Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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41709 Publications

Single pairing spike-timing dependent plasticity inBiFeO3 memristors with a time window of 25ms to 125µs

Du, N.; Kiani, M.; Mayr, C. G.; You, T.; Buerger, D.; Skorupa, I.; Schmidt, O. G.; Schmidt, H.

Abstract

Memristive devices are popular among neuromorphic engineers for their ability to emulate forms of spike-driven synaptic plasticity by applying specific voltage and current waveforms at their two terminals. In this paper, we investigate spike-timing dependent plasticity(STDP) with a single pairing of one presynaptic voltage spike and one post-synaptic voltage spike in a BiFeO3 memristive device. In most memristive materials the learning window is primarily a function of the material characteristics and not of the applied waveform. In contrast, we show that the analog resistive switching of the developed artificial synapses allows to adjust the learning time constant of the STDP function from 25ms to 125μs via the duration of applied voltage spikes. Also, as the induced weight Change may degrade, we investigate the remanence of the resistance change for several hours after analog resistive switching, thus emulating the processes expected in biological synapses. As the power consumption is a major constraint in neuromorphic circuits, we show methods to reduce the consumed energy per setting pulse to only 4.5 pJ in the developed artificial synapses.

Keywords: BiFeO3 memristor; artificial synapse; single Pairing STDP; memory consolidation; learning window; low-power device

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Permalink: https://www.hzdr.de/publications/Publ-23091


An Energy-Efficient, BiFeO3-Coated Capacitive Switch with Integrated Memory and Demodulation Functions

You, T.; Selvaraj, L. P.; Zeng, H.; Luo, W.; Du, N.; Buerger, D.; Skorupa, I.; Prucnal, S.; Lawerenz, A.; Mikolajick, T.; Schmidt, O. G.; Schmidt, H.

Abstract

A capacitive switching behavior is observed in a Si 3 N 4 /p-Si-based metal–insulator–semiconductor (MIS) structure due to the electron tunneling at the Si 3 N 4 /p-Si interface. A BiFeO 3 (BFO) layer is deposited on Si 3 N 4 /p-Si by pulsed laser deposition technique to obtain the memcapacitive effect as the distribution of positive charges in the Si 3 N 4 layer can be stabilized by the polarization charge of the ferroelectric BFO coating layer. The capacitive switching behavior of the Al/BFO/Si 3 N 4 /p-Si/Au MIS structure is also sensitive to both intensity and wavelength of the illumination, which offers the possibility to create a photodetector for both intensity and color detection. Thus, the presented device has the potential application for future information storage and visible light communications. As an example, a photocapacitive demodulator with capability of decoding both wavelength and intensity information of the incident light is demonstrated.

Keywords: analog resistive switch; flexible barrier height; BiFeO3; fast and energy-efficient resistive switching

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Permalink: https://www.hzdr.de/publications/Publ-23089


Sorption of trivalent lanthanides and actinides onto montmorillonite: Macroscopic, thermodynamic and structural evidence for ternary hydroxo and carbonato surface complexes on multiple sorption sites

Marques Fernandes, M.; Scheinost, A. C.; Baeyens, B.

Abstract

The credibility of long-term safety assessments of radioactive waste repositories may be greatly enhanced by a molecular level understanding of the sorption processes onto individual minerals present in the near- and far-field. In this study we couple extensive macroscopic sorption experiments to surface complexation modelling and spectroscopic tools including extended X-ray absorption fine structure (EXAFS) and time-resolved laser fluorescence spectroscopies (TRLFS), in order to elucidate the uptake mechanism of trivalent actinides and lanthanides (Ln/AnIII) on montmorillonite in the absence and presence of dissolved carbonate. Based on the experimental sorption isotherms, the previously developed 2SPNE SC/CE sorption model needed to be complemented with an additional surface complexation reaction on to a weak site (ºSWOEu2+) for the carbonate-free system. In the presence of carbonate, the previously published model required refinement by reducing the strong-site capacity and by adding the formation of Ln/AnIII-carbonato complexes both on strong and weak sites. EXAFS spectra collected of selected Am sorption samples and TRLFS spectra of selected Cm sorption samples corroborate the model assumptions by showing the existence of different surface complexation sites and evidencing the formation of Ln/AnIII carbonate surface complexes. In the absence of carbonate and at low loadings, Ln/AnIII form strong innersphere sorption complexes through binding to three Al(O,OH)6 octahedra, most likely by occupying vacant sites in the octahedral layers of montmorillonite, which are exposed on {010} and {110} edge faces. At higher loadings, Ln/AnIII bind to only one Al octahedron, forming a weaker, edge-sharing sorption complex. In the presence of carbonate, we identified a ternary mono-carbonato Ln/AnIII complex binding directly to one Al(O,OH)6 octahedron, thereby revealing that type-A ternary complexes form with one or two carbonato groups pointing away from the surface into the solution phase; these complexes form on weak sites only at the observable concentration range, in line with the small amount of strong-site complexes suggested by the complexation model. When the solubility of carbonates was exceeded, formation of an Am carbonate hydroxide could be identified. The excellent agreement between the thermodynamic model parameters developed by fitting a large set of macroscopic data, and the spectroscopically identified mechanisms, demonstrates the mature state of the 2SPNE SC/CE model for predicting and quantifying the retention of Ln/AnIII elements by montmorillonite-rich clay rocks.

Keywords: Americium; Sorption; clay rocks; montmorillonite; TRLFS; EXAFS; surface complexation modeling

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Permalink: https://www.hzdr.de/publications/Publ-23088


Magnetoelectric effect and phase transitions in CuO in external magnetic fields

Wang, Z.; Qureshi, N.; Yasin, S.; Mukhin, A.; Ressouche, E.; Zherlitsyn, S.; Skourski, Y.; Geshev, J.; Ivanov, V.; Gospodinov, M.; Skumryev, V.

Abstract

Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of ≈50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric features are uncovered, establishing CuO as prototype multiferroic with abundance of competitive magnetic interactions.

Involved research facilities

  • High Magnetic Field Laboratory (HLD)

Permalink: https://www.hzdr.de/publications/Publ-23086


Room temperature deposition of highly dense TiO2 thin films by Filtered Cathodic Vacuum Arc

Guillen, E.; Heras, I.; Rincon Llorente, G.; Lungwitz, F.; Alcon-Camas, M.; Escobar-Galindo, R.

Abstract

A systematic study of TiO2 films deposited by dc filtered cathodic vacuum arc (FCVA) was carried out by varying the deposition parameters in a reactive oxygen atmosphere. The influence of the oxygen partial pressure on film properties is analyzed. Composition was obtained by Rutherford backscattering spectroscopy (RBS) measurements, which also allow us to obtain the density of the films. Morphology of the samples was studied by scanning electron microscopy (SEM) and their optical properties by ellipsometry. Transparent, very dense and stoichiometric TiO2 films were obtained by FCVA at room temperature.

Involved research facilities

Related publications

  • Contribution to proceedings
    SPIE Optics + Photonics 2015, 09.-13.08.2015, San Diego, USA
    Proceedings of SPIE 9558
    DOI: 10.1117/12.2189503

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Permalink: https://www.hzdr.de/publications/Publ-23085


Cyclotron Resonance in InAs/AlSb Quantum Wells in Magnetic Fields up to 45 T

Spirin, K. E.; Krishtopenko, S. S.; Sadofyev, Y. G.; Drachenko, O.; Helm, M.; Teppe, F.; Knap, W.; Gavrilenko, V. I.

Abstract

Electron cyclotron resonance in InAs/AlSb heterostructures with quantum wells of various widths in pulsed magnetic fields up to 45 T are investigated. Our experimental cyclotron energies are in satisfactory agreement with the results of theoretical calculations performed using the eight-band kp Hamiltonian. The shift of the cyclotron resonance (CR) line, which corresponds to the transition from the lowest Landau Level to the low magnetic-field region, is found upon varying the electron concentration due to the negative persistent photoconductivity effect. It is shown that the observed shift of the CR lines is associated with the finite width of the density of states at the Landau levels.

Keywords: cyclotron resonance; quantum wells

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Permalink: https://www.hzdr.de/publications/Publ-23083


Magnetic anisotropy and reduced neodymium magnetic moments in Nd3Ru4Al12

Gorbunov, D.; Henriques, M. S.; Andreev, A. V.; Eigner, V.; Gukasov, A.; Fabrèges, X.; Skourski, Y.; Petricek, V.; Wosnitza, J.

Abstract

This paper addresses the electronic properties of Nd3Ru4Al123Ru4Al12.We performed magnetization measurements on a single crystal in static and pulsed magnetic fields as well as neutron-diffraction experiments. Nd3Ru4Al12 is a strongly anisotropic uniaxial ferromagnet with a Curie temperature of 39 K. The magnetic moments are aligned collinearly along the [001] axis. The magnetic structure of Nd3Ru4Al12 has orthorhombic symmetry for which the crystallographic Nd site is split into two magnetically inequivalent positions, Nd1 and Nd2. The Nd1 and Nd2 atoms exhibit reduced magnetic moments, 0.95 and 2.66 μB, as compared to the free Nd3+-ion value (3.28 μB). We discuss this finding in terms of crystal-field effects and competing exchange and anisotropy interactions. Since the single-ion mechanism in Nd3Ru4Al12 leads to uniaxial anisotropy and the two-ion mechanism of the actinide analog U3Ru4Al12 is known to lead to planar anisotropy, this paper demonstrates the decisive influence of these different mechanisms on the magnetic anisotropy.

Involved research facilities

  • High Magnetic Field Laboratory (HLD)

Permalink: https://www.hzdr.de/publications/Publ-23082


Structural stability study of the mixed La0.7-xLuxEu0.3PO4 solid solutions by Extended X-ray Absorption Spectroscopy

Lozano-Rodriguez, M. J.; Arinicheva, Y.; Holthausen, J.; Neumeier, S.; Scheinost, A. C.

Abstract

Monazite and related solid solutions are considered as potential candidates for the immobilization of actinides for the disposal of high-level nuclear waste. Because of their low solubility, high resistance to radiation damage and high chemical flexibility, phosphate materials are matter of study 1. Recently many efforts have been putting to find effective ways to synthesis and characterize lanthanide phosphates through diverse methods3. However, a deeper understanding on structural determinations concerning the local atomic arrangement of surrogate elements, i.e., europium, in phosphates matrices is still in progress. In order to correlate the high chemical flexibility of monazite/xenotime structures with local atomic ordering, mixed La0.7-xLuxEu0.3PO4 solid solutions (x=0, 0.35, 0.5, 0.7) have been study by Extended X-ray Absorption Spectroscopy. La L1, Eu L3 and Lu L3-edges were measured for each phosphate solid solution series. It has been found that for each measured element, the local atomic environment has tendency to adopt an environment closer to monazite or xenotime depending strongly to the x loading. The results obtained by XRPD, XANES and EXAFS are discussed with respect to its chemical flexibility in potential host matrix for sequestering long-lived radionuclides.

Keywords: EXAFS; phosphate solid solutions; monazite; lanthanides

Involved research facilities

Related publications

  • Invited lecture (Conferences)
    Scientific Basis for Nuclear Waste Management XXXIX, 02.-06.11.2015, Montpellier, France

Permalink: https://www.hzdr.de/publications/Publ-23081


Determination of local structure in mixed lanthanoid phosphate solid solutions by X-ray absorption spectroscopy

Lozano-Rodriguez, M. J.; Arinicheva, Y.; Heuser, J.; Kvashnina, K.; Neumeier, S.; Scheinost, A. C.

Abstract

In the 21st century, scientific challenges are required for the disposal of high-level nuclear waste in deep geological formations. Ceramic waste forms like monazite and xenotime, are natural lanthanoid phosphate minerals containing U and Th. When light lanthanoids are involved the monazite structure forms; when heavy lanthanoids are present, the xenotime structure predominates. X-ray absorption spectroscopy was carried out to elucidate the structural changes induced by different lanthanoids when mixed lanthanoids phosphates are formed. EXAFS measurements for La1-xEuxPO4 solid solutions around La and Eu, show the flexibility of the monazite structure when two light lanthanoids are hosted. The La-O distances in the first coordination shell and the first metal-metal distances decrease with increasing Eu content; while the Eu-O local coordination remains unchanged. Additionally, HR-XANES shows an increase on the white line in the absorption coefficient with increasing Eu content, suggesting a change in the valence electron distribution. However, EXAFS measurements of Sm1-xTbxPO4 solid solutions around the Sm, show significant differences between low and high Tb content.

Keywords: EXAFS; phosphate solid solutions; lanthanoids

Involved research facilities

Related publications

  • Lecture (Conference)
    E-MRS 2015 Spring Meeting, 11.-15.05.2015, Lille, France

Permalink: https://www.hzdr.de/publications/Publ-23080


Modern Approaches in Ion Beam Analysis - Challenges and ongoing Developments

Heller, R.

Abstract

The general trend in technology and science to create, process and analyze small structures on a nm scale or even on an atomic scale leads to new challenges in modern ion beam analysis (IBA). This is accompanied by higher demands on the lateral resolution as well as by the demand on high precision determination of elemental compositions on an atomic depth scale. Further the complexity of processing materials in micro and nano electronics is increasing continuously. Thus elemental analysis is no longer limited to the determination of a few particular atomic species and a division of measurement tasks into „light element in a heavy matrix“ or vise versa becomes obsolete in this way. 

Thinner but more complex layer structures are closely related to an increased sensitivity on external impacts. Even the transport of a sample to the place of analysis under ambient conditions can lead to unwanted (chemical) modifications at the surface. Thus IBA under in-situ conditions has developed into an established method within recent years. Furthermore in technological developments not only the state of a system after processing but the process itself may be of particular interest. “Online” IBA under process conditions is thus more and more desired.

Analysis methods of classical IBA like RBS (Rutherford Backscattering Spectrometry), ERD (Elastic Recoil Detection Analysis), PIXE (Particle Induced X-Ray Emission) or PIGE (Particle Induced Gamma Emission), either applied as broad beam or in terms of a micro probe, can therefore rapidly reach their limits. Even the use of modern magnetic spectrometers with depth resolution of a few nano-meters may in particular situations not be sufficient.

Searching for approaches to all these modern measurement tasks often brings IBA to the physical limits and thus complicates quantitative analysis. This for instance may manifest in energy depended charge fractions of the projectile, deviations from classical Rutherford cross-sections, the modification of the sample by the probing beam, etc. All these effects have to be carefully taken into consideration when interpreting analysis results.

The present contribution will give an overview on the demands and difficulties resulting from the described demands on modern IBA. Ongoing development of new IBA techniques and approaches at the Ion Beam Center at HZDR will be presented and discussed. Those are covering

  • the realization of IBA within a Helium ion microscope,
  • the unification of different IBA techniques in complex experimental chambers including in-situ capabilities,
  • the increase of efficiency of lateral resolved PIXE measurements,
  • the controlled implantation of single ions into surfaces with nm precision, as well as
  • concepts for a new low-energy ion laboratory.

Keywords: Ion Beam Analysis; IBA; TBS; ERD; PIXE; lateral resolved IBA

Involved research facilities

Related publications

  • Invited lecture (Conferences)
    50th Zakopane School of Physics, 18.-23.05.2015, Zakopane, Polen

Permalink: https://www.hzdr.de/publications/Publ-23079


Carbon:nickel nanocomposite templates - predefined stable catalysts for diameter-controlled growth of single-walled carbon nanotubes

Melkhanova, S.; Kunze, T.; Haluska, M.; Hübner, R.; Keller, A.; Abrasonis, G.; Gemming, S.; Krause, M.

Abstract

Carbon: nickel (C:Ni) nanocomposite templates (NCTs) were used as catalyst precursors for diameter-controlled growth of single-walled carbon nanotubes (SWCNTs) by chemical vapor deposition (CVD). Two NCT types of 2 nm thickness were prepared by ion beam co-sputtering without (type I) or with assisting Ar+ ion irradiation (type II). NCT type I comprised Ni-rich nanoparticles (NPs) with defined diameter in an amorphous carbon matrix, while NCT type II was a homogenous C:Ni film. Based on the Raman spectra of more than 600 individual SWCNTs, the diameter distribution obtained from both types of NCT was determined. SWCNTs with a selective, monomodal diameter distribution are obtained from NCT type I. About 50% of the SWCNTs have a diameter of (1.36 ± 0.10) nm. In contrast to NCT type I, SWCNTs with a non-selective, relatively homogeneous diameter distribution are obtained from NCT type II. From both catalyst templates predominantly separated as-grown SWCNTs are obtained. They are free of solvents or surfactants, exhibit a low degree of bundling and contain negligible amounts of MWCNTs. The study demonstrates the advantage of predefined catalysts for diameter-controlled SWCNT synthesis in comparison to in situ formed catalysts.

Keywords: Nanocomposites; single-walled carbon nanotubes; diameter selective growth; predifined stable catalyst templates

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Permalink: https://www.hzdr.de/publications/Publ-23078


Comprehensive real time characterization of AlTiO(x)N(y) thin films at high temperatures

Heras, I.; Guillén, E.; Krause, M.; Wenisch, R.; Lungwitz, F.; Escobar-Galindo, R.

Abstract

AlTiN, AlTiO, and AlTiO(x)N(y) thin films were investigated in order to understand the influence of the oxygen to nitrogen ratio on the failure mechanisms at high temperatures. The thin films were deposited by cathodic vacuum arc and characterized in-situ following the methodology proposed for comprehensive environmental testing of optical properties in thin films using the HZDR cluster tool [1].
This multi-chamber material processing and analysis system enables the detailed analysis of the temperature dependence of composition, chemical bonding, and optical properties of thin films. The methodology combines the sequential study of the optical constants by spectroscopic ellipsometry, compositional analysis using ion beam analysis techniques and structure analysis by Raman spectroscopy. All characterizations of AlTiO(x)N(y) thin films were carried out in situ without sample exposure to undefined atmospheres. The samples were heated in vacuum from room temperature to 800°C inside the different chambers and in parallel, to elucidate the influence of the ambience on the degradation process. Moreover, ex-situ annealing in air was performed. Ellipsometry, Raman and ERDA results show the influence of the initial oxygen content in the sample with the inward diffusion of oxygen into the coating and the oxidation resistance at high temperatures.

[1] I. Heras, E. Guillén, R. Wenisch, M. Krause, R. Escobar Galindo, J.L. Endrino - Comprehensive environmental testing of optical properties in thin films. Procedia CIRP. 22 (2014) 271–276

Keywords: In situ analysis; cluster tool; new energy materials; solar-selective coatings

Involved research facilities

Related publications

  • Lecture (Conference)
    78th IUVSTA Workshop, 05.-09.10.2015, Braga, Portugal

Permalink: https://www.hzdr.de/publications/Publ-23077


New energy materials - concepts, structure formation and in situ analysis

Krause, M.

Abstract

An overview on the current activities on new energy materials is given. It includes concepts, structure formation, and in situ analysis of nanocomposite materials for solar energy applications.

Keywords: New energy materials; in situ analysis; nanocomposites

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Related publications

  • Lecture (Conference)
    Ehrenkolloquium für Prof. Hans Kuzmany, 10.09.2015, Wien, Österreich

Permalink: https://www.hzdr.de/publications/Publ-23076


Solar selective coatings based on carbon:transition metal nanocomposites

Heras, I.; Guillén, E.; Krause, M.; Pardo, A.; Endrino, J.-L.; Escobar, R.

Abstract

The design of efficient and stable solar selective coatings for Concentrating Solar Power (CSP) central receivers requires a comprehensive knowledge about the incorporated materials. In this work solar selective coatings were grown by filtered cathodic vacuum arc (FCVA) deposition. The complete stacks consist of an infrared reflection layer, an absorber layer of C:ZrC nanocomposites and an antireflection layer. The Carbon-transition metal nanocomposites were studied as absorber materials because they show appropriate optical properties, i.e. high absorption in the solar region and low thermal emittance. Furthermore metal carbides are thermally and mechanically stabile in air at high temperatures. In order to optimize the absorber layer, the metal content was controlled by adjusting the pulse ratio between the two arc sources. The elemental composition of the absorber layers was determined by Ion Beam Analysis. X-Ray diffraction (XRD) measurements show the formation of metal carbides when the metal content is high enough. The optical properties of the deposited coatings were characterized by spectroscopic ellipsometry (SE). The reflectance spectra of the complete selective coating were simulated with the optical software CODE. Bruggeman effective medium approximation (EMA) was employed to average the dielectric functions of the two components which compose the nanocomposite in the absorber layer. Good agreement was found between simulated and measured reflectance spectra of the solar selective multilayer.

Keywords: Solar selective coatings; carbide thin films; high temperature applications; cathodic arc deposition; simulation of optical properties

Involved research facilities

Related publications

  • Lecture (Conference)
    SPIE: Optics + Photonics 2015, 09.-13.08.2015, San Diego, United States
  • Contribution to proceedings
    SPIE Optics + Photonics for Sustainable Energy, 09.-13.08.2015, San Diego, USA
    Proceedings Volume 9559, High and Low Concentrator Systems for Solar Energy Applications X
    DOI: 10.1117/12.2189515

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Permalink: https://www.hzdr.de/publications/Publ-23075


Effect of hydrogen on magnetic properties of Fe60Al40 alloy thin films

Ehrler, J.; Bali, R.; Otalora, C.; Yildirim, O.; Anwand, W.; Trinh, T. T.; Liedke, M. O.; Cornelius, S.; Böttger, R.; Heller, R.; Potzger, K.

Abstract

The influence of hydrogen on magnetic properties and defects of metallic thin films based on FeAl alloys was investigated. Therefore, hydrogen ions have been implanted with different fluences into disordered Fe60Al40 thin films. Magneto-optical Kerr effect showed an increase of coercivity with increasing ion fluence. The treatment of paramagnetic Fe60Al40 thin films in a climate chamber, on the other hand, led to an establishment of ferromagnetism. Elastic Recoil Detection Analysis showed no hydrogen in the implanted and climate chamber treated samples. However, positron annihilation measurements suggest storage of hydrogen in vacancies. The origin of ferromagnetism was investigated with further methods.

Keywords: Hydrogen; Defects; Implantation; Climate Chamber; FeAl; MOKE; VSM; RBS; ERD; PAS

Involved research facilities

Related publications

  • Poster
    PSI Summer School 2015, 15.-21.08.2015, Zuoz, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-23074


Magnetohydrodynamic effects in liquid metal batteries

Stefani, F.; Galindo, V.; Kasprzyk, C.; Landgraf, S.; Seilmayer, M.; Starace, M.; Weber, N.; Weier, T.

Abstract

Liquid metal batteries (LMBs) consist of two liquid metal electrodes and a molten salt ionic conductor sandwiched between them. The density ratios allow for a stable stratification of the three layers. LMBs were already considered as part of energy conversion systems in the 1960s and have recently received renewed interest for economical large-scale energy storage. In this paper, we concentrate on the magnetohydrodynamic aspects of this cell type with special focus on electro-vortex flows and possible effects of the Tayler instability.

  • Contribution to proceedings
    Liquid Metal Processing & Casting Conference 2015, 20.-24.09.2015, Leoben, Austria
    LMPC 2015 - Proceedings of the 2015 International Symposium on Liquid Metal Processing and Casting, Leoben: ASMET, 978-3-901384-63-9, 347-354
  • Open Access Logo IOP Conference Series: Materials Science and Engineering 143(2016), 012024
    DOI: 10.1088/1757-899X/143/1/012024
    Cited 24 times in Scopus

Permalink: https://www.hzdr.de/publications/Publ-23073


Contactless inductive flow tomography: basic principles and first applications in the experimental modelling of continuous casting

Stefani, F.; Eckert, S.; Ratajczak, M.; Timmel, K.; Wondrak, T.

Abstract

Contactless inductive flow tomography (CIFT) aims at reconstructing the flow structure of a liquid metal from the magnetic fields measured at various positions outside the fluid body which are induced by the flow under the influence of one or multiple applied magnetic fields. We recap the basic mathematical principles of CIFT and the results of an experiment in which the propeller-driven three-dimensional flow in a cylindrical had been reconstructed. We also summarize the recent activities to utilize CIFT in various problems connected with the experimental simulation of the continuous casting process. These include flow reconstructions in single-phase and two-phase flow problems in the Mini-LIMMCAST model of slab-casting, studies of the specific effects of an electromagnetic stirrer attached to the Submerged Entry Nozzle (SEN), as well as first successful applications of CIFT on the background of a strong electromagnetic brake field. We conclude by discussing some remaining obstacles for the deployment of CIFT in a real caster.

  • Contribution to proceedings
    Liquid Metal Processing & Casting Conference 2015, 20.-24.09.2015, Leoben, Austria
    LMPC 2015 - Proceedings of the 2015 International Symposium on Liquid Metal Processing and Casting, Leoben: ASMET, 978-3-901384-63-9, 339-346
  • Open Access Logo IOP Conference Series: Materials Science and Engineering 143(2016), 012023
    DOI: 10.1088/1757-899X/143/1/012023
    Cited 4 times in Scopus

Permalink: https://www.hzdr.de/publications/Publ-23072


Evolution of Crystal Structure During the Initial Stages of ZnO Atomic Layer Deposition

Boichot, R.; Tian, L.; Richard, M.-I.; Crisci, A.; Chaker, A.; Cantelli, V.; Coindeau, S.; Lay, S.; Ouled, T.; Guichet, C.; Chu, M. H.; Aubert, N.; Ciatto, G.; Blanquet, E.; Thomas, O.; Deschanvres, J.-L.; Fong, D. D.; Renevier, H.

Abstract

A complementary suite of in situ synchrotron X-ray techniques is used to investigate both structural and chemical evolution during ZnO growth by atomic layer deposition. Focusing on the first 10 cycles of growth, we observe that the structure formed during the coalescence stage largely determines the overall microstructure of the film. Furthermore, by comparing ZnO growth on silicon with a native oxide with that on Al2O3(001), we find that even with lattice-mismatched substrates and low deposition temperatures, the crystalline texture of the films is dependent strongly on the nature of the interfacial bonds.

Keywords: ALD; ZnO; in-situ X-ray scattering

Permalink: https://www.hzdr.de/publications/Publ-23071


Nickel-Enhanced Graphitic Ordering of Carbon Ad-Atoms During Physical Vapor Deposition

Wenisch, R.; Hübner, R.; Munnik, F.; Gemming, S.; Abrasonis, G.; Krause, M.

Abstract

The mechanism of graphitic ordering of atomic C on Ni was investigated at temperatures ranging from room temperature to 550°C. The C/Ni films were prepared by ion beam sputtering. Their structure has been determined by Rutherford backscattering spectrometry/nuclear reaction analysis, X-ray photoelectron spectroscopy, Raman spectroscopy and cross-sectional transmission electron microscopy. A temperature-induced and a Ni-induced enhancement of graphitic ordering is demonstrated. The Ni-effect is responsible for the formation of a bi-layer structure of the C films at higher deposition temperatures. In the bi-layers, C forms graphenic planes parallel to the Ni surface within a thickness range of 1-2 nm. Further deposited C grows preferentially perpendicular to the surface. The results are discussed on the basis of hyperthermal atom deposition, surface diffusion, metal-induced crystallization and dissolution-precipitation. Our findings point to a dominating role of surface diffusion-assisted crystallization in the carbon ordering process.

Involved research facilities

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Permalink: https://www.hzdr.de/publications/Publ-23070


Quantitative detection of microscopic lithium distributions with neutrons

Neri, G.; Gernhäuser, R.; Lichtinger, J.; Winkler, S.; Seiler, D.; Bendel, M.; Kunze-Liebhäuser, J.; Brumbarov, J.; Portenkirchner, E.; Renno, A. D.; Rugel, G.

Abstract

The importance of lithium in the modern industrial society is continuously increasing. Spatially resolved detection of tritium particles from 6Li(n,α)3H nuclear reactions is used to reconstruct microscopic lithium distributions. Samples are exposed to a flux of cold neutrons. Emitted charged particles are detected with a PSD. Introducing a pinhole aperture between target and detector, the experimental setup works like a “camera obscura”, allowing to perform spatially resolved measurements. Tritium detection analysis was successfully used to reconstruct the lithium content in self-organized TiO2-x-C and Si/TiO2-x-C nanotubes electrochemically lithiated, for the first time. Titanium dioxide nanotubes are a candidate for a safe anode material in lithium-ion batteries. Also lithium distributions in geological samples, so called “pathfinder-minerals” containing lithium, like lepidolite from a pegmatite, were analyzed. With this development we present a new precision method using nuclear physics for material science.
Supported by the DFG (GE 2296/1-1).

Keywords: microscopic lithium distribution; PNAA

  • Lecture (Conference)
    DPG-Frühjahrstagung des Fachverbandes Physik der Hadronen und Kerne (HK), 14.-18.03.2016, Darmstadt, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-23069


Multiphase Flow Measurement and Visualization for Fluid Dynamics Research at Helmholtz-Zentrum Dresden-Rossendorf

Schleicher, E.

Abstract

The presentation will give a short overview on our research activities in the field of multiphase flow investigations and focus on advanced measurement techniques developed in-house.
The content can be summarized to:

  • introduction of HZDR:
o Who we are?
o Where we come from?
o Basic structure of German research organization.
  • Institute of fluid dynamics: structure and research topics
  • Advanced measurement techniques to create CFG grate data for multiphase flow experiments,
functional principle and application examples of:
o wire-mesh sensors,
o gamma-ray computed tomography,
o ultra-fast X-ray tomography.

Involved research facilities

  • TOPFLOW Facility
  • Lecture (others)
    Institutsseminar bei CSIRO, Melbourne, 17.11.2015, Melbourne, Australien

Permalink: https://www.hzdr.de/publications/Publ-23068


An algorithm for refined reconstruction of the phase interface for two-phase gas/liquid annular flows using wire-mesh data

Aydin, T. B.; Brito, R.; Schleicher, E.; Pereyra, E.; Sarica, C.

Abstract

Wire-Mesh sensors (WMS) are used to measure the instantaneous distributions of the phases with high temporal and spatial resolutions with conducting and non-conducting fluids. The experimental data acquired from WMS can be used in a variety of ways to obtain detailed information on the two-phase flow topology including the quantitative visualization of the phase interface.

The current study explores the applicability of a phase interface identification algorithm for two-phase annular flows. The experimental data used in the development of the algorithm is acquired by a WMS with a 16×16 wires for water/air two-phase annular flow at Tulsa University Horizontal Well Artificial Lift Projects (TUHWALP) in 0.05 m (2-in.), 1° upward inclined pipe. The superficial gas and liquid velocities are set to 26 m/s and 0.075 m/s, respectively.

Initially, the WMS data is transformed from a Cartesian coordinate system (with a mesh size of 16×16) onto a polar coordinate system with a mesh size of (MRF×16)×(MRF×16) using a weighted average interpolation, where MRF is the mesh refinement factor. The interpolated data (defined in the polar coordinate system) enables the identification of the phase interface based on the local liquid holdup profile along the radial direction at a given tangential position using the centre of mass concept.

The data interpolation scheme conserves the liquid mass within the pipe cross-section as the mesh refinement factor is increased. However, large values of the refinement factor yield in outliers in the phase interface coordinates. The significance of these outliers is more pronounced if the WMS measurements are affected by the end effects such as in capacitance based measurements. The current algorithm is also capable of producing satisfactory results under stratified flow conditions.

Keywords: Phase interface reconstruction; two-phase; annular flow; wire-mesh

  • Contribution to proceedings
    7th International Symposium on Process Tomography, 01.-03.09.2015, Dresden, Deutschland
    Proceedings of the 7th International Symposium on Process Tomography
  • Poster
    7th International Symposium on Process Tomography, 01.-03.09.2015, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-23067


Preparation and characterization of high purity Ti thin films by high power impulse magnetron sputtering deposition

Meško, M.; Munnik, F.; Heller, R.; Grenzer, J.; Hübner, R.; Krause, M.

Abstract

The increased ion-to-atom ratio in high power impulse magnetron sputtering (HiPIMS) allows directional deposition and film densification by the bombarding ions [1]. Recently, Andersson et al. showed HiPIMS gasless self-sputtering operation and proposed this method for the synthesis of ultraclean metal coatings through self-ion-assisted deposition [2]. In the present work we investigated Ti thin films prepared by direct current magnetron sputtering (dcMS) and HiPIMS with respect to their element composition, surface roughness, and microstructure. Ti films were deposited on Si/SiO2 substrates at room temperature. The base pressure prior to the two hours depositions was 5x10-5 Pa. The film thicknesses were determined by profilometry after the deposition and are 800 nm and 200 nm for dcMS and HiPIMS respectively. It is shown that Ti thin films prepared by HiPIMS do not suffer from bulk contamination like dcMS films (Fig. 1). In particular, the impurity levels for O, N and C are below the detection limit (0.3 – 0.5 at.%) of elastic recoil detection analysis (ERDA) and the hydrogen content was measured to 0.5 at.% for the HiPIMS case. Compared to the dcMS films, we observed an element specific reduction of impurities by a factor 3- 4 for N and H; and a factor of 20 for O. This suggests the presence of at least two sources of impurities. Unlike in [2], the HiPIMS self-sputtering regime was sustained in Ar gas. The high purity of Ti films can be partly explained by gas rarefaction and the cleaning effect of the bombarding ions. Moreover, densification effects presumably suppress post-deposition oxidation. The compositional effects are correlated with differences in the film microstructure revealed by SEM, XRD and TEM analysis. A more sensitive analytical method is needed to evaluate the actual impurity levels of O, N, and C in the deposited HiPIMS films.
[1] U. Helmersson, M. Lattemann, J. Bohlmark, A. P. Ehiasarian, J. T. Gudmundsson “Ionized physical vapor deposition (IPVD): A review of technology and applications” (2006) Thin Solid Films 513, 1–24
[2] J. Andersson and A. Anders “Gasless sputtering: Opportunities for ultraclean metallization, coatings in space, and propulsion” (2008) Appl. Phys. Lett. 92, 221503

Keywords: HiPIMS; self-sputtering; impurity level; Ti thin films

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  • Poster
    6th International Conference on HIPIMS 2015, 10.-11.06.2015, Braunschweig, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-23066


Carbide Formation and Optical Properties in Carbon: Transition Metal Nanocomposites Thin Films

Heras, I.; Guillen, E.; Krause, M.; Pardo, A.; Endrino, J. L.; Escobar Galindo, R.

Abstract

Transition metals with carbon deposited by physical vapor deposition techniques, lead the formation of metal nano-clusters or nanocrystalline metallic carbides embedded in a carbon matrix. Interstitial carbides are stable at high temperature, have high melting points and possess a high reflectivity. In contrast, the resulting carbon: transition metal nanocomposites show optical selective properties such as good absorptance in the visible with high reflectance in the infrared. These properties make them very attractive for applications where high temperature resistant materials with selective optical properties are required.
In this study, carbon: transition metal nanocomposites were grown using a physical vapor deposition system incorporating two pulsed filtered cathodic arc sources, one provided with a graphite cathode and the other with a metallic cathode (Zr, V or Mo). The metal content in the composite was controlled by adjusting the pulse ratio between the two sources, and determined by Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). Comprehensive structure characterization was carried out using a combination of X-ray diffraction (XRD), Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). Optical characterization has been done using both ellipsometry and spectrophotometer measurements in order to obtain the optical constants and the reflectance spectra of the samples.
Together with experimental characterization, a computer program is used to simulate the reflectance spectra of different carbon: transition metal films. Bruggeman effective medium theory was used to average the dielectric functions of the two components which compose the film. According to our simulations, the resulting reflectance of the nanocomposite films is strongly affected by the metal content, independently if it results in metallic nano-clusters or nanocrystalline metallic carbides. Simulated spectra were compared with the measured reflectance of the deposited films obtaining good agreement between simulations and experimental results.

Keywords: Nanocomposites; Optical properties; Carbon

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  • Lecture (Conference)
    2015 MRS Spring Meeting & Exhibit, 06.-10.04.2015, San Francisco, United States

Permalink: https://www.hzdr.de/publications/Publ-23065


Subcritical excitation of the current-driven Tayler instability by super-rotation

Rüdiger, G.; Schultz, M.; Gellert, M.; Stefani, F.

Abstract

It is known that in a hydrodynamic Taylor-Couette system, uniform rotation or a rotation law with positive shear (“super-rotation”) is linearly stable. It is also known that a conducting fluid under the presence of a sufficiently strong axial electric-current becomes unstable against nonaxisymmetric disturbances. It is thus suggestive that a cylindrical pinch formed by a homogeneous axial electric-current is stabilized by rotation laws with dOmega/dR ≥ 0. For magnetic Prandtl number Pm not equal 1 and for slow rotation, however, rigid rotation and super-rotation support the instability by lowering the critical Hartmann numbers. This double-diffusive instability of superrotation even exists for toroidal magnetic fields with rather arbitrary radial profiles, the current-free profile B_phi∝1/R included. The sign of the azimuthal drift of the nonaxisymmetric hydromagnetic instability pattern strongly depends on the magnetic Prandtl number. The pattern counterrotates with the flow for Pm ≪ 1 and it corotates for Pm ≫ 1 while for rotation laws with negative shear, the instability pattern migrates in the direction of the basic rotation for all Pm. An axial electric-current of minimal 3.6 kA flowing inside or outside the inner cylinder suffices to realize the double-diffusive instability for super-rotation in experiments using liquid sodium as the conducting fluid between the rotating cylinders. The limit is 11 kA if a gallium alloy is used

Permalink: https://www.hzdr.de/publications/Publ-23064


Ferromagnetism of nonstoichiometric manganese monosilicides at room temperature

Semisalova, A. S.; Chernoglazov, K. Y.; Perov, N. S.; Gan'Shina, E. A.; Granovsky, A. B.; Zenkevich, A. V.; Zhou, S.; Rylkov, V. V.

Abstract

Ferromagnetic Si-Mn alloys attract increasing interest due to their interesting properties - recently it was found that these alloys prepared by PLD method exhibit unusual magnetic characteristics which cannot be adequately interpreted within the framework of available theoretical models. Curie temperature TC in nonstoichiometric Si1-xMnx alloys slightly enriched in Mn (x ~ 0.52-0.55) was shown to be on order of magnitude higher (TC ~ 300 K) in comparison to the stoichiometric MnSi (TC ~ 30 K). The mechanism of high-temperature ferromagnetism is still not clear. The ferromagnetic exchange is associated with the formation of defects with localized magnetic moments coupled via spin fluctuations of itinerant electrons in the host. Also we suppose that structural defects have a strong influence on the formation of metastable phases with enhanced ferromagnetic response. In this contribution the recent experimental results on ferromagnetism of PLD deposited SiMn alloys are summarized.

  • Poster
    DPG Frühjahrstagung (Spring Meeting) of the Condensed Matter Section (SKM), 17.-20.03.2015, Berlin, Germany

Permalink: https://www.hzdr.de/publications/Publ-23063


Correlations of RBM, G line, and D line frequencies of individual SWCNTs grown from carbon:nickel nanocomposite templates

Melkhanova, S.; Kunze, T.; Haluska, M.; Hübner, R.; Keller, A.; Gemming, S.; Krause, M.

Abstract

Carbon:nickel nanocomposite templates were used for CVD growth of separated, individual single-walled carbon nanotubes (SWCNTs) free of solvents or surfactants. The as-grown carbon nanotubes were characterized by laser-energy dependent Raman spectroscopy and atomic force microscopy. Raman spectra showing a single radial breathing mode (RBM) line were analyzed with respect to correlations of RBM, G+ line and D line frequencies for SWCNT diameters covering the range of 0.8 nm to 1.6 nm. Opposite line shifts were found for RBM and G+ lines of the individual SWCNTs. However, the line shifts of the G+ line are smaller than the standard deviation of the G+ position for SWCNTs with almost the same RBM frequency, i.e. 1 1/cm compared to ± 1 1/cm for the whole diameter range. The D line often shows a complex shape including up to three components, which makes the identification of correlations with RBM frequencies possible only in selected cases.

Keywords: Nanocomposites; thin films; carbon nanotubes; Raman spectroscopy

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  • Poster
    29th International Winterschool for Electronic Properties of Novel Materials, 07.-14.03.2015, Kirchberg, Austria

Permalink: https://www.hzdr.de/publications/Publ-23062


Maldistribution susceptibility of monolith reactors: Case study of glucose hydrogenation performance

Schubert, M.; Haase, S.; Lange, R.; Kost, S.; Salmi, T.; Hampel, U.

Abstract

In this work an ultrafast electron beam modality was applied for the first time to characterise the gas-liquid Taylor flow inside each channel of an opaque honeycomb monolith structure (65 cpsi) for u_(G,S)=0.1…0.5 m/s and u_(L,S)=0.2 m/s. Significant spatial and temporal deviations in the phase holdup as well as in the gas bubble and liquid slug lengths were found. In order to evaluate the impact of Taylor flow maldistribution on the reactor performance, the data of more than 125,000 unit cells were used to simulate the reactor productivity in the hydrogenation of glucose. The results verify that a monolith reactor solely designed by using superficial velocities and empirical correlations for gas bubble and liquid slug lengths fails significantly in achieving high product selectivity and the desired conversion. The developed methods are a solid base to design and select proper distributors ensuring the favourable flow configurations for specific chemical processes.

Keywords: monolith reactor; Taylor flow; X-ray tomography; reactor modelling; glucose hydrogenation

Involved research facilities

  • TOPFLOW Facility

Permalink: https://www.hzdr.de/publications/Publ-23061


Controlled immobilization of His-tagged proteins for protein-ligand interaction experiments using Ni2+-NTA layer on glass surfaces

Cherkouk, C.; Rebohle, L.; Lenk, J.; Keller, A.; Ou, X.; Laubec, M.; Neuber, C.; Haase-Kohn, C.; Pietzsch, J.; Skorupa, W.

Abstract

Gold surfaces functionalized with nickel-nitrilotriacetic acid (Ni2+-NTA) as self-assembled monolayers (SAM) to immobilize histidine (His)-tagged biomolecules are broadly reported in the literature. However, the increasing demand of using microfluidic systems and biosensors takes more and more advantage on silicon technology which provides dedicated glass surfaces and substantially allows cost and resource savings. Here we present a novel method for the controlled oriented immobilization of His-tagged proteins on glass surfaces functionalized with a Ni2+-NTA layer. Exemplarily, the protein pattern morphology after immobilization on the Ni2+-NTA layer of His6-tagged soluble receptor for advanced glycation endproducts (sRAGE) was investigated and compared to non-oriented immobilization of sRAGE on amino SAM by using scanning electron microscopy (SEM). Moreover, we demonstrated interaction of immobilized sRAGE with three structurally different ligands, S100A12, S100A4, and glycated low density lipoproteins (glycLDL), by means of peak-force tapping atomic force microscopy (PF-AFM). We showed a clear discrimination of different protein-ligand orientations by differential height measurements.

Keywords: His-tagged proteins; glycated low density lipoproteins; microfluidic systems and biosensors; S100 proteins; self-assembled monolayers; soluble receptor for advanced glycation endproducts

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Permalink: https://www.hzdr.de/publications/Publ-23060


Quantitative observation of tracer transport with high-resolution PET

Kulenkampff, J.; Gründig, M.; Zakhnini, A.; Lippmann-Pipke, J.

Abstract

Transport processes in natural porous media are typically heterogeneous over various scales. This heterogeneity is caused by the complexity of pore geometry and molecular processes. Heterogeneous processes, like diffusive transport, conservative advective transport, mixing and reactive transport, can be observed and quantified with quantitative tomography of tracer transport patterns. Positron Emission Tomography (PET) is by far the most sensitive method and perfectly selective for positron-emitting radiotracers, therefore it is suited as reference method for spatiotemporal tracer transport observations.
The number of such PET-applications is steadily increasing. However, many aplications are afflicted by the low spatial resolution (3 - 5 mm) of the clinical scanners from cooperating nuclear medical departments. This resolution is low in relation to typical sample dimensions of 10 cm, which are restricted by the mass attenuation of the material. In contrast, our GeoPET-method applies a high-resolution scanner with a resolution of 1 mm, which is the physical limit of the method and which is more appropriate for samples of the size of soil columns or drill cores. This higher resolution is achieved at the cost of a more elaborate image reconstruction procedure, especially considering the effects of Compton scatter. The result of the quantitative image reconstruction procedure is a suite of frames of the quantitative tracer distribution with adjustable frame rates from minutes to months. The voxel size has to be considered as reference volume of the tracer concentration. This continuous variable includes contributions from structures far below the spatial resolution, as far as a detection threshold, in the pico-molar range, is exceeded.
Exemples from a period of almost 10 years (Kulenkampff et al. 2008a, Kulenkampff et al. 2008b) of development and application of quantitative GeoPET-process tomography are shown. Theses examples include differnt transport processes, like conservative flow, reactive transport, and diffusion (Kulenkampff et al, 2015). Such experimental data are complementary to the outcome of model simulations based upon structural µCT-images. The PET-data can be evaluated with respect to specific process parameters, like effective volume and flow velocity distribution. They can further serve as a basis for establishing intermediate-scale simulation models which directly incorporate the observed specific response functions, without requiring modeling on the pore scale at the highest possible spatial resolution.
Kulenkampff, J., Gründig, M., Richter, M., Wolf, M., Dietzel, O.: First applications of a small-animal-PET scanner for process monitoring in rocks and soils. Geophysical Research Abstracts, Vol. 10, EGU2008-A-03727, 2008a.
Kulenkampff, J., Gründig, M., Richter, M., and Enzmann, F.: Evaluation of positron emission tomography for visualisation of migration processes in geomaterials, Physics and Chemistry of the Earth, 33, 937-942, 2008b.
Kulenkampff, J., Gruendig, M., Zakhnini, A., Gerasch, R., and Lippmann-Pipke, J.: Process tomography of diffusion with PET for evaluation anisotropy and heterogeneity, Clay Minerals, accepted 2015, 2015.

  • Lecture (Conference)
    European Geosciences Union General Assembly 2016 (EGU 2016), 17.-22.04.2016, Wien, Österreich

Permalink: https://www.hzdr.de/publications/Publ-23058


MHD effects in continuous casting and liquid metal batteries

Stefani, F.

Abstract

This invited talk gives an overview about the recent activities of the Magnetohydrodynamics Department at HZDR related to continuous casting and liquid metal batteries. Particular focus is laid on the development and first experimental tests of the contactless inductive flow tomography (CIFT) for the continuous casting of steel, and the simulation of various MHD phenomena in liquid metal batteries.

  • Invited lecture (Conferences)
    Liquid metal processing and casting conference, LMPC 2015, 20.-24.09.2015, Leoben, Austria

Permalink: https://www.hzdr.de/publications/Publ-23057


The DRESDYN project: Theoretical background and planned experiments

Stefani, F.

Abstract

The DREsden Sodium facility for DYNamo and thermohydraulic studies (DRESDYN) is a platform for large scale experiments related to geo- and astrophysics as well as to various industrial liquid metal applications. The most ambitious parts of DRESDYN are a homogeneous hydromagnetic dynamo driven solely by precession, and a large Tayler-Couette type experiment for the combined investigation of the magnetorotational instability (MRI) and the Tayler instability (TI). We present recent numerical results on precession driven flows in cylinders and their dynamo action. We also discuss some new theoretical results on various versions of the magnetorotational instability and the Tayler instability, including magnetically triggered instabilities of rotating flows with positive shear. The progress of the construction of the DRESDYN building and of the design of the various experiments is delineated.

  • Invited lecture (Conferences)
    The Dynamo Effect in Astrophysical and Laboratory Plasmas, 07.-09.12.2015, Princeton, USA

Permalink: https://www.hzdr.de/publications/Publ-23056


Liquid metal experiments on dynamo action, magnetorotational instability and current-driven instabilities

Stefani, F.

Abstract

Magnetic fields of planets, stars and galaxies are generated by self-excitation in moving electrically conducting fluids. Once produced, cosmic magnetic fields can play an active role in cosmic structure formation by destabilizing rotational flows that would be otherwise hydrodynamically stable. For a long time, both effects, i.e. hydromagnetic dynamo action and magnetically triggered flow instabilities, have been the subject of purely theoretical investigations. This situation changed in 1999 when the threshold of magnetic-field self-excitation was exceeded in the two liquid sodium experiments in Riga and Karlsruhe [1,2]. Since 2006, the VKS dynamo experiment in Cadarache has successfully reproduced many features of geophysical interest such as reversals and excursions. In the same year, the helical version of the magnetorotational instability (MRI) was observed in the PROMISE experiment in Dresden-Rossendorf [3]. More recently, the azimuthal MRI was found at the same facility [4]. First evidence of the current-driven Tayler instability in a liquid metal was obtained, too [5].
The lecture gives an overview about liquid metal experiments on dynamo action and magnetically triggered instabilities. It concludes with an overview about future experiments, including a precession driven dynamo and a large-scale Tayler-Couette experiment to be set-up in the framework of the DRESDYN project [6].

1. Gailitis, A., Lielausis, O., Platacis, E., Gerbeth, G., Stefani, F., Rev. Mod. Phys. 74 (2002), 973-990
2. Stefani, F., Gailitis, A., Gerbeth, G., ZAMM - Z. Angew. Math. Mech. 88 (2008), 930-954
3. Stefani, F. et al., Phys. Rev. Lett. 97 (2006), 184502
4. Seilmayer, M. et al., Phys. Rev. Lett. 113 (2014), 024505
5. Seilmayer, M. et al., Phys. Rev. Lett. 108 (2012), 244501
6. Stefani, F. et al, Magnetohydrodynamics 48 (2012), 103-113

  • Invited lecture (Conferences)
    Russian Conference on Magnetohydrodynamics, 22.-25.06.2015, Perm, Russia

Permalink: https://www.hzdr.de/publications/Publ-23055


The present status of the DRESDYN project, and some recent theoretical results

Stefani, F.

Abstract

Magnetic fields of planets, stars and galaxies are produced by self-excitation in moving electrically conducting fluids. Once produced, cosmic magnetic fields can play an active role in cosmic structure formation by destabilizing rotational flows that would be otherwise hydrodynamically stable. For a long time, hydromagnetic dynamo action and magnetically triggered flow instabilities, have been the subject of purely theoretical investigations. This situation changed in 1999 when the threshold of magnetic-field self-excitation was exceeded in the two liquid sodium experiments in Riga and Karlsruhe. Since 2006, the VKS dynamo experiment in Cadarache has successfully reproduced many features of geophysical interest such as reversals and excursions. In the same year, the helical version of the magnetorotational instability (MRI) was observed in the PROMISE experiment in Dresden-Rossendorf. More recently, the azimuthal MRI was found at the same facility. First evidence of the current-driven Tayler instability in a liquid metal was obtained, too.
The lecture gives a short overview about liquid metal experiments on dynamo action and magnetically triggered instabilities. The main part is devoted to future experiments, including a precession driven dynamo and a large-scale Tayler-Couette experiment to be set-up in the framework of the DRESDYN project at HZDR.

  • Lecture (Conference)
    GdR Dynamo 2015, 08.-12.06.2015, Bangalore, India

Permalink: https://www.hzdr.de/publications/Publ-23054


Industrial application of radiation

Hampel, U.

Abstract

The presentation gives an overview over various diagnostic applications of radiation in industry reaching from thickness gauging, over multiphase flow metering in the oil and gas production to tomographic techniques on industrial plant components, like chemical reactors or distillation units. The talk comprises physical and mathematical fundamentals, technical system designs and data as well as image processing aspects. Different applications on industrial and scientific flow analysis problems are being discussed in detail.

Keywords: gamma radiation; X-ray diagnostics; tomography; industrial processes; multiphase flow

Involved research facilities

  • TOPFLOW Facility
  • Lecture (others)
    PhD Seminar Series, 12.01.2016, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-23053


Protracted fluvial recovery from medieval earthquakes, Pokhara, Nepal

Stolle, A.; Bernhardt, A.; Schwanghart, W.; Andermann, C.; Schönfeldt, E.; Seidemann, J.; Adhikari, B. R.; Merchel, S.; Rugel, G.; Fort, M.; Korup, O.

Abstract

River response to strong earthquake shaking in mountainous terrain often entails the flushing of sediments delivered by widespread co-seismic landsliding. Detailed mass-balance studies following major earthquakes in China, Taiwan, and New Zealand suggest fluvial recovery times ranging from several years to decades. We report a detailed chronology of earthquake-induced valley fills in the Pokhara region of western-central Nepal, and demonstrate that rivers continue to adjust to several large medieval earthquakes to the present day, thus, challenging the notion of transient fluvial response to seismic disturbance. The Pokhara valley features one of the largest and most extensively dated sedimentary records of earthquake-triggered sedimentation in the Himalayas, and independently augments paleo-seismological archives obtained mainly from fault trenches and historic documents.
New radiocarbon dates from the catastrophically deposited Pokhara Formation document multiple phases of extremely high geomorphic activity between ~700 and ~1700 AD, preserved in thick sequences of alternating fluvial conglomerates, massive mud and silt beds, and cohesive debris-flow deposits. These dated fan-marginal slackwater sediments indicate pronounced sediment pulses in the wake of at least three large medieval earthquakes in ~1100, 1255, and 1344 AD. We combine these dates with digital elevation models, geological maps, differential GPS data, and sediment logs to estimate the extent of these three pulses, which are characterized by sedimentation rates of ~200 mm yr-1 and peak rates as high as 1,000 mm yr-1. Some 5.5 to 9 km3 of material infilled the pre-existing topography, and is now prone to ongoing fluvial dissection along major canyons. Contemporary river incision into the Pokhara Formation is rapid (120-170 mm yr-1), triggering widespread bank erosion, channel changes, and very high sediment yields of the order of 103 to 105 t km-2 yr-1, which by far outweigh bedrock denudation rates inferred from cosmogenic 10Be inventories in river sands. The rapid infill of about a dozen tributary valleys displaced river channels, and caused them to re-incise into bedrock along steep epigenetic gorges. We conclude that the Pokhara Formation offers a unique archive of medieval earthquakes as well as the associated protracted fluvial response that may have been ongoing for up to 900 years.

Keywords: earthquake; AMS; dating; radiocarbon; Be-10

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  • Lecture (Conference)
    European Geosciences Union (EGU) General Assembly, Vienna, Austria, 17.-22.04.2016., 17.-22.04.2016, Wien, Österreich

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Permalink: https://www.hzdr.de/publications/Publ-23052


Geomorphic legacy of medieval Himalayan earthquakes in the Pokhara Valley

Schwanghart, W.; Bernhardt, A.; Stolle, A.; Hoelzmann, P.; Adhikari, B. R.; Andermann, C.; Tofelde, S.; Merchel, S.; Rugel, G.; Fort, M.; Korup, O.

Abstract

The Himalayas and their foreland belong to the world’s most earthquake-prone regions. With millions of people at risk from severe ground shaking and associated damages, reliable data on the spatial and temporal occurrence of past major earthquakes is urgently needed to inform seismic risk analysis. Beyond the instrumental record such information has been largely based on historical accounts and trench studies. Written records provide evidence for damages and fatalities, yet are difficult to interpret when derived from the far-field. Trench studies, in turn, offer information on rupture histories, lengths and displacements along faults but involve high chronological uncertainties and fail to record earthquakes that do not rupture the surface. Thus, additional and independent information is required for developing reliable earthquake histories.
Here, we present exceptionally well-dated evidence of catastrophic valley infill in the Pokhara Valley, Nepal. Bayesian calibration of radiocarbon dates from peat beds, plant macrofossils, and humic silts in fine-grained tributary sediments yields a robust age distribution that matches the timing of nearby M>8 earthquakes in ~1100, 1255, and 1344 AD. The upstream dip of tributary valley fills and X-ray fluorescence spectrometry of their provenance rule out local sediment sources. Instead, geomorphic and sedimentary evidence is consistent with catastrophic fluvial aggradation and debris flows that had plugged several tributaries with tens of meters of calcareous sediment from the Annapurna Massif >60 km away.
The landscape-changing consequences of past large Himalayan earthquakes have so far been elusive. Catastrophic aggradation in the wake of two historically documented medieval earthquakes and one inferred from trench studies underscores that Himalayan valley fills should be considered as potential archives of past earthquakes. Such valley fills are pervasive in the Lesser Himalaya though high erosion rates reduce preservation potential. Further studies may wish to seek such remnants of prehistoric earthquakes using extensive sedimentological work as well as numerical age control.

Keywords: earthquake; Nepal; radiocarbon; AMS; Be-10

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  • Lecture (Conference)
    European Geosciences Union (EGU) General Assembly, 17.-22.04.2016, Wien, Österreich

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Permalink: https://www.hzdr.de/publications/Publ-23051


Making SiMn and TiO2 ferromagnetic at room temperature

Semisalova, A. S.; Rylkov, V. V.; Nikolaev, S. N.; Tugushev, V. V.; Zhou, S.; Potzger, K.; Smekhova, A.; Perov, N.; Granovsky, A.

Abstract

During the last two decades the enormous efforts were put into the creation, understanding and manipulation of room temperature ferromagnetism (RT FM) in semiconductors. The utilization of spin functionality hand in hand with electrical charge-based electronics opens the wide field of phenomena combining brand-new physics and extensive potential for applications in the next generation logic device and storage. In this talk, recent experimental results on RT FM in Si1-xMnx and Ti1-xCo(V)xO2, the promising materials for hybrid semiconductor spintronics will be reported. Si1-xMnx mosaic (polycrystalline) thin films prepared by pulsed laser deposition exhibit unusual magnetic characteristics - The Curie temperature TC in nonstoichiometric Si1-xMnx alloys (x ~ 0.52-0.55) is one order of magnitude higher (TC ~ 300 K) as compared to stoichiometric MnSi (TC ~ 30 K). The mechanism of the high-temperature FM is still not clear. The FM exchange is associated with the formation of defects with localized magnetic moments coupled via spin fluctuations of itinerant electrons in the host. Also we suppose that structural defects caused, in particular, by small sizes of crystallites have a strong influence on the formation of Si1-xMnx phase with high temperature FM. In case of TiO2, Co and V dopants have been used to create a RT FM dilute magnetic oxide. The study of magnetic, magnetotransport and magneto-optical properties of magnetron sputtered Ti1-xCo(V)xO2–δ (x ∼ 1 at. %) revealed a different mechanism responsible for ferromagnetic response at RT. For Ti1-xVxO2−δ the magnetic properties are determined mainly by structural defects, whereas in Ti1-xCoxO2−δ the magnetic moments of Co play a main role.

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  • Invited lecture (Conferences)
    9th International Conference on Magnetic and Superconducting Materials, 01.-3.05.2015, Antalya, Turkey

Permalink: https://www.hzdr.de/publications/Publ-23050


Approval procedures for clinical trials in the field of radiation oncology

Simon, M.; Habeck, M.; Büttner, D.; Habeck, U.; Nölling, T.; Krause, M.; Brix, G.; Willich, N.; Wenz, F.; Schmidberger, H.; Debus, J.; Baumann, M.

Abstract

BACKGROUND AND PURPOSE:

Application of ionizing radiation for the purpose of medical research in Germany needs to be approved by the national authority for radiation protection (Bundesamt für Strahlenschutz, BfS). For studies in the field of radiation oncology, differentiation between use of radiation for "medical care (Heilkunde)" versus "medical research" frequently leads to contradictions. The aim of this article is to provide principle investigators, individuals, and institutions involved in the process, as well as institutional review or ethics committees, with the necessary information for this assessment. Information on the legal frame and the approval procedures are also provided.
METHODS:

A workshop was co-organized by the German Society for Radiation Oncology (DEGRO), the Working Party for Radiation Oncology (ARO) of the German Cancer Society (DKG), the German Society for Medical Physics (DGMP), and the German Cancer Consortium (DKTK) in October 2013. This paper summarizes the results of the workshop and the follow-up discussions between the organizers and the BfS.
RESULTS:

Differentiating between "Heilkunde" which does not need to be approved by the BfS and "medical research" is whether the specific application of radiation (beam quality, dose, schedule, target volume, etc.) is a clinically established and recognized procedure. This must be answered by the qualified physician(s) ("fachkundiger Arzt" according to German radiation protection law) in charge of the study and the treatments of the patients within the study, taking into consideration of the best available evidence from clinical studies, guidelines and consensus papers. Among the important parameters for assessment are indication, total dose, and fractionation. Radiation treatments applied outside clinical trials do not require approval by the BfS, even if they are applied within a randomized or nonrandomized clinical trial. The decision-making by the "fachkundigem Arzt" may be supported on request by an opinion given by the DEGRO Expert Committee for clinical trials.
CONCLUSION:

An important aim for promoting clinical research and patient care in radiation oncology is to further professionalize planning and implementation of clinical trials in this field. Correct assessment, at an early stage, whether a trial needs to be approved by the BfS may reduce unneccesary costs and reduce the time needed for the approval procedure for those trials which need to be assessed by the BfS.

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Evaluating the use of optimally respiratory gated 18F-FDG-PET in target volume delineation and its influence on radiation doses to the organs at risk in non-small-cell lung cancer patients

Wijsman, R.; Grootjans, W.; Troost, E. G.; van der Heijden, E. H.; Visser, E. P.; de Geus-Oei, L.-F.; Bussink, J.

Abstract

OBJECTIVE:

This radiotherapy planning study evaluated tumour delineation using both optimally respiratory gated and nongated fluorine-18 fluorodeoxyglucose-PET (F-FDG-PET).
METHODS:

For 22 non-small-cell lung tumours, both scans were used to create the nongated and gated (g) gross tumour volumes (GTVg) together with the accompanying clinical target volumes (CTV) and planning target volumes (PTV). The size of the target volumes (TV) was evaluated and the accompanying radiotherapy plans were created to study the radiation doses to the organs at risk (OAR).
RESULTS:

The median volumes of GTVg, CTVg and PTVg were statistically significantly smaller compared with the corresponding nongated volumes, resulting in a median TV reduction of 0.5 cm (interquartile range 0.1-1.2), 1.5 cm (-0.2 to 7.0) and 2.3 cm (-0.5 to 11.3) for the GTVg, CTVg and PTVg, respectively. For the OAR, only the percentage of lung (GTV included) receiving at least 35 Gy was significantly smaller after gating, with a median difference in lung volume receiving at least 35 Gy of 5.7 cm (interquartile range -0.8 to 30.50).
CONCLUSION:

Compared with nongated F-FDG-PET, the TVs obtained with optimally respiratory gated F-FDG-PET were significantly smaller, however, without a clinically relevant difference in radiation dose to the OAR.

Permalink: https://www.hzdr.de/publications/Publ-23048


Determination of the exchange stiffness in ultrathin magnetic films by magnonic patterning and ferromagnetic resonance

Lenz, K.; Langer, M.; Wagner, K.; Sebastian, T.; Schultheiss, H.; Lindner, J.; Fassbender, J.

Abstract

In ultrathin films of below 20 nm thickness, it is hardly possible to determine the exchange constant A, since perpendicular standing spin waves (PSSWs) are shifted up to inaccessibly high energies. In this work, a method is presented to analytically determine the exchange stiffness constant D = 2A/MS using ferromagnetic resonance (FMR) and magnonic patterning. Usual FMR measurements, however, are not influenced by the value of D, since no exchange energy is involved in uniform precession. To overcome this problem a coupling mechanism, such as two-magnon scattering (TMS), can be employed to couple exchange dominated in-plane spin waves with the uniform mode.
In our approach lateral magnetic surface patterning was carried out to artificially induce TMS. Subsequent FMR measurements give access to the spin wave spectra of backward volume modes, and thus, to the exchange stiffness constant D.

Keywords: Magnetism; FMR; magnonics

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    603. WE-Heraeus Seminar: Magnonics: Spin Waves Connecting Charges, Spins and Photons, 05.-8.1.2016, Bad Honnef, Deutschland

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Spontaneous pattern formation on ion irradiated semiconductor surfaces

Facsko, S.; Ou, X.; Wang, X.; Hübner, R.; Grenzer, J.

Abstract

Low energy ion irradiation of semiconductor surfaces induces the formation of periodic surface patterns under particular conditions. These nanostructured surfaces exhibit periodici- ties in the range of a few tens to hundreds of nanometers and are promising templates for producing nanostructured thin films [1]. During ion irradiation the surfaces are driven out of equilibrium by continuous creation of displacements in the sub-surface region. At room tem- perature (RT) the accumulation of created displacements leads to amorphization of the irradi- ated semiconductor surfaces. Under these conditions periodic ripple patterns with wave vec- tor parallel to the ion beam direction are observed frequently for ion irradiation at incidence angles between 50° and 70° to the surface normal [2]. At normal incidence dot or hole pat- terns with hexagonal symmetry are observed for specific semiconductors, i.e. GaSb [3], InSb, GaP, or for special irradiation conditions, e.g. Ga+ or Bi3+ irradiation of Ge [4, 5].
In Fig. 1 different patterns on ion irradiated Ge (001) surfaces are shown. Although the Ge (001) surface is thermodynamically stable at all temperature used in the experiments, ion irradiation induces a surface instability which is counterbalanced by surface smoothing via different relaxation mechanisms, e.g. surface diffusion, ion enhanced surface diffusion, sur- face viscous flow, etc. As a result a wavelength selection in the surface roughness manifests itself as a periodic surface pattern. For off-normal angle of incidence ripple patterns are
At higher temperatures than RT, however, point defects created by the displacements in the ion collision cascade can diffuse longer distances, thus, vacancies and interstitial recom- bine or diffuse to the surface more effectively. Eventually, at temperatures higher than the recrystallization temperature, defects in the sub-surface region are annealed or diffuse to the surface before a second ion creates new defects in the same area and the surface remains crystalline. However, the average density of surface vacancies and ad-atoms is much higher than the corresponding densities in thermal equilibrium resulting in a much higher entropy. In this regime, ion irradiation creates an excess of vacancies on the crystalline surface due to sputtering. Thus, the surfaces morphology is determined primarily by vacancy kinetics alt- hough the kinetics of ad-atoms also play an important role.
In this contribution we present investigations of the evolution of Ge surfaces with dif- ferent surface orientation irradiated at temperatures above the recrystallization temperature. The irradiations were done with 1 keV Ar+ ions at normal incidence at temperatures above 250°C which has been established to be the temperature at which the Ge surface remains crystalline even after prolonged irradiation. The samples were cut from epi-ready Ge wafers with (001) and (111) surface orientation. Irradiations were performed in a UHV chamber with a base pressure in the range of 10-8 mbar with a beam from a Kaufman ion source. During irradiation the chamber is flooded with Ar up to a pressure of 3x10-4 mbar. The flux was 1.7x1015 cm-2s-1 and the applied fluence was in the range of 1017 – 1019 cm-2.
The formation of these patterns on crystalline surfaces can be understood in analogy to the formation of 3D structures in homoepitaxy. In molecular beam epitaxy (MBE) the contin- uous deposition of atoms can lead to growth of self-organized 3D nanostructures [5]. One of the possible surface instability, which is responsible for the formation of islands or mounds is caused by the Ehrlich-Schwoebel (ES) barrier, i.e. an additional diffusion barrier for ad- atoms to cross terrace steps. Due to this effect the arriving atoms are trapped on a terraces and can again nucleate to form new terraces.
The same mechanism is also active on ion irradiated surfaces when the temperatures is above the recrystallization temperature. In this case bulk defects are dynamically annealed and amorphization is prevented. Now, ion sputtering is creating vacancies on the crystalline surface and the surfaces morphology is determined by vacancy kinetics. The diffusion of va- cancies is also biased by the ES barrier like the diffusion of ad-atoms. Consequently, the 3D growth turns into 3D erosion. The resulting structures are inverse pyramids which are grow- ing into the surface. The symmetry of these patterns is given by the crystal symmetry. In Fig. 3 zooms of AFM images and the 2D slope distributions of the surface patterns on Ge (001) and Ge (111) are shown, respectively. The detailed facet analysis of the patterns by the 2D slope distribution reveals that on Ge (001) {105} facets with a polar angle of 11° exhibiting a four-fold symmetry are formed, whereas on Ge (111) {356} facets with a polar angle of 15° are formed with a three-fold symmetry. These facets are not know to be thermodynamically stable facets in growth conditions. The {105} facets have only been observed in heteroepi- taxy, where they are stabilize by strain due to the lattice mismatch. In the case of ion erosion no strain is expected [8]. Hence, it can be concluded that these are non-equilibrium facets which are determined by the kinetics of vacancies induce by ion irradiation.
For the description of the pattern formation and evolution in reverse epitaxy a continuum equation can be used which combines the effects of ion irradiation and effective diffusion
currents due to the ES barrier on the crystalline surface. For normal incidence irradiation it is know that smoothing mechanisms dominate thus we can omit an instability term induced by the curvature dependent sputtering or ion induced mass redistribution [9]. By choosing the adequate ES barrier induced surface currents and including also a conserved Kardar-Parisi- Zhang term a remarkable qualitative agreement to the experiments is achieved for both sur- face orientations. Ge (001) and Ge (111), respectively (see Fig. 4) [7].

1. J. Fassbender, T. Strache, M.O. Liedke, D. Marko, S. Wintz, K. Lenz, A. Keller, S. Facsko, I. Monch, J. McCord, New Journal of Physics 11, 125002 (2009).
2. W.L. Chan and E. Chason, J. Appl. Phys. 101, 121301 (2007).
3. S. Facsko, T. Dekorsy, C. Koerdt, C. Trappe, H. Kurz, A. Vogt, and H.L. Hartnagel, Science 285, 1551 (1999).
4. M. Fritzsche, A. Muecklich, S. Facsko, Appl. Phys. Lett. 100, 223108 (2012).
5. Böttger, K.-H. Heinig, L. Bischoff, B. Liedke, R. Hübner, and W. Pilz, Phys. Status Solidi (RRL), 501 (2013).
6. C. Teichert, Phys. Rep. 365, 335 (2002).
7. X. Ou, A. Keller, M. Helm, J. Fassbender, and S. Facsko, Phys. Rev. Lett. 111, 016101 (2013).
8. X. Ou and S. Facsko, Nucl. Instr. Meth. B 341, 13 (2014).
9. C.S. Madi, E. Anzenberg, K.F. Ludwig, and M.J. Aziz, Phys. Rev. Lett. 106, 066101 (2011).

Keywords: ion induced nanopatterns

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  • Invited lecture (Conferences)
    22nd International Conference on Ion-Surface Interactions, 19.-22.08.2015, Moskow, Russia
  • Lecture (Conference)
    8th International Meeting on Recent Developments in the Study of Radiation Effects in Matter, 21.-23.09.2015, Kerteminde, Denmark

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Reverse Epitaxy on Semiconductor Surfaces

Facsko, S.; Ou, X.; Hübner, R.; Grenzer, J.; Heinig, K.-H.

Abstract

Arrays of semiconductor nanostructures are emerging as building blocks for next generation of electronic and optoelectronic nano-devices. In molecular beam epitaxy (MBE) the continuous deposition of atoms can lead to growth of self-organized 3D nanostructures. One of the possible surface instabilities, which is responsible for this kind of growth, is caused by the Ehrlich-Schwoebel (ES) barrier, i.e. an additional diffusion barrier for ad-atoms to cross terrace steps [1]. The arriving atoms are trapped on terraces and can again nucleate to form new terraces. This mechanism leads to the growth of pyramidal mounds on the surface with facets corresponding to energetically favored crystal planes. An analogous mechanism is also observed on ion irradiated surfaces. However, ion sputtering leads to the erosion of the surfaces and at room temperature semiconductor surfaces become amorphous. At these conditions various periodic patterns are observed. [2,3] For device fabrication, a crystalline surface of high quality is indispensable.
In this talk, we demonstrate single crystal elemental (Si and Ge) and compound semiconductor (III-V) nanostructure pattern formation based on a “reverse epitaxy” process. Vacancies created during ion beam irradiation at elevated temperature distribute according to the crystallographic anisotropy, which results in an orientation-dependent pattern formation on single crystal semiconductor surfaces. This process shows nicely the equivalence of epitaxy with deposited adatoms and “reverse epitaxy” with ion induced surface vacancies on semiconductors. The formation of these patterns is interpreted as the result of a surface instability due to an Ehrlich-Schwoebel barrier for ion induced surface vacancies. The potential application of reverse epitaxy on fabrication of UUV optical grating and of metallic nanowires will be discussed.

[1] P. Politi, G. Grenet, A. Marty, A. Ponchet, J. Villain, Phys. Rep. 324, 271 (2000).
[2] S. Facsko, T. Dekorsy, C. Koerdt, C. Trappe, H. Kurz, A. Vogt, and H. L. Hartnagel, Science 285, 1551 (1999).
[3] W. L. Chan and E. Chason, J. Appl. Phys. 101, 121301 (2007).

Keywords: ion induced nanopatterns

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    8th International Workshop on Nanoscale Pattern Formation at Surfaces, 12.-16.07.2015, Krakow, Poland

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First clinical application of a prompt gamma based in vivo proton range verification

Richter, C.; Pausch, G.; Barczyk, S.; Priegnitz, M.; Keitz, I.; Thiele, J.; Smeets, J.; Vander Stappen, F.; Bombelli, L.; Fiorini, C.; Hotoiu, L.; Perali, I.; Prieels, D.; Enghardt, W.; Baumann, M.

Abstract

Background and Purpose: To improve precision of particle therapy, in vivo range verification is highly desirable. Methods based on prompt gamma rays emitted during treatment seem promising but have not yet been applied clinically. Here we report on the worldwide first clinical application of prompt gamma imaging (PGI) based range verification.
Material and Methods: A prototype of a knife-edge shaped slit camera was used to measure the prompt gamma ray depth distribution during a proton treatment of a head and neck tumor for seven consecutive fractions. Inter-fractional variations of the prompt gamma profile were evaluated. For three fractions in-room control CTs were acquired and evaluated for dose relevant changes.
Results: The measurement of PGI profiles during proton treatment was successful. Based on the PGI information, inter-fractional global range variations were in the range of ±2 mm for all evaluated fractions. This is in agreement with the control CT evaluation showing negligible range variations of about 1.5 mm.
Conclusions: For the first time, range verification based on prompt gamma imaging was applied for a clinical proton treatment. With the translation from basic physics experiments into clinical operation, the potential to improve the precision of particle therapy with this technique has increased considerably.

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Interaction of Highly Charged Ions with Surfaces and Nanomembranes

Facsko, S.; Wilhelm, R. A.; Gruber, E.; Ritter, R.; Heller, R.; Aumayr, F.

Abstract

Highly charged ions (HCI) release a large amount of potential energy (the stored ionization energy) when interacting with solids. This energy is deposited into a very small volume directly at the surface via multiple charge exchanges on a fs time scale leading to a highly excited electronic system. Especially ionic crystals have shown a predisposition to potential energy effects due to their low conductivity and their strong electron phonon coupling. On CaF_2 surfaces the formation of hillocks induced by the potential energy of a single highly charged Xe^{q+} ion has been observed for charge states higher than q > 27. The formation of these hillocks can be attributed to local melting [1]. In contrast, on surfaces of KBr one monolayer deep pits are formed by defect mediated desorption also showing a threshold behavior in the pit formation [2].
The interaction of HCI with thin membranes is particularly interesting because the pre-equilibrium interaction regime can be accessed for thicknesses below a few nm. In 1 nm carbon nano membranes (CNM) for instance, holes are produced by the passage of highly charged Xe^{q+} ions [3]. For the formation of these holes a threshold in the potential energy of the HCI exists that depends on the kinetic energy. In order to elucidate the formation mechanism we examined the charge state and the energy loss of the Xe^{q+} ions after their passage through the CNM. Surprisingly, two distinct exit charge distributions were observed [4]. Part of the ions are passing the membrane with almost now charge loss, whereas the other part looses most of their charge. Apparently, the measured charge distribution reflects two different impact parameter regimes. Ions with trajectories far away of any C atom of the membrane can stabilize only few electrons and exit therefore in a high charge state, whereas ions with trajectories close to a C atom can capture a large amount of electrons and exit the membrane in a low charge state. The different impact parameter regimes are also connected to different energy losses: ions with large impact parameters are practically not stopped, whereas ions in close collisions exhibit high stopping force which is strongly dependent on the incident charge state.
[1] A. El-Said, R. Wilhelm, R. Heller, S. Facsko, C. Lemell, G. Wachter, J. Burgdorfer, R. Ritter, and F. Aumayr, Phys. Rev. Lett. 109, 117602 (2012).
[2] R. Heller, S. Facsko, R.A. Wilhelm, and W. Moller, Phys. Rev. Lett. 101, 096102 (2008).
[3] R. Ritter, R.A. Wilhelm, M. Stöger-Pollach, R. Heller, A. Mücklich, U. Werner, H. Vieker, A. Beyer, S. Facsko, A. Gölzhäuser, F. Aumayr, Appl. Phys. Lett. 102, 063112 (2013).
[4] R.A. Wilhelm, E. Gruber, R. Ritter, R. Heller, S. Facsko, F. Aumayr, Phys. Rev. Lett. 112, 153201 (2014).

Keywords: highly charged ions

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    MRS Spring Meeting, 06.-10.04.2015, San Francisco, USA

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Transition from Pits to Mounds in Ion Induced Patterning of Germanium

Facsko, S.; Ou, X.

Abstract

Low energy ion irradiation drives surfaces out of equilibrium by continuous creation of displacements in the sub-surface region. At room temperature the accumulation of displacements leads to the amorphization of the irradiated surfaces and self-organized ripple pattern perpendicular to the ion beam direction are formed for incidence angles higher than 50° [1]. At normal incidence irradiation smoothing dominates and no pattern are observed for low energy ion irradiation. At higher temperatures, point defects created by the displacements in the ion collision cascade can diffuse longer distances, thus vacancies and interstitial recombine more effectively or diffuse to the surface. Finally, at temperatures higher than the recrystallization temperature, all defects in the sub-surface region are annealed before an ion creates new defects and the surface remains crystalline. The average density of surface vacancies and ad-atoms on the surface is, however, much higher than the corresponding densities in thermal equilibrium resulting in a much higher entropy.
In this regime, ion irradiation creates an excess of vacancies on the crystalline surface due to sputtering and the surfaces morphology is determined primarily by their kinetics. The diffusion of vacancies is biased by the Ehrlich-Schwoebel barrier, i.e. an additional barrier for crossing terrace steps, similar to the diffusion barrier of ad-atoms known from growth by molecular beam epitaxy. Consequently, ion sputtering leads to the erosion of 3D structures in a “reverse epitaxy” process. The resulting patterns are arrays of inverse pyramids growing into the Ge surface [1,2]. The morphology of these patterns is given by the crystal symmetry of the surface. Hence, checkerboard patterns appear on the Ge (001) surface Here, we show that the inverse pyramid pattern on Ge(001) surface, which is observed for normal incidence ion irradiation at higher temperatures, turns into a pyramidal mound pattern at incidence angles between 50° and 70° with respect to the surface normal, and finally, into ripple patterns above 80° incidence. All irradiations were performed at 350° C with 1 keV Ar+ at a fluence of 1x1018 cm-2 from a Kaufman ion source.
The observed transition from pit to mound patterns in reverse epitaxy can be understood by assuming a transition from vacancy dominated pattern formation to ad-atom dominated pattern formation. Therefore, at incidence angles above 50° the pattern resemble mound patterns observed in growth. Furthermore, the transition to ripples patterns at higher incidence angles is ascribed to a shadowing instability at these grazing incidence angles.

[1] A. Keller and S. Facsko, Materials 2010, Vol. 3, Pages 4811-4841 3, 4811 (2010).
[2] X. Ou, A. Keller, M. Helm, J. Fassbender, and S. Facsko, Phys. Rev. Lett. 111, 016101 (2013).
[3] X. Ou and S. Facsko, Nucl. Instr. Meth. B 341, 13 (2014).

Keywords: ion induced nanostructures

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    21st International Workshop on Inelastic Ion-Surface Collisions, 18.-23.10.2015, San Sebastian, Spanien

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Pre-equilibrium Dynamics of Highly Charged ions at Surfaces and Carbon Nanomembranes

Facsko, S.; Wilhelm, R.; Gruber, E.; Heller, R.; Aumayr, F.

Abstract

The interaction of HCI with thin membranes is particularly interesting because the pre-equilibrium interaction regime can be accessed for thicknesses below a few nm. In 1 nm carbon nano membranes (CNM) for instance, holes are produced by the passage of highly charged Xe$^{q+}$ ions. For the formation of these holes a threshold in the potential energy of the HCI exists that depends on the kinetic energy. In order to elucidate the formation mechanism we examined the charge state and the energy loss of the Xe$^{q+}$ ions after their passage through the CNM. Surprisingly, two distinct exit charge distributions were observed. Part of the ions are passing the membrane with almost now charge loss, whereas the other part looses most of their charge. Apparently, the measured charge distribution reflects two different impact parameter regimes. Ions with trajectories far away of any C atom of the membrane can stabilize only few electrons and exit therefore in a high charge state, whereas ions with trajectories close to a C atom can capture a large amount of electrons and exit the membrane in a low charge state. The different impact parameter regimes are also connected to different energy losses: ions with large impact parameters are practically not stopped, whereas ions in close collisions exhibit high stopping force which is strongly dependent on the incident charge state.

Keywords: highly charged ions

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    Kolloquiumsvortrag, 12.11.2015, Leipzig, Deutschland

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Defects in zinc oxide grown by pulsed laser deposition

Ling, F. C. C.; Wang, Z.; Ho, L. P.; Younas, M.; Anwand, W.; Wagner, A.; Su, S. C.; Shan, C. X.

Abstract

ZnO films are grown on c-plane sapphire using the pulsed laser deposition method. Systematic studies on the effects of annealing are performed to understand the thermal evolutions of the defects in the films. Particular attention is paid to the discussions of the ZnO/sapphire interface thermal stability, the Zn-vacancy related defects having different microstructures, the origins of the green luminescence (~2.4-2.5 eV) and the near band edge (NBE) emission at 3.23 eV.

Keywords: ZnO; pulsed laser deposition; Zn-vacancy; green luminescence; near band edge emission; positron annihilation spectroscopy

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Ferromagnetism of MnxSi1-x (x~0.5) films grown in the shadow geometry by pulsed laser deposition method

Nikolaev, S.; Semisalova, A.; Rylkov, V.; Tugushev, V.; Zenkevich, A.; Vasiliev, A.; Pashaev, E.; Chernoglazov, K.; Chesnokov, Y.; Likhachev, I.; Perov, N.; Matveyev, Y.; Novodvorskii, O.; Kulatov, E.; Bugaev, A.; Wang, Y.; Zhou, S.

Abstract

The results of a comprehensive study of magnetic, magneto-transport and structural properties of nonstoichiometric MnxSi1-x (x=0.51-0.52) films grown by the Pulsed Laser Deposition (PLD) technique onto Al2O3(0001) single crystal substrates at T = 340 C are present. A highlight of used PLD method is the non-conventional ("shadow") geometry with Kr as a scattering gas during the sample growth. It is found that the films exhibit high-temperature (HT) ferromagnetism (FM) with the Curie temperature TC ~ 370 K accompanied by positive sign anomalous Hall effect (AHE); they also reveal the polycrystalline structure with unusual distribution of grains in size and shape. It is established that HT FM order is originated from the bottom interfacial self-organizing nanocrystalline layer. The upper layer adopted columnar structure with the lateral grain size >50 nm, possesses low temperature (LT) type of FM order with ТС ~ 46 K and contributes essentially to the magnetization at T < 50 K. Under these conditions, AHE changes its sign from positive to negative at T < 30K. We attribute observed properties to the synergy of distribution of MnxSi1-x crystallites in size and shape as well as peculiarities of defect-induced FM order in shadow geometry grown polycrystalline MnxSi1-x (x~0.5) films.

Keywords: Si-Mn alloys; High-temperature ferromagnetism; Anomalous Hall effect

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Modifications of gallium phosphide single crystals using slow highly charged ions and swift heavy ions

El-Said, A. S.; Wilhelm, R. A.; Heller, R.; Akhmadaliev, S.; Schumann, E.; Sorokin, M.; Facsko, S.; Trautmann, C.

Abstract

GaP single crystals were irradiated with slow highly charged ions (HCI) using 114 keV 129Xe(33–40)+ and with various swift heavy ions (SHI) of 30 MeV I9+ and 374 MeV–2.2 GeV 197Au25+. The irradiated surfaces were investigated by scanning force microscopy (SFM). The irradiations with SHI lead to nanohillocks protruding from the GaP surfaces, whereas no changes of the surface topography were observed after the irradiation with HCI. This result indicates that a potential energy above 38.5 keV is required for surface nanostructuring of GaP. In addition, strong coloration of the GaP crystals was observed after irradiation with SHI. The effect was stronger for higher energies. This was confirmed by measuring an increased extinction coefficient in the visible light region.

Keywords: GaP; Swift heavy ions; Slow highly charged ions; Nanostructures

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A setup for transmission measurements of low energy multiply charged ions through free-standing few atomic layer films

Smejkal, V.; Gruber, E.; Wilhelm, R. A.; Brandl, L.; Heller, R.; Facsko, S.; Aumayr, F.

Abstract

We report the design and testing of a setup for transmission measurements of multiply charged ions through free-standing films with a thickness of a few atomic layers. The in- vestigation thereof can yield deeper insight into charge equilibration and pre-equilibrium stopping phenomena which can ultimately be used to specifically tailor and modify these materials.

Keywords: transmission measurements; highly charged ions; ion surface interaction; graphene; carbon nano membranes; ion charge loss; ion energy loss; equilibrium charge state

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Polarity dependence of Mn incorporation in (Ga,Mn)N superlattices

Tropf, L.; Kunert, G.; Jakieła, R.; Wilhelm, R. A.; Figge, S.; Grenzer, J.; Hommel, D.

Abstract

In the context of recent efforts to combine high Mn concentrations in (Ga,Mn)N with a pronounced p- type carrier density, (Ga,Mn)N/GaN:Mg-superlattices have been fabricated using plasma-assisted mole- cular beam epitaxy. Profiles of the dopant atomic densities in the heterostructures are obtained by secondary ion mass spectroscopy. They show an abrupt drop of two to three orders of magnitude in both Mn and Mg concentrations after the first GaN:Mg layer above a critical Mg-flux. Scanning electron microscopy before and after selective etching reveals a polarity inversion from originally Ga-face to N- face GaN in samples in which high Mg fluxes were applied. From our observations, we are able to draw an analogy between the impurity incorporation laws of Mg and Mn.

Keywords: Doping; Molecular beam epitaxy; Nitrides; Magnetic materials

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Interaction of multiply charged ions with single layer graphene Part I: Charge exchange and energy loss (Conference Paper)

Smejkal, V.; Gruber, E.; Kralik, M.; Wilhelm, R. A.; Heller, R.; Facsko, S.; Aumayr, F.

Abstract

The exit charge state distribution and the energy loss of slow multiply charged ions transmitted through single layers of graphene and 1 nm thick carbon nanomembranes is analyzed.

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Imaging-Based Treatment Adaptation in Radiation Oncology

Troost, E. G.; Thorwarth, D.; Oyen, W. J.

Abstract

In many tumor types, significant effort is being put into patient-tailored adaptation of treatment to improve outcome and preferably reduce toxicity. These opportunities first arose with the introduction of modern irradiation techniques (e.g., intensity-modulated radiotherapy) combined with functional imaging for more precise delineation of target volume. On the basis of functional CT, MRI, and PET results, radiation target volumes are altered during the course of treatment, or subvolumes inside the primary tumor are defined to enhance the dosing strategy. Moreover, the probability of complications to normal tissues is predicted using anatomic or functional imaging, such as in the use of CT or PET to predict radiation pneumonitis. Besides focusing, monitoring, and adapting photon therapy for solid tumors, PET also has a role in verifying proton-beam therapy. This article discusses the current state and remaining challenges of imaging-based treatment adaptation in radiation oncology.

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Is integrated transit planar portal dosimetry able to detect geometric changes in lung cancer patients treated with volumetric modulated arc therapy?

Persoon, L. C.; Podesta, M.; Hoffmann, L.; Sanizadeh, A.; Schyns, L. E.; de Ruiter, B. M.; Nijsten, S. M.; Muren, L. P.; Troost, E. G.; Verhaegen, F.

Abstract

BACKGROUND:

Geometric changes are frequent during the course of treatment of lung cancer patients. This may potentially result in deviations between the planned and actual delivered dose. Electronic portal imaging device (EPID)-based integrated transit planar portal dosimetry (ITPD) is a fast method for absolute in-treatment dose verification. The aim of this study was to investigate if ITPD could detect geometric changes in lung cancer patients.
MATERIALS AND METHODS:

A total of 460 patients treated with volumetric modulated arc therapy (VMAT) following daily cone beam computed tomography (CT)-based setup were visually inspected for geometrical changes on a daily basis. Forty-six patients were subject to changes and had a re-CT and an adaptive treatment plan. The reasons for adaptation were: change in atelectasis (n = 18), tumor regression (n = 9), change in pleural effusion (n = 8) or other causes (n = 11). The ITPDs were calculated on both the initial planning CT and the re-CT and compared with a global gamma (γ) evaluation (criteria: 3%\3mm). A treatment fraction failed when the percentage of pixels failing in the radiation fields exceeded 10%. Dose-volume histograms (DVHs) were compared between the initial plan versus the plan re-calculated on the re-CT.
RESULTS:

The ITPD threshold method detected 76% of the changes in atelectasis, while only 50% of the tumor regression cases and 42% of the pleural effusion cases were detected. Only 10% of the cases adapted for other reasons were detected with ITPD. The method has a 17% false-positive rate. No significant correlations were found between changes in DVH metrics and γ fail-rates.
CONCLUSIONS:

This study showed that most cases with geometric changes caused by atelectasis could be captured by ITPD, however for other causes ITPD is not sensitive enough to detect the clinically relevant changes and no predictive power of ITPD was found.

Permalink: https://www.hzdr.de/publications/Publ-23025


Single organ metastatic disease and local disease status, prognostic factors for overall survival in stage IV non-small cell lung cancer: Results from a population-based study.

Hendriks, L. E.; Derks, J. L.; Postmus, P. E.; Damhuis, R. A.; Houben, R. M.; Troost, E. G.; Hochstenbag, M. M.; Smit, E. F.; Dingemans, A. M.

Abstract

PURPOSE:

To analyse the prognostic impact on overall survival (OS) of single versus multiple organ metastases, organ affected, and local disease status in a population based stage IV non-small cell lung cancer (NSCLC) cohort.
METHODS:

In this observational study, data were analysed of all histologically confirmed stage IV NSCLC patients diagnosed between 1 January 2006 and 31 December 2012 registered in the Netherlands Cancer Registry. Location of metastases before treatment was registered. Multivariable survival analyses [age, gender, histology, M-status, local disease status, number of involved organs, actual organ affected] were performed for all patients and for an (18)fluorodeoxyglucose-positron emission tomography ((18)FDG-PET)-staged subgroup.
RESULTS:

11,094 patients were selected: 60% male, mean age 65years, 73% adenocarcinoma. Median OS for 1 (N=5676), 2 (N=3280), and ⩾3 (N=2138) metastatically affected organs was 6.7, 4.3, 2.8months, respectively (p<0.001). Hazard ratio (HR) for 2 versus 1 organ(s) was 1.33 (p<0.001), for ⩾3 versus 1 organ(s) 1.91 (p<0.001). Results were confirmed in the (18)FDG-PET-staged cohort (N=1517): patients with single organ versus 2 and ⩾3 organ metastases had higher OS (8.6, 5.7, 3.8months, HR 1.40 and 2.17, respectively, p<0.001). In single organ metastases, OS for low versus high TN-status was 8.5 versus 6.5months [HR 1.40 (p<0.001)]. (18)FDG-PET-staged single organ metastases patients with low TN-status had a superior OS than those with high TN-status (11.6 versus 8.2months, HR 1.62, p<0.001).
CONCLUSION:

Patients with single organ metastases stage IV NSCLC have a favourable prognosis, especially in combination with low TN status. They have to be regarded as a separate subgroup of stage IV disease.

Keywords: Local disease status; Metastases; Non-small cell lung cancer; Prognosis; Stage IV

Permalink: https://www.hzdr.de/publications/Publ-23023


Increasing the Therapeutic Ratio of Stereotactic Ablative Radiotherapy by Individualized Isotoxic Dose Prescription

Zindler, J. D.; Thomas, C. R.; Hahn, S. M.; Hoffmann, A. L.; Troost, E. G. C.; Lambin, P.

Abstract

To obtain a favorable tradeoff between treatment benefits and morbidity ("therapeutic ratio"), radiotherapy (RT) dose is prescribed according to the tumor volume, with the goal of controlling the disease while respecting normal tissue tolerance levels. We propose a new paradigm for tumor dose prescription in stereotactic ablative radiotherapy (SABR) based on organ-at-risk (OAR) tolerance levels called isotoxic dose prescription (IDP), which is derived from experiences and limitations of conventionally fractionated radiotherapy. With IDP, the radiation dose is prescribed based on the predefined level of normal tissue complication probability of a nearby dose-limiting OAR at a prespecified dose-volume constraint. Simultaneously, the prescribed total tumor dose (TTD) is maximized to the technically highest achievable level in order to increase the local tumor control probability (TCP). IDP is especially relevant for tumors located at eloquent locations or for large tumors in which severe toxicity has been described. IDP will result in a lower RT dose or a treatment scheduled with more fractions if the OAR tolerance level is exceeded, and potential dose escalation occurs when the OAR tolerance level allows it and when it is expected to be beneficial (if TCP < 90%). For patients with small tumors at noneloquent sites, the current SABR dose prescription already results in high rates of local control at low toxicity rates. In this review, the concept of IDP is described in the context of SABR.

Permalink: https://www.hzdr.de/publications/Publ-23022


Interactive Plasma Simulations on Next Generation Supercomputers for Everybody

Huebl, A.; Widera, R.; Zenker, E.; Worpitz, B.; Burau, H.; Pausch, R.; Grund, A.; Matthes, A.; Garten, M.; Eckert, C.; Debus, A.; Bussmann, M.

Abstract

Subject to change: we will cover our fundamental, performance portable building blocks (alpaka) that power kernels in PIConGPU and PMacc. PMacc is our particle-mesh library with reusable, light-weight containers and event scheduling for many/multi-core hardware. Combining our open source libraries with C++ meta-programming and our open data standard suitable for extreme I/O load in HPC (openPMD) we will visualize the whole plasma simulation environment with a live simulation.

Keywords: Simulation; Plasma; LPA; GPU; PIConGPU; HPC; Open

  • Invited lecture (Conferences)
    Swiss Platform for Advanced Scientific Computing (PASC) Conference 2016, 08.-10.06.2016, Lausanne, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-23021


Functional DNA origami nanostructures for nanoelectronics and Photonics

Teschome, B.; Facsko, S.; Keller, A.; Kerbusch, J.

Abstract

Nanodevices based on DNA origami-based nanowires

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  • Lecture (Conference)
    Group meeting Aarhus University, 22.05.2015, Aarhus, Denmark

Permalink: https://www.hzdr.de/publications/Publ-23020


Arrangement and characterization of functional DNA origami nanostructures for nanoelectronics

Teschome, B.; Facsko, S.; Kerbusch, J.; Hübner, R.; Gothelf, K. V.; Keller, A.

Abstract

In this work, we will highlight some of results from our work on the arrangement and characterization of functional DNA origami nanostructures for nanoelectronics. First, a new compelling approach to generate ordered arrays of DNA origami nanotubes on topographically patterned Si surfaces will be introduced. Then, the high-yield synthesis of high-density gold nanoparticle (AuNP) arrangements on DNA origami nanotubes with few unbound background nanoparticles will be presented. The high yield of AuNP assembly was achieved by careful control of the buffer concentration and the hybridization time on Si surface. Finally, also the assembly of heterogeneous nanostructures, i.e. 5 nm gold nanoparticles (AuNPs) and 10 nm semiconductor quantum dots (QDs), on a single DNA origami will be demonstrated.

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  • Invited lecture (Conferences)
    IHRS NanoNet Annual Workshop 2015, 30.09.-02.10.2015, Lohmen, Germany

Permalink: https://www.hzdr.de/publications/Publ-23019


Above Room Temperature Ferromagnetism in Dilute Magnetic Oxide Semiconductors

Semisalova, A. S.; Orlov, A.; Smekhova, A.; Gan’Shina, E.; Perov, N.; Anwand, W.; Potzger, K.; Lähderanta, E.; Granovsky, A.

Abstract

In this chapter, we will survey early and recent experimental results on magnetic properties of dilute magnetic oxide semiconductors, focusing on TiO2-δ:Co and TiO2-δ:V. Room temperature ferromagnetism was observed in both types of thin film samples fabricated by RF sputtering, but their magnetic properties appeared to be quite different. Magnetic moments in case of TiO2-δ:Co are mostly associated with local polarization of Co ions and induced defects. There is an evidence of intrinsic ferromagnetism in the case of low Co content (<1 at.%). Room temperature ferromagnetism was observed in TiO2-δ:V at V content from 3 up to 18 at.% in the whole resistivity range from 10e-3 up to 10e6 Ω cm. Positron annihilation spectroscopy revealed a correlation between magnetization and concentration of the negatively charged defects in TiO2-δ:V thin films. The origin of room temperature ferromagnetism in these systems is discussed. Besides, the recent research findings in ZnO-based magnetic semiconductors are briefly discussed with focus on defect-induced ferromagnetism.

Keywords: Dilute magnetic oxides; Dilute magnetic semiconductors; Above room temperature ferromagnetism; Defect-induced ferromagnetism; Positron annihilation spectroscopy

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  • Book chapter
    Zhukov, A.: Novel Functional Magnetic Materials. Fundamentals and Applications. Springer Series in Materials Science 231, Switzerland: Springer International Publishing, 2016, 978-3-319-26104-1, 187-219
    DOI: 10.1007/978-3-319-26106-5_5
    Cited 5 times in Scopus

Permalink: https://www.hzdr.de/publications/Publ-23018


Structure-Correlated Exchange Anisotropy in Oxidized Co80Ni20 Nanorods

Liebana-Vinas, S.; Wiedwald, U.; Elsukova, A.; Perl, J.; Zingsem, B.; Semisalova, A.; Salgueirino, V.; Spasova, M.; Farle, M.

Abstract

Rare earth-free permanent magnets for applications in electro-magnetic devices promise better sustainability and availability and lower prices. Exploiting the combination of shape, magnetocrystalline and exchange anisotropy in 3D-metals can pave the way to practical application of nanomagnets. In this context, we study the structural and magnetic properties of Co80Ni20 nanorods with a mean diameter of 6.5 nm and a mean length of 52.5 nm, prepared by polyol reduction of mixed cobalt and nickel acetates. Structural analysis shows crystalline rods with the crystallographic c-axis of the hexagonal close-packed (hcp) phase parallel to the long axis of the Co80Ni20 alloy rods, which appear covered by a thin oxidized face-centered cubic (fcc) shell. The temperature dependence of the surprisingly high coercive field and the exchange bias effect caused by the antiferromagnetic surface oxide indicate a strong magnetic hardening due to alignment of anisotropy axes. We identify a temperature dependent local maximum of the coercive field at T = 250 K, which originates from noncollinear spin orientations in the ferromagnetic core and the antiferromagnetic shell. This might be useful for building four way magnetic switches as a function of temperature.

Permalink: https://www.hzdr.de/publications/Publ-23017


Discrimination, correlation and provenance of Bed I tephrostratigraphic markers, Olduvai Gorge, Tanzania, based on multivariate analyses of phenocryst compositions

Habermann, J. M.; Mchenry, L. J.; Stollhofen, H.; Tolosana-Delgado, R.; Stanistreet, I. G.; Deino, A. L.

Abstract

The chronology of Pleistocene flora and fauna, including hominin remains and associated Oldowan industries in Bed I, Olduvai Gorge, Tanzania, is primarily based on 40Ar/39Ar dating of intercalated tuffs and lavas, combined with detailed tephrostratigraphic correlations within the basin. Although a high-resolution chronostratigraphy has been established for the eastern part of the Olduvai Basin, the western subbasin is less well known due in part to major lateral facies changes within Bed I combined with discontinuous exposure. We address the correlation difficulties using the discriminative power of the chemical composition of the major juvenile mineral phases (augite, anorthoclase, plagioclase) from tuffs, volcaniclastic sandstones, siliciclastic units, and lavas. We statistically evaluate these compositions, obtained from electron-microprobe analyses, applying principal component analysis and discriminant analysis to develop discriminant models that successfully classify most Bed I volcanic units. The correlations resulting from integrated analyses of all target minerals provide a basin-wide Bed I chemostratigraphic framework at high lateral and vertical resolution, consistent with the known geological context, which expands and refines the geochemical databases currently available. Correlation of proximal ignimbrites at the First Fault with medial and distal Lower Bed I successions of the western basin enables assessment of lateral facies and thickness trends that corroborate Ngorongoro Volcano as the primary source for Lower Bed I, whereas Upper Bed I sediment supply is mainly from Olmoti Volcano. Compositional similarity between Tuff IA, Bed I lava, and Mafic Tuffs II and III single-grain fingerprints, together with north- and northwestward thinning of Bed I lava, suggests a common Ngorongoro source of these units. The techniques applied herein improve upon previous work by evaluating compositional affinities with statistical rigor rather than primarily relying on visual comparison of bivariate plots.

Keywords: Olduvai; Ngorongoro fan; Principal component analysis; Discriminant analysis; Chemostratigraphy

Permalink: https://www.hzdr.de/publications/Publ-23015


Lectures concerning uranium pollution, time-resolved laser-induced fluorescence spectroscopy (TRLFS), and radiometric dating

Baumann, N.

Abstract

Within a comprehensive course in Radioecology, 5 special lectures were held in the Department of Applied Radiation and Isotopes, Faculty of Science, Kasetsart University, Chatuchak, Bangkok. Topics of these 5 lectures were “Kinds of radiation and radioactive decay”, “Natural occurring radionuclides and natural radiation”, “Age determination by radioactive decay and mass spectrometry”, “Speciation of radionuclides and time-resolved laser-induced fluorescence spectroscopy (TRLFS)”, and “Uranium as basement of nuclear energy production”. As a precondition for obtaining Credit Points for the students in the audience, a questionnaire was created concerning these 5 topics.

Keywords: Uranium; TRLFS; Radiometric Dating

  • Invited lecture (Conferences)
    Course in Radioecology, Department of Applied Radiation and Isotopes, Faculty of Science, Kasetsart University, 22.-26.02.2016, Chatuchak, Bangkok, Thailand

Permalink: https://www.hzdr.de/publications/Publ-23014


Single crystal growth, structural characteristics and magnetic properties of chromium substituted M-type ferrites

Shlyk, L.; Vinnik, D. A.; Zherebtsov, D. A.; Hu, Z.; Kuo, C.-Y.; Chang, C.-F.; Lin, H.-J.; Yang, L.-Y.; Semisalova, A. S.; Perov, N. S.; Langer, T.; Pöttgeng, R.; Nemrava, S.; Niewa, R.

Abstract

Two different types of fluxes, namely sodium based and chloride based fluxes were used to grow Cr substituted barium and strontium hexaferrite ferrite crystals, (Sr,Ba)Fe12 − xCrxO19 at comparatively low temperatures of about 1300 °C. The sodium based flux led to growth of larger crystals up to 5 mm, but with only minor Cr contents x ≤ 0.07. From the chloride based flux the obtained Cr contents are significantly higher with x = 5.7 (Sr) and x = 3.4 (Ba), however, crystals reach only sizes in the sub-mm range. X-ray absorption spectroscopy data support exclusively isovalent substitution of Fe3+ by Cr3+ even for very low Cr contents. 57Fe Mößbauer spectroscopy reveals Cr to preferentially occupy the six-fold by oxygen coordinated site at 12k and, to a lower degree, 2a and 4f2 in space group P63/mmc. All characteristic magnetic properties drop upon Cr substitution, e. g., the Curie temperature from 728 K for pure BaFe12O19 to 465 K for BaFe8.6Cr3.4O19, the saturation magnetization from 71 emu/g to 29.7 emu/g and the coercive field from 363 Oe to 45 Oe.

Keywords: Hexaferrites; Chromium; Mößbauer; Magnetism

Permalink: https://www.hzdr.de/publications/Publ-23013


Layer-to-layer compression and enhanced optical properties of few-layer graphene nanosheet induced by ion irradiation

Shang, Z.; Tan, Y.; Zhou, S.; Chen, F.

Abstract

We report on the first experimental study of the layer-to-layer compression and enhanced optical properties of few-layer graphene nanosheet by applying ion irradiation. The deformation of graphene layers is investigated both theoretically and experimentally. It is observed that after the irradiation of energetic ion beams, the space between separate graphene layers is reduced due to layer-to-layer compression, resulting in tighter contact of the graphene sheet with the surface of the substrate. This processing enables enhanced interaction of the graphene with the evanescent-field wave near the surface, which induces reinforced polarization-dependent light absorption of the graphene. Utilizing the ion-bombarded graphene nanosheets as saturable absorbers, we have realized efficient Q-switched waveguide lasing with enhanced performance through the interaction of the graphene and evanescent field.

Keywords: Graphene; Absorption; Q switching; Optical properties; Waveguides

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Permalink: https://www.hzdr.de/publications/Publ-23012


Detectability of local range shifts in double scattered proton irradiation with a prompt gamma slit camera

Priegnitz, M.; Nenoff, L.; Barczyk, S.; Vander Stappen, F.; Hotoiu, L.; Smeets, J.; Fiedler, F.; Pausch, G.; Richter, C.

  • Poster
    55th Annual Conference PTCOG 55, 22.-28.05.2016, Prague, Czech Republic

Permalink: https://www.hzdr.de/publications/Publ-23011


Evaluation of four Sigma-1 PET radiotracers in nonhuman primates

Cai, Z.; Baum, E.; Bois, F.; Holden, D.; Lin, S.-F.; Chen, Y.; Fischer, S.; Jia, H.; Brust, P.; Huang, Y.

Abstract

Objectives: Sigma-1 receptor (S1R) is resident to mitochondrial-associated endoplasmic reticulum and plasma membranes with implications in a variety of diseases including Alzheimer's disease, ALS, and cancer. Previous PET S1R radiotracers are characterized by slow kinetics that impedes their use for human brain imaging. Recently a series of spirocyclic piperidine-based ligands showed great promise as S1R PET imaging probes, based on their high selectivity towards S1R and good binding characteristics in rodents or porcine. [1-3] Here, we report the first monkey PET imaging studies of four ligands (1-4) in this series to assess their pharmacokinetic and in vivo binding properties, and to select the most suitable tracer for advancing to humans.
Methods:
Each tracer was injected as a bolus (~5 mCi) to the same rhesus monkey.
Baseline scans were obtained on a Siemens FOCUS 220 scanner over 4 h. Two hour blocking scans were performed with administration of SA4503 (0.5 mg/kg) [4] before tracer injection. Arterial blood was drawn during each scan for metaboite analysis by HPLC and construction of the plasma input functions. Regional brain time-activity curves (TACs) were analyzed by one-tissue (1T), two-tissue (2T), and multilinear analysis-1 (MA1) models to obtain regional volumes of distribution (VT). The free fraction (fp) in plasma was meassured via ultrafiltration method. Log D of each tracer was also determined.
Results:
Fast metabolism of the tracers was observed in rhesus monkeys, with ~ 35%, 18%, and 19% parent fraction, respectively, for 1 (2), 3 and 4 at 60 min post-injection. Plasma fP values were 2%, 8%, and 17%, for 1 (2), 3 and 4, consistent with their respective measured Log D values of 2.80, 2.55, and 2.50. In the brain, all four tracers showed high and fast uptake. Tissue activity washout was rapid for 2 and 4, and much slower for 1 and 3, in line with their respective in vitro S1R binding affinities. Both the 1T and MA1 kinetic models provided good fits of regional TACs, and reliable VT estimates with low errors. Across all regions, 1T VT values were greatest for 3, follwed by 1, 4, and 2. The highest VT values were in the cingulate gyrus for all tracers. Ligand 4 showed the greatest differential uptake across different brain regions. SA4503 at the dose of 0.5 mg/kg blocked ~85% (2) and ~95% (4) of radiotracer binding, respectively.

  • Lecture (Conference)
    SNMMI 2016 Annual Meeting, 11.-15.06.2016, San Diego, California, USA
  • Abstract in refereed journal
    Journal of Nuclear Medicine 57(2016)2, 81P

Permalink: https://www.hzdr.de/publications/Publ-23010


A Johann-type X-ray emission spectrometer at the Rossendorf beamline

Kvashnina, K. O.; Scheinost, A. C.

Abstract

This paper gives a detailed description of the Johann-type X-ray emission spectrometer recently installed and tested at the Rossendorf Beamline (ROBL) of the European Synchrotron Radiation Facility (ESRF). The spectrometer consists of a single spherically bent crystal analyzer and avalanche photodiode detector positioned on the vertical Rowland cycle of 1m diameter. The hard X-ray emission spectrometer (~ 5 – 25 keV) operates at atmospheric pressure and covers the Bragg angles of 65°–89°. The instrument has been tested at high and intermediate incident energies, i.e. at the Zr K-edge and at the Au L3-edge, in the second experimental hutch of ROBL. The spectrometer is constructed for studying nuclear materials and environmental applications by high energy resolution X-ray absorption and X-ray emission spectroscopies.

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Permalink: https://www.hzdr.de/publications/Publ-23009


Evaluation of Actinide(IV)-Silica colloids mobility under repository conditions - the NuWaMa Project

Hildebrand, H.; Weiss, S.; Zaenker, H.; Kulenkampff, J.; Lippmann-Pipke, J.; Videnska, K.; Červinka, R.

Abstract

Colloidal transport in the near-field and far-field of repositories is considered as one potential pathway for migration of (radio-)toxic components in case of groundwater intrusion. Recently, the in-situ formation of actinide(IV)-silica colloids (dp < 20 nm) was discovered under typical conditions for nuclear waste repositories in granitic formations (Dreissig et al. (2011), Hennig et al. (2013), Husar et al. (2015). These colloids show long-term stability over years and could therefore play a significant role in radionuclide migration since silica is an ubiquitous compound. So far, there is only little knowledge about the reactive transport of actinide(IV)-silica colloids under repository conditions. Within the NuWaMa project (intended start in January 2016), a new close collaboration between the Helmholtz-Zentrum Dresden - Rossendorf (HZDR) and the ÚJV Řež will be established and joint research focused on this problem will be intensified. First transport experiments using packed columns with crushed granite and distilled water amended with Th(IV)-silica colloids gave hints of mobility of the colloids under certain conditions. These experiments are conducted using conventional analytics such as ICP/MS and light scattering techniques for detection of the colloids in the column effluent. The aim of the project is also to evaluate this transport in more detail, including also by positron emission tomography (PET) with its unrivaled sensitivity and robustness for non-destructive quantitative spatio-temporal measurements. HZDR empowered “GeoPET” for its applicability in opaque/geological media (e.g. Kulenkampff et al., (2008), Kulenkampff et al.,(2015) , see Fig. 1). Zirconium radionuclides (positron-emitter and analogues for tetravalent actinides)shall be used for visualization of colloidal transport on column scale and, if applicable, also in a fractured rock sample.
The aim of the study, applied techniques and first results are intended to be shown as a poster presentation.

  • Poster
    BELBaR-Conference 2016 "Clay Colloids in Aqueous Systems", 03.-04.02.2016, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-23007


Structure and chemistry of surface-doped Pt:SnO2 gas sensing materials

Degler, D.; Pereira De Carvalho, H. W.; Kvashnina, K.; Grunwaldt, J.-D.; Barsan, U. W. N.

Abstract

Surface-doped Pt:SnO2 was synthesized by impregnation of calcined SnO2 made by an aqueous sol-gel route. The structure of the introduced Pt-dopant and its behaviour during gas exposure were examined by in-situ and operando X-ray absorption spectroscopy. The results reveal that Pt forms a nano-sized PtO2 phase, which was not found for bulk and surface doped materials, studied previously. In a comparative investigation of undoped and Pt-doped SnO2 gas sensors the performance and the surface chemistry were investigated, the latter one using operando FT-IR spectroscopy. The results prove a strong influence of the different Pt structures on the surface chemistry of SnO2, providing the basis for an understanding on the varying sensor performance of differently synthesized Pt:SnO2 gas sensing materials

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Permalink: https://www.hzdr.de/publications/Publ-23006


Crystal-field and covalency effects in uranates: x-ray spectroscopic study

Butorin, S.; Kvashnina, K.; Smith, A.; Popa, K.; Martin, P.

Abstract

The electronic structure of U(V) and U(VI) containing uranates NaUO3 and Pb3UO6 was studied using an advanced technique, namely x-ray absorption spectroscopy (XAS) in the high-energyresolution uorescence-detection (HERFD) mode. Thanks to a significant reduction of the core-hole lifetime broadening, the crystal-field splitting of the 5f shell were probed directly in the HERFDXAS spectra collected at the U 3d edge, which is not possible with conventional XAS. In addition, the charge-transfer satellites resulting from the U 5f-O 2p hybridization were clearly resolved. The crystal-field parameters, 5f occupancy, and degree of covalency of the chemical bonding in these uranates were estimated using the Anderson impurity model by calculating the U 3d HERFD-XAS, conventional XAS, core-to-core (U 4f-to-3d transitions) resonant inelastic x-ray scattering (RIXS) and U 4f x-ray photoelectron spectra, respectively. The crystal field was found to be strong in these systems, while the 5f occupancy was determined to be 1.32 and 0.84 electrons in the ground state for NaUO3 and Pb3UO6, respectively, thus indicating a significant covalent character for these compounds.

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Permalink: https://www.hzdr.de/publications/Publ-23005


High-resolution x-ray absorption spectroscopy as a probe of crystal-field and covalency effects in actinide compounds

Butorin, S. M.; Kvashnina, K.; Vegelius, J. R.; Meyer, D.; Shuh, D. K.

Abstract

Applying the high-energy-resolution uorescence-detection (HERFD) mode of x-ray absorption spectroscopy (XAS) we were able for the first time to probe the crystalline electric field (CEF) splittings of the 5f shell directly in the HERFD-XAS spectra of actinides. Using ThO2 as an example, the data measured at the Th 3d edge were interpreted within framework of the Anderson impurity model. Since the charge-transfer satellites were also resolved in the HERFD-XAS spectra, their analysis revealed that ThO2 is not an ionic compound as previously believed. The Th 6d occupancy in the ground state was estimated to be as twice as much compared to that for the Th 5f states. We demonstrate that HERFD-XAS now allows for characterization of the CEF interaction and degree of covalency in the ground state of actinide compounds as it is extensively done for 3d transition metal systems.

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Permalink: https://www.hzdr.de/publications/Publ-23004


Valence fluctuations of europium in the boride Eu4Pd29+xB8

Gumeniuk, R.; Schnelle, W.; Ahmida, M.; Abd-Elmeguid, M.; Kvashnina, K.; Tsirlin, A.; Leithe-Jasper, A.; Geibel, C.

Abstract

We synthesized a high quality sample of the boride Eu4Pd29+xB8 (x = 0.76) and studied its structural and physical properties. Its tetragonal structure was solved by direct methods and confirmed to belong to the Eu4Pd29B8 type. All studied physical properties indicate a valence fluctuating Eu state, with a valence decreasing continuously from about 2.9 at 5 K to 2.7 at 300 K. Maxima in the T dependence of the susceptibility and thermopower at around 135 K and 120 K, respectively, indicate a valence fluctuation energy scale on the order of 300 K. Analysis of the susceptibility evidences some inconsistencies when using the ionic interconfigurational fluctuation (ICF) model, thus suggesting a stronger relevance of hybridization between 4f and valence electrons compared to standard valence-fluctuating Eu systems.

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Permalink: https://www.hzdr.de/publications/Publ-23003


Kondo effect and thermoelectric transport in CePd3Bex

Gumeniuk, R.; Schnelle, W.; Kvashnina, K.; Leithe-Jasper, A.

Abstract

The physical properties of the series CePd3Bex (0 ≤ x ≤ 0.47) have been studied. Introducing Be into CePd3 results in a drastic reduction of the Seebeck coefficient from 100 μV K−1 at 300 K to -2 μV K−1, respectively. Paramagnetism of Ce3+ free ions and metallic conduction dominate the physical properties. A structural transition at x = 0.25 is accompanied by a significant lowering of the Kondo temperature and leads to a successive suppression of the thermoelectric performance of CePd3Bex with increasing x.

Keywords: crystal structure; Kondo effect; thermoelectric properties; XAS

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Permalink: https://www.hzdr.de/publications/Publ-23002


Creating and imaging nanosized magnets using HIM and TEM holography

Hlawacek, G.; Röder, F.; Bali, R.; Wintz, S.; Hübner, R.; Bischoff, L.; Lichte, H.; Potzger, K.; Lindner, J.; Fassbender, J.

Abstract

Besides imaging, gas field ion source (GFIS) based microscopes [1] are used for materials modification. This usually is based on the use of high fluence to either mill the sample material or implant Nobel gas ions into the target material [2]. Here, we present a novel route utilizing a Helium Ion Microscope (HIM) to form nano–sized magnets of arbitrary shape using very low fluences (6 × 1014 cm−2) of 20 keV–25 keV Neon ions. The fine Neon beam available in the HIM is used to locally switch 40nm thin Fe60Al40 films from the well ordered paramagnetic B2 structure into the ferromagnetic A2 structure [3, 4]. Planar structures potentially useful for applications such as spin valves or other spin–transport devices have been formed this way. Kerr Microscopy and off–axis TEM holography has been used to analyze the resulting magnetic nano–structures. Results on the energy depended depth of magnetization as well as on the lateral definition of the magnetic structures due to scattering are presented.

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  • Lecture (Conference)
    62nd AVS meeting, 18.-23.10.2015, San Jose, USA

Permalink: https://www.hzdr.de/publications/Publ-23000


High resolution surface patterning with the Helium Ion Microscope

Hlawacek, G.

Abstract

In the past years Helium Ion Microscopy (HIM) [1] has become a mature imaging and nano-modification technique. The method is best known for its high resolution imaging capabilities. In addition it provides excellent charge compensation capabilities and a high surface sensitivity [2]. With the introduction of Ne as an working gas for the used Gas Field Ion Source (GFIS) also fast and high resolution nanomachining has become possible. In the following I would like to give a brief introduction of the technique. Subsequently, I will present examples of materials modification with a highly focused Helium or Neon beam.

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  • Invited lecture (Conferences)
    XXII International Conference on Ion-Surface Interactions, 20.-24.08.2015, Moskau, Russia

Permalink: https://www.hzdr.de/publications/Publ-22999


Application of Helium Ion Microscopy to surface science problems

Hlawacek, G.; Veligura, V.; Jankowski, M.; van Gastel, R.; Wormeester, R.; Zandvliet, H. J. W.; Poelsema, B.

Abstract

Helium Ion Microscopy (HIM) is well known for its high lateral resolution and unique nanomachining capabilities. In addition it is a very surface sensitive technique and therefore ideally suited to answer scientific questions in surface and interface science. I will give a brief introduction of the technique followed by a selection of problems related to surface and interface science.
The high surface sensitivity of HIM allowed us to measure the thickness of thin carbon layers present on gold nanorods. On the other hand one can use back scattered helium (BSHe) particles to reveal buried interfaces such as the diffusion front of a Pd2Si layer covered by more than 100 nm of SiO2.
The orientation of a sample can be determined using channeling. I will show that with a simple geometrical model channeling directions can be predicted with sufficient accuracy to align the He beam parallel to low index directions. Exploiting channeling into a crystalline sample the background from the substrate can be suppressed, thus enhancing the surface sensitivity even further. This has been used in a recent study of the surface reconstruction observed in the case of a few ML of Ag deposited on Pt(111). Based on a change of the work function of 25meV across the atomically flat terraces we can distinguish Pt rich from Pt poor areas and visualize the single atomic layer high steps between the terraces. Utilizing channeling/dechanneling and the exceptional surface sensitivity of the HIM we can measure the periodicity of the hcp/fcc pattern formed in the 2 ML thick Ag alloy layer. A periodicity of 6.65 nm along the <-1-12> surface direction has been measured. In terms of crystallography a hcp domain is obtained through a lateral displacement of a part of the outermost layer by 1/√3 of a nearest neighbor spacing along <-1-12>.

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  • Invited lecture (Conferences)
    1st International Conference on Applied Surface Science, 27.-30.07.2015, Shanghai, China

Permalink: https://www.hzdr.de/publications/Publ-22998


On the way to a quantitative analytical ion microscopy

Klingner, N.; Hlawacek, G.; Heller, R.; Facsko, S.

Abstract

Analytical Ion Micrsocope

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  • Lecture (Conference)
    Gas Ion Microscopy User Group Meeting 2015, 13.07.2015, Dublin, Ireland

Permalink: https://www.hzdr.de/publications/Publ-22997


Helium Ion Microscopy

Hlawacek, G.

Abstract

In the past years Helium Ion Microscopy (HIM) [1] has become a mature imaging and nano-modification technique. The method is best known for its high resolution imaging capabilities. In addition it provides excellent charge compensation capabilities and high surface
sensitivity [2]. With the introduction of Ne as an working gas for the used gas field ion source (GFIS) also fast high resolution nanomachining has become possible. In the following I would like to give a brief introduction to the technique. Subsequently I want to highlight the
importance of channeling to achieve the best possible imaging conditions and maximize the surface sensitivity. Finally I want to present selected results on high and low fluence He and Ne milling and implantation.

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  • Invited lecture (Conferences)
    mmc2015, 29.06.-02.07.2015, Manchester, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-22996


Materials analysis using channeling and ionoluminescence in a helium ion microscope

Hlawacek, G.

Abstract

Helium ion Microscopy1 is a versatile microscopy technique that provides high resolution imaging and nano-machining in combination with a high surface sensitivity and large depth of focus. It utilizes a narrow beam of He+ ions to achieve a lateral resolution of less than 0.5 nm. Backscattered Helium ions (BSHe) and secondary electrons (SE) can be used to obtain an image of the specimen.
When using crystalline samples channeling of the particles can occur. This effect can be exploited in several ways in the HIM. First of all it is possible to map out the different channeling directions and intensities and thus obtain information on the crystal structure of the sample. A simple geometrical model is introduced that can predict the channeling directions and relative intensities observed in the HIM2. By exploiting channeling and making use of the dechanneling contrast thin surface layers can be made visible in SE as well BSHe images3. We used this to observe composition and structural changes in a 2 ML thin silver layer on Pt(111). Work function differences as small as 40 meV between Ag and Pt rich areas on the surface reveal the position of mono—atomic surface steps. A regular arrangement of areas with reduces the channeling probability reveals the surface reconstruction of the top 2—3 ML which has a periodicity of only 5.8 nm.
Ionoluminescence on the other hand allows to obtain information on defects in the bulk of the material. I will show results obtained for a variety of materials including semiconductors4, rare earth containing perovskites and ionic crystals. The types of defects were identified and the influence of the scanning conditions on the IL signal has been investigated5. We used IL to map out the interaction volume of the beam in NaCl, and demonstrate the possibility of subsurface patterning. In our setup using a 35 keV He+ beam and NaCl only 3 vac/nm-2 are needed to obtain a detectable IL signal6.

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  • Invited lecture (Conferences)
    IBA2015, 14.-19.06.2015, Opatija, Croatia

Permalink: https://www.hzdr.de/publications/Publ-22995


Helium Ion Microscopy of atomic steps and surface reconstruction

Hlawacek, G.; Jankowski, M.; van Gastel, R.; Wormeester, H.; Zandvliet, H.; Poelsema, B.

Abstract

Helium Ion Microscopy (HIM) [1] is well known for its exceptional imaging and nanofabrication capabilities. HIM has an unprecedented surface sensitivity, and channeling can be utilized to maximise the signal to noise ratio. We demonstrate the resolving power of the technique using a thin (2 ML) silver layer on Pt(111). The obtained HIM results are compared to results obtained by low energy electron microscopy, spot profile analysis low energy electron diffraction (SPA-LEED), and atomic force microscopy phase contrast. In HIM single atom layer high steps can be visualized as a result of a work function change—across the otherwise atomically flat terraces—of only ~20 meV. By utilizing the dechanneling contrast mechanism [2] also the surface reconstruction of this thin surface layer can be revealed. We find a threefold periodic structure of channeling (fcc stacking) and dechanneling (hcp stacking) areas.
The periodicity—measured along the h112i surface direction—of this structure is 5.8 nm. This is in excellent agreement with values obtained by SPA-LEED [3].

  • Lecture (Conference)
    Physics Boat, 09.-11.06.2015, Helsinki, Stockholm, Finland, Sweden

Permalink: https://www.hzdr.de/publications/Publ-22994


He-ion Microscopy

Hlawacek, G.

Abstract

Tutorial on Helium Ion Microscopy

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  • Invited lecture (Conferences)
    Physics boat 2015, 08.06.2015, Helsinki, Finland

Permalink: https://www.hzdr.de/publications/Publ-22993


Relaxation dynamics in graphene: Surprising Coulomb scattering effects

Winnerl, S.

Abstract

We report on Coulomb scattering effects in graphene and Landau-quantized graphene.

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  • Lecture (others)
    3rd Workshop of the SPP "Graphene", 17.-20.05.2015, Kremmen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22992


Relaxation dynamics in graphene studied by THz radiation from the free-electron laser FELBE

Winnerl, S.

Abstract

The free-electron laser FELBE, which is operated as a user facility, provides tunable radiation in the mid infrared and terahertz spectral range (wavelength: 4 – 230 µm) in form of ps pulses. It is driven by a superconducting accelerator that enables continuous pulsing operation at a repetition rate of 13 MHz, making it highly attractive for many experiments. We briefly review a few types of experiments including non-perturbative nonlinear spectroscopy and near-field microscopy on systems like excitons in semiconductor quantum wells or electrons confined in quantum dots. Mainly we discuss time-resolved spectroscopy on graphene and in particular Landau quantized graphene. Here evidence for extremely efficient Auger scattering is found that can actually deplete a level that at the same time is optically pumped.

Keywords: Graphene; ultrafast dynamics; FEL

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  • Lecture (others)
    Semninar an der Uni Manchester, 25.11.2015, Manchester, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-22991


Coulomb scattering in the vicinity of the Dirac point in graphene

Winnerl, S.

Abstract

After a brief introduction into time-resolved spectroscopy we focus on polarization-resolved pump-probe experiments at different infrared photon energies. Here an anisotropy induced by the polarization of the pump beam is observed. For photon energies below the optical photon energy (~200 meV) the relaxation dynamics is dominated by Coulomb scattering. The experiments show directly that non-collinear scattering, which leads to an isotropic distribution, is remarkably slow at low fluences, namely on a timescale of 5 ps. The findings are in good agreement with microscopic theory. The timescale of this thermalization is very attractive for applications where hot carriers in graphene are exploited in detectors or modulators of infrared radiation.

Keywords: Graphene; ultrafast dynamics; Coulomb scattering

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  • Invited lecture (Conferences)
    International workshop on many-body phenomena in graphene, 26.-27.10.2015, Gothenburg, Schweden

Permalink: https://www.hzdr.de/publications/Publ-22990


Coulomb Scattering in Graphene

Winnerl, S.; Mittendorff, M.; Otto, J.; Wendler, F.; Winzer, T.; Malic, E.; Knorr, A.; Schneider, H.; Helm, M.

Abstract

Coulomb interaction is the main mechanism that transforms a nonequilibrium carrier distribution in graphene into a hot Fermi-Dirac distribution. In many experiments carriers are assumed to be thermalized on a timescale well below 100 fs. However, we have recently observed that the carrier distribution is strongly anisotropic in k-space when graphene is excited with near infrared pulses (photon energy 1.5 eV, duration 30 fs). The anisotropy induced by the polarization of light vanishes on a timescale of 150 fs due to scattering with optical phonons [1]. Exciting with photon energies of 75 meV, i.e. well below the optical phonon energy (~200 meV), strongly quenches the scattering via phonons and allows one to study pure Coulomb scattering in the vicinity of the Dirac point. At low fluences the transition from the anisotropic distribution to an isotropic one is very slow (~5 ps at 10 K) since Coulomb scattering is predominantly collinear and thus preserves the anisotropic distribution. At higher fluences the strength of Coulomb scattering increases, resulting in a faster decay of the anisotropy. In a second experiment we study the relaxation dynamics in Landau quantized graphene [2].
By applying circularly polarized radiation in the pump-probe experiments individual low-index Landau level transitions can be addressed. Here a surprising effect is revealed, namely a change in sign of a pump-probe signal with respect to the expectation considering single-particle interactions only (cf.Fig.1). This is caused by strong Auger scattering that depletes the zeroth Landau level even though it is optically pumped at the same time.

[1] M. Mittendorff. et al. Nano Lett., 14 2014, 1504
[2] M. Mittendorff. et al. Nature Phys., 11 2015, 75

Keywords: Graphene; ultrafast spectroscopy; carrier dynamics; Coulomb scattering; Landau quantization

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  • Lecture (Conference)
    Nanospectroscopy for two-dimensional materials, 08.-10.09.2015, Chemnitz, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22989


Strong Auger scattering in Landau-quantized graphene

Winnerl, S.; Mittendorff, M.; Wendler, F.; Malic, E.; Knorr, A.; Schneider, H.; Helm, M.

Abstract

While the carrier dynamics in graphene in absence of magnetic fields is well researched in a large spectral range ranging from UV to THz, the dynamics in Landau quantized graphene is almost unexplored. We investigate the carrier dynamics within the system of Landau levels (LLs) of index n = -1, n = 0 and n = 1 by pump-probe experiments complemented by microscopic modelling. Using circularly polarized mid-infrared radiation (photon energy 75 meV) allows one to selectively excite the two energetically degenerate transitions LL-1 → LL0 and LL-1 → LL0, respectively. Surprisingly, induced transmission is observed in one configuration of pumping and probing with opposite configuration (cf. Fig 1 (c) and (d)). Considering single particle Pauli blocking, one would expect induced absorption in this case. The sign change indicates that LL0 is depleted by strong Auger scattering, even though it is optically pumped at the same time.
We discuss the role of carrier-carrier and carrier-phonon scattering in Landau quantized graphene and provide an outlook on the application potential of this system for tunable THz lasers.

References
[1] M. Mittendorff, F. Wendler, E. Malic, A. Knorr, M. Orlita, M. Potemski, C. Berger, W. A. de Heer, H. Schneider, M. Helm, and S. Winnerl, " Nature Phys. 11, 75 (2015).

Keywords: Graphene; Landau quantization; Auger scattering; Terahertz

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  • Lecture (Conference)
    Graphene Week 2015, 22.-26.06.2015, Manchester, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-22988


Einblick in die Relaxationsdynamik von Graphen mittels Infrarot-Kurzzeitspektroskopie: überraschende Coulomb-Streueffekte

Winnerl, S.

Abstract

Graphen, ein zweidimensionaler Kohlenstoffkristall weist eine Bandstruktur ohne Bandlücke auf. Im Bereich kleiner Energien ist die Dispersion linear und Elektronen und Löcher verhalten sich perfekt symmetrisch. Ihr Verhalten entspricht masselosen Dirac-Fermionen. Nach einem kurzen Überblick über grundlegende Eigenschaften von Graphen und dem Anwendungspotential dieses Materials stellen wir einige unserer Experimente zur Ladungsträgerdynamik vor.
Wird Graphen mit kurzen, linear polarisierten nahinfraroten Lichtpulsen angeregt, so beobachten wir eine im k-Raum anisotrope Ladungsträgerverteilung. Erstaunlicher Weise bleibt diese Anisotropie für 150 fs erhalten, obwohl die Elektron-Elektron Streuzeit in dem System viel kürzer ist. Wir zeigen, dass Energie- und Impulserhaltung für diese Anisotropie-erhaltende Wirkung der Coulomb-Streuung verantwortlich sind und eine isotrope Ladungsträgerverteilung durch Elektron-Phonon-Streuung erreicht wird. Verwenden wir zur Anregung Strahlung mit Photonenergie kleiner als die optische Phonon-Energie, so bleibt die Anisotropie auf der Skala von 1 – 10 ps erhalten. Für diese Experimente dient der Freie-Elektronenlaser FELBE als Strahlungsquelle.
Schließlich diskutieren wir die Ladungsträgerdynamik zwischen Landau-Niveaus von Graphen im Magnetfeld. Hierfür wurde zirkular polarisierte Infrarotstrahlung von FELBE eingesetzt. Hier gibt es aufgrund der Coulomb-Wechselwirkung einen sehr verblüffenden Effekt, nämlich dass ein Landauniveau entvölkert wird, obwohl es durch optisches Pumpen mit Ladungsträgern befüllt wird.
Zusammenfassend zeigen unsere Experimente, dass man mit Licht unterschiedlicher Wellenlänge und unterschiedlicher Polarisationszustände tiefe Einblicke in die Dynamik stark Coulomb-wechselwirkenden Elektronen in Graphen erhalten kann.

Keywords: Graphen; Relaxationsdynamik; Kurzzeit-Spektroskopie

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  • Invited lecture (Conferences)
    26. Edgar Lüscher Seminar 2015, 07.02.2015-13.01.2016, Klosters, Schweiz

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Terahertz spectroscopy at HZDR

Schneider, H.

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  • Invited lecture (Conferences)
    International consortium on terahertz photonics and optoelectronics conference, 16.-17.12.2015, Moskau, Russland

Permalink: https://www.hzdr.de/publications/Publ-22986


Applications of Helium Ion Microscopy

Hlawacek, G.

Abstract

HIM is well known for its exceptional imaging and nanofabrication capabilities. After a brief introduction of the gas field ion source and the ion microscope, I will present a wide range of results obtained with either the Twente UHV Orion+ or the NanoFab at the HZDR in Dresden. Special emphasis will be given to the use of channeling and the role of defects created by the energetic ion beam. Ionoluminescence is used to obtain information on the latter. Helium Ion Microscopy has an unprecedented surface sensitivity. Recent results obtained on thin silver layers on Pt(111) demonstrate that work function differences as small as ~20 meV as well as surface reconstructions can be visualized. Finally, some preliminary results of Neon based materials modification and cross section preparation will be presented.

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  • Invited lecture (Conferences)
    49th Annual Meeting of the Israel Society for Microscopy, 17.-18.05.2015, Bar Ilan, Israel

Permalink: https://www.hzdr.de/publications/Publ-22985


Microscopy of (electronic) materials

Hlawacek, G.

Abstract

Microscopy of (electronic) materials

  • Invited lecture (Conferences)
    Festveranstaltung 30 Jahre Werkstoffe der Elektronik, 25.03.2015, Leoben, Österreich

Permalink: https://www.hzdr.de/publications/Publ-22984


Exploiting channelling in Helium Ion Microscopy

Hlawacek, G.

Abstract

Helium ion Microscopy1⁠ is a versatile microscopy technique that provides high resolution imaging and nano-machining in combination with a high surface sensitivity and large depth of focus. It utilizes a narrow beam of He+ ions to achieve a lateral resolution of less than 0.5 nm. Backscattered Helium ions (BSHe) and secondary electrons (SE) can be used to obtain an image of the specimen. While the first one will provide information of the bulk the latter is extremely surface sensitive2⁠.
When using crystalline samples channeling of the particles can occur. This effect can be exploited in several ways in the HIM. First of all it is possible to map out the different channeling directions and intensities and thus obtain information on the crystal structure of the sample. A simple geometrical model is introduced that can predict the channeling directions and relative intensities observed in the HIM. Such a map of the channeling directions for a fcc metal is presented in figure 13⁠. Channeling is also important for many imaging applications. The contrast on thin surface layers in SE mode can be enhanced when channeling is considered. For BSHe images the situation is more complicated as the signal is dominated by the bulk. Only heavy element adlayers on light element substrates can easily be imaged in BSHe mode. However, the dechanneling contrast also allows the visualization of light elements on heavy element substrates4⁠. In figure 2 a thin organic layer on a silicon wafer is made visible in SE and BSHe mode. By exploiting channeling and making use of the dechanneling contrast thin surface adlayers can be made visible in SE as well BSHe images5⁠.

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  • Invited lecture (Conferences)
    EBSD 2015, 30.-31.03.2015, Glasgow, United Kingdom

Permalink: https://www.hzdr.de/publications/Publ-22983


Mechanisms of metal induced crystallization analyzed by in situ Rutherford Backscattering Spectroscopy

Wenisch, R.; Hanf, D.; Lungwitz, F.; Heller, R.; Hübner, R.; Gemming, S.; Krause, M.

Abstract

Metal induced crystallization (MIC) is a promising technique for low temperature thin film transistor fabrication and graphene synthesis. In MIC, a transition metal acts as seed for the crystallization of an amorphous group IV element. Bond screening near the interface and facilitation of nucleation are recently discussed as mechanisms for MIC. So far, in situ studies have been performed using X-ray diffraction, which is sensitive to the degree of crystallinity but lacks depth resolution. A better insight into the MIC mechanisms requires depth resolved in situ studies in order to determine the concentration profiles of the diffusing components.

Here, the Si/Ag and C/Ni bilayer systems are studied. They are annealed at temperatures of up to 750 °C. Simultaneously, the layer composition and the compositional profiles are investigated with in situ Rutherford backscattering spectroscopy revealing the diffusion kinetics of the components. Both, the quick initial nucleation and the ensuing growth processes are investigated. Further characterization is performed employing in vacuo Raman spectroscopy revealing the phase structure of the resulting films and scanning electron microscopy to investigate the surface structure.

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  • Poster
    DPG Frühjahrstagung, 15.-20.03.2015, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-22982


In situ Study of Metal Induced Crystallization Processes for Low-Dimensional Materials Synthesis

Wenisch, R.; Heller, R.; Hanf, D.; Hübner, R.; Lungwitz, F.; Schumann, E.; Gemming, S.; Krause, M.

Abstract

Metal induced crystallization (MIC) is a promising technique for thin film transistor fabrication and graphene synthesis. In MIC, a transition metal catalyzes the crystallization of the amorphous phase of a group IV element by bond screening near the interface and facilitation of nucleation. So far, in situ studies have been performed using X-ray diffraction which is sensitive to the degree of crystallinity. However this technique lacks depth resolution and is therefore unable to track diffusion and layer exchange.
Here, the Si/Ag and C/Ni bilayer systems are studied. The samples are annealed at temperatures of up to 750 °C. Simultaneously, depth profiles of the elements are investigated by in situ Rutherford backscattering spectroscopy revealing the diffusion kinetics. The changes in the phase structure are explored by in situ Raman spectroscopy. Both the quick initial nucleation and ensuing growth processes are investigated. Scanning electron microscopy provides insight to the surface morphology.

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  • Poster
    International Winterschool on Electronic Properties of Novel Materials, 08.-13.03.2015, Kirchberg in Tirol, Österreich

Permalink: https://www.hzdr.de/publications/Publ-22981


Ion Beam Analysis in the Helium Ion Microscope

Heller, R.; Klingner, N.; Facsko, S.; von Borany, J.; Gnauck, P.; Hlawacek, G.

Abstract

Helium ion microscopes (HIM) have turned into a frequently used imaging device in several laboratories around the world. Beside a sub nano-meter resolution and its high field of depth the latest generation of HIM devices (Zeiss Orion NanoFab) offers the ability to make use of Neon ions enabling additional opportunities in terms of surface modifications on the nm scale [1].

While the image generation in a HIM is based on evaluating the amount of secondary electrons the information carried by the back-scattered He/Ne projectiles (BSP) is not taken into consideration at the moment. Thus the HIM offers excellent topographic imaging capabilities but chemical information (in terms of elemental composition) of the surface is not accessible. Nevertheless back-scattered particles carry that information and may be used to provide additional contrast mechanism(s). First attempts to measure BSP energy spectra were done by Sijbrandij et al. [2] and gave evidence for the general feasibility but also revealed that a quantitative chemical analysis of thin layers would require development of more sophisticated detection concepts than those used in their experiments (silicon surface barrier detector).

In the present contribution we show the development and the implementation of a Time-of-Flight back-scattering spectrometry (ToF-BS) technique within our HIM. Pulsing the primary ion beam by using the existing beam blanker with a customized pulsing electronics enables us to generate pulses as short as below 10ns. BSP detection is done by means of a micro channel plate detector. Our measurements demonstrate that this technique is capable to achieve an energy resolution as good as 2keV (for 30keV He incident ions) by simultaneously keeping the spatial resolution in the order of a few 100nm. We further show that with some slight modification the presented setup can be utilized to acquire ToF spectra of sputtered particles as well, thus enabling lateral resolved ToF-SIMs within the HIM.

[1] G. Hlawacek, V. Veligura, R. van Gastel, and B. Poelsema, J. Vac. Sci. Technol. B 32(2), 2014, 020801.
[2] S. Sijbrandij, B. Thompson, J. Notte, B. W. Ward and N. P. Economou, J. Vac. Sci. Technol. B, 26(6), 2008, 2103-2106

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  • Lecture (Conference)
    22nd International Conference on Ion Beam Analysis, 16.06.2015, Opatija, Croatia

Permalink: https://www.hzdr.de/publications/Publ-22980


KMC simulation of irradiation-induced nanostructure evolution in Oxide Dispersion Strengthened Fe-Cr alloys

Liedke, B.; Posselt, M.; Murali, D.; Abdou, J. M.; Claisse, A.; Olsson, P.

Abstract

Oxide Dispersion Strengthened (ODS) steels are considered as one of the most promising candidates for structural materials in next generation nuclear fusion reactors and future nuclear fission reactors [1]. The ODS materials consist of a ferritic or ferritic/martensitic Fe-Cr matrix filled with yttria-based oxide particles and is fabricated during mechanical alloying and hot consolidation processes. It is well known that their extraordinary properties such as high-temperature creep strength as well as high dose ion/neutron irradiation resistance are due to formation of small Y-Ti-O clusters with a size of few nanometers. Besides their significant effect on reduction of dislocations and grain-boundaries mobility, the nanoclusters also act as traps for point defects like vacancies, interstitials and helium, which may be typically generated in a nuclear reactor. It is still under debate what the formation mechanisms of the nanoclusters are and why they prove such high temperature and radiation damage stability.
Experimental methods typically applied to investigate the issues stated above cannot fully reflect the atomic-scale of the nanoclusters, as well as the mechanisms related to their formation, evolution and destruction upon radiation damage. Therefore, atomistic computer experiments can significantly contribute to a general understanding.
In this work, kinetic Monte Carlo (KMC) technique is applied to study evolution of Y-Ti-O nanoclusters in a bcc-Fe and FeCr matrix. Starting from a uniform distribution of O, Y, Ti atoms in the matrix at first a stationary state is produced by high temperature annealing. Such a state is characterized by a certain population of Y-Ti-O nanoclusters. Then vacancies and interstitials are introduced in order to simulate ion and neutron irradiation taking into account realistic conditions, and the evolution of the nanostructure is studied. The parameters for the atomic interactions used in KMC were obtained recently by first-principle Density-Functional-Theory calculations and applied in Metropolis Monte Carlo simulations on energetics, structure and composition of the Y-Ti-O nanoclusters [2].
1. G. R. Odette, M. J. Alinger, B. D. Wirth, Annu. Rev. Mater. Res. 38, 471 (2008)
2. M. Posselt, D. Murali, B. K. Panigrahi, Modelling Simul. Mater. Sci. Eng. 22, 085003 (2014)

Keywords: Oxide Dispersion Strengthened steels; ODS; kinetic Monte-Carlo; density functional theory

  • Lecture (Conference)
    Advances in Materials and Processing Technologies (AMPT), 14.-17.12.2015, Madird, Spain

Permalink: https://www.hzdr.de/publications/Publ-22979


Development of a new radiofluorinated quinoline analog for PET imaging of phosphodiesterase 5 (PDE5) in brain.

Liu, J.; Wenzel, B.; Dukic-Stefanovic, S.; Teodoro, R.; Ludwig, F.-A.; Deuther-Conrad, W.; Schröder, S.; Chezal, J.-M.; Moreau, E.; Brust, P.; Maisonial-Besset, A.

Abstract

Phosphodiesterases (PDEs) are enzymes that play a major role in cell signalling by hydrolysing the second messengers cyclic adenosine monophosphate (cAMP) and/or cyclic guanosine monophosphate (cGMP) throughout the body and brain. Altered cyclic nucleotide-mediated signalling has been associated with a wide array of disorders, including neurodegenerative disorders. Recently, PDE5 has been shown to be involved in neurodegenerative disorders such as Alzheimer's disease but its precise role has not been elucidated yet. To visualize and quantify the expression of this enzyme in brain, we developed a radiotracer for specific PET imaging of PDE5. A quinoline based lead compound has been structurally modified resulting in the fluoroethoxymethyl derivative ICF24027 with high inhibitory activity towards PDE5 (IC50 = 1.89 nM). Radiolabelling with fluorine-18 was performed by a one-step nucleophilic substitution reaction using a tosylate precursor (RCY(EOB) = 12.9 ± 1.8%; RCP >99%; SA(EOS) = 70-126 GBq/µmol). In vitro autoradiographic studies of [18F]ICF24027 on different mouse tissue as well as on porcine brain slices demonstrated a moderate specific binding to PDE5. In vivo studies in mice revealed that [18F]ICF24027 was metabolized under formation of brain penetrable radiometabolites making the radiotracer not suitable for PET imaging of PDE5 in brain.

Keywords: PDE5; PET imaging; Fluorine-18; quinoline; Micellar chromatography

Permalink: https://www.hzdr.de/publications/Publ-22978


NMR shift and relaxation measurements in pulsed high-field magnets up to 58 T

Kohlrautz, J.; Reichardt, S.; Green, E. L.; Kühne, H.; Wosnitza, J.; Haase, J.

Abstract

Nuclear magnetic resonance (NMR) experiments at fields up to 58 T in pulsed magnets at the Dresden High Magnetic Field Laboratory are reported. The challenge to resolve NMR shifts in these timedependent fields is addressed for the first time, and it is shown that this can indeed be accomplished with high precision with an internal reference. As a result, signal averaging is possible during a single magnetic field pulse, but also for multiple pulses. Thus, even very weak signals can in principle be recorded and their shifts can be determined. In a second set of experiments, the measurement of nuclear relaxation is investigated. Using adiabatic inversion with the inherent time dependence of the magnetic field and small-angle inspection, it is shown that relaxation measurements are possible, as well. The shift experiments were performed with 27Al NMR on a mixture of aluminum metal and a Linde type A zeolite. For the relaxation studies, 27Al NMR and 69Ga NMR on the metals aluminum and gallium were preformed, respectively.

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  • High Magnetic Field Laboratory (HLD)

Permalink: https://www.hzdr.de/publications/Publ-22977


Electron spin resonance modes in a strong-leg ladder in the Tomonaga-Luttinger liquid phase

Ozerov, M.; Maksymenko, M.; Wosnitza, J.; Honecker, A.; Landee, C. P.; Turnbull, M. M.; Furuya, S. C.; Giamarchi, T.; Zvyagin, S.

Abstract

Magnetic excitations in the strong-leg quantum spin ladder compound (C7H10N)2CuBr4 (known as DIMPY) in the field-induced Tomonaga-Luttinger spin-liquid phase are studied by means of high-field electron spin resonance (ESR) spectroscopy. The presence of a gapped ESR mode with unusual nonlinear frequency-field dependence is revealed experimentally. Using a combination of analytic and exact-diagonalization methods, we compute the dynamical structure factor and identify this mode with longitudinal excitations in the antisymmetric channel. We argue that these excitations constitute a fingerprint of the spin dynamics in a strong-leg spin-1/2 Heisenberg antiferromagnetic ladder and owe their ESR observability to the uniform Dzyaloshinskii-Moriya
interaction.

Involved research facilities

  • High Magnetic Field Laboratory (HLD)

Permalink: https://www.hzdr.de/publications/Publ-22976


Years: 2023 2022 2021 2020 2019 2018 2017 2016 2015


Pages: [1.] [2.] [3.] [4.] [5.] [6.] [7.] [8.] [9.] [10.] [11.] [12.] [13.] [14.] [15.] [16.] [17.] [18.] [19.] [20.] [21.] [22.] [23.] [24.] [25.] [26.] [27.] [28.] [29.] [30.] [31.] [32.] [33.] [34.] [35.] [36.] [37.] [38.] [39.] [40.] [41.] [42.] [43.] [44.] [45.] [46.] [47.] [48.] [49.] [50.] [51.] [52.] [53.] [54.] [55.] [56.] [57.] [58.] [59.] [60.] [61.] [62.] [63.] [64.] [65.] [66.] [67.] [68.] [69.] [70.] [71.] [72.] [73.] [74.] [75.] [76.] [77.] [78.] [79.] [80.] [81.] [82.] [83.] [84.] [85.] [86.] [87.] [88.] [89.] [90.] [91.] [92.] [93.] [94.] [95.] [96.] [97.] [98.] [99.] [100.] [101.] [102.] [103.] [104.] [105.] [106.] [107.] [108.] [109.] [110.] [111.] [112.] [113.] [114.] [115.] [116.] [117.] [118.] [119.] [120.] [121.] [122.] [123.] [124.] [125.] [126.] [127.] [128.] [129.] [130.] [131.] [132.] [133.] [134.] [135.] [136.] [137.] [138.] [139.] [140.] [141.] [142.] [143.] [144.] [145.] [146.] [147.] [148.] [149.] [150.] [151.] [152.] [153.] [154.] [155.] [156.] [157.] [158.] [159.] [160.] [161.] [162.] [163.] [164.] [165.] [166.] [167.] [168.] [169.] [170.] [171.] [172.] [173.] [174.] [175.] [176.] [177.] [178.] [179.] [180.] [181.] [182.] [183.] [184.] [185.] [186.] [187.] [188.] [189.] [190.] [191.] [192.] [193.] [194.] [195.] [196.] [197.] [198.] [199.] [200.] [201.] [202.] [203.] [204.] [205.] [206.] [207.] [208.] [209.] [210.] [211.] [212.] [213.] [214.] [215.] [216.] [217.] [218.] [219.] [220.] [221.] [222.] [223.] [224.] [225.] [226.] [227.] [228.] [229.] [230.] [231.] [232.] [233.] [234.] [235.] [236.] [237.] [238.] [239.] [240.] [241.] [242.] [243.] [244.] [245.] [246.] [247.] [248.] [249.] [250.] [251.] [252.] [253.] [254.] [255.] [256.] [257.] [258.] [259.] [260.] [261.] [262.] [263.] [264.] [265.] [266.] [267.] [268.] [269.] [270.] [271.] [272.] [273.] [274.] [275.] [276.] [277.] [278.] [279.] [280.] [281.] [282.] [283.] [284.] [285.] [286.] [287.] [288.] [289.] [290.] [291.] [292.] [293.] [294.] [295.] [296.] [297.] [298.] [299.] [300.] [301.] [302.] [303.] [304.] [305.] [306.] [307.] [308.] [309.] [310.] [311.] [312.] [313.] [314.] [315.] [316.] [317.] [318.] [319.] [320.] [321.] [322.] [323.] [324.] [325.] [326.] [327.] [328.] [329.] [330.] [331.] [332.] [333.] [334.] [335.] [336.] [337.] [338.] [339.] [340.] [341.] [342.] [343.] [344.] [345.] [346.] [347.] [348.] [349.] [350.] [351.] [352.]