink

Experimental data on a three dimensional flow field around an obstacle in a vertical pipe
Al Issa, S.; Prasser, H.-M.; Beyer, M.;
In many industrial applications, i.e. constrictions, expansions and various valves, three dimensional two-phase flows occur. Two-phase flows over an asymmetric obstacle create pronounced 3D structures, including: flow separation at the obstacle edge, recirculation zone and vortex formation in the wake of them as well as strong shear fields with curved streamlines and large angles.
The presentation starts with a description of the test section, the measuring technique and the boundary conditions of the required experiments. Furthermore, algorithms and equations for estimation of the axial liquid and the lateral bubble velocities were given. The results of the data treatment are shown in cross section views of the tube in well defined distances under and above the obstacle. Additionally, comparisons of void fractions and liquid velocities of selected experiments were discussed. In this context it is possible to identify from the plots the stagnant point upstream the obstacle and the flow separation zone as well as recirculation and reattachment phenomena.
The ability to build 3D sets of data makes it useable to validate CFD codes and proof new implemented models. The work will be continued.
Keywords: Two-phase flow, asymmetric obstacle, flow separation, recirculation zone, vortex, lateral bubble velocity, CFD code
  • Lecture (Conference)
    Multiphase Flow Workshop, 27.-29.06.2006, Dresden, Germany
  • Contribution to proceedings
    Multiphase Flow Workshop, 27.-29.06.2006, Dresden, Germany
    CD - Multiphase Flow Workshop: 28.06.2006, paper 05, II-5

Publ.-Id: 8689 - Permalink

Quantum Well Infrared Photodetector: Physics and Applications
Schneider, H.; Liu, H. C.;
Addressed to both students as a learning text and scientists/engineers as a reference, this book discusses the physics and applications of quantum-well infrared photodetectors (QWIPs). It is assumed that the reader has a basic background in quantum mechanics, solid-state physics, and semiconductor devices. To make this book as widely accessible as possible, the treatment and presentation of the materials is simple and straightforward. The topics for the book were chosen by the following criteria: they must be well-established and understood; and they should have been, or potentially will be, used in practical applications. The monograph discusses most aspects relevant for the field but omits, at the same time, detailed discussions of specialized topics such as the valence-band quantum wells.
Keywords: Intersubband, Quantum-well infrared photodetector, QWIP, Thermal Imaging
  • Book (Authorship)
    Heidelberg: Springer-Verlag, 2006
    250 Seiten

Publ.-Id: 8688 - Permalink

Quadratic autocorrelation of free-electron laser radiation and photocurrent saturation in two-photon quantum well infrared photodetectors
Schneider, H.; Drachenko, O.; Winnerl, S.; Helm, M.; Walther, M.;
Using the free-electron laser facility FELBE, the authors have studied the influence of the intensity on the quadratic autocorrelation measured with two-photon quantum well infrared photodetectors (QWIPs). At high illumination powers, the shape of the autocorrelation trace is affected by photocurrent saturation of the two-photon QWIP. They describe the saturation mechanism by different analytical models taking account of the photocurrent nonlinearity in analogy to linear QWIPs and give conditions where true quadratic behavior can be observed. While these studies were carried out at 77 K, properties of two-photon QWIPs at room temperature will also be addressed.
Keywords: quantum-well infrared photodetector, QWIP, quadratic autocorrelation, GaAs/AlGaAs, free-electron laser

Publ.-Id: 8687 - Permalink

FELBE: a new infrared free-electron-laser user facility
Helm, M.; Michel, P.;
no abstract available
Keywords: free-electron laser
  • Notiziario Neutroni e Luce di Sincrotrone 11(2006)2, 33-35

Publ.-Id: 8686 - Permalink

Spectroscopic ellipsometry study of nickel induced crystallization of a-Si
Pereira, L.; Águas, H.; Beckers, M.; Martins, R. M. S.; Fortunato, E.; Martins, R.;
The aim of this work is to present a spectroscopic ellipsometry study focused on the annealing time effect on nickel metal induced crystallization of amorphous silicon thin films. For this purpose silicon layers with 80 and 125 nm were used on the top of which a 0.5 nm Ni thick layer was deposited. The ellipsometry simulation using a Bruggemann Effective Medium Approximation shows that films with 80 nm reach a crystalline fraction of 72% after 1 h annealing, appearing to be full crystallized after 2 h. No significant structural improvement is detected for longer annealing times. On the 125 nm samples the crystalline volume fraction after 1 h is only around 7%, requiring 5 h to get a similar crystalline fraction than the one achieved with the thinner film. This means that the time required for full crystallization will be strongly determined by the Si layer thickness. Using a new fitting approach the Ni content within the films was also determined by SE and related to the silicon film thickness.
Keywords: Crystallization; Optical spectroscopy; Rutherford backscattering; Microstructure
  • Journal of Non-Crystalline Solids (2006)352, 1204-1208

Publ.-Id: 8685 - Permalink

Planar channeling radiation from electrons in quartz
Wagner, W.; Azadegan, B.; Grigoryan, L. S.; Pawelke, J.;
The possibility to stimulate channeling radiation by external excitation of ultrasonic waves in a piezoelectric single crystal is predicted by theory. The experimental verification of this effect requires the knowledge of the undisturbed spectrum of channeling radiation. For the first time, planar channeling radiation from 17, 25 and 32 MeV electrons channeled in a quartz crystal has been measured. The analysis performed bases on the application of the theory of channeling radiation. Real planar continuum potentials of quartz, eigenvalues of channeling states and transition energies have been calculated and radiation intensities were evaluated. The corresponding calculations proved to be necessary for the unambiguous identification of the spectra of channeling radiation registered from a hexagonal crystal.
Keywords: channeling radiation, quartz crystal

Publ.-Id: 8684 - Permalink

Quasi-Particle Description of Strongly Interacting Matter: Towards a Foundation
Bluhm, M.; Kämpfer, B.; Schulze, R.; Seipt, D.;
We confront our quasi-particle model for the equation of state of strongly interacting matter with recent first-priciple QCD calculations. In particular, we test its applicability at finite baryon densities by comparing with Taylor expansion coefficients of the pressure for two quark flavours. We outline a chain of approximations starting from the Phi-functional approach to QCD which motivates the quasi-particle picture.
  • Contribution to proceedings
    Hot Quarks 2006, 15.-20.05.2006, Villasimius, Sardinia, Italy
  • European Physical Journal C 49(2007)1, 205-211
    DOI: 10.1140/epjc/s10052-006-0056-y

Publ.-Id: 8683 - Permalink

Isentropic Equation of State of Two-Flavour QCD in a Quasi-Particle Model
Bluhm, M.; Kämpfer, B.; Schulze, R.; Seipt, D.;
We examine the isentropic QCD equation of state within a quasi-particle model being adjusted to first principle QCD calculations of two quark flavours. In particular, we compare with Taylor expansion coefficients of energy and entropy densities and with the isentropic trajectories describing the hydrodynamical expansion of a heavy-ion collision fireball.
Keywords: QCD Equation of State, Quasi-particle model
  • Contribution to proceedings
    International Workshop on Hot and Dense Matter in Relativistic Heavy Ion Collisions, 09.-12.04.2006, Budapest, Hungary
  • Acta Physica Hungarica A 27(2006), 397-402

Publ.-Id: 8682 - Permalink

An investigation of target poisoning during reactive magnetron sputtering using ion beam analysis and energy resolved mass spectroscopy
Güttler, D.; Abendroth, B.; Möller, W.;
The effect of target poisoning is commonly observed in reactive magnetron sputtering, where a metallic target is sputtered in reactive gas atmosphere. This phenomenon can be described quite well in terms of sputter rate, reactive gas pressure and pumping speed, however the details of the processes on the target are not yet understood. In this work, an experimental setup is presented that combines energy resolved mass spectroscopy with quantitative in situ nuclear reaction analysis (NRA) of the reactive species target coverage. By adjusting the position of the magnetron, locally resolved information is obtained across the target surface. Experiments have been performed for the reactive deposition of TiN in an Ar/N2 gas mixture at varying process parameters, with special emphasis on the transition from metallic to poisoned target mode.
In the centre of the race track the nitrogen coverage is significantly smaller than on the remaining part of the target surface. The maximum amount of retained nitrogen significantly exceeds one adsorbed monolayer, which is attributed to nitrogen ion implantation and recoil implantation of adsorbed nitrogen.
The energy distribution of the neutral species is clearly composed from particles originating from the gas atmosphere and sputtered particles from the magnetron target. Significant differences in the energy distribution of the sputtered atoms are observed between the centre and the erosion zone of the target. The ratio of sputtered N/Ti atoms reflects characteristically the metallic or compound mode of operation.
  • Lecture (Conference)
    Workshop "Oberflächentechnologie mit Plasma- und Ionenstrahlprozessen", 14.-16.03.2006, Mühlleithen, Deutschland

Publ.-Id: 8681 - Permalink

Target poisoning during magnetron sputtering: Real-time in-situ analysis and collisional computer simulation
Güttler, D.; Abendroth, B.; Grötzschel, R.; Depla, D.; Möller, W.;
The evolution of reactive gas uptake at the target surface has been investigated by real-time in-situ diagnostics during magnetron sputtering. Using a planar circular DC magnetron for reactive sputter deposition of TiN from a Ti target in an argon/nitrogen gas mixture, the target uptake of nitrogen was determined at varying gas flow of nitrogen using the 14N(d,a)12C nuclear reaction, directly demonstrating the target "poisoning" effect. The expected hysteresis behaviour at increasing/decreasing nitrogen gas flow is confirmed. In the centre of the racetrack, the nitrogen uptake is significantly smaller than on the remaining target surface. Within the precision of the measurement, the nitrogen content remains unaltered after switching off the magnetron, indicating the absence of a significant mobile fraction of nitrogen in the target. The maximum amount of retained nitrogen significantly exceeds one adsorbed monolayer, which is attributed to nitrogen ion implantation and recoil implantation of adsorbed nitrogen. This is quantitatively reproduced by TRIDYN collisional computer simulations.
  • Poster
    VEIT 2005, 14th International Summer School on Vacuum, Electron and Ion Technologies, 12.-16.09.2005, Sunny Beach, Bulgaria

Publ.-Id: 8680 - Permalink

Relaxation dynamics of interminiband transitions and electron cooling in doped GaAs/AlGaAs superlattices
Stehr, D.; Winnerl, S.; Helm, M.; Andrews, A. M.; Roch, T.; Strasser, G.;
Semiconductor superlattices represent a key model system for artificial crystals and are an essential component of novel infrared devices such as detectors and quantum cascade lasers. Hence their optical and transport properties have been investigated extensively during the past two decades. However, unlike for quantum well structures where considerable knowledge on the intersubband relaxation dynamics has been obtained, so far no experimental work has been published on the interminiband relaxation dynamics in superlattices.

We have studied the transient transmission of a doped GaAs/Al0.3Ga0.7As superlattice in pump-probe experiments [1]. The superlattice with thickness of 9.0 nm and 2.5 nm of the wells and barriers, respectively, was n-doped in the center of the wells, resulting in a doping density of 1.51016 cm-3 averaged over one superlattice period. Picosecond infrared pulses in the range from 4 µm to 22 µm were generated at 13 MHz repetition rate by the free-electron laser FELBE at the Forschungszentrum Rossendorf. In particular, the experiments were performed at the absorption maxima of the superlattice at 9.0 µm and 15.8 µm. These wavelengths are the spectral positions of the van Hove singularities of the joint density of states in the center and at the edge of the mini-Brillouin zone, respectively.

The measured pump-probe signals consist of a fast component due to the bleaching of the interminiband transition and subsequent relaxation and thermalization, and a slower component due to cooling of the heated electron system. The fast component decays typically around 2-2.5 ps, in reasonable agreement with published theoretical values [2]. The slower component due to cooling is positive for excitation at 9.0 µm and negative at 15.8 µm and shows a strong temperature and excitation density dependence with cooling times ranging from 5 to 50 ps. This behavior is consistent with the temperature dependence of the linear absorption spectrum, i.e. yielding higher or lower transmission for increasing electron temperature. The effect provides an internal thermometer for the miniband electrons on a picosecond timescale.

We will also report on data from superlattices with higher doping density, which shows the influence of electron-electron scattering.

[1] D. Stehr et al., submitted to Appl. Phys. Lett.
[2] F. Compagnone, A. Di Carlo, and P. Lugli, Appl. Phys. Lett. 80, 920 (2002)
  • Lecture (Conference)
    28th International Conference on the Physics of Semiconductors, 24.-28.07.2006, Wien, Austria
  • Contribution to proceedings
    28th International Conference on the Physics of Semiconductors, 24.-28.07.2006, Wien, Austria
    AIP Conference Proceedings 893, 485-486

Publ.-Id: 8679 - Permalink

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