Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

"Online First" included
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Only approved publications

35836 Publications

The He ion microscope—a high resolution tool for the nano–world

Hlawacek, G.

Invited presentation ANP retreat

  • Invited lecture (Conferences)
    ANP retreat, 19.-20.06.2014, Tecklenburg, Germany

Publ.-Id: 21432

Resistive Switching in Titanium dioxide: Comparison of thermally oxidized and magnetron sputtered films

Blaschke, D.; Cornelius, S.; Zahn, P.; Gemming, S.; Skorupa, I.; Scheumann, B.; Scholz, A.; Potzger, K.

Resistive RAM devices based on TiO2 are promising candidates for the next generation memory storage devices.
We compared TiO2 thin films from two different preparation methods with respect to crystallinity and resistive switching behavior.
While thermal oxidation of 100nm Ti on Pt/Ti/SiO2/Si substrates leads to polycrystalline rutile TiO2 layers, dc-magnetron sputter deposition of films on Nb:STO substrates leads to epitaxial anatase TiO2 structure.
In case of the rutile films, unipolar switching occurred, which points to a filamentary mechanism based on the formation of Magnéli phases [1]. The epitaxial anatase films, however, showed bipolar switching, which we correlated with the modification of the metal/oxide interface due to the drift of oxygen vacancies in the applied electric field [2].

The project is funded by the Initiative and Networking Fund of the Helmholtz Association (VI MEMRIOX, VH-VI-422).

[1] Deok-Hwang Kwon et al., “Atomic structure of conducting nanofilaments in TiO2 resistive switching memory”, Nature Nanotechnology 5, 148 – 153 (2010)

[2] J. Joshua Yang et al., “Memristive switching mechanism for metal/oxide/metal nanodevices”, Nature Nanotechnology 3, 429 – 433 (2008)

Keywords: resistive switching; Titanium dioxide; thermal oxidation; magnetron sputtering

  • Lecture (Conference)
    DPG-Frühjahrstagung der Sektion Kondensierte Materie (SKM), 30.03.-04.04.2014, Dresden, Deutschland

Publ.-Id: 21431

Beam transport and bunch compression at TARLA

Aksoy, A.; Lehnert, U.

The Turkish Accelerator and Radiation Laboratory in Ankara (TARLA) will operate two InfraRed Free Electron Lasers (IR-FEL) covering the range of 3–250 μm. The facility will consist of an injector fed by a thermionic triode gun with two-stage RF bunch compression, two superconducting accelerating ELBE modules operating at continuous wave (CW) mode and two independent optical resonator systems with different undulator period lengths. The electron beam will also be used to generate Bremsstrahlung radiation. In this study, we present the electron beam transport including beam matching to the undulators and the shaping of the longitudinal phase space using magnetic dispersive sections.

Keywords: Beam transport; Beam undulator matching; Bunch compression; FEL

Publ.-Id: 21430

5 years of Helium Ion Microscopy

Hlawacek, G.

Presentation at UTwente on Helium Ion Microscopy

  • Lecture (others)
    Colloquium PIN/UTwente, 17.04.2014, Enschede, Netherlands

Publ.-Id: 21429

Functionalization of nanoparticles for nanoelectronics

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

Nowadays, DNA origami has become a key technique for designing well-defined nanostructures with any desired shape and for the controlled arrangement of nanostructures with few nanometer resolution. These unique features of DNA origami nanostructures make them promising candidates for use as a scaffolds in nanoelectronics and nanophotonics device fabrication. In recent years, a number of studies have shown the precise organization of functional nanoparticles on various shapes of DNA origami. Most of these studies used, however, homogeneous nanostructures such as either metallic or semiconducting nanoparticles. In this work, we demonstrate the assembly of heterogeneous nanostructures, i.e. 5 nm gold nanoparticles (AuNPs) and 10 nm semiconductor quantum dots (QDs), on a single DNA origami. First, a streptavidin-conjugated QD was assembled on the center of biotin functionalized and immobilized DNA origami nanotube, then DNA coated AuNPs were hybridized along the right and left side of the QD-modified nanotube. The high yield of AuNP assembly was achieved by careful control of the buffer concentration and the hybridization time on Si surface.

Keywords: DNA nanostructures; DNA origami; self-assembly; nanoparticles; nanoelectronics

  • Poster
    IHRS NanoNet Annual Workshop 2014, 29.-30.09.2014, Lohmen, Germany

Publ.-Id: 21428

In Situ Observations during Chemical Vapor Deposition of Hexagonal Boron Nitride on Polycrystalline Copper

Kidambi, P. R.; Blume, R.; Kling, J.; Wagner, J. B.; Baehtz, C.; Weatherup, R. S.; Schloegl, R.; Bayer, B. C.; Hofmann, S.

Using a combination of complementary in situ X-ray photoelectron spectroscopy and X-ray diffraction, we study the fundamental mechanisms underlying the chemical vapor deposition (CVD) of hexagonal boron nitride (h-BN) on polycrystalline Cu. The nucleation and growth of h-BN layers is found to occur isothermally, i.e., at constant elevated temperature, on the Cu surface during exposure to borazine. A Cu lattice expansion during borazine exposure and B precipitation from Cu upon cooling highlight that B is incorporated into the Cu bulk, i.e., that growth is not just surface-mediated. On this basis we suggest that B is taken up in the Cu catalyst while N is not (by relative amounts), indicating element-specific feeding mechanisms including the bulk of the catalyst. We further show that oxygen intercalation readily occurs under as-grown h-BN during ambient air exposure, as is common in further processing, and that this negatively affects the stability of h-BN on the catalyst. For extended air exposure Cu oxidation is observed, and upon re-heating in vacuum an oxygen-mediated disintegration of the h-BN film via volatile boron oxides occurs. Importantly, this disintegration is catalyst mediated, i.e., occurs at the catalyst/h-BN interface and depends on the level of oxygen fed to this interface. In turn, however, deliberate feeding of oxygen during h-BN deposition can positively affect control over film morphology. We discuss the implications of these observations in the context of corrosion protection and relate them to challenges in process integration and heterostructure CVD.

Keywords: CVG; Graphene; Borazine

Publ.-Id: 21427

Thickness-dependent metal-insulator transition in epitaxial SrRuO3 ultrathin films

Shen, X.; Qiu, X.; Su, D.; Zhou, S.; Li, A.; Wu, D.

Transport characteristics of ultrathin SrRuO3 films, deposited epitaxially on TiO2-terminated SrTiO3 (001) single-crystal substrates, were studied as a function of film thickness. Evolution from a metallic to an insulating behavior is observed as the film thickness decreases from 20 to 4 unit cells. In films thicker than 4 unit cells, the transport behavior obeys the Drude low temperature conductivity with quantum corrections, which can be attributed to weak localization. Fitting the data with 2-dimensional localization model indicates that electron-phonon collisions are the main inelastic relaxation mechanism. In the film of 4 unit cells in thickness, the transport behavior follows variable range hopping model, indicating a strongly localized state. Magnetoresistance measurements reveal a likely magnetic anisotropy with the magnetic easy axis along the out-of-plane direction.

Keywords: SrRuO3; metal-insulator transition

Publ.-Id: 21426

64Cu-labeled agents for molecular imaging of cancer

Stephan, H.

The development of multi-functional complexing agents for radiometal nuclides with a view of nuclear medical application represents a field of research that is intensively dealt with and has rapidly been developing. In this context, ligands that form highly stable metal complexes and additionally possess several different functional groups are of particular interest. This enables the simultaneous introduction of targeting, solubilizing and, for example, fluorescent units into the relevant metal complexes. This allows, on the one hand, adjusting defined features of solubility and of selective binding properties with a view of in vivo application with controllable targeting. There is the possibility of dual labeling, on the other, for the simultaneous application of different imaging techniques on the basis of radioactive labeling for SPECT (single-photon emission computed tomography) / PET (positron emission tomography) and of a fluorescent unit for optical imaging. In that way, tumors can be localized by means of PET, for instance, and, during subsequent surgery, the fluorescent label of the compound applied will permit to clearly differentiate between healthy and malign tissue (fluorescence guided surgery). Besides, the application of fluorescent compounds allows investigating cellular processes and exactly localizing these compounds in cell compartments. Here, we discuss specific examples of multi-functional ligand systems and nanomaterials, enabling simultaneous imaging of cancer using 64Cu PET and fluorescence imaging.

  • Invited lecture (Conferences)
    7th Asian Biological Inorganic Chemistry Conference (AsBIC7), 30.11.-05.12.2014, Gold Coast, Australia
  • Contribution to proceedings
    7th Asian Biological Inorganic Chemistry Conference (AsBIC7), 30.11.-05.12.2014, Gold Coast, Australia
    Proceedings AsBIC7

Publ.-Id: 21425

Interaction of Eu(III) with mammalian cells: Cytotoxicity, uptake and speciation as a function of Eu(III) concentration and nutrient composition

Sachs, S.; Heller, A.; Weiß, S.; Bok, F.; Bernhard, G.

In case of the release of heavy metal ions (e.g., rare earth elements or radionuclides) into the environment, detailed knowledge about their toxic behavior in biological systems is necessary to assess and to prevent adverse health effects for humans. In the present work, we investigated the interaction of europium with FaDu cells (human squamous cell carcinoma cell line) combining analytical methods (solubility studies, ultrafiltration, and ultracentrifugation), time-resolved laser-induced fluorescence spectroscopy and thermodynamic modeling with in-vitro cell experiments under well-defined chemical conditions. Both the cytotoxicity of Eu(III) onto FaDu cells and its cellular uptake are mainly concentration-dependent and only slightly time-dependent. Moreover, they are governed by its chemical speciation in the nutrient medium. In complete cell culture medium, i.e., in the presence of fetal bovine serum, Eu(III) is stabilized in solution in a wide concentration range by complexation with serum proteins resulting in a low cytotoxicity and very low cellular Eu(III) uptake. In contrast to that, in the serum-free medium, Eu(III) precipitates and forms hardly soluble phosphate species, exhibiting a significant higher cytotoxicity and a slightly higher cellular uptake. The presence of a tenfold excess of citrate in the serum-free medium causes the formation of Eu(HCit)23− complexes in addition to the dominating Eu(III) phosphate species, resulting in a decreased Eu(III) cytotoxicity and cellular uptake. Thus, the results of this study underline the crucial role of the chemical speciation of a metal ion for its toxicity and bioavailability and demonstrate that, in the studied case of europium, soluble species are less toxic and bioavailable than precipitating ones.

Keywords: Toxicology; heavy metal speciation; lanthanides; actinides; FaDu cells; TRLFS

Publ.-Id: 21424

Optical ridge waveguides in Nd:CNGG disorder laser crystal produced by combination of carbon ion irradiation and precise diamond blade dicing

Luan, Q.; Tan, Y.; Akhmadaliev, S.; Zhou, S.; Yu, H.; Zhang, H.; Chen, F.

Ridge waveguides have been produced in Nd:CNGG disorder laser crystal by using precise diamond blade dicing of carbon ion irradiated planar waveguide. The propagation loss of the ridge waveguide is measured to be ∼3.8 dB/cm at the wavelength of 632.8 nm. The micro-Raman spectrum indicates that the microstructure of the Nd:CNGG crystal has no significant change after the carbon ion irradiation. The microphotoluminescence feature has been found well preserved in the waveguide structure. The thermal stability of the waveguide has been investigated, showing relatively stable feature below 260 °C.

Keywords: Diamond blade dicing; Disorder laser crystals; Ion irradiation; Optical waveguide

Publ.-Id: 21423

On the development of a novel non-peptidic 18F-labeled radiotracer for in vivo imaging of oxytocin receptors with positron emission tomography

Wenzel, B.; Mollitor, J.; Deuther-Conrad, W.; Kranz, M.; Dukic-Stefanovic, S.; Günther, R.; Teodoro, R.; Ludwig, F.-A.; Fischer, S.; Smits, R.; Steinbach, J.; Hoepping, A.; Brust, P.

The peptide oxytocin is synthesized in the hypothalamus and acts as neurotransmitter to regulate a diverse range of CNS functions. Its receptor (OTR) is expressed in specific brain areas related to psychiatric diseases. So far, a non-invasive investigation of the OTR in brain is hampered by a lack of suitable radiotracers. To develope a PET ligand with high affinity toward OTR, we synthesized a series of fluorinated non-peptidic small molecules and performed radiofluorination of a selected candidate to investigate its in vivo properties in mice and pigs.

Binding affinities of the compounds to the human OTR were determined by radioligand displacement studies. The radiosynthesis of [18F]ABX163 (KD = 12.3 nM) was performed via thermal and microwave heating (see scheme 1). Metabolism and organ distribution of the radiotracer were studied in female CD-1 mice. Dynamic PET scans were performed in mice (animal PET/MR; 60 min) and in one female piglet (PET; 120 min).

Using microwave heating for the synthesis of [18F]ABX163 provided higher RCY in shorter reaction time compared to thermal heating (RCY 25.4 ± 3.1% (n=5); SA 35-160 GBq/µmol; RP > 97%). Both organ distribution and dynamic PET imaging studies revealed limited uptake of the radiotracer in mouse brain (SUVmean = 0.04). Besides, significant uptake in the pituitary gland was observed (SUV55 min p.i. = 0.85), which indicates target-specific binding of [18F]ABX163. By a dynamic PET study in pig, a mean SUV of 0.43 was estimated for the whole brain at 120 min p.i. A two fold higher uptake was observed in the olfactory bulb (SUV120 min p.i. = 0.73), a region with high expression of OTR. In vitro autoradiography studies on rat and pig brain slices revealed an interaction of [18F]ABX163 with several off target receptors.

Due to the low brain uptake and the insufficient selectivity, [18F]ABX163 is not suitable for imaging of OTR in living brain.

Acknowledgment: The authors thank the SAB (Sächsische AufbauBank) for funding this project.

  • Poster
    ISRS2015 - 21st International Symposium on Radiopharmaceutical Sciences, 26.-31.05.2015, Columbia, Missouri, USA
  • Open Access Logo Abstract in refereed journal
    Journal of Labelled Compounds and Radiopharmaceuticals 58(2015)1, 173
    DOI: 10.1002/jlcr.3302_2
  • Journal of Medicinal Chemistry 59(2016)5, 1800-1817
    Online First (2016) DOI: 10.1021/acs.jmedchem.5b01080

Publ.-Id: 21422

LC-MS supported investigations on the metabolism of (+)-[18F]flubatine, a radiotracer for imaging of α4β2 nicotinic acetylcholine receptors by PET

Ludwig, F.-A.; Fischer, S.; Smits, R.; Deuther-Conrad, W.; Hoepping, A.; Patt, M.; Sabri, O.; Brust, P.; Steinbach, J.

Objectives: (+)-[18F]flubatine [1] is currently being investigated for imaging of α4β2 nicotinic acetylcholine receptors (nAChRs) in patients with early Alzheimer’s disease. Accompanying these studies, we performed LC-MS supported in vitro and in vivo investigations to detect and characterize radiometabolites in plasma and urine.
Methods: Liver microsomes from mouse (MLM) or human (HLM) were incubated for 90 or 120 min in PBS or TRIS buffer with 10 µM (+)-, and, for comparison, (-)-flubatine and synthesized references. Further, an anaesthesized pig was infused with (+)-flubatine (67 µg/kg). Plasma and urine samples were taken after 30 and 45 min, respectively. Structures of metabolites in these in vitro and in vivo samples were elucidated by LC-MS.
For comparison MLM and HLM were incubated with 7 MBq (+)-[18F]flubatine and the radiometabolite patterns were monitored by radio-HPLC. Additionally, plasma (15, 30 min p.i) and urine (90-130 min p.i.) samples of human subjects receiving 283 (259-299) MBq (+)-[18F]flubatine i.v. were investigated.
Results: After microsomal incubations, up to seven metabolites exclusively hydroxylated at the azabicyclic ring system were detected (Fig.1A), among them some which were glucuronidated subsequently. Accordingly, seven hydroxylated metabolites and one glucuronide of (+)-flubatine were found in pig. Radiometabolites of (+)-[18F]flubatine obtained after HLM and MLM incubations corresponded to those of (+)-flubatine. Plasma (15, 30 min p.i.) and urine (90-130 min p.i.) samples obtained from humans showed 92-99% and 88-99% (n=7) of unchanged tracer, respectively. Two radiometabolites could be detected in both plasma and urine and were finally assigned to 1 and 3 (Fig.1B).
Conclusions: (+)-[18F]Flubatine showed high metabolic stability in human. The structures of the two radiometabolites found in plasma and urine could be assigned by the presented approach.
Acknowledgements: Supported by the Helmholtz Validation Found.
References: [1] Smits R, et al. (2014) Bioorg Med Chem, 22, 804-12

  • Lecture (Conference)
    ISRS2015 - 21st International Symposium on Radiopharmaceutical Sciences, 26.-31.05.2015, Columbia, Missouri, USA
  • Open Access Logo Abstract in refereed journal
    Journal of Labelled Compounds and Radiopharmaceuticals 58(2015)1, 33
    DOI: 10.1002/jlcr.3302_1

Publ.-Id: 21421

Defect-induced magnetism in graphite through neutron irradiation

Wang, Y.; Pochet, P.; Jenkins, C. A.; Arenholz, E.; Bukalis, G.; Gemming, S.; Helm, M.; Zhou, S.

We have investigated the variation in the magnetization of highly ordered pyrolytic graphite (HOPG) after neutron irradiation, which introduces defects in the bulk sample and consequently gives rise to a large magnetic signal. We observe strong paramagnetism in HOPG, increasing with the neutron fluence. The induced paramagnetism can be well correlated with structural defects by comparison with density-functional theory calculations. In addition to the in-plane vacancies, the transplanar defects also contribute to the magnetization. The lack of any magnetic order between the local moments is possibly due to the absence of hydrogen/nitrogen chemisorption, or the magnetic order cannot be established at all in the bulk form.

Keywords: Defect-induced magnetism; graphite; neutron irradiation

Publ.-Id: 21420

Novel 18F-labelled triazine derivatives for PET imaging of phosphodiesterase 2A

Schröder, S.; Wenzel, B.; Deuther-Conrad, W.; Teodoro, R.; Egerland, U.; Kranz, M.; Fischer, S.; Höfgen, N.; Steinbach, J.; Brust, P.

Objectives: Phosphodiesterases (PDEs) are enzymes which degrade the second messengers cAMP and cGMP thereby affecting cellular functions. PDE2A is involved in the pathophysiology of Alzheimer´s disease and cancer. Therefore PDE2A inhibitors are suggested as potential therapeutics. Accordingly we aim to develop an 18F-labelled radioligand for PET imaging of PDE2A.
Methods: Based on a triazine compound [1] (TA1) novel fluoroalkylated derivatives (TA2-5, see Table 1) were synthesized and their affinity and selectivity towards PDE2A were determined. 18F-labelling of selected candidates was accomplished by nucleophilic substitution in acetonitrile using tosylate precursors. In vitro autoradiographic studies on rat brain sections were performed with [18F]TA3 and [18F]TA4 under control and blocking conditions. For PET/MR studies of [18F]TA3 in mice the radiosynthesis was performed in a TRACERlabTM FX F-N module. In vivo metabolism studies of [18F]TA3 and [18F]TA4 in mouse plasma and brain samples were carried out by conventional extraction procedure as well as by direct injection of the samples into a micellar HPLC system.
Table 1: Structures and affinity data of TA1-5, autoradiographic image of [18F]TA3.

IC50PDE2A 4.5 nM 10.4 nM 11.4 nM 7.3 nM 3.0 nM
IC50PDE10A 670 nM 77 nM 318 nM 913 nM > 1000 nM

Results: [18F]TA3, [18F]TA4 and [18F]TA5 were successfully synthesized with labelling yields of 40 - 70%, radiochemical yields of 30 - 45% and specific activities of ≥ 60 GBq/µmol. In vitro autoradiographic experiments showed region-specific accumulation of [18F]TA3 (see Table 1) and [18F]TA4 with higher binding density in cortex and striatum than in cerebellum, which is consistent with the distribution pattern of PDE2A in rat brain. PET/MR studies of [18F]TA3 in mice exhibited a fast wash out of radioactivity from the striatum while constantly increasing uptake was observed in the cerebellum. Metabolism studies of [18F]TA3 and [18F]TA4 at 30 minutes p.i. revealed a significant brain concentration of radiometabolites (≥ 40%).
Conclusions: Due to the brain accumulation of radiometabolites, the new radioligands [18F]TA3 and [18F]TA4 are exclusively suitable for in vitro imaging of PDE2A. Further in vitro and in vivo characterization of the highly affine and selective PDE2A radioligand [18F]TA5 is currently in progress.
Reference: [1] Stange et al.: Triazine Derivatives as Inhibitors of Phosphodiesterases; Patent WO2010/054253 A1.

  • Poster
    ISRS2015 - 21st International Symposium on Radiopharmaceutical Sciences, 26.-31.05.2015, Columbia, Missouri, USA
  • Open Access Logo Abstract in refereed journal
    Journal of Labelled Compounds and Radiopharmaceuticals 58(2015)1, 221
    DOI: 10.1002/jlcr.3302_2

Publ.-Id: 21419

Instrumentation for diagnostics and control of laser-accelerated proton (ion) beams

Bolton, P. R.; Borghesi, M.; Brenner, C.; Carroll, D. C.; de Martinis, C.; Flacco, A.; Floquet, V.; Fuchs, J.; Gallegos, P.; Giove, D.; Green, J. S.; Green, S.; Jones, B.; Kirby, D.; Mckenna, P.; Neely, D.; Nuesslin, F.; Prasad, R.; Reinhardt, S.; Roth, M.; Schramm, U.; Scott, G. G.; Ter-Avetisyan, S.; Tolley, M.; Turchetti, G.; Wilkens, J. J.

Suitable instrumentation for laser-accelerated proton (ion) beams is critical for development of integrated, laser-driven ion accelerator systems. Instrumentation aimed at beam diagnostics and control must be applied to the driving laser pulse, the laser-plasma that forms at the target and the emergent proton (ion) bunch in a correlated way to develop these novel accelerators. This report is a brief overview of established diagnostic techniques and new developments based on material presented at the first workshop on 'Instrumentation for Diagnostics and Control of Laser-accelerated Proton (Ion) Beams' in Abingdon, UK. It includes radiochromic film (RCF), image plates (IP), micro-channel plates (MCP), Thomson spectrometers, prompt inline scintillators, time and space-resolved interferometry (TASRI) and nuclear activation schemes. Repetition-rated instrumentation requirements for target metrology are also addressed. © 2013 Associazione Italiana di Fisica Medica.

Keywords: Radiation Oncology; Diagnostics; Dosimetry; Laser-acceleration; Proton beam

Publ.-Id: 21418

Magnetoresistance and Resistance Relaxation of Nanostructured La-Ca-MnO Films in Pulsed Magnetic Fields

Žurauskiene, N.; Balevicius, S.; Pavilonis, D.; Stankevic, V.; Plaušinaitiene, V.; Zherlitsyn, S.; Herrmannsdörfer, T.; Law, J. M.; Wosnitza, J.

The results of magnetoresistance (MR) and resistance relaxation of nanostructured La1−xCaxMnO3 films, with different composition x grown by metal–organic chemical vapor deposition technique, are presented and compared with the La0.83Sr0.17MnO3 films. The MR was investigated in pulsed magnetic fields up to 60 T in the temperature range 1.5–294 K while the relaxation processes were studied in pulsed fields up to 10 T and temperatures in the range of 80–300 K. It was demonstrated that at low temperatures the MR has higher values in the LCMO films in comparison with the LSMO ones, while at room temperatures, the highest MR values are obtained for the LSMO films. The fast (∼100 μs) and slow (∼ms) resistance relaxation processes were observed after the magnetic field pulse was switched off. It was shown that the fast process could be analyzed using the Kolmogorov–Avrami–Fatuzzo model, considering the reorientation of magnetic domains into their equilibrium state, while the slow process—by the Kohlrausch–Williams–Watts model considering the interaction of the magnetic moments in disordered grain boundaries having pin-glass properties. It was concluded that La1−xCaxMnO3 films having a higher sensitivity and lower memory effects and should be favored for the development of fast pulsed magnetic field sensors operating at low temperatures.

Publ.-Id: 21417

Large field-induced irreversibility in Ni-Mn based Heusler shape-memory alloys: A pulsed magnetic field study

Nayak, A. K.; Salazar Mejia, C.; D'Souza, S. W.; Chadov, S.; Skourski, Y.; Felser, C.; Nicklas, M.

We present a pulsed magnetic field study on the magnetic and magnetostriction properties of Ni-Mn-Z (Z = In, Sn, and Sb) based Heusler shape-memory alloys. These materials generally display a field-induced magnetostructural transition that could lead to an irreversible phase transition, when measured near themartensitic transition temperature. Here, we show that independently of the transition temperature, the critical field for the phase transition sensitively depends on the main-group element in the sample. Irrespective of their compositions, all samples display a magnetization of around 2μB/f.u. in the martensite phase and about 6μB/f.u. in the cubic austenite phase. Our magnetic and magnetostriction measurements at low temperatures exhibit a partial or complete arrest of the high-field austenite phase below the reverse martensitic transition. This results in a large irreversibility with a hysteresis width as high as 24 T. We introduce a theoretical model to discuss the experimental results.

Publ.-Id: 21416

Magnetic pyroxenes LiCrGe2O6 and LiCrSi2O6: Dimensionality crossover in a nonfrustrated S = 3/2 Heisenberg model

Janson, O.; Nénert, G.; Isobe, M.; Skourski, Y.; Ueda, Y.; Rosner, H.; Tsirlin, A. A.

The magnetism of magnetoelectric S = 3/2 pyroxenes LiCrSi2O6 and LiCrGe2O6 is studied by density functional theory calculations, quantum Monte Carlo (QMC) simulations, neutron diffraction, as well as low- and high-field magnetization measurements. In contrast with earlier papers, we find that the two compounds feature remarkably different, albeit nonfrustrated magnetic models. In LiCrSi2O6, two relevant exchange integrals, J1 ≃ 9 K along the structural chains and Jic1 ≃ 2 K between the chains, form a two-dimensional anisotropic honeycomb lattice. In contrast, the spin model of LiCrGe2O6 is constituted of three different exchange couplings. Surprisingly, the leading exchange Jic1 ≃ 2.3 K operates between the chains, while J1 ≃ 1.2 K is about two times smaller. The additional interlayer coupling Jic2 ≃ J1 renders this model three dimensional. QMC simulations reveal excellent agreement between our magnetic models and the available experimental data. Underlying mechanisms of the exchange couplings, magnetostructural correlations, as well as implications for other pyroxene systems are discussed.

Publ.-Id: 21415

Evolution of Spin Wave Modes in Periodically Perturbed Thin Films

Langer, M.; Gallardo, R.; Banholzer, A.; Schneider, T.; Wagner, K.; Landeros, P.; Lenz, K.; Lindner, J.; Fassbender, J.

The transition from a continuous thin film to a magnonic crystal is studied by ferromagnetic resonance (FMR).
Ion irradiation as well as reactive ion beam etching were used to realize a periodic modulation of the sample surface after patterning by electron beam lithography.
Mode-splitting in the FMR spectra has been investigated dependent on the size of the perturbations and compared to available analytical perturbation theory. Numerical simulations have been carried out to identify the spin waves corresponding to the mode spectra as well as to understand deviations between measurement and analytical theory for large perturbations.

Keywords: ferromagnetic resonance; fmr; spin waves; magnonics; magnonic crystals; magnetization dynamics; micromagnetic simulations; ion irradiation; patterned films

  • Lecture (others)
    Gruppenseminar der AG Küpper an der TU Osnabrück, 10.-11.12.2014, Osnabrück, Deutschland

Publ.-Id: 21414

Photon emission within the linear sigma model

Wunderlich, F.; Kämpfer, B.

Soft-photon emission rates are calculated within the linear sigma model. The investigation is aimed at answering the question to which extent the emissivities map out the phase structure of this particular effective model of strongly interacting matter.

Publ.-Id: 21413

Ultrafast spectroscopy on Landau quantized graphene: Evidence for strong Auger scattering

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

While the carrier dynamics in graphene in absence of magnetic fields is well researched in a large spectral range ranging from UV to THz, the dynamics in Landau quantized graphene is almost unexplored. We investigate the carrier dynamics within the system of Landau levels (LLs) of index n = -1, n = 0 and n = 1 by pump-probe experiments complemented by microscopic modelling. Using circularly polarized terahertz radiation (at 20 THz) allows one to excite the two energetically degenerate transitions LL-1 → LL0 and LL-1 → LL0, respectively, selectively. Surprisingly, induced transmission is observed in one configuration of pumping and probing with opposite configuration. Considering single particle Pauli blocking, one would expect induced absorption in this case. The sign change indicates that LL0 is depleted by strong Auger scattering, even though it is optically pumped at the same time.

Keywords: Graphene; ultrafast spectroscopy; Landau quantization; Auger scattering

  • Invited lecture (Conferences)
    5th International Symposium on Terahertz Nanoscience, 01.-05.12.2014, Schoelcher, Martinique, France

Publ.-Id: 21412

Ge(1-x)Sn(x) alloy synthesized by ion-implantation: from epitaxial thin films to crystalline nanostructures

Gao, K.; Prucnal, S.; Baehtz, C.; Huebner, R.; Skorupa, W.; Helm, M.; Zhou, S.

Group IV semiconductor alloys have drawn substantial attention for their potential applications in optoelectronic devices capable of integration with the existing silicon-based IC circuitry. Monocrystalline Ge1-xSnx alloys are promising for electronic and optical applications in virtue of their high carrier mobility and possibility of direct bandgap transition.
In this contribution we present the monocrystalline Ge1-xSnx thin film and nanostructure synthesized by ion implantation, by which the low solubility of Sn in Ge can be overcome. Sn was implanted into commercial Ge wafers at room temperature. After implantation, cross-sectional transmission electron microscopy (TEM) image reveals a 70 nm thick Sn-doped porous structure on 80 nm thick Sn-doped amorphous Ge layer. The implantation induced amorphized layer has been recrystallized after ultrashort thermal process. By nanosecond pulsed laser melting (PLM), high quality monocrystalline Ge1-xSnx thin films were obtained through a bottom-up liquid phase epitaxial process. On the other hand, solid phase recrystallization induced by millisecond flash lamp annealing (FLA) results in crystalline Ge1-xSnx porous nanostructures. Depending on the FLA condition, the structure can be polycrystalline or monocrystalline.
Field emission scanning electron microscopy was applied to measure the surface morphology. High resolution transmission electron microscopy and Rutherford backscattering and channeling analysis confirmed the monocrystalline structure of the Ge1-xSnx layer. The crystallinity and the lattice expansion due to Sn doping were determined by X-ray diffraction and micro-Raman spectroscopy. Our investigation provides an efficient, IC-compatible technique to prepare high quality monocrystalline Ge1-xSnx alloys.

Keywords: GeSn alloy; ion implantation; ultrashort annealing

  • Poster
    Trends in Nanotechnology 2014, 27.-31.10.2014, Barcelona, Spain

Publ.-Id: 21411

Faceted nanostructure arrays with extreme regularity controlled vacancy by vacancy

Ou, X.; Heinig, K.-H.; Hübner, R.; Grenzer, J.; Wang, X.; Helm, M.; Fassbender, J.; Facsko, S.

Semiconductor quantum dots and wires are important building blocks for future electronic and optoelectronic devices. The common way of producing semiconductor nanostructures is by molecular beam epitaxy (MBE). Recently, we have shown self-assembling by a subtractive process induced by high fluence ion irradiations [X. Ou et al., Phys. Rev. Lett. 111 (2013) 016101], where vacancies created by ion impacts nucleate and finally lead to 3D morphology patterns. Here, we show that for III-V semiconductors with zinc-blende crystal structure a symmetry-breaking driving force exists on (001) surfaces which leads to extremely regular nanogroove patterns oriented along the [11 ̅0] direction without any defect over large areas. These faceted stripe structures are formed due to different energetics as well as kinetics of the reconstructed surfaces of GaAs and InAs with extremely regularity due to enhanced, ion-assisted surface diffusion. In contrast, on group IV (Si, Ge) semiconductors with diamond structure patterns with three-fold, four-fold, and six-fold symmetry, depending on the surface orientation, have been found.

Keywords: nanopatterning; surface facets; low-energy ion irradiation; vacancy kinetics

Publ.-Id: 21410

Optical characterisation of plasmonic nanostructures on planar substrates using second harmonic generation

Persechini, L.; Verre, R.; Smith, C. M.; Fleischer, K.; Shvets, I. V.; Ranjan, M.; Facsko, S.; Mcgilp, J. F.

Polarization dependent second{harmonic generation (SHG) measurements were performed ex situ on plasmonic nanostructures grown by self{assembly on nanopatterned templates. These exploratory studies of Ag nanoparticles show that SHG is highly sensitive to the local elds associated with the morphology of the NP layer, with the substrate making little or no contribution. The anisotropic polarized SH response is easily detected under non{resonant conditions and shows promise as a complementary technique for the in situ characterization of anisotropic nanoparticle arrays. In particular, the ratio of two parameters related to the p{polarized SH response arising from p{ and s{polarized excitation showed over an order of magnitude difference between isotropic and anisotropic NPs. While these measurements involved rotating the sample to access orthogonal azimuths, the results show that a simple xed normal incidence geometry could be used for in situ measurements of mirror plane symmetry breaking, associated with anisotropic nanostructure morphology, in situations where rotating the sample may be neither desirable nor easily accomplished.

Keywords: Second Harmonic Generation; plasmonic structures; metallic nanoparticles

Publ.-Id: 21409

Qualification of multiphase CFD for nuclear reactor safety – status and perspective

Lucas, D.

The thermal-hydraulic part of safety analyses for design based accidents mainly relies on 1D system codes. This enables a safe design and operation of existing nuclear reactors. However due to the 3D characteristics of fluid flows some additional conservatism is required. Especially for large vessels, as the Reactor Pressure Vessel, Computational Fluid Dynamics (CFD) may provide more realistic information on relevant flow and heat transfer phenomena. Single phase CFD is widely used for industrial applications, e.g. in automotive and aviation industries. However CFD is not mature for two-phase flows, which may be relevant e.g. in Loss Of Coolant Accident scenarios leading to steam-water flows in the primary circuit of a Pressurized Water Reactor. This paper discusses the present shortcomings and strategies to improve the situation.

Keywords: multiphase flow; CFD; multi-fluid approach; flow morphology; CFD-grade experiment

  • Contribution to proceedings
    10th International Topical Meeting on Nuclear Thermal Hydraulics, Operation and Safety (NUTHOS-10), 14.-18.12.2014, Naha, Okinawa, Japan
    Paper KN-07
  • Invited lecture (Conferences)
    10th International Topical Meeting on Nuclear Thermal Hydraulics, Operation and Safety (NUTHOS-10), 14.-18.12.2014, Naha, Okinawa, Japan

Publ.-Id: 21408

A droplet entrainment model for horizontal gas/liquid flows

Höhne, T.

One limitation today in simulating horizontal segregated flows is that there is no treatment of droplet formation mechanisms at wavy surfaces. For self-generating waves and slugs, the interfacial momentum exchange and the turbulence parameters have to be modelled correctly. Furthermore, understanding the mechanism of droplet entrainment for heat and mass transfer processes is of great importance in the chemical and nuclear industry.
The development of general computational fluid dynamics (CFD) models, which are closer to physics and include less empiricism, is a long-term objective of the activities of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) research programs. Such models are an essential precondition for the application of CFD codes to the modelling of flow related phenomena in the chemical and nuclear industries. The new formulation for the interfacial drag at the free surface and turbulence parameters within the algebraic interfacial area density model (AIAD) is one result of these HZDR activities. The AIAD approach allows the use of different physical models depending on the local morphology inside a macro-scale multi-fluid framework.
A further step of improvement of modelling interfaces is the consideration of droplet entrainment mechanisms. The proposed entrainment model assumes that due to liquid turbulence the interface gets rough and wavy leading to the formation of droplets. The new approach is validated against existing horizontal two-phase flow data from the HAWAC channel.

Keywords: CFD; horizontal flow; AIAD; droplet entrainment; two-phase flow; HAWAC; HZDR

  • Contribution to proceedings
    ICONE-23, 23rd International Conference on Nuclear Engineering, 17.-21.5.2015, Chiba, Japan
    Proceedings of ICONE-23
  • Lecture (Conference)
    ICONE-23, 23rd International Conference on Nuclear Engineering, 17.-21.5.2015, Chiba, Japan

Publ.-Id: 21407

BiFeO3 bilayer structures for implementing beyond von-Neumann computing

You, T.; Shuai, Y.; Luo, W.; Du, N.; Buerger, D.; Skorupa, I.; Huebner, R.; Henker, S.; Mayr, C.; Schueffny, R.; Mikolajick, T.; Schmidt, O. G.; Schmidt, H.

The conventional von-Neumann architecture, which physically separates processing and memory operations, is limited in so much as the processor cannot execute a program faster than instructions and data can be fetched from and returned to memory[1]. Resistive switching devices[2] are considered as one of the most promising candidates for carrying out the processing and storage simultaneously and at the same device cell. In this work, we present a BiFeO3:Ti/BiFeO3 bilayer structure which shows stable and nonvolatile resistive switching behaviour under both positive and negative bias. With the same writing bias, the bilayer structure shows different resistance state for the different polarity of reading bias. The resistance states are distinguishable and stable enough for the practical applications. For the logic applications, the polarity of reading bias can be used as an additional logic variable, which makes it feasible to program and store all 16 Boolean logic functions simultaneously and into a same single bilayer structure cell in three logic cycles. [1] C. D. Wright, et al., Adv. Funct. Mater., 2013, 23, 2248 [2] A. Bogusz, T. You, et al., accepted in Proc. IEEE (2013)

  • Lecture (Conference)
    78. Jahrestagung der DPG und DPG-Frühjahrstagung der Sektion Kondensierte Materie (SKM), 30.03.-04.04.2014, Dresden, Deutschland

Publ.-Id: 21406

Spin-transfer dynamics in magnetic tunnel junctions with an out-of-plane magnetized free layer and in-plane polarizer

Kowalska, E.; Sluka, V.; Fowley, C.; Aleksandrov, Y.; Lindner, J.; Fassbender, J.; Deac, A. M.

Spin-torque nano-oscillators may be exploited for wireless communication applications [1-3]. It has recently been demonstrated that devices with an in-plane (IP) magnetized polarizer and out-of-plane (OOP) free layer allow for maximizing the output power, as the full parallel(P)-to-antiparallel (AP) resistance variation can be exploited in the limit of 90° precession angle [1]. However, in this geometry stable precession can only be sustained if the spin-transfer torque (STT) shows an asymmetric dependence on the angle between the free and the polarizing layer, like in metallic devices [1]. Nevertheless, recent experimental reports showed that spin-transfer driven dynamics can also be sustained in similarly designed MgO-based magnetic tunnel junctions (MTJs), with high output powers, but lacking an intrinsic STT angular asymmetry [4-5].
Here, we explore potential mechanisms for sustaining steady-state precession in MgO-MTJs with IP polarizer and OOP free layer. To this end, we analytically solve the Landau-Lifshitz-Gilbert-Slonczewski equation for a typical device with circular cross-section, under perpendicular applied fields and currents (Fig. 1). We assume that the precession trajectory is approximately circular around the direction perpendicular to the plane, as set by the effective field [1]. Since the magnitude of the STT is determined by the voltage across the barrier, at each point along the trajectory, we convert the current into an equivalent voltage value, taking into account the bias dependence of the resistance for the instant angle between the magnetic moments of the two layers. We assume the bias dependence of the AP state resistance to be linear, the P state resistance to be constant and a simple cosine angular dependence of the resistance with bias. We find that for constant current, the bias dependence of the resistance inherently induces an STT angular dependence asymmetry that it is sufficient to sustain precession and high output powers for relatively low values of applied current and field.
[1] W. H. Rippard, A. M. Deac, M. R. Pufall, et al., Physical Review B 81, 014426 (2010).
[2] A. M. Deac, A. Fukushima, H. Kubota, et al., Nature Physics 4, 308 (2008).
[3] S. I. Kiselev, J. C. Sankey, I. N. Krivorotov, et al., Nature 425, 380 (2003).
[4] H. Kubota, K. Yakushiji, A. Fukushima, et al., Applied Physics Express 6, 103003 (2013).
[5] T. Taniguchi, H. Arai, S. Tsunegi, et al., Applied Physics Express 6, 123003 (2013).

Keywords: spin-torque nano-oscillator (STNO); MgO-based magnetic tunnel junctions; tunnel magnetoresistance (TMR); spin dynamics

  • Lecture (Conference)
    The 59th Annual Magnetism and Magnetic Materials (MMM) Conference, 03.-07.11.2014, Honolulu, USA

Publ.-Id: 21405

Detection of thermal spin-transfer torque with ferromagnetic resonance technique utilizing a metallic microresonator

Kowalska, E.; Banholzer, A.; Fowley, C.; Sluka, V.; Aleksandrov, Y.; Grebing, J.; Lindner, J.; Fassbender, J.; Deac, A. M.

The revolution in mobile Information-Communication Technology (ICT) devices evident today is based on three principal trends: (i) constant miniaturization for increased portability; (ii) exponential increase in the volume of data stored and transmitted; and (iii) reduced power consumption for extended autonomy and Green-ICT applications. Currently, ICT devices are based on semiconductor technology, which has been experiencing a tremendous increase in performance and density, following a continuous miniaturization trend known as Moore’s law. Nevertheless, semiconductor technology faces severe limitations on the time horizon 2019 and beyond [1]. Indeed, as the gate length of semiconductor transistors shrinks below the design rule of 20 nm, leakage currents considerably reduce their performance, thereby calling for a new generation of alternative approaches: the so-called “More than Moore” technologies. One such approach having so far demonstrated great promise focuses on spin-transfer devices, which exploit the spin degree of freedom of the conduction electrons in order to manipulate their electronic properties. A variety of applications are being explored in this context, including non-volatile memory and wireless communication, with some being close to commercialization [2].
An even more recent research area: spincaloritronics, which aims at investigating new phenomena that can enable inherently generated Joule heating to be functionalized in microelectronic circuit design, bringing about a new generation of Green-ICT devices. Indeed, it has been theoretically predicted and demonstrated that temperature gradients can induce spin-currents and spin-accumulation in ferromagnets (known as the spin-Seebeck effect) [3], as well as tunneling between two magnetic layers separated by an insulator (magneto-Seebeck effect) [3], spin-injection from a ferromagnet to a semiconductor (spin Seebeck tunneling) [4] and so on. It has also been suggested that spin currents generated by temperature gradients in magnetic tunnel junctions can induce spin-transfer torques large enough to cause switching [3], although experimental evidence remains elusive.
This proposal focuses on fundamental research aimed at experimentally demonstrating that thermal gradients can generate spin-transfer torques in MgO-based magnetic tunnel junctions (MTJs). Specifically, we propose a novel approach which will allow us to provide proof-of-concept even if the magnitude of the torques that can be realistically achieved is considerably lower than predicted theoretically.
[3] G.E.W. Bauer, E. Saitoh and B.J. van Wees, Nature Mater. 11, 391 (2012) and references therein.
[4] R. Jansen, Nature Mater. 11, 400 (2012) and references therein.

Keywords: spin caloritronics; ferromagnetic resonance; micro-resonators

  • Poster
    SpinCaloritronics VI, School & Conference, 14.-18.07.2014, Irsee, Germany

Publ.-Id: 21404

Experimental news from a theoretical state: The "ppK"

Epple, E.; Adamczewski-Musch, J.; Arnold, O.; Atomssa, E. T.; Behnke, C.; Berger-Chen, J. C.; Biernat, J.; Blanco, A.; Blume, C.; Böhmer, M.; Bordalo, P.; Chernenko, S.; Deveaux, C.; Dybczak, A.; Fabbietti, L.; Fateev, O.; Fonte, P.; Franco, C.; Friese, J.; Fröhlich, I.; Galatyuk, T.; Garzón, J. A.; Gill, K.; Golubeva, M.; Guber, F.; Gumberidze, M.; Harabasz, S.; Hennino, T.; Hlavac, S.; Höhne, C.; Holzmann, R.; Ierusalimov, A.; Ivashkin, A.; Jurkovic, M.; Kämpfer, B.; Karavicheva, T.; Kardan, K.; Koenig, I.; Koenig, W.; Kolb, B. W.; Korcyl, G.; Kornakov, G.; Kotte, R.; Krása, A.; Krebs, E.; Kuc, H.; Kugler, A.; Kunz, T.; Kurepin, A.; Kurilkin, A.; Kurilkin, P.; Ladygin, V.; Lalik, R.; Lapidus, K.; Lebedev, A.; Lopes, L.; Lorenz, M.; Mahmoud, T.; Maier, L.; Mangiarotti, A.; Markert, J.; Metag, V.; Michel, J.; Müntz, C.; Münzer, R.; Naumann, L.; Palka, M.; Parpottas, Y.; Pechenov, V.; Pechenova, O.; Petousis, V.; Pietraszko, J.; Przygoda, W.; Ramstein, B.; Rehnisch, L.; Reshetin, A.; Rost, A.; Rustamov, A.; Sadovsky, A.; Salabura, P.; Scheib, T.; Schmidt-Sommerfeld, K.; Schuldes, H.; Sellheim, P.; Siebenson, J.; Silva, L.; Sobolev, Y. G.; Spataro, S.; Ströbele, H.; Stroth, J.; Strzempek, P.; Sturm, C.; Svoboda, O.; Tarantola, A.; Teilab, K.; Tlusty, P.; Traxler, M.; Tsertos, H.; Vasiliev, T.; Wagner, V.; Wendisch, C.; Wirth, J.; Wüstenfeld, J.; Zanevsky, Y.; Zumbruch, P.

We present p+p data, measured by the HADES spectrometer at EKin(proton)=3.5 GeV. We have analyzed the final state p+ p→ pLK+ in view of a possible intermediate state p+p -> KbarNN + K+. The KbarNN is the lightest candidate for a cluster of an anti-kaon bound to nucleons, and a current topic of theoretical and experimental interest. We have analyzed our data with a Partial-Wave Analysis that includes also the production of intermediate N*-resonances (→ K+Lambda). The result of the fit describes the data well and a statistical test showed no major deviations between model and data which could be attributed to a production of a kaonic nuclear bound state. We have, thus, started to focus on the determination of an upper limit of its production cross section.

  • Open Access Logo Proceedings of Science (2014), PoS(Bormio2014)049


Publ.-Id: 21403

Spin-transfer dynamics in magnetic tunnel junctions with an out-of-plane magnetized free layer and in-plane polarizer

Kowalska, E.; Sluka, V.; Fowley, C.; Aleksandrov, Y.; Bernert, K.; Lindner, J.; Fassbender, J.; Deac, A. M.

Spin-torque nano-oscillators (STNOs) are novel spintronics devices which may be exploited in order to design energy-efficient, frequency-tunable receivers and transmitters for wireless technology purposes [1-3]. Indeed, such devices are considerably smaller than conventional oscillators, having lateral dimensions in the range of a few tens of nanometers. Moreover, it has been demonstrated that STNOs can generate signals with high quality factors (in metallic point-contacts) and output powers in order of μW (in MgO-based magnetic tunnel junctions (MTJs) nanopillars). These levels are compatible with applications, thus justifying the interest which they have attracted as potential mobile Information-Telecommunication Technology devices.
To date, most studies focusing on spin-transfer driven dynamics have been carried out on devices with both the free and the reference layers magnetized in-plane. In this configuration, under application-desirable conditions (i.e., close to zero applied field), steady-state precession mainly occurs on clam-shell trajectories centered on the direction defined by the in-plane shape anisotropy. Consequently, only a fraction of the full magnetoresistance amplitude translates into the radio-frequency output power. However, it has been demonstrated that devices utilizing an in-plane (IP) magnetized polarizer (also acting as read-out layer) and out-of-plane (OOP) magnetized free layer allow for the full parallel-to-antiparallel resistance variation to be exploited in the limit of 90° precession angle [1]. In this particular geometry, it has been shown that steady-state precession can only be sustained if the spin-transfer torque exhibits an asymmetric dependence on the angle between the free and the polarizing layer [1].
Nevertheless, it has been very recently demonstrated experimentally that spin-transfer driven dynamics can also be sustained in similarly designed MgO-based MTJs, in spite of the fact that such devices do not exhibit any asymmetry in the spin-torque angular dependence [4-5].
Here, we explore potential mechanisms for sustaining steady-state precession in MgO-Based MTJs with IP polarizing and OOP free layer. To this end, we analytically solve the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation for a nano-pillar MTJ with circular cross-section, under a constant applied current (Fig. 1). We take into account both the in-plane and the field-like spin-torque terms, whose magnitude is determined not by the current, but by the corresponding voltage across the barrier. Steady-state precession can be sustained if the in-plane spin-torque term and the damping torque compensate over a full precession period.
Our results show that the stable dynamics occur only for negative current, for electrons flowing from the free to the reference layer (Fig. 2). According to our calculations, at small finite fields, the precession angle increases gradually from around zero to 90° with increasing current. High output powers (in the limit of 90° precession) can be obtained for relatively low values of applied current and field, which is beneficial from the point of view of STNOs application.

[1] W. H. Rippard, A. M. Deac, M. R. Pufall, et al., Physical Review B 81, 014426 (2010).
[2] A. M. Deac, A. Fukushima, H. Kubota, et al., Nature Physics 4, 308 (2008).
[3] S. I. Kiselev, J. C. Sankey, I. N. Krivorotov, et al., Nature 425, 380 (2003).
[4] T. Taniguchi, H. Arai, S. Tsunegi, et al., Applied Physics Express 6, 123003 (2013).
[5] H. Kubota, K. Yakushiji, A. Fukushima, et al., Applied Physics Express 6, 103003 (2013).

Keywords: spin-torque nano-oscillator (STNO); MgO-based magnetic tunnel junctions; tunnel magnetoresistance (TMR); spin dynamics

  • Poster
    INTERMAG 2014 - IEEE International Magnetics Conference, 04.-08.05.2014, Dresden, Germany

Publ.-Id: 21402

Grain alignment in thin silicon films by impurity controlled grain growth

Endler, R.; Grenzer, J.; Hübner, R.; Rebohle, L.; Prucnal, S.; Skorupa, W.

The high potential of the out-of-plane grain alignment, especially for the improvement of the device characteristics that are connected to the quality of the Si-SiO2 interface, has been investigated since the 1980's.
The growth of aligned grains during the crystallization of amorphous silicon thin films strongly influences the electrical properties of thin film silicon based devices.
We investigated the influence of the phosphorus concentration on the surface energy driven {111} grain growth during low-temperature crystallization of 100 nm thick silicon films.
Amorphous silicon thin films were deposited by electron beam evaporation on an oxidized silicon wafer. Afterwards the films were implanted either by beam-line implantation or plasma immersion ion implantation (PIII). Finally the films were crystallized by furnace annealing at 600 °C in N2 atmosphere.
The samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Cross-sectional and plan-view TEM investigations demonstrate that the lateral grain size increases continuously with the impurity concentration. XRD investigations identified an optimal impurity concentration of ~10^20 ions/cm^3, i.e. a maximum of the integrated intensity and a minimum FWHM of the rocking curve of the (111) reflection.
Contrary to the seed selection through ion channeling technique, our method provides a grain alignment independent to the presence of pre-aligned seeds in the as-implanted film.

Keywords: silicon thin films; ion Implantation; grain alignment; grain growth

  • Lecture (Conference)
    E-MRS 2014 Spring Meeting (Symposium T : Non-classical nucleation and crystallization), 26.-30.05.2014, Lille, France

Publ.-Id: 21401

Thermally assisted all-optical helicity dependent switching of ferrimagnetic amorphous Fe100-xTbx thin films

Hassdenteufel, A.; Hebler, B.; Schubert, C.; Liebig, A.; Teich, M.; Schmidt, J.; Helm, M.; Aeschlimann, M.; Albrecht, M.; Bratschitsch, R.

We observe all-optical switching (AOS) in ferrimagnetic Fe100-xTbx alloy films below, above, and in samples without a magnetic compensation point. AOS is linked to a low remanent magnetization MR and associated with laser heating up to the Curie temperature. Above MR = 220 emu/cc AOS is replaced by pure thermal demagnetization.

Keywords: All-optical switching; Alloy film; Helicities; Magnetic compensation; Remanent magnetization; Thermal demagnetization

  • Contribution to proceedings
    International Conference on Ultrafast Magnetism, UMC 2013, 28.10.-01.11.2013, Strasbourg, France
    Springer Proceedings in Physics: Springer, 978-331907742, 238-240
    DOI: 10.1007/978-3-319-07743-7_74

Publ.-Id: 21400

Extracellular polymeric substances govern the surface charge of biogenic elemental selenium nanoparticles

Jain, R.; Jordan, N.; Weiss, S.; Foerstendorf, H.; Heim, K.; Kacker, R.; Hübner, R.; Kramer, H.; van Hullebusch, E. D.; Farges, F.; Lens, P. N. L.; (Editors)

The origin of the organic layer covering colloidal biogenic elemental selenium nanoparticles (BioSeNPs) is not known, particularly in the case when they are synthesized by complex microbial communities. This study investigated the presence of extracellular polymeric substances (EPS) on BioSeNPs. The role of EPS in capping the extracellularly avialble BioSeNPs was also examined. FT-IR spectroscopy and colorimetric measurements confirmed the presence of functional groups characteristic of proteins and carbohydrates on the BioSeNPs, suggesting the presence of EPS. Chemical synthesis of elemental selenium nanoparticles in the presence of EPS, extracted from selenite fed anaerobic granular sludge, yielded stable colloidal spherical selenium nanoparticles. Furthermore, extracted EPS, BioSeNPs and chemically synthesized EPS capped selenium nanoparticles had similar surface properties, as shown by ζ-potential versus pH profiles and iso-electric point measurements. This study shows that the EPS of anaerobic granular sludge forms the organic layer present on the BioSeNPs synthesized by these granules. The EPS also govern the surface charge of these BioSeNPs, thereby contributing to their colloidal properties, hence affecting their fate in the environment and the efficiency of bioremediation technologies.

Keywords: EPS; BioSeNPs; surface charge; capping; FT-IR

Publ.-Id: 21399

Self-assembled growth of Ni nanoparticles in amorphous alumina matrix

Jerčinović, M.; Radić, N.; Buljan, M.; Grenzer, J.; Marion, I. D.; Kralj, M.; Radović, I. B.; Hübner, R.; Dubček, P.; Salamon, K.; Bernstorff, S.

We present the formation of ordered 3D lattice of Ni nanocrystals/nanoparticles in amorphous alumina matrix achieved by a self-assembly process during a single-step magnetron sputtering deposition of Ni/Al2O3 multilayer at room temperature. The structure of the films was analyzed using Grazing Incidence Small and Wide Angle X-ray Scattering, Transmission Electron Microscopy, Atomic Force Microscopy, Grazing Incidence Wide Angle X-ray Scattering, and Time-of-Flight Elastic Recoil Detection Analysis measurements. The self-assembly is driven by surface morphology effects and it results in a body-centered tetragonal (BCT) lattice of Ni particles with crystalline face-centered cubic (FCC) internal structure in amorphous Al2O3 matrix. The size distribution of Ni NPs is narrow, and the material has good mechanical properties due to the alumina matrix. We show that the NP sizes and separations can be easily tuned by a suitable choice of the deposition conditions. The quality of the ordering achieved in the alumina matrix is found to be significantly better than the ordering of Ni particles in silica. The obtained results are important for the understanding of the self-assembly process of metallic particles in amorphous matrices and the applications of such materials. The prepared materials are potentially interesting for spintronic applications.

  • Poster
    E-MRS 2014 Spring Meeting, 26.-30.05.2014, Lille, France

Publ.-Id: 21398

Electronic structure and morphology of chromium oxide thin films grown by reactive pulsed magnetron sputtering

Gago, R.; Vinnichenko, M.; Hübner, R.; Redondo-Cubero, A.; Vázquez, L.

Chromium oxide appears in different phases such as CrO (II), Cr2Or3 (III) CrO2 (IV), and CrO3 (VI); the number in brackets denoting the valence of the Cr atoms. This structural variety results in a wide range of applications. For example, Cr2O3 is the most stable oxide under normal conditions and exhibits high hardness and low friction, being used as protective coating in magnetic recording devices or solar absorber materials.1 CrO2 is a half-metallic ferromagnet and, therefore, good candidate for Spintronics.2 Finally, CrO3 is highly toxic, corrosive, and carcinogenic; but widely used in electroplating.3 Clearly, practical applications demand processing with a precise control of the phase formation. In this work, CrOx films were grown by pulsed magnetron sputtering with different Ar/O2 mixtures and substrate temperatures up to 500ºC. The samples were analyzed by Rutherford backscattering, ellipsometry, atomic force microscopy, transmission and scanning electron microscopies, X-ray diffraction and X-ray absorption spectroscopy. On unheated substrates, films exhibit X-ray amorphous character and a direct correlation between the O2 content in the gas, [O2], and the growth rate and stoichiometry. Remarkably, a range of mixed-valence oxides is observed, with higher valence Cr states promoted with [O2]. On the contrary, substrate heating favors Cr2O3 formation and, accordingly, single-phase nanocrystalline (nc) films are identified. The amorphous films are compact, displaying a surface morphological transition from granular-like to smooth with [O2]. Upon heating, the morphology develops into a porous fibrillar-like structure, as evidenced by empty voids between nc-Cr2O3 grains. These results show a clear correlation between the bonding structure and the microstructure of the different CrOx structures. REFs: 1 Hones et al. Surf. Coat. Technol. 120-121 (1999) 277; 2 Stewart et al. Phys. Rev. B 79 (2009) 144414; 3 Sarto et al. Carcinogenesis 3 (1982) 1011.

Keywords: chromium oxide; pulsed magnetron sputtering; phase formation; XANES; morphology

  • Poster
    14th International Conference on Plasma Surface Engineering, PSE 2014, 15.-19.09.2014, Garmisch-Partenkirchen, Deutschland

Publ.-Id: 21397

Th-U-total Pb timing constraints on the emplacement of the granitoid pluton of Stolpen, Germany

Lisowiec, K.; Budzyń, B.; Słaby, E.; Schulz, B.; Renno, A. D.

Monazite from the Stolpen monzogranite (SE Germany) was studied to constrain the Th-U-total Pb age of pluton formation. Monazite grains demonstrate subtle to distinct patchy zoning related to slight compositional variations. Textural and compositional characteristics indicate that the monazite formed in a single magmatic event in a slightly heterogeneous system, and was only weakly affected by secondary alteration, which did not disturb the Th-U-Pb system. Chemical dating of the monazite gave a consistent age of 299 ± 1.7 Ma. The current study presents the first geochronological data for the Stolpen granite. It provides evidence that Stolpen is the youngest Variscan granitic intrusion in the Lusatian Granodiorite Complex and indicates that magmatic activity related to post-collisional extension in this region lasted at least 5my longer than previously assumed.

Keywords: Monazite, Th-U-Pb chemical dating; Lusatian Granodiorite Complex; Stolpen Granite; Variscan granitoids

Publ.-Id: 21396

Direct measurement of the anisotropy field in CoMnGa thin films

Fowley, C.; Ouardi, S.; Kubota, T.; Yildirim, O.; Neudert, A.; Lenz, K.; Sluka, V.; Lindner, J.; Law, J. M.; Mizukami, S.; Fecher, G. H.; Felser, C.; Deac, A. M.

High anisotropy MnGa-based ferrimagnetic thin films have received heavy attention recently as they possess high uniaxial anisotropy combined with low saturation magnetisation tuneable properties and low Gilbert damping [1-3]. These materials are therefore useful as both magnetic recording elements (due to their high anisotropy) as well as active elements in next generation spin-transfer torque devices (due to their high ferromagnetic resonance frequencies and low Gilbert damping).

Unfortunately, due to their rather high intrinsic anisotropies (up to several tens of Teslas) common measurement methods such as SQUID magnetometers and conventional ferromagnetic resonance techniques are in the worst case unsuitable for characterisation. The anisotropy field is usually characterised by extrapolation of the in-plane magnetisation curve beyond the limit of the measurement device, e.g. SQUID [2].

Here we use the techniques of high-field extraordinary Hall effect (EHE) up to 60 T, conventional SQUID (up to 7 T) and time-resolved magneto-optical Kerr effect (TR-MOKE) to characterise MnGa and CoMnGa thin films grown by sputtering on MgO (001) substrates. While conventional SQUID is used to extract the saturation magnetisation, EHE is employed in order to directly measure the magnitude of the in-plane anisotropy field and reconstruct the tilt angle of magnetisation with respect to the film normal. TR-MOKE is used to probe the high frequency response of these materials.

MnGa films (figure 1) are found to have a saturation magnetisation of 0.36 MA/m, coercivity of 0.25 T, an anisotropy field of 4.5 T, and anisotropy energy of +0.8 MJ/m³. From TR-MOKE, the precession frequency is determined to be of order 125-150 GHz and a Gilbert damping factor of 0.015.

CoMnGa (figure 2) films were found to have a saturation magnetisation of 0.2 MA/m, a coercivity of 1 T (figure 2 (a)), an anisotropy field of 18 T (figure 2 (b)) and an anisotropy energy of +1.8 MJ/m³. From the anisotropy field the estimated precession frequency is of order 500 GHz.

[1] Balke B., et al., Appl. Phys. Lett, 90, 152504 (2007);
[2] Kurt H., et al., Phys. Rev. B, 83, 020405 (2011);
[3] Ma Q.L., et al., Appl. Phys. Lett. 101, 032402 (2012).

Keywords: High anisotropy materials; Heusler alloys; extraordinary Hall effect; time resolved magento optical Kerr effect; high magnetic fields

  • Lecture (Conference)
    Intermag Dresden 2014, 04.-08.05.2014, Dresden, Germany

Publ.-Id: 21395

Re-Melting of Magnesium Chips

Ohmann, S.; Ditze, A.; Scharf, C.

Compact and loose magnesium chips, were processed by means of re-melting. The re-melting has been successfully performed without the addition of flux between temperatures of 580 °C and 600 °C. At this temperature range, the exothermic reaction of magnesium with the oxygen present in the surrounding atmosphere was avoided. Results show that more than 95 % of the magnesium chips were able to be recovered as metal. At higher temperatures, the ignition of magnesium takes place and as a result, additional dross at the surface occurs. Experiments were performed at different scales to obtain production parameters for the recycling process. Larger particle size of magnesium chips were able to be faster remelted than the smaller ones. In the case of added lime for oil removal, the yield of recovered magnesium was lower due to the reaction towards magnesium foam. The ability of re-melting at low temperatures without the need for flux demonstrates the possibility of recovering virtually all of the metal from the chips.

Keywords: Magnesium; Chips; Recycling; Remelting; Analysis

  • ERZMETALL-World of Metallurgy 67(2014)6, 330-338

Publ.-Id: 21394

Structure and Giant Inverse Magnetocaloric Effect of Epitaxial Ni-Co-Mn-Al Films

Teichert, N.; Kucza, D.; Yildirim, O.; Yuzuak, E.; Dincer, I.; Behler, A.; Helmich, L.; Boehnke, A.; Klimova, S.; Waske, A.; Elerman, Y.; Huetten, A.

The structural, magnetic, and magnetocaloric properties of epitaxial Ni-Co-Mn-Al thin flms with different compositions have been studied. The fims were deposited on MgO(001) substrates by co-sputtering on heated substrates. All films show a martensitic transition where the transition temperatures are strongly dependent on the composition. The structure of the martensite phase was shown to be 14M. The metamagnetic martensitic transition occurs from a strong ferromagnetic austenite to a weak magnetic martensite. The structural properties of the films were investigated by atomic force microscopy and temperature dependent X-ray diraction. Magnetic and magnetocaloric properties were analyzed using temperature dependent and isothermal magnetization measurements. We found that Ni41Co10:4Mn34:8Al13:8 films show giant inverse magnetocaloric effects with magnetic entropy change of 5.8 J /kg K for deltaH = 1 T.


Publ.-Id: 21393

Influence of film thickness and composition on the martensitic transformation in epitaxial Ni–Mn–Sn thin films

Teichert, N.; Auge, A.; Yuzuak, E.; Dincer, I.; Elerman, Y.; Krumme, B.; Wende, H.; Yildirim, O.; Potzger, K.; Huetten, A.

Two series of epitaxial Ni–Mn–Sn thin films of different thickness are investigated for the thickness and composition dependence of the martensitic transformation. Thin films ranging in thickness from 20 to 200 nm (series A) and 10 to 100 nm (series B) were prepared by magnetron cosputtering and deposited on heated MgO(001) substrates. The structural characterization was done by temperature-dependent X-ray diffraction measurements. Magnetization and resistivity measurements were performed to investigate the transformation characteristics. We find a strong influence 20 of the film thickness on the relative amount of material undergoing the martensitic transformation, the temperature range of the transformation, and the transformation temperatures. The main contribution originates from the rigid substrate which delays the transformation of the Ni–Mn–Sn near the interface and even leads to a layer of residual austenite at low temperatures. Another issue are size effects which presumably broaden the martensitic transformation and decrease the transformation temperatures. By variation of the thin film composition we find changes of the substrate influence due to a different mismatch between the lattice of MgO and austenite. A better phase compatibility between martensite and austenite, denoted by k2, not only results in a smaller hysteresis but is also beneficial for the transformation of material close to the substrate.

Keywords: Martensitic transformation; Magnetic thin films; Ferromagnetic shape memory alloys; Magnetocaloric effect

Publ.-Id: 21392

Search for supernova-60Fe in Earth’s microfossil record

Ludwig, P.; Bishop, S.; Egli, R.; Chernenko, V.; Deneva, B.; Faestermann, T.; Famulok, N.; Fimiani, L.; Guzman, J. M. G.; Hain, K.; Korschinek, G.; Frederichs, T.; Hanzlik, M.; Rugel, G.; Merchel, S.

Supernova (SN) explosions eject copious amounts of material into the interstellar medium. It is possible that supernova ejecta are incorporated into solar system reservoirs. 60Fe is an ideal isotope to search for such a signature, since it is produced in massive stars and can be ejected in their subsequent SN explosions, and has almost no terrestrial background. For this study, the ratio of 60Fe/Fe was determined with AMS using the GAMS setup in Garching, Germany, in a set of samples extracted from two Pacific Ocean sediment cores of 0 to 7 Ma of age. The Fe samples were obtained using a novel chemical extraction technique targeting specifically magnetofossils, chains of magnetite crystals produced by magnetotactic bacteria, and other small-grained Fe-bearing minerals, to prevent signal dilution. Our findings reveal a 60Fe signature in the age range of 1.8-2.5 Ma, which is attributed to input of SN ejecta.

Keywords: accelerator mass spectrometry; supernova

  • Lecture (Conference)
    DPG-Frühjahrstagung der Sektion AMOP, 23.-27.03.2015, Heidelberg, Deutschland

Publ.-Id: 21391

Studying the constancy of galactic cosmic rays using cosmogenic noble gases and radionuclides in iron meteorites

Smith, T.; Leya, I.; Merchel, S.; Rugel, G.; Pavetich, S.; Wallner, A.; Fifield, K.; Tims, S.; Korschinek, G.

Cosmogenic noble gases and radionuclides in meteorites are the only tools that provide information about the cosmic ray exposure (CRE) history of meteorites. In space, meteoroids are irradiated by galactic cosmic rays (GCR), which produces, among others, stable and radioactive cosmogenic nuclides. It has been demonstrated that periodic variations in the GCR intensity induce periodic peaks in the CRE age histograms. Therefore, searching for periodic peaks in CRE histograms enables one to obtain information about GCR fluency variations. Since expected GCR fluency variations have periodicities of a few hundred million years, one needs meteorites irradiated for at least that long. Iron meteorites, which have CRE ages ranging from a few million to a few billion years, are the best candidates. So far we measured noble gases and radionuclides in 28 iron meteorites by noble gas mass spectrometry and accelerator mass spectrometry. First CRE age histograms have been established and will be presented. Further analyses are ongoing and will improve the statistical interpretation, providing new information on the temporal variability of the GCR fluency.

Keywords: accelerator mass spectrometry; GCR; cosmic radiation

  • Lecture (Conference)
    DPG-Frühjahrstagung der Sektion AMOP, 23.-27.03.2015, Heidelberg, Deutschland

Publ.-Id: 21390

Efficiency enhancement of gas evolving electrolysis

Kryk, H.; Mutschke, G.; Rzehak, R.; Weier, T.; Lucas, D.; Hampel, U.

Scientific Background:
The efficiency of gas evolving electrolysis depends to a large extend on the hydrodynamics in the electrolytic cell where the gas fraction in the cell leads to increased electrical cell resistance and in turn to an increased cell voltage. Moreover, local flow phenomena like flow instabilities and bubble adhesion at electrodes / membranes may hinder the mass transfer and reduce the lifetime of membranes and electrodes. All those effects come increasingly to bear in case of enhancing the space-time yield by increasing the current density. Despite the local nature of flow effects, the design and optimization of electrolytic cells is mostly done by means of integral parameters. For further enhancement of electrolysis efficiency, the consideration of local phenomena is indispensable. The expertise of HZDR in development and application of multiphase flow measurement techniques as well as in modelling and simulation of multiphase flow using CFD may contribute to overcome technological obstacles.

State of Development:
HZDR has a long tradition in development of multiphase flow sensor and imaging techniques for in-detail flow analyses, with a focus on measurement with high resolution in space and time (see IP-situation). Flow measuring techniques are used successfully to investigate and optimize hydrodynamics in chemical as well as power engineering processes. Examples are investigations on liquid phase dispersion and flow velocity profiles in pilot-scale electrolytic cells using laser induced fluorescence [1]. Furthermore, the institute has experiences in application of MHD methods for active flow control in electrolysis processes [2]. The activities on modeling and simulation focus on the qualification of CFD-codes for multiphase flows. In close cooperation with ANSYS CFX, new modeling and simulation concepts for poly-dispersed bubbly flow including phase transfer have been developed [3].

HZDR holds 15 patents families for following measuring techniques, which are intended to use within the proposed project: X-ray tomography (4), needle probes (5), field-focusing sensor (5), flow microscope (1)

Proposed Project: Optimization of hydrodynamics and mass transfer in hydrogen evolving electrolyzers
by multiphase flow modeling and active flow control

We offer the following activities to overcome the above-mentioned obstacles:
Experimental investigation of local two-phase flow phenomena in lab-scale and pilot-scale electrolytic cells in order to gain information on integral and local gas fraction, bubble behavior and flow velocity profiles
Investigation on local mass transfer in electrolytic cells by measuring of local species concentration, transmembrane ion transport, local electrical parameters and G/L mass transfer
Development, test and validation of different cell designs, internals and process parameters (e.g. inflow conditions, mixing elements, flow control) based on the above-mentioned measurements
Development and test of active flow control methods (e.g. magnetic fields) for gas bubble detachment and mass transfer intensification
CFD modeling / simulation of two-phase flow in electrolytic cells including local effects (e.g. flow instabilities) as basis for model-based cell design / optimization

[1] Schubert, M.; Kryk, H.; Hessel, G.; Friedrich, H.-J.; Residence Time Measurements in Pilot-Scale Electrolytic Cells: Application of Laser-Induced Fluorescence; Chem. Eng. Comm., 197:1172–1186, 2010
[2] Weier, T.; Landgraf, S.; The two-phase flow at gas-evolving electrodes: bubble-driven and Lorentz-force-driven convection; European Physical Journal - Special Topics 220(2013), 313-322
[3] Ziegenhein, T.; Rzehak, R.; Krepper, E.; Lucas, D.; Numerical simulation of polydispersed flow in bubble columns with the inhomogeneous Multi-Size-Group (iMUSIG) model; Chem.Ing.Tech. 85(2013)7, 1080-1091

Keywords: Electrolysis; gas evolution; multiphase flow; measurement techniques; fast X-ray tomography; numerical simulation; Euler-Euler method; phase-field method; power-to-gas; electrochemistry

  • Invited lecture (Conferences)
    Bosch-Helmholtz-Research Day 2014, 05.-06.05.2014, Stuttgart, Deutschland

Publ.-Id: 21389

Amphiphilic polymer coating, radiolabel functionalization and in vitro behaviour of ultrasmall iron oxide nanoparticles in different tumour cell lines

Pombo Garcia, K.; Zarschler, K.; Bergmann, R.; Steinbach, J.; Stephan, H.; Barreto, J. A.; Spiccia, L.; Graham, B.

There is currently great interest in the application of nanoparticles for cancer imaging [1]. Due to their small size, biocompatibility and unique magnetic properties, ultrasmall superparamagnetic iron oxide nanoparticles (USPIONs) are one of the most attractive candidates as advanced biomedical materials [2]. A number of different methods have been used to transfer USPIONs into aqueous solution, one of the most successful of which has been the embedding of an amphiphilic polymer within the hydrophobic coating of the as-synthesized particles [3]. This strategy also provides a ready route to further chemical modification and functionalization of the outer surface with different molecules such as radiolabels, which may be used to help understand the biodistribution of the nanoparticles in vivo [4, 5].
We report the synthesis of functionalized USPIONs with a 5 nm core and stabilized by octylamine-modified polyacrylic acid (OPA) for potential use in magnetic resonance imaging (MRI). To assess their in vitro and in vivo behaviour, a 64Cu(II) chelator, N-(4-aminophenyl)-2-[4,7-bis(2-pyridylmethyl)-1,4,7-triazacyclononan-1-yl]acetamide (ami-no-dmptacn) was conjugated to the OPA chains for radioactivity-based detection and the nanoparticles tested on a range of different tumor cell lines (FaDu, MDA-MB435S, A431). Transmission Electron Microscopy (TEM) and Dynamic Light Scattering (DLS) confirmed that the OPA-USPIONs are relatively monodisperse and stable in aqueous media. Temperature-dependent cellular uptake experiments indicate that the OPA-USPIONs can be internalized into cells via an energy-dependent cellular pathway, however no evidence of cytotoxicity is observed upon incubating the particles with the three different tumor cell lines for up to 72 h.
[1] J. A. Barreto et al., Adv. Mater. 2011, 23, H18.
[2] R. Costo et. al., Langmuir 2011, 28, 178.
[3] A. Quarta et al., Nanoscale 2012, 4, 3319.

  • Poster
    GDCh Wissenschaftsforum, 01.-04.09.2013, Darmstadt, Deutschland

Publ.-Id: 21388

Multidentate zwitterionic polymer modified ultrasmall iron oxide nanoparticles as a multimodal diagnostic imaging tool

Pombo Garcia, K.; Zarschler, K.; Stephan, H.; Spiccia, L.; Graham, B.

The application of ultrasmall superparamagnetic iron oxide nanoparticles (USPIONs) as versatile diagnostic probes for biomedical imaging, including magnetic resonance imaging (MRI) and positron emission tomography (PET), requires hydrophilic and biocompatible surface coatings [1]. Novel coating strategies involving zwitterionic species limit nonspecific adsorption of biomolecules to the surface of nanoparticles and enable them to evade phagocytosis. This opens up promising routes for the development of clinically-approved nanoparticle-based theranostic agents with fewer off-target effects [2].We have generated a novel nanoparticle platform by introducing a zwitterionic polymer layer onto the magnetite particle surface (ZW-USPIONs). The ZW-USPIONs exhibit remarkable colloidal stabilities under extreme conditions, including high ionic strength, a wide pH range, and complex biological fluids. Furthermore, we have demonstrated that conjugation of biomolecules and/or bifunctional chelators for radiolabelling is feasible. Specifically, an epidermal growth factor (EGFR)-specific targeting vector and a 64Cu radiolabel for PET imaging have been
coupled to the carboxylic groups of ZW-USPIONs [3]. In vitro evaluation of the ZW-USPIONS using an MTT assay indicate low cytotoxicity in a range of human cells for nanoparticle concentrations up to 100 µg/mL. A very low degree nanoparticle-protein complex formation upon incubation of the ZW-USPIONs with human serum has been confirmed. Engineering the surface charge and functionalization of nanoparticles using multidentate zwitterionic polymers thus
provides a powerful strategy to optimise their surface properties for biological/medical applications, including as multimodal diagnostic imaging agents.

1. García KP, Zarschler K, Barreto JA, Hesse J, Spiccia L, Graham B, Stephan H, RSC Adv. 3, 22443 (2013).
2. García KP, Zarschler K, Barbaro L, Barreto JA, O'Malley W, Spiccia L, Stephan H, Graham B, Small. doi: 10.1002/smll.201303540 (2014).
3. Viehweger K, Barbaro L, García KP, Joshi T, Geipel G, Steinbach J, Stephan H, Spiccia L, Graham B. Bioconjugate Chem. doi: 10.1021/bc5001388 (2014).

  • Lecture (Conference)
    5th EuCheMS Chemistry Congress, 31.08.-04.09.2014, Istanbul, Turkey

Publ.-Id: 21387

Zwitterionic-coated ultrasmall iron oxide nanoparticles for magnetic resonance imaging

Pombo Garcia, K.; Zarschler, K.; Stephan, H.; Weiß, S.; Uhlarz, M.; Hübner, R.; Spizzia, L.; Graham, B.

Ultrasmall superparamagnetic iron oxide nanoparticles (USPIONS) have been applied in vitro and in vivo as contrast agents to improve the sensitivity of magnetic resonance imaging (MRI). Most of the clinically approved iron-containing particles are used as MRI contrast agents for the liver. This kind of particles is accumulated in the liver as a result of opsonization and scavenging by the mononuclear phagocyte system. Currently, novel coating strategies are applied to evade phagocytosis and to pave the way to evolve contrast agents with fewer off-target effects. We have developed a novel nanoparticle platform by introducing a zwitterionic polymer layer onto the magnetite particle surface (ZW-USPIONS). Firstly, magnetite (Fe3O4) crystals of about 5 nm were synthesized via thermal decomposition of iron oleate in presence of oleyl alcohol. Subsequently, the hydrophobic magnetite nanoparticles were stabilized using a zwitterionic polymeric layer to render them water-soluble and to provide an extraordinary stability over a broad pH range and different ionic strength. Purification of the polymer-coated nanoparticles via ultracentrifugation becomes a critical step for getting rid of the unbound polymer and to produce monodisperse samples with a defined hydrodynamic diameter by intensity of ca.15 nm measured by dynamic light scattering and to give a nearly neutral zeta potential in a pH range of 6.8 to 9. Thorough characterization of the ZW-USPIONS has been performed. X-ray diffraction studies show the presence of magnetite (Fe3O4). The dependence of magnetisation on temperature and magnetic field in static fields up to 7 Tesla was determined by using a commercial SQUID magnetometer. High-resolution transmission electron microscopy (HR-TEM) confirms well-defined narrow-sized particles. The presence of a tiny polymer layer around the crystals was also studied by elemental analysis, Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The in vitro cytotoxicity of the ZW-USPIONS was evaluated by 3-[4,5-dimethylthialzol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. Toxic response was not observed with nanoparticle concentrations up to 100 µg/mL in a range of human cell lines. SDS-PAGE and Gel electrophoresis studies were performed to analyse the composition of the nanoparticles-protein complex formed upon incubation with human serum. The zwitterionic-coated nanoparticles have shown anti-fouling properties and a significant decrease of non-specific bounded proteins compared to negatively and positively charged coatings. These properties render the functionalized ZW-USPIONS suitable for being used as MRI-agents.

  • Poster
    10th International Conference on the Scientific and Clinical Applications of Magnetic Carriers, 10.-14.06.2014, Dresden, Deutschland

Publ.-Id: 21386

Scatter analysis and correction for ultrafast X-ray tomography

Wagner, M.; Barthel, F.; Zalucky, J.; Bieberle, M.; Hampel, U.

Ultrafast X-ray computed tomography (CT) is an imaging technique with high potential for the investigation of the hydrodynamics in multiphase flows. For correct determination of the phase distribution of such flows a high accuracy of the reconstructed image data is essential. In X-ray CT, radiation scatter may cause disturbing artefacts. Since the scattering is not considered in standard reconstruction algorithms, additional methods are necessary to correct the detector readings or to prevent the detection of scattered photons. In this paper, we present an analysis of the scattering background for the ultrafast X-ray CT imaging system ROFEX at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and propose a correction technique based on collimation and deterministic simulation of first-order scattering.

Keywords: X-ray tomography; scatter analysis; cupping artefacts

Publ.-Id: 21385

Time of flight measurement in heavy-ion collisions with the HADES RPC TOF wall

Kornakov, G.; Arnold, O.; Atomssa, E. T.; Behnke, C.; Belyaev, A.; Berger-Chen, J. C.; Biernat, J.; Blanco, A.; Blume, C.; Böhmer, M.; Bordalo, P.; Chernenko, S.; Deveaux, C.; Dybczak, A.; Epple, E.; Fabbietti, L.; Fateev, O.; Fonte, P.; Franco, C.; Friese, J.; Fröhlich, I.; Galatyuk, T.; Garz´On, J. A.; Gill, K.; Golubeva, M.; Gonz´Alez-D´Iaz, D.; Guber, F.; Gumberidze, M.; Harabasz, S.; Hennino, T.; Höhne, C.; Holzmann, R.; Ierusalimov, A.; Ivashkin, A.; Jurkovic, M.; Kämpfer, B.; Karavicheva, T.; Kardan, K.; Koenig, I.; Koenig, W.; Kolb, B. W.; Korcyl, G.; Kotte, R.; Krasa, A.; Krebs, E.; Krizek, F.; Kuc, H.; Kugler, A.; Kunz, T.; Kurepin, A.; Kurilkin, A.; Kurilkin, P.; Ladygin, V.; Lalik, R.; Lang, S.; Lapidus, K.; Lebedev, A.; Lopes, L.; Lorenz, M.; Maier, L.; Mangiarotti, A.; Markert, J.; Metag, V.; Michel, J.; Müntz, C.; Münzer, R.; Naumann, L.; Palka, M.; Pechenov, V.; Pechenova, O.; Petousis, V.; Pietraszko, J.; Przygoda, W.; Ramstein, B.; Rehnisch, L.; Reshetin, A.; Rost, A.; Rustamov, A.; Sadovsky, A.; Salabura, P.; Scheib, T.; Schmidt-Sommerfeld, K.; Schuldes, H.; Sellheim, P.; Siebenson, J.; Silva, L.; Sobolev, Y. G.; Spataro, S.; Ströbele, H.; Stroth, J.; Strzempek, P.; Sturm, C.; Svoboda, O.; Tarantola, A.; Teilab, K.; Tlusty, P.; Traxler, M.; Tsertos, H.; Vasiliev, T.; Wagner, V.; Wendisch, C.; Wirth, J.; Wüstenfeld, J.; Yurevich, S.; Zanevsky, Y.

This work presents the analysis of the performance of the RPC ToF wall of the HADES, located at GSI Helmholtzzentrum f¨ur Schwerionenforschung, Darmstadt. The behavior of the detector is studied in Au+Au collisions at 1.23 AGeV. A main characteristic of the detector is that all the active areas were designed to be electrically shielded in order to operate in high occupancies of the chambers. Here we show the achieved performance regarding efficiency and timing capabilities at different occupancies of this special design after the applied offline corrections to the data. Also the stability of the intrinsic time resolution over time of data taking is presented.

Publ.-Id: 21384

Influence of ϕ mesons on negative kaons in Ni+Ni collisions at 1.91A GeV beam energy

Piasecki, K.; Herrmann, N.; Averbeck, R.; Andronic, A.; Barret, V.; Basrak, Z.; Bastid, N.; Benabderrahmane, M. L.; Berger, M.; Buehler, P.; Cargnelli, M.; Čaplar, R.; Crochet, P.; Czerwiakowa, O.; Deppner, I.; Dupieux, P.; Dželalija, M.; Fabbietti, L.; Fodor, Z.; Gasik, P.; Gašparić, I.; Grishkin, Y.; Hartmann, O. N.; Hildenbrand, K. D.; Hong, B.; Kang, T. I.; Kecskemeti, J.; Kim, Y. J.; Kirejczyk, M.; Kiš, M.; Koczon, P.; Kotte, R.; Lebedev, A.; Leifels, Y.; Le Fèvre, A.; Liu, J. L.; Lopez, X.; Manko, V.; Marton, J.; Matulewicz, T.; Münzer, R.; Petrovici, M.; Rami, F.; Reischl, A.; Reisdorf, W.; Ryu, M. S.; Schmidt, P.; Schüttauf, A.; Seres, Z.; Sikora, B.; Sim, K. S.; Simion, V.; Siwek-Wilczyńska, K.; Smolyankin, V.; Suzuki, K.; Tymiński, Z.; Wagner, P.; Weber, I.; Widmann, E.; Wiśniewski, K.; Xiao, Z. G.; Yushmanov, I.; Zhang, Y.; Zhilin, A.; Zinyuk, V.; Zmeskal, J.

ϕ and K− mesons from Ni+Ni collisions at the beam energy of 1.91A GeV have been measured by the FOPI spectrometer, with a trigger selecting central and semi-central events amounting to 51% of the total cross section. The phase space distributions, and the total yield of K−, as well as the kinetic energy distribution and the total yield of ϕ mesons are presented. The ϕ\K− ratio is found to be 0.44±0.07(stat)+0.18−0.12(syst), meaning that about 22% of K− mesons originate from the decays of ϕ mesons, occurring mostly in vacuum. The inverse slopes of direct kaons are up to about 15 MeV larger than the ones extracted within the one-source model, signalling that a considerable share of gap between the slopes of K+ and K− could be explained by the contribution of ϕ mesons to negative kaons.

Publ.-Id: 21383

Lifting shell structures in the dynamically assisted Schwinger effect in periodic fields

Otto, A.; Seipt, D.; Blaschke, D.; Kämpfer, B.; Smolyansky, S. A.

The dynamically assisted pair creation (Schwinger effect) is considered for the superposition of two periodic electric fields acting a finite time interval. We find a strong enhancement by orders of magnitude caused by a weak field with a frequency being a multitude of the strong-field frequency. The strong low-frequency field leads to shell structures which are lifted by the weaker high-frequency field. The resonance type amplification refers to a new, monotonously increasing mode, often hidden in some strong oscillatory transient background which disappears during the smoothly switching off the background fields, thus leaving a pronounced residual shell structure in phase space.

Publ.-Id: 21382

Accounting for the analytical properties of the quark propagator from Dyson-Schwinger equation

Dorkin, S. M.; Kaptari, L. P.; Kämpfer, B.

An approach based on combined solutions of the Bethe-Salpeter (BS) and Dyson-Schwinger (DS) equations within the ladder-rainbow approximation in the presence of singularities is proposed to describe the meson spectrum as quark-antiquark bound states. We consistently implement into the BS equation the quark propagator functions from the DS equation, with and without pole-like singularities, and show that, by knowing the precise positions of the poles and their residues, one is able to develop reliable methods of obtaining finite interaction BS kernels and to solve the BS equation numerically. We show that, for bound states with masses M < 1 GeV, there are no singularities in the propagator functions when employing the infrared part of the Maris-Tandy kernel in truncated BS-DS equations. For M > 1 GeV, however, the propagator functions reveal pole-like structures. Consequently, for each type of mesons (unflavored, strange and charmed) we analyze the relevant intervals of M where the pole-like singularities of the corresponding quark propagator influence the solution of the BS equation and develop a framework within which they can be consistently accounted for. The BS equation is solved for pseudo-scalar and vector mesons. Results are in a good agreement with experimental data. Our analysis is directly related to the future physics programme at FAIR with respect to open charm degrees of freedom.

Publ.-Id: 21381

The interaction of slow highly charged ions with carbon nano membranes

Heller, R.; Wilhelm, R. A.; Gruber, E.; Ritter, R.; Stöger-Pollach, M.; Mücklich, A.; Werner, U.; Beyer, A.; Vieker, H.; Facsko, S.; Gölzhäuser, A.; Aumayr, F.

The ability of highly charged ions to induce various kinds of nanostructures at solid surfaces was intensively investigated within recent years [1-6]. Whether these nanostructures were hillocks, craters or of caldera type they all had one thing in common: a pronounced dependency of their size on the projectiles potential energy and to a minor extend on their velocity. Thus by tuning the incident ions charge state the size of nanostructures can be precisely adjusted.
The mechanisms of nano structuring by HCI impact were intensively investigated and especially for insulating surfaces of alkali and earth alkali halide crystals plausible models could be derived from extensive experimental studies [1-4]. Thereby it turned out that the confinement of initial excitations induced by the relaxing projectile as well as the coupling of electronic excitations to the phononic system of the target play a major role in the formation of nanostructures [5]. A strong local confinement of electronic excitation is not exclusively present in insulators, but can also be achieved by reducing the dimensions of the interaction volume.
In the present contribution we will show recent results of investigations on the nano structure formation by HCIs on 1nm thick freestanding carbon nano membranes (CNMs). We could successfully demonstrate the ability of HCI to create nano pores in CNMs with a diameter of a few nm, which is controllable by the projectile’s charge state [6].
Besides these findings the use of a 2D material as a target offers another unique opportunity that was not available in all previous investigations on bulk materials: the chance to have a look onto the projectile after transmission through the surface.
We could access the HCIs charge exchange and energy transfer to the surface during interaction by observing its energy and charge state distribution by means of an electrostatic analyzer (Fig.1). As a result of these investigations we will present energy loss and charge spectra of low energy (4keV...135keV) Xenon ions of
various charge states (q=5+...30+) when passing through CNMs as well as through freestanding sheets of graphene. A microscopic model to explain the experimental findings with special emphasis on the non-equilibrium charge state dependent energy loss of HCIs will be presented and discussed.

[1] A.S. El-Said, R. Heller, W. Meissl, R. Ritter, S. Facsko et al., Phys. Rev. Lett. 100, 237601 (2008)
[2] R. Heller, S. Facsko, R. A. Wilhelm and W. Möller, Phys. Rev. Lett. 101, 096102 (2008)
[3] A.S. El-Said, R. Heller, F. Aumayr and S. Facsko, Phys. Rev. B 82, 033403 (2010)
[4] A.S. El-Said, R. A. Wilhelm, R. Heller, S. Facsko et al., Phys. Rev. Lett. 109, 117602 (2012)
[5] F. Aumayr et al., J. Phys.:Cond. Mat. 23, 393001 (2011)
[6] R. Ritter, R. A. Wilhelm, M. Stöger-Pollach, R. Heller
et al., Appl. Phys. Lett. 102, 063112 (2013)

Keywords: Highly Charged Ions; Stopping; Nano membranes; charged exchange; energy loss

  • Invited lecture (Conferences)
    20th International Workshop on Inelastic Ion-Surface Collisions (IISC-20), 16.-21.02.2014, Adelaide, Australia

Publ.-Id: 21380

Wilson coefficients and four-quark condensates in QCD sum rules for medium modifications of D mesons

Buchheim, T.; Hilger, T.; Kämpfer, B.

Wilson coefficients of light four-quark condensates in QCD sum rules are evaluated for pseudo-scalar D mesons, thus, pushing the sum rules toward mass dimension six. Contrary to the situation for q¯q mesons the impact of the four-quark condensates for vacuum as well as in-medium situations is found to be rather small within the Borel window used in previous analyses. The complete four-quark condensate contributions enable to identify candidates for an order parameter of spontaneous chiral symmetry breaking/restoration as well as to evaluate stability criteria of operator product expansions.

Publ.-Id: 21379

The Bain library: A Cu-Au buffer template for a continuous variation of lattice parameters in epitaxial films

Kauffmann-Weiss, S.; Hamann, S.; Reichel, L.; Siegel, A.; Alexandrakis, V.; Heller, R.; Schultz, L.; Ludwig, A.; Faehler, S.

Smallest variations of the lattice parameter result in significant changes in material properties. Whereas in bulk, lattice parameters can only be changed by composition or temperature, coherent epitaxial growth of thin films on single crystals allows adjusting the lattice parameters independently. Up to now only discrete values were accessible by using different buffer or substrate materials. We realize a lateral variation of in-plane lattice parameters using combinatorial film deposition of epitaxial Cu-Au on a 4-in. Si wafer. This template gives the possibility to adjust the in-plane lattice parameter over a wide range from 0.365 nm up to 0.382 nm.

Keywords: epitaxial growth; magnetic layers; RBS

Publ.-Id: 21378

Threshold and Efficiency for Pore Formation in 1nm Thick Carbon Nanomembranes by Slow Highly Charged Ions

Wilhelm, R. A.; Gruber, E.; Ritter, R.; Heller, R.; Beyer, A.; Turchanin, A.; Klingner, N.; Hübner, R.; Stöger-Pollach, M.; Vieker, H.; Hlawacek, G.; Gölzhäuser, A.; Facsko, S.; Aumayr, F.

The perforation of 1nm thick carbon nanomembranes by slow highly charged ions is studied with respect to their pore formation efficiency. It is found that a threshold in potential energy of the highly charged ions of about 10keV must be exceeded in order to form round pores with diameters of a few nm. Above this energy threshold the efficiency for a single ion to form a pore increases from 70% to nearly 100% with increasing charge state. These findings are verified by two independent methods, namely the analysis of individual membranes stacked together during irradiation and the detailed analysis of exit charge state spectra utilizing an electrostatic analyzer.

Keywords: slow highly charged ion; HCI; carbon nanomembranes; CNM; ion charge state; charge exchange; nanostructures

Publ.-Id: 21377

Digital Pulse Processing for CdZnTe Detectors

Födisch, P.; Berthel, M.; Kormoll, T.; Hueso-González, F.; Lange, B.; Pausch, G.; Kaever, P.

Segmented Cadmium Zinc Telluride (CdZnTe) detectors are promising candidates for a high resolution radiation detection system due to their good energy and spatial resolution. They are also well suited for medical imaging in a Compton camera application. This first Compton camera prototype is set up with a double sided strip detector in the scatter layer. Such an electrode layout enables a fast timing at the expense of efficiency losses. As a Compton camera needs an adequate number of valid scattering events with precise timing, a pixelated electrode layout performs best. The analog readout electronics for a 8 x 8 pixeldetector (20 x 20 x 5mm3) has been replaced by a digital FPGA-based system. All detector signals are sampled by charge sensitive preamplifiers with 100MS/s and 14 bit resolution. The pulse heights are extracted with an adjustable Moving Window Deconvolution in combination with a slow shaping IIR filter (Recursive Moving Average). To generate a trigger, we implemented a digital constant fraction discriminator (DCFD) on the fast shaped signal. This signal is derived by applying a FIR filter to the deconvoluted cathode signal. The algorithm uses a symmetric [−1, 1] coefficient structure for fast computation. Our results show that the accuracy of the DCFD can be improved by optimizing the filter length. Both algorithms for energy and timestamp calculation have been evaluated with measurements of a 22Na source and at the ELBE accelerator with a pulsed beam. The initial implementation results in an energy resolution of about 5% (FWHM at 511 keV) for a randomly selected pixel and a timing resolution better than 12 ns FWHM for all events up to 10MeV.

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


Publ.-Id: 21376

Spin wave emission from magnetic vortex cores measured by time-resolved magnetic x-ray microscopy

Wintz, S.; Tyberkevych, V.; Weigand, M.; Schultheiss, K.; Raabe, J.; Lindner, J.; Erbe, A.; Slavin, A.; Fassbender, J.

The investigation of spin wave dynamics in nanomagnetic systems is a key topic of modern magnetism. Here we demonstrate that the excitation of short propagating spin waves is possible in a stacked vortex pair system with opposite circulations and parallel cores. The emitted spin waves (L~100nm) were directly imaged by means of time-resolved scanning transmission x-ray microscopy (Maxymus). The observed frequencies were only limited by the width of the x-ray pulses in multi-bunch mode.

Keywords: spin waves vortex x-ray microscopy

  • Poster
    From PICO to FEMTO workshop, 26.-27.01.2015, Berlin, Deutschland

Publ.-Id: 21375

Observation of Binary Vortex Core States in Magnetic Multilayers

Wintz, S.; Im, M.-Y.; Banholzer, A.; Schneider, T.; Weigand, M.; Raabe, J.; Mattheis, R.; Erbe, A.; Fischer, P.; Fassbender, J.

Topological spin textures such as skyrmions or vortices are attracting significant attention because of their fundamentally interesting magnetostatic and dynamic properties. In particular, magnetic vortices have been studied intensively during the past decade. Such a spin vortex consists of a planar, flux-closing magnetization curl that tilts out of the plane in the central core. Although being relatively small, the core is of crucial importance for the overall vortex structure. For a stacked trilayer geometry, two basic vortex core configurations can be expected, namely parallel (PL) and antiparallel (AP) cores. In the present contribution, we address the direct observation of both PL and AP core configurations in trilayer vortex pairs. Transmission x-ray microscopy is used to directly image the congruent cores in a Co(48)/Ru(0.8)/Ni81Fe19(43) (nm) trilayer stack. Moreover, switching between both configurations in the same vortex pair was achieved by applying high frequency in-plane magnetic fields.

Keywords: vortex core x-ray microscopy

  • Poster
    DPG Frühjahrstagung der Sektion kondensierte Materie, 15.-20.02.2015, Berlin, Deutschland

Publ.-Id: 21374

In-situ ion beam irradiation: X-ray scattering & diffraction experiments

Roshchupkina, O. D.; Baehtz, C.; Facsko, S.; Bischoff, L.; Posselt, M.; Grenzer, J.

Ion beam techniques are widely used in semiconductor industry e.g. for introducing dopant atoms into materials. Ion implantation is characterized by fast dynamic processes associated with the evolution of collision cascades resulting in formation of defects such as vacancies, interstitials, etc. As a consequence, normally a strained layer that expands in the direction normal to the substrate surface is formed. This is due to the point that the bulk material prevents any lateral macroscopic expansion and as a result the thin irradiated layer is subjected to an in-plane biaxial compressive stress. Ion irradiation is a very fast process and it is almost impossible to monitor it in-situ with the present x-ray sources. However, the accumulation of damage and the diffusion of defects in implanted species are much slower processes and can be studied in-situ.

An in-situ ion beam implantation experiment was set up at ROBL/MRH at ESRF. For this purpose an ion gas source with a maximal acceleration voltage of 5keV was mounted on a sputtering chamber. To realize sufficient volume damage the ion energy was further raised to 20keV by increasing the electrostatic potential of the irradiated sample using an additional power supply. Samples were irradiated at room temperature using He+.

Measuring reciprocal space maps the different behavior of the strain evolution from the accumulation of defects, over the formation of a strained layer to a complete (X-ray) amorphous layer of single crystal substrates of Si and Al2O3 was studied.

  • Lecture (Conference)
    DPG-Frühjahrstagung der Sektion Kondensierte Materie (SKM), 30.03.-04.04.2014, Dresden, Germany

Publ.-Id: 21373

Kooperationspartnerschaft: Die Helmholtz-Gesellschaft und die TU Bergakademie

Meyer, B.; Sauerbrey, R.; Gutzmer, J.

Die Technische Universität Bergakademie trägt den Namenszusatz "Die Ressourcenuniversität. Seit 1765."

Keywords: Keine Keywords

  • Acamonta 21(2014), 20-23

Publ.-Id: 21372

Direct Condensation and Entrainment Steam Experiments at the TOPFLOW-DENISE facility

Seidel, T.; Beyer, M.; Lucas, D.

In a hypothetical Small Break Loss of Coolant Accident (SB-LOCA) in a Pressurized Water Reactor (PWR), the Reactor Pressure Vessel wall (RPV) may be exposed to thermal stress, since Emergency Core Cooling Systems (ECCS) injects cold water. The loads on the primary loop and RPV walls are determined by mixing processes with the surrounding hot water and by the condensation of steam on the surface.

For the development and validation of CFD-models, experiments have to meet a high standard of reproducibility, measurement certainty and temporal and local resolution. The pressure tank technology of the TOPFLOW facility allows conducting such experiments at reasonable effort.
The Direct Condensation and Entrainment Installation for Steam Experiments (DENISE) is made for CFD-grade condensation experiments at up to 50 bars pressure. Subcooled water is injected into the DENISE-basin in three different configurations to generate stratified flow, jet and plunging jet (steam entrainment with a jet) experiments with condensation.
The experimental facility is presented along with the high degree of instrumentation. High speed camera, a particular LED illumination, infrared observation, micro thermocouples, coriolis flow meters and movable thermal lances were used.

The available set of experiments is shown together with a comparison of surface waviness and fragmentation to temperature distributions and to the resulting condensation rates.
The results will be compared to the available literature.

Keywords: direct condensation; steam; water; pressure; experiment; CFD; computational fluid dynamics; SB-LOCA; thermal shock; PTS; stratified; jet; plunging; entrainment

  • Contribution to proceedings
    The 16th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-16), 30.08.-04.09.2015, Chicago, IL, USA
  • Lecture (Conference)
    The 16th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-16), 30.08.-04.09.2015, Chicago, IL, USA
  • atw - International Journal for Nuclear Power 60(2015)10, 599-602

Publ.-Id: 21371

Production of three-dimensional quantum dot lattice of Ge/Si core-shell quantum dots and Si/Ge layers in an alumina glass matrix

Buljan, M.; Radic, N.; Sancho-Paramon, J.; Janicki, V.; Grenzer, J.; Bogdanovic-Radovic, I.; Siketic, Z.; Ivanda, M.; Utrobicic, A.; Huebner, R.; Weidauer, R.; Vales, V.; Endres, J.; Car, T.; Jercinovic, M.; Rosko, J.; Bernstorff, S.; Holy, V.

We report on the formation of Ge/Si quantum dots with core/shell structure that are arranged in a three-dimensional body centered tetragonal quantum dot lattice in an amorphous alumina matrix. The material is prepared by magnetron sputtering deposition of Al2 O3 /Ge/Si multilayer. The inversion of Ge and Si in the deposition sequence results in the formation of thin Si/Ge layers instead of the dots. Both materials show an atomically sharp interface between the Ge and Si parts of the dots and layers. They have an amorphous internal structure that can be crystallized by an annealing treatment. The light absorption properties of these complex materials are significantly different compared to films that form quantum dot lattices of the pure Ge, Si or a solid solution of GeSi. They show a strong narrow absorption peak that characterizes a type II confinement in accordance with theoretical predictions. The prepared materials are promising for application in quantum dot solar cells.

Keywords: Ge/Si core shell; 3D QD lattices; Type II confinement; Absorption Submitted

Publ.-Id: 21370

Experimente und Simulation von Wasser- und Stofftransport an einem für Sachsen typischen Grundwassersystem

Peuker, L. C.

Es ist kein Abstract vorhanden.

  • Study thesis
    HZDR, Rossendorf, 2014
    Mentor: Dr. J. Lippmann-Pipke
    25 Seiten

Publ.-Id: 21369

Power of the sun is all we need for a power plant

Liedke, B.

Abstract was not needed

Keywords: sponge-like absorber; nanostructure; PV; science slam

  • Lecture (others)
    Science slam in Köln, 07.11.2014, Rathaus Köln, Deutschland

Publ.-Id: 21368

Laser-wakefield acceleration and brilliant radiation sources -- A brief virtual tour of our work the Helmholtz-Center Dresden-Rossendorf

Debus, A.; Irman, A.; Couperus, J. P.; Jochmann, A.; Zarini, O.; Köhler, A.; Bussmann, M.; Kluge, T.; Pausch, R.; Steiniger, K.; Hübl, A.; Zeil, K.; Metzkes, J.; Krafft, S.; Richter, C.; Siebold, M.; Bock, S.; Helbig, U.; Röser, F.; Loeser, M.; Schramm, U.; Cowan, T.; Sauerbrey, R.

We show a broad overview on our experimental facilities at the HZDR (ELBE, Draco, Penelope) with the experimental and theoretical activities with respect to RF and laser-accelerated electrons and derived secondary radiation sources. On the experimental part we feature the Thomson source PHOENIX at ELBE and our spectrometer diagnostics for measuring electron bunch durations at sub-fs resolution at single shots. On the theoretical part we feature synthetic diagnostics in radiative particle-in-cell simulations for linking experimental observations and theory and our project of an optical free-electron laser based on traveling-wave Thomson scattering. (This is a summary, as there was no official abstract.)

Keywords: PHOENIX; OFEL; TWTS; Traveling-wave Thomson scattering; electron bunch duration measurements; radiative PIC; synthetic diagnostics; Laser wakefield accelerator; LWFA

  • Lecture (others)
    Invitation to give a seminar talk, 21.-28.7.2014, Austin, TX, United States

Publ.-Id: 21367

Diffraction and Small Angle Scattering in 2D and 3D

Grenzer, J.; Kharchenko, A.; Gateshki, M.; Holz, T.

The development of new materials is today closely related to the “creation” of new functional nanostructures. Structural investigations are the key to establish a connection between the functional and structural properties that generate this function. This knowledge makes it possible to design new materials with precisely predetermined properties. The function of nanostructures is not only determined by their internal structure, but in large part by their morphology and surface properties.
The development of advanced optical components and 2D detectors enable today measurements that were some years ago only possible using synchrotron radiation. The most important factor is the enhancement of the signal-to-noise ratio using a well optimized setup depending on the concrete measurement problem.
For example, the development of micro-focus sources in the combination of high-performance optics and especially the new semiconductor area detectors (here we used: 2x2 á 256x256 pixels with 55μm pixel size) has established the possibility of GISAXS investigations increasingly in the laboratory. The measurements were performed on a system equipped with a two-dimensional side-by-side optics using a Q-Q diffractometer. It was ensured that the primary beam width remains almost constant over the entire dynamic range and that no secondary maxima occur. The advantage of such an optimized approach is that using the same device both small-angle scattering and as well as additional necessary diffraction experiments without any change in the setup of the diffractometer are possible.
A great advantage of GISAXS is the investigation of buried nanostructures that can be investigated without any additional preparation. Based on thin films prepared by an energetic ion assisted by PVD process, we illustrate the potential of laboratory GISAXS studies.
2D detectors are very efficient for the measurement of large reciprocal space maps at medium resolution in reciprocal space. As an example we will show the investigation of Sn-Ge-Si layers on a Si(001) substrate that are used in modern detectors for telecom applications.
In this contribution we will demonstrate the advantages as well as the special care that is needed during use of modern semiconductor pixel detectors.

  • Poster
    Workshop on X-ray Scattering Methods for Thin Film Characterization, 25.-26.09.2014, Prague, Czech Republic

Publ.-Id: 21366

From electron beams to full-scale laser-plasmas -- Obtaining radiation ``sky-maps'' of LWFA within particle-in-cell codes.

Debus, A.; Pausch, R.; Burau, H.; Couperus, J. P.; Cowan, T.; Hübl, A.; Irman, A.; Köhler, A.; Schramm, U.; Widera, R.; Bussmann, M.

Spectra from laser-wakefield acceleration are readily available in experiment, but are challenging to model ab-initio. Yet, beyond the basic properties of synchrotron light sources, the emitted spectra from laser-plasmas include the complete phase-space dynamics and thus are applicable as a powerful tool to quantitatively resolve plasma structures on micrometer and femtosecond scales.

Classical, Liénard-Wiechert-type radiation models that include coherence and polarization properties are based on electron trajectory data, which in full-scale laser-plasmas are on the order of hundreds of TBs to several PBs and require about 10^18 kernel calculations for spectral computation in a single LWFA simulation. Since, such radiation calculations clearly cannot be performed in a post-processing step after a PIC simulation, we include the calculation of these radiation spectra into our multi-GPU particle-in-cell code PIConGPU as a synthetic diagnostic (code is available as open source).

We present current LWFA and laser plasma results, for which we calculated angularly resolved spectra ranging from infrared to X-ray wavelengths. Such an extensive treatment of plasma radiation across billions of macro particles makes it possible to explore temporally resolved plasma radiation spectra on linear and logarithmic photon energy scales over large solid angles ("sky-maps"). These 3D, radiative laser-plasma simulations run on current high-performance GPU clusters and scale up to petaflop performance.

Keywords: radiative PIC; plasma radiation; Laser wakefield accelerator; LWFA; Liénard Wiechert potentials; synthetic diagnostics; particle-in-cell code

  • Lecture (Conference)
    16th Advanced Accelerator Concepts Workshop (AAC 2014), 13.-18.7.2014, San Jose, United States

Publ.-Id: 21365

X-ray Diffraction and Scattering from Nanostructures

Grenzer, J.

Nowadays, the development of new materials is often associated with specific properties of functionalized nano structures. X-ray investigations are a very important tool to find the link between the functional (magnetism, luminescence) and the corresponding structural properties (size, orientation etc.) that are generating this function. This knowledge makes it possible to design new materials with specific properties.
This tutorial will show how modern X-ray scattering methods are used in material science. Beside standard X-ray diffraction techniques we will show that with up-to-date laboratory setups X-ray methods can be applied that were some years ago only possible using synchrotron radiation. The advantage and peculiarities of different geometries, 1- and 2-dimensional detectors will be discussed, e.g. they are very efficient for the measurement of large reciprocal space maps at medium resolution.
Different examples will be shown like the investigation of semiconductor nanostructures, of fluorescence up-converting nano particles potentially used in medical applications or grazing incidence diffraction measurements of thin magnetic metallic films.

  • Invited lecture (Conferences)
    Workshop on X-ray Scattering Methods for Thin Film Characterization, 25.-26.09.2014, Prague, Czech Republic

Publ.-Id: 21364

How to bring optical free-electrons lasers to table-top with Traveling-wave Thomson scattering

Debus, A.; Steiniger, K.; Bussmann, M.; Pausch, R.; Cowan, T.; Irman, A.; Jochmann, A.; Sauerbrey, R.; Schramm, U.

Optical FELs (OFELs) based on Traveling-wave Thomson scattering (TWTS) optimally exploit the high spectral photon density in high-power laser pulses by spatially stretching the laser pulse and overlapping it with the electrons in a side scattering setup. The introduction of a laser pulse-front tilt provides for interaction lengths appropriate for FEL operation. With careful dispersion control, electrons witness an undulator field of almost constant strength and wavelength over hundreds to thousands of undulator periods, thus giving enough time for self-amplified spontaneous emission (SASE) to seed the FEL instability and the realization of large laser gains.

The TWTS OFEL provides undulator wavelengths on the order of the laser wavelength, sub-meter gain lengths and optimum conditions for optical synchronization. The TWTS OFEL has several advantages over other compact FEL concepts, as it neither requires electron beam focusing nor material for producing or containing the undulator field in the interaction region.

Here, we emphasize similarities and differences of TWTS-OFELs to conventional SASE-FELs and discuss possible experimental scenarios with respect to challenges for high-power lasers and LWFA or RF-driven electron beam sources. Since TWTS-OFELs are highly scalable and tunable from EUV to hard X-rays in very different interaction configurations, we compare these different regimes including their experimental trade-offs.

Keywords: Optical free-electron laser; OFEL; Traveling-wave Thomson scattering; TWTS

  • Lecture (Conference)
    16th Advanced Accelerator Concepts Workshop (AAC 2014), 13.-18.7.2014, San Jose, United States

Publ.-Id: 21363

GISAXS Investigation using a Standard Laboratory Diffractometer

Grenzer, J.; Kharchenko, A.; Gateshki, M.; Holz, T.

The development of new materials is today closely related to the "creation" of new functional nanostructures. Structural investigations are the key to establish a connection between the functional and structural properties that generate this function. This knowledge makes it possible to design new materials with precisely predetermined properties. The function of nanostructures is not only determined by their internal structure, but in large part by their morphology and surface properties.
Small angle X-ray scattering in grazing incidence (GISAXS) has the advantage over imaging microscopy (TEM), that usually no complex sample preparation is necessary and larger sample volumes can be analyzed. GISAXS allows the morphology of near-surface structures as well as their (inner) electron density distribution to be determined. GISAXS studies have long been almost exclusively performed at specialized synchrotron beamlines, as the requirements on e.g. on the beam quality were only hard to met with conventional laboratory equipment. The development of microfocus sources in the combination of high-performance optics and especially the new semiconductor area detectors established GISAXS investigations increasingly in the laboratory.

  • Poster
    "XTOP" 2014, 15.-19.09.2014, Grenoble & Villard de Lans, France

Publ.-Id: 21362

The evolution and dynamics of damage accumulation due to ion beam implantation studied by X-ray diffraction

Grenzer, J.; Baehtz, C.; Roshchupkina, O. D.; Posselt, M.

In the last years, the development of XFEL facilities have stimulated the discussions on the application of highly intense, ultrafast and/or coherent X-ray pulses for nonconventional in-situ studies in order to improve the understanding of fundamental and atomic-level ultrafast, processes like athermal materials modification by intense fs- or ps-laser or ion pulses generating collision cascades. A predictive understanding of such processes on the performance of advanced (nano) materials on their fabrication and live time would have a strong influence on the development of new materials and technologies. We will discuss the layout of a potential experiment at the European XFEL that could it make possible to probe the cascade dynamics of individual cascades in the (sub-)picosecond temporal regime.

Today, with the present x-ray sources it is almost impossible to monitor a collision cascade in-situ. However, the accumulation of damage and the diffusion of defects in implanted species are much slower processes and can be studied in-situ already today.

Ion beam modification of materials consists usually of two steps (i) the ion implantation and (ii) a subsequent thermal treatment. The first step is characterized by the continuous formation and relaxation of collision cascades over the irradiation time leading to a super-saturation of different types of defects (vacancies, self-interstitials, clusters, etc.) and the resulting kinetic processes like defect diffusion, the build-up of strain etc., on the time scales ranging from ns up to minutes and even hours. The second step is directed towards a reduction of radiation damage formed by ion implantation. It is determined by kinetic processes whose duration is mainly given by the length of the thermal treatment, such as damage recovery, recrystallization of amorphous regions, migration and/or clustering or segregation of point defects and dopant atoms. As a result the material undergoes a strong modification that determines the way how its properties of are changed.
An in-situ ion beam implantation experiment was set up at ROBL/MRH at ESRF. For this purpose an ion gas source with a maximum acceleration voltage of 5keV was mounted on a sputtering chamber. To guarantee a sufficient volume damage the ion energy was further raised to 20keV. Si (001) samples were irradiated at room temperature using He+ at an ion flux of about 1013ions/cm2s. In-situ/in-operando subsequent reciprocal space maps (RSM) were measured to study the evolution of the implanted layer. The time resolution for one RSM as shown in figure 1 was 100ms resulting an acquisition time for one map of below 1min. The crystal truncation rod vanished within the first seconds of He-ion bombardment. In the following the Si (004) reflection broadens, forming a layer peak that gives clearly a hint of increasing strain in the material. After 50 minutes (>1016ions/cm2) a steady-state that corresponds to a heavily damaged or amorphized Si layer was reached.

  • Lecture (Conference)
    "XTOP" 2014, 15.-19.09.2014, Grenoble & Villard de Lans, France

Publ.-Id: 21361

LWFA at HZDR - Stability by understanding more data

Debus, A.; Pausch, R.; Steiniger, K.; Hübl, A.; Burau, H.; Widera, R.; Bussmann, M.; Cowan, T.; Couperus, J. P.; Zarini, O.; Jochmann, A.; Irman, A.; Schramm, U.

After a brief update on the current experimental facilities (DRACO & ELBE) we show how measuring plasma radiation can help understand the dynamics of Laser wakefield accelerators. As an example results of a LWFA simulation including plasma radiation using PIConGPU is shown. The radiation was calculated in many direction, in order to reconstruct a spatial image of the radiation sources within the LWFA structure -- corresponding to an experimental imaging diagnostic of plasma self-emission. This reconstruction enables to pinpoint position and time of LWFA electron injection. As an outlook of what to do with high quality LWFA electrons the project of an optical free electron laser is presented. (This is a summary, as there was no official abstract.)

Keywords: Laser wakefield acceleration; LWFA; plasma radiation; radiative PIC; optical free electron laser; Traveling-wave Thomson scattering; TWTS

  • Lecture (Conference)
    LAOLA Workshop, 06.-7.10.2014, Wismar, Deutschland

Publ.-Id: 21360

He+ Ion Beam Implantation studied by In-situ X-ray Diffraction

Roshchupkina, O. D.; Baehtz, C.; Bischoff, L.; Posselt, M.; Grenzer, J.

Ion beam techniques are widely used in semiconductor industry for introducing dopant atoms into materials. Ion implantation is characterized by fast dynamic processes associated with the evolution of collision cascades resulting in formation of different types of defects such as vacancies, interstitials, etc. At large fluencies a strained layer that expands in the direction normal to the substrate surface is formed. This is due to the point that the bulk material prevents any lateral macroscopic expansion and as a result a thin irradiated layer is subjected to an in-plane biaxial compressive stress.

The penetration of an Ion into a solid is a very fast process of picoseconds and it is almost impossible to monitor it in-situ with present x-ray sources. However, the accumulation of damage and the diffusion of defects or the implanted species are much slower processes and can be studied in-situ already today.
An in-situ ion beam implantation experiment was set up at ROBL/MRH at ESRF using an ion gas source resulting in a maximal ion energy of 20keV. Si(001) samples were irradiated at room temperature using He+ with an ion flux of max. 1013ions/cm2s. In-situ/in-operando reciprocal space maps were measured to study the strain evolution. The time resolution for one high resolution reciprocal space maps as shown in figure 1 was 100ms resulting an acquisition time for one map of below 1min.

  • Poster
    The 13th conference on Surface X-ray and Neutron Diffraction, 07.-11.07.2014, Hamburg, Germany

Publ.-Id: 21359

In-situ X-ray experiments for material science

Grenzer, J.

Kein Abstract vorhanden.

  • Invited lecture (Conferences)
    Workshop „Advanced X-ray Diffraction for Semiconductor Materials and Technology“, 03.-04.04.2014, Frankfurt/Oder, Germany

Publ.-Id: 21358

Structural evolution upon thermal annealing for Fe ion irradiated Si(100)

Khanbabaee, B.; Grenzer, J.; Facsko, S.; Pietsch, U.

Off-normal irradiation of Si(100) by Fe ions leads to the surface patterning. In order to understand the mechanism of pattern formation, chemical reactions between Fe and Si atoms have to be considered to influence the surface instability required for pattern formation. Since the as-irradiated surface area is amorphous, we examined the recrystallization process of the Fe-Si layer formed by off-normal 20 keV irradiation using a Si(100) substrate comparing the effect of low (1X10^16 ions cm^-2) and high (5x10^17 ions cm-2) fluencies, where only the higher fluence leads to patterned surface. The samples were annealed up to a temperature of 800°C and characterized by in-situ grazing incidence X-ray diffraction (GI-XRD). Depth profiling by GI-XRD confirmed that ε-FeSi was formed close to the surface changing to a ß-FeSi2 phase with lower Fe content at larger depths. While the polycrystalline ß-FeSi2 phase dominates for higher ion fluencies, a nearly equal ratio between ε-FeSi and ß-FeSi2 is found for lower ones. Our results suggest that phase distribution is related to the Fe concentration profile and can be considered as the relevant factor in the process of pattern formation.

  • Lecture (Conference)
    DPG-Frühjahrstagung der Sektion Kondensierte Materie (SKM), 30.03.-04.04.2014, Dresden, Germany

Publ.-Id: 21357

Radiative Particle in Cell simulations on laser-plasma interactions

Debus, A.; Pausch, R.; Hübl, A.; Steiniger, K.; Schmitt, F.; Burau, H.; Widera, R.; Pugmire, D.; Juckeland, G.; Nagel, W. E.; Bussmann, M.; Schramm, U.; Cowan, T.; Sauerbrey, R.

We show how to simulate electromagnetic radiation from plasmas using the particle-in-cell code PIConGPU. After an introduction of the methods we present results from Laser-Wakefield simulations and a large scale Kelvin-Helmholtz-simulation (7.2 PFLOP/s) performed on the TITAN cluster in Oakridge. Towards the conclusion we show that these methods also translate to modeling optical free-electron lasers based on Traveling-wave Thomson scattering. (This is a summary, as there was no official abstract.)

Keywords: plasma radiation; laser wakefield acceleration; LWFA; Kelvin-Helmholtz instability; KHI; Traveling-wave Thomson scattering; TWTS; optical free-electron laser

  • Lecture (Conference)
    LA3NET TW3: Novel Acceleration Techniques,, 28.-30.4.2014, Dresden, Deutschland

Publ.-Id: 21356

Ion irradiation combined with nanoindentation as a screening test procedure for irradiation hardening

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

Ion irradiation has long been recognized as a means to efficiently approximate neutron damage in structural materials. Likewise, nanoindentation has long been recognized as a tool to probe the mechanical behaviour of thin layers. The combination of both techniques in order to establish a screening test procedure for irradiation hardening requires consideration of a number of details. The objective is to specify one among several possible variants of such a screening test.
Important constituents of an approach based on ion irradiation and nanoindentation as a screening test procedure for the irradiation response of structural materials exposed to heavy neutron irradiation are: (1) the design of the ion irradiation experiments, e.g. using the MD code SRIM, (2) nanoindentation testing over a large range of indentation depths, (3) elimination of the indentation size effect, elimination of the substrate effect and pile-up correction, (4) careful documentation of the selected options and parameters.
A version of the approach sketched above was applied to unirradiated, ion-irradiated and neutron-irradiated 9%Cr ferritic/martensitic 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.

Keywords: Ion irradiation; neutron irradiation; nanoindentation; T91; screening

Publ.-Id: 21355

Mean-field model of the von Kármán sodium dynamo experiment using soft iron impellers

Nore, C.; Leorat, J.; Guermond, J.-L.; Giesecke, A.

It has been observed that dynamo action occurs in the von-Karman-Sodium (VKS) experiment only when the rotating disks and the blades are made of soft iron. The purpose of this paper is to numerically investigate the role of soft iron in the VKS dynamo scenario. This is done by using a mean field model based on an axisymmetric mean flow, a localized permeability distribution, and a localized α-effect modeling the action of the small velocity scales between the blades. The action of the rotating blades is modeled by an axisymmetric effective permeability field. Key properties of the flow giving to the numerical magnetic field a geometric structure similar to that observed experimentally are identified. Depending on the permeability of the disks and the effective permeability of the blades, the dynamo that is obtained is either oscillatory or stationary. Our numerical results confirm the leading role played by the ferromagnetic impellers. A scenario for the VKS dynamo is proposed.

Keywords: dynamo

Publ.-Id: 21354

Recent LIMMCAST Results on the Modeling of Steel Casting

Gerbeth, G.; Eckert, S.; Timmel, K.; Wondrak, T.

Model experiments with low melting point liquid metals are an important tool to investigate the flow structure and related transport processes in melt flows relevant for metallurgical applications. We present recent results from the three LIMMCAST facilities working either with room-temperature GaInSn or with the alloy SnBi at temperatures of 200-400°C. The main value of cold metal laboratory experiments consists in the capabilities to obtain quantitative flow measurements with a reasonable spatial and temporal resolution, which is essential for code validation. Experimental results are presented covering the following phenomena: contactless electromagnetic tomography of the flow in the mold, flow monitoring by a multitude of ultrasonic sensors, mold flow under the influence of an electromagnetic brake, injection of argon bubbles through the stopper rod, X-ray visualization of gas bubble two-phase flow in the nozzle and in the mold.

Keywords: Continuous casting; physical modeling; flow measurements; magnetic field; flow control; electromagnetic brake

  • Journal for Manufacturing Science and Production 15(2015)1, 131-139
    Online First (2015) DOI: 10.1515/jmsp-2014-0036
  • Lecture (Conference)
    2015 TMS Annual Meeting & Exhibition, 15.-19.03.2015, Orlando, USA

Publ.-Id: 21353

Observation and manipulation of dipole-forbidden exciton transitions in semiconductors

Schneebeli, L.; Boettge, C. N.; Breddermann, B.; Kira, M.; Koch, S. W.; Rice, W. D.; Kono, J.; Zybell, S.; Winnerl, S.; Bhattacharyya, J.; Esser, F.; Schneider, H.; Helm, M.; Ewers, B.; Chernikov, A.; Koch, M.; Chatterjee, S.; Khitrova, G.; Gibbs, H.; M. Andrews, A.; Strasser, G.

We discuss recent experimental and theoretical results that report on the observation of dipole-forbidden intra-exciton transitions in semiconductors via terahertz excitation. Additional manipulation capabilities are gained through the application of a magnetic field.

  • Lecture (Conference)
    CLEO-QELS, 08.-13.06.2014, San José, USA

Publ.-Id: 21351

YMnO3-based photocapacitive detectors in the visible light regime

Bogusz, A.; Choudhary, O. S.; Selvaraj, L.; John, V.; Bürger, D.; Skorupa, I.; Lawerenz, A.; Schmidt, O. G.; Schmidt, H.

YMnO3 is one of the few materials that exhibit ferroelectricity and antiferromagnetism. Ferroelectricity and antiferromagnetism in case of YMnO3 can be observed up to 900 K and 80 K, respectively. The remanent polarization of YMnO3 amounts to 2 uC/cm2 and metal-YMnO3-metal thin film structures can be switched between a high resistance state (HRS) and a low resistance state (LRS). In case of YMnO3 thin film, the transition from HRS to LRS (set process) occurs at the voltages 10 V and higher, while the transition from LRS to HRS (reset process) is triggered at the smaller voltages. This unipolar resistive switching is nonvolatile and has a resistance ratio of 5 orders of magnitude [1]. In pure YMnO3, absorption occurs throughout the entire visible light region, resulting in its black color [2]. This work investigates the effect of light-irradiation on the capacitance of YMnO3-based metal-ferroelectric-insulator-semiconductor (MFIS) structures. The thickness and optical constants of all layers of the MFIS diodes have been investigated using spectral ellipsometry measurements with a VASE ellipsometer assuming the refraction index of SiN to be 1.95 [3]. The DC bias for the capacitance measurements was swept from +10 to -20 V and back under different light-irradiation at a sweep rate of 230 mV/s. It has been found that under dark conditions two nonvolatile capacitance minima exists at -11 and at -3.55 V, possibly when the YMnO3 is in the LRS and HRS state, respectively. If we rewrite the +10 and -20 V branch in shorter period of time then, low capacitance state (LCS) is non-volatile and pseudo volatile, respectively. Under illumination the capacitance at the two minima increases in the visible spectral range, depending on the wavelength illumination, YMnO3 thickness and capacitance state.
[1] A. Bogusz et al., IEEE Xplore (2013), DOI:10.1109/ISCDG.2013.656319
[2] A. E. Smith et al., J. Am. Chem. Soc. 131, 17084 (2009)
[3] A. Laades et al., Phys. Stat. Sol. C 9, 2124 (2012)

  • Poster
    8th Workshop on Ellipsometry, 10.-12.03.2014, Dresden, Germany

Publ.-Id: 21350

Near-surface density profiling of Fe ion irradiated Si (100) using extremely asymmetric x-ray diffraction by variation of the wavelength

Khanbabaee, B.; Facsko, S.; Doyle, S.; Pietsch, U.

In this work, we report on correlations between surface density variations and ion parameters during ion beam-induced surface patterning process. The near-surface density variations of irradi- ated Si(100) surfaces were investigated after off-normal irradiation with 5 keV Fe ions at different fluences. In order to reduce the x-ray probing depth to a thickness below 5 nm, the extremely asym- metrical x-ray diffraction by variation of wavelength was applied, exploiting x-ray refraction at the air-sample interface. Depth profiling was achieved by measuring x-ray rocking curves as function of varying wavelengths providing incidence angles down to 0°. The density variation was extracted from the deviations from kinematical Bragg angle at grazing incidence angles due to refraction of the x-ray beam at the air-sample interface. The simulations based on the dynamical theory of x-ray diffraction revealed that while a net near-surface density decreases with increasing ion fluence which is accompanied by surface patterning, there is a certain threshold of ion fluence to surface density modulation. Our finding suggests that the surface density variation can be relevant with the mechanism of pattern formation.

Keywords: ion irradiation; surface patterning; x-ray diffraction

Publ.-Id: 21349

Photocapacitance change in YMnO3 based MFIS in the visible light regime

Choudhary, O. S.; Bogusz, A.; Selvaraj, L.; John, V.; Bürger, D.; Skorupa, I.; Lawerenz, A.; Schmidt, O. G.; Schmidt, H.

YMnO3 is one of the few materials that exhibit ferroelectricity and antiferromagnetism. Metal-YMnO3-metal thin film structures can be switched between a high resistance state (HRS) and a low resistance state (LRS), when a positive and negative writing bias is applied, respectively. This work investigates the effect of light-irradiation on the capacitance of YMnO3-based metal-ferroelectric-insulatorsemiconductor (MFIS) structures. The DC bias for the capacitance measurements was swept from +10 to -20 V and back under different light-irradiation at a sweep rate of ca. 103 mV/s. It has been found that under dark conditions two nonvolatile capacitance minima exists at -11 and at -3.55 V, possibly when the YMnO3 is in the LRS and HRS state, respectively. If we rewrite the +10 and -20 V branch in shorter period of time then, low capacitance state (LCS) is non-volatile and pseudo volatile, respectively. Under illumination the capacitance at the two minima increases in the visible spectral range, depending on the wavelength illumination, YMnO3 thickness and YMnO3 capacitance state.

  • Poster
    DPG Spring Meeting, 30.03.-04.04.2014, Dresden, Germany

Publ.-Id: 21348

Sequence dependence of electron-induced DNA strand breakage revealed by DNA nanoarrays

Keller, A.; Rackwitz, J.; Cauet, E.; Liévin, J.; Körzdörfer, T.; Rotaru, A.; Gothelf, K. V.; Besenbacher, F.; Bald, I.

The electronic structure of DNA is determined by its nucleotide sequence, which is for instance exploited in molecular electronics. Here we demonstrate that also the DNA strand breakage induced by low-energy electrons (18 eV) depends on the nucleotide sequence. To determine the absolute cross sections for electron induced single strand breaks in specific 13 mer oligonucleotides we used atomic force microscopy analysis of DNA origami based DNA nanoarrays. We investigated the DNA sequences 59-TT(XYX)3TT with X 5 A, G, C and Y 5 T, BrU 5-bromouracil and found absolute strand break cross sections between 2.66 ? 10214 cm2 and 7.06 ? 10214 cm2. The highest cross section was found for 59-TT(ATA)3TT and 59-TT(ABrUA)3TT, respectively. BrU is a radiosensitizer, which was discussed to be used in cancer radiation therapy. The replacement of T by BrU into the investigated DNA sequences leads to a slight increase of the absolute strand break cross sections resulting in sequence-dependent enhancement factors between 1.14 and 1.66. Nevertheless, the variation of strand break cross sections due to the specific nucleotide sequence is considerably higher. Thus, the present results suggest the development of targeted radiosensitizers for cancer radiation therapy.


Publ.-Id: 21347

Resistive switching of flash lamp crystallized YMnO3 thin films prepared on Pt/Ti/SiO2/Si substrates by low temperature pulsed laser deposition

Bogusz, A.; Prucnal, S.; Skorupa, W.; Blaschke, D.; Abendroth, B.; Stöcker, H.; Skorupa, I.; Bürger, D.; Schmidt, O. G.; Schmidt, H.

Use of multiferroic oxides as a switching medium presents an opportunity to add the additional or novel functionalities into the switching device. Typically, the growth temperatures of such oxides are above 600°C and so far CMOS compatibility has not been achieved. YMnO3 exhibits unipolar resistive switching [1] however its high crystallization temperature (above 750°C) imposes difficulties in preparation of thin films on metal-coated substrates. This work presents the results of electrical and structural characterization of YMnO3 thin films grown on Pt/Ti/SiO2/Si substrates by pulsed laser deposition at 400°C and crystallized by flash lamp annealing (FLA). It is shown that the FLA process with optimized parameters allows the preparation of polycrystalline YMnO3 films without deformation of the Pt/Ti electrode and interdiffusion processes in the YMnO3/Pt/Ti/SiO2/Si stack.
[1] A. Bogusz et al., IEEE Xplore (2013), DOI:10.1109/ISCDG.2013.656319

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

Publ.-Id: 21346

Atomistic modeling of ion-beam and thermally induced processes in Si and Ge

Liedke, B.; Heinig, K.-H.; Schmidt, B.; Bulutay, C.; Böttger, R.; Posselt, M.; Bracht, H.

The abstract was not needed

Keywords: Kinetic Monte-Carlo; atomistic simulation; silicon; germanium; molecular dynamics; phase separation; self organization

  • Lecture (others)
    Seminar at the KTH Royal Institute of Technology - Reactor Physics, 12.11.2014, Stockholm, Sweden

Publ.-Id: 21345

Nonvolatile unipolar resistive switching in YMnO3 thin films

Bogusz, A.; Prucnal, S.; Skorupa, W.; Blaschke, D.; Abendroth, B.; Stöcker, H.; Skorupa, I.; Bürger, D.; Schmidt, O. G.; Schmidt, H.

Due to its promising applications, resistive switching in oxides known also as a memristive effect, has gained a lot of attention both from scientists and industry. The multiferroic oxides as a switching medium offer a novel functionalities of the switching devices. Typically, the growth temperatures of such oxides are above 600°C and so far CMOS compatibility has not been achieved. As an example, the multiferroic YMnO3 exhibits unipolar resistive switching [1]. However, its high crystallization temperature (above 750°C) imposes difficulties in preparation of thin films on metal-coated substrates. This work compares the results of electrical and structural characterization of YMnO3 thin films grown on Pt/Ti/SiO2/Si substrates by pulsed laser deposition with two different approaches. In the first one, the polycrystalline YMnO3 films are deposited at 800°C. In the second approach, amorphous films deposited at 400°C are crystallized by millisecond range flash lamp annealing. It is shown that the ultra short annealing allows the preparation of polycrystalline YMnO3 films without deformation of the Pt/Ti electrode which exhibits improved endurance of resistive switching.

  • Lecture (Conference)
    Workshop AK Materialien für nichtflüchtige Speicher, 28.-29.04.2014, Chemnitz, Germany

Publ.-Id: 21344

Evaluation of the new radioligand [18F]AQ-28A by small animal PET/MR demonstrates increase of PDE10A expression in striatum and brown adipose tissue (BAT) of obese mice

Wagner, S.; Kranz, M.; Hankir, M.; Deuther-Conrad, W.; Scheunemann, M.; Teodoro, R.; Wenzel, B.; Fischer, S.; Egerland, U.; Fenske, W. K.; Hesse, S.; Höfgen, N.; Steinbach, J.; Brust, P.

Objectives: Phosphodiesterase 10A (PDE10A) hydrolyses cAMP and cGMP. It is abundantly expressed in striatum, where it modulates intracellular dopamine signaling. There is evidence that PDE10A is involved in the regulation of whole body energy balance [1], but changes in its expression associated with obesity have not been described so far. To investigate this issue, we developed the new PDE10A radioligand [18F]AQ-28A and performed preliminary PET/MR studies in various animal models of obesity.
Methods: [18F]AQ-28A was synthesized by nucleophilic aromatic substitution from a nitro precursor. Ex vivo and in vitro autoradiography of [18F]AQ-28A was performed on mice and pig brain, respectively. Mice brain and plasma samples (30 min p.i) were investigated by radio-HPLC. Uptake of [18F]AQ-28A in striatum and brown adipose tissue (BAT) of adult female CD-1 mice (n=5) was studied by dynamic animal PET/MR before (control) and after a high fat diet for 12 weeks as well as in genetically obese leptin deficient (ob/ob) mice (n=5).
Results: [18F]AQ-28A was synthesized fully automated with a radiochemical yield of 31.0±7.0% (n=3), a specific activity of 65.9±19.9 GBq/μmol (n=3) and a radiochemical purity of >98%. Ex vivo and in vitro, distribution patterns of [18F]AQ-28A in mouse and pig brain corresponded to the expression of PDE10A. In vivo, 89% and 64% of intact tracer accounted for total radioactivity in brain and plasma of mice at 30 min p.i. Dynamic PET/MR studies revealed a target specific accumulation in striatum (max. SUVmean=2.04). Follow-up studies after high fat diet showed a 130% higher SUV in BAT and a 30% higher SUV in striatum of obese mice in comparison to controls. Similar radioligand accumulation was observed in genetically obese mice with 86% parent fraction in BAT at 30 min p.i.
Conclusions: We have successfully used the new radioligand [18F]AQ-28A for PET imaging of PDE10A in various animal models of obesity. Small animal PET/MR studies demonstrated for the first time that an increase of the PDE10A expression in the striatum of obese mice is accompanied by an even stronger increase in BAT suggesting that PDE10A is a potential therapeutic target.
References: [1] Nawrocki AR, et al. (2014) Diabetes, 63: 300.

  • Lecture (Conference)
    21st International Symposium on Radiopharmaceutical Sciences - ISRS2015, 26.-31.05.2015, Columbia, Missouri, USA
  • Open Access Logo Abstract in refereed journal
    Journal of Labelled Compounds and Radiopharmaceuticals 58(2015)1, 52
    DOI: 10.1002/jlcr.3302_1

Publ.-Id: 21343

Radiation effects in structural steels for nuclear applications: an atomistic study

Bakaev, A.; van Neck, D.; van Oost, G.; Terentyev, D.

Keywords: radiation effects; ferritic steels; austenitic steels; ab initio; molecular dynamics

  • Lecture (Conference)
    KTH seminar, 18.11.2014, Stockholm, Sweden

Publ.-Id: 21342

Segregation-induced embrittlement in RPV steels: assessment by atomistic simulations

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

Segregation-induced embrittlement in RPV steels: assessment by atomistic simulations

Keywords: ferrtitic steels; embrittlement; hardening

  • Lecture (Conference)
    FWI seminar, 14.08.2014, HZDR, Germany

Publ.-Id: 21341

Dislocation mechanism of deuterium retention in tungsten under plasma implantation

Dubinko, V. I.; Terentyev, D.; Grigorev, P.; Bakaev, A.; van Oost, G.; Gao, F.; van Neck, D.; Zhurkin, E. E.

We have developed a new theoretical model for deuterium (D) retention in tungsten-based alloys on the basis of its being trapped at dislocations and transported to the surface via the dislocation network with parameters determined by ab initio calculations. The model is used to explain experimentally observed trends of D retention under sub-threshold implantation, which does not produce stable lattice defects to act as traps for D in conventional models. Saturation of D retention with implantation dose and effects due to alloying of tungsten with, e.g. tantalum, are evaluated, and comparison of the model predictions with experimental observations under high-flux plasma implantation conditions is presented.

Keywords: dislocations; retention; plasma; hydrogen isotopes; rate theory

Publ.-Id: 21340

Deuterium accumulation in tungsten under low-energy high-flux plasma exposure

Grigorev, P. Y.; Dubinko, V. I.; Terentyev, D. A.; Bakaev, A.; Zhurkin, E. E.

The accumulation of deuterium implanted in tungsten is simulated within the framework of kinetic diffusion theory. The influence of the tungsten microstructure (dislocation density and impurity concentration) on the process of deuterium capture and accumulation is considered. It is established that, under the chosen irradiation conditions, deuterium accumulation in the near-surface region is determined by capture at defects formed during implantation. The deuterium concentration gradient, together with the material microstructure, determines its accumulation in tungsten. Variation in the dislocation density and impurity concentration does not affect the simulation results, which is, first, related to the fact that the model used does not contain alternative mechanisms for the formation and growth of vacancy clusters under the subthreshold irradiation mode. The simulation results are compared with experimental data, and ways of improving the model are discussed in order to explain the deuterium-saturation effect for high fluences (more than 1023 m−2).

Keywords: tungsten; retention; hydrogen isotopes; plasma; rate theory

  • Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques 8(2014)2, 234-238
    DOI: 10.1134/S102745101402013X

Publ.-Id: 21339

Many-body central force potentials for tungsten

Bonny, G.; Terentyev, D.; Bakaev, A.; Grigoriev, P.; van Neck, D.

Tungsten and tungsten-based alloys are the primary candidate materials for plasma facing components in fusion reactors. The exposure to high-energy radiation, however, severely degrades the performance and lifetime limits of the in-vessel components. In an effort to better understand the mechanisms driving the materials' degradation at the atomic level, large-scale atomistic simulations are performed to complement experimental investigations. At the core of such simulations lies the interatomic potential, on which all subsequent results hinge. In this work we review 19 central force many-body potentials and benchmark their performance against experiments and density functional theory (DFT) calculations. As basic features we consider the relative lattice stability, elastic constants and point-defect properties. In addition, we also investigate extended lattice defects, namely: free surfaces, symmetric tilt grain boundaries, the 1/2〈1 1 1〉{1 1 0} and 1/2〈1 1 1〉 {1 1 2} stacking fault energy profiles and the 1/2〈1 1 1〉 screw dislocation core. We also provide the Peierls stress for the 1/2〈1 1 1〉 edge and screw dislocations as well as the glide path of the latter at zero Kelvin. The presented results serve as an initial guide and reference list for both the modelling of atomically-driven phenomena in bcc tungsten, and the further development of its potentials.

Keywords: tungsten; interatomic potentials; classical molecular dynamics; ab initio

Publ.-Id: 21338

Carbon-vacancy interaction controls lattice damage recovery in Iron

Terentyev, D.; Heinola, K.; Bakaev, A.; Zhurkin, E. E.

Ab initio techniques are applied to assess the positron lifetime of carbon–vacancy (C–V) complexes in iron for the first time. Positron lifetime is extremely sensitive to C–V arrangement and multiplicity. Following the ab initio lifetime data, a C–V complex can be detected as a single or clustered vacancy, or remain indistinguishable from bulk. Combining ab initio data with kinetic rate theory, we modelled annealing of irradiated Fe–C alloys and performed one-to-one comparison with experiment, which revealed a good agreement.

Keywords: Iron; Interstitial carbon; Lattice damage; Recovery; Annealing

Publ.-Id: 21337

Dislocations mediate hydrogen retention in tungsten

Terentyev, D.; Dubinko, V.; Bakaev, A.; Zayachuk, Y.; van Renterghem, W.; Grigorev, P.

In this letter, a comprehensive mechanism for the nucleation and growth of bubbles on dislocations under plasma exposure of tungsten is proposed. The mechanism reconciles long-standing experimental observations of hydrogen isotopes retention, essentially defined by material microstructure, and so far not fully explained. Hence, this work provides an important link to unify material's modelling with experimental assessment of W and W-based alloys as candidates for plasma facing components.

Keywords: hydrogen isotopes; retention; tungsten; ab initio; DFT; rate theory

Publ.-Id: 21336

Effect of carbon decoration on the absorption of 〈 100 〉 dislocation loops by dislocations in iron

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

This work closes a series of molecular dynamics studies addressing how solute/interstitial segregation at dislocation loops affects their interaction with moving dislocations in bodycentred cubic Fe-based alloys. We consider the interaction of 〈1 0 0〉 interstitial dislocation loops decorated by different numbers of carbon atoms in a wide temperature range. The results reveal clearly that the decoration affects the reaction mechanism and increases the unpinning stress, in general. The most pronounced and reproducible increase of the unpinning stress is found in the intermediate temperature range from 300 up to 600 K. The carbon-decoration effect is related to the modification of the loop–dislocation reaction and its importance at the technologically relevant neutron irradiation conditions is discussed.

Keywords: dislocation; carbon; iron; segregation

Publ.-Id: 21335

Atomistic simulation of the interaction between mobile edge dislocations and radiation-induced defects in Fe-Ni-Cr austenitic alloys

Bakaev, A.; Terent’Ev, D. A.; Grigor’Ev, P. Y.; Zhurkin, E. E.

The classical molecular dynamics method is employed to simulate the interaction of edge dislocations with interstitial Frank loops (2 and 5 nm in diameter) in the Fe-Ni10-Cr20 model alloy at the temperatures T = 300–900 K. The examined Frank loops are typical extended radiation-induced defects in austenitic steels adapted to nuclear reactors, while the chosen triple alloy (Fe-Ni10-Cr20) has the alloying element concentration maximally resembling these steels. The dislocation-defect interaction mechanisms are ascertained and classified, and their comparison with the previously published data concerning screw dislocations is carried out. The detachment stress needed for a dislocation to overcome the defect acting as an obstacle is calculated depending on the material temperature, defect size, and interaction geometry. It is revealed that edge dislocations more efficiently absorb small loops than screw ones. It is demonstrated that, in the case of small loops, the number of reactions accompanied by loop absorption increases with temperature upon interaction with both edge and screw dislocations. It is established that Frank loops are stronger obstacles to the movement of screw dislocations than to the movement of edge ones.

Keywords: classical molecular dynamics; plastic behaviour; austenitic steels; dislocations

  • Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques 8(2014)2, 220-228
    DOI: 10.1134/S1027451014020062

Publ.-Id: 21334

Basic Properties of Minor Alloying Elements in High-Cr Ferritic Steels: An Ab Initio Study

Bakaev, A.; Terentyev, D.; Bonny, G.; Klaver, T. P. C.; Olsson, P.; van Neck, D.

Basic properties of minor alloying elements, namely Mo, W, Nb, Ta, V, Mn, Si entering the conventional and reduced-activation structural Fe–(9–12)Cr steels have been analyzed using ab initio calculations. The electronic structure calculations were applied to study the interaction of minor alloying elements with a number of important and well defined lattice structures, such as point defects, the 1/2<111> screw dislocation core, high angle symmetric grain boundaries and free surfaces. The studied elements were classified according to their similarities and discrepancies regarding the interaction with the above mentioned defects. The refractory alloying elements are found to follow the same trend whereas Mn and Si exhibit peculiar behavior with respect to the interaction with both point and extended lattice defects. The obtained results are discussed and compared with previously published ab initio and available experimental data.

Keywords: minor alloying elements; ferritic steels; ab initio; DFT; lattice defects

Publ.-Id: 21333

Interaction of carbon-vacancy complex with minor alloying elements of ferritic steels

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

Interstitial carbon, dissolved in bcc matrix of ferritic steels, plays an important role in the evolution of radiation-induced microstructure since it exhibits strong interaction with vacancies. Frequent formation and break-up of carbon–vacancy pairs, occurring in the course of irradiation, affect both kinetics of the accumulation of point defect clusters and carbon spatial distribution. The interaction of typical alloying elements (Mn, Ni, Cu, Si, Cr and P) in ferritic steels used as structural materials in nuclear reactors with a carbon–vacancy complex is analyzed using ab initio techniques. It is found that all the considered solutes form stable triple clusters resulting in the increase of the total binding energy by 0.2–0.3 eV. As a result of the formation of energetically favourable solute–carbon–vacancy triplets, the dissociation energy for vacancy/carbon emission is also increased by ~0.2–0.3 eV, suggesting that the solutes enhance thermal stability of carbon–vacancy complex. Association of carbon–vacancy pairs with multiple solute clusters is found to be favorable for Ni, Cu and P. The energetic stability of solute(s)–carbon–vacancy complexes was rationalized on the basis of pairwise interaction data and by analyzing the variation of local magnetic moments on atoms constituting the clusters.

Keywords: ferritic steels; carbon; solute-rich clusters; ab initio; DFT

Publ.-Id: 21332

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