Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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35174 Publications

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

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

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

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

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

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

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 Logo Wissenschaftlich-Technische Berichte / Helmholtz-Zentrum Dresden-Rossendorf; FZR-456 2006
    ISSN: 1437-322X


Publ.-Id: 8717

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Theory of impurity states in coupled quantum wells and superlattices and their infrared absorption spectra

Stehr, D.; Helm, M.; Metzner, C.; Wanke, M. C.

The problem of shallow impurities in confined semiconductor systems has been extensively investigated over the past two decades. It is well known that in two dimensions the binding energy is increased and the degeneracy of the states is lifted due to the symmetry breaking. Yet virtually all calculations were based on variational approaches for the impurity levels; some more sophisticated calculations, on the other hand, were mostly done solely for the ground state and other low-lying states.

Here we present an essentially exact calculation, treating the quantum well (QW) and impurity potential on the same footing in a unified framework [1], by diagonalizing the fully three-dimensional Schrödinger equation exactly on a 100 x 100 nm grid (however neglecting electron-electron interaction). This results in some thousand states. To facilitate comparison with existing experiments we calculate the infrared (IR) absorption spectra by properly evaluating the matrix elements between all relevant states, taking into account their proper thermal occupation.

These calculations have lead to some remarkable insights:

In a superlattice (SL; we use 20 QWs for the calculation) we show that there is not only a an excited 2pz state slightly below the second miniband [2], but rather an excited, resonant impurity band which partly overlaps the second miniband [1]. Consequently the low-temperature IR absorption spectrum in not too highly doped SLs exhibits a transition from the 1s ground state to this resonant impurity band, showing two maxima related to the band edges. This implies that there are impurity states not only attached to the lower miniband edge, but also to the higher miniband edge. The localized nature of the high-energy peak can only be identified by analyzing the final-state wavefunction in the xy plane, since it occurs at nearly the same spectral position as the kz=0 interminiband peak. This observation requires re-interpretation of previous interminiband absorption experiments and is important for the experimental diagnostics of the magnetic-field induced metal-insulator transition. Note that the excited impurity band does not result from impurity levels overlapping in the xy plane, but from the state coupling in z-direction, just like the miniband.

By studying the energy levels and IR spectra for systems with a varying number of quantum wells, we can map out the transition from coupled QWs to a SL, i.e. from sharp intersubband and/or impurity lines to band-like spectra including their van Hove singularities. In a coupled double QW, where the 1-3 intersubband transition is symmetry forbidden, the related impurity transition can nevertheless be observed.

A particularly spectacular example is provided by the analysis of previously not understood experimental IR absorption spectra of a quadruple QW system. The complex temperature dependent absorption pattern is reproduced by our theory with a remarkable degree of accuracy.

[1] D. Stehr et al., Phys. Rev. Lett. 95, 257401 (2005)
[2] M. Helm et al., Phys. Rev. B 48, 1601 (1993)

  • Poster
    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, 243-244

Publ.-Id: 8678

Use of models for lift, wall and turbulent dispersion forces acting on bubbles for poly-disperse flows

Lucas, D.; Krepper, E.; Prasser, H.-M.

Closure laws are needed for the qualification of CFD codes for two-phase flows. In case of bubbly and slug flow, forces acting on the bubbles usually model the momentum transfer between the phases. These forces depend on the liquid flow field as well as on the size and the shape of the bubbles. The validation of these forces base on a detailed experimental database for vertical pipe flows, which contains data on the radial distribution of bubbles of different size and local bubble size distributions. A one-dimensional model resolving the variables in radial direction and considering a large number of bubble size classes, is used for an analysis of several bubble force models. Results of this model were compared to results of 3D simulations done by the commercial code CFX-5.7 for simplified cases with only one bubble class. The effects of the number of bubbles classes as well as the effect of the lateral extension of the bubbles were analysed. For the validation of bubble force models measured bubble size distributions were taken as an input for the model. On basis of the assumption of equilibrium of the lateral bubble forces, radial volume fraction profiles were calculated separately for each bubble class. In the result of the validation of different models for the bubble forces, a set of Tomiyama lift and wall force, deformation force and Favre averaged turbulent dispersion force was found to reflect the experimental data with best agreement. Some discrepancies remain at high liquid superficial velocities.

Keywords: bubbly flow; bubble forces; CFD; pipe flow

Publ.-Id: 8677

Off-normal ion erosion of silicon – simultaneous observation of two ripple modes

Keller, A.; Roßbach, S.; Facsko, S.; Möller, W.

It is well known that oblique low and mediate energy (typically 0.1 – 100 keV) ion erosion of solid surfaces can lead to the formation of periodic ripple patterns with wavelength ranging from 10 to 1000 nm. These structures were found on a large variety of materials, such as semiconductors, metals, and insulating surfaces [1]. The first attempt to describe the formation of surface ripples was made by Bradley and Harper [2]. In their framework the ripples are either parallel or perpendicular to the direction of the ion beam, depending on the angle of incidence. This was also shown experimentally on HOPG [3].
In the presented work we studied the formation of ripples on Si(100) surface under sub-keV Ar+ bombardment using ex-situ AFM. In addition to previous experimental observations two simultaneous ripple modes are found with direction perpendicular and parallel to the ion beam, respectively. The dependence of each mode on ion energy, fluence, flux, and incidence angle is investigated. The two modes have wavelengths of some ten and a few hundred nanometres and show different temporal behaviour. Finally, our observations are compared to simulations based on the damped Kuramoto-Sivashinsky equation [4].

[1] M. A. Makeev et al., Nucl. Inst. Meth. Phys. Res. B 197 (2002), 185
[2] R. Bradley and J. Harper, J. Vac. Sci. Technol. A 6 (1988), 2390
[3] S. Habenicht et al., Phys. Rev. B 60 (1999), R2200
[4] S. Facsko et al., Phys. Rev. B 69 (2004), 153412

  • Lecture (Conference)
    22nd International Conference on Atomic Collisions in Solids, 21.-26.07.2006, Berlin, Deutschland

Publ.-Id: 8676

Computational studies on the design of an electromagnetic pump for the process of aluminum investment casting

Galindo, V.; Eckert, S.; Gerbeth, G.; Willers, B.

The mould filling process of aluminum investment casting consists basically of the liquid metal flow in a U-bend. It shows a high pouring velocity at the beginning and decreasing flow velocity values during the course of the process. The main problem is the occurrence of large velocity values at the beginning of the casting process, leading to an accumulated generation of vortices inside the pouring channel. A high rate of turbulence in the flow is supposed to entail the transported impurities, oxides or gas bubbles from the walls and the free surface, respectively, into the bulk of the casting patterns. As a result, the mechanical properties of the casting products are deteriorated.
We present results on the design and application of electromagnetic pumps to control the velocity of the aluminum melt during the pouring process. The induced electromagnetic force in the melt caused by the coil system were calculated using the commercial finite-element code OPERA-3d (Vector Fields Ltd.). The free surface problem which occurs in the riser of the casting unit was taken into account by a Volume-of-Fluids Method. Three-dimensional transient calculations using the commercial finite-element code FIDAP (FLUENT Inc.) were carried out for a simplified model system. Parallel to the simulations model experiments have been performed using the low melting eutectic GaInSn.
The casting unit was modeled by a perspex model [1], which also allows the monitoring of the filling process. Ultrasonic Doppler velocimetry measurements were applied in the model in order to validate the numerical calculations [2]. A comparison between numerical and experimental results showed an excellent agreement.
The main goal of this study is the optimization of the pump design depending on the geometry of the coil system in oder to achieve the best possible homogeneity in the electromagnetic force distribution with an integral flow rate of sufficient strength. Inhomogeneities in the force distribution can lead to undesirable fluid vortices in the melt, which on this part can induce a detriment of the material properties after solidification.


[1] A. Cramer, S. Eckert, V. Galindo, G. Gerbeth, W. Willers, W. Witke,
Liquid metal model experiments on casting and solidification processes
Journal of Materials Science 39 (2004) 7285-7294
[2] S. Eckert, G. Gerbeth, Th. Gundrum, F. Stefani, W. Witke,
New approaches to determine the velocity field in metallic melts,
Proceedings of the 4th Int. Conference on Electromagnetic Processing of
Materials (EPM 2003), Lyon (France), October 14-17, 2003, pp. 601-608

  • Lecture (Conference)
    7th World Congress on Computational Mechanics, 16.-22.07.2006, Los Angeles, California, United States

Publ.-Id: 8675

Nanostructures by ion beams

Schmidt, B.

Ion beam techniques, including conventional broad beam ion implantation, ion beam synthesis and ion irradiation of thin layers as well as local ion implantation with fine-focused ion beams (FIB) have been applied in different fields of micro- and nanotechnology. The ion beam synthesis of nanoparticels in high dose ion implanted solids is explained as phase separation of nanostructures from a super-saturated solid state through precipitation and Ostwald–ripening during subsequent thermal treatment of the ion implanted samples. A special topic will be addressed to self-organization processes of nanoparticles during ion irradiation of flat and curved solid state interfaces. As an example of silicon nanocrystal application the fabrication of silicon nanocrystal non-volatile memories will be described. Finally, the fabrication possibilities of nanostructures, like nanowires and chains of nanoparticles (e.g. CoSi2), by ion beam synthesis using a focused Co+ ion beam will be demonstrated and possible applications will be mentioned.

Keywords: Ion implantation; Ion beam synthesis; Ion irradiation; nanoclusters; nanowires

  • Radiation Effects and Defects in Solids 162(2007)3-4, 171-184
    DOI: 10.1080/10420150601132743
  • Invited lecture (Conferences)
    Workshop on Ion Beam Studies of Nanomaterials: Synthesis, Modification and Characterization, 26.06.-01.07.2006, Trieste, Italy

Publ.-Id: 8674

In-beam PET measurements of biological half-lives of {12}C irradiation induced beta+-activity

Fiedler, F.; Sellesk, M.; Crespo, P.; Jülich, R.; Parodi, K.; Pawelke, J.; Pönisch, F.; Enghardt, W.

One of the long-standing problems in carbon-ion therapy is the monitoring of the treatment, i.e. of the delivered dose to a given tissue volume within the patient. Over the last 8 years, in-beam positron emission tomography (PET) has been used at the experimental carbon ion treatment facility at the Gesellschaft für Schwerionenforschung (GSI) Darmstadt and has become a valuable quality assurance tool. In order to determine and evaluate the correct delivery of the patient dose, a simulation of the positron emitter distribution has been compared to the measurement. One particular effect is the blurring as well as the reduction of the measured activity distribution via washout. The objective of this study is the investigation of tissue dependent effective half lives from patient data. We find no significant dependence of the effective half life on the Hounsfield unit but on the local dose. The biological half-live within the high dose region is longer than in the low dose region. Furthermore, the influence of the overall treatment time on the kinetics of the positron emitter is reported. There are indications for a metabolic response of the tissue on the irradiation. Taking into account the biological half-life in the simulation leads to an improvement of the quality of the PET-images in some cases.

Keywords: in-beam PET; washout

Publ.-Id: 8673

Excited and ground state properties of LaSrMnO4: A combined x-ray spectroscopic study

Kuepper, K.; Klingeler, R.; Reutler, P.; Büchner, B.; Neumann, M.

The electronic properties of the parent compound of the single layered manganites La1−xSr1+xMnO4, namely LaSrMnO4 has been investigated in a detailed spectroscopic study.
We apply different complementary x-ray spectroscopic techniques in the soft x-ray regime. X-ray photoelectron spectroscopy and normal like x-ray emission spectroscopy were used to reveal a detailed picture of the total and partial densities of states in this compound. Furthermore we apply resonant x-ray emission spectroscopy to the Mn L2,3 edges. The spectra exhibit a rich multiplet structure. Resonant x-ray emission spectroscopy at the O K edge is used to study the local partial densities of states of the in plane and out of plane oxygen atoms. The results are discussed along available band structure calculations as well as charge transfer multiplet calculations.

Keywords: PACS numbers: 71.20.-b; 75.47.Lx; 78.70.En; 79.60.-i

Publ.-Id: 8672

One-step reductive etherification of 4-[18F]fluoro-benzaldehyde with decaborane

Funke, U.; Jia, H.; Fischer, S.; Scheunemann, M.; Steinbach, J.

Reductive coulling reactions between 4-[18F]fluoro-benzaldehyde ([18F]19 and different alcoholsby use of decaborane (B10H14)as reducing agent have the potential to synthesize 4-[18F]fluoro-benzylethers in one step. [18F]1 was sythesized from 4-trimethylammonium benzaldehyde (triflate salt) via a standard fluorination procedure (K[18F]F/Kryprofix 222) in dimethylformamide at 90°C for 25 min and purified by solid-phase extraction. Subsequently, reductive etherifications of [18F]1 were performed as one-step reactions with primary and secondary alcohols, mediated by B10H14 in acetonitrile at 60°C. Various 4-[18F]fluorobenzyl ethers (6 examples are shown) were obtained within 1-2h reaction time in decay-corrected radiochemical yields of 12-45%.

Keywords: 4[18F]fluoro-benzylether; fluorine-18; reductive etherification; decarborane; labelling

  • Journal of Labelled Compounds and Radiopharmaceuticals 49(2006), 745-755
    DOI: 10.1002/jlcr.1087

Publ.-Id: 8671

On the efficiency of MHD drag reduction

Gerbeth, G.; Shatrov, V.

Permanent magnets and high electric current densities are often used to achieve reasonable Lorentz forces for a magnetohydrodynamic flow control. This choice, however, usually leads to a low energetic efficiency for the flow control of seawater. We present results of direct numerical simulations of turbulent channel flow [1] drag reduction using electromagnetic forces. The Lorentz force is created by a permanent magnetic field and an electric current from electrodes placed on the bottom wall. The Reynolds number Re, the magnetic interaction parameter N and the load factor k are the control parameters of the problem.
In the first case we used a spanwise oscillating force. The efficiency is the ratio between saved and used power [2]. We found that the efficiency at the load factor k = 4 is about 100 times larger than at k = 1000.
In the second case a streamwise Lorentz force [3] was used. The flow is accelerated near the bottom and the thrust force applied to the bottom wall is directed against the mean flow. The energetic efficiency is the ratio between the mechanical power necessary to move the flat plate with a given velocity and the used electric power. The efficiency is more than 0.9 at k = 0.8.
The main result is that a significant efficiency improvement is possible if load factors k in the order of 1 are used.
1. R.D. Moser, J. Kim and Nagi N. Mansour, "Direct numerical simulation of turbulent channel flow up to Ret = 590," Phys. Fluids, v. 11, no. 4, p. 943-945, 1999.
2. T.W. Berger, J. Kim, C. Lee and L. Lim, "Turbulent boundary layer control utilizing the Lorentz force," Phys. Fluids, v. 12, no. 3, p. 631-649, 2000.
3. A. Gailitis and O. Lielausis, "On the possibility of drag reduction of a flat plate in an electrolyte," Applied Magnetohydrodynamics, Rep. of the Phys. Inst., AN Latv. SSR, v. 12, p. 143-146, (in Russ.), 1961.

Keywords: MHD; drag reduction

  • Lecture (Conference)
    7th World Congress on Computational Mechanics, 17.-22.07.2006, Los Angeles, United States

Publ.-Id: 8670

The GDT as neutron source in a sub-critical system for transmutation?

Noack, K.; Rogov, A.; Ivanov, A. A.; Kruglyakov, E. P.

In the last decade, a great progress was made in developing projects of sub-critical fission systems dedicated to transmutation of nuclear waste. In contrast to a fission reactor, such a device is fed with neutrons from an outer source in order to sustain a steady-state power generation. The Budker Institute of Nuclear Physics has made the proposal of a powerful 14 MeV neutron source based on a gas dynamic trap (GDT). This neutron source is primarily thought as irradiation facility for fusion material research. So, the question raises, whether the GDT based neutron source could be a candidate to efficiently drive such a sub-critical system too. The contribution pursues this question using results of first neutron transport calculations. The calculations were made for a simplified model of an actinides burner, which has been developed for an international benchmark exercise performed under the auspices of the Nuclear Energy Agency (NEA) of the Organization for Economic Cooperation and Development (OECD). Important parameters of the burner are compared for two cases - when driven by a spallation or by the GDT neutron source. From this comparison some advices for further improvements of the GDT neutron source are concluded.

Keywords: transmutation of nuclear waste; sub-critical system; minor actinides burner; plasma neutron source; gas dynamic trap

  • Invited lecture (Conferences)
    Open Magnetic Systems for Plasma Confinement, 17.-21.07.2006, Tsukuba, Japan
  • Fusion Science and Technology 51(2007), 65-68

Publ.-Id: 8668

Spin-up and spin-down dynamics driven by a single rotating magnetic field pulse

Nikrityuk, P. A.; Eckert, S.; Eckert, K.

This paper presents a study concerning the transient dynamics of the flow field inside a liquid metal column created by an application of a rotating magnetic field (RMF) in form of a single pulse. The flow structure is governde by an impulsive spin-up form the rest state which is followed by a spin-down phase under the inertia of the fluid. A distinctive influence of pulse length the transient fluid flow has been found. Our results reveal that the recirculating flow in the radial-meridional plane shows periodic reversals. This phenomena is especially pronounced if the pulse length of the electromagnetic forcing corresponds to the so-called initial adjustment phase.

Keywords: electromagnetic stirring; rotating magnetic field; UDV flow measurment

Publ.-Id: 8667

Synthesis and evaluation of novel multimeric neurotensin(8-13) analogs

Hultsch, C.; Pawelke, B.; Bergmann, R.; Wüst, F.

Neurotensin(8–13) is a hexapeptide with subnanomolar affinity to the neurotensin receptor 1 which is expressed with high incidence in several human tumor entities. Thus, radiolabeled neurotensin(8–13) might be used for tumor targeting. However, its application is limited by insufficient metabolic stability. The present study aims at improving metabolic stability by the synthesis of multimeric neurotensin(8–13) derivatives rather than commonly employed chemical modifications of the peptide itself. Thus, different dimeric and tetrameric peptides carrying C- or N-terminal attached neurotensin(8–13) moieties have been synthesized and their binding affinity toward the neurotensin receptor has been determined. The results demonstrate that branched compounds containing neurotensin(8–13) attached via its C-terminus only show low receptor affinities, whilst derivatives with neurotensin(8–13) attached via the N-terminus show IC50 values in the nanomolar range. Moreover, within the multimeric neurotensin(8–13) derivatives with neurotensin(8–13) attached via the N-terminus an increasing number of branching units lead to higher binding affinities toward the neurotensin receptor.

Keywords: Neurotensin; Multimer; Peptides; Tumor targeting

  • Bioorganic & Medicinal Chemistry 14(2006)17, 5913-5920

Publ.-Id: 8666

Flow control in bubble-driven liquid metal flows by means of external magnetic fields

Zhang, C.; Eckert, S.; Gerbeth, G.

In our experiments we investigated the consequences of an application of both DC and AC magnetic fields on the velocity field of bubble plumes. A restructuring of the entire flow field can be observed if a bubble plume is exposed to a DC magnetic field. The application of a transverse field leads not only to a general damping of the flow, but also favours the occurrence of vortices aligned parallel to the magnetic field direction. AC magnetic fields can be applied to generate flow structures being different from the recirculating flow known from classical bubble plumes. A tailoring of the flow using magnetic fields obviously allows a control of the heat and mass transfer in bubble plumes.

Keywords: bubble plume; DC magnetic field; AC travelling magnetic field; electromagnetic stirring; velocity measurements; ultrasound Doppler technique

  • Contribution to proceedings
    EPM2006, 5th International. Conference on Electromagnetic Processing of Materials, 23.-27.10.2006, Sendai, Japan
    Proceedings: The Iron and Steel Institue of Japan (ISIJ), 4-930980-55-0 C3057, 242-247
  • Lecture (Conference)
    EPM2006, 5th International. Conference on Electromagnetic Processing of Materials, 23.-27.10.2006, Sendai, Japan

Publ.-Id: 8665

Velocity measurements in metallic melts driven by AC magnetic fields in a square vessel

Pal, J.; Eckert, S.; Zhang, C.; Gerbeth, G.

Model experiments are an important tool to understand the details of the flow structure and the transport properties of flows occurring in real-scale metallurgical facilities as well as to validate the multitude of numerical codes for flow simulation. The present study considers the electromagnetic stirring of a liquid metal by a rotating magnetic field (RMF). Measurements of the velocity field in a square column filled with the eutectic GaInSn have been carried out utilising the ultrasound Doppler velocimetry (UDV). In this paper the spatiotemporal properties of both the primary, swirling and the secondary flow arising from a sinusoidal modulation of the RMF will be presented.

Keywords: electromagnetic stirring; rotating magnetic field; velocity measurements; ultrasound Doppler technique

  • Contribution to proceedings
    5th International Conference on Electromagnetic Processing of Materials, 23.-27.10.2006, Sendai, Japan
    Proceedings: The Iron and Steel Institute of Japan (ISIJ), 4-930980-55-0 C3057, 725-730
  • Lecture (Conference)
    EPM 2006, 23.-27.10.2006, Sendai, Japan

Publ.-Id: 8664

The influence of electromagnetically-driven convection on temperature field and solute distribution during directional solidification

Willers, B.; Nikrityuk, P. A.; Eckert, S.; Eckert, K.

Solidification experiments as well as numerical simulations were carried out considering the directional solidification of Pb Sn alloys from a water cooled copper chill. A rotating magnetic field (RMF) was applied for melt agitation. The comparison between numerical simulations and solidification experiments delivered a good agreement. Our results disclose that the forced convection causes distinct modifications of the temperature and concentration field. The electromagnetic stirring promotes the columnar-to-equiaxed transition and causes a considerable grain refinement. Indications for a flow-induced fragmentation have been found in the microstructure of the stirred samples.

Keywords: solidification; fluid flow; electromagnetic stirring; rotating magnetic field

  • Contribution to proceedings
    EPM2006, 5th Int. Conference on Electromagnetic Processing of Materials, 23.-27.10.2006, Sendai, Japan
    Proceedings: The Iron and Steel Institute of Japan (ISIJ), 4-930980-55-0 C3057, 407-412
  • Lecture (Conference)
    EPM2006, 5th International Symposium on Electromagnetic Processing of Materials, 23.-27.10.2006, Sendai, Japan

Publ.-Id: 8663

Buoyancy driven coolant mixing studies of natural circulation flows at the ROCOM test facility using ANSYS CFX

Höhne, T.; Kliem, S.; Rohde, U.; Weiss, F.-P.

Coolant mixing in the cold leg, downcomer and the lower plenum of pressurized water reactors is an important phenomenon mitigating the reactivity insertion into the core. Therefore, mixing of the de-borated slugs with the ambient coolant in the reactor pressure vessel was investigated at the four loop 1:5 scaled ROCOM mixing test facility. Thermal hydraulics analyses showed, that weakly borated condensate can accumulate in particular in the pump loop seal of those loops, which do not receive safety injection. After refilling of the primary circuit, natural circulation in the stagnant loops can re-establish simultaneously and the de-borated slugs are shifted towards the reactor pressure vessel (RPV).
In the ROCOM experiments, the length of the flow ramp and the initial density difference between the slugs and the ambient coolant was varied. From the test matrix experiments with 0 resp. 2% density difference between the de-borated slugs and the ambient coolant were used to validate the CFD software ANSYS CFX. To model the effects of turbulence on the mean flow a higher order Reynolds stress turbulence model was employed and a mesh consisting of 6.4 million hybrid elements was utilized. Only the experiments and CFD calculations with modeled density differences show a stratification in the downcomer. Depending on the degree of density differences the less dense slugs flow around the core barrel at the top of the downcomer. At the opposite side the lower borated coolant is entrained by the colder safety injection water and transported to the core. The va¬li¬¬dation proves that ANSYS CFX is able to simulate appropriately the flow field and mixing effects of coolant with different densities.

Keywords: PWR; Boron Dilution; ROCOM

  • Contribution to proceedings
    14th International Conference on Nuclear Engineering, 16.-20.07.2006, Miami, United States
    CD-ROM, paper ICONE 14- 89120
  • Lecture (Conference)
    14th International Conference on Nuclear Engineering, 16.07.2006, Miami, United States

Publ.-Id: 8662

Competition between damage buildup and dynamic annealing in ion implantation into Ge

Posselt, M.; Bischoff, L.; Grambole, D.; Herrmann, F.

Channeling implantation of Ga into Ge is performed at two very different ion fluxes (1011 and 1019 cm-2 s-1), at two temperatures (room temperature and 250 0C), and at five different fluences. The fluence dependence of the range profiles and of the implantation damage is strongly influenced by defect accumulation and dynamic annealing. At 250 0C a significant reduction of the ion-beam-induced defects is already observed 1 s after an ion impact, and the maximum lifetime of the defects is less than 10 s. On the other hand, at room temperature no significant annealing is found within the first 10 s after ion impact. The measured Ga depth profiles are reproduced very well by atomistic computer simulations.

Keywords: Germanium; ion implantation; defects; computer simulation

  • Applied Physics Letters 89(2006), 151918

Publ.-Id: 8661

Alkaline earth elements in the uranium phosphate system - a TRLFS study

Geipel, G.

wird nachgereicht

  • Lecture (others)
    Kolloquium, 01.08.2006, Tokyo, Japan

Publ.-Id: 8659

Uranium speciation in water - lectures learned from the nature

Geipel, G.

wird nachgereicht

  • Lecture (others)
    Seminar, 31.07.2006, Tokyo, Japan

Publ.-Id: 8658

Interaction of Cu(II) with dendritic ligands studied by TRLFSand some remarks on luminescence of uranium(V)

Geipel, G.

wird nachgereicht

  • Lecture (others)
    Seminar, 01.08.2006, Tokyo, Japan

Publ.-Id: 8657

Evaluation of Research Institutes in Germany - Basics, Procedures, Results

Joehnk, P.

  • Invited lecture (Conferences)
    Workshop "Mechanism and criteria for efficiency of evaluation of research institutions", 14.07.2006, Moskva, Russia

Publ.-Id: 8656

Systematics of pion emission in heavy ion collisions in the 1A GeV regime

Reisdorf, W.; Stockmeier, M.; Andronic, A.; Benabderrahmane, M. L.; Hartmann, O. N.; Herrmann, N.; Hildenbrand, K. D.; Kim, Y. J.; Kis, M.; Koczon, P.; Kress, T.; Leifels, Y.; Lopez, X.; Merschmeyer, M.; Schüttauf, A.; Alard, J. P.; Barret, V.; Basrak, Z.; Bastid, N.; Caplar, R.; Crochet, P.; Dupieux, P.; Dzelalija, M.; Fodor, Z.; Grishkin, Y.; Hong, B.; Kang, T. I.; Kecskemeti, J.; Kirejczyk, M.; Korolija, M.; Kotte, R.; Lebedev, A.; Matulewicz, T.; Neubert, W.; Petrovici, M.; Rami, F.; Ryu, M. S.; Seres, Z.; Sikora, B.; Sim, K. S.; Simion, V.; Siwek-Wilczynska, K.; Smolyankin, V.; Stoicea, G.; Tyminski, Z.; Wisniewski, K.; Wohlfarth, D.; Xiao, Z.; Xu, H. S.; Yushmanov, I.; Zhilin, A.

Using the large acceptance apparatus FOPI, we study pion emission in the reactions (energies in A GeV are given in parentheses): 40Ca+40Ca (0.4, 0.6, 0.8, 1.0, 1.5, 1.93), 96Ru+96Ru (0.4, 1.0, 1.5), 96Zr+96Zr (0.4, 1.0, 1.5), 197Au+197Au (0.4, 0.6, 0.8, 1.0, 1.2, 1.5). The observables include longitudinal and transverse rapidity distributions and stopping, polar anisotropies, pion multiplicities, transverse momentum spectra, ratios (pi+/pi-) of average transverse momenta and of yields, directed flow, elliptic flow. The data are compared to earlier data where possible and to transport model simulations.

Publ.-Id: 8654

Synthesis of metal nanoparticles using bacterial surface layers (S-layers) as biological templates

Pollmann, K.; Merroun, M.; Raff, J.; Fahmy, K.; Hennig, C.; Selenska-Pobell, S.; Herrmannsdörfer, T.

The regular structured paracrystalline surface layers (S-layers), which are one of the most common surface structures found in bacteria and archaea, are promising biological templates for the production of metal nanoclusters. Most of them are composed of protein monomers with the ability to self assemble in two-dimensional arrays. They possess regularly arranged pores of identical size that enable the exchange of ions and small molecules between the living cells and their environment.
In the present work the purified S-layer sheets of the uranium mining waste pile isolate B. sphaericus JG-A12 were used as biological template to produce Pd, Au, and Pt nanoparticles of a size of 0.8-1 nm. Such metal nanoparticles may have strongly altered properties compared to their bulk counterparts making them interesting for the development of new materials and applications in the field of catalysis. The deposition of metal nanoclusters was performed via a two step process, consisting of biosorption by exposition of the S-layer proteins to a hydrolyzed solution of metal salt (I) and of metal reduction (II) by the addition of a strong reducing agent. The interaction of Pd(II) with the S-layer of B. sphaericus JG-A12 and the formed Pd-nanoclusters were analyzed using EXAFS-spectroscopy and ATR-FT-IR-spectroscopy. The analyses demonstrated the complexation of Pd(II) to carboxyl groups. The metal nanoclusters were investigated using EXAFS spectroscopy and SQUID magnetometry. The Pd and Pt nanoclusters reveal interesting magnetic properties in a wide temperature and magnetic field range.

  • Poster
    E-MRS 2006 Spring Meeting, 29.05.-02.06.2006, Nice, Frankreich

Publ.-Id: 8653

Defects in N+ ion-implanted ZnO single crystals studied by positron annihilation and Hall effect

Brauer, G.; Anwand, W.; Skorupa, W.; Kuriplach, J.; Melikhova, O.; Cizek, J.; Prochazka, I.; von Wenckstern, H.; Brandt, M.; Lorenz, M.; Grundmann, M.

High quality ZnO single crystals of dimensions 10 x 10 x 0.5 mm3, grown by a hydrothermal approach, have been implanted by 40 keV N+ ions to a fluence of 1 x 1015 cm-2 at room temperature. Their proper-ties revealed by positron annihilation and Hall effect measurements are given in the as-grown and as-irradiated states, and after post-implantation annealing in an oxygen ambient at 200 °C and 500 °C.

Keywords: positron annihilation; zinc oxide; ion implantation; Hall effect; defects; annealing

  • Physica Status Solidi (C) 4(2007), 3642-3645
  • Lecture (Conference)
    14th International Conference on Positron Annihilation (ICPA-14), 23.-28.07.2006, Hamilton/Ontario, Canada
  • Lecture (Conference)
    The 4th International Wokshop on ZnO and Related Materials, 03.-06.10.2006, Giessen, Deutschland
  • 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: 8652

Electrical characterization of deep acceptor states in nitrogen implanted ZnO single crystals

von Wenckstern, H.; Schmidt, H.; Pickenhain, R.; Biehne, G.; Brandt, M.; Brauer, G.; Lorenz, M.; Grundmann, M.

nicht vorhanden

  • Lecture (Conference)
    E-MRS Spring Meeting 2006, 29.06.-02.07.2006, Nice, France

Publ.-Id: 8651

Optical and microelectrical characterization of ZnO single crystals implanted with group V elements

Brandt, M.; von Wenckstern, H.; Benndorf, G.; Lenzner, J.; Schmidt, H.; Lorenz, M.; Grundmann, M.; Braunstein, G.; Brauer, G.

nicht vorhanden

  • Lecture (Conference)
    Frühjahrstagung der Deutschen Physikalischen Gesellschaft, 04.-09.03.2006, Dresden, Deutschland

Publ.-Id: 8650

Electrical spectroscopy of acceptor states in N+ implanted ZnO single crystals

von Wenckstern, H.; Schmidt, H.; Pickenhain, R.; Biehne, G.; Brandt, M.; Brauer, G.; Lorenz, M.

nicht vorhanden

  • Lecture (Conference)
    DPG Spring Meeting, 04.-09.03.2006, Dresden, Deutschland

Publ.-Id: 8649

Recent results of slow positron implantation spectroscopy to characterize vacancy-type damage in ion-implanted 6H-SiC

Brauer, G.

nicht vorhanden

  • Lecture (Conference)
    Seminar an der University of Hong Kong, Physics Department, 13.01.2006, Hong Kong, China
  • Lecture (Conference)
    Seminar an der Chinese University of Hong Kong, Department of Electronic Engineering, 17.01.2006, Hong Kong, China

Publ.-Id: 8648

Die intensive Positronenquelle EPOS am Forschungszentrum Rossendorf

Krause-Rehberg, R.; Brauer, G.

nicht vorhanden

  • Lecture (others)
    Kolloquium der Friedrich-Schiller-Universität Jena, 02.12.2005, Jena, Deutschland

Publ.-Id: 8647

Verwendung von Mikroorganismen zur Synthese von Nanopartikeln

Pollmann, K.

Mikroorganismen können in verschiedenster Weise für die Synthese von Nanopartikeln genutzt werden. Hierzu gehören die enzymatische Reduktion von Metallionen durch Bakterien und die Verwendung von Biomolekülen wie DNA, Aktinfilamente, S-Layer-Proteinen als Matrizes für die Synthese. Der Vortrag gibt einen Überblick über diese Mechanismen, die Verwendungsmöglichkeiten und unsere bisherigen Arbeiten auf diesem Gebiet.

  • Lecture (others)
    Mikrobiologisches Kolloquium, 03.07.2006, Mainz, Deutschland

Publ.-Id: 8646

Influence of hydrogen on the toughness of irradiated reactor pressure vessel steels

Müller, G.; Uhlemann, M.; Ulbricht, A.; Böhmert, J.

The influence of hydrogen on the mechanical behaviour of different reactor pressure vessel (RPV) steels was investigated by tensile tests in correla-tion to the chemical composition, the neutron fluence, the hydrogen charg-ing condition, the strain rate, and the temperature. Small-angle neutron scattering (SANS) experiments, hydrogen analyses and thermal desorp-tion investigations were performed to prove the evidence of hydrogen trapping at irradiation defects. An increasing susceptibility to hydrogen embrittlement indicated by reduction of area was observed at room tem-perature with in-situ hydrogen-charged specimens when loaded by low strain rates or with specimens, which had been irradiated at low tempera-ture. Generally, the susceptibility increases with increasing strength of the steels. At 250°C hydrogen embrittlement was not evident. The results do neither prove that irradiation defects are favoured traps for hydrogen nor give evidence that hydrogen affects the RPV integrity under normal oper-ating conditions.

Keywords: hydrogen embrittlement; reactor pressure vessel steel; irradiation; fluence; tensile test; toughness; hydrogen charging

Publ.-Id: 8645

Preparation and Biological Characterization of Isomeric 188Re(V) Oxocomplexes with Tetradentate S4 Ligands Derived from DMSA for Labeling of Biomolecules

Seifert, S.; Heinrich, T.; Jentschel, C.; Smuda, C.; Bergmann, R.; Pietzsch, H.-J.

A new type of tetradentate S4 ligand has been synthesized by bridging two molecules of meso 2,3-dimercapto succinic acid for stable binding and easy conjugation of rhenium-188 to biologically interesting structures. The stereoisomeric tetrathiolato S4 ligands form very robust anionic five-coordinated oxorhenium(V) and oxotechnetium(V) complexes. Two routes for the preparation of the 188Re(V) oxocomplexes with (iBu)2N(O)C-C(SH)-C(SH)-C(O)NH-(CH2)3–NH-(CH2)3–NHC(O)-C(SH)-C(SH)-C(O)N(iBu)2 (ligand 1) and its hydrophilic crown ether derivative (ligand 2) were tested and optimized. Several isomers were separated by HPLC from the preparation solutions and characterized in vitro and in vivo. The identity of the species obtained was determined by comparison with the HPLC profiles of reference 185/187Re analogues which were characterized by ESI-MS. All of them were absolutely stable in rat and human plasma solutions. Challenge experiments with cysteine corroborated the high inertness of the isomers towards ligand exchange reactions. Various in vivo samples, taken off at different times from blood, intestine and urine of rats, confirmed the high in vivo stability of the 188Re-S4 complexes. Biodistribution studies using male Wistar rats were performed and resulted in a high uptake and fast clearance from the liver of the more lipophilic cis and trans isomers of complex I (log Po/w between 1.5 and 1.7), whereas the isomers of the hydrophilic complex II (log Po/w about -1.75) were rapidly excreted via the renal and the hepatobiliary pathway. The low level of radioactivity in the stomach confirms good in vivo stability. Thus, these new 188Re-S4 complexes fulfill the requirements for a stable and high specific activity labeling of biomolecules with rhenium-188.

Publ.-Id: 8644

Safety aspects of the process control of Grignard reactions

Kryk, H.; Hessel, G.; Schmitt, W.; Tefera, N.

Grignard reactions comprise considerable hazard potentials due to the spontaneous heat release during the initiation of the exothermic reactions and the high reactivity of the Grignard compounds. To establish industrially applicable methods for an objective detection of the reaction start-up and for the accumulation of the organic halide during the process, calorimetric studies of a special Grignard reaction in a pressurised vessel were carried out using several on-line monitoring methods. In general, the process signal profiles, FTIR measurements and balance-based on-line monitoring systems are applicable to provide the operator with additional information on the process state. Further experiments at adiabatic and isothermal conditions show significant influences of impurities (i.e. water) on the thermal process behaviour.

Keywords: Chemical processes; Grignard reaction; On-line monitoring; Process control; Safety; Chemical analysis

  • Poster
    ISCRE 19, 19th International Symposium on Chemical Reaction Engineering, 03.-06.09.2006, Potsdam, Germany
  • Contribution to proceedings
    ISCRE 19, 19th International Symposium on Chemical Reaction Engineering, 03.-06.09.2006, Potsdam, Germany
    Book of Abstracts, 362-363
  • Chemical Engineering Science 62(2007), 5198-5200

Publ.-Id: 8643

Colloid Generation and Stability in Simulated Reduced Groundwaters

Zänker, H.; Weiß, S.; Sachs, S.

The focus was on the investigation of U(IV) colloids during this phase of the project. First, a technique to reduce U(VI) to U(IV) was developed. Colloid formation and solubility of U(IV) in acidic HClO4/NaClO4 solutions were investigated by coulometric titration. Quantification of traces of U(VI) by laser fluorescence spectroscopy proved that the tetravalent state of uranium had been maintained. Laser-induced breakdown detection (LIBD) was applied for the detection of traces of uranium colloids as the pH was increased. The pH values at the onset of colloid formation were used for thermodynamic calculations aimed at determining the solubility products of crystalline and amorphous uranium dioxides. In acidic solutions, the UO2·xH2O(am) colloids showed colloidal stability over a period of at least 10 months during which they did not aggregate and precipitate. Relatively high zeta potentials were found for the UO2 colloids in the acidic and in the alkaline pH regions; the point of zero charge was at 6.9. No indications of the formation of mixed U(IV)-Al(III) complexes were found. There was also no influence of Al(III) on the UO2(cr) solubility.

Keywords: Uranium(IV); intrinsic colloids; pseudocolloids

  • Lecture (others)
    2nd FUNMIG RTDC-2 Progress Meeting Karlsruhe, August 2, 2006, 02.08.2006, Karlsruhe, Germany

Publ.-Id: 8641

Synthesis and biological evaluation of tricarbonyl Re(I) and Tc(I) complexes anchored by poly(azolyl)borates: application on the design of radiopharmaceuticals for the targeting of 5-HT1A receptors

Garcia, R.; Gano, L.; Maria, L.; Paulo, A.; Santos, I.; Spies, H.

The building blocks fac-[99mTc{kappa3-HB(timMe)3}(CO)3] and fac-[99mTc{kappa3-R(mu-H)B(timMe)2}(CO)3] [R is H (4a), Ph (5a); timMe is 2-mercapto-1-methylimidazolyl] were obtained almost quantitatively by reacting fac-[99mTc(CO)3(H2O)3]+ with the corresponding scorpionate. These compounds cross the intact blood-brain barrier in mice, with significant retention in the case of 4a and 5a. Using 4a as the lead structure, we have synthesized the functionalized complexes fac-[M{kappa3-H(mu-H)B(timBu-pip)2}(CO)3] [M is Re (8), 99mTc (8a); timBu-pip is methyl[4-((2-methoxyphenyl)-1-piperazinyl)butyl](2-mercapto-1-methylimidazol-5-yl)methanamide] and fac-[M{kappa3-H(mu-H)B(timMe)(timBu-pip)}(CO)3] [M is Re (9), 99mTc (9a)] and evaluated their potential as radioactive probes for the targeting of brain 5-HT1A serotonergic receptors. The Re complexes exhibit excellent affinity [IC50=0.172 +/- 0.003 nM (8); IC50=0.65 +/- 0.01 nM (9)] for the 5-HT1A receptor. The radioactive congeners (99mTc) have shown an initial brain uptake of 1.38 +/- 0.46%ID g-1 (8a) and 0.43 +/- 0.12%ID g-1 (9a), but suffer from a relatively fast washout.

Keywords: Technetium; Rhenium; Poly(azolyl)borates; 5HT1A receptor; Bivalent approach

Publ.-Id: 8640

Properties of p- and w-mesons in dense and hot nuclear matter near the critical pion mode softening

Bunatian, G.; Kämpfer, B.

  • Open Access Logo Forschungszentrum Rossendorf; FZR 08 1992
    ISSN: 1436-3976


Publ.-Id: 8639

Velocity measurements and concentration field visualizations in copper electrolysis under the influence of Lorentz forces and buoyancy

Weier, T.; Cierpka, C.; Hüller, J.; Gerbeth, G.

Velocity measurements and shadowgraph visualizations for copper electrolysis under the influence of a magnetic field are reported. Experiments in a rectangular cell show the expected strong correlation between flow features and limiting current density. The flow can be understood as driven by the interplay of Lorentz forces and buoyancy. For a cylindrical cell with only slightly non-parallel electric and magnetic field lines, the presence and importance of the Lorentz force is demonstrated by velocity measurements.

Keywords: electrolysis; Lorentz force; copper deposition

  • Magnetohydrodynamics 42(2006)4, 379-387

Publ.-Id: 8638

Development of dendritic ligands for binding metallic radionuclides

Röhrich, A.; Noll, S.; Stephan, H.; Steinbach, J.; Bergamini, G.; Balzani, V.

Two star-like cyclam derivatives 1 and 2 possessing four sugar units on the periphery have been prepared. UV-vis measurements point to slow formation of stable 1:1 complexes of Cu(II) with 1 and 2. Preliminary studies show that the dendritic ligands also complex 99mTc.

  • Poster
    7th International Symposium on Technetium in Chemistry and Nuclear Medicine, 06.-09.09.2006, Bressanone, Italy
  • Contribution to external collection
    U. Mazzi: Technetium, Rhenium and Other Metals in Chemistry and Nuclear Medicine, Padova: SGEditoriali, 2006, 88-89884-04-5, 233-234

Publ.-Id: 8637

Synthesis, characterization and evaluation of novel chelating agents for copper radionuclides

Stephan, H.; Juran, S.; Walther, M.; Steinbach, J.; Born, K.; Comba, P.

Three different hexadentate bispidine ligands L4 – L6 have been prepared. Studies which have been performed to label these ligands with 67Cu indicate a rapid formation of stable complexes under mild conditions (room temperature, aqueous solution). Challenge experiments of these complexes in the presence of a high excess of competing ligands such as glutathione, histidine and cyclam gave no evidence of radiocopper exchange. The 67Cu complexes of the bispidine ligands investigated are also stable in rat plasma at least for 24 h.

  • Poster
    7th International Symposium on Technetium in Chemistry and Nuclear Medicine, 06.-09.09.2006, Bressanone, Italy
  • Contribution to external collection
    U. Mazzi: Technetium, Rhenium and Other Metals in Chemistry and Nuclear Medicine, Padova: SGEditoriali, 2006, 88-89884-04-5, 219-222

Publ.-Id: 8636

Stability aspects of higher dimensional gravitational models with nonlinear scalar curvature coupling R4

Günther, U.

Subject of the talk is a multidimensional gravitational model with scalar curvature nonlinearity R4. It is assumed that the higher dimensional spacetime manifold of this model undergoes a spontaneous compactification to a manifold with warped product structure. The main attention is paid to the stability of the extra-dimensional factor spaces and it is shown that for certain parameter regions the system allows for a freezing stabilization of these spaces. The most interesting fact, which is demonstrated, is a dependence of the stability region (in parameter space) on the total dimension D=dim(M) of the higher dimensional spacetime M. For D>8 the stability region consists of a single (absolutely stable) sector which is shielded from a conformal singularity (and an antigravity sector beyond it) by a potential barrier of infinite height and width. This sector is smoothly connected with the stability region of a curvature-linear model. For D<8 an additional (metastable) sector exists which is separated from the conformal singularity by a potential barrier of finite height and width so that systems in this sector are prone to collapse into the conformal singularity. This second sector is not smoothly connected with the first (absolutely stable) one. Several limiting cases and the possibility for inflation are discussed. The talk is mainly based on:
Class. Quantum Grav. 22, (2005), 3135-3167, hep-th/0409112

Keywords: cosmology; higher dimensional gravity; stabilization of extra dimensions; conformal singularity; gravitational collapse

  • Invited lecture (Conferences)
    Eleventh Marcel Grossmann Meeting on recent developments in theoretical and experimental general relativity, gravitation and relativistic field theories., 23.-29.07.2006, Berlin, Germany

Publ.-Id: 8635

Mn-silicide nanoparticles: the origin of ferromagnetism in Mn-implanted Si?

Zhou, S.; Potzger, K.; Zhang, G.; Mücklich, A.; Eichhorn, F.; Grötzschel, R.; Schmidt, B.; Skorupa, W.; Helm, M.; Fassbender, J.

Recently Si and Ge semiconductors doped with Mn are reported to be ferromagnetic [1, 2]. The ferromagnetism is attributed to the coupling between Mn ions and carriers. However some MnxSiy and MnxGey compounds are ferromagnetic. A careful structural analysis is crucial to clarify the ferromagnetism in Mn doped Si or Ge. In this work, Mn ions were introduced to p-Si by ion implantation. Structural and ferromagnetic properties of Mn-implanted Si were investigated by Rutherford backscattering, x-ray diffraction (XRD), transmission electron microscopy (TEM) and superconducting quantum interference device magnetometer (SQUID). Synchrotron irradiation XRD and TEM revealed the formation of Mn4Si7 nanoparticles already in the as implanted samples (Fig. 1). Depending on the Mn-fluence they grew up from 5 nm to 20 nm upon rapid thermal annealing. No significant evidence is found for Mn substituting Si sites either in the as-implanted or annealed samples. By SQUID, hysteretic loops were observed, while the coecivity and remanence are drastically decreased with increasing temperature. The magnetization upon zero-field cooled and field cooled procedure shows a characteristic of a magnetic nanoparticle system. Together with structural analysis, we attributed the observed ferromagnetism to Mn-silicides nanoparticles.

(1) Y. D. Park, et al., Science 295, 651 (2002).
(2) M. Bolduc, et al., Phys. Rev. B 71, 033302 (2005).

  • Poster
    IBMM 2006, 18.-22.09.2006, Taormina, Italy

Publ.-Id: 8634

A brane model, its AdS-dS states and their agitated extra dimensions

Günther, U.; Vargas Moniz, P.; Zhuk, A.

We consider multidimensional gravitational models with a nonlinear scalar curvature term and form fields. It is assumed that the higher dimensional spacetime undergoes a spontaneous compactification to a warped product manifold. Particular attention is paid to models with quadratic scalar curvature terms and a Freund-Rubin-like ansatz for solitonic form fields. It is shown that for certain parameter ranges the extra dimensions are stabilized for any sign of the internal space curvature, the bulk cosmological constant and of the effective four-dimensional cosmological constant. Moreover, the effective cosmological constant can satisfy the observable limit on the dark energy density.

Keywords: cosmology; higher dimensional gravity; stabilization of extra dimensions

Publ.-Id: 8633

Remarks on dimensional reduction of multidimensional cosmological models

Günther, U.; Zhuk, A.

Multidimensional cosmological models with factorizable geometry and their dimensional reduction to effective four-dimensional theories are analyzed on sensitivity to different scalings. It is shown that a non-correct gauging of the effective four-dimensional gravitational constant within the dimensional reduction results in a non-correct rescaling of the cosmological constant and the gravexciton/radion masses. The relationship between the effective gravitational constants of theories with different dimensions is discussed for setups where the lower dimensional theory results via dimensional reduction from the higher dimensional one and where the compactified space components vary dynamically.

Keywords: cosmology; higher dimensional gravity; stabilization of extra dimensions

Publ.-Id: 8632

Boron Dilution Transients during natural circulation flow in PWR – experiments and CFD simulations

Höhne, T.; Kliem, S.; Rohde, U.; Weiss, F.-P.

Partial depletion of the primary circuit during a postulated small break loss of coolant accident can lead to the interruption of one-phase flow natural circulation. In this case, the decay heat is removed from the core in the reflux-condenser mode. In case of the partial non-availability of the safety injection system, weakly borated condensate can accumulate in particular in the pump loop seal of those loops, which do not receive this safety injection. After refilling of the primary circuit, natural circulation re-establishes and the lower-borated slugs are shifted towards the reactor pressure vessel (RPV). Mixing in the downcomer and the lower plenum is an important phenomenon mitigating the reactivity insertion into the core in this postulated scenario. Therefore, mixing of the lower-borated slugs with the ambient coolant in the RPV was investigated at the four loop 1:5 scaled ROCOM mixing test facility. In these experiments, the length of the flow ramp and the initial density difference between the slugs and the ambient coolant was varied. From the test matrix an experiment with 2% density difference between the de-borated slugs and the ambient coolant was used to validate the CFD software ANSYS CFX. To model the effects of turbulence on the mean flow a Reynolds stress turbulence model was employed and a hybrid mesh consisting of 3.6 million nodes and 6.4 million elements was used. The experiment and CFD calculation show a stratification in the downcomer. The less dense slugs flow around the core barrel at the top of the downcomer. At the opposite side the lower borated coolant is entrained by the colder safety injection water and transported to the core. The validation proves that ANSYS CFX is able to simulate appropriately the flow field and mixing effects of coolant with different densities.

Keywords: PWR; CFD; ROCOM; Boron Dilution

Publ.-Id: 8631

Einfluss von Uran auf die Archae-Diversität in Bodenproben aus der Haberlandhalde

Reitz, T.

Der Einfluss von Uranylnitrat und Natriumnitrat auf eine in einer Uranabfallhalde natürlich vorkommende archaeale Gemeinschaft wurde mit Hilfe molekularbiologischer Methoden auf der Grundlage der 16S rRNA-Genanalyse untersucht. Es zeigte sich, dass sich die archaeale Diversität der Originalprobe auf einige wenige Abstammungslinien mesophiler Crenarchaeota begrenzte.
In allen Versuchsansätzen, unabhängig von der zugegebenen Menge des Uranyl- bzw. Natriumnitrats, der Inkubationsdauer und der Sauerstoffverfügbarkeit, konnte eine Verschiebung der archaealen Populationen zu einer Gruppe eng verwandter, mikrodiverser Crenarchaeota nachgewiesen werden.
Unterschiedliche Nährmedien wurden hergestellt um Vertreter dieser Gruppe anzureichern und zu isolieren. Dabei gelang es mesophile Crenarchaeota der oben genannten Gruppe in zwei Nährmedien zu kultivieren. Die bakterielle Begleitflora beider Archaea-Kulturen war geprägt durch Vertreter der Klassen Bacilli und Clostridia.
In einem weiteren Teil der Arbeit wurde die Biosorptionskinetik von Uran(VI) an den Zellen des hyperthermophilen Crenarchaeons Pyrobaculum islandicum (DSM 4184) sowie des bakteriellen Haldenisolats Paenibacillus sp. JG35+U4-B1 untersucht. Es konnte gezeigt werden, dass bei beiden Stämmen eine schnelle Sättigung (< 30 min) der Zelloberflächen mit Uran bei Raumtemperatur eintritt. Die Uranbindungskapazität von P. islandicum lag mit etwa 28 mg pro g Trockenmasse deutlich unter der des bakteriellen Isolats Paenibacillus sp. JG35+U4-B1 mit 65 mg Uran/g TM. Ein Grund dafür könnte sein, dass die Versuchsbedingungen nicht dem physiologischen Temperaturoptimum des hypothermophilen Archaeons entsprechen.

  • Other report
    Technische Universität Dresden: Diplomarbeit (Studiengang Biologie), 2006
    83 Seiten

Publ.-Id: 8630

Winzlinge im Kampf gegen Uran

Bittner, D.; Flemming, K.; Raff, J.

Im Rahmen der Praktikumsarbeit "Winzlinge im Kampf gegen Uran" wurde ein Internetbeitrag zu „Forschung populär – Von Schülern für Schüler“ über die Arbeiten zur Identifizierung von Bakterien, der Untersuchung ihrer Wechselwirkungen mit Schwermetallen und Radionukliden sowie deren Nutzung zur Reinigung belasteter Abwässer erstellt. Ziel ist es, einzelne Arbeitsgebiete einem breiten Publikum vorzustellen.

Publ.-Id: 8629

FZR activities on in-vessel melt retention

Altstadt, E.; Willschütz, H.-G.

An overview of the modelling activities of FZR in the field of in-vessel corium melt retention (IVR) is given. The thermal and mechanical models and their application to various IVR scenarios are described. The thermo-chemical interaction between molten corium and reactor vessel steel is considered in the calculations.

Keywords: In-vessel melt retention; Finite element model; corrosion; VVER-1000; FOREVER experiments

  • Lecture (others)
    2nd meeting of the ISTC-project "Reactor Core Melt", 05.07.2006, Moskva, Russia

Publ.-Id: 8628

Monocrystalline spinel nanotube fabrication based on the Kirkendall effect

Fan, H. J.; Knez, M.; Scholz, R.; Nielsch, K.; Pippel, E.; Hesse, D.; Zacharias, M.; Gösele, U.

There is a deep interest in methods to fabricate hollow anocrystals for potential application as high-efficiency catalysts or drug-delivery agents. Tubular one-dimensional nanocrystals have been prepared for a wide variety of materials, including semiconductors1,2, metals3,4, ferroelectrics5,6 and magnetite7. They can be produced by rolling up layered materials or via an axial growth in a rolled-up form8–10, coating pores in templates11 or by eliminating the core of a core–shell nanowire1,7. The Kirkendall effect, a classical phenomenon in metallurgy12, was recently applied to explain the formation of hollow spherical nanocrystals13–17. Although the experimental demonstration and theoretical treatment mainly concern binary compounds and planar interfaces or nanoscale spherical interfaces, the fabrication route provided by the Kirkendall effect should be generic, and should also work for high-aspect-ratio hollow cylinders (that is, nanotubes) or even more complex superstructures. In this letter, we report, for the first time, on ultra-long single-crystal ZnAl2O4 spinel nanotubes (total diameter: ∼40 nm, wall thickness: ∼10 nm) fabricated through a spinel-forming interfacial solid-state reaction of core–shell ZnO–Al2O3 nanowires involving the Kirkendall effect. Our results simultaneously represent an extension of applying the Kirkendall effect in fabricating hollow nano-objects from zero-dimensional to multidimensional, and from binary to ternary systems.

Keywords: nanowires; nanotubes; kirkendall effect

Publ.-Id: 8627

Synthesis and radiopharmacological characterization of 2β-carbo-2’-[18F]fluoroethoxy-3β-(4-bromo-phenyl)tropane ([18F]MCL-322) as a potential PET radiotracer for imaging the dopamine transporter (DAT)

Wüst, F.; Berndt, M.; Strobel, K.; van den Hoff, J.; Peng, X.; Neumeyer, J. L.; Bergmann, R.

The fluoroalkyl-containing tropane derivative 2β-carbo-2’-fluoroethoxy-3β-(4-bromo-phenyl)tropane (MCL-322) is a highly potent and selective ligand for the dopamine transporter (DAT). The compound was labeled with the short-lived positron emitter 18F in a single step by nucleophilic displacement of the corresponding tosylate precursor MCL-323 with n.c.a. [18F]fluoride. The positron emission tomography (PET) radiotracer 2βcarbo-2’-[18F]fluoroethoxy-3β-(4-bromo-phenyl)tropane [18F]MCL-322 was obtained in decay-corrected radiochemical yields of 30-40 % at a specific radioactivity of 1.6-2.4 Ci/µmol (60-90 GBq/µmol) at the end-of-synthesis (EOS). Small animal PET, ex vivo and in vivo biodistribution experiments in rats demonstrated a high uptake in the striatum (3.2 % ID/g) 5 min after injection, which increased to 4.2 % ID/g after 60 min. The uptake in the cere¬bellum was 1.8 % ID/g and 0.6 % ID/g after 5 min and 60 min post injection, respectively. Specific binding to DAT of [18F]MCL-322 was confirmed by blocking ex¬periments using the high affinity DAT ligand GBR 12909. The radiopharmacological characterization was completed with metabolite and autoradiographic studies confirming the highly selective uptake of [18F]MCL-322 in the striatum.
It is concluded that the simple single-step radiosynthesis of [18F]MCL-322 and the promising radiopharmacological data make [18F]MCL-322 an attractive candidate for the further development of a PET radiotracer potentially suitable for clinical DAT imaging in the human brain.

Publ.-Id: 8626

Aktuelle Themen der Reaktorsicherheitsforschung in Deutschland - KTG-Fachtagung 2006

Grauf, E.; Stoll, U.; Teschendorff, V.; Weiß, F.-P.

Der Fachtag fand am 03. und 04. April 2006 im Forschungszentrum Rossendorf (FZR) e.V. bei Dresden statt.
Die Veranstaltung griff mit der Borverdünnung in Druckwasserreaktoren bzw. mit der Verstopfung der Sumpfansaugsiebe durch freigesetztes Isolationsmaterial zwei Themen der Reaktorsicherheit auf, die auch in aktuellen Aufsichtsverfahren eine Rolle spielen. Eingebettet in den internationalen Kontext beleuchteten die Veranstalter die sicherheitstechnische Bedeutung dieser Themen für die deutschen Anlagen und stellten die Auswirkungen auf die zu erbringenden Sicherheitsnachweise und den Anlagenbetrieb dar. Dabei kamen Gutachter, Vertreter der Forschung, Hersteller und Betreiber gleichermaßen zu Wort.
Der Fachtag vermittelte den Teilnehmern aber insbesondere, welche Beiträge die privat und öffentlich finanzierte Reaktorsicherheitsforschung zur Aufklärung der jeweiligen Ereignisabläufe und ihrer sicherheitstechnischen Bedeutung geleistet hat.
In diesem Forschungskontext spielen, auch international, die Methoden der so genannten Computational Fluid Dynamics (CFD) eine zunehmende Rolle. Deshalb war eine Sitzung den Grundlagen, Möglichkeiten und Grenzen von CFD-Methoden gewidmet. Dabei wurden u. a. Anwendungen zur Borvermischung und zum Verhalten von Mineralwolle im Sumpf präsentiert.

  • atw - International Journal for Nuclear Power (2006)6, 405-409

Publ.-Id: 8625

Strong improvement of the electroluminescence stability of SiO2:Gd layers by potassium co-implantation.

Prucnal, S.; Sun, J. M.; Reuther, H.; Skorupa, W.; Buchal, C.

Metal-Oxide-Silicon-Based light emitting diodes with Gd implanted SiO2 layers exhibit strong ultraviolet electroluminescence at 316 nm coming from Gd3+ ions. Co-implantation of potassium ions into the SiO2:Gd layer leads to elimination of the luminescent peak from point defects in SiO2 layers. The electropositive potassium ions compensate the negative charge trapping in the gate oxide and thus improve the electrical stability of the ultraviolet emission from Gd3+ ions. The operating time of the devices was increased more than 30 times by using potassium co-implantation and millisecond flash lamp annealing.

Keywords: ion implantation; MOSLEDs; electroluminescence; silicon dioxide

  • Electrochemical and Solid State Letters 10(2007), 330

Publ.-Id: 8624

Dipole-strength distributions up to the particle-separation energies and photodissociation of Mo isotopes

Schwengner, R.; Benouaret, N.; Beyer, R.; Dönau, F.; Erhard, M.; Frauendorf, S.; Grosse, E.; Junghans, A. R.; Kosev, K.; Klug, J.; Nair, C.; Nankov, N.; Rusev, G.; Schilling, K. D.; Wagner, A.

Dipole-strength distributions in the nuclides 92$Mo, 98Mo and 100Mo have been investigated in photon-scattering experiments at the superconducting electron accelerator ELBE of the Forschungszentrum Rossendorf.
A simulation of gamma cascades was performed in order to estimate the distribution of inelastic transitions to low-lying states and thus to deduce the correct dipole-strength distributions up to the neutron-separation energies. The obtained absorption cross sections connect smoothly to (gamma,n) cross sections and give novel information about the low-energy tail of the Giant Dipole Resonance below the neutron-separation energies. The experimental cross sections are compared with predictions of a Quasiparticle-Random-Phase Approximation in a deformed basis.

Photoactivation experiments were performed at various electron energies to study the 92Mo(gamma,n), 92Mo(gamma,p) and 92Mo(gamma,alpha) reactions. The deduced activation yields are compared with theoretical predictions.

Keywords: Photon scattering; photoabsorption cross section; dipole strength; Random-Phase-Approximation; photoactivation; p-nuclei; reaction yields

  • Contribution to proceedings
    International Conference on Collective Motion in Nuclei under Extreme Conditions, 20.-23.6. 2006, Sankt Goar, Deutschland
  • Nuclear Physics A 788(2007), 331c-336c

Publ.-Id: 8623

Excess vacancy generation in SiGe

Kögler, R.

The excess vacancy generation in SiGe by ion implantation was investigated. In contrast to theory the excess vacancy generation in SiGe was observed to be higher than in Si. The reason is the incomplete vacancy-interstitial recombination of defects in SiGe.

Keywords: Ion implantation; implantation defects; excess defects; vacancies; Si; SiGe

  • Lecture (Conference)
    Europien Materials Research Society Conference 2006 - E-MRS2006 -, 29.05.-02.06.2006, Nizza, France

Publ.-Id: 8622

Ion beam Engineering in the initial stage of SIMOX processing

Kögler, R.

Different modes of defect engineering were applied for synthesis of SiO2 nanoclusters in Si. In particular He ion pre-implantation and simultaneous dual-implantation of Si and O ions were performed. It was demonstrated that He pre-implantation is especially suitable for creation of a well defined narrow oxide layer.

Keywords: Silicon-on-insulator; SOI; SiO2 nanocluster; defect engineering; ion beam synthesis

  • Lecture (Conference)
    Ion Implantation - Jahrestagung, 16.-17.05.06, Berlin, Deutschland

Publ.-Id: 8621

Kinetics of precipitation in neutron irradiated steel

Gokhman, A.; Bergner, F.; Ulbricht, A.

Gegenstand der Vortrags ist die ratentheoretische Modellierung der Entwicklung der Größenverteilung von Cu-reichen Defekt-Fremdatom-Clustern in Cu-angereicherten Fe-Basis-Modelllegierungen unter Neutronenbestrahlung. Die berechneten Clustergrößenverteilungen werden mit Ergebnissen von Neutronenkleinwinkelstreumessungen verglichen.

  • Lecture (Conference)
    Mathematical problems to describe the phase transformation of the 1-st kind, 02.-08.07.2006, Bugas, Ukraina

Publ.-Id: 8620

A novel needle probe based on high-speed complex permittivity measurements for investigation of dynamic fluid flows

Da Silva, M. J.; Schleicher, E.; Hampel, U.

For the investigation of multiphase or multicomponent flows, which are of interest, for instance, in oil extraction and processing or in chemical engineering, there are only few suitable measuring techniques. For this reason, we have developed a high-speed complex permittivity needle probe. Such probes are able to distinguish the different phases or components of a flow by measuring the complex value of the electrical permittivity at a high data rate (up to 20000 samples/s). The performance of the system, as well as its ability to differentiate organic substances, has been analyzed. A time-resolved experiment in an oil–water–gas flow, as well as a two-substance mixing experiment in a stirred tank, is presented.

Keywords: complex permittivity measurement; heterogeneous mixture; multiphase flow; multicomponent flow; needle probe


Publ.-Id: 8619

EXAFS investigation of U(VI), U(IV) and Th(IV) sulfato complexes in aqueous solution

Hennig, C.; Schmeide, K.; Brendler, V.; Moll, H.; Tsushima, S.; Scheinost, A.

The structures of U(VI), U(IV) and Th(IV) sulfato complexes were investigated by LIII edge EXAFS spectroscopy in aqueous solutions with total sulfate concentrations, [SO42-]total, ranged from 0.05 to 3 M in the pH range 1.0-2.7. In equimolar U(VI)/[SO42-]total solutions, the species distribution is dominated by monodentate sulfate coordination with a U-Smon distance of 3.57±0.02 Å. With increasing U(VI)/[SO42-]total ratio bidentate coordination becomes dominant with a U-Sbid distance of 3.11±0.02 Å. The aqueous Th(IV) sulfate comprises both, monodentate and bidentate coordination with Th-S distances of 3.14±0.02 and 3.81±0.02 Å, respectively. A similar coordination is obtained for U(IV) sulfato complexes at pH 1 with U-S distances of 3.08±0.02 Å and 3.67±0.02 Å. By increasing the pH value to 2, a U(IV) precipitate occurs, where sulfate act exclusively as bridging ligand with a U-S distance 3.67±0.02 Å.

Keywords: EXAFS; uranium; thorium; sulfate; coordination

  • Inorganic Chemistry 46(2007)15, 5882-5892
  • 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, 978-92-64-99006-7, 219-224

Publ.-Id: 8618

Evidence for double-electron excitations in X-ray absorption spectra of actinides

Hennig, C.

A systematic investigation of double-electron resonances in L3 edge X-ray absorption spectra of actinides in aqueous solution is presented. Actinide species in valence states IV and III were investigated by using experimental data of Th4+, U4+, Np4+, Pu3+, and Am3+ hydrates. The double-electron excitation was identified as a L3N6,7 shake-up effect. Energy positions of the double-electron features were found in good agreement with the Z+1 approximation. The double-electron resonances undergo a larger chemical shift as the L3 edge.

Keywords: double-electron excitation; shake-up; Z+1 approximation; actinides

Publ.-Id: 8617

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