Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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34121 Publications

A review of rare earth minerals flotation: Monazite and Xenotime

Chehreh Chelgani, S.; Rudolph, M.; Leistner, T.; Gutzmer, J.; Peuker, U. A.

This paper reviews rare earth minerals (monazite and xenotime) separation by flotation. A wide range of monazite and xenotime flotation test results are reviewed including: reasons of variation in the point of zero charges on these minerals, the effects of various flotation conditions on zeta potential and the point of zero charges on the surface of monazite and xenotime, interactions of collectors and depressants on the surface of these mineral phases during flotation separation, relationship between surface chemistry of the minerals and different types of collector adsorptions, effects of the conditioning temperature on flotation of rare earth minerals. This review summarizes various approaches for selective separation of monazite and xenotime by flotation for further research in future.

Keywords: Flotation; Rare earth; Monazite; Xenotime; IEP; Hydroxamate; Sodium oleate

Permalink: https://www.hzdr.de/publications/Publ-21570
Publ.-Id: 21570


Charge trapping of Ge-nanocrystals embedded in TaZrOx dielectric films

Lehninger, D.; Seidel, P.; Geyer, M.; Schneider, F.; Klemm, V.; Rafaja, D.; von Borany, J.; Heitmann, J.

Ge-nanocrystals (NCs) were synthesized in amorphous TaZrOx by thermal annealing of co-sputtered Ge-TaZrOx layers. Formation of spherical shaped Ge-NCs with small variation of size, areal density, and depth distribution was confirmed by high-resolution transmission electron microscopy. The charge storage characteristics of the Ge-NCs were investigated by capacitance-voltage and constant-capacity measurements using metal-insulator-semiconductor structures. Samples with Ge-NCs exhibit a maximum memory window of 5 V by sweeping the bias voltage from −7 V to 7 V and back. Below this maximum, the width of the memory window can be controlled by the bias voltage. The fitted slope of the memory window versus bias voltage characteristics is very close to 1 for samples with one layer Ge-NCs. A second layer Ge-NCs does not result in a second flat stair in the memory window characteristics. Constant-capacity measurements indicate charge storage in trapping centers at the interfaces between the Ge-NCs and the surrounding materials (amorphous matrix/tunneling oxide). Charge loss occurs by thermal detrapping and subsequent band-to-band tunneling. Reference samples without Ge-NCs do not show any memory window

Permalink: https://www.hzdr.de/publications/Publ-21569
Publ.-Id: 21569


Towards a novel THz-based monitor for sub picosecond electron bunches working at MHz repetition rates and low bunch charges

Kovalev, S.; Green, B.; Golz, T.; Kuntzsch, M.; Fisher, A.; Stojanovic, N.; Gensch, M.

Femtosecond level diagnostic and control of sub-picosecond electron bunches is an important topic in modern accelerator research. At the same time new quasi-cw linear electron accelerators are the drivers of many future 4th Generation lightsources such as X-ray free electron lasers. A high duty cycle, high stability and online pulse to pulse diagnostic of these new accelerators are crucial ingredients to the success of these large scale facilities. A novel THz based online monitor concept is presented that has the potential to give access to pulse to pulse information on bunch form, arrival time and energy at high repetition rate and down to sub pC charges. It has been shown experimentally that pulse to pulse arrival time measurements can be used to perform high temporal resolution and dynamic range experiments, removing the influence of synchronization problems between the accelerator and external laser systems.

  • Poster
    4th workshop on longitudinal instrumentation for future accelerators, 15.-16.01.2015, Villigen, Switzerland

Permalink: https://www.hzdr.de/publications/Publ-21567
Publ.-Id: 21567


Re-Green Indicator System

Dirlich, S.

The presentation deals with the development of an indicator system for the sustainability assessment of green policies dedicated to retrofitting of buildings, urban districts, and entire cities.
Based on existing building sustainability assessment schemes a specific indicator system is developed meeting the requirements of the Re-Green project.
The flexible and modular structure of the system allows an adaptation of the sustainability assessment to the particular circumstances and conditions of the region/city to be evaluated.

Keywords: sustainability assessment; green building; retrofit

  • Invited lecture (Conferences)
    FutureBuild 2014, 05.-06.11.2014, Sheffield, Großbritannien

Permalink: https://www.hzdr.de/publications/Publ-21566
Publ.-Id: 21566


Automated mineralogical analysis of secondary raw materials – challenges and pitfalls

Dressler, S.; Renno, A. D.; Bachmann, K.; Schaefer, J.

Secondary raw materials are becoming increasingly more important in ensuring the stability of critical metal supply. Ashes, slags, dusts and other industrial residues are produced in large quantities.
Precise and accurate chemical and mineralogical data, knowledge of distribution of valuable and deleterious elements in the single phases as well as information about homogeneity and grain size distribution of the minerals are crucial for the development of new extraction technologies.
Gaining these essential information can be achieved by using SEM-based automated mineralogical analysis. However, the large particle size range, the dominance of very small grain sizes (< 5 µm) and the diversity of phases are challenging for the analysis. Furthermore, in contrast to natural materials the analysis of secondary materials faces the challenge of developing new methods for non-natural extreme combinations of elements and phases. Initial results of ash and slag samples will be presented and evaluated.

  • Poster
    ANAKON, 23.-26.03.2015, Graz, Österreich

Permalink: https://www.hzdr.de/publications/Publ-21565
Publ.-Id: 21565


Role of electric and magnetic dipole strength functions in the 114Cd(gamma,gamma') and 113Cd(n, gamma) reactions

Massarczyk, R.; Schramm, G.; Belgya, T.; Schwengner, R.; Beyer, R.; Bemmerer, D.; Elekes, Z.; Grosse, E.; Hannaske, R.; Junghans, A. R.; Kis, Z.; Kögler, T.; Lorenz, C.; Schmidt, K.; Szentmiklosi, L.; Wagner, A.; Weil, J. L.

The distribution of the electromagnetic dipole strength below the neutron separation energy and its influence on the photon distribution after neutron capture was investigated in a twin experiment for the compound nucleus 114Cd. The experimentally deduced spectra after photon scattering on 114Cd and the neutron capture in 113Cd were analyzed in matters of electro magnetic strength function and nuclear level density. By measuring the photoabsorption cross section at the bremsstrahlung facility gammaELBE at the Helmholtz-Zentrum Dresden-Rossendorf it was possible to deduce information about the distribution of dipole strength below the neutron separation energy. The deexcitation spectrum after cold-neutron capture in 113 Cd was calculated and different magnetic dipole strength function models were tested with the help of the statistical code gammaDEX.

Keywords: Photon scattering; nuclear resonance fluorescence; strength functions; cross sections

Permalink: https://www.hzdr.de/publications/Publ-21564
Publ.-Id: 21564


High magnetic field study of the Dy2Fe17Hx compounds with x = 0-3.8

Isnard, O.; Andreev, A. V.; Heczko, O.; Skourski, Y.

The Dy2Fe17Hx compounds crystallizing in a hexagonal Th2Ni17-like structure are studied on aligned powder by magnetization measurements carried out in steady (up to 5 T) and pulsed (60 T) magnetic fields at 4.2-300 K. Dy2Fe17 is a ferrimagnet with TC = 375 K and a spontaneous moment of 16.5 μB/f.u. Magnetization curves recorded for two different crystal orientations for Dy2Fe17Hx polycrystalline oriented samples show that the insertion of hydrogen in the Dy2Fe17 crystal lattice significantly modifies the magnetization, Curie temperature and intersublattice exchange interactions of the Dy2Fe17 phase. The high-field behavior of Dy2Fe17 is compared with that of its hydrides derivatives. Despite different types of magnetic anisotropy, the high-field behavior of the hydrides and the parent compound is qualitatively similar. Depending upon the composition, they can exhibit easy plane or else conical type anisotropy as for x = 0 and 3, respectively. The low temperature spontaneous magnetization exhibits a moderate composition dependence, it first decreases continuously upon increasing the hydrogen composition up to x = 3 then slightly increases for x = 3.8 reflecting the Fe sublattice evolution. A mean Fe moment of about 2.1 μB is derived for Dy2Fe17H3.8 composition, this magnetization value is close to that of the original Dy2Fe17 compound. It is found that the intersublattice coupling between the Fe and Dy sublattices is reduced as illustrated by the decrease of the nDy-T coefficient from about 3.3 down to 2.82 T f.u./μB for Dy2Fe17 and Dy2Fe17H3.8, respectively.

Permalink: https://www.hzdr.de/publications/Publ-21563
Publ.-Id: 21563


PIConGPU - The World’s Fastest Particle-in-Cell Plasma Simulation

Pausch, R.; Burau, H.; Bussmann, M.; Debus, A.; Eckert, C.; Garten, M.; Huebl, A.; Juckeland, G.; Kluge, T.; Knespel, M.; Schmitt, F.; Schumann, C.; Steiniger, K.; Widera, R.; Worpitz, B.

PIConGPU is currently the fastest particle-in-cell code available. This talk explains how using GPUs increased the performance of this simulation code making parameter scans of previously hard to compute plasma setups possible. Additionally newly developed techniques are discussed and an outlook on simulations of foil targets and high harmonic generation are presented.

Keywords: PIConGPU; particle-in-cell code; GPU; high harmonic generation

  • Invited lecture (Conferences)
    Workshop on PIConGPU and high harmonic generation, 12.-16.01.2015, Jena, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21562
Publ.-Id: 21562


Highly transparent and conductive Al-doped ZnO films synthesized by pulsed laser co-ablation of Zn and Al targets assisted by oxygen plasma

You, Q.; Cai, H.; Gao, K.; Hu, Z.; Guo, S.; Liang, P.; Sun, J.; Xu, N.; Wu, J.

Highly optically transparent and electrically conductive Al-doped ZnO (AZO) thin films were deposited by pulsed laser co-ablation of a Zn target and an Al target in an oxygen plasma. Zn ablation resulted in the reactive deposition of ZnO films assisted by the plasma, while Al ablation provided the growing ZnO films with Al dopants. The morphology, composition and structure as well as the optical and electrical properties were characterized and the effects of Al doping and annealing treatment were investigated. The deposited AZO films have a hexagonal wurtzite structure with deteriorated crystal quality which can be improved by annealing. The AZO films are highly transparent from ultraviolet up to 1450 nm and present an obvious blue shift in absorption edge and a widening of band gap compared with undoped ZnO. The electrical properties were also improved after annealing with the resistivity decreasing by over two orders of magnitude because of the increase of free carrier concentration. The variation in the carrier concentration also affects the absorption edge and the band gap of the films as well as the transparency in the infrared region. Meanwhile, this method offers an approach for in-situ doping preparation of other doped compound films with different dopant concentrations.

Keywords: Al-doped ZnO; Co-ablation deposition; Electrical properties; In-situ doping; Optical properties; Thin films

Permalink: https://www.hzdr.de/publications/Publ-21561
Publ.-Id: 21561


Study of the p\vec{d}→n{pp}s charge-exchange reaction using a polarised deuterium target

Gou, B.; Mchedlishvili, D.; Bagdasarian, Z.; Barsov, S.; Garbonell, J.; Chiladze, D.; Dymov, S.; Engels, R.; Gaisser, M.; Gebel, R.; Grigoryev, K.; Hartmann, M.; Kacharava, A.; Khoukaz, A.; Kulessa, P.; Kulikov, A.; Lehrach, A.; Li, Z.; Lomidze, N.; Lorentz, B.; Macharashvili, G.; Merzliakov, S.; Mielke, M.; Mikirtychyants, M.; Mikirtychyants, S.; Nioradze, M.; Ohm, H.; Prasuhn, D.; Rathmann, F.; Serdyuk, V.; Shmakova, V.; Ströher, H.; Tabidze, M.; Trusov, S.; Tsirkov, D.; Uzikov, Y.; Valdau, Y.; Wang, T.; Weidemann, C.; Wilkin, C.; Yuan, X.

The vector and tensor analysing powers, Ayand Ayy, of the p d→n{pp}scharge-exchange reaction have been measured at a beam energy of 600MeV at the COSY-ANKE facility by using an unpolarised proton beam incident on an internal storage cell target filled with polarised deuterium gas. The low energy recoiling protons were measured in a pair of silicon tracking telescopes placed on either side of the target. Putting a cut of 3MeV on the diproton excitation energy ensured that the two protons were dominantly in the 1S0state, here denoted by {pp}s. The polarisation of the deuterium gas was established through measurements in parallel of proton–deuteron elastic scattering. By analysing events where both protons entered the same telescope, the charge-exchange reaction was measured for momentum transfers q ≥160MeV/c. These data provide a good continuation of the earlier results at q ≤140MeV/cobtained with a polarised deuteron beam. They are also consistent with impulse approximation predictions with little sign evident for any modifications due to multiple scatterings. These successful results confirm that the ANKE deuteron charge-exchange programme can be extended to much higher energies with a polarised deuterium target than can be achieved with a polarised deuteron beam.

Permalink: https://www.hzdr.de/publications/Publ-21560
Publ.-Id: 21560


High-resolution Positron-Emission-Tomography for ultrasensitive spatio-temporal monitoring of tracer transport in porous media

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

Positron-Emission-Tomography (PET) has favourable characteristics for monitoring the propagation of tracers in opaque media on the drill core scale. PET directly yields quantitative tracer concentrations, without retroaction on the process, with ultimate sensitivity (less than 109 atoms per voxel) and selectivity, with negligible matrix effect, and with expedient spatial resolution. It only requires radiolabelling of the observed substance with a positron-emitting isotope and therefore allows investigating processes in fluids without physical or compositional contrasts.
PET is a widely applied nuclear medical technique for 3D functional imaging. Since 1988 [1], some exemplary applications for the visualization of flow processes in various rock formations and other non-medical applications were conducted with clinical PET scanners with low spatial resolution and with short living radionuclides (e.g. 18F, T1/2=109.77 min).
In contrast, we take advantage of a high-resolution PET-scanner, originally dedicated for biomedical research on small animals, with a resolution at the physical limit of 1 mm [2]. Also, we are able to conduct long-time experiments in our laboratory, applying radionuclides with longer lifetimes (e.g. 22Na, T1/2=2.602 a).
Our applications cover conservative and reactive flow of solutes and particles in soils and rocks, as well as diffusion processes in clays [3]. We strive to characterize heterogeneous transport effects on the basis of our visualisations of the propagation of the tracer. Frequently, we observe the evolution of strongly localized pathways, which depict the reduced effective volume, coming along with a reduced efficiency of the internal surface area for sorption processes. Our data can be used also for experimental calibration and verification of computer simulations of transport processes.
REFERENCES
[1] van den Bergen, E.A., Jonkers, G., Strijckmans, K., Goethals, P., 1989. Industrial Applications of Positron Emission Computed Tomography. Nuclear Geophysics 3, pp. 407-418.
[2] Zakhnini, A., Kulenkampff, J., Sauerzapf, S., Pietrzyk, U. and Lippmann-Pipke, J., 2013. Monte Carlo simulations of GeoPET experiments: 3D images of tracer distributions (18F, 124I and 58Co) in Opalinus Clay, anhydrite and quartz. Computers & Geosciences, 57: 183-196.
[3] Kulenkampff, J.; Gründig, M.; Korn, N.; Zakhnini, A.; Barth, T.; Lippmann-Pipke, J., 2013. Application of high-resolution positron-emission-tomography for quantitative spatiotemporal process monitoring in dense material. 7. World Congress on Industrial Process Tomography, Krakow, Poland, free available on http://www.isipt.org/world-congress/7/902.html

  • Poster
    Interpore Conference, 18.-21.05.2015, Padova, Italy

Permalink: https://www.hzdr.de/publications/Publ-21559
Publ.-Id: 21559


Examples of XRF and PIXE imaging with few microns Resolution using SLcam® – a Color X-Ray Camera

Nowak, S. H.; Bjeoumikhov, A.; von Borany, J.; Buchriegler, J.; Munnik, F.; Petric, A.; Renno, A. D.; Radtke, A.; Reinholz, U.; Scharf, O.; Strüder, L.; Wedell, R.; Ziegenrücker, R.

The Color X-ray Camera, type SLcam®, can detect X-ray radiation with spatial and energy resolution. The device is used as an X-ray detector in applications at synchrotrons and on PIXE beamlines. The unique properties of SLcam®, a combination of a single photon counting CCD with a polycapillary optics objective, allows a fast overview over a large detection area with first results visible almost immediately.
The maximal field of view exceeds 1 cm² and the spatial resolution can Approach several microns when using a sub-pixel algorithm.

Keywords: SLcam; HSPIXE; High-Speed PIXE; PIXE; Proton; X-Ray

Permalink: https://www.hzdr.de/publications/Publ-21558
Publ.-Id: 21558


Defect-induced magnetism in SiC: Interplay between ferromagnetism and paramagnetism

Wang, Y.; Liu, Y.; Wendler, E.; Hübner, R.; Anwand, W.; Wang, G.; Chen, X.; Tong, W.; Yang, Z.; Munnik, F.; Bukalis, G.; Chen, X.; Gemming, S.; Helm, M.; Zhou, S.

Defect-induced ferromagnetism has triggered a lot of investigations and controversies. The major issue is that the induced ferromagnetic signal is so weak that it can be sufficiently accounted for by trace contamination. To resolve this issue, we studied the variation of the magnetic properties of SiC after neutron irradiation with fluence covering four orders of magnitude. A large paramagnetic component has been induced and scales up with defect concentration, which can be well accounted for by uncoupled divacancies. However, the ferromagnetic contribution is still weak and only appears in the low fluence range of neutrons or after annealing treatments. First-principles calculations hint towards a mutually exclusive role of the concentration of defects: A higher defect concentration favors a larger magnetic interaction at the expense of spin polarization. Combining both experimental and first-principles calculation results, the defect-induced ferromagnetism can probably be understood as a local effect which cannot be scaled up with the volume.

Keywords: SiC; Defect-induced magnetism; Neutron irradiation

Permalink: https://www.hzdr.de/publications/Publ-21557
Publ.-Id: 21557


Subthreshold Xi- Production in Collisions of p(3.5 GeV)+Nb

Agakishiev, G.; Arnold, O.; Balanda, A.; Belver, D.; Belyaev, A. V.; Berger-Chen, J. C.; Blanco, A.; Böhmer, M.; Boyard, J. L.; Cabanelas, P.; Chernenko, S.; Dybczak, A.; Epple, E.; Fabbietti, L.; Fateev, O. V.; Finocchiaro, P.; Fonte, P.; Friese, J.; Fröhlich, I.; Galatyuk, T.; Garzon, J. A.; Gernhäuser, R.; Göbel, K.; Golubeva, M.; Gonzalez-Diaz, D.; Guber, F.; Gumberidze, M.; Heinz, T.; Hennino, T.; Holzmann, R.; Ierusalimov, A.; Iori, I.; Ivashkin, A.; Jurkovic, M.; Kämpfer, B.; Karavicheva, T.; Koenig, I.; Koenig, W.; Kolb, B. W.; Kornakov, G.; Kotte, R.; Krasa, A.; Krizek, F.; Krücken, R.; Kuc, H.; Kühn, W.; Kugler, A.; Kurepin, A.; Ladygin, V.; Lalik, R.; Lang, S.; Lapidus, K.; Lebedev, A.; Liu, T.; Lopes, L.; Lorenz, M.; Maier, L.; Mangiarotti, A.; Markert, J.; Metag, V.; Michalska, B.; Michel, J.; Müntz, C.; Naumann, L.; Pachmayer, Y. C.; Palka, M.; Parpottas, Y.; Pechenov, V.; Pechenova, O.; Pietraszko, J.; Przygoda, W.; Ramstein, B.; Reshetin, A.; Rustamov, A.; Sadovsky, A.; Salabura, P.; Schmah, A.; Schwab, E.; Siebenson, J.; Sobolev, Y. G.; Spataro, S.; Spruck, B.; Ströbele, H.; Stroth, J.; Sturm, C.; Tarantola, A.; Teilab, K.; Tlusty, P.; Traxler, M.; Trebacz, R.; Tsertos, H.; Vasiliev, V.; Wagner, T.; Weber, M.; Wendisch, C.; Wüstenfeld, J.; Yurevich, S.; Zanevsky, Y. V.

Results on the production of the double strange cascade hyperon Xi(-) are reported for collisions of p(3.5 GeV) + Nb, studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18 at GSI Helmholtzzentrum for Heavy-Ion Research, Darmstadt. For the first time, subthreshold Xi(-) production is observed in proton-nucleus interactions. Assuming a Xi(-) phase-space distribution similar to that of. hyperons, the production probability amounts to P Xi- = [2.0 +/-0.4(stat) +/- 0.3(norm) +/- 0.6(syst)] x 10(-4) resulting in a Xi(-)/d(Lambda + Sigma(0)) ratio of P Xi-/P Lambda+Sigma 0 = [1.2 +/-0.3(stat) +/- 0.4(syst)] x 10(-2). Available model predictions are significantly lower than the measured Xi(-) yield.

Permalink: https://www.hzdr.de/publications/Publ-21556
Publ.-Id: 21556


Biosorption of uranium on the cells of the halophilic archaea Halobacterium noricense DSM 15987 under highly saline conditions

Bader, M.; Drobot, B.; Müller, K.; Stumpf, T.; Cherkouk, A.

For the long-term storage of radioactive waste in salt rock formations it is important to know how microorganisms can affect the performance of a repository. It is possible that in salt rock indigenous microbes are able to influence the oxidation state, the speciation and hence the solubility of actinides. So the migration of actinides can be reduced or enhanced by different processes as for example by biosorption with cellular ligands [1].
The biosorption capacity of the archaea Halobacterium noricense DSM 15987, which was used as a reference strain due to its worldwide occurrence in salt rock, was investigated. The strain was isolated from an Austrian salt mine [2] and similar species were also found in the WIPP (Waste Isolation Pilot Plant, Carlsbad, New Mexico, USA) [3]. This rod shaped microorganism is a halophilic one which requires at least 2.1 M NaCl whereat the optimum is 3.0 M NaCl [2].
Independent of the uranium concentration (10 – 120 µM) Halobacterium noricense DSM 15987 shows good sorption capabilities at room temperature (RT). Constantly, 90 % of added uranium was bound on the cells. Looking at the time dependence of biosorption with a constant uranium concentration (100 µM) an asymptotic sorption curve was obtained. This accumulation kinetic indicates a two stage process.
Directly after the addition of uranium it is rapidly sorbed by the cells. After 3 h 35 % of uranium was accumulated. The second slower step is reached after 42 h, where 90 % of added uranium was bound to the cells. Under these conditions this corresponds to 37.5 ± 0.7 mg U(VI) per 1 g dry biomass. A slightly faster sorption can be seen at higher temperatures especially at 50 °C. There the maximal sorption of 90 % is already reached after 24 h.
Interestingly, with increasing incubation time, uranium concentration and also temperature an agglomeration of the cells occurred. Live/Dead staining showed that agglomerated cells were mostly alive but nearly all single cells were dead. So we conclude that this agglomeration process is a kind of stress response to protect the cells themselves for environmental challenges.
Furthermore, the supernatant and the washed cell suspension were analysed with TRLFS to characterize the binding of uranium to the cells. Despite the high quenching effect of chloride to uranium fluorescence a luminescence signal could be detected. The reason is the high quantum yield of some formed complexes. With parallel factor analysis (Parafac) we could assume that two uranium-cell-complexes were formed. Species 1 is difficult to interpret due to the unspecific form of the spectrum. Further investigations with reference substances have to be done. But comparing with literature we assign species 2 to a carboxylic cell-uranyl-complex [4]. These results were confirmed by IR spectroscopy where as well carboxylic vibrations could be detected.
The archaeum Halobacterium noricense DSM 15987 shows good uranium sorption efficiencies. Independent of the occurring agglomeration always 90 % of the added uranium was bound to the cells. The binding of uranium to carboxylic groups on the cell wall was proven with TRLFS and IR spectroscopy. A binding of uranium to phosphate groups is still under investigation.
Thanks to Sabrina Gurlit for analysing the samples with ICP-MS and Karsten Heim for recording the IR-spectra.

REFERENCES
1. Morris, K. et al., “Biogeochemical cycles and remobilisation of the actinide elements” Interactions of microorganisms with radionuclides, Volume 2, Page 101-141 (2002).
2. Gruber, C. et al., “Halobacterium noricense sp. nov., an archaeal isolate from a bore core of an alpine Permian salt deposit, classification of Halobacterium sp. NRC-1 as a strain of H. salinarum and emended description of H. salinarum” Extremophiles, 8, Page 431-439 (2004).
3. Swanson, J. S. et al., “Status Report on the Microbial Characterization of Halite and Groundwater Samples from the WIPP” Status report Los Alamos National Laboratory, Page 1ff. (2012).
4. Vogel, M. et al., “Biosorption of U(VI) by the green algae Chlorella vulgaris in dependence of pH value and cell activity” Science of the Total Environment , Page 384-395. (2010).

  • Lecture (Conference)
    ABC-Salt IV Workshop 2015, 14.-15.04.2015, Heidelberg, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21555
Publ.-Id: 21555


Carbon p Electron Ferromagnetism in Silicon Carbide

Wang, Y.; Liu, Y.; Wang, G.; Anwand, W.; Jenkins, C.; Arenholz, E.; Munnik, F.; Gordan, O.; Salvan, G.; Zahn, D. R. T.; Chen, X.; Gemming, S.; Helm, M.; Zhou, S.

Ferromagnetism can occur in wide-band gap semiconductors as well as in carbon-based materials, when specific defects are introduced. It is thus desirable to establish a direct relation between the defects and the resulting ferromagnetism. Here, we contribute to revealing the origin of defect-induced ferromagnetism using SiC as a prototypical example. We show that the long-range ferromagnetic coupling can be attributed to the p electrons of the nearest-neighbor carbon atoms around VSiVC divacancies. Thus, the ferromagnetism is traced down to its microscopic, electronic origin.

Keywords: SiC; Defect induced magnetism; XMCD

Permalink: https://www.hzdr.de/publications/Publ-21554
Publ.-Id: 21554


(V,Nb):TiO2 - a transparent conductor and ferromagnetic semiconductor !?

Cornelius, S.

Both the property combinations a) transparent conductivity and b) diluted magnetic semiconductivity are based on the concept of dispersed substitutional doping of a suitable semiconductor with dopants that provide free electrons and/or carry a magnetic moment. The implications of current point defect theory with respect to defect formation enthalpies in anatase TiO2 are discussed in context with experimental results on electrical, optical and magnetic properties of V and Nb doped TiO2 thin films prepared by reactive magnetron sputtering. While transparent conductivity could be achieved via Nb doping, V doped films remain paramagnetic (in contrast to theoretical predictions).

Keywords: anatase; titanium oxide; magnetron sputtering; transparent conductive oxide; doping; magnetic semiconductor

  • Lecture (Conference)
    Deti.2 project meeting, 22.10.2014, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21553
Publ.-Id: 21553


Dopant activation and charge transport limits in transparent conductive (Al,Ga):ZnO and Nb:TiO2

Cornelius, S.

Transparent conductive oxides (TCOs) are a class of wide bandgap metal oxides in which high densities of free charge carriers can be induced in such a way, that a unique combination of high optical transmittance in the visible spectral range and metal-like electrical conductivity is achieved. The influence of the oxygen partial pressure, substrate temperature, dopant concentration and substrate type on the properties (Al,Ga) doped ZnO and Nb doped TiO2 thin films has been studied systematically. The correlations between electrical, structural and optical properties as well as the elemental composition of the films have been characterized by a combination of Hall-effect, XRD, spectroscopic ellipsometry, spectral photometry, RBS and PIXE techniques including complementary XANES and TEM studies. The results are discussed with a focus on understanding the dopant incorporation into the ZnO and TiO2 host materials as well as finding physical limits for the resistivity based on charge transport models for degenerate polar semiconductors.

Keywords: transparent conductive oxide; zinc oxide; doping; titanium oxide; anatase; electron mobility; reactive sputtering; magnetron sputtering

  • Invited lecture (Conferences)
    Institutsseminar, Institut für experimentelle Physik II, Universität Leipzig, 26.11.2014, Leipzig, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21552
Publ.-Id: 21552


Formation and propagation of laser-driven plasma jets in an ambient medium studied with X-ray radiography and optical diagnostics

Diziere, A.; Pelka, A.; Ravasio, A.; Loupias, B.; Falize, E.; Kuramitsu, Y.; Sakawa, Y.; Morita, T.; Pikuz, S.; Yurchak, R.; Koenig, M.

In this paper, we present experimental results obtained on the LULI2000 laser facility regarding structure and dynamics of astrophysical jets propagating in interstellar medium. The jets, generated by using a cone-shaped target, propagate in a nitrogen gas that mimics the interstellar medium. X-ray radiography as well as optical diagnostics were used to probe both high and low density regions. In this paper, we show how collimation of the jets evolves with the gas density.

Permalink: https://www.hzdr.de/publications/Publ-21551
Publ.-Id: 21551


Bacterial diversity in bentonites, engineered barrier for deep geological disposal of radioactive wastes

Lopez-Fernandez, M.; Cherkouk, A.; Vílchez-Vargas, R.; Sandoval, R.; Pieper, D.; Boon, N.; Sánchez-Castro, I.; Merroun, M.

The long-term disposal of radioactive wastes in a deep geological repository is the accepted international solution for the treatment and management of these special residues. The microbial community of the selected host rocks and engineered barriers for the deep geological repository may affect the performance and the safety of the radioactive waste repository. In this work the bacterial community of bentonite formations of Almeria (Spain), selected as reference material for bentonite engineered barriers in the disposal of radioactive wastes, was studied. 16S rRNA gene-based approaches were used to study the bacterial community of the bentonite samples by traditional clone libraries and Illumina-sequencing. A high bacterial diversity was found by both techniques, with phylotypes belonging to 13 different bacterial phyla: Acidobacteria, Actinobacteria, Armatimonadetes, Bacteroidetes, Choloroflexi, Cyanobacteria, Deinococcus-Thermus, Firmicutes, Gemmatimonadetes, Planctomycetes, Proteobacteria, Nitrospirae and Verrucomicrobia. The dominant groups of the community were mainly represented by Proteobacteria and Bacteroidetes.
Some of the identified genera were previously described for their capacity to interact with iron and different heavy metals as well as radionuclides, as for example, Acidovorax, Ralstonia, Variovorax and Sphingomonas. The results obtained in this work demonstrate the high bacterial diversity of these Spanish bentonite formations, and their possible negative effect on the structure of the clay minerals as well as interactions with different radionuclides, which may affect the safety of the deep geological repository of radioactive wastes.

Keywords: Spanish bentonite; bacterial diversity; Illumina sequencing; Cloning and sequencing

Permalink: https://www.hzdr.de/publications/Publ-21550
Publ.-Id: 21550


Investigation of ionoluminescence of semiconductor materials using helium ion microscopy

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

Helium ion microscopy has been employed to investigate the ionoluminescence of various semiconduc- tors. We have verified the possibility of application of this technique for high-resolution ionolumines- cence analysis of this kind of materials. In this work the ionoluminescence signal was induced by a sub- nanometer Heþ beam with an energy of 35 keV. Several types of semiconductor samples were investigated: bulk materials, nanowires and quantum dots. All samples were found to exhibit ionoluminescence. However, the ionoluminescence signal rapidly degrades under the ion irradiation. The signal degradation was found to depend not only on the sample's composition, but also on its size. The ionoluminescence emission spectra were recorded and emission peaks identified.

Keywords: Ionoluminescence Helium ion microscopy Semiconductors Nanowires Quatum dots

Downloads:

Permalink: https://www.hzdr.de/publications/Publ-21549
Publ.-Id: 21549


Experimental studies of contact angle hysteresis phenomena on polymer surfaces — Toward the understanding and control of wettability for different applications

Grundke, K.; Pöschel, K.; Synytska, A.; Frenzel, R.; Drechsler, A.; Nitschke, M.; Cordeiro, A. L.; Uhlmann, P.; Welzel, P. B.

Contact angle hysteresis phenomena on polymer surfaces have been studied by contact angle measurements using sessile liquid droplets and captive air bubbles in conjunction with a drop shape method known as Axisymmetric Drop Shape Analysis — Profile (ADSA-P). In addition, commercially available sessile drop goniometer techniques were used. The polymer surfaces were characterized with respect to their surface structure (morphology, roughness, swelling) and surface chemistry (elemental surface composition, acid–base characteristics) by scanning electron microscopy (SEM), scanning force microscopy (SFM), ellipsometry, X-ray photoelectron spectroscopy (XPS) and streaming potential measurements.
Heterogeneous polymer surfaces with controlled roughness and chemical composition were prepared by different routes using plasma etching and subsequent dip coating or grafting of polymer brushes, anodic oxidation of aluminium substrates coated with thin polymer films, deposition techniques to create regular patterned and rough fractal surfaces from core–shell particles, and block copolymers. To reveal the effects of swelling and reorientation at the solid/liquid interface contact angle hysteresis phenomena on polyimide surfaces, cellulose membranes, and thermo-responsive hydrogels have been studied. The effect of different solutes in the liquid (electrolytes, surfactants) and their impact on contact angle hysteresis were characterized for solid polymers without and with ionizable functional surface groups in aqueous electrolyte solutions of different ion concentrations and pH and for photoresist surfaces in cationic aqueous surfactant solutions.
The work is an attempt toward the understanding of contact angle hysteresis phenomena on polymer surfaces aimed at the control of wettability for different applications.

Keywords: Contact angle hysteresis; Polymer surfaces; Morphological and chemical heterogeneity Swelling and reorientation at the solid/liquid interface; Different solutes in the liquid

Permalink: https://www.hzdr.de/publications/Publ-21548
Publ.-Id: 21548


Flow measurements in continuous casting models by means of contactless inductive flow tomography under the influence of electromagnetic brakes

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

Most of the steel in the world is produced by continuous casting, where liquid metal flows from a tundish through a submerged entry nozzle into a copper mould. The mould is cooled by water, so a solid shell starts to form at the mould walls. The resulting steel strand is pulled out of the mould continuously and solidifies completely.
In industry it is well-known that an unstable flow in the mould has negative effects on the resulting steel’s quality. Electromagnetic brakes (EMBr) are expected to dampen instabilities, although their impact on the flow can hardly be examined directly in liquid steel. In spite of the casting method’s economical and industrial importance, only a few simple measurement techniques are available to investigate the actual flow patterns in the mould. A more sophisticated technique for liquid metal flow measurements could help to resolve the open issues in the mould flow.
The contactless inductive flow tomography (CIFT) is a technique that measures the mean global flow of an electrically conducting melt by exposing it to a magnetic excitation field and measuring the flow induced perturbations of that field outside the melt. The velocity profile can then be calculated by solving an inverse problem, using adequate regularization techniques to deal with the non-uniqueness.
We present preliminary results for a physical model of a mould with a rectangular cross-section of 140 × 35 mm 2 in the presence of an EMBr. Additionally we show flow reconstructions for a 400 × 100 mm 2 mould, demonstrating the upward scalability of CIFT.

  • Lecture (Conference)
    3rd International Workshop on Measuring Techniques for Liquid Metal Flows (MTLM2015), 15.-17.04.2015, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21547
Publ.-Id: 21547


Hyperdoping silicon with selenium: solid vs. liquid phase epitaxy

Zhou, S.; Liu, F.; Prucnal, S.; Gao, K.; Khalid, M.; Baehtz, C.; Posselt, M.; Skorupa, W.; Helm, M.

Chalcogen-hyperdoped silicon shows potential applications in silicon-based infrared photodetectors and intermediate band solar cells. Due to the low solid solubility limits of chalcogen elements in silicon, these materials were previously realized by femtosecond or nanosecond laser annealing of implanted silicon or bare silicon in certain background gases. The high energy density deposited on the silicon surface leads to a liquid phase and the fast recrystallization velocity allows trapping of chalcogen into the silicon matrix. However, this method encounters the problem of surface segregation. In this paper, we propose a solid phase processing by flash-lamp annealing in the millisecond range, which is in between the conventional rapid thermal annealing and pulsed laser annealing. Flash lamp annealed selenium-implanted silicon shows a substitutional fraction of ≈ 70% with an implanted concentration up to 2.3%. The resistivity is lower and the carrier mobility is higher than those of nanosecond pulsed laser annealed samples. Our results show that flash-lamp annealing is superior to laser annealing in preventing surface segregation and in allowing scalability.

Keywords: Ion implantation; Solid phase epitaxy; Deep level impurity; Si

Permalink: https://www.hzdr.de/publications/Publ-21546
Publ.-Id: 21546


Ion irradiation and nanoindentation as a screening test for irradiation effects on neutron-irradiated ferritic/martensitic high-Cr steels

Heintze, C.; Akhmadaliev, S.; Altstadt, E.; Bergner, F.

A combined approach based on ion irradiation and nanoindentation as a screening test procedure for the irradiation response of structural materials foreseen for components exposed to heavy neutron irradiation has been selected and specified in detail. Important constituents of the approach are:
• Design of the ion irradiation experiments using the MD code SRIM and taking into account recommendations according to [1], detailed documentation of the irradiation experiments and the SRIM calculations,
• Nanoindentation testing over a large range of indentation depths (from about 5% to 100% of the thickness of the ion-irradiated layer) to gain information on the indentation size effect and the substrate effect, analysis according to the Oliver-Pharr method [2,3],
• Elimination of the indentation size effect (ISE) based on an unirradiated reference sample of the same material using a suitable model (e.g. [4]), elimination of the substrate effect.

A version of the approach specified above was applied to unirradiated, ion-irradiated and neutron-irradiated 9%Cr F/M steel T91 (MATTER reference material). The findings indicate that the approach based on ion irradiation and nanoindentation is suitable as a screening test for F/M steels exposed to neutron irradiation.

References
[1] R. E. Stoller, M. B. Toloczko, G. S. Was, A. G. Certain, S. Dwaraknath, F. A. Garner, Nucl. Instrum. Methods Phys. Res. B 310 (2013) 75–80.
[2] W. C. Oliver, G. M. Pharr, J. Mater. Res., 7 (1992) 1564–1583.
[3] W. C. Oliver, G. M. Pharr, J. Mater. Res., 19 (2004) 3–20.
[4] W. D. Nix, H. Gao, J. Mech. Phys. Solids, 46 (1998) 411–425.

Keywords: ion irradiation; neutron irradiation; nanoindentation; irradiation hardening; ferritic/martensitic steels; T91

  • Lecture (Conference)
    MATTER - “MATerials TEsting and Rules“ Project Final Workshop, 20.-22.10.2014, Rom, Italy

Permalink: https://www.hzdr.de/publications/Publ-21544
Publ.-Id: 21544


The Zn-vacancy related green luminescence and donor-acceptor pair emission in ZnO grown by pulsed laser deposition

Wang, Z.; Su, S. C.; Younas, M.; Ling, F. C. C.; Anwand, W.; Wagner, A.

Low temperature (10 K) photoluminescence study shows that green luminescence (GL) peaked at 2.47 eV and near band edge (NBE) emission at 3.23 eV are introduced in undopd ZnO grown by pulsed laser deposition (PLD) after the 900°C annealing. The NBE emission exhibiting blue shift with increasing temperature is assigned to the transitions of donor-acceptor-pair (DAP)/free-electron-to-acceptor (FA). Positron annihilation spectroscopy (PAS) study shows that the introduction of the GL is correlated with the formation of the Zn vacancy-related defect (VZn). Comparing the transition energies of VZn obtained by the previous first principle calculation [Janotti and Van de Walle, Phys. Rev. B 76, 165202 (2007)], the GL is associated with the transition from the conduction band to the ε(-/2-) state of VZn and the DAP/FA emission involves the acceptor level ε(0/-) of VZn.

Keywords: ZnO; green luminescence; Zn vacancy

Permalink: https://www.hzdr.de/publications/Publ-21543
Publ.-Id: 21543


Influence of the discharge regime on Ti thin films growth and properties in dc, single pulsed and chopped high power impulse magnetron sputtering

Meško, M.; Heller, R.; Hübner, R.; Krause, M.

High power impulse magnetron sputtering (HiPIMS) has been becoming an appealing deposition process for synthesis of high quality coatings over the last decade. It utilizes average power similar to the dc magnetron sputtering (DCMS) applied on magnetron target, which is however concentrated in short pulses. This ensures generation of dense plasmas with high fraction of ionized film forming species. Control over the energy and trajectory of ionized sputtered species can lead to the formation of coatings with improved properties. Despite of certain advantages the HiPIMS process has often a lower deposition rate compared to DCMS. This is one reason which hinders the HiPIMS process from further exploration in industrial applications. However more recently it has been shown that deposition rates of Ti thin films can be significantly increased by so called chopped-HiPIMS (c-HiPIMS) technique [1]. In this case a single HiPIMS pulse is decomposed into several individual pulses with microsecond pulse off-times. C-HiPIMS can be especially effective in suppression of thermal spikes on the target. The effect of thermal spikes is most pronounced during long single HiPIMS pulses. It negatively influences mobility of arriving ad-atoms leading to the formation of larger grains and rougher surface of Ti thin films [2]. In present work we compare the density, crystallinity, roughness, and microstructure of Ti thin films prepared by dc, single pulsed, and c-HiPIMS. Much attention has been paid on role of microsecond pulse off-times on Ti thin films properties. Plasma parameters have been measured to supplement our investigations.
References:
[1] P. M. Barker, E. Lewin, and J. Patscheider “Modified high power impulse magnetron sputtering process for increased deposition rate of titanium” J. Vac. Sci. Technol. A 31 (2013) 0606041
[2] F. J. Jing, T. L. Yin, K. Yukimura, H. Sun, Y. X. Leng, and N. Huang “Titanium film deposition by high-power impulse magnetron sputtering: Influence of pulse duration” Vacuum 86 (2012) 2114

Keywords: HiPIMS; Ti thin film; microstructure; plasma parameters

  • Poster
    14th International Conference on Plasma Surface Engineering, 15.-19.09.2014, Garmisch-Partenkirchen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21542
Publ.-Id: 21542


The effect of He on irradiation hardening of Fe-9Cr-based non-ODS and ODS alloys at 300°C

Heintze, C.; Kögler, R.; Bergner, F.; Hernández Mayoral, M.

The influence of helium on the irradiation hardening and embrittlement of reduced-activation ferritic/martensitic Cr-steels and their oxide dispersion strengthened variants under fusion-relevant irradiation conditions is still a concern. While the fact that He can influence the mechanical properties is well established [1,2], the underlying mechanisms are not fully understood [1,2]. In this work the effect of He and displacements per atom (dpa) on the irradiation-induced hardening of an Fe-9at%Cr alloy, Eurofer97 and an oxide dispersion strengthened variant of Eurofer (ODS-Eurofer) at 300°C was studied. Self-ion irradiation was applied to simulate the neutron-irradiation-induced damage. To separate the effect of helium different irradiation modes were applied. Apart from single-beam irradiations with He or self-ions, only, Helium was implanted prior to (pre-implantation), simultaneously (dual-beam irradiation) or following the (post-implantation) self-ion irradiation. The ion irradiated layer was characterized by means of nanoindentation. We conclude, that:
• there is a significant interaction between dpa and He
• pre-implantation of He followed by self-ion irradiation is not suitable to replace simultaneous irradiations or to simulate neutron irradiation induced damage
• nano-oxides in ODS-Eurofer mitigate the effect of helium on irradiation hardening

[1] H. Trinkaus, B.N. Singh, J. Nucl. Mater. 323 (2003) 229
[2] R. Schäublin, Y.L. Chiu, J. Nucl. Mater. 362 (2007) 152

Keywords: irradiation hardening; ODS; dual-beam; ion irradiation; nanoindentation

  • Poster
    2nd International Workshop on ODS Materials, 26.-27.06.2014, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21541
Publ.-Id: 21541


Using nature's genius for pollutant detection

Raff, J.; Weinert, U.; Nikolaus, N.; Guenther, T.; Strehlitz, B.; Pollmann, K.

In all environments single cell organisms such as bacteria are directly affected by changing and sometimes extreme environmental conditions. This includes not only pH, temperature and salinity but also the presence of toxic ions, compounds and complexes. It is thusly essential for these microorganisms to possess a robust and selectively permeable cell surface. For this purpose, many bacteria form a proteinaceous cell envelope, the so called surface layer (S-layer). This cell envelope has different functions in different organisms for example the binding of toxic metals and metalloids to protect cells from being damaged by these elements. On other cells, S-layers may act as immobilization matrix for exoenzymes, as molecular sieve or as ion and molecule trap or they protect the cell from being affected by other bacteria or by lytic enzymes. S-layers are composed of identical protein or glycoprotein monomers, which are able to self-assemble to highly ordered monomolecular layers. They form para-crystalline sheets in suspension, on interfaces and on surfaces. Furthermore, on the surface of such a layer different functional groups are available which can be modified without the loss of its structure. The protein layers are in general mechanically and chemically highly stable. These properties make S-layers very interesting building blocks for the construction of new bioinspired nanomaterials and nanocoatings. Using the two-dimensional protein arrays in combination with layer-by-layer technique different kinds of technical surfaces can be functionalized.
This technique is used to design a new kind of sensory layers which will allow detecting small amounts of analytes with high selectivity. This sensory device will consist of an S-layer coating, a selective receptor and two fluorescence dyes. Aptamers were used as compound specific receptors . These are short, single stranded nucleic acid oligomers that meet the requirements for a more selective and sensitive detection of pollutants in nature, medicine and industry. Aptamers are able to recognize almost all classes of substrates and bind them in analogy to antigen-antibody interactions. By using an in vitro selection and amplification technique aptamers can be developed for pollutants like heavy metals, pharmaceuticals but also for proteins and complex targets like viruses and microbial spores. Currently, several aptamers for different antibiotics have been selected and experiments confirmed their high selectivity for single antibiotics or groups of antibiotics. As third component two fluorescence dyes allowing a FRET as signal transducer system will be coupled onto the S-layer lattice. This setup can be combined in different ways to optimize the sensitivity and selectivity of the sensor. Comparable to the binding of exoenzymes on S-layer carrying cells the three sensor components can also be linked to S-layer coated technical surfaces. In first experiments we coupled the model aptamer, anti-thrombin-aptamer, on S-layer proteins and proved its functionality after being linked to the protein. Furthermore, we modified S-layer proteins with a FRET pair and proved the energy transfer between them. We used a FRET pair containing a green and red fluorescence dye and succeed to detect a FRET between the S-layer linked fluorescence dyes. The FRET efficiency was 40 %. Because of the regular arranged functional groups on the S-layer lattice, coupling of the sensor components can be done in a defined and reproducible way.
In further work we will combine all components, aptamers and fluorescence dyes, on the S layer proteins. The aptamer will bind the specific analyte, affecting also the fluorescence dyes and disturbing FRET because of their close proximity to each other. In result a sensory layer is developed which uses the high specificity of aptamers and fluorescence dyes for an easy detection due to an optical signal. Additionally, in the future other techniques such as phage surface display will be used to select also peptide based binding molecules. Hopefully, this will allow detecting even more and also other kinds of pollutants not being bound by aptamers.

Keywords: S-layer; aptamer; biosensor; FRET

  • Invited lecture (Conferences)
    Rapid Methods Europe 2014, 03.03.-02.04.2014, Noordwijkerhout, Niederland

Permalink: https://www.hzdr.de/publications/Publ-21540
Publ.-Id: 21540


Transport in mesoscopic conductors

Erbe, A.

Electronic transport in mesoscopic structures

The size of electrical components has been scaled down by several orders of magnitude during the past decades arriving at structure sizes of only tens of nanometers in modern circuits. Electrical current on the nanocale obeys different laws than in macroscopic conductors. The resistance of macroscopic conductors is described by Ohm’s law, assuming diffusive charge transport carried by the conduction electrons. At small length scales this concept cannot be applied any more, electrons move ballistically at these length scales and transport needs to be described using a scattering approach. We will review the concepts used to describe mesoscopic conductors and give examples for typical structures that can be treated using these concepts.

Molecular electronics

The use of single molecules as active elements in electrical circuits may serve as alternative technology for building integrated circuits on the nanoscale. In this presentation we want to give an overview on various techniques that have been used successfully to contact single molecules and to characterize them electrically. Especially the comparison between different techniques shows that a single measurement is always prone to artifacts originating from the unknown microscopic details of the junctions. It is therefore necessary to perform a statistically relevant number of measurements in order to resolve molecular properties. Using these techniques various properties of the molecules can be studied. Special examples are the influence of conformational changes of the molecules, differences between various coupling endgroups of the molecules, effects of light-irradiation onto the molecular junctions, and the influence of self-organization in DNA nanostructures.

  • Invited lecture (Conferences)
    PIER summer school, 06.-09.10.2014, Hamburg, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21539
Publ.-Id: 21539


Introduction to transport in confined geometries an across point contacts

Erbe, A.

The size of electrical components has been scaled down by several orders of magnitude during the past decades arriving at structure sizes of only tens of nanometers in modern circuits. Electrical current on the nanocale obeys different laws than in macroscopic conductors. The resistance of macroscopic conductors is described by Ohm’s law, assuming diffusive charge transport carried by the conduction electrons. At small length scales this concept cannot be applied any more, electrons move ballistically at these length scales and transport needs to be described using a scattering approach. We will review the concepts used to describe mesoscopic conductors and give examples for typical structures that can be treated using these concepts.
Mechanically controllable breakjunctions (MCBJs) are excellent tools to form stable metallic contacts consisting of single atoms, only. In these devices, a tiny metallic bridge is broken by mechanically bending the underlying substrate. During the breaking, the number of atoms taking part in the conduction is reduced gradually, leading to a step-wise decrease of the conductance of the whole junction. This behavior is understood by taking the atomic orbitals, which contribute to the conductance mechanism, into account1. We will show how the experimental results showing this behavior can be obtained and compared to theoretical predictions. Apart from being a fascinating model system for mesoscopic conductance, MCBJs can be used for making contacts to other nanoscale conductors, such as single molecules. We will give a brief introduction on the transport mechanisms involved in single molecule conductance and show measurements of such systems using MCBJs2.

1. Scheer, E. et al. The signature of chemical valence in the electrical conduction through a single-atom contact. Nature 394, 154–157 (1998).
2. Zotti, L. A. et al. Revealing the Role of Anchoring Groups in the Electrical Conduction Through Single-Molecule Junctions. Small 6, 1529–1535 (2010).

  • Invited lecture (Conferences)
    MAINZ summer school, 25.-29.08.2014, Mainz, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21538
Publ.-Id: 21538


Benchmarking of reactive transport visualisation (PET) by numerical modelling with COMSOL Multiphyiscs and PhreeqC

Lippmann-Pipke, J.; Kulenkampff, J.; Lippold, H.; Stuhlfauth, C.; Gerasch, R.; Gründig, M.

For about a decade we apply positron emission tomography (PET) as molecular imaging modality for non-destructive process visualisation in geological material on laboratory scale [1]. Sequential PETimages directly yield the spatiotemporal concentration distribution of a PET radiotracer (3D+t) in the course of the process, with a resolution of ~1 mm and tracer sensitivity to some ten thousand atoms per 1 mm³ voxel. Our PET data sets are typically complemented by μCT-images. We demonstrated its advantages for elucidating heterogeneous processes on different time scales, conservative and reactive flow experiments, including the transport of nano-particles. We considered our 3D+t data sets as highly valuable for benchmarking of reactive transport modelling results. In the recent past we have consequently gained some experiences in aligning differently complex data sets (up to 3D, (non-)reactive) with available modelling tools. As suitable for aligning our data with well-established transport codes we identified Comsol Multiphysics for the transport process parameter estimation, PhreeqC for the geochemical reaction simulation, as well as couplings of both for reactive transport processes in 2D at the mm to cm scale. Benchmarks in 3D+t have their strength in emphasising on the role of heterogeneity on macroscopic reaction rates. Here, retroactions from geochemical reactions on the effective transport pathways in geological media (dissolution, precipitation, filtration, etc.) cause spatial and temporal variations of their transport properties - processes verifiable by our non-destructive PET method.
We aim at developing into this highest complexity level of reactive transport benchmarking; the backcoupling of reaction on structural transport parameter values (porosity, permeability).
REFERENCES
[1] Kulenkampff, J. et al., 2013. Application of high-resolution positron-emission-tomography for quantitative spatiotemporal process monitoring in dense material. 7. World Congress on Industrial Process Tomography, Krakow, Poland, http://www.isipt.org/worldcongress/7/902.html (free access after registration)

  • Poster
    Interpore Conference, 18.-21.05.2015, Padova, Italy

Permalink: https://www.hzdr.de/publications/Publ-21537
Publ.-Id: 21537


Development of a Compton Camera for online ion beam range verification via prompt gamma detection

Aldawood, S.; Böhmer, M.; Bortfeldt, J.; Castelhano, I.; Dedes, G.; Lutter, R.; Gernhäuser, R.; Kolff, H. V. D.; Lang, C.; Maier, L.; Petzoldt, J.; Römer, K.; Pausch, G.; Fiedler, F.; Schaart, D. R.; Parodi, K.; Thirolf, P. G.; Auer, M.

Precise and preferably online ion beam range verification is a mandatory prerequisite to fully exploit the advantages of hadron therapy in cancer treatment. Our aim is to develop an imaging system based on a Compton camera designed to detect prompt gamma-rays induced by nuclear reactions between the ion beam and biological tissue. The Compton camera prototype consists of a stack of six customized double-sided Si-strip detectors (DSSSD, 50x50 mm2,128 strips/side) acting as scatterer, while the absorber is formed by a monolithic LaBr3:Ce scintillator crystal (50x50x30mm3) read out by a position-sensitive multi-anode photomultiplier (Hamamatsu H9500). This camera has the ability to not only detect the scattered photon, but it can also track the scattered Compton electron due to the way of designing and arranging the scatter detectors, and the multi-MeV energy of the incidentprompt gamma-rays[1].Thereflectively wrapped LaBr3:Ce detector was characterized with calibration sources, showing an excellent performance of this crystal. The time resolution was determined to be 273ps and the relative energy resolution at 662 keV was found to be 3.8%.The study of the DSSSD detectors together with the full Compton camera properties are inprogress both in the laboratory as well as at the online facilities.

References
[1] C. Lang et al., Journal of Instrumentation 9 (2014) P01008.
This work is supported by the DFG Cluster of Excellence MAP (Munich-Centre for Advanced Photonics)

Keywords: Compton Camera; range verification; particle therapy

  • Poster
    International Workshop on Range Assessment and Dose Verification in Particle Therapy, 29.-30.09.2014, Dresden, Germany

Permalink: https://www.hzdr.de/publications/Publ-21536
Publ.-Id: 21536


Entwicklung von Elektromagneten mit kurzen, hohen Strompulsen für die laserbasierte Protonentherapie

Schürer, M.; Herrmannsdörfer, T.; Karsch, L.; Kroll, F.; Masood, U.; Sobiella, M.; Pawelke, J.

  • Poster
    5. Dresdner Medizintechnik Symposium, 01.-03.12.2014, Dresden, Deutschland
  • Contribution to proceedings
    5. Dresdner Medizintechnik Symposium, 01.-03.12.2014, Dresden, Deutschland
    A. Förster, J. Füssel, M. Gelinsky, E. Koch, H. Malberg, W. Vonau (Hrsg.): Biomedizinische Technik – Von der Grundlagenforschung zum Transfer. Reports on Biomedical Engineering, Stuttgart: Steinbeis-Edition, ISBN 978-3-95663-018-7, 94

Permalink: https://www.hzdr.de/publications/Publ-21535
Publ.-Id: 21535


Turbulent amplification of magnetic fields in laboratory laser-produced shock waves

Meinecke, J.; Doyle, H. W.; Miniati, F.; Bell, A. R.; Bingham, R.; Crowston, R.; Drake, R. P.; Fatenejad, M.; Koenig, M.; Kuramitsu, Y.; Kuranz, C. C.; Lamb, D. Q.; Lee, D.; Macdonald, M. J.; Murphy, C. D.; Park, H.-S.; Pelka, A.; Ravasio, A.; Sakawa, Y.; Schekochihin, A. A.; Scopatz, A.; Tzeferacos, P.; Wan, W. C.; Woolsey, N. C.; Yurchak, R.; Reville, B.; Gregori, G.

X-ray and radio observations of the supernova remnant Cassiopeia A reveal the presence of magnetic fields about 100 times stronger than those in the surrounding interstellar medium. Field coincident with the outer shock probably arises through a nonlinear feedback process involving cosmic rays. The origin of the large magnetic field in the interior of the remnant is less clear but it is presumably stretched and amplified by turbulent motions. Turbulence may be generated by hydrodynamic instability at the contact discontinuity between the supernova ejecta and the circumstellar gas9. However, optical observations of Cassiopeia A indicate that the ejecta are interacting with a highly inhomogeneous, dense circumstellar butt bank formed before the supernova explosion. Here we investigate the possibility that turbulent amplification is induced when the outer shock overtakes dense clumps in the ambient medium. We report laboratory experiments that indicate the magnetic field is amplified when the shock interacts with a plastic grid. We show that our experimental results can explain the observed synchrotron emission in the interior of the remnant. The experiment also provides a laboratory example of magnetic field amplification by turbulence in plasmas, a physical process thought to occur in many astrophysical phenomena.

Permalink: https://www.hzdr.de/publications/Publ-21534
Publ.-Id: 21534


Experimental demonstration of an inertial collimation mechanism in nested outflows

Yurchak, R.; Ravasio, A.; Pelka, A.; Pikuz, S.; Falize, E.; Vinci, T.; Koenig, M.; Loupias, B.; Benuzzi-Mounaix, A.; Fatenejad, M.; Tzeferacos, P.; Lamb, D. Q.; Blackman, E. G.

Interaction between a central outflow and a surrounding wind is common in astrophysical sources powered by accretion. Understanding how the interaction might help to collimate the inner central outflow is of interest for assessing astrophysical jet formation paradigms. In this context, we studied the interaction between two nested supersonic plasma flows generated by focusing a long-pulse high-energy laser beam onto a solid target. A nested geometry was created by shaping the energy distribution at the focal spot with a dedicated phase plate. Optical and x-ray diagnostics were used to study the interacting flows. Experimental results and numerical hydrodynamic simulations indeed show the formation of strongly collimated jets. Our work experimentally confirms the “shock-focused inertial confinement” mechanism proposed in previous theoretical astrophysics investigations.

Permalink: https://www.hzdr.de/publications/Publ-21533
Publ.-Id: 21533


Fabrication of highly efficient transparent metal thin film electrodes using Direct Laser Interference Patterning

Eckhardt, S.; Müller-Meskamp, L.; Siebold, M.; Fabián Lasagni, A.

The demand of highly efficient transparent electrodes without the use of rare earth materials such as indium requires a new generation of thin metallic films with both high transparency and electrical conductivity. For this purpose, Direct Laser interference Patterning was used to fabricate periodic hole-like surface patterns on thin metallic films in order to improve their optical transparency by selective laser ablation of the material and at the same time keeping the electrical properties at an acceptable level.
Metallic films consisting of aluminum and copper with film thicknesses ranging between 5 and 40 nm were deposited on glass substrates and treated with nanosecond and picosecond pulse laser system..In order to analyze the processability of the films, the laser ablation threshold for each material as function of the layer thickness and pulse duration was firstly determined. After analyzing these initial experiments, the samples were structured with a 1.7 μm spatial period hole-like-pattern using three beam direct laser interference patterning. The structural quality of the fabricated structures was analyzed as function laser energy density (laser fluence) using scanning electron microscopy (SEM), atom force microscopy (AFM). Finally, optical and electrical properties of the films were characterized using optical spectroscopy, as well as surface impedance measurements.

Keywords: direct laser interference patterning; metallic thin films; large area surface functionalization

  • Contribution to proceedings
    SPIE Photonics West - LASE, 07.-12.02.2015, San Francisco, USA
    Laser-Based Micro- and Nanoprocessing IX, 9351
    DOI: 10.1117/12.2082537

Permalink: https://www.hzdr.de/publications/Publ-21532
Publ.-Id: 21532


Realizing a reference setup for irradiation experiments with laser-accelerated proton pulses (at the Draco laser)

Obst, L.; Zeil, K.; Metzkes, J.; Kraft, S.; Schramm, U.

By focusing an ultra-short high-intensity laser pulse on a solid target, pulses of protons and other positively charged ions with energies of several 10 MeV per nucleon are generated. The properties of these particle beams such as their energy and absolute number are highly dependent on experimental conditions like laser and target parameters. In order to achieve principal comparability between different experimental campaigns at the Draco laser system at the Helmholtz-Zentrum Dresden-Rossendorf, a reference setup for the laser ion acceleration experiment was established. A configuration is sought in which proton beams of reproducible characteristics are generated. To ensure a high stability of the proton spectra, the application of longer focal length parabolas (f ~ 1000 mm) will be tested for this setup, according preparatory studies being presented in this talk.

Keywords: accelerators; laser-driven proton acceleration; high-intensity lasers; beams and electromagnetism

  • Lecture (Conference)
    DPG-Tagung 2014, 17.-21.03.2014, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21531
Publ.-Id: 21531


Comparative investigation of three dose rate meters for their viability in pulsed radiation fields.

Gotz, M.; Karsch, L.; Pawelke, J.

Pulsed radiation fields, characterized by microsecond pulse duration and correspondingly high pulse dose rates, are increasingly used in therapeutic, diagnostic and research applications. Yet, dose rate meters which are used to monitor radiation protection areas or to inspect radiation shielding are mostly designed, characterized and tested for continuous fields and show severe deficiencies in highly pulsed fields. Despite general awareness of the problem, knowledge of the specific limitations of individual instruments is very limited, complicating reliable measurements. We present here the results of testing three commercial dose rate meters, the RamION ionization chamber, the LB 1236-H proportional counter and the 6150AD-b scintillation counter, for their response in pulsed radiation fields of varied pulse dose and duration. Of these three the RamION proved reliable, operating in a pulsed radiation field within its specifications, while the other two instruments were only able to measure very limited pulse doses and pulse dose rates reliably.

Permalink: https://www.hzdr.de/publications/Publ-21530
Publ.-Id: 21530


Laserbasierte Teilchenbeschleunigung und deren Anwendung in der Strahlentherapie

Pawelke, J.

  • Invited lecture (Conferences)
    Referententagung des Strahlenschutzseminar in Thüringen e.V., 15.-16.03.2013, Heyda, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21529
Publ.-Id: 21529


Concept of a compact gantry for laser based proton therapy.

Karsch, L.; Bussmann, M.; Enghardt, W.; Kroll, F.; Masood, U.; Pawelke, J.

  • Poster
    52th Particle Therapy Cooperative Group (PTCOG) Meeting, 02.-08.06.2013, Essen, Deutschland
  • Contribution to proceedings
    52th Particle Therapy Cooperative Group (PTCOG) Meeting, 02.-08.06.2013, Essen, Deutschland
    Proceedings of the 52th Particle Therapy Cooperative Group (PTCOG) Meeting, 312

Permalink: https://www.hzdr.de/publications/Publ-21528
Publ.-Id: 21528


Einfluss der Pulslänge auf die Sättigungskorrektur von Ionisationskammern in gepulsten Strahlungsfeldern.

Gotz, M.; Karsch, L.; Pawelke, J.

  • Lecture (Conference)
    44. Jahrestagung der Deutschen Gesellschaft für Medizinische Physik (DGMP), 18.-21.09.2013, Köln, Deutschland
  • Contribution to proceedings
    44. Jahrestagung der Deutschen Gesellschaft für Medizinische Physik (DGMP), 18.-21.09.2013, Köln, Deutschland
    H. Treuer (Ed.): Medizinische Physik 2013, Deutsche Gesellschaft für Medizinische Physik e. V., ISBN 978-3-9816002-1-6, 318-321

Permalink: https://www.hzdr.de/publications/Publ-21527
Publ.-Id: 21527


Optimierung der Bestrahlungsplanungsstrategie für laserbeschleunigte Protonen.

Findeisen, A.; Karsch, L.; Masood, U.; Pawelke, J.

  • Poster
    44. Jahrestagung der Deutschen Gesellschaft für Medizinische Physik (DGMP), 18.-21.09.2013, Köln, Deutschland
  • Contribution to proceedings
    44. Jahrestagung der Deutschen Gesellschaft für Medizinische Physik (DGMP), 18.-21.09.2013, Köln, Deutschland
    H. Treuer (Ed.): Medizinische Physik 2013, Deutsche Gesellschaft für Medizinische Physik e. V., ISBN 978-3-9816002-1-6, 714-715

Permalink: https://www.hzdr.de/publications/Publ-21526
Publ.-Id: 21526


Proton therapy in Dresden: present and future

Pawelke, J.

  • Invited lecture (Conferences)
    International Conference on the Future of Radiation Oncology: Imaging, Dosimetry, Biology and Therapy (7th Berder Workshop), 25.-28.09.2013, Berder Island, France

Permalink: https://www.hzdr.de/publications/Publ-21525
Publ.-Id: 21525


Review on radiobiological studies using laser driven particle beams:From in vitro experiments to human tumor irradiations on mice

Pawelke, J.

  • Invited lecture (Conferences)
    International Workshop on Microbeam Probes of Cellular Radiation Response (11th Microbeam Workshop), 03.-04.10.2013, Bordeaux, Frankreich

Permalink: https://www.hzdr.de/publications/Publ-21524
Publ.-Id: 21524


Development of a compact particle therapy facility with laser- driven ion beams via novel pulse powered gantry systems

Masood, U.; Bussmann, M.; Baumann, M.; Cowan, T. E.; Enghardt, W.; Herrmannsdörfer, T.; Hofmann, K. M.; Kaluza, M.; Karsch, L.; Kroll, F.; Schramm, U.; Schürer, M.; Wilkens, J. J.; Pawelke, J.

  • Contribution to proceedings
    Joint Conference in Medical Physics, 07.-10.09.2014, Zürich, Schweiz
    Proceedings of the Joint Conference in Medical Physics, 27-28
  • Lecture (Conference)
    Joint Conference in Medical Physics, 07.-10.09.2014, Zürich, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-21523
Publ.-Id: 21523


A treatment planning study to assess the feasibility and the limitations of laser-driven proton therapy

Hofmann, K. M.; Masood, U.; Pawelke, J.; Wilkens, J. J.

  • Contribution to proceedings
    Joint Conference in Medical Physics, 07.-10.09.2014, Zürich, Schweiz
    Proceedings of the Joint Conference in Medical Physics, 29
  • Lecture (Conference)
    Joint Conference in Medical Physics, 07.-10.09.2014, Zürich, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-21522
Publ.-Id: 21522


Wechselwirkung langsamer hochgeladener Ionen mit Ionenkristalloberflächen und ultradünnen Kohlenstoffmembranen

Wilhelm, R. A.

In der vorliegenden Arbeit wurde die Strukturbildung durch Einzeleinschlag von langsamen hochgeladenen Ionen (v < 5·10^5 m/s, Q < 40) auf Ionenkristalloberflächen und freistehenden amorphen Kohlenstofffolien untersucht. Mittels experimenteller Methoden der Rasterkraft-, Transmissionselektronen- und Heliumionenmikroskopie wurden die in jüngster Zeit gewonnenen Modellbeschreibungen für Nanostrukturbildung an Oberflächen erweitert. Beim Auftreffen eines langsamen hochgeladenen Ions auf die (001)-Oberfläche von KCl können Nanostrukturen mit Ausdehnungen bis zu einigen 10nm entstehen. Die damit verknüpften Desorptionsausbeuten von bis zu 3000 Atomen pro Ion aus der Oberfläche werden vor allem durch die Deposition der potentiellen Energie in sehr oberflächennahe Schichten erklärt. Die kinetische Energie der Ionen wirkt jedoch unterstützend auf die Entstehung und Vergrößerung der Lochstrukturen. Die Entstehung der Strukturen durch die potentielle Energie kann konsistent im Bild der defektinduzierten Desorption erklärt werden.
Für die CaF2(111)-Oberfläche konnte gezeigt werden, dass auch unter ununterbrochenen Ultrahochvakuumbedingungen Nanohügel durch Ionenbeschuss erzeugt werden können. Die Größenverteilung beobachteter Nanohügel ist dabei jedoch um einen Faktor 3 kleiner im Vergleich zu atmosphärenexponierten Proben. Der Einfluss der Atmosphäre auf die beobachteten Strukturgrößen wird diskutiert. Für niedrigere Ladungszustände als nötig für Nanohügelerzeugung wurden ioneninduzierte Kristalldefekte durch nasschemisches Ätzen der Oberfläche nachgewiesen. Diese Tatsache verbindet die Beschreibung der Strukturbildung durch elektronische Anregungen in Alkali- und Erdalkalihaliden.
Für den Durchgang von hochgeladenen Ionen durch 1 nm dicke, freistehende und amorphe Kohlenstofffolien zeigte sich, dass Poren durch Einzelioneneinschlag entstehen, sobald ein Schwellwert von etwa Q = 25 in der Ionenladung überschritten wurde. Erstmalig konnte gezeigt werden, dass sich bei der Transmission der Ionen zwei unterschiedliche Ladungszustandsverteilungen bilden. Die Ausbildung der beiden Verteilungen wird durch einen stark stoßparameterabhängigen Ladungsaustausch beschrieben. Neben dem Einfluss der potentiellen Energie bei der Porenbildung ist auch der Einfluss der kinetischen Energie untersucht worden. Um den Anteil nuklearer und elektronischer kinetischer Energieverluste in Abhängigkeit des Ionenladungszustandes abzuschätzen, wurde ein Modell im Rahmen der Beschreibung statistischer Atome bzw. Ionen aufgestellt. Auf Basis dieses Modells wird der Einfluss der kinetischen Energiedeposition und die damit verbunde- ne Synergie aus potentieller und kinetischer Energie bei der Strukturbildung diskutiert.

The formation of nano-structures due to the impact of individual slow highly charged ions (v < 5 · 105 m/s, Q < 40) on surfaces of ionic crystals and 1 nm thick, freestanding and amorphous carbon membranes is investigated in this work. By an experimental approach using atomic force, transmission elec- tron and helium ion microscopy recent model descriptions for the formation processes of highly charged ion induced nano-structures are expanded. Upon impact of slow highly charged ions on the (001)-surface of KCl pit-like structures with sizes up to a few 10nm are found. The corresponding desorption yields of a few thousand atoms per ion are mainly attributed to the deposition of potential energy in a shallow region near the surface. The kinetic energy has only a minor effect on the formation processes. The structure formation due to potential energy deposition is described consis- tently in the framework of a defect induced desorption model.
For the CaF2(111) surface it is shown, that even without breaking the vac- uum between irradiation and analyses ion induced nano-hillocks are present. However, the sizes of the nano-hillocks are about a factor of 3 smaller than the sizes after exposure to atmosphere. The influence of the atmosphere conditions on the structure sizes is discussed. For lower charge states than needed for nano-hillock formation ion-induced defects are revealed by wet- chemical etching of the exposed surfaces. The existence of defects for lower charge states combines the model descriptions of nano-structure formation by strong electronic excitations for alkali and earth-alkali halide surfaces. Pore formation due to highly charged ion impact on 1 nm thick, freestanding and amorphous carbon films is observed for ions with a charge state above a threshold of about Q = 25. It has been shown for the first time that two distinct exit charge state distributions are formed upon transmission of the ions through the membrane. The formation of the distributions is explained
by a strongly impact parameter dependent charge exchange. Besides the influence of the potential energy on the pore formation also the importance of the kinetic energy is investigated. To distinguish between nuclear and electronic losses in the case of highly charged ions at low velocities a model for charge state dependent energy loss is proposed. Based on the descrip- tion of statistical atoms and ions, respectively, the synergy of potential and kinetic energy upon nano-structure formation is discussed.

  • Doctoral thesis
    TU Dresden, 2014
    Mentor: Prof. Jürgen Fassbender / Dr. Stefan Facsko
    149 Seiten

Permalink: https://www.hzdr.de/publications/Publ-21521
Publ.-Id: 21521


In vivo dose response to laser driven electron beams

Oppelt, M.; Baumann, M.; Bergmann, R.; Beyreuther, E.; Brüchner, K.; Hartmann, J.; Kaluza, M.; Karsch, L.; Krause, M.; Laschinsky, L.; Lessmann, E.; Nicolai, M.; Reuter, M.; Sävert, A.; Schnell, M.; Schürer, M.; Pawelke, J.

  • Contribution to proceedings
    Joint Conference in Medical Physics, 07.-10.09.2014, Zürich, Schweiz
    Proceedings of the Joint Conference in Medical Physics, 191
  • Lecture (Conference)
    Joint Conference in Medical Physics, 07.-10.09.2014, Zürich, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-21520
Publ.-Id: 21520


Laserbasierte Partikeltherapie.

Pawelke, J.

Laserbasierte Partikeltherapie.

  • Invited lecture (Conferences)
    Symposium “Innovationen in der Radioonkologie”, 24.09.2014, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21519
Publ.-Id: 21519


Development of short-pulsed high-field electromagnetic dipoles for laser-based proton therapy

Schürer, M.; Herrmannsdörfer, T.; Karsch, L.; Kroll, F.; Masood, U.; Pawelke, J.

Introduction
During the last years, the new technology of laser based particle acceleration was developed at such a rate that medical application for cancer therapy becomes conceivable. Promising more compact and economic accelerators, the laser technology however generates intense ultra-short (~ ps) pulsed proton beams with large divergence and broad energy spectrum. Within the German joint research project “onCOOPtics” the clinical applicability of such pulsed proton beams is investigated including the development of a laser accelerator and a suitable beam transport.
Methods
A compact beam transport system was designed enabling an efficient transport of proton pulses from generation to treatment site. The initially divergent proton beam is captured by a cylindrical electromagnet (solenoid), deflected by 45° dipole magnets and formed by quadrupole magnets, whereas the spectrum is shaped by adaptable lead apertures. For realization, electromagnetic dipoles with magnetic fields of up to 10 T are required to deflect up to 220 MeV protons. These field strengths are achieved by in-house developed non-ferrous dipoles that consist of 80 copper coils in 12 layers and are operated at peak currents of up to 20 kA. To handle the high currents and the generated heat the dipoles are externally cooled and operated in 1 ms short pulses synchronized with the laser repetition frequency.
Results
The prototype of a short-pulsed electromagnetic dipole magnet was designed and manufactured. Results of the experimental characterization and first performance tests at a conventional Tandem accelerator are under way.
Conclusion
Pulsed electromagnetic dipoles as crucial components of a compact beam line for laser-accelerated protons are engineered. Following validation of their suitability at a conventional accelerator the dipoles will be implemented and further tested at a laser accelerator. Together with improvement of the dipole the design of quadrupoles will start.

  • Open Access Logo Abstract in refereed journal
    Biomedical Engineering / Biomedizinische Technik 59(2014)Suppl 1, 891
    DOI: 10.1515/bmt-2014-5011
  • Lecture (Conference)
    48. Jahrestagung der Deutschen Gesellschaft für Biomedizinische Technik, 08.-10.10.2014, Hannover, Deutschland
  • Poster
    48. Jahrestagung der Deutschen Gesellschaft für Biomedizinische Technik, Hannover, Germany, 8-10 Oct 2014, 08.-10.10.2014, Hannover, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21518
Publ.-Id: 21518


Development of laser-driven proton beam therapy

Masood, U.; Bussmann, M.; Cowan, T.; Enghardt, W.; Herrmannsdörfer, T.; Kaluza, M.; Krause, M.; Pawelke, J.; Sauerbrey, R.; Schramm, U.; Baumann, M.

  • Open Access Logo Abstract in refereed journal
    Annals of Oncology 25(2014)Suppl 4, iv546-iv547
    DOI: 10.1093/annonc/mdu358.3
  • Lecture (Conference)
    Congress of the European Society for Medical Oncology (ESMO) on “Precision Medicine in Cancer Care”, 26.-30.09.2014, Madrid, Spanien
  • Poster
    Congress of the European Society for Medical Oncology (ESMO) on “Precision Medicine in Cancer Care”, 26.-30.09.2014, Madrid, Spanien

Permalink: https://www.hzdr.de/publications/Publ-21517
Publ.-Id: 21517


Negative magneto- and electroresistance of silicon films with superconducting nanopreciptates - the role of inelastic cotunneling

Heera, V.; Fiedler, J.; Schmidt, B.; Hübner, R.; Voelskow, M.; Skrotzki, R.; Skorupa, W.

The electronic transport properties of insulating Si:Ga films with superconducting, Ga-rich nanopreciptates are investigated in dependence on temperature, current, and magnetic field. The large negative magnetoresistance, observed below the critical temperature, can be explained by Cooper pair breaking and subsequent tunneling of the fermionic quasiparticles. Localization due to quantum interferences of bosons or fermions, as recently discussed, seems not to be the reason for the insulating state and the large magnetoresistance. Cooper pair tunneling is blocked by the high Coulomb barrier. The quasiparticles can overcome the barrier by inelastic cotunneling that results in nonlinear current-voltage characteristics and negative electroresistance. Since the experimental results obtained for the Si:Ga film resemble that of many other films with superconducting nanoprecipitates the conclusions drawn here could be quite general.

Keywords: disordered films; superconducting precipitates; insulating state; magnetotransport measurement; negative magnetoresistance; inelastic cotunneling

Permalink: https://www.hzdr.de/publications/Publ-21516
Publ.-Id: 21516


Comparison study of in vivo dose response to laser driven versus conventional electron beam.

Oppelt, M.; Baumann, M.; Bergmann, R.; Beyreuther, E.; Brüchner, K.; Hartmann, J.; Karsch, L.; Krause, M.; Laschinsky, L.; Leßmann, E.; Nicolai, M.; Reuter, M.; Richter, C.; Sävert, A.; Schürer, M.; Schnell, M.; Woithe, J.; Kaluza, M.; Pawelke, J.

The long-term goal to integrate laser-based particle accelerators into radiotherapy clinics not only requires technological development of high-intensity lasers and new techniques for beam detection and dose delivery, but also characterization of the biological consequences of this new particle beam quality, i.e. ultra-short, ultra-intense pulses. In the present work, we describe successful in vivo experiments with laser-driven electron pulses by utilization of a small tumour model on the mouse ear for the human squamous cell carcinoma model FaDu. The already established in vitro irradiation technology at the laser system JETI was further enhanced for 3D tumour irradiation in vivo in terms of beam transport, beam monitoring, dose delivery and dosimetry in order to precisely apply a prescribed dose to each tumour in full-scale radiobiological experiments. Tumour growth delay was determined after irradiation with doses of 3 and 6 Gy by laser-accelerated electrons. Reference irradiation was performed with continuous electron beams at a clinical linear accelerator in order to both validate the dedicated dosimetry employed for laser-accelerated JETI electrons and above all review the biological results. No significant difference in radiation-induced tumour growth delay was revealed for the two investigated electron beams. These data provide evidence that the ultra-high dose rate generated by laser acceleration does not impact the biological effectiveness of the particles.

Permalink: https://www.hzdr.de/publications/Publ-21515
Publ.-Id: 21515


Scaling of TNSA-accelerated proton beams with laser energy and focal spot size

Obst, L.; Zeil, K.; Metzkes, J.; Kraft, S.; Schramm, U.

By focusing an ultra-short high-intensity laser pulse on a solid target, pulses of protons and other positively charged ions with energies of several 10 MeV per nucleon are generated. The properties of these particle beams such as their energy and absolute number are highly dependent on experimental conditions like laser and target parameters. In order to achieve principal comparability between different experimental campaigns at the Draco laser system at the Helmholtz-Zentrum Dresden-Rossendorf, a reference setup for the laser ion acceleration experiment was established. A configuration is sought in which proton beams of reproducible characteristics are generated. To ensure a high stability of the proton spectra, the application of longer focal length parabolas (f ~ 1000 mm) will be tested for this setup, according preparatory studies being presented in this paper.

Keywords: accelerators; laser-driven proton acceleration; high-intensity lasers; beams and electromagnetism

  • Open Access Logo Contribution to proceedings
    IPAC'14 - 5th International Particle Accelerator Conference, 15.-20.06.2014, Dresden, Deutschland
    IPAC2014 - Proceedings, TUPME033
  • Poster
    IPAC'14 - 5th International Particle Accelerator Conference, 15.-20.06.2014, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21514
Publ.-Id: 21514


Energetic stability of solute–carbon–vacancy complexes in bcc iron

Bakaev, A.; Terentyev, D.; Zhurkin, E. E.; van Neck, D.

The strong binding between a vacancy and carbon in bcc iron plays an important role in the evolution of radiation-induced microstructure. Our previous ab initio study points to the fact that the vacancy–carbon (V–C) pair can serve as a nucleus for the solute-rich clusters. Here, we continue the ab initio study by considering the interaction of mixed solute clusters (Mn, Ni and Si) with the V–C pair, and the interaction of typical alloying elements of Fe-based steels (i.e., Mn, Ni, Cu, Si, Cr and P) with di-carbon–vacancy pair (V–C2). We have identified the sequence of growth of Ni, Si and Mn solute-rich clusters nucleating on the V–C pair. The mixed-solute–V–C configurations are found to be less stable clusters than pure-solute–V–C clusters with the energy difference up to 0.22 eV per four atoms. The V–C2 pair is found to be as strong nucleation site for the solute-rich clusters as the V–C pair. Only Si solute atom stands out from the trend showing a weaker affinity to the V–C2 complex by 0.09 eV compared to the attraction to the V–C pair. The overall results point to the importance of taking into account the existence of both V–C and V–C2 complexes in studying the formation of solute-rich clusters in Fe-based steels for nuclear applications.

Keywords: Ferritic steels; Minor alloying elements; Carbon; Solute clusters; Ab initio calculations

Permalink: https://www.hzdr.de/publications/Publ-21513
Publ.-Id: 21513


The pH dependence of Am(III) complexation with acetate: an EXAFS study

Froehlich, D. R.; Skerencak-Frech, A.; Bauer, N.; Rossberg, A.; Panak, P. J.

The complexation of acetate with Am(III) is studied as a function of the pH (1–6) by extended X-ray absorption fine-structure (EXAFS) spectroscopy. The molecular structure of the Am(III)–acetate complexes (coordination numbers, oxygen and carbon distances) is determined from the raw k3-weighted Am LIIIedge EXAFS spectra. The results show a continuous shift of Am(III) speciation with increasing pH value towards the complexed species. Furthermore, it is verified that acetate coordinates in a bidentate coordination mode to Am(III) (Am—C distance: 2.82 0.03 A ° ). The EXAFS data are analyzed by iterative transformation factor analysis to further verify the chemical speciation, which is calculated on the basis of thermodynamic constants, and the used structural model. The experimental results are in very good agreement with the thermodynamic modelling.

Keywords: americium; acetate; coordination chemistry; EXAFS; ITFA

Permalink: https://www.hzdr.de/publications/Publ-21512
Publ.-Id: 21512


A new look at the structural properties of trisodium uranate Na3UO4

Smith, A. L.; Raison, P. E.; Martel, L.; Prieur, D.; Charpentier, T.; Wallez, G.; Suard, E.; Scheinost, A. C.; Hennig, C.; Martin, P.; Kvashnina, K. O.; Cheetham, A. K.; Konings, R. J. M.

The crystal structure of the trisodium uranate, which forms following the interaction between sodium and hyperstoichiometric urania, has been solved for the first time using powder X-ray and neutron diffraction, X-ray Absorption Near-Edge Structure spectroscopy, and solid state 23Na Multi-Quantum Magic Angle Spinning Nuclear Magnetic Resonance. The compound, isostructural with Na3BiO4, has a monoclinic symmetry, in space group P2~c. Moreover, it has been shown that this structure can accommodate some cationic disorder, with up to 16(2)% sodium on the uranium site, corresponding to the composition -Na3(U1−x,Nax)O4 (0

Keywords: Sodium uranate; X-ray diffraction (XRD); Neutron diffraction; XANES; MQMAS NMR

Permalink: https://www.hzdr.de/publications/Publ-21511
Publ.-Id: 21511


Pushing the Limits of Ferromagnetic Resonance: Detection of Single sub-100-nm Nanodots

Lenz, K.; Banholzer, A.; Narkowicz, R.; Grebing, J.; Stienen, S.; Lindner, J.; Fassbender, J.

Spintronic devices like MRAM, STNOs, or magnonic crystals are based on various types of magnetic nanostructures. Hence, it is crucial to know their magnetic properties, e.g., to allow for proper simulation for further development. The magnetic characterization in terms of magnetic damping, resonance modes, or magnetic anisotropy of single nanosized objects is very challenging, nevertheless of utmost importance, as otherwise bulk or film parameters need to be used for simulations.
Ferromagnetic resonance (FMR) is in principle the ultimate technique to measure such parameters. Unfortunately, conventional FMR based on resonant cavities and even modern broadband coplanar waveguide FMR lacks the sensitivity to measure single sub-micron-sized nano elements. Usually the detection limit of FMR, i.e. the minimum number of spins, which can be detected, is about 1012 spins for permalloy. Up to now, arrays of such elements had to be prepared to overcome this limit. However, great care on a homogeneous sample preparation has to be taken. For example, already slight inhomogeneities between the array's elements render the individual resonant modes of the nanostructures invisible, due to linewidth broadening.
For the analysis of single nanoobjects a much higher sensitivity is required. Using our recently developed microresonators [1,2] we show how single nanoelements down to sample diameters of 100 nm can be measured.
Taking the signal-to-noise ratio achieved so far into account, we extrapolate the detection limit to 105 spins. The uniform excitation mode as well as various localized modes like e.g. edge modes can be observed. Their state can be visualized with micromagnetic simulations.
References:
[1] A. Banholzer, et al., Nanotechnology 22, 295713 (2011).
[2] R. Narkowicz et al., Rev. Sci. Instrum. 79, 084702 (2008).

Keywords: ferromagnetic resonance; nanostructures; damping; spinwaves; microresonators

  • Lecture (Conference)
    59th Annual Magnetism and Magnetics Materials Conference, 04.-07.11.2014, Honolulu, Hawaii, USA

Permalink: https://www.hzdr.de/publications/Publ-21510
Publ.-Id: 21510


Light emitting diode based on (In,Ga)As/GaAs coaxial multi-shell nanowires monolithically integrated on silicon

Dimakis, E.; Jahn, U.; Ramsteiner, M.; Tahraoui, A.; Grandal, J.; Trampert, A.; Biermanns-Föth, A.; Pietsch, U.; Riechert, H.; Geelhaar, L.

We demonstrate the use of III-arsenide nanowires as light emitting diodes (LEDs) monolithically integrated on Silicon. LEDs made of (In,Ga)As/GaAs coaxial multi-shell nanowires were grown catalyst-free directly on Si(111) by molecular beam epitaxy (MBE). The active region consists of a single (In,Ga)As/GaAs quantum well in the radial direction (Fig. 1-a). Correlating the emission properties of the quantum wells (Fig. 1-b) with the growth kinetics on the ( ) side-walls, we were able to identify the optimal growth conditions for coherently strained quantum wells with high-quality interfaces and homogeneous structure (in terms of chemical composition and shell thicknesses) along the nanowire axes. Shell-doping methods were successfully employed for the realization of p- and n-type GaAs shells, while a planarization scheme with transparent ohmic contacts allowed massive biasing in parallel configuration of the free-standing nanowires on the Silicon substrate. Rectifying operation and room-temperature electroluminescence were obtained (Fig. 1-c), proving the great potential of this technology.

  • Invited lecture (Conferences)
    Nanowires 2013, 12.11.2013, Weizmann Institute of Science, Rehovot, Israel

Permalink: https://www.hzdr.de/publications/Publ-21509
Publ.-Id: 21509


Magnetostatics and Dynamics of Ion Irradiated NiFe/Ta Multilayer Films Studied by Vector Network Analyzer Ferromagnetic Resonance

Markó, D.

In the present work, the implications of ion irradiation on the magnetostatic and dynamic properties of soft magnetic Py/Ta (Py = Permalloy: Ni80Fe20) single and multilayer lms have been investigated with the main objective of nding a way to determine their saturation magnetization. Both polar magneto-optical Kerr eect (MOKE) and vector network analyzer ferromagnetic resonance (VNA-FMR) measurements have proven to be suitable methods to determine 0MS, circumventing the problem of the unknown eective magnetic volume that causes conventional techniques such as SQUID or VSM to fail. Provided there is no perpendicular anisotropy contribution in the samples, the saturation magnetization can be determined even in the case of strong interfacial mixing due to an inherently high number of Py/Ta interfaces and/or ion irradiation with high uences.
Another integral part of this work has been to construct a VNA-FMR spectrometer capable of performing both azimuthal and polar angle-dependent measurements using a magnet strong enough to saturate samples containing iron. Starting from scratch, this comprised numerous steps such as developing a suitable coplanar waveguide design, and writing the control, evaluation, and tting software.
With both increasing ion uence and number of Py/Ta interfaces, a decrease of saturation magnetization has been observed. In the case of the 10Py samples, an immediate decrease of 0MS already sets in at small ion uences. However, for the 1Py and 5Py samples, the saturation magnetization remains constant up to a certain ion uence, but then starts to rapidly decrease. Ne ion irradiation causes a mixing and broadening of the interfaces. Thus, the Py/Ta stacks undergo a transition from being polycrystalline to amorphous at a critical uence depending on the number of interfaces. The saturation magnetization is found to vanish at a Ta concentration of about 10{15 at.% in the Py layers. The samples possess a small uniaxial anisotropy, which remains virtually unaected by the ion uence, but slightly reduces with an increasing number of Py/Ta interfaces. In addition to magnetostatics, the dynamic properties of the samples have been investigated as well. The Gilbert damping parameter increases with both increasing number of Py/Ta interfaces and higher ion uences, with the former having a stronger in uence. The inhomogeneous linewidth broadening B0 increases as well with increasing number of Py/Ta interfaces, but slightly decreases for higher ion uences.

Keywords: ferromagnetic resonance; FMR; thin films; multilayers; ferromagnetism

  • Doctoral thesis
    TU Dresden, 2010
    Mentor: Jürgen Fassbender
    107 Seiten

Permalink: https://www.hzdr.de/publications/Publ-21508
Publ.-Id: 21508


From 2D CoCrPt:SiO2 films with perpendicular magnetic anisotropy to 3D nanocones — A step towards bit patterned media

Ball, D. K.

Due to the ever-increasing worldwide consumption of memory for digital information, new technologies for higher capacity and faster data storage systems have been the focus of research and development. A step towards achieving higher data storage densities for magnetic recording media is the concept of bit patterned media, where the magnetic recording layer is divided up into magnetically isolated bit units. This approach is one of the most promising technologies for increasing data storage densities and could be implemented by nanostructuring the wafer. Therefore, the fabrication of the appropriate nanostructures on a small scale and then be able to manufacture these structures on an industrial scale is one of the problems where science and industry are working on a solution. In addition, the answer to the open question about the influence that patterning on the nano length scale has on the magnetic properties is of great interest.
The main goal of this thesis is to answer the open question, which magnetic properties can be tailored by a modification of the surface texture on the nanometre length scale. For this purpose the following properties: anisotropy, remanence, coercivity, switching field distribution, saturation magnetisation, Gilbert damping, and inhomogeneous linebroadening were compared between planar two dimensional thin ferromagnetic films and three dimensional magnetic structures. In addition, the influences of the tailored morphology on the intergranular or the exchange coupling between the structures, which is called interdot exchange coupling, was investigated. For the ferromagnetic thin films, the focus of the investigations was on the granular CoCrPt:SiO2 and [Co/Pd] layer, which currently are the state-of-the-art material for magnetic data storage media. These materials are characterised by their high coercivity and high perpendicular anisotropy, which has a low spatial distribution in the preferred direction of magnetisation.
In this work the pre-structured GaSb(001) substrate with self-assembled periodic nanocone structures at the surface are used. The preparation by ion beam erosion of these structures is simple, fast, and highly reproducible and therefore this method is particularly beneficial for fundamental research. To compare the 2D thin films with the 3D magnetic structures, besides the pre-structured specimen, planar samples were also fabricated. The first sample series prepared was coated by Py. Due to the fact that the magnetic properties of this material are well-known, it was also possible to do some OOMMF simulations in addition to the VNA-FMR and MOKE measurements.
Afterwards two planar samples with CoCrPt and CoCrPt:SiO2 were prepared. The planar CoCrPt:SiO2 samples were Co+ ion implanted to study the influence of such irradiation on the intergranular and interdot exchange coupling, switching field distribution, and in particular on the spin dynamics. Moreover, both samples were measured by TRMOKE in order to obtain information about the spin dynamics.
Subsequently, the perpendicular storage media materials CoCrPt:SiO2 and [Co/Pd] were deposited on a prestructured GaSb(001) nanocone substrate surface. These sample series were measured by MOKE, SQUID, and vector-VSM. The measurements demonstrate the influence of the periodicity and height of the nanocones on the intergranular and interdot exchange coupling. They also show the reorientation of the magnetization with respect to the curvature of the substrate template and furthermore, the morphology-induced influences on the magnetic domains.
From the comparison between the results for the planar and the pre-structured samples, a decrease of the interdot exchange coupling was observed, which scales together with the periodicity of the nanocone pattern. In addition, it was shown that for all samples with thin magnetic films on nanocones,the magnetization aligns along the curvature of the underlying nanocone structure. For Py on nanocones, planar granular CoCrPt:SiO2, and planar granular CoCrPt, measurements by VNA-FMR and TRMOKE could be carried out, which yielded information about the spin dynamics. The results obtained for both of the planar sample are comparable to values from the literature for the Gilbert damping. The results for the Py samples showed that the commonly used 2D model resonance condition is, in case of a 3D magnetic structure, no longer valid due to the alignment of the magnetisation along the underlying substrate structure and therefore an new model has to be derived.

Keywords: Ferromagnetic resonance; VSM; ferromagnetism; MOKE; nanostructures; spin waves

  • Doctoral thesis
    TU Dresden, 2013
    Mentor: Jürgen Fassbender
    132 Seiten

Permalink: https://www.hzdr.de/publications/Publ-21507
Publ.-Id: 21507


Method for evaluation of upgrading by liberation and separation

Leißner, T.; Mütze, T.; Atanasova, P.; Bachmann, K.; Peuker, U. A.

A method is presented for the evaluation of mineral processing by liberation and upgrading. The method bases on the plot of recovery of valuables versus the recovery of gangue (Fuerstenau upgrading curve). The locking curve of a feed material was plotted together with the upgrading curve in a Fuerstenau diagram. The assessment of liberation and upgrading is done by a comparison of surfaces formed by the curves and some characteristic lines of the diagram. This plot allows the calculation of two new quantitative measures for liberation and upgrading.
Both parameters can be combined to a third parameter describing the quality of the whole technical setup for mineral processing by subtracting the upgrading parameter from the liberation parameter. The third parameter shows a positive value in cases of upgrading determined processes, a negative value in cases of liberation affected processes and about zero for processes affected equally by liberation and upgrading. Thus it is easily possible to distinguish between poor results in the mineral processing caused by insufficient liberation or poor results caused by insufficient upgrading. Preliminary results from two case studies, performed on two different ore types, are very promising illustrating the practical use of such an approach.

Keywords: flotation modeling; optimization; process development

  • Contribution to proceedings
    XXVII International Congress on Mineral Processing, 20.-24.10.2014, Santiago de Chile, Chile
    Proceedings of the XXVII International Mineral Processing Congress – IMPC 2014, 1-10

Permalink: https://www.hzdr.de/publications/Publ-21506
Publ.-Id: 21506


Morphology-Induced Magnetic Phenomena Studied by Broadband Ferromagnetic Resonance

Körner, M.

In the present work, the influence of the morphology of thin ferromagnetic films on their static as well as dynamic magnetic properties was investigated by means of broadband ferromagnetic resonance (FMR). Using an ion beam erosion process the surface of the substrates was periodically modulated (ripples), where the modulation wavelength is determined by the ion energy. In this way a well-controllable roughness profile evolves ranging from a few ten up to several hundreds of nanometers in wavelength. The substrate’s surface profile in turn is repeated by films grown on top offering an easy and fast approach to investigate morphology influences on the magnetic properties. This work aims on modifications of the magnetic anisotropy as well as the FMR linewidth of the magnetic relaxation process.
Prior to magnetic investigations the existing FMR setup was extended to measure FMR spectra at a fixed microwave frequency while sweeping the external magnetic field. Furthermore, a software toolbox was developed to perform the data processing and evaluation.
Starting with the morphology influence on the magnetic anisotropy 10 nm thin Fe, Co, and Ni81Fe19 (Permalloy Py) films were deposited on rippled Si substrates. Due to Si displacements during ion erosion and natural oxidation the rippled Si substrates exhibit an amorphous surface causing a polycrystalline material growth. This leads to a suppression of magneto-crystalline anisotropy leaving only morphology-induced anisotropy contributions.
Here, a uniaxial magnetic anisotropy (UMA) was observed that aligns its easy axis with the ripple ridges, whereas its strength decays with increasing ripple wavelength for all materials. From thickness-dependent measurements two characteristic regions were determined with competing uniaxial volume and surface anisotropy contributions. Underlined by micromagnetic simulations a dominant volume contribution was found in the thin region accompanied by magnetic moments nearly following the surface corrugation. In the thick region the UMA is controlled by dipolar stray fields at the surface.
In contrast to Si, ion eroded MgO keeps its crystal structure offering epitaxial growth of 10 nm thin single-crystalline Fe films. Consequently, a superposition of morphology-induced UMA and magneto-crystalline cubic anisotropy was observed. The direction of the ripple ridges is predetermined by the incident ion beam, which allows to freely orient the UMA’s direction with respect to the cubic anisotropy, offering a possibility for anisotropy engineering. In comparison to the planar reference case rippled magnetic films exhibit lower intrinsic and extrinsic relaxation contributions.
For the final part, 30 nm Py was grown on rippled Si covering modulation wavelengths l ranging from 27 to 432 nm. Using magnetic force microscopy and holography measurements the dipolar stray fields above and inside the magnetic layer were characterized. For l 222 nm, the stray fields act as scattering centers for spin waves triggering two-magnon scattering (TMS). This causes an apparent line broadening generating distinct peaks in the frequency-dependent linewidth whose position can be tuned by altering l. These effects are understood in the framework of a perturbation theory of spin waves in periodically perturbed films recently presented in the literature. Furthermore, the in-plane angular dependence of the linewidth revealed a two-fold symmetry, which is not present for vanishing TMS at small l.

Keywords: ferromagnetic resonance; ferromagnetism; FMR; thin films; spin waves

  • Doctoral thesis
    TU Dresden, 2013
    Mentor: Jürgen Fassbender
    185 Seiten

Permalink: https://www.hzdr.de/publications/Publ-21505
Publ.-Id: 21505


Polytypism in GaAs nanowires: determination of the interplanar spacing of wurtzite GaAs by X-ray diffraction

Köhl, M.; Schroth, P.; Minkevich, A. A.; Hornung, J.-W.; Dimakis, E.; Somaschini, C.; Geelhaar, L.; Aschenbrenner, T.; Lazarev, S.; Grigoriev, D.; Pietsch, U.; Baumbach, T.

In GaAs nanowires grown along the cubic [111]c direction, zinc blende and wurtzite arrangements have been observed in their stacking sequence, since the energetic barriers for nucleation are typically of similar order of magnitude. It is known that the interplanar spacing of the (111)c Ga (or As) planes in the zinc blende polytype varies slightly from the wurtzite polytype. However, different values have been reported in the literature. Here, the ratio of the interplanar spacing of these polytypes is extracted based on X-ray diffraction measurements for thin GaAs nanowires with a mean diameter of 18–25 nm. The measurements are performed with a nano-focused beam which facilitates the separation of the scattering of nanowires and of parasitic growth. The interplanar spacing of the (111)c Ga (or As) planes in the wurtzite arrangement in GaAs nanowires is observed to be 0.66% 0.02% larger than in the zinc blende arrangement.

Keywords: GaAs; nanowires; polytypism; X-ray diffraction; nanofocus

Permalink: https://www.hzdr.de/publications/Publ-21504
Publ.-Id: 21504


Analyse eines Messplatzes für frequenzaufgelöste magnetooptische Kerr-Effektmessungen (FR-MOKE) sowie dessen Ansteuerung auf der Basis von LabVIEW

Trache, M.

keines Vorhanden

Keywords: ferromagnetism; ferromagnetic resonance; MOKE; FR-MOKE

  • Diploma thesis
    Wilhelm Büchner Hochschule, 2012
    Mentor: Kilian Lenz
    80 Seiten

Permalink: https://www.hzdr.de/publications/Publ-21503
Publ.-Id: 21503


Untersuchung der elektrischen und magnetischen Eigenschaften von BiFeO3-Filmen

Noack, H.

keines vorhanden.

Keywords: ferromagnetism; ferromagnetic resonance; ferroelectrics

  • Diploma thesis
    Wilhelm Büchner Hochschule, 2011
    Mentor: Kilian Lenz
    94 Seiten

Permalink: https://www.hzdr.de/publications/Publ-21502
Publ.-Id: 21502


Magnetische Charakterisierung dünner Keilschichten mittels frequenzaufgelöstem magnetooptischen Kerr-Effekt

Hoffmann, S.

Im Rahmen dieser Arbeit wurden erstmalig ein Probensystem aus zwei ferromagnetischen Fe3Si Schichten und einer keilförmigen Magnesiumoxid (MgO) Zwischenschicht, mittels der Kombination von ferromagnetischer Resonanz (FMR) und magnetooptischer Detektion mit Hilfe des Kerr-Effekts (MOKE), am FR-MOKE Aufbau untersucht. An verschiedenen Stellen der Probe wurden frequenzabhängige Messungen durchgeführt, um daraus Rückschlüsse auf die magnetischen Eigenschaften des Materials zu ziehen. Durch den Vergleich mit Literaturwerten konnte gezeigt werden, dass mithilfe dieser Messmethode durchaus zuverlässige Messungen durchgeführt werden können. Anhand der aufgenommen Spektren wurde der Gradient in der Probe nachgewiesen, sowie die Abhängigkeit der Interlagenaustauschkopplung von der Zwischenschichtdicke verdeutlicht.

Keywords: magnetism; ferromagnetic resonance; MOKE; thin films

  • Bachelor thesis
    TU Ilmenau, 2014
    Mentor: Kilian Lenz
    50 Seiten

Permalink: https://www.hzdr.de/publications/Publ-21501
Publ.-Id: 21501


Impact of strain induced by polymer curing in benzocyclobutene embedded semiconductor nanostructures

Bussone, G.; Dimakis, E.; Grifone, R.; Biermanns, A.; Tahraoui, A.; Carbone, D.; Geelhaar, L.; Schülli, T. U.; Pietsch, U.

Polymers such as benzocyclobutene are commonly used as embedding materials for semiconductor nanostructures. During the curing process of the polymer up to 250 °C, a significant impact of strain can be induced on the embedded semiconductor material due to different thermal expansion coefficients. This strain has been revealed by X-ray diffraction in free-standing GaAs nanowires grown on a silicon substrate, embedded in a polymer matrix. It will be shown that this strain is released during the X-ray irradiation if additionally an external static electric field is applied.

Keywords: benzocyclobutene; polymer matrix; X-ray diffraction; embedded semiconductor nanostructures; GaAs nanowires; strain

Permalink: https://www.hzdr.de/publications/Publ-21499
Publ.-Id: 21499


Untersuchung der magnetischen Eigenschaften von Permalloy-Filmen mittels frequenzaufgelöstem magnetooptischen Kerr-Effekt (FR-MOKE)

Rupp, S.

Im Rahmen dieser Arbeit wurden dünne ferromagnetische Filme aus Permalloy (Ni80Fe20) mit eine Kombination aus ferromagnetischer Resonanz (FMR) und magnetooptischem Kerr-Effekt (MOKE) auf ihre dynamischen Eigenschaften hin untersucht. Das Hauptmerkmal dieser frequenzaufgelösten FR-MOKE Messung lag hierbei auf der Bestimmung der Linienbreite zur Charakterisierung der wirksamen Dämpfung der Magnetiserungspräzession und des g-Faktors für Permalloy. Die Ergebnisse im Einzelnen sind: 1. Die Linienbreite kann im Bereich von 1-25 GHz bestimmt werden. 2. Die Linienbreite steigt mit der Mikrowellenfrequenz f linear an und besitz einen geringen inhomogenen Anteil. 3. Die Amplitude der Lorentzkurve fällt mit 1/f ab. 4. Bestimmung des g-Faktors g=2,085(11), für Permalloy. 5.
Die Dämpfung ist konstant und beträgt alpha=0,007.

Keywords: ferromagnetism; ferromagnetic resonance; thin films; MOKE

  • Bachelor thesis
    nta Naturwissenschaftlich-Technische Akademie Prof. Dr. Grübler gGmbH, 2014
    Mentor: Kilian Lenz
    54 Seiten

Permalink: https://www.hzdr.de/publications/Publ-21498
Publ.-Id: 21498


Role of liquid indium in the structural purity of wurtzite InAs nanowires that grow on Si(111)

Biermanns, A.; Dimakis, E.; Davydok, A.; Sasaki, T.; Geelhaar, L.; Takahasi, M.; Pietsch, U.

InAs nanowires that grow catalyst-free along the [111] crystallographic orientation are prone to wurtzite-zincblende polytypism, making the control of the crystal phase highly challenging. In this work, we explore the dynamic relation between the growth conditions and the structural composition of the nanowires using time-resolved X-ray scattering and diffraction measurements during the growth by molecular beam epitaxy. A spontaneous buildup of liquid indium is directly observed in the beginning of the growth process and associated with the simultaneous nucleation of InAs nanowires predominantly in the wurtzite phase. The highly arsenic-rich growth conditions that we used limited the existence of the liquid indium to a short time interval, which is defined as the nucleation phase. After their nucleation, the nanowires grow in the absence of liquid indium, and with a highly defective wurtzite structure. Complementary ex-situ diffuse X-ray scattering measurements and modeling revealed that this structural degradation is due to the formation of densely spaced stacking faults. Thus, high wurtzite phase purity is associated with the presence of liquid indium. This finding implies that pure wurtzite nanowires may be obtained only if the growth is performed under the continuous presence of liquid indium at the growth interface, that is, in the vapor–liquid–solid mode.

Keywords: nanowires; InAs; Si; molecular beam epitaxy (MBE); X-ray diffraction

Permalink: https://www.hzdr.de/publications/Publ-21497
Publ.-Id: 21497


Two-dimensional electron gas in monolayer InN quantum wells

Pan, W.; Dimakis, E.; Wang, G. T.; Moustakas, T. D.; Tsui, D. C.

We report in this letter experimental results that confirm the two-dimensional nature of the electron systems in a superlattice structure of 40 InN quantum wells consisting of one monolayer of InN embedded between 10 nm GaN barriers. The electron density and mobility of the two-dimensional electron system (2DES) in these InN quantum wells are 51015cm2 (or 1.251014cm2 per InN quantum well, assuming all the quantum wells are connected by diffused indium contacts) and 420 cm2/Vs, respectively. Moreover, the diagonal resistance of the 2DES shows virtually no temperature dependence in a wide temperature range, indicating the topological nature of the 2DES.

Permalink: https://www.hzdr.de/publications/Publ-21496
Publ.-Id: 21496


Evolution of polytypism in GaAs nanowires during growth revealed by time-resolved in situ x-ray diffraction

Schroth, P.; Köhl, M.; Hornung, J.-W.; Dimakis, E.; Somaschini, C.; Geelhaar, L.; Biermanns, A.; Pietsch, U.; Bauer, S.; Lazarev, S.; Baumbach, T.

In III-V nanowires the energetic barriers for nucleation in zinc blende or wurtzite arrangement are typically of similar order of magnitude. As a result, both arrangements can occur in a single wire. Here, we investigate the evolution of this polytypism in self-catalyzed GaAs nanowires on Si(111) grown by molecular beam epitaxy with time-resolved in situ X-ray diffraction. We interpret our data in the framework of a height dependent Markov model for the stacking in the nanowires. This way, we extract the mean sizes of faultless wurtzite and zinc blende segments -- a key parameter of polytypic nanowires -- and their temporal evolution during growth. Thereby, we infer quantitative information on the differences of the nucleation barriers including their evolution without requiring a model of the nucleus.

Permalink: https://www.hzdr.de/publications/Publ-21495
Publ.-Id: 21495


Status of the ELBE SRF Gun II

Arnold, A.

As in 2007 the first 3.5 cell superconducting radio frequency (SRF) gun was taken into operation, it turned out that the specified performance has not been achieved. However, to demonstrate the full potential of this new type of electron source, a second and slightly modified SRF gun II was built in collaboration with Thomas Jefferson National Accelerator Facility (TJNAF). We will report on commissioning and first results of the new gun, which includes in particular the characterization of the most important RF properties as well as their comparison with previous vertical test results.

Keywords: superconducting electron injector; SRF gun; ELBE

  • Lecture (others)
    HOPE / SINEMP project meeting, 03.-04.11.2014, Siegen, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21494
Publ.-Id: 21494


System Integration and Application of CdZnTe Detectors in a Compton Camera for Medical Use

Fiedler, F.; Enghardt, W.; Golnik, C.; Hueso-Gonzalez, F.; Kormoll, T.; Schumann, A.; Pausch, G.; Petzold, J.; Rohling, H.; Roemer, K.; Schoene, S.

Ion beams offer advantages over conventional treatment modalities, such as photons. Because of the way ions deposit their energy on their path through tissue they allow for an increased dose deposition in the tumor volume and reduce the collateral damage to the surrounding healthy tissue. However, the range of these particles is sensitive to small density changes in the irradiated volume. Deviations will lead to a misalignment of the deposited dose maximum and the tumor. It is therefore highly desirable to verify the particle range in-vivo and in realtime by means of a detector system independent from the treatment device. One approach is to monitor the prompt gamma-ray emissions from excited nuclei that originate in the interaction of projectile and target nuclei. A Compton camera could be one of the feasible technical solutions for such a monitoring system. To set up a clinical applicable device a comprehensive modelling of the creation of secondary radiation as well as of the detection process is required. Furthermore, a sophisticated reconstruction of the data is essential. This paper will present three different prototypes of such a detector system made of CdZnTe and scintillation detectors. Also we will show measurements on the detector performance as well as reconstructed images of the radiation sources.

Keywords: Compton camera; in-vivo dose imaging; proton therapy; CdZnTe detectors

  • Invited lecture (Conferences)
    IEEE NSS MIC, 21st Symposium on room-temperature Semiconductor and Gamma-Ray detectors, 08.-15.11.2014, Seattle, WA, USA

Permalink: https://www.hzdr.de/publications/Publ-21493
Publ.-Id: 21493


Broadband, diode pumped Yb-doped fused silica laser

Röser, F.; Loeser, M.; Albach, D.; Siebold, M.; Grimm, S.; Brand, D.; Schwuchow, A.; Langner, A.; Schötz, G.; Schönfeld, D.; Schramm, U.

We report on the fabrication, optical properties and lasing characteristics of Yb-doped fused silica in bulk volume. The glass rods were manufactured by sintering of Yb-doped fused silica granulates and subsequent homogenization. Samples of various thicknesses containing doping levels of 0.27 mol% and 0.39 mol%, respectively, were investigated. The glass shows a high optical quality with refractive index variations in the 10 ppm range. We successfully demonstrated cw lasing with a maximum optical to optical efficiency of 60 % and slope efficiencies of about 70 % with respect to absorbed pump power for all samples. The laser cavity could be tuned in a wavelength range of 100 nm. The large amplification bandwidth of fused silica was verified by gain distribution measurements in a double-pass amplifier configuration.

Keywords: Lasers, diode-pumped; Lasers, ytterbium; Lasers, solid-state; Laser materials

Permalink: https://www.hzdr.de/publications/Publ-21492
Publ.-Id: 21492


Comparison of a Separated Flow Response to Localized and Global type Disturbances

Monnier, B.; Williams, D.; Weier, T.; Albrecht, T.

POD modes and coefficients obtained from an inclined flat plate with Lorentz force actuator are compared to that of a pitching wing model (NACA 0009). The pitching wing model emulates an impulsive disturbance by pitching from 15 deg to 17 deg and then back to 15 deg in a very short time interval. There is also a negative pitching case, 15 deg to 13 then back to 15 deg. The Lorentz force actuator produced a spatially localized disturbance, whereas the pitching wing produces a spatially global disturbance. We found very close similarities in the POD modes and the time-varying coefficients. We also found that POD mode 2 tracks the negative of the lift force in both cases.

Keywords: active flow control; Lorentz force; pitching wing

  • Lecture (Conference)
    53rd AIAA Aerospace Sciences Meeting, 05.-09.01.2015, Kissimmee , Florida, USA
  • Contribution to proceedings
    53rd AIAA Aerospace Sciences Meeting, 05.-09.01.2015, Kissimmee, Florida, USA
    AIAA papers
    DOI: 10.2514/6.2015-1056

Permalink: https://www.hzdr.de/publications/Publ-21491
Publ.-Id: 21491


Critical exponents and intrinsic broadening of the field-induced transition in NiCl2 · 4SC(NH2)2

Wulf, E.; Hüvonen, D.; Schönemann, R.; Kühne, H.; Herrmannsdörfer, T.; Glavatskyy, I.; Gerischer, S.; Kiefer, K.; Gvasaliya, S.; Zheludev, A.

The field-induced ordering transition in the quantum spin system NiCl2 · 4SC(NH2)2 is studied by means of neutron diffraction, ac magnetometry, and relaxation calorimetry. The interpretation of the data is strongly influenced by a finite distribution of transition fields in the samples, which was present but disregarded in previous studies. Taking this effect into account, we find that the order-parameter critical exponent is inconsistent with the BEC universality class even at temperatures below 100 mK. All results are discussed in comparison with previous measurements and with recent similar studies of disordered Ni(Cl1−xBrx)2 · 4SC(NH2)2.

Permalink: https://www.hzdr.de/publications/Publ-21490
Publ.-Id: 21490


Multigap superconductivity in locally noncentrosymmetric SrPtAs: An 75As nuclear quadrupole resonance investigation

Brückner, F.; Sarkar, R.; Günther, M.; Kühne, H.; Luetkens, H.; Neupert, T.; Reyes, A. P.; Kuhns, P. L.; Biswas, P. K.; Stürzer, T.; Johrendt, D.; Klauss, H.-H.

We report detailed 75As nuclear quadrupole resonance investigations of the locally noncentrosymmetric superconductor SrPtAs. The spin-lattice relaxation studies prove weakly coupled multigap superconductivity. A retardation of the decay in 1/T1T evidences a nodeless (fully gapped) superconducting state on the complex multipocket Fermi surface, which is consistent with an anisotropic s-wave order parameter and with proposed unconventional f-wave and chiral d-wave symmetries. A quantitative analysis of these models favors the unconventional f-wave state.

Permalink: https://www.hzdr.de/publications/Publ-21489
Publ.-Id: 21489


Nanowire spin torque oscillator driven by spin orbit torques

Duan, Z.; Smith, A.; Yang, L.; Youngblood, B.; Lindner, J.; Demidov, V. E.; Demokritov, S. O.; Krivorotov, I. N.

Spin torque from spin current applied to a nanoscale region of a ferromagnet can act as negative magnetic damping and thereby excite self-oscillations of its magnetization. In contrast, spin torque uniformly applied to the magnetization of an extended ferromagnetic film does not generate self-oscillatory magnetic dynamics but leads to reduction of the saturation magnetization. Here we report studies of the effect of spin torque on a system of intermediate dimensionality—a ferromagnetic nanowire. We observe coherent self-oscillations of magnetization in a ferromagnetic nanowire serving as the active region of a spin torque oscillator driven by spin orbit torques. Our work demonstrates that magnetization selfoscillations can be excited in a one-dimensional magnetic system and that dimensions of the active region of spin torque oscillators can be extended beyond the nanometre length scale.

Permalink: https://www.hzdr.de/publications/Publ-21488
Publ.-Id: 21488


Calculation of Electronic Structure and Transport properties of Donor – Acceptor Polymers

Günther, F.; Gemming, S.; Seifert, G.

Donor-acceptor (DA) polymers have been found to be good materials for organic electronics since they provide interesting features like mechanical flexibility and high impact resistance.
Moreover, they offer the possibility to produce devises by low cost roll-to-roll printing techniques.
Thus, they are highly promising candidates for organic thin-film transistors and solar cells.
For applications, however, these materials should fulfill several demands such as ambient stability, good solubility, and good film-forming properties.
The charge carrier mobility is one of the most important quantities.
In order to analyse the influence of the molecular structure on these properties, we investigate DA polymers using first-principals methods as density functional theory (DFT).
In particular, density functional based tight binding (DFTB) is used to study large systems.

In our study we analyse the electronic structures of the isolated monomers which are the building blocks of the DA polymers, of finite oligomers up to a length of 20 conjugated units, and of infinite long polymers using periodic boundary conditions.
Furthermore, we investigate the morphological properties by studying systems of several polymer chains with different crossing angles and relative shifts.

Using Marcus transfer theory, we then calculated the electronic transport properties based on hopping processes.
Here, we focus on the coupling matrix elements and their dependence on the stacking configuration.
Using a Boltzmann-like approach for evaluating an average value of this quantity, we obtain good agreements to experimental trends.
Especially the dominating transport type is nicely reproduced by our approach.

  • Lecture (others)
    Arbeitsgruppenseminar der Professur für Theoretische Chemie, 20.11.2014, Dresden, Deutschland
  • Lecture (others)
    Seminar der Professur Theorie ungeordneter Systeme, 17.12.2014, Chemnitz, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21487
Publ.-Id: 21487


Optical Synchronization and Electron Bunch Diagnostic at the CW-Linac ELBE

Kuntzsch, M.

no abstract submitted

Keywords: Synchronization ELBE BAM BCM Diagnostic

  • Invited lecture (Conferences)
    DESY MSK Seminar, 09.01.2015, Hamburg, Germany

Permalink: https://www.hzdr.de/publications/Publ-21486
Publ.-Id: 21486


Optimising the spatial structure of BLN protein models by means of "partial distortion''-quench cycles

Günther, F.; Möbius, A.; Schreiber, M.

The prediction of the spatial structure of a protein based on its amino acid sequence is a challenging problem.
Corresponding theoretical studies of the protein folding require highly efficient structure optimisation tools.
Here we investigate whether and to what extent the thermal cycling (TC) algorithm [1] is appropriate for determining low energy structures of the BLN protein model by J.D. Honeycutt and D. Thirumalai [2].
In our simulations for 46-, 58-, and 69-bead sequences, the TC algorithm reliably finds the global minimum within reasonable computing time.
In comparison to the multi-start local search and simulated annealing approaches, TC turns out to be far more efficient.

In the present work, the BLN model with rigid bonds is studied in detail for the first time.
Comparing these results to data for the extended model by Berry et al. [3], where stiff springs are substituted for the rigid bonds, we observe several level crossings when varying the spring constant, even for quite hard springs.

[1] A. Möbius et al. , Phys. Rev. Lett. 79 (1997) 4297.
[2] J.D. Honeycutt and D. Thirumalai, Biopolymers 32 (1992) 695.
[3] R.S. Berry et al. , Proc. Natl. Acad. Sci. USA 94 (1997) 9520.

  • Lecture (Conference)
    DPG Spring Meeting 2014, 30.03.-04.04.2014, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21485
Publ.-Id: 21485


Handling Domain Decomposition in Massively Parallel Implementations of Stochastic Lattice Models

Kelling, J.; Ódor, G.; Gemming, S.

Nanopatterning of surfaces and bulk materials is very important from molecular electronics to photovoltaics. But, in order to understand the underlying physics of self-organization, large scale atomistic simulations are crucial. Only stochastic models can bridge the gap from nano to micro, enabling simulations of micron-sized volumes, billions of atoms and study long-time evolution. Random site-selection is essential but can be harmed by domain decomposition in GPGPU. We present solutions by example of a dimer-model for KPZ surface growth.

Keywords: Statistical Physics; Domain Decompostion

Permalink: https://www.hzdr.de/publications/Publ-21484
Publ.-Id: 21484


Mesoscale magnetism

Hoffmann, A.; Schultheiss, H.

Magnetic interactions give rise to a surprising amount of complexity due to the fact that both static and dynamic magnetic properties are governed by competing short-range exchange interactions and long-range dipolar coupling. Even though the underlying dynamical equations are well established, the connection of magnetization dynamics to other degrees of freedom, such as optical excitations, charge and heat flow, or mechanical motion, make magnetism a mesoscale research problem that is still wide open for exploration. Synthesizing magnetic materials and heterostructures with tailored properties will allow to take advantage of magnetic interactions spanning many length-scales, which can be probed with advanced spectroscopy and microscopy and modeled with multi-scale simulations. This review highlights some of the current basic research topics in mesoscale magnetism, which beyond their fundamental science impact are also expected to influence applications ranging from information technologies to magnetism based energy conversion.

Keywords: Magnetic soliton; Magnetomechanical coupling; Magnonics; Optical magnetization switching; Permanent magnet; Spin caloritronics; Spin Hall effect; Spin texture; Spin transfer torque

Permalink: https://www.hzdr.de/publications/Publ-21483
Publ.-Id: 21483


Calculation of Electronic Structure and Transport in Polymers

Günther, F.; Seifert, G.; Luschtinetz, R.; Gemming, S.

Organic thin-film transistors producible by low-cost roll-to-roll manufacturing processes seem to be highly promising for flexible electronics. Therefore, high mobility semiconducting polymers with ambient stability, good solubility and film-forming properties are needed. Our study is turned to the calculation of the electronic transport properties through polymers consisting of conjugated thiophene based donor units and di-ketopyrrolo-pyrrol based acceptor units. The structure and electronic properties of thiophene-based molecular stacks is studied by means of quantum- mechanical calculations. We demonstrate how the functionalization of thiophenes can increase the dispersion interaction and promote the parallel- stacked arrangement of the molecules, which is essential to get efficient charge transport channels in the direction perpendicular to the plane of the thiophene rings. Moreover, we found that the π-π-stacking is the main driving force in the self-assembly of the molecules in the film. These results are the basis for further studies of the hopping transport properties of this promising organic semiconducting material.
For the description of the electronic transport we apply a Greens function method, including Coulomb and inelastic tunneling to a simple one-particle hopping Hamiltonian by calculating the matrix elements through overlap integrals of orbitals obtained from density-functional calculations.

  • Poster
    XXIII International Materials Research Congress, 17.-21.08.2014, Cancun, Mexico

Permalink: https://www.hzdr.de/publications/Publ-21482
Publ.-Id: 21482


Transport Calculations for Si4 Clusters with Gold single Atom Contacts

Kelling, J.; Kerbusch, J.; Erbe, A.; Dietsche, R.; Ganteför, G.; Scheer, E.; Zahn, P.; Gemming, S.

We present results for electronic transport through Si4 clusters contacted by single-atom gold contacts attached to <111> fcc-gold leads. The calculations were performed using density functional theory and the non-equilibrium Green's function approach for transport. The simulation setup mimics contacts in mechanically controllable break-junction experiments, which provide data for comparison.

  • Poster
    DPG-Frühjahrstagung der Sektion Kondensierte Materie, 15.-20.03.2015, Berlin, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21481
Publ.-Id: 21481


Ab inito study on morphology of donor-acceptor polymers

Günther, F.; Gemming, S.; Seifert, G.

Donor-acceptor (DA) polymers have been found to be good materials for organic electronics since they provide interesting features like mechanical flexibility and high impact resistance.
Moreover, they offer the possibility to produce devises by low cost roll-to-roll printing techniques. Thus, they are highly promising candidates for organic thin-film transistors and solar cells. For applications, however, these materials should fulfill several demands such as ambient stability, good solubility, and good film-forming properties. The charge carrier mobility is one of the most important quantities.
In order to analyze the influence of the molecular structure on these properties, we investigate DA polymers using first-principals methods as density functional theory (DFT). In particular, density functional based tight binding (DFTB) is used to study large systems.
Starting from isolated monomers being the building blocks of the DA polymers the total energy, the atomic charges, the difference between highest occupied and lowest unoccupied molecular orbitals, and the reorganization energies are calculated.
Furthermore, oligomers up to a length of 100 conjugated units as well as infinite long polymers are considered. Finally, the morphological properties are investigated by studying systems of several polymer chains with different crossing angles and relative shifts.

  • Poster
    IHRS NanoNet Annual Workshop 2014, 29.-30.09.2014, Rathen, Sächsische Schweiz, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21480
Publ.-Id: 21480


Structure optimization of BLN protein models combining local-search, evolutionary, and genetic approaches

Günther, F.; Möbius, A.; Schreiber, M.

Establishing relations between the amino acid sequence of a protein and its spatial structure is a very important and challenging task.
When analysing the forces affecting the protein folding, the investigation of the related free energy landscape is of central importance.
Thereby, to obtain the ground state and low-energy metastable states, highly effcient structure optimization tools are needed.
Here, we study the properties of genetic local-search type optimization approaches, in particular of thermal cycling (TC) [1] and systematic-crossover localsearch (SCLS) algorithms.
For this aim, we focus on the BLN protein model, designed by J.D. Honeycutt and D. Thirumalai [2]. We consider 46-, 58-, and 69-bead sequences from Refs. [2,3]; the 69-bead sequence seems to be the largest BLN model treated in the liteature up to now.
In all these cases, the TC and SCLS algorithms reliably find the global minimum within reasonable computing times.
Both algorithms proved to be far more effcient than multi-start local-search and simulated annealing approaches.
In the present work, to the best of our knowledge, the BLN model with rigid bond lengths is studied in detail for the first time.
We compare our results to the properties of the extended model by Berry et al. [4], in which stiff spring-like bonds are substituted for the rigid bonds:
The hardening of the spring constants causes several level crossings of the metastable states.
For the 46-bead model, this concerns even the ground state.

[1] A. Möbius, A. Neklioudov, A. Diaz-Sanchez, K.H. Hoffmann, A. Fachat, and M. Schreiber, Phys. Rev. Lett. 79 (1997) 4297.
[2] J.D. Honeycutt and D. Thirumalai, Biopolymers 32 (1992) 695.
[3] M.T. Oakley, D.J.Wales, and R.L. Johnston, J. Phys. Chem. B 115 (2011) 11525.
[4] R.S. Berry, N. Elmaci, J.P. Rose, and B. Vekhter, Proc. Natl. Acad. Sci. USA 94 (1997) 9520.

  • Poster
    Conference of Middle European Coorperation in Statistical Physics, 08.-10.04.2014, Coventry, England

Permalink: https://www.hzdr.de/publications/Publ-21479
Publ.-Id: 21479


Electronic Transport through Au-contacted PEEB

Kelling, J.; Gemming, S.

Transport through the organic molecule PEEB has been investigated using Density functional Theory (DFT) and the Non-Equilibrium Green's Function (NEGF) approach.

Keywords: electronic transport; molectular electronics; dft

  • Poster
    NanoNet Workshop, 29.-30.09.2014, Rathen, Sächsische Schweiz, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21478
Publ.-Id: 21478


Calculation of electronic transport properties through polymeres

Günther, F.; Gemming, S.; Seifert, G.

Organic thin-film transistors producible by low-cost roll-to-roll manufacturing processes seem to be highly promising for flexible electronics.
Therefore, high mobility semiconducting polymers with ambient stability, good solubility and film-forming properties are needed.
Our study is turned to the calculation of the electronic transport properties through polymers consisting of conjugated thiophene based donor units and di-ketopyrrolo-pyrrol based acceptor units.
Therefore, we use empirical hopping equations such as Marcus transfer theory in order to determine the mobility of holes and electrons.
The parameters required for this are taken from first-principles calculations such as density functional theory and Hartree-Fock approaches.

  • Poster
    DPG Spring Meeting 2014, 30.03.-04.04.2014, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21477
Publ.-Id: 21477


EcoMetals - innovative eco-efficient biohydrometallurgy process for the recovery of strategic and rare metals: primary and secondary resources

Kutschke, S.; Bodénan, F.; Schaefer, J.; Guezennec, A.-G.; Janneck, E.; Möckel, R.; Petiot, C.; Pollmann, K.

The German-French project EcoMetals focuses on metal production from base (copper) and rare metal- bearing primary and secondary resources in Europe. The main objectives are the development of alternative methods in mineral processing involving pretreatment steps, biohydrometallurgical and metal recovery processes, their up scaling and finally the determination of economic and sustainable performances through process integration, life cycle analysis and economic assessment.
German Kupferschiefer dumps and ores from Mansfeld and Lausitz region, Polish Kupferschiefer concentrates and French complex polymetallic deposits will be used as case studies for scientific breakthrough and integrated process demonstration activity.

Keywords: copper; rare metals; black shale; bioleaching

  • Lecture (Conference)
    Biohydrometallurgy '14, 09.-11.06.2014, Falmouth, United Kingdom and Ireland

Permalink: https://www.hzdr.de/publications/Publ-21476
Publ.-Id: 21476


Recyclingpotenziale von Galliumarsenid

Kutschke, S.; Zeidler, O.; Matys, S.; Pollmann, K.; Scharf, C.

Zusammenfassung zu Stand der Arbeiten zum Galiumarsenid-Recycling, Darstellung bilogischer und hydrometallurigischer Ansätze

  • Lecture (others)
    21. Kolloquium Abfall & Altlasten aktuell, 11.12.2014, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21475
Publ.-Id: 21475


Stacked topological spin textures as emitters for multidimensional spin wave modes

Sluka, V.; Weigand, M.; Kakay, A.; Erbe, A.; Tyberkevych, V.; Slavin, A.; Deac, A.; Lindner, J.; Fassbender, J.; Raabe, J.; Wintz, S.

In the present contribution we will show that in a vortex pair system with uniaxial magnetic anisotropy, spin waves of different symmetries and dimensionalities can be excited.

Keywords: vortex spin wave

  • Lecture (Conference)
    IEEE International Magnetics Conference (Intermag), 11.-15.05.2015, Beijing, China

Permalink: https://www.hzdr.de/publications/Publ-21474
Publ.-Id: 21474


Fast Timing with BGO (and other Scintillators) on Digital Silicon Photomultipliers for Prompt Gamma Imaging

Petzoldt, J.; Roemer, K.; Kormoll, T.; Berthel, M.; Dreyer, A.; Enghardt, W.; Fiedler, F.; Hueso-Gonzalez, F.; Golnik, C.; Kirschke, T.; Wagner, A.; Pausch, G.

Particle therapy is supposed to be an advanced treatment modality compared to conventional radiotherapy because of the well-defined range of the ions. Prompt gamma rays, produced in nuclear reactions between ion and nuclei, can be utilized for real-time range verification to exploit the full potential of particle therapy. Several devices have been investigated in the field of Prompt Gamma Imaging (PGI), like Slit and Compton Cameras. The latter need very high detection efficiency as well as good time and energy resolution, requiring a versatile scintillation detector. In positron emission tomography (PET), LSO and LYSO are known for their good timing resolution, while the lower cost alternative BGO shows worse performance. In PGI however, where gamma rays have energies up to 10 MeV, the light output of a scintillator is up to 20 times larger compared to PET with E gamma = 511 keV. This reduces the statistical contribution of the time resolution, which is the dominant part in case of BGO. Thus, BGO could be a reasonable alternative to LSO/LYSO for applications in PGI. Hence, experiments at the ELBE accelerator at Helmholtz-Zentrum Dresden Rossendorf (Germany) were performed using digital silicon photomultiplier (dSiPM) from Philips with monolithic BGO and LYSO crystals and for completeness with GAGG and CeBr3. The time resolution of BGO compared to the faster scintillators will be presented for a wide range of trigger- and validation levels as well as validation lengths of the dSiPM. Timing resolutions below 300 ps were obtained for BGO, while LYSO and CeBr3 achieve about 170 ps.

Keywords: fast timing; digital silicon photomultiplier; particle therapy; range verification

  • Lecture (Conference)
    2014 IEEE Nuclear Science Symposium and Medical Imaging Conference (2014 NSS/MIC), 08.-15.11.2014, Seattle, USA
  • Contribution to proceedings
    2014 IEEE Nuclear Science Symposium and Medical Imaging Conference (2014 NSS/MIC), 08.-15.11.2014, Seattle, USA
    Proceedings of 2014 NSS/MIC

Permalink: https://www.hzdr.de/publications/Publ-21473
Publ.-Id: 21473


In-Vivo Range Verification Based on Prompt Gamma-Ray Timing Measurements

Golnik, C.; Hueso-Gonzalez, F.; Enghardt, W.; Mueller, A.; Fiedler, F.; Kormoll, T.; Ostendorf, R.; Petzold, J.; Roemer, K.; Dendooven, P.; Pausch, G.

We present a novel approach for real time range verification in particle therapy based on prompt gamma ray timing (PGT) spectroscopy. The measurement approach relies on the fact that high-energy particles need, dependent on their range, a varying transit time through the irradiated material. This transit time is about 1-2 ns in the case of protons with a range of 5-20 cm. Timing spectroscopy of promptly emitted gamma rays, measured with an arrangement resembling a typical time-of-flight (TOF) setup, encode these transit times and thus give a real time access to the particle range. We show that statistical momenta of PGT distributions such as the center of gravity and the variance incorporate the information on the particle range with a millimeter precision, although measurement uncertainties may cover the described effect at first sight. Typical uncertainties are limited by the detector time resolution and the particle bunch spread, and are finally determined by the spectrum stability. In contrast to other prompt gamma imaging techniques, the PGT method is based on straight timing spectroscopy with a single scintillation detector. Therefore, neither passive nor electronic collimation is required. The proposed idea is verified by an analysis of experimental data taken during proton irradiation experiments at the AGOR facility at KVI-CART in Groningen (The Netherlands). The measurement results are in excellent agreement with Monte Carlo based modeling of the experimental setup. We conclude that precise range assessment is feasible within a few seconds of irradiation due to the direct measurement method.

Keywords: particle therapy; range verification; prompt gamma timing

  • Lecture (Conference)
    2014 IEEE Nuclear Science Symposium and Medical Imaging Conference (2014 NSS/MIC), 08.-15.11.2014, Seattle, USA

Permalink: https://www.hzdr.de/publications/Publ-21472
Publ.-Id: 21472


Particle Range Retrieval in Heterogeneous Phantoms with the Prompt Gamma Timing Method at a Clinical Proton Accelerator

Hueso-Gonzalez, F.; Golnik, C.; Berthel, M.; Dreyer, A.; Enghardt, W.; Fiedler, F.; Heidel, K.; Janssens, G.; Kormoll, T.; Petzoldt, J.; Prieels, D.; Priegnitz, M.; Roemer, K. E.; Smeets, J.; Sobiella, M.; Vander Stappen, F.; Wagner, A.; Weinberger, D.; Pausch, G.

The characteristic dose profile of accelerated ions has opened up new horizons in the context of cancer treatment. However, particle range uncertainties strongly constrain the potential of ion beam therapy. Despite of worldwide efforts, a detector system for range and dose delivery assessment in real-time is not yet available for clinical routine. Complementary to the active- and passively collimated prompt gamma ray imaging systems for range assessment under development in several research centers, the prompt gamma ray timing method has been recently proposed. Based on the measurable transit time of ions through matter, the emission times of prompt gamma rays encode essential information about the depth-dose profile. In a collaboration between OncoRay, Helmholtz-Zentrum Dresden-Rossendorf and IBA, the prompt gamma ray timing method was tested for the first time at a clinical proton accelerator (Westdeutsches Protonentherapiezentrum Essen) with different phantoms. Several fast scintillation detectors were used to acquire prompt gamma ray timing distributions at various geometries and proton energies. From the resulting distributions, particle range differences of around 5 millimeters in heterogeneous phantoms were observed. In conclusion, we proved the feasibility of the prompt gamma ray timing method for range verification in a clinical radiation environment and realistic phantoms, which reassures this novel approach as a promising alternative in the field of prompt gamma based in vivo dosimetry.

Keywords: particle therapy; range verification; prompt gamma timing

  • Poster
    2014 IEEE Nuclear Science Symposium and Medical Imaging Conference (2014 NSS/MIC), 08.-15.11.2014, Seattle, USA
  • Contribution to proceedings
    2014 IEEE Nuclear Science Symposium and Medical Imaging Conference (2014 NSS/MIC), 08.-15.11.2014, Seattle, USA
    Proceedings of 2014 NSS/MIC

Permalink: https://www.hzdr.de/publications/Publ-21471
Publ.-Id: 21471


Vacancy controlled spontaneous pattern formation on semiconductor surfaces

Facsko, S.

In molecular beam epitaxy (MBE) the continuous deposition of atoms can lead to growth of self-organized 3D nanostructures. One of the possible surface instability, 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. The arriving atoms are trapped on a terraces and can again nucleate to form new terraces. 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 periodic patterns are observed which are oriented perpendicular or parallel to the ion beam direction or are isotropic dot or hole patterns for normal incidence of the ion beam.
At temperatures above the recrystallization temperature of the material, 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 primarily by vacancy kinetics. The diffusion of vacancies is also biased by the ES barrier like the diffusion of ad-atoms. Consequently, the 3D growth turns into a 3D erosion. The resulting structures are inverse pyramids which are growing into the surface. The symmetry of these patterns is given by the crystal symmetry. Hence, checkerboard patterns appear on the Ge (001) surface, oriented in the <100> directions (Fig. 1 left). On the other hand, on the Ge (111) surface facets with a three fold symmetry evolve (Fig. 1 right).
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. By including also a conserved Kardar-Parisi-Zhang term a remarkable qualitative agreement to the experiments is achieved.

Keywords: reverse epitaxy; nanostructures; ion irradiation; self-organized patterning

  • Lecture (others)
    Seminar, 12.11.2014, Stuttgart, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-21470
Publ.-Id: 21470


Faceting semiconductor surfaces by reverse epitaxy

Facsko, S.; Ou, X.; Helm, M.; Fassbender, J.

In molecular beam epitaxy (MBE) the continuous deposition of atoms can lead to growth of self-organized 3D nanostructures. One of the possible surface instability, 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. The arriving atoms are trapped on a terraces and can again nucleate to form new terraces. 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 periodic patterns are observed which are oriented perpendicular or parallel to the ion beam direction or are isotropic dot or hole patterns for normal incidence of the ion beam.
At temperatures above the recrystallization temperature of the material, 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 primarily by vacancy kinetics. The diffusion of vacancies is also biased by the ES barrier like the diffusion of ad-atoms. Consequently, the 3D growth turns into a 3D erosion. The resulting structures are inverse pyramids which are growing into the surface. The symmetry of these patterns is given by the crystal symmetry. Hence, checkerboard patterns appear for instance on the Ge (001) surface, oriented in the <100> directions. On the other hand, on the Ge (111) surface facets with a three fold symmetry evolve. For high ion fluences the patterns also exhibit facets, which correspond to low index crystal planes.
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. By including also a conserved Kardar-Parisi-Zhang term a remarkable qualitative agreement to the experiments is achieved.

Keywords: reverse epitaxy; ion irradiation; nanopatterning; nanostructures

  • Invited lecture (Conferences)
    16th International Conference on Thin Films, 13.-16.10.2014, Dubrovnik, Kroatien

Permalink: https://www.hzdr.de/publications/Publ-21467
Publ.-Id: 21467


Energy Loss and Charge Exchange of Highly Charged Ions in Carbon Nanomembranes

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

During the interaction of highly charged ions with solids the ions potential energy, i.e. the stored ionization energy, is released via multiple charge exchanges on a fs time scale. Thus, HCIs reach charge equilibrium after passing only a few nanometers of the solid. The dependence of the charge state on the stopping force of the ions is therefore not accessible in irradiation experiments with bulk material. In order to investigate this pre-equilibrium regime films of just a few nanometers have to be used.
We examined the charge state and the energy loss of highly charged Xe ions after their passage through 1 nm thick carbon nanomembranes. Surprisingly, two distinct exit charge distributions were observed [1]. Part of the ions are passing the membrane with almost now charge loss, whereas the other part loose 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 enough 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.
The charge distribution and energy loss of Xeq+ ions of different incident charge states up to q=30 will be presented and the implication for the formation of holes in these nanomembranes by the HCIs [2] will be discussed.

[1] R.A. Wilhelm, E. Gruber, R. Ritter, R. Heller, S. Facsko, F. Aumayr, Phys. Rev. Lett. 112, 153201 (2014).
[2] 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).

Keywords: highly charged ions; nano membranes; energy loss

  • Lecture (Conference)
    International Conference on Ion Beam Modification of Materials, 15.-19.09.2014, Leuven, Belgien

Permalink: https://www.hzdr.de/publications/Publ-21466
Publ.-Id: 21466


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