Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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35836 Publications

Jahn-Teller effect problems via ultrasonic experiments. Application to the impurity crystal CdSe:Cr

Averkiev, N. S.; Bersuker, I. B.; Gudkov, V. V.; Zhevstovskikh, I. V.; Sarychev, M. N.; Zherlitsyn, S.; Yasin, S.; Korostelin, Y. V.; Surikov, V. T.

Based on the data analysis of ultrasonic experiments, a novel approach has been developed to explore Jahn-Teller effect (JTE) problems in non-cubic crystals with JT centers without involving additional experimental data beyond the information about the electronic term and crystal symmetry. Distinguished from cubic crystals, the axis of symmetry of the bulk non-cubic crystal do not necessarily coincide with those of the local impurity center, thus complicating the relation between the distortions produced by the ultrasound wave and the JTE active modes. We analysed the problem with corresponding calculations for the wurtzite-type hexagonal crystal CdSe:Cr2+, in which the chromium ion substitutes the cadmium one in the tetrahedral environment, resulting in its electronic ground state 5T2(e2t2). Experimental investigation of this system by ultrasound at frequencies of 28-105 MHz in the temperature range of 4-180 K, yields a peak in the attenuation of the ultrasound below 40 K for the normal modes related to the c 11, c 44, c 55, c 55, and c 66 elastic moduli. The peak has been interpreted as the manifestation of the JTE, similar to the one, observed in cubic crystals doped with 3d ions. However, no anomalies of attenuation have been detected for the mode related to the c 33 elastic modulus, in contradiction to the theoretical predictions based on the previous method, worked out for cubic crystals. In the new method we obtained direct relations between the deformations, related to the crystal moduli, and the local JT modes, calculated the partition functions for each of the three possible JTE problems for systems with an electronic T term, T⊗e, T⊗t2 and T⊗(e + t2) revealed how these deformations alter the vibronic energy levels responsible for the relaxations in the JT centers. It emerged that in the wurtzite crystal under consideration, only in the T⊗e problem the deformation related to the elastic moduli c 33 displaces all the vibronic energy level uniformly, without relaxation possibilities, thus supporting the new method and explaining the experimental observations.

Publ.-Id: 28831

Changing the properties of GaAs via strain engineering in core/shell nanowires

Balaghi, L.; Bussone, G.; Grifone, R.; Hübner, R.; Grenzer, J.; Shan, S.; Fotev, I.; Pashkin, A.; Ghorbani-Asl, M.; Krasheninnikov, A.; Schneider, H.; Helm, M.; Dimakis, E.

III-V compound semiconductors have fueled many breakthroughs in photonics owing to their direct optical band gap and the possibility to tailor it in ternary or quaternary alloys by selecting the chemical composition appropriately. More recently, III-V semiconductors in the form of free-standing nanowires have found new strengths for a wide range of future applications in nanotechnology, including nano-photonics. Here we explore the great possibilities for strain engineering in core/shell nanowires as an alternative route to tailor the optical band gap of III-V semiconductors without changing their chemical composition. In particular, we demonstrate that the GaAs core in GaAs/InxGa1-xAs or GaAs/InxAl1-xAs core/shell nanowires can sustain unusually large misfit strains that would have been impossible in equivalent thin-film heterostructures, and undergoes a significant modification of its electronic proper-ties.

Core/shell nanowires were grown in the self-catalyzed mode on SiOx/Si(111) substrates by molecular beam epitaxy [1, 2]. Strain analysis was performed using synchrotron X-ray diffraction and Raman scat-tering spectroscopy, and showed that for a thin enough core, the magnitude and the spatial distribution of the built-in misfit strain can be regulated via the composition and the thickness of the shell. Beyond a critical shell thickness, we obtain a heavily tensile-strained core and an almost strain-free shell. The tensile strain of the core exhibits a predominantly-hydrostatic character and causes the reduction of the GaAs band gap energy (Figure 1) in accordance with our theoretical predictions using deformation-potential theory and first-principle calculations. For 7 % of strain (x = 0.54), the band gap energy was reduced to 0.87 eV at 300 K, i.e. a remarkable reduction of 40 %. This is particularly important for ap-plications in optical fiber telecommunications because the emission from strained GaAs nanowires can now cover the O-band and potentially the S-band of telecommunication wavelengths.

Besides the optical band gap, a similar reduction is expected for the effective mass of free electrons in tensile-strained GaAs. The corresponding electron mobility was estimated by time-domain terahertz spectroscopy to be in the range of 4000 – 5000 cm2/V·s at 300 K (core diameter = 22 nm, x = 0.39–0.49). These values are the highest reported, even in comparison to GaAs/AlxGa1-xAs nanowires with double the core thickness. This means that high-mobility transistors could now be possible with strained GaAs nanowires.

All in all, our results demonstrate that strained GaAs in core/shell nanowires can resemble the electronic properties of InxGa1-xAs, which makes it suitable for near-infrared nano-photonics. The use of a binary alloy instead of a ternary one would be advantageous because phenomena like phase separation, surface segregation or alloy disorder that typically exist in ternary alloys and limit the performance of photonic or electronic devices, become now irrelevant.

  • Invited lecture (Conferences)
    Nanostructures for Photonics, 07.05.2018, Saint Petersburg, Russia

Publ.-Id: 28829

Broadband photo-excited coherent acoustic frequency combs and mini-Brillouin-zone modes in a MQW-SESAM structure

Li, C.; Gusev, V.; Dimakis, E.; Dekorsy, T.; Hettich, M.

A multiple quantum-well semiconductor saturable absorber mirror (MQW-SESAM) structure has been investigated by femtosecond pump-probe laser spectroscopy at a central wavelength of around 1050 nm. Coherent acoustic phonons are generated and detected over a wide frequency range from ~15 GHz to ~800 GHz. In the optical absorption region, i.e., in the multiple quantum wells (In0.27Ga0.73As), acoustic frequency combs centered at ~365 GHz, with a comb spacing of ~33 GHz, are generated. Most importantly, in the transparent region, i.e., in the distributed Bragg reflector, which is formed by a non-doped long-period semiconductor GaAs/Al0.95Ga0.05As superlattice, the mini-Brillouin-zone center, as well as zone-edge acoustic modes, are observed. The mini-zone-center modes with a fundamental frequency of 32 GHz can be attributed to the spatial modulation of the pump optical interference field with a period very close to that of the distributed Bragg reflector, in combination with the periodic spatial modulation of the electrostriction coefficient in the distributed Bragg reflector. The excitation of mini-zone-edge modes is attributed to the stimulated subharmonic decay of the fundamental center modes. Their subsequent back-folding to the mini-Brillouin-zone center makes them Raman active for the probe light.

Keywords: coherent acoustic phonons; pump-probe spectroscopy; quantum well; semiconductor; superlattice; mini-Brillouin-zone; high-speed asynchronous optical sampling

  • Open Access Logo Applied Sciences 9(2019), 289

Publ.-Id: 28828

Observation of sidearm splitting studied by high resolution X-ray radiography

Shevchenko, N.; Grenzer, J.; Keplinger, O.; Rack, A.; Eckert, S.

The local dynamics of dendritic sidearms during the growth stage are studied by in-situ radiography observations at high spatial resolution of < 1 µm. A flat sample of a Ga-In alloy is solidified top-down applying a vertical temperature gradient. The evolving dendritic microstructure is visualized using synchrotron X-ray imaging at the beamline ID19 (ESRF, France). The experimental investigations on the dendrite evolution revealed a transition from a four-fold symmetry to a hyperbranched dendritic morphology. Both, the sidearm-splitting phenomena – responsible for this morphological transition – as well as the arm growth dynamics are characterized by image processing.

Keywords: Dendritic growth; Solidification; Sidearm splitting; Synchrotron X-ray imaging


Publ.-Id: 28827

Development and radiosynthesis of the first 18F-labeled inhibitor of monocarboxylate transporters (MCTs)

Sadeghzadeh, M.; Moldovan, R.-P.; Fischer, S.; Wenzel, B.; Ludwig, F.-A.; Teodoro, R.; Deuther-Conrad, W.; Jonnalagadda, S.; K. Jonnalagadda, S.; Gudelis, E.; Šačkus, A.; R. Mereddy, V.; R. Drewes, L.; Brust, P.

Monocarboxylate transporters 1 and 4 (MCT1 and MCT4) are involved in tumor development and progression. Their expression levels are related to clinical disease prognosis. Accordingly, both MCTs are promising drug targets for treatment of a variety of human cancers. The non-invasive imaging of these MCTs in cancers is regarded to be advantageous for assessing MCT-mediated effects on chemotherapy and radiosensitization using specific MCT inhibitors. Herein, we describe a method for the radiosynthesis of [18F]FACH ((E)-2-cyano-3-{4-[(3-[18F]fluoropropyl)(propyl) amino]-2-methoxyphenyl}acrylic acid), as a novel radiolabeled MCT1/4 inhibitor for imaging with PET. A fluorinated analog of α-cyano-4-hydroxycinnamic acid (FACH) was synthesized and the inhibition of MCT1 and MCT4 was measured via an [14C]lactate uptake assay. Radiolabeling was performed via a two-step protocol comprising the radiosynthesis of the intermediate (E)/(Z)-[18F]tert-Bu-FACH (tert-butyl (E)/(Z)-2-cyano-3-{4-[(3-[18F]fluoropropyl)(propyl)amino]-2-methoxyphenyl}acrylate) followed by deprotection of the tert-butyl group. The radiofluorination was successfully implemented using either K[18F]F-K2.2.2-carbonate or [18F]TBAF. The final deprotected product [18F]FACH was only obtained when [18F]tert-Bu-FACH was formed by the latter procedure. After optimization of the deprotection reaction, [18F]FACH was obtained in high radiochemical yields (39.6 ± 8.3%, EOB) and radiochemical purity (>98%).

Keywords: Radiofluorination; [18F]FACH; Positron emission tomography (PET); Monocarboxylate transporters (MCTs); α-cyano-4-hydroxycinnamic acid (α-CHC)

  • Open Access Logo Journal of Labelled Compounds and Radiopharmaceuticals 62(2019)8, 411-424
    DOI: 10.1002/jlcr.3739


Publ.-Id: 28825

Structural and optical properties of pulsed-laser deposited crystalline β-Ga2O3 thin films on silicon

Berencén, Y.; Xie, Y.; Wang, M.; Prucnal, S.; Rebohle, L.; Zhou, S.

Crystalline β-Ga2O3 thin films on (100)- and (111)-oriented Si substrates are produced by pulsed laser deposition. The as-deposited thin films are demonstrated to be polycrystalline and contain a slight deficit of oxygen atoms as measured by x-ray diffraction spectroscopy and Rutherford backscattering spectrometry, respectively. The crystallographic orientation of the Si substrate is found to play no role on the ultimate properties of the films. A direct optical band gap of 4.8 eV is determined by temperature-dependent photoluminescence excitation (PLE). Temperature-dependent PLE spectra reveal the existence of a deep acceptor level of around 1.1 eV with respect to the valence band related to self-trapped holes. We experimentally demonstrate that point defects in O-poor β-Ga2O3 thin films act as deep donors and the optical transitions are found to take place via recombination of electrons from one of the intrinsic deep donor levels with self-trapped holes located at 1.1 eV above the valence band. The 3.17 eV ultraviolet photoluminescence is proven to be related to self-trapped holes in a small polaron state between two O(II)-s sites, whereas the two blue (2.98, 2.72 eV) and the green (2.39 eV) luminescence bands are mainly originated from gallium-oxygen vacancy pairs in the (1-) charge state, gallium vacancies in the (2-) charge state and neutral oxygen interstitials, respectively.

Keywords: β-Ga2O3 thin film on Si; pulsed laser deposition; photoluminescence; substrate orientation


Publ.-Id: 28824

Geological 3D model of the Thaba Chrome Mine, Bushveld Complex, South Africa

Schwäbe, A.; Gutzmer, J.; Bachmann, K.

This 3D geologic-stratigraphic modelling project is based on comprehensive drilling data originating from the Thaba mining lease area in the Western Bushveld Complex, South Africa. The geometric shape and distribution of manifold ultramafic and mafic rock types of the cyclic units of Lower and Upper Critical Zone of the Rustenburg Layered Suite is subject of this study. These 2.06 Ga old Paleoproterozoic successions occur as layered sequences at Thaba Mine and are characterized by shallow inclination towards SE (dipping angles 15° to 27°), distinct lateral and vertical continuity of the layering and the regular stratigraphic order of the horizons.
Therewith, the requirements for the 3D geologic modelling approach are complied. The technique is called ‘Integrated Stratigraphic Modelling’ and is utilized by Maptek’s Vulcan modelling suite. The project is started by compiling a drilling database of more than 400 boreholes that host collar, survey, geology, and assay data. The present rock types of the source data are classified into five classes (chromitite, ‘silicate’, alteration, structure, burden) and a stratigraphic list of 42 horizons is developed based on the positions of chromitite horizons in the drilling data. Preparations for the modelling include modifications of original drilling data by applying the succession of horizons on the stratigraphy field in the geology table of the database.
The developed stratigraphic horizon list is utilized by the software to sequence and interpolate floor and roof positions of each horizon. In total, the model consists of 21 chromitite layers (LG1 to MG4b) and 21 mafic interlayers. Interlayer horizons with the suffix -‘SIL’ comprise all rock types between two chromitite layers. Pyroxenite, harzburgite, and norite are most dominant; the higher in the stratigraphic column the more likely are rock types that contain cumulus plagioclase, e.g. anorthosites. Altered rock types such as troctolites and serpentinites are also occurring relatively frequent, but also discordant IRUP bodies as well as dykes and pegmatitic rocks.
Chromitite layers are representing the basal parts of cyclic units with average thicknesses between a few centimeters and one meter, in some cases up to a few meters. Especially Lower Group chromitites are occurring with relative constant thicknesses over the entire study area. The farms Schilpadnest, south-west Zwartkop, and Elandskuil are parts of the study area where the layering is developed best. Middle Group chromitites and their mafic host rocks are distributed much more discontinuous on the farms Zwartkop (north-east), Roodedam, and Middellaagte.
A pronounced lateral variation of the distribution of chromitites and host rock interlayers is recognizable in different parts of the study area, particularly areas of intense faulting between the farms Zwartkop and Middellaagte plus between Middellaagte and Elandskuil. Additionally, the consistency of the modelling results is reduced in areas were dominantly short boreholes occur, especially on the north-east section of farm Zwartkop.

  • Master thesis
    TU BA Freiberg, 2018
    Mentor: Jens Gutzmer
    90 Seiten

Publ.-Id: 28823

A new single crystal diffractometer at BM20/ESRF

Hennig, C.; Ikeda-Ohno, A.; Radoske, T.; Scheinost, A. C.

The Institute of Resource Ecology / Helmholtz-Zentrum Dresden-Rossendorf operates since 20 years the Rossendorf Beamline (ROBL/BM20) at the European Synchrotron Radiation Facility (ESRF). The ESRF will interrupt the user operation for a large upgrade between January 2019 and July 2020. This time will be used to refurbish the existing experiments and to extend the experimental capacities including a diffractometer for single crystal diffraction.
This diffractometer intends to fill the gap between small molecule and large molecule crystallography. The photon flux of up to 1012 photons/sec allow the structure determination of small single crystals. The analysis of complex intergrown crystals and electron density studies is possible. The energy range of 5-35 keV allows the use of anomalous dispersion. In-situ experiments will be supported.
The objective requires the combination of a large detector, precise sample position and sufficient space for additional equipment. The diffractometer consists of an adjustable granite table with a metal frame which carry the detector. It follows a design of SNBL/ESRF and is manufactured by Instrument Design Technology Ltd/UK. The Bragg reflexes will be registered with a silicon Pilatus3 X 2M single photon counting detector. Samples will be mounted on a kappa goniometer. A microscope will be placed in a large distance 170 mm from the crystal, which allows to install a cryo cooler (80-400 K), a heater (1200 K), and a Vortex X90 CUBE silicon drift detector with a FalconX1 processor. The data extraction with will be performed with CRYSALIS. Individual components are already tested with X-ray beam.
The new single crystal diffractometer will be accessible starting from August 2020.

Keywords: single crystal diffraction; small molecule and large molecule crystallography; SCXRD; Rossendorf Beamline

  • Lecture (Conference)
    Annual Meeting of the German Crystallographic Society, 25.-28.03.2019, Leipzig, Germany
  • Abstract in refereed journal
    Zeitschrift für Kristallographie Supplement 39(2019), S16-04

Publ.-Id: 28822

Multivariate geochemical classification of chromitite seams in the Bushveld Complex, South Africa

Bachmann, K.; Menzel, P.; Tolosana-Delgado, R.; Schmidt, C.; Hill, M.; Gutzmer, J.

The Bushveld Complex, the largest layered mafic-ultramafic intrusion worldwide, is host of numerous, laterally continuous and chemically similar chromitite seams. Based on their stratigraphic position the seams are subdivided into a lower, middle and upper group (LG, MG and UG). Within these groups the seams are numbered successively – from the base to the top of each group. Attempts of discriminating between single seams based on their composition have failed – mainly due to the significant overlap of compositional fields, e.g. of chromitite mineral assemblages and chromite mineral chemistry between (neighboured) seams. In this contribution a tailored and easy to use multivariate classification scheme for the chromitite seams is proposed, based on a comprehensive classification routine for the LG and MG chromitites. This routine allows a clear attribution with known uncertainty of eight distinct chromitite seams. The study was carried out at the Thaba Mine, a chromite mine located on the western limb of the Bushveld Complex. The classification is based on a large geochemical database (N = 1205) from Thaba Mine. It comprises of a hierarchical discrimination approach relying on linear discriminant analysis and involves five distinct steps. Using default homogeneous prior probabilities, classification results are excellent for the first discrimination steps (LGs vs. MGs, 97 %; LG-6 vs. LG-6A, 94 %) and very good for the following steps (MG-1/2 vs. MG-3/4, 86 %; MG-1 vs. MG-2, 92 %; MG-3 vs. MG-4, 93 %; MG-4 vs. MG-4Z, 97 %; MG-4 vs. MG-4A, 88 %). The classification scheme was tested using the same sample set as a training set with unknown composition. Overall classification results for unknown samples belonging to one of the seams are 81 %. Hence, the classification scheme is at least valid for the Thaba mine. The approach may, however, be extended across the entire Bushveld, provided that an appropriate geochemical data set is available.

Keywords: Linear discriminant analysis; PGE; Thaba Mine; Lower group chromitites; Middle group chromitites; compositional data analysis


Publ.-Id: 28821

Variations in Composition, Texture, and Platinum Group Element Mineralization in the Lower Group and Middle Group Chromitites of the Northwestern Bushveld Complex, South Africa

Kaufmann, F. E. D.; Hoffmann, M. C.; Bachmann, K.; Veksler, I. V.; Trumbull, R. B.; Hecht, L.

Small-scale variations in mineral chemistry, textures and platinum group element (PGE) mineralization were investigated in the Lower and Middle Group chromitite layers LG6, LG6a, MG1, MG2, MG2 II from vertical drill core profiles at the Thaba mine in the northwestern limb of the Bushveld Complex. We present detailed geochemical profiles of chromite composition and chromite crystal-size distribution curves to shed light on the processes of chromite accumulation and textural modification as well as mineralization. Multiple samples within each layer were assayed for platinum-group element concentrations and the respective platinum-group mineral association was determined by mineral liberation analysis (MLA).
There is strong evidence for post-cumulus changes in the chromitites. The crystal size distribution curves suggest that the primary chromite texture was coarsened by a combination of adcumulus growth and textural equilibration, while compaction of the crystal mush played only a minor role. Mineral compositions were also modified by post-cumulus processes, but because of the very high modal amount of chromite and its local preservation in orthopyroxene oikocrysts, that phase retained much primary information. Vertical variations of chromite composition within chromitite layers, and from one layer to another do not support the idea of chromite accumulation from crystal-rich slurries nor crystal settling from a large magma chamber. Instead, we favor a successive buildup of chromitite layers by repeated injections of relatively thin layers of chromite-saturated magmas, with in-situ crystallization occurring at the crystal mush-magma interface. The adcumulus growth of chromite grains to form massive chromitite required addition of Cr to the layers, which we attribute to downward percolation from the overlying magma.
The PGE concentrations are elevated in all chromitite layers compared to adjacent silicate rocks, and show a systematic increase upwards from LG6 (ave. 807 ppb Ir+Ru+Rh+Pt+Pd+Au) to MG2 II (ave. 2062 ppb). There are also significant internal variations in all layers, with enrichments at hanging and/or footwalls. The enriched nature of chromitites in PGE compared to host pyroxenites is a general feature, independent of the layer thickness. The MLA results distinguish two principal groups of PGE mineral associations: the LG6, LG6 and MG1 are dominated by the malanite series, laurite and PGE sulfarsenides, while the MG2 and MG2 II layers are characterized by laurite and PGE sulfides, as well as Pt-Fe-Sn and PGE-Sb-Bi-Pb alloys. Differences in the PGE associations are attributed to post-cumulus alteration of the MG2 and MG2 II layer, while the chromitites below, particularly LG6 and LG6a, contain the primary association.

Keywords: Bushveld Complex; chromitites; PGE mineralization; crystal size distribution; Thaba Mine; MLA

Publ.-Id: 28820

On defects role in enhanced perpendicular magnetic anisotropy in Pt/Co/Pt, induced by ion irradiation

Jakubowski, M. M.; Liedke, M. O.; Butterling, M.; Dynowska, E.; Sveklo, I.; Milińska, E.; Kurant, Z.; Böttger, R.; von Borany, J.; Maziewski, A.; Wagner, A.; Wawro, A.

Modifications of magnetic and magneto-optical properties of Pt/Co(dCo)/Pt upon Ar+ irradiation (with energy 1.2, 5 and 30 keV) and fluence, F at the range from 2e13-2e16 Ar+/cm^2) were studied. Two ‘branches’ of increased perpendicular magnetic anisotropy (PMA) and enhanced magneto-optical response are found on two-dimensional (dCo, F) diagrams. The difference in F between ‘branches’ is driven by ion energy. Structural features correlated with magnetic properties have been analysed thoroughly by X-ray diffraction, Rutherford backscattering spectrometry and positron annihilation spectroscopy. Experimental results are in agreement with TRIDYN numerical calculations of irradiation-induced layers intermixing. Our work discusses particularly structural factors related to crystal lattice defects and strain, created and modified by irradiation, co-responsible for the increase in PMA.

Keywords: thin films; magnetic measurements; vacancy formation; Rutherford backscattering, RBS; atom, molecule, and ion impact; positron spectroscopies

Publ.-Id: 28819

Spin currents and spin-transfer torques

Deac, A. M.

To-date, the best known (or most successfully implemented) spin-based device is the hard-disk read-head. Indeed, the discovery of giant magnetoresistance enabled a paradigm shift in the miniaturization of magnetic storage technology, which was disruptive enough to earn a Nobel for the two researchers who carried out the initial studies [1]. In a nutshell, giant magnetoresistance refers to the fact that the electrical properties of a multilayer containing at least two magnetic layers depend on the orientation of their magnetic moment. For instance, if the magnetic layers are cobalt, iron or nickel (or their alloys), the resistance of the structure is maximum when the magnetic moments are antiparallel to each other, and minimum when they are parallel.
More recently, it has been demonstrated the inverse phenomenon can also be observed: the relative orientation of the magnetic moments of two ferromagnetic layers can be manipulated by applying an electrical bias (i.e. a current or a voltage) across the structure. This is a consequence of spin-momentum transfer between the conduction electrons and the magnetization of the layer they are travelling across, which effectively induces a torque on the magnetization, the so-called ‘spin-transfer torque’ or ‘spin-torque’ [2-6]. Two main effects can be induced exploiting this torque: the magnetic moment of a given layer can be switched to a chosen direction – for instance, from parallel to antiparallel to the magnetization of the second layer – or it can be induced to gyrate around a given direction for as long as the electrical bias is applied. Today, spin-transfer switching is the write scheme for non-volatile, ultra-fast Spin-Transfer Torque Random Access Memory (STT-RAM) devices. STT-RAM can be designed so that they can scale down to more than one fifth of all other available technologies, including SRAM [7,8]. Spin-transfer driven precession [6] has been suggested as working principle for other spin-based nanoelectronics devices currently under consideration, which range from tuneable, low input power radio-frequency oscillators wireless communication, to magnetic field sensors, negative resistors, amplifiers, write heads and random number generators. Indeed, the frequency of such devices can be adjusted simply by changing the applied bias, and they provide sufficient power [9] while at the same time being about 50 times smaller than present devices used in mobile telecommunication [10]. Moreover, novel materials hold the promise of pushing the frequency limit beyond what present-day technology can achieve [11]. Possible applications include anti-collision systems for cars, remote hospitals and immersive audio-video entertainment systems.
The talk will present the different projects focusing on spin currents and spin-transfer-torque induced phenomena being pursued by the Spintronics Group at the Helmholtz-Zentrum Dresden-Rossendorf (Germany).

[2] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996).
[3] L. Berger, Phys. Rev. B 54, 9353 (1996).
[4] M. D. Stiles, A. Zangwill, Phys. Rev. B 66, 014407 (2002).
[5] J. A. Katine, F. J. Albert, R. A. Buhrman et al., Phys. Rev. Lett. 84, 3149 (2000).
[6] S. I. Kiselev, J. C. Sankey, I. N. Krivorotov et al., Nature (London) 425, 380 (2003).
[9] A. Deac, A. Fukushima, H. Kubota, et al., Nature Phys. 4, 803 (2008).
[10] P. Villard, U. Ebels, D. Houssameddine, et al., IEEE J. Solid-State Circuits 45, 214 (2010).
[11] S. Mizukami, F. Wu, A. Sakuma, et al., Phys. Rev. Lett. 106, 117201 (2011).

Keywords: magnetism; spin currents; spin-transfer torques

  • Lecture (others)
    Seminar, 04.02.2019, Melbourne, Australia

Publ.-Id: 28818

[11C]-Methionine-PET/MRI is superior to MRI alone for detecting residual tumor burden in glioblastoma multiforme undergoing radical radiochemotherapy – analysis of a prospective trial

Beuthien-Baumann, B.; Seidlitz, A.; Platzek, I.; Petr, J.; Kotzerke, J.; Jentsch, C.; Löck, S.; Zessin, J.; Krex, D.; Zöphel, K.; Schackert, G.; van den Hoff, J.; Baumann, M.; Krause, M.

kein Abstrakt vorhanden

Keywords: [11C]Methionin; Glioblastoma; Positronen-Emissions-Tomographie

  • Abstract in refereed journal
    European Journal of Nuclear Medicine and Molecular Imaging 45(2018)S1, OP-088

Publ.-Id: 28817

Microstructure of accumulative roll bonded high purity aluminium laminates

Chekhonin, P.; Zöllner, D.; Zimmer, E.; Scharnweber, J.; Romberg, J.; Skrotzki, W.

Aluminium sheets of high purity were produced by accumulative roll bonding (ARB) at room temperature. The microstructure of the sheets up to 16 ARB cycles was analyzed by scanning electron microscopy. In all sheets discontinuous dynamic recrystallization occurred leading to coarse grains. In general, the grain size decreases with increasing number of applied ARB cycles, but remains much larger than the theoretical layer thickness after 6 or more ARB cycles. It is shown for the first time, how the interfaces introduced by ARB have a significant effect on the elongated grain shape by a combined experimental-numerical-study: The resulting microstructure is qualitatively discussed with regard to defects introduced at the interfaces by the ARB process, while two-dimensional Potts model simulations yield very good qualitative agreements with the experiments and underpin the importance of the ARB interfaces as barriers for the motion of grain boundaries.

Keywords: accumulative roll bonding; microstructure; dynamic recrystallization; modelling


Publ.-Id: 28816

Contact of a tumour with the pleura is not associated with regional recurrence following stereotactic ablative radiotherapy for early stage non-small cell lung cancer

Wink, K. C. J.; Löck, S.; Rossi, M.; van Baardwijk, A.; Belderbos, J.; de Ruysscher, D.; Troost, E. G. C.

Background and purpose: The aim was to investigate the incidence of isolated regional failure following stereotactic ablative radiotherapy (SABR) and risk factors for recurrence. Materials and methods: Early stage non-small cell lung cancer (NSCLC) patients treated with SABR were included in this retrospective cohort study, with isolated regional recurrence (IRR) as primary endpoint, distant recurrence (DR) and overall survival (OS) as secondary endpoints. Survival analyses were performed using the cumulative incidence function (IRR and DR) or the Kaplan–Meier method (OS) and Cox proportional hazards modelling for univariate and multivariate analyses. The prognostic effect of contact between the tumour and the pleura was investigated using the CT scans used for SABR planning. Results: A total of 554 patients were included, of whom 494 could be analysed for IRR. The median follow-up for surviving patients was 48.1 months. Twenty-one patients developed an IRR (4%). The cumulative incidence of IRR and DR after 1-, 2-, and 5 years was 2%, 3%, 7% and 8%, 15% and 21%, respectively. Two year OS was 71%. The presence and type of pleural contact was not associated with any of the studied outcomes. Conclusion: The presence, type and length of pleural contact as surrogate for visceral pleural invasion were not predictive for outcome. Further studies focussing on risk factors for occult nodal involvement, (I)RR, distant metastases and mortality in early stage NSCLC are warranted for the development of risk adapted diagnostic, treatment and follow-up strategies as more younger, operable and fitter patients receive SABR. © 2018 Elsevier B.V.

Keywords: Early stage NSCLC; Isolated regional recurrence; Radiotherapy; SABR; Visceral pleural invasion

Publ.-Id: 28814

Optically detected spin-mechanical resonance in silicon carbide membranes

Poshakinskiy, A. V.; Astakhov, G.

Hybrid spin-mechanical systems are a promising platform for future quantum technologies. Usually they require application of additional microwave fields to project integer spin to a readable state. We develop a theory of optically detected spin-mechanical resonance associated with half-integer spin defects in silicon carbide (SiC) membranes. It occurs when a spin resonance frequency matches a resonance frequency of a mechanical mode, resulting in a shortening of the spin relaxation time through enhanced spin-phonon coupling. The effect can be detected as an abrupt reduction of the photoluminescence intensity under optical pumping without application of microwave fields. We propose all-optical protocols based on such spin-mechanical resonance to detect external magnetic fields and mass with ultra-high sensitivity. We also discuss room-temperature nonlinear effects un- der strong optical pumping, including spin-mediated cooling and heating of mechanical modes. Our approach suggests a new concept for quantum sensing using spin-optomechanics.

Keywords: Quantum sensing; spin-mechanical systems; silicon carbide; point defects


Publ.-Id: 28813

Influence of microscopic precipitate structures on macroscopic pattern formation in reactive flows in a confined geometry

Balog, E.; Bittmann, K.; Schwarzenberger, K.; Eckert, K.; de Wit, A.; Schuszter, G.

Thanks to the coupling between chemical precipitation reactions and hydrodynamics, new dynamic phenomena may be obtained and new types of materials can be synthesized. Here we experimentally investigate how the characteristic microscopic crystal properties affect the macroscopic pattern obtained. To shed light on such interactions, different reactant solutions are radially injected into a calcium chloride solution at different volumetric flow rates in a confined geometry. Depending on the reactants used and the flow conditions, deformed precipitate membranes have been observed due to reaction-driven viscous fingering. In such cases we show that upon injection a large number of small particles is produced in situ by the reaction at the miscible interface between the two reactant solutions. Therefore, a colloidal gel composed of those tiny particles is pushed forward by the injected aqueous solution giving rise to a viscosity gradient-driven hydrodynamic instability.


Publ.-Id: 28812

Multiphase Flow Imaging - From Fundamentals to Applications

Hampel, U.

The invited lecture discusses fundamentals and applications of tomographic imaging for multiphase flow measurement. A focus is given to the fields of oil and gas production, chemical engineering and nuclear engineering. Th etals addresses open quesions and future needs and exemplarily demonstrates the use of tomographic imaging techniques in fundmental and engineering research at HZDR.

Keywords: multiphase flow; flow measurement; flow imaging; tomograpy; chemical engineering; nuclear engineering; oil and gas production

  • Invited lecture (Conferences)
    SPIE Student Chapter, 12.02.2019, Dresden, Deutschland

Publ.-Id: 28811

Robust Bayesian inference using Bayes spaces

van den Boogaart, K. G.

In the world of geoanalytics and mining statistics we are often confronted with indirect and uncertain measurments, where there might be considerable bias in the observations itself. This contribution addresses a new view to robust Bayesian inference, which might be suitable to address such problems.

Bayes spaces are spaces of distributions and likelihoods, similar to compositions and the Aitchison simplex. In these spaces the updating a prior to a posterior is a vector addition. This simple structure allows it to reconsider questions from Bayesian statistical analysis. E.g. a posteriors from different priors have a constant difference independent of the actual observation.

In this contribution we will consider robustness against uncertainty in model assumptions and data errors. The structure allows it to introduce uncertainty about the prior knowledge by replacing a single prior by convex set of possible priors and a single model of likelihoods by convex sets of possible models. A Bayesian update is than convex Minkowski sum of the two sets, which can be explictly computed and analysed. We will show in examples how model uncertainty and possible data errors can be expressed.

We will also discuss the uncertainty introduced by this approach. While uncertainty measured in the geometry of the Bayes spaces measured as diameter convex result will diverge at rate n, we can typically observe a constant residual uncertainty in the model estimation.

Keywords: Robust Statistics; Bayesian inference; Minkovski Sum

  • Lecture (Conference)
    IAMG2019 - 20th Annual Conference of the International Association for Mathematical Geosciences, 10.-16.08.2019, University Park, Pennsylvenia, USA

Publ.-Id: 28810

Estimating the distribution of the field for Multiple Point Statistics

van den Boogaart, K. G.; Selia, S. R. R.; Tolosana Delgado, R.

Multiple Point statistics typically provides a known distribution of the random field by means of the training image. Classical Geostatistics estimates the variogram, which is only an aspect of the distribution of the field. Both thus might use an inappropriate description of the distribution of the random field. The only exception are the high order cummulants methods and spline methods using a completly nonparametric approach. This contribution addresses the possibility to estimate the distributions for Nongaussian Random fields at the example of categorical random fields in a multiple point statistics setting.

The core idea is to discribe possible characteristics of fields by using small training patches, which can be combinded to span a space of possible random field distribution models. The specific combination is selected by a distribution valued parameter, which can be estimated from an a sampled random fields using an estimation procedure based on observation likelihoods.

Similarly to the difficulty in estimating the shape parameter of the Matern variogram there is little power in this procedure to estimate the roughness of boundaries. We will thus introduce a prior preweighting of the patches according to our physical assumptions about the boundaries.

The same procedure allows to measure, how good the high order statistics of final simulation fit to the orignal observations. We will use this to check the conditional simulations for distributional consistency with the conditioning set.

Keywords: MPS; Geostatistical Simulation; Object Simulation; High Order Statistics

  • Lecture (Conference)
    IAMG 2019 - 20th Annual Conference of the International Association for Mathematical Geosciences, 10.-16.08.2019, Pennsylvania, USA

Publ.-Id: 28809

Predictive Geometallurgy

Gutzmer, J.; Birtel, S.; Büttner, P.; Bachmann, K.; Kern, M.; Frenzel, M.

For centuries the German proverb “Vor der Hacke ist es duster” has aptly described the lack of knowledge about ore volumes, grades and beneficiation characteristics during the incremental progress of mining operations. Although much progress has been made constraining ore volumes and grades by following rigorous exploration drilling programs and applying appropriate geostatistical and spatial modelling tools, there still remains considerable technical risk when exploration turns into exploitation. This is illustrated by the observation that ca. 70% of mines perform below the prediction of their feasibility study (Wood, 2018). This underperformance is usually attributed to deficiencies in the collection of tangible geoscientific data needed to design the mine and the minerals processing plant (Wood, 2018).
Geometallurgy is an interdisciplinary approach that aims to connect the data available from geosciences with the information required to predict the performance of technologies used for ore extraction and mineral beneficiation. Tangible resource characteristics – beyond grade and tonnage - are quantified to create a model that links the geology of an ore deposit with the performance achieved during mining, mineral processing and extractive metallurgy. Successful geometallurgical programs may thus be used to mitigate the risk of production planning and plant design. However, the tools of geometallurgy have thus far mostly been used by the mining industry to improve metal recoveries and to monitor process efficiency of mineral processing plants only.
Present research goes beyond these current applications of geometallurgy. Predictive geometallurgical models for complex ore bodies and even anthropogenic raw materials are being developed by interdisciplinary teams including expertise in exploration, resource characterization, minerals processing, geostatistics and spatial modelling. Case studies will be presented in this contribution that illustrate the approach taken. These examples include (1) the recovery of Sn from a historic flotation tailings storage facility; (2) the recovery of PGE as a by-product of chromite exploitation; and (3) the intelligent use of quantitative mineral abundance and mineral association data to predict the prospects of success of sensor-based sorting.
Results obtained in the three case studies illustrate the prospects of increasing resource and energy efficiency in the mining industry. Innovative approaches are of general applicability and can be easily extended to other metals and ore deposit types. The results clearly illustrate the value of conducting comprehensive geometallurgical assessments already during the latter stages of exploration; the initial process of constructing a predictive geometallurgical model will, of course, benefit greatly from regular follow-up during the phase of active exploitation.

Keywords: geometallurgy; geosciences; minerals processing; metallurgy

  • Invited lecture (Conferences)
    4th GOOD Meeting, 23.-25.01.2019, Bremen, Germany
  • Contribution to proceedings
    4th GOOD Meeting, 23.-25.01.2019, Bremen, Germany
    4th GOOD Meeting Abstract Volume, Bremen

Publ.-Id: 28808

Corrigendum to “Synchrotron tomographic quantification of the influence of Zn concentration on dendritic growth in Mg-Zn alloys” [Acta Mater. 156 (2018) 287-296]

Phillion, A.; Shuai, S.; Guo, E.; Wang, J.; Jing, T.; Ren, Z.; Neumann-Heyme, H.; Beckermann, C.; Lee, P.

In solidification science, the solid-liquid interfacial area density is a key metric that characterizes the overall semi-solid morphology in a general sense. This interfacial area density can be defined in two different ways... [Abstract not available for Corrigenda]


Publ.-Id: 28807

Geometric reconstruction of 3D dendrite evolution from 2D transmission radiography data by a simple phase-field method

Neumann-Heyme, H.

For the in-situ observation of dynamical processes radiographic imaging possess significant advantages over tomographic reconstruction in terms of e.g. time resolution and data handling. However, on the other hand essential spatial information is lost in the projected 2D image. The proposed method demonstrates, how in the case of continuously growing, coherent structures such as dendrites their time evolution can be utilized in recovering the 3D morphology. In addition, the reconstruction incorporates some prior knowledge including the smoothness and preferential growth directions of the interface. The capabilities of the method are assessed for different situations based on simulated experiments of dendritic growth. Finally, the reconstruction of evolving dendrites from flat sample synchrotron experiments is shown.

  • Lecture (Conference)
    55th Annual Technical Meeting of the Society of Engineering Science (SES2018), 10.-12.10.2018, Madrid, Spain

Publ.-Id: 28806

4D particle tracking velocimetry to analyze bubble-particles collisions and flotation recovery at low Stokes numbers

Sommer, A.-E.; Heitkam, S.; Eckert, K.

Froth flotation is a fundamental technique to separate minerals. Hydrophobized target particles attach to the fluidic interface of gas bubbles rising in a suspension. The success of the process depends on both the surface chemistry for the hydrophobization of particles and the hydrodynamics for an encounter between bubble and particle. In the first part of the talk on overview about flotation research and modeling is given.
The second part of the talk is devoted to own research on the hydrodynamics in model cells. To quantify this performance in terms of recovery, the number of target particles at various times in a reference volume is measured. One of the remaining challenges in this field is the flotation of fine particles with a size below 10 µm. Caused by their small inertia, the particles follow the streamlines around the bubble and no collision occurs [1]. This work focuses on the measurement of the collision probability of particles with a small inertia at the bubble surface to advance our understanding of relevant microprocesses and its influence on the flotation recovery. With a 4D particle tracking velocimetry device the particle and bubble trajectories were measured simultaneously with a high temporal (1000 fps) and spatial resolution (0.03 mm/pixels). We developed an algorithm to evaluate the flotation recovery based on the collision and attachment probability [2]. The three-phase flow within a rectangular bubble column consisted of fluorescent polystyrene particles (33 µm, 1.05 g/cm3), a bubble chain (1-7 mm) and deionized water with methanol. The variation of the bubble diameter and methanol concentration led to a change of the fluid flow around the bubble (Re = 100 - 1200) and the particle hydrophobization. The results show the preferred collision of the particles at the rear of the bubble due to a higher acceleration within the vortices in the wake.

[1] Yoon and Luttrell, Mineral Processing and Extractive Metallurgy Review 5, 101 (1989).
[2] AE Sommer, M Nikpay, S Heitkam, M Rudolph, K Eckert, Minerals Engineering 124, 116-122 (2018)

Keywords: flotation; particle image velocimetry

  • Invited lecture (Conferences)
    Permsker Wissenschaftliche Lesung, 24.-28.9.2018, Perm, Rusland

Publ.-Id: 28805

Coarsening and refinement phenomena in dendritic solidification

Neumann-Heyme, H.; Eckert, K.; Beckermann, C.

Curvature-driven interface motion plays an important role in the formation of the final microstructure during dendritic solidification. Usually, such motion results in a coarser microstructure via coalescence or retraction of dendrite sidebranches \cite{ref1}. Under certain conditions, however, the microstructure can be refined due to curvature-driven pinching events that lead to dendrite fragmentation. Such pinching events are a strong function of the size and shape of the initial dendrite structure \cite{ref2}. In the present study, two- and three-dimensional phase-field simulations are performed to investigate coarsening and refinement phenomena during directional solidification of alloys. The phase-field model is solved using a finite element library that permits adaptive mesh refinement and exhibits wide parallel scalability on supercomputing facilities. A semi-implicit time integration scheme is used to allow for adaptive time stepping, which is important in particular, since curvature-driven interface motion occurs on significantly larger time scales than the initial growth. The present talk will focus on some characteristics of the applied model and physical insights that were obtained.

  • Lecture (Conference)
    5th GAMM Workshop on Phase-Field Modeling, 08.-09.02.2018, TU Dresden, Germany

Publ.-Id: 28804

Evaluation of Brain Nuclear Medicine Imaging Tracers in a Murine Model of Sepsis-Associated Encephalopathy

Szöllősi, D.; Hegedűs, N.; Veres, D. S.; Futó, I.; Horváth, I.; Kovács, N.; Martinecz, B.; Dénes, Á.; Seifert, D.; Bergmann, R.; Lebeda, O.; Varga, Z.; Kaleta, Z.; Szigeti, K.; Máthé, D.

Purpose: The purpose of this study was to evaluate a set of widely used nuclear medicine imaging agents as possible methods to study the early effects of systemic inflammation on the living brain in a mouse model of sepsis-associated encephalopathy (SAE). The lipopolysaccharide (LPS)-induced murine systemic inflammation model was selected as a model of SAE.
Procedures: C57BL/6 mice were used. A multimodal imaging protocol was carried out on each animal 4 h following the intravenous administration of LPS using the following tracers: [99mTc][2,2-dimethyl-3-[(3E)-3-oxidoiminobutan-2-yl]azanidylpropyl]-[(3E)-3-hydroxyiminobutan-2-yl]azanide ([99m
Tc]HMPAO) and ethyl-7-[125I]iodo-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]ben-zodiazepine-3-carboxylate ([125I]iomazenil) to measure brain perfusion and neuronal damage, respectively; 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) to measure cerebral glucose uptake.
We assessed microglia activity on another group of mice using 2-[6-chloro-2-(4-[125I]iodo-phenyl)-imidazo[1,2-a]pyridin-3-yl]-N-ethyl-N-methyl-acetamide ([125I]CLINME). Radiotracer uptakes were measured in different brain regions and correlated. Microglia activity was also assessed using immunohistochemistry. Brain glutathione levels were measured to investigate oxidative stress.
Results: Significantly reduced perfusion values and significantly enhanced [18F]FDG and [125I]CLINME uptake was measured in the LPS-treated group. Following perfusion compensation, enhanced [125I]iomazenil uptake was measured in the LPS-treated group’s hippocampus and cerebellum. In this group, both [18F]FDG and [125I]iomazenil uptake showed highly negative correlation to perfusion measured with ([99mTc]HMPAO uptake in all brain regions. No significant differences were detected in brain glutathione levels between the groups. The CD45 and P2Y12 double-labeling immunohistochemistry showed widespread microglia activation in the LPS-treated group.
Conclusions: Our results suggest that [125I]CLINME and [99mTc]HMPAO SPECT can be used to detect microglia activation and brain hypoperfusion, respectively, in the early phase (4 h post injection) of systemic inflammation. We suspect that the enhancement of [18F]FDG and [125I]iomazenil uptake in the LPS-treated group does not necessarily reflect neural hypermetabolism and the lack of neuronal damage. They are most likely caused by processes emerging during neuroinflammation, e.g., microglia activation and/or immune cell infiltration.

Keywords: Systemic infection; Neuroinflammation; Microglia activation; LPS; [99mTc]HMPAO; [18F]FDG; [125I]iomazenil; [125I]CLINME; SPECT/CT; PET/MRI

Publ.-Id: 28803

Effects of electron beam generated lattice defects on the periodic lattice distortion structure in 1T-TaS2 and 1T-TaSe2 thin layers

Kinyanjui, M. K.; Björkman, T.; Lehnert, T.; Köster, J.; Krasheninnikov, A.; Kaiser, U.

We have investigated the influence of electron beam generated defects on the structure of periodic lattice distortions (PLDs) which accompany charge density wave modulations in 1T -TaS2 and 1T -TaSe2 . Lattice defects were generated through irradiation with high-energy electrons in a transmission electron microscope (TEM). Using atomically resolved high-resolution TEM imaging, we investigate the PLD structure and the changes in this structure with prolonged exposure to the electron beam. We observe the formation of dislocationlike topological defects in the PLD structure. Prolonged exposure to the electron beam also leads to an increase in density of these defects. This is also accompanied by an increase in structural disorder of the PLD. Density functional theory calculations were also performed in order to understand sulfur (S) and selenium (Se) vacancy defect formation in 1T -TaSe2 and 1T -TaS2 and their effects on the PLD structure. The formation energy of Se/S vacancies was calculated to be lowest for the highly displaced S/Se atoms in the vicinity of PLD maxima. Vacancies formed at the less displaced sites near the PLD minima were found to have lower formation energy. The calculations also showed that an increase in the S/Se vacancies leads to the formation of dislocations and an increase in disorder in the PLD structures. This supports the experimental observations.

Keywords: TEM; 2D materials; first-principles calculations


Publ.-Id: 28802

Investigation of density variations in molded wood tubes using gamma-ray CT and correlation with load-bearing behavior

Hartig, J. U.; Bieberle, A.; Engmann, C.; Haller, P.

It is well known that mechanical properties of wood correlate with the density. Since wood is a naturally grown material, variations in the density distribution still exist in timber elements leading to a non-uniform distribution of mechanical properties. To investigate the density distribution in timber elements at the meter scale non-destructively, the gamma-ray computed tomography (CT) scanner, firstly introduced in 2007 by Hampel et al., has been applied. The CT scanner offers a spatial resolution of about 1-2 mm. Nevertheless, small single structures like cracks or branches can be revealed up to a size of several micrometers.
As object of interest, a molded wooden tube (MWT) [3] with a length of 3 m and a diameter of 0.3 m made of beech (Fagus sylvatica) is used. The MWT is produced in a thermo-hydro-mechanical process incorporating densification and recovery of wood transverse to the grain. Thus, besides naturally grown density variations also variations due to the production process of the MWT occur.
In order to verify the assumption that the mechanical properties correlate with the density, an axial compression test is performed with the MWT previously scanned with CT. The spatial deformations on the surface of the MWT were measured by photogrammetry and digital image correlation (DIC) is applied to determine the strain distribution.
The density and geometry data gathered by CT is also used to create a finite element (FE) model. Based on the density data, the elastic properties of the respective elements are defined. The axial compression test is simulated and the results in terms of the strain distributions are compared to the experimental data determined by DIC.
The results of the investigations showed that computed tomography is highly suitable for the non-destructive determination of the density distribution in structural elements of timber. Thus, besides for research purposes CT scanning might be used also in the future for industrial grading of timber elements.

Keywords: molded wooden tube; gamma-ray CT

  • Contribution to proceedings
    International Conference on Computational Methods in Wood Mechanics - CompWood 2019, 17.-19.06.2019, Växjö, Sweden

Publ.-Id: 28801

Characterization of continuous wave laser-induced thermal gradients in magnetic tunnel junctions integrated into microresonators via COMSOL simulations

Cansever, H.; Lindner, J.; Huebner, T.; Niesen, A.; Reiss, G.; Fassbender, J.; Deac, A. M.

Spin caloritronics still is a vivid field and aims to investigate static and dynamic effects on magnetic structures due to spin-currents generated by thermal gradients [1]. In magnetic tunnel junctions, magnetization dynamics can be induced by bias voltage as well as thermal gradients [2]. In most research, COMSOL simulations are used to estimate the overall temperature of the magnetic tunnel junction as well as the thermal gradient over the insulating barrier [3-5]. Here, we perform COMSOL simulations using the 2D heat transfer module for specific Co2FeAl/MgO(2nm)/CoFeB magnetic tunnel junctions which are integrated into so-called microresonators [6]. Microresonators have been recently used as alternative approach to investigate the magnetization dynamics of the free-layer within magnetic tunnel junctions, induced by a thermal gradient by means of its ferromagnetic resonance response [6]. Utilizing microresonators for ferromagnetic resonance detection allow for the detection of signals from micron/nano-sized object under laser heating in terms of linewidth and resonance field and thus provide the possibility to detect influences of a thermal gradient on the magnetization dynamics far below the threshold of magnetic switching. The heat diffusion over all layers are modeled by starting with a 2D (vertical) rectangular shape in which we consider the MTJ stack with the MgO-substrate and backside metallization as part of the microresonators shown in Fig 1. Moreover, we consider an air ‘layer’ and the metal-contacts defining the microresonator on top of the MgO-substrate. Upon rotation of this two-dimensional shape around the central vertical z-axis of the MTJ, we obtain a 3D cylinder in which the heat profile is simulated (see Fig 2). The simulation parameters for the materials were chosen similar to those in [3,4]. In the simulation, the fundamental properties of layers i.e. thermal conductivity, heat capacity and material density are used to obtain a temperature profile of the magnetic structure. According to the simulation results, the thermal conductivity of the insulating barrier (MgO) and top metal thicknesses influence the thermal gradient, while uniform heating is strongly affected by the surrounding material of the microresonator which is mainly made from copper (high thermal conductivity). The simulation results provide insight into the heat profile of the whole structure and in particular demonstrate that not only changing the magnetic object itself but also modifying the structure of the surrounding materials yields a handle to tune and optimize the thermal gradient.
Figure 1. 2D sketch of MTJ structure integrated into a microresonator for COMSOL modelling. Heat source, i.e. cw- laser is applied to magnetic layers through the top-metal. The temperature of the bottom of the whole structure is set to 293.15 K.
Figure 2. (a) Temperature profile across the MTJ integrated in a microresonator with the applied power of 145 mW inset (b) 3D cylindrical image of MTJ structure.

[1] Bauer G E W, Saitoh E and van Wees B J 2012 Nat. Mater. 11 391
[2] Jia X, Xia K and Bauer G E W 2011 Phys. Rev. Lett.107 176603
[3] Walter M et al 2011 Nat. Mater. 10 742
[4] Huebner T, Boehnke A, Martens U, Thomas A, Schmalhorst J M, Reiss G, Münzenberg M and Kuschel T 2016 Phys. Rev. B 93 224433
[5] T Huebner et al 2018 J. Phys. D: Appl. Phys. 51 224006
[6] H Cansever et al 2018 J. Phys. D: Appl. Phys. 51 224009

Keywords: COMSOL Simulation; magnetic tunnel junction; microresonator; ferromagnetic resonance

  • Lecture (Conference)
    Joint MMM-Intermag Conference 2019 Washington D.C., 14.-18.01.2019, Washington D.C., The United States of America

Publ.-Id: 28800

New collective mode in superconducting cuprates uncovered by Higgs spectroscopy

Chu, H.; Kim, M.-J.; Katsumi, K.; Kovalev, S.; Dawson, R. D.; Schwarz, L.; Yoshikawa, N.; Kim, G.; Putzky, D.; Li, Z. Z.; Raffy, H.; Germanskiy, S.; Deinert, J.-C.; Awari, N.; Ilyakov, I.; Green, B.; Chen, M.; Bawatna, M.; Christiani, G.; Logvenov, G.; Gallais, Y.; Boris, A. V.; Keimer, B.; Schnyder, A.; Manske, D.; Gensch, M.; Wang, Z.; Shimano, R.; Kaiser, S.

The complexity of competing interactions in high-temperature superconductors provides a fertile ground for collective modes of different origins. Their coupling to the superconducting order parameter may give important insight into the microscopic pairing mechanism. One prominent example in cuprates is the magnetic resonant mode, whose experimental observation spawned theoretical investigations of pairing scenarios mediated by antiferromagnetic fluctuations. Now, phase-resolved nonlinear terahertz spectroscopy of the superconducting Higgs mode offers a new way to reveal the coupling between the collective modes and the superconducting order parameter.
Using this technique, we discover a new collective mode distinct from the heavily damped Higgs mode in different families of cuprates. We discuss the origin of this mode and characterize its interplay with the Higgs mode. Our results demonstrate Higgs spectroscopy as a new approach to uncover interactions directly relevant to superconductivity. This technique opens up entirely new avenues for understanding unconventional superconductivity and calls for supporting theoretical work to unlock its full power.

Keywords: Cuprate high-temperature superconductors; Higgs mode; nonlinear THz spectroscopy

Publ.-Id: 28799

The new Felsenkeller 5 MV underground accelerator: Status and Program

Bemmerer, D.

Experimental nuclear astrophysics aims to study, in the laboratory, the nuclear reactions taking place in stars. However, at the energies relevant to stellar burnings, the relevant cross sections are strongly reduced by the repulsive Coulomb barrier. As a result, ion beam experiments in underground laboratories shielded from cosmic ray effects are needed in order to gain precise data. The Felsenkeller 5 MV accelerator, below 45 m rock in Dresden, is the first such accelerator on the MV scale in Europe. The laboratory was jointly built by HZDR and TU Dresden and opened in 2018. Both an internal and an external ion source have already been tested successfully underground. The accelerator itself is under commissioning, as well as a high-sensitivity radioactivity counting setup by TU Dresden. The talk will summarise the science case and the status for the new laboratory.

Keywords: Nuclear Astrophysics

  • Invited lecture (Conferences)
    Institutsseminar (Kolloquium), 24.01.2019, Dresden, Deutschland

Publ.-Id: 28797

Felsenkeller 5 MV underground ion accelerator status December 2018

Bemmerer, D.

I review the status of the Felsenkeller 5 MV underground accelerator in view of the CELLAR network of underground labs.

Keywords: Nuclear Astrophysics; Low-Level Radioactivity Measurements

  • Lecture (Conference)
    CELLAR / JEILORA Meeting, 05.-07.12.2018, Monaco, Monaco

Publ.-Id: 28796

Nuclear Astrophysics: Nucleosynthesis and Chemical Evolution Studies

Bemmerer, D.

I review Nuclear Astrophysics: Nucleosynthesis and Chemical Evolution Studies.

Keywords: Nuclear Astrophysics

  • Invited lecture (Conferences)
    Astroparticle Physics in Germany: Status and Perspectives, 19.09.2018, Mainz, Deutschland

Publ.-Id: 28795

Felsenkeller 5 MV underground accelerator at the 5th International Solar Neutrino Conference

Bemmerer, D.

I review the status of the 5 MV underground accelerator at Felsenkeller, Dresden/Germany.

Keywords: Underground physics; Nuclear Astrophysics

  • Invited lecture (Conferences)
    5th International Solar Neutrino Conference, 13.06.2018, Dresden, Deutschland

Publ.-Id: 28794

Felsenkeller 5 MV underground ion accelerator for nuclear astrophysics

Bemmerer, D.; Cowan, T. E.; Grieger, M.; Hensel, T.; Junghans, A. R.; Koppitz, M.; Ludwig, F.; Rimarzig, B.; Reinicke, S.; Schwengner, R.; Stöckel, K.; Szücs, T.; Takács, M. P.; Turkat, S.; Wagner, A.; Zuber, K.

A 5 MV Pelletron accelerator with both an internal and an external ion source providing for intensive 1H+, 4He+, and 12C+ beams is being installed in the Felsenkeller underground site in Dresden, shielded from cosmic rays by 45 m rock overburden. Civil construction has recently been completed. The technical features of the new laboratory, test results, and the scientific program will be summarized. In addition to in-house research by HZDR and TU Dresden, the new accelerator will be open for outside users, both from Germany and worldwide.

Keywords: Nuclear astrophysics; Felsenkeller

  • Lecture (Conference)
    Frühjahrstagung Hadronen und Kerne, 26.02.2018, Bochum, Deutschland

Publ.-Id: 28793

Laser pulse-length effects in trident pair production

Hernandez Acosta, U.; Kämpfer, B.

Laser pulses facilitate multiphoton contributions to the trident pair production e_L^- \to e_L^- + e_L^+ + e_L^-, where the label L indicates a laser field dressed electron (e^-) or positron (e^+ ). We isolate the impact of the pulse envelope in the trident S matrix element, formulated within the Furry picture, in leading order of a series expansion in the classical non-linearity parameter a_0. Generally, the Fourier transform of the envelope carries the information on the pulse length, which becomes an easily tractable function in the case of a cos^2 pulse envelope. The transition to a monochromatic laser wave can be handled in a transparent manner, as also the onset of multiphoton effects for short pulses can be factorized out and studied separately.

Publ.-Id: 28792

Improved astrophysical rate for the 18O(p,α)15N reaction by underground measurements

Bruno, C. G.; Aliotta, M.; Descouvemont, P.; Best, A.; Davinson, T.; Bemmerer, D.; Boeltzig, A.; Broggini, C.; Caciolli, A.; Cavanna, F.; Chillery, T.; Ciani, G. F.; Corvisiero, P.; Depalo, R.; Di Leva, A.; Elekes, Z.; Ferraro, F.; Formicola, A.; Fülöp, Z.; Gervino, G.; Guglielmetti, A.; Gustavino, C.; Gyürky, G.; Imbriani, G.; Junker, M.; Lugaro, M.; Marigo, P.; Menegazzo, R.; Mossa, V.; Pantaleo, F. R.; Piatti, D.; Prati, P.; Stöckel, K.; Straniero, O.; Strieder, F.; Szücs, T.; Takács, M. P.; Trezzi, D.

The 18O(p,α)15N reaction affects the synthesis of 15N, 18O and 19F isotopes, whose abundances can be used to probe the nucleosynthesis and mixing processes occurring deep inside asymptotic giant branch (AGB) stars. We performed a low-background direct measurement of the 18O(p,α)15N reaction cross-section at the Laboratory for Underground Nuclear Astrophysics (LUNA) from center of mass energy E_CM= 340 keV down to E_CM = 55 keV, the lowest energy measured to date corresponding to a cross-section of less than 1 picobarn/sr. The strength of a key resonance at center of mass energy E_r = 90 keV was found to be a factor of 10 higher than previously reported. A multi-channel R-matrix analysis of our and other data available in the literature was performed. Over a wide temperature range, T=0.01-1.00 GK, our new astrophysical rate is both more accurate and precise than recent evaluations. Stronger constraints can now be placed on the physical processes controlling nucleosynthesis in AGB stars with interesting consequences on the abundance of 18O in these stars and in stardust grains, specifically on the production sites of oxygen-rich Group II grains.

Keywords: Stellar hydrogen burningHydrostatic stellar nucleosynthesis

Publ.-Id: 28791

Hyperspectral Feature Extraction Using Sparse and Smooth Low-Rank Analysis

Rasti, B.; Ghamisi, P.; Ulfarsson, M. O.

In this paper, we develop a hyperspectral feature extraction method called sparse and smooth low-rank analysis (SSLRA). First, we propose a new low-rank model for hyperspectral images (HSIs) where we decompose the HSI into smooth and sparse components. Then, these components are simultaneously estimated using a nonconvex constrained penalized cost function (CPCF). The proposed CPCF exploits total variation penalty, ℓ1 penalty, and an orthogonality constraint. The total variation penalty is used to promote piecewise smoothness, and, therefore, it extracts spatial (local neighborhood) information. The ℓ1 penalty encourages sparse and spatial structures. Additionally, we show that this new type of decomposition improves the classification of the HSIs. In the experiments, SSLRA was applied on the Houston (urban) and the Trento (rural) datasets. The extracted features were used as an input into a classifier (either support vector machines (SVM) or random forest (RF)) to produce the final classification map. The results confirm improvement in classification accuracy compared to the state-of-the-art feature extraction approaches.


Publ.-Id: 28790

Gamma-Durchstrahlungsverfahren zur Prozessanalyse und Fehlersuche in Rohrleitungen, Behältern und Kolonnen

Jentsch, T.; Joonha, J.; Thereska, J.; Brisset, P.; Verhasselt, S.

Ein optimaler und auslegungskonformer Betrieb ist das Ziel eines jeden Anlagenbe-treibers. Unregelmäßigkeiten sollten möglichst ohne Betriebsunterbrechung detek-tiert und lokalisiert werden können.
Die Einflüsse auf die Betriebsweise von Rohrleitungen und Chemieanlagen sind sehr vielfältig. Ablagerungen in Rohrleitungen führen beispielsweise zu erhöhten Druckverlusten oder gar zu Durchsatzeinbußen; Ablagerungen in Behältern zum Verlust wertvollen Lagervolumens.
Ursachen für das Fehlverhalten von Kolonnen sind häufig Beschädigungen von Einbauten, die bei Packungs- oder Füllkörperkolonnen zu einer ungleichmäßigen Fluidverteilung über den Kolonnenquerschnitt führen können. Typische Probleme von Bodenkolonnen sind das Fluten einzelner Böden, das Mitreißen von Flüssig-keit, das Schäumen oder das Durchregnen.
Mit Hilfe von Gamma-Durchstrahlungsverfahren, die auf der Schwächung der von einer umschlossenen Quelle emittierten ionisierenden Strahlung basieren, sind in der Lage, die beschriebenen Fehler zu detektieren und zu lokalisieren.
Im Beitrag werden nach einer kurzen Erläuterung der physikalischen Grundlagen und des Messprinzips an Hand von Praxisbeispielen die verschiedenen Anwen-dungsmöglichkeiten und Grenzen des Gamma-Durchstrahlungsverfahrens vorge-stellt.

Keywords: Gamma-Durchstrahlungsverfahren; Prozessanalyse; chemische Industrie; Kolonnen; zerstörungsfreie Prüfung; Gamma column scanning; trouble shooting

  • Invited lecture (Conferences)
    18. Seminar „Aktuelle Fragen der Durchstrahlungsprüfung und des Strahlenschutzes“, 28.02.2019, Kassel, Deutschland

Publ.-Id: 28789

LiDAR Data Classification Using Spatial Transformation and CNN

He, X.; Wang, A.; Ghamisi, P.; Li, G.; Chen, Y.

Light detection and ranging (LiDAR) is a useful data acquisition technique, which is widely used in a variety of practical applications. The classification of LiDAR-derived rasterized digital surface model (LiDAR-DSM) is a fundamental technique in LiDAR data processing. In recent years, deep learning methods, especially convolutional neural networks (CNNs), have shown their capability in remote sensing areas, including LiDAR data processing. Traditional deep models empirically use a fixed neighborhood system as input to the network. Therefore, the weight and height of the input rectangle may not be optimal. In order to modify such handcrafted setting, a spatial transformation network is used here to identify optimal inputs. The transformed inputs are fed into a well-designed CNN to obtain the final classification results. Furthermore, morphological profiles are combined with spatial transformation CNN to further improve the classification accuracy. The proposed frameworks are tested on two LiDAR-DSMs (i.e., the Recology and Houston data sets). The experimental results show that the proposed models provide competitive results compared to the state-of-the-art methods. Furthermore, the proposed optimal input identification approach can also be found beneficial for other remote sensing applications.

Publ.-Id: 28788

The IR-truncated PT-symmetric V = ix3 model and its asymptotic spectral scaling graph

Günther, U.; Stefani, F.

The PT-symmetric V = ix3 model over the real line is infra-red (IR) truncated and considered as Sturm-Liouville problem over a finite interval. Structures hidden in the Airy function setup of the V = ix3 model are combined with WKB techniques developed by Bender and Jones in 2012 for the derivation of the real part of the spectrum of theV = ix3 model. Via WKB and Stokes graph analysis, the location of the complex spectral branches of the ix3 model as well as those of more general V = -(ix)2n+1 models over finite intervals are obtained. Splitting the related action functions into purely real scale factors and scale invariant integrals allows to extract underlying asymptotic spectral scaling graphs. These (structurally very simple) scaling graphs are geometrically invariant and cutoff-independent so that the IR limit can be formally taken. Moreover an increasing length scale can be associated with a spectral UV-IR renormalization group flow on this scaling graph. It is shown that the eigenvalues of the IR-complete V = ix3 model can be bijectively mapped onto a finite segment of the scaling graph asymptotically approaching a (scale invariant) PT phase transition region. In this way, a simple heuristic picture and complementary explanation for the unboundedness of projector norms and C-operator for the ix3 model are provided and the lack of quasi-Hermiticity of the ix3 Hamiltonian over the real line appears physically plausible. Possible directions of further research are briefly sketched.

Keywords: PT Quantum Mechanics; PT phase transition; spectral branch points; exceptional points; ix3 model; WKB techniques; IR truncation; IR completion; asymptotic spectral scaling graphs; spectral UV-IR renormalization group flow

  • Invited lecture (Conferences)
    Discrete-18, organized by CERN and the Austrian Academy of Sciences, 26.-30.11.2018, Wien, Österreich

Publ.-Id: 28787

The IR-truncated PT-symmetric V = ix3 model and its asymptotic spectral scaling graph

Günther, U.; Stefani, F.

The PT-symmetric quantum mechanical V = ix3 model over the real line is infra-red (IR) truncated and considered as Sturm-Liouville problem over a finite interval of the real line. Via WKB and Stokes graph analysis, the location of the complex spectral branches of the V = ix3 model as well as those of more general V = -(ix)2n+1 models over finite intervals are obtained. Underlying asymptotic spectral scaling graphs are extracted which are scale-invariant so that the IR completion can be performed. Implications for the V = ix3 model over the full real line are discussed.

Keywords: PT Quantum Mechanics; PT phase transition; spectral branch points; exceptional points; ix3 model; WKB techniques; IR truncation; asymptotic spectral scaling graphs

  • Invited lecture (Conferences)
    Analytic and algebraic methods in physics XV, 10.-13.09.2018, Prague, Czech Republic

Publ.-Id: 28786

The IR-truncated PT-symmetric V=ix3 model and its asymptotic spectral scaling graph

Günther, U.; Stefani, F.

The PT-symmetric V=ix3 model over the real line is IR truncated and considered as Sturm-Liouville problem over a finite interval. Combining structures hidden in the Airy function setup of the V=ix model with WKB techniques developed by Bender and Jones in 2012 for the derivation of the real part of the spectrum of the ix3 model, a WKB and Stokes graph analysis for the complex spectral branches of the ix3 model as well as those of more general V=-(ix)2n+1 models over finite intervals is performed. Complementary insights into the spectra of these models are obtained by splitting the spectral branch-structure into purely real scale factors and asymptotic spectral scaling graphs. It turns out that the corresponding (structurally very simple) scaling graphs are geometrically invariant and cutoff-independent so that the infra-red (IR) limit can be formally taken. These graphs have invariantly existing PT phase transition regions. In this way, a simple heuristic picture and complementary explanation for the unboundedness of the C-operator and the lack of quasi-Hermiticity of the ix3 Hamiltonian over the real line is provided.

Keywords: PT-symmetric Quantum Mechanics; PT phase transition; spectral branch points; exceptional points; ix3 model; WKB techniques; IR truncation; C-operator; unboundedness; quasi-Hermiticity

  • Invited lecture (Conferences)
    Pseudo-Hermitian Hamiltonians in Quantum Physics (PHHQP) XVIII, 04.-13.06.2018, Bangalore, India

Publ.-Id: 28785

Curing processes in ultra low-k materials by positron annihilation spectroscopy

Liedke, M. O.; Köhler, N.; Butterling, M.; Hirschmann, E.; Attallah, A. G.; Krause-Rehberg, R.; Schulz, S. E.; Wagner, A.

The first results on in-situ investigations of pore formation in ultra low-k dielectrics during a curing process, i.e., a porogen removal by vacuum annealing will be presented. The main focus is to obtain insight into initial stages of pore networks formation up to their full development. The in-situ annealing and Doppler broadening positron annihilation spectroscopy measurements have been conducted on our Apparatus for In-situ Defect Analysis (AIDA) - the end-station of a slow positrons beamline at HZDR. In addition, positron lifetime spectroscopy has been utilized, where mono-energetic pulsed positron beam (MePS) serves as a probe to evaluate pore sizes, their concentration and distribution as a function of curing temperature and time. The MePS facility has partly been funded by the Federal Ministry of Education and Research (BMBF) with the grant PosiAnalyse (05K2013). The AIDA system was funded by the Impulse- und Networking fund of the Helmholtz-Association (FKZ VH-VI-442 Memriox) and through the Helmholtz Energy Materials Characterization Platform (03ET7015)

Keywords: positron; low-k; curing; AIDA; defects; pores

  • Lecture (Conference)
    DPG Frühjahrstagung Berlin, 11.03.2018, Berlin, Deutschland

Publ.-Id: 28784

Vacancy-mediated magnetic phase-transitions

Liedke, M. O.; Butterling, M.; Quintana, A.; Menéndez, E.; Ehrler, J.; Bali, R.; Hirschmann, E.; Sireus, V.; Nogués, J.; Sort, J.; Wagner, A.

Two thin film systems exhibiting vacancy mediated magnetic phase transitions will be discussed in detail, i.e., Co3O4 and Fe60Al40.
In applications, substituting electric currents, which are nowadays used to operate spintronic devices, with electric fields, would result in a reduction of both the energy consumption and cost [1]. Co3O4 is a candidate for a tunable, non-volatile energy-efficient functional material whose magnetic properties can be controlled by electric voltage. In our current investigations the as-grown Co3O4 films consist of a paramagnetic (PM) phase only, which is transformed to a ferromagnetic (FM) state by electrolyte-gated and defect-mediated O and Co transport. A negative voltage reduces Co3O4 to Co (FM: ON), resulting in a phase separated material with Co- and O-rich regions. Applying a positive bias, the process is reversed oxidizing Co back to Co3O4 (PM: OFF). We will show that atoms migration is driven by rather complex vacancy states and a clear increase of the grain boundaries volume after negative biasing assists to O transport. Moreover, concomitantly with the PM phase transition due to the positive biasing the structural defects picture reverses to a large extent as well, which manifests as reduction in volume of both vacancy clusters and grain boundaries.
B2-Fe60Al40 phase is paramagnetic, and strong ferromagnetism can be induced via disordering to the A2-Fe60Al40 phase [2]. Disordering implies the formation of anti-site defects [3], which correlates with an increased Fe coordination. The concentration and size of open volume defects can play an important role in reordering kinetics. Three different initial order states have been investigated: (i) as-grown, partially disordered Fe60Al40, (ii) the as-grown films after Ne+ irradiation, and (iii) Ne-irradiated B2-Fe60Al40. Since, reordering directly affects magnetization saturation; the extent of the diffusion process can be traced via magnetometry at slightly elevated temperature of 400 K. We show that immobile large vacancy clusters with a high thermal activation barrier are dominant in the as-grown film and hinder ordering. Ion irradiation breaks down these pinning sites, thereby strongly accelerating thermal diffusion and reordering. These results provide insights into thermal reordering processes in binary alloys, and the consequent effect on magnetic properties. Doppler broadening and positron annihilation lifetime spectroscopy have been used as a probe for both electric field driven ionic transport of Co and O via different type of defects in Co3O4 systems as well as vacancy-mediated ordering in Fe60Al40.
[1] Y. Shiota, et al. Nature Mater. 11, 39 (2012). [2] M.O. Liedke, et al., J. Appl. Phys. 117, 163908 (2015).
[3] R. Bali, et al., Nano Lett. 14, 435 (2014).

Keywords: magnetism; positron; ion; irradiation; phase transitions; defects; electrical fields

  • Invited lecture (Conferences)
    18th International Conference on Positron Annihilation (ICPA-18), 19.08.2018, Orlando, USA

Publ.-Id: 28783

Magnetic ordering and open volume defects – phase transitions in ion irradiated Fe60Al40 thin films

Liedke, M. O.; Ehrler, J.; Bali, R.; Butterling, M.; Hirschmann, E.; Wagner, A.

Fe60Al40 exhibit the so-called disorder induced ferromagnetism, where anti-site disorder (ASD) promotes ferromagnetic A2-phase (disordered) over paramagnetic B2-phase (ordered). Both phases can be - in a controllable fashion - driven by ion irradiation or annealing, respectively. The main physical origin correlates strongly with ASD [R. Bali, et al., Nano Lett. 14, 435 (2014)]. Nevertheless, the concentration and size of open volume defects can be of crucial importance in determining the kinetics of the reordering processes. To unravel the influence of vacancy clusters, three different initial order states have been investigated: (i) as-sputtered, (ii) as-grown irradiated with Ne+ and (iii) B2 ordered films, obtained via 773 K annealing and Ne-irradiated. Open volume defects in the treated samples were investigated with Doppler broadening and positron annihilation lifetime spectroscopy. Furthermore, since the reordering directly affects Ms, the extent of the diffusion process can be traced via magnetometry at slightly elevated temperature of 400 K. We show that immobile large vacancy clusters are dominant in the as-grown films; these complexes present only in the as-sputtered film possess a high thermal activation barrier and hinder ordering. Ion irradiation breaks down these pinning defects strongly accelerating thermal diffusion and reordering. These results provide insights into thermal reordering processes in binary alloys, and the consequent effect on magnetic behavior.

Keywords: magnetism; positron; ordering; ion; irradiation; phase transition; defects

  • Lecture (Conference)
    9th Joint European Magnetic Symposia (JEMS-9), 03.09.2018, Mainz, Deutschland

Publ.-Id: 28782

On the use of stacks of fission-like targets for neutron capture experiments

Guerrero, C.; Lerendegui-Marco, J.; Eberhardt, K.; Düllmann, C. E.; Junghans, A.; Lommel, B.; Mokry, C.; Quesada, J. M.; Runke, J.; Thörle-Pospiech, P.; The N_Tof Collaboration

The measurement of neutron induced reactions on unstable isotopes is of interest in many fields, from nuclear energy to astrophysics or applications; in particular transuranic isotopes are essential for the development of innovative nuclear reactors and for the management of the radioactive waste. In such measurements, the quality of the associated radioactive target is crucial for the success of the experiment, but in many cases the geometry, amount of mass and encapsulation of the target are not optimal, leading to limited results. In this work we propose to produce high quality radioactive targets for capture as a stack of thin targets using the techniques usually employed for fission measurements. In particular, we have succeeded in making a 242Pu target of nearly 100 mg by combining seven thin (~1 mg/cm2) fission-like targets with 45 mm in diameter achieving a total backing thickness of only 70 m of aluminum. The target has been shown to perform successfully in experiments at both a neutron time-of-flight facility (n_TOF at CERN) and a thermal neutron beam (BRR at KFKI), providing the most accurate data from thermal up to 250 keV to date.

Publ.-Id: 28781

Dual-Energy CT for more accurate stopping power prediction & Importance of range verification

Richter, C.

Direct dual-energy CT based stopping power prediction (DirectSPR) allows the reduction of the currently clinical used range uncertainty. The clinical benefits of DirectSPR, its extensive validation and implementation will be presented together with a quantification of the range reduction potential. Furthermore, steps towards in-man validation of the SPR prediction with the IBA Prompt-Gamma-Imaging slit camera will be highlighted.

  • Invited lecture (Conferences)
    IBA Proteus User Meeting 2019, 01.-03.02.2019, Miami, USA

Publ.-Id: 28780

Strahlenschutzaspekte bei der Errichtung und Inbetriebnahme des kanadischen Zyklotrons TR-FLEX im Helmholtz-Zentrum Dresden-Rossendorf

Preusche, S.; Naumann, B.; Kaspari, W.

  • Invited lecture (Conferences)
    Strahlenschutz in Medizin, Forschung und Industrie, 11.-12.12.2018, Aschaffenburg, Deutschland

Publ.-Id: 28779

Entwicklung von hochaffinen 18F-markierten Radiotracern zur molekularen Bildgebung des Adenosin-A2A-Rezeptors mittels Positronen-Emissions-Tomographie

Lai, T. H.; Schröder, S.; Moldovan, R.-P.; Ludwig, F.-A.; Fischer, S.; Dukic-Stefanovic, S.; Deuther Conrad, W.; Steinbach, J.; Brust, P.

Ziel: Die Bildgebung des Adenosin-A2A-Rezeptors mittels PET gilt hinsichtlich der Frühdiagnostik neurodegenerativer und neuroonkologischer Erkrankungen als vielversprechend. Für die Entwicklung neuer 18F-Tracer basierend auf einer Pyrazolo[2,3-d]pyrimidin-Leitverbindung [1] wurden 21 fluorierte Derivate synthetisiert. Aufgrund der Affinitätswerte wurden die 2- bzw. 4-Fluorbenzylderivate 1 (Ki(hA2A) = 5,3 nM; Ki(hA1) = 220 nM) und 2 (Ki(hA2A) = 2,1 nM; Ki(hA1) = 147 nM) als potentielle A2A-Radiotracer ausgewählt.

Methodik: Für die Radiosynthese von [18F]1 wurden drei verschiedene Markierungsstrategien entwickelt. Strategien A und B erfolgten über 2 bzw. 4 Stufen unter Verwendung von [18F]Fluorbenzaldehyd. Dieser wurde für die reduktive Aminierung (A) bzw. nach Reduktion und Bromierung für die finale Benzylierung (B) verwendet. Strategie C basiert auf einer einstufigen 18F Markierung eines Boronsäurepinacol-Präkursors mit [18F]TBAF in Gegenwart von Cu(OTf)2(py)4 in DMA/n-BuOH und wurde sowohl für die Radiosynthese von [18F]1 als auch [18F]2 angewendet. Die In vitro-Evaluierung erfolgte mittels Autoradiographie in Hirnschnitten von der Maus.

Ergebnisse: Die mehrstufigen Markierungsverfahren A und B ergaben radiochemische Ausbeuten von lediglich 1,4 bzw. 10% [18F]1 (nicht isoliert). Mittels der einstufigen Strategie C konnten [18F]1 und [18F]2 mit einer RCA von 51+6 bzw. 8+1% (EOB), einer AM von 135+64 bzw. 132 GBq/μmol (EOS) und einer RCR >98% erhalten werden. In-vitro-Autoradiographiestudien zeigten nur für [18F]2 eine spezifische Bindung im Striatum, die mit A2A- Liganden blockierbar war.

Schlussfolgerung: Es wurde eine effiziente einstufige 18F-Markierung von zwei neuen hochaffinen A2A-Radiotracern etabliert. In ersten In-vitro-Studien erwies sich [18F]2 für die bildgebende Darstellung des Adenosin-A2A-Rezeptores als geeignet. Gegenstand aktueller Arbeiten sind weitere In-vitro- und In-vivo-Untersuchungen.

Literatur: [1] Gillespie et al., Bioorg Med Chem Lett 2008, 18, 2924-2929.

Keywords: Adenison; A2A; Radiotracer; 18F; PET

  • Lecture (Conference)
    Nuklearmedizin 2019 - 57. Jahrestagung der Deutschen Gesellschaft für Nuklearmedizin, 03.-06.04.2019, Bremen, Deutschland
    DOI: 10.1055/s-0039-1683493

Publ.-Id: 28778

Detection of Extraterrestrial ⁶⁰Fe in Antarctica with AMS

Koll, D.; Faestermann, T.; Korschinek, G.; Merchel, S.; Welch, J. M.; Kipfstuhl, S.

The long-lived radioactive isotope ⁶⁰Fe with a half-life of 2.6 Myr is mainly produced by stellar nucleosynthesis and ejected into space by core-collapse supernovae. Former investigations by Accelerator Mass Spectrometry (AMS) showed a supernova signal on Earth 1.7-3.2 Myr ago.

Considering an enrichment of the solar neighborhood in long-lived radionuclides by previous supernovae, deposition of ⁶⁰Fe on Earth could be currently ongoing. To investigate this case, 500 kg of Antarctic snow were analyzed by Accelerator Mass Spectrometry with the 14 MV tandem accelerator and the Gas-filled Analyzing Magnet System (GAMS) at the Maier-Leibnitz-Laboratorium in Garching, Germany.

Indeed, ⁶⁰Fe was discovered in Antarctic snow and by the measurement of ⁵³Mn, which is dominantly produced by cosmic ray interactions with solar system objects, the origin of these ⁶⁰Fe atoms could be deduced.

Keywords: AMS; supernovae

  • Lecture (Conference)
    83. Jahrestagung der DPG und DPG-Frühjahrstagung der Sektion Atome, Moleküle, Quantenoptik und Plasmen (SAMOP) in Zusammenarbeit mit der Deutschen Gesellschaft für Massenspektrometrie DGMS 2019, 10.-15.03.2019, Rostock, Deutschland

Publ.-Id: 28777

Universal Limits of Thermopower and Figure of Merit from Transport Energy Statistics

Zahn, P.

The search for new thermoelectric materials aims at improving their power and efficiency, as expressed by thermopower S and figure of merit ZT.
By considering a very general transport spectral function w(ε), expressions for the S and ZT can be derived, which contain the statistical weights of an effective distribution function only, see Ref. [1]. The assumption of a Lorentzian shape with width kBT resulting from the electron-phonon coupling allows to estimate an upper limit of S and ZT regardless the microscopic mechanisms of the transport process.
The limit of |S| is given by 1.88 in units of kB/e, which is about 160 μV/K, and the limit for ZT is about 1.11 (red dots in the figure).
The work was partially supported by the Initiative and Networking Fund of the German Helmholtz Association, International Helmholtz Research School NanoNet (VH-KO-606), the Helmholtz Exzellenznetzwerk cfaed (ExNet-0026), and the DFG Priority Program 'Nanostructured Thermoelectrics' (ZA264/3-2). We thank S. Gemming for helpful discussions.

Keywords: Thermoelectrics; Thermopower; Figure of Merit; universal limit; transport spectral function

  • Poster
    667. WE-Heraeus-Seminar 'System-oriented approach to thermoelectrics: Materials – Interfaces – Devices', 08.-11.04.2018, Bad Honnef, Deutschland
  • Contribution to WWW

Publ.-Id: 28776

Burning in the Tail: Implications for a Burst Oscillation Model

Chambers, F.; Watts, A.; Keek, L.; Cavecchi, Y.; Garcia Gonzalez, F.

Accreting neutron stars (NSs) can exhibit high-frequency modulations, known as burst oscillations, in their light curves during thermonuclear X-ray bursts. Their frequencies can be offset from the spin frequency of the NS (known independently) by several Hz, and can drift by 1-3 Hz. One plausible explanation for this phenomenon is that a wave is present in the bursting ocean that decreases in frequency (in the rotating frame) as the burst cools. The strongest candidate is the buoyant r-mode; however, models for the burning ocean background used in previous studies over-predict frequency drifts by several Hz. Using new background models (which include shallow heating, and burning in the tail of the burst) the evolution of the buoyant r-mode is calculated. The resulting frequency drifts are smaller, in line with observations. This illustrates the importance of accounting for the detailed nuclear physics in these bursts.

Publ.-Id: 28775

Experiments and Simulations of the Magnetized Spherical Couette Problem

Garcia Gonzalez, F.; Stefani, F.

The magnetized spherical Couette system models experiments that are being carried out at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). A liquid metal is confined within two differentially rotating spheres and immersed in a magnetic field parallel to the axis of rotation. Preliminary simulations of periodic and quasiperiodic flows, arising at the first bifurcations, will be presented. The aim is to study in detail the instabilities observed in the experiments and previous numerical studies. This study will reveal how the flow patterns depend on control parameters, reproducing thus different physical situations of the HZDR experiments.

  • Poster
    Fifty years after Roberts' MHD: Dynamos and planetary flows today, 16.-17.11.2017, London, United Kingdom

Publ.-Id: 28774

Experiments and Simulations of the Magnetized Spherical Couette Problem

Garcia Gonzalez, F.; Stefani, F.

The magnetized spherical Couette system models experiments that are being carried out at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). A liquid metal is confined within two differentially rotating spheres and immersed in a magnetic field parallel to the axis of rotation. Preliminary simulations of periodic and quasiperiodic flows, arising at the first bifurcations, will be presented. The aim is to study in detail the instabilities observed in the experiments and previous numerical studies. This study will reveal how the flow patterns depend on control parameters, reproducing thus different physical situations of the HZDR experiments.

  • Poster
    GDRI Dynamo meeting 2017, 27.-29.11.2017, Paris, France

Publ.-Id: 28773

New axisymmetric helical magnetorotational instability in dissipative rotating flows with positive shear

Mamatsashvili, G.; Stefani, F.; Hollerbach, R.; Rüdiger, G.

We present a new type of axisymmetric magnetorotational instability which is capable of destabilizing viscous and resistive magnetized flows with radially increasing angular velocity. Using short-wavelength WKB approach, supported by 1D linear stability calculations in Taylor-Couette flow, we show that this instability works only when a combination of axial and azimuthal magnetic fields is applied and when the magnetic Prandtl number is different from one. It might have grave consequences for the stability of the equator-near parts of the solar tachocline, and for the dynamo action in this region

  • Lecture (Conference)
    MHD Days and GdRI Dynamo Meeting, 26.-28.11.2018, Dresden, Germany

Publ.-Id: 28772

Experiments and Simulations of the Magnetized Spherical Couette Problem

Garcia Gonzalez, F.; Stefani, F.

The magnetized spherical Couette system models experiments that are being carried out at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). A liquid metal is confined within two differentially rotating spheres and immersed in a magnetic field parallel to the axis of rotation. Preliminary simulations of periodic and quasiperiodic flows, arising at the first bifurcations, will be presented. The aim is to study in detail the instabilities observed in the experiments and previous numerical studies. This study will reveal how the flow patterns depend on control parameters, reproducing thus different physical situations of the HZDR experiments.

  • Poster
    Alexander von Humboldt Foundation Network Meeting 2018, 19.-21.02.2018, Potsdam, Deutschland

Publ.-Id: 28771

A Tayler-Spruit type model of a tidally synchronized solar dynamo

Stefani, F.; Giesecke, A.; Weber, N.; Weier, T.

We consider a solar dynamo model of Tayler-Spruit type whose Omega-effect is conventionally produced by a solar-like differential rotation but whose alpha-effect is assumed to be periodically modulated by planetary tidal forcing. This resonance-like effect relies on the tendency of the current-driven Tayler instability to produce intrinsic helicity oscillations which can be synchronized by periodic tidal perturbations. Specifically, we focus on the 11.07 years alignment periodicity of the tidally dominant planets Venus, Earth, and Jupiter, whose empirical synchronization with the solar dynamo is illustrated. The typically emerging dynamo modes are dipolar fields, oscillating with a 22.14 years period or pulsating with a 11.07 years period, but also quadrupolar fields with corresponding periodicities. In the absence of any constant part of alpha, we prove the subcritical nature of this periodic Tayler-Spruit type dynamo. Phase coherent transitions between dipoles and quadrupoles, which are reminiscent of the observed behaviour during the Maunder minimum, can be easily triggered by long-term variations of dynamo parameters. Further interesting features of the model are the typical second intensity peak and the intermittent appearance of reversed helicities in both hemispheres

  • Poster
    MHD Days and GdRI Dynamo Meeting, 26.-28.11.2018, Dresden, Germany

Publ.-Id: 28770

Rotating Waves in Spherical Geometry: Thermal Convection in Thin Rotating Shells and the Magnetized Spherical Couette System

Garcia Gonzalez, F.; Sánchez, J.; Net, M.; Chambers, F.; Watts, A.; Stefani, F.

Fluid dynamics plays an important role in many geophysical and astrophysical objects such as planets and stars. For instance, convection can occur in neutron stars' oceans formed by very thin layers of helium or hydrogen, which are subject to the influence of strong temperature gradients and rotation. In addition, instabilities observed in differentially rotating flows in the presence of a magnetic field (magnetized spherical Couette flows) were attributed to the magnetorotational instability (MRI), which is presently considered the most promising candidate to explain the transport mechanism of angular momentum in accretion disks around black holes and protostars. In this study, bifurcation diagrams of the first instabilities occurring in the two mentioned set-ups will be presented. They were obtained by means of continuation techniques. The arising flow patterns will be described. In both cases, pseudo-spectral high order methods as well as high order time integration methods are used for the time evolution of the Navier-Stokes equations.

  • Lecture (Conference)
    International Conference on Spectral and High-Order Methods ICOSAHOM'18., 09.-13.07.2018, London, United Kingdom

Publ.-Id: 28769

The DRESDYN project: Liquid metal experiments on dynamo action and magnetorotational instability

Stefani, F.

The dynamo effect in moving electrically conducting fluids is at the root of magnetic field generation in planets and stars. Yet, cosmic magnetic fields play also an active role in the formation of central objects, such as protostars and black holes, by destabilizing accretion disks that would be hydrodynamically stable. While often studied separately, dynamo action and magnetically triggered instabilities may also occur together in such highly non-linear processes as the MRI dynamo or the Tayler-Spruit dynamo.

The DRESDYN project at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) serves as a platform for continuing the liquid metal experiments of the last two decades which were related to dynamo action and magnetically triggered flow instabilities. After a short survey of the dynamo experiments in Riga, Karlsruhe and Cadarache, and the various MRI experiments at the PROMISE facility at HZDR, I discuss the preparatory status of a large-scale precession experiment and a Taylor-Couette experiment for investigating various forms of the MRI and their combinations with the Tayler instability. Special focus will be laid on the numerical predictions of both experiments, as well as on some recent findings concerning the relation of non-modal growth in rotating flows with dissipation-induced instabilities, such as helical and azimuthal MRI for negative and positive shear.

  • Invited lecture (Conferences)
    Waves, Turbulence, and Large-scale Structures in Rotating Magnetic Fluids: Above & Beyond Geophysical Fluid Dynamics, 10.-14.09.2018, Boulder, USA

Publ.-Id: 28768

A tidally synchronized Tayler-Spruid type model of the solar dynamo

Stefani, F.; Giesecke, A.; Weber, N.; Weier, T.

We present a solar dynamo model of the Tayler-Spruit type whose Omega-effect is produced, as usual, by differential rotation but whose alpha-effect is assumed as being periodically modulated by planetary tidal forcing. This resonance-like effect has its rationale in the tendency of the current-driven Tayler instability to undergo intrinsic helicity oscillations which can be synchronized by periodic tidal perturbations. Specifically, we focus on the 11.07 years periodicity of the alignment of the tidally dominant planets Venus, Earth, and Jupiter. In the framework of a simple one-dimensional numerical model we prove the subcritical character of this Tayler-Spruit type dynamo. The typical dynamo modes are dipole fields, oscillating with a 22.14 year period, but also quadrupole fields pulsating with an 11.07 years period. Transitions between these field topologies are reminiscent of the observed behavior during the Maunder minimum. Further interesting features of the model are the emergence of mid-term fluctuations, and the intermittent appearance of reversed helicities in both hemispheres. With minor model modifications, the correct direction of the butterfly diagram comes out as a robust feature, too.

  • Lecture (Conference)
    Third Russian Conference on Magnetohydrodynamics, 18.-21.06.2018, Perm, Russia

Publ.-Id: 28767

Experiments and Simulations on the Magnetized Spherical Couette Problem

Garcia Gonzalez, F.; Stefani, F.

Experiments on the magnetized spherical Couette system are presently being carried out at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). A liquid metal (GaInSn) is confined within two differentially rotating spheres and exposed to a magnetic field parallel to the axis of rotation. Bifurcation diagrams for rotating waves, obtained with continuation methods when only the magnetic field is increased, are presented. This allows us to carefully investigate the time-scales of the nonlinear saturation of the radial jet, return flow, and shear layer instabilities, as found in previous studies. In addition, modulated rotating waves, obtained at secondary bifurcations, are exhaustively studied by means of direct numerical simulations, with main focus on their spatio-temporal symmetries. We find that at moderate differential rotation the modulated rotating waves give rise to several types of chaotic flows, but only for the radial jet instability. With this study we reveal how the flow patterns and time-scales depend on the magnetic field, reproducing thus different physical situations of the HZDR experiments.

  • Lecture (Conference)
    MHD Days and GdRI Dynamo Meeting, 2018, 26.-28.11.2018, Dresden, Deutschland

Publ.-Id: 28766

A Tayler-Spruit model of the solar dynamo with tidal synchronization

Stefani, F.; Giesecke, A.; Weber, N.; Weier, T.

We present a solar dynamo model of the Tayler-Spruit type whose Omega-effect is coventionally produced by differential rotation but whose alpha-effect is assumed as being periodically modulated by planetary tidal forcing. This resonance-like effect has its rationale in the tendency of the current-driven Tayler instability to undergo intrinsic helicity oscillations which can be synchronized by periodic tidal perturbations. Specifically, we focus on the 11.07 years periodicity of the alignment of the tidally dominant planets Venus, Earth, and Jupiter. In the framework of a simple one-dimensional numerical model we prove the subcritical character of this Tayler-Spruit type dynamo. The typical dynamo modes are dipole fields, oscillating with a 22.14 year period, but also quadrupole fields pulsating with an 11.07 years period. Transitions between these field topologies are reminiscent of the observed behavior during the Maunder minimum. Further interesting features of the model are the emergence of mid-term fluctuations, and the intermittent appearance of reversed helicities in both hemispheres. With minor model modifications, the correct direction of the butterfly diagram comes out as a robust feature, too.

  • Invited lecture (Conferences)
    Planetary-Stellar Connection: The Sun's Lesson, 07.-09.05.2018, Freiburg im Breisgau, Germany

Publ.-Id: 28765

Thermal convection in rotating spherical shells: Temperature-dependent internal heat generation using the example of triple-𝛼 burning in neutron stars

Garcia Gonzalez, F.; Chambers, F.; Watts, A.

We present an extensive study of Boussinesq thermal convection including a temperature-dependent internal heating source, based on numerical three-dimensional simulations. The temperature dependence mimics triple-α nuclear reactions and the fluid geometry is a rotating spherical shell. These are key ingredients for the study of convective accreting neutron star oceans. A dimensionless parameter Raₙ, measuring the relevance of nuclear heating, is defined. We explore how flow characteristics change with increasing Raₙ and give an astrophysical motivation. The onset of convection is investigated with respect to this parameter and periodic, quasiperiodic, chaotic flows with coherent structures, and fully turbulent flows are exhibited as Raₙ is varied. Several regime transitions are identified and compared with previous results on differentially heated convection. Finally, we explore (tentatively) the potential applicability of our results to the evolution of thermonuclear bursts in accreting neutron star oceans.


Publ.-Id: 28764

Self-excitation in a helical liquid metal flow: the Riga dynamo experiments

Gailitis, A.; Gerbeth, G.; Gundrum, T.; Lielausis, O.; Lipsbergs, G.; Platacis, E.; Stefani, F.

The homogeneous dynamo effect is at the root of magnetic field generation in cosmic bodies, including planets, stars and galaxies. While the underlying theory had increasingly flourished since the middle of the 20th century, hydromagnetic dynamos were not realized in the laboratory until 1999. On 11 November 1999, this situation changed with the first observation of a kinematic dynamo in the Riga experiment. Since that time, a series of experimental campaigns has provided a wealth of data on the kinematic and the saturated regime. This paper is intended to give a comprehensive survey about these experiments, to summarize their main results and to compare them with numerical simulations.


Publ.-Id: 28763

Continuation and stability of rotating waves in the magnetized spherical Couette system: secondary transitions and multistability

Garcia Gonzalez, F.; Stefani, F.

Rotating waves (RW) bifurcating from the axisymmetric basic magnetized spherical Couette (MSC) flow are computed by means of Newton–Krylov continuation techniques for periodic orbits. In addition, their stability is analysed in the framework of Floquet theory. The inner sphere rotates while the outer is kept at rest and the fluid is subjected to an axial magnetic field. For a moderate Reynolds number Re = 10^3 (measuring inner rotation), the effect of increasing the magnetic field strength (measured by the Hartmann number Ha) is addressed in the range Ha ∈ (0, 80) corresponding to the working conditions of the HEDGEHOG experiment at Helmholtz-Zentrum Dresden-Rossendorf. The study reveals several regions of multistability of waves with azimuthal wavenumber m = 2, 3, 4, and several transitions to quasi-periodic flows, i.e modulated rotating waves. These nonlinear flows can be classified as the three different instabilities of the radial jet, the return flow and the shear layer, as found in the previous studies. These two flows are continuously linked, and part of the same branch, as the magnetic forcing is increased. Midway between the two instabilities, at a certain critical Ha, the non-axisymmetric component of the flow is maximum.


Publ.-Id: 28762

The DRESDYN project: liquid metal experiments on dynamo action and magnetorotational instability

Stefani, F.; Gailitis, A.; Gerbeth, G.; Giesecke, A.; Gundrum, T.; Rüdiger, G.; Seilmayer, M.; Vogt, T.

Magnetic fields of planets, stars and galaxies are generated by self-excitation in moving electrically conducting fluids. Once produced, magnetic fields can play an active role in cosmic structure formation by destabilising rotational flows that would be otherwise hydro-dynamically stable. For a long time, both hydromagnetic dynamo action as well as magnetically triggered flow instabilities had been the subject of purely theoretical research. Meanwhile, however, the dynamo effect has been observed in large-scale liquid sodium experiments in Riga, Karlsruhe and Cadarache. In this paper, we summarise the results of liquid metal experiments devoted to the dynamo effect and various magnetic instabilities such as the helical and the azimuthal magnetorotational instability and the Tayler instability. We discuss in detail our plans for a precession-driven dynamo
experiment and a large-scale Tayler–Couette experiment using liquid sodium, and on the prospects to observe magnetically triggered instabilities of flows with positive shear.


  • Secondary publication expected

Publ.-Id: 28761

Numerical simulation of multicomponent flows with the presence of density gradients for the upgrading of advanced turbulence models

Huang, M.; Höhne, T.

The turbulence effects during the buoyancy-driven mixing was investigated at a vertical mixing (VeMix) test facility, which was developed to investigate the mixing of high borated and low borated coolant in nuclear reactor. Additional buoyancy terms are included in buoyancy-modified turbulence models, which have been implemented in the CFD code ANSYS CFX and validated with experimental data captured by optical methods and conductivity measurement technology. The physicality of the flow phenomena and the vortical oscillations analyzed by Fourier tranformation in both the experiments and simulations show good agreement under different flow conditions. The influence of different buoyancy models were investigated in detail and optimal models for simulations at similar flow conditions have been selected.

Keywords: Multicomponent flow; CFD; turbulence models; SBES; RANS; LES


Publ.-Id: 28760

Simulation der Strömungsverhältnisse in einem DWR (Vor-Konvoi)

Höhne, T.; Kliem, S.

Ziel der Untersuchung war der Nachweis möglicher Wirbel zwischen der 8.-9. Abstandshalterebene eines Vorkonvoi-Kerns, die zu Temperatur-Wechselbelastungen führen könnten. Es wurde eine 3D-CFD Modell genutzt, die instationäre Rechnung wurde mit einem geeignetem hybriden RANS-LES SBES Turbulenzmodell durchgeführt. Die CFD-Rechnung war off-line gekoppelt mit einem Neutronenkinetikprogramm (DYN3D). Es erfolgte eine Nutzung bekannter Druckverluste über der Kernhöhe zur Festlegung von Permeabilität und Widerstandskoeffizienten. Hierbei erfolgte eine Absicherung durch Vergleichsrechnungen mit einem Modell aus der Literatur. Die neutronenkinetische Rechnung wurde unter Normalbetriebsbedingungen mit einer prototypischen Leistungsverteilung im Kern bei EOC durchgeführt. Die Ergebnisse der Berechnung zeigen keine großflächigen dominanten Wirbel im oberen Kernbereich. Der Kern wirkt als Gleichrichter und dämpft großräumige Wirbel. Die Analysen umfassten mehrere Höhenebenen im Kern und zeigen, dass in einigen Bereichen des Kernquerschnitts eine nach oben zunehmend gerichtete laterale Strömung zu den Austrittsstutzen auftritt. In anderen Bereichen des Kernquerschnitts tritt dagegen kaum Querströmung auf. Die Geschwindigkeiten sind Leerrohrgeschwindigkeiten. Um diese in Strömungsgeschwindigkeiten umzurechnen, müssen diese durch die Porosität (ca. 0.4 in vertikaler und ca. 0.25 in horizontaler Richtung) geteilt werden. Die berechnete Leerrohrgeschwindigkeit von 2 m/s entspricht einer Strömungsgeschwindigkeit im Kern von ca. 5 m/s. Analog ist die Transversalgeschwindigkeit von 0.05 m/s (Leerrohr) einer Strömungsgeschwindigkeit von 0.2 m/s gleichzusetzen.
Folgende Einschränkungen des Modells gelten jedoch: Im Modell sind alle Brennelemente gleich behandelt und Querströmungen auf Grund unterschiedlicher axialer Druckverluste bei verschiedenen BE-Typen können nicht dargestellt werden. Die komplexe Struktur der BEs (bspw. Strömungsfahnen in AHs) könnte einen Einfluss auch auf die Bildung großräumiger Wirbel haben. Dieser Effekt könnte nur mit einer sehr hohen Anzahl Gitterelemente aufgelöst werden (mehrere Milliarden). Dies ist momentan rechentechnisch noch nicht möglich. Auch der mögliche Einfluss von Zweiphasenströmungen wurde nicht betrachtet.

Keywords: Vor-Konvoi; CFX; DYN3D; RPV

  • Open Access Logo Wissenschaftlich-Technische Berichte / Helmholtz-Zentrum Dresden-Rossendorf; HZDR-099 2019
    ISSN: 2191-8708, eISSN: 2191-8716
  • Lecture (Conference)
    CFD-Verbund, 12.-13.03.2019, Garching, Deutschland

Publ.-Id: 28758

Example of computations for boron dilution (system code, CFD and neutronic assessment)

Höhne, T.; Kliem, S.

Analysis of core responses during boron dilution scenarios requires use of different codes
Realistic boundary conditions especially for the coolant mixing can mitigate the consequences of higher slug volumes
Coupled neutron kinetic/thermal hydraulic core calculations needed for assessment of consequences
According to the results no violation of safety criteria observed

Keywords: DYN3D; ROCOM; Konvoi; PWR

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Publ.-Id: 28757

Numerical simulation of density driven flows (PTS)

Höhne, T.

Buoyancy driven mixing was investigated under simulated natural circulation conditions at the test facility ROCOM.
A transition matrix from momentum to buoyancy-driven flow experiments was selected for validation of the CFD software ANSYS CFX.
Buoyancy generated source and dissipation terms were proposed and introduced into the balance equations for the turbulent kinetic energy .
The results of the experiments and of the numerical calculations show that mixing strongly depends on buoyancy effects: At higher Froude numbers (Fr>1.5) the injected slug propagates in the circumferential direction around the core barrel. With lower Froude numbers (Fr<0.85) buoyancy effects reduce this circumferential propagation at lower mass flow rates and/or higher density differences. The ECC water falls in an almost vertical path and reaches the lower downcomer sensor directly below the inlet nozzle.

Keywords: buoyancy-driven flow; PTS; ROCOM; ECC

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Publ.-Id: 28756

Numerical simulation of boron dilution and cold-water transients

Höhne, T.

Boron Dilution scenarios were indentified for the use of 3D flow and mixing tools
Buoyancy driven mixing was investigated under simulated natural circulation conditions at the test facilities ROCOM and GMTF.
Hybrid meshes consisting at least of 2-8 million nodes were used.
The experiment and CFD calculations show in both cases significant mixing effects due to the density differences.
The ANSYS CFX calculations show a good qualitative agreement with the data.
CFD simulation of fibre material transport in a PWR core under loss of coolant conditions
Implementation of strainer model for the spacer grid

Keywords: CFX; PWR; ROCOM; Mixing

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Publ.-Id: 28755

ROCOM Experiments – 2

Höhne, T.; Kliem, S.

Goal: Investigation of coolant mixing at constant flow rates in the primary circuit

Mass flow rate in the loop (10 - 160 %)
Number of operating loops
Status of non-operating loops (reverse flow or closed)
Friction losses at core inlet (lower, nominal and enhanced)

Keywords: ROCOM; Mixing Scalar; RPV; Konvoi

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Publ.-Id: 28754

ROCOM Experiments – 1

Höhne, T.; Kliem, S.

Goal: Investigation of coolant mixing at constant flow rates in the primary circuit

Mass flow rate in the loop (10 - 160 %)
Number of operating loops
Status of non-operating loops (reverse flow or closed)
Friction losses at core inlet (lower, nominal and enhanced)

Keywords: ROCOM; Mixing Scalar; RPV; Konvoi

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Publ.-Id: 28753

Overview of the Mixing Test Facility ROCOM

Höhne, T.; Kliem, S.; Prasser, H.-M.

PWR is equipped with 2 or more loops (German KONVOI: 4)
Boron dissolved in the coolant acts as neutron absorber
Hypothetical accidents with creation of lower borated slugs in single loops
Importance of mixing of coolant with different boron content

Keywords: ROCOM; Konvoi; PWR; RPV; Wire Mesh Sensor

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Publ.-Id: 28752

Theory and Practice PTS

Höhne, T.; Lucas, D.

PTS is an important issue of Nuclear Safety Research
Thermohydraulic part: more reliable simulations on mixing needed  3D  CFD
Two-phase PTS is one of the most challenging problems for CFD simulations.
In principle simulations are possible, but with too large uncertainties (even for single regions and phenomena).
There are still open question regarding the most appropriate models to be used for a simulation of two-phase PTS.
Step by step improvement of single effect models is ongoing.
New experimental data with high resolution in space and time are needed for model development and validation.

Keywords: PTS; RPV; NPP

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Publ.-Id: 28751

Theory and Practice Boron Dilution Transients

Höhne, T.; Kliem, S.

Development of calculation tools for the modeling of hypothetical accidents in NPPs
Conduction of calculations to show that safety criteria are fulfilled (safety goals)
For existing reactors
For future reactors
Two examples
Overcooling transient
Boron dilution accident

Keywords: Overcooling transient; PWR; Boron dilution; RPV; NPP

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Publ.-Id: 28750

PWR Design Attributes Relevant to PTS and Boron Dilution

Höhne, T.

The primary system (also called the Reactor Coolant System) consists of the reactor vessel, the steam generators, the reactor coolant pumps, a pressurizer, and the connecting piping. A reactor coolant loop is a reactor coolant pump, a steam generator, and the piping that connects these components to the reactor vessel. The primary function of the reactor coolant system is to transfer the heat from the fuel to the steam generators. A second function is to contain any fission products that escape the fuel.
The following drawings show the layout of the reactor coolant systems for three pressurized water reactor vendors. All of the systems consist of the same major components, but they are arranged in slightly different ways. For example, Westinghouse has built plant with two, three, or four loops, depending upon the power output of the plant. The Combustion Engineering plants and the Babcock & Wilcox plants only have two steam generators, but they have four reactor coolant pumps.

Keywords: PWR; NPP; Primary System

  • Invited lecture (Conferences)
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan
  • Contribution to proceedings
    TC Expert Mission on Analysis of Heterogeneity in Coolant and Transient Boron Dilution Temperature Distribution, 11.-15.02.2019, Islamabad, Pakistan

Publ.-Id: 28749

FDG-PET/MRI in patients with pelvic recurrence of rectal cancer: first clinical experiences

Plodeck, V.; Rahbari, N.; Weitz, J.; Radosa, C.; Laniado, M.; Hoffmann, R.; Zoephel, K.; Beuthien-Baumann, B.; Kotzerke, J.; van den Hoff, J.; Platzek, I.

To determine the value of 18F-FDG-PET/MRI in the diagnosis and management of patients with pelvic recurrence of rectal cancer.

Forty-four patients (16 women, 28 men) with a history of rectal cancer who received FDG-PET/MRI between June 2011 and February 2017 at our institution were retrospectively enrolled. Three patients received two FDG-PET/MRIs; thus a total of 47 examinations were included. Pelvic recurrence was confirmed either with histology (n = 27) or imaging follow-up (n = 17) (> 4 months). Two readers (one radiologist, one nuclear medicine physician) interpreted the images in consensus. Pelvic lesions were assessed regarding FDG uptake and morphology. Sensitivity, specificity, positive and negative predictive values as well as accuracy of PET/MRI in detecting recurrence were determined.

In 47 FDG-PET/MRIs 30 suspicious pelvic lesions were identified, 29 of which were malignant. Two patients underwent resection and had histologically proven pelvic recurrence without showing suspicious findings on FDG-PET/MRI. Changes in management due to FDG-PET/MRI findings had been implemented in eight patients. Eighty per cent (16/20) of resected patients had histologically negative resection margins (R0), one patient had uncertain resection margins. Sensitivity of FDG-PET/MRI in detecting recurrence was 94%, specificity 94%, positive/negative predictive value and accuracy were 97%, 90% and 94%, respectively.

FDG-PET/MRI is a valuable tool in the diagnosis and staging of pelvic recurrence in patients with rectal cancer.

Keywords: Positron-emission tomography; Magnetic resonance imaging; Rectal cancer; local Neoplasm Recurrence

Publ.-Id: 28748

Investigation of the ion induced magnetization in FeRh

Semisalova, A.; Barton, C.; Bali, R.; Böttger, R.; Thomson, T.; Potzger, K.; Lenz, K.; Lindner, J.; Fassbender, J.

Structurally B2-ordered equiatomic FeRh thin films are known for unique properties such as a temperature, magnetic field, and spin polarized current driven phase transition from the antiferro- to the ferromagnetic state. The strain and structural disorder also influences the magnetic properties of FeRh, which opens a new way for controllable modification of properties at the micro- and nanoscale. Namely, structural modification by ion beam irradiation was shown to be an effective tool for tuning the phase transition temperature in FeRh as well as the saturation magnetization [1-3]. Here, we present a detailed study of magnetic properties of ion irradiated 40 nm thick FeRh films using magnetometry and broadband ferromagnetic resonance technique. The structurally ordered films were deposited epitaxially on MgO(001) substrates using magnetron sputtering. The irradiation was performed with 25 keV Ne ions at fluences of 0.1 – 4 ions/nm2 leading to a controllable reduction of the order parameter. The ion beam induced magnetization of FeRh at room temperature was shown to be as high as 1300 kA/m. Ferromagnetic resonance measurements performed at frequencies up to 40 GHz show that the Gilbert damping in structurally disordered ferromagnetic FeRh films is comparable to Py films. Such a relatively low damping in combination with the highly tunable saturation magnetization appears promising for further experiments on magnetization dynamics and spin wave propagation in FeRh thin films and nanostructures fabricated using ion beam irradiation.
[1] N. Fujita et al., J. Appl. Phys. 107 (2010) 09E302
[2] A. Heidarian et al., Nucl. Instr. Meth. B 358 (2015) 251-254
[3] S.P. Bennett et al., Mater. Res. Lett. 6 (2018) 106-112

Keywords: FeRh; disorder; FMR; Gilbert damping; ion irradiation

  • Lecture (Conference)
    21st International Conference on Ion Beam Modification of Materials IBMM 2018, 24.-29.06.2018, San Antonio, Texas, USA

Publ.-Id: 28747

Probing the exchange coupling in the complex modified Ho-Fe-B compounds by high-field magnetization measurements

Tereshina, I. S.; Pyatakov, A. P.; Tereshina-Chitrova, E. A.; Gorbunov, D. I.; Skourski, Y.; Law, J. M.; Paukov, M. A.; Havela, L.; Doerr, M.; Zvezdin, A. K.; Andreev, A. V.

By examining the Ho2Fe14B case, we explored the influence of substitution and absorption atoms on the high-field behavior of magnetization of rare-earth (R)-Fe intermetallics. The value of the first critical field shows that the inter-sublattice exchange interactions remain practically unchanged when the substitution takes place in the R sublattice (replacement of up to 50 % of Ho by Nd). On the contrary, hydrogen absorption by Ho2Fe14B and Ho1Nd1Fe14B of the maximum possible hydrogen concentration 5.5 at./f.u. decreases the strength of the R-Fe Exchange by 30%. Remarkably, the influence of hydrogenation is stronger in the compound modified by substitution.

Publ.-Id: 28746

2D Materials Under Ion Irradiation: In-situ Experiments and the Role of the Substrate

Hlawacek, G.; Kretschmer, S.; Maslov, M.; Ghaderzadeh, S.; Ghorbani-Asl, M.; Krasheninnikov, A. V.

Helium ion Microscopy (HIM) is frequently used for the fabrication of 2D nanostructures in graphene, MoS 2 and other materials. While some of the experiments are carried out with freestanding materials most of the work is done on supported material. While the defect production is understood for the former case, it is not fully understood in the latter setup. We used a combination of analytical potential molecular dynamics and Monte Carlo simulations to elucidate the role of the different damage channels, namely primary ions, backscattered atoms and sputtered substrate atoms.
Using this approach we looked at the defect production by helium and neon ions in MoS 2 and graphene supported by SiO 2 at typical energies used in HIM. We show that depending on ion species and energy defect production for supported 2D materials can be dominated by sputtered atoms from the support, rather than direct damage induced by the primary ion beam. We also evaluated the consequences of these additional damage mechanisms on the achievable lateral resolution for HIM based defect engineering and nano-fabrication in 2D materials. The obtained results agree well with experimental results obtained by in-situ and ex-situ characterization of defects in graphene and MoS 2 .

Keywords: him; 2D; Simulations

  • Lecture (Conference)
    AVS International Symposium & Exhibition, 21.-26.10.2018, Long Beach, USA

Publ.-Id: 28744

Organized Single Si Quantum Dots in tiny SiO2 volumes: Self-alignment for Single Electron Transistors

Heinig, K.-H.; von Borany, J.; Prüfer, T.; Xu, X.; Möller, W.; Ahmed, G.; Tiron, R.; Gregor, H.; Bischoff, L.; Engelmann, H.-J.; Facsko, S.

Room temperature (RT) operation of Single Electron Transistors (SETs) is based on two conditions: (i) The Coulomb blockade energy of charging a dot must be smaller than kT, i.e. a Si quantum dot must be <5nm. (ii) The electron tunneling distance from the Si dot embedded in SiO2 to an electrode must be <1.5nm. Such dimensions are beyond the limits of top-down processes like Electron Beam Lithography (EBL) and Reactive Ion Etching (RIE).
As we demonstrate by atomistic computer simulations, a functional nanostructure for RT-SETs can be achieved by bottom-up processes, self-organization and self-alignment: Phase separation in a tiny volume ~(10nm)3 of metastable SiOx results in the formation of a single Si precipitate in SiO2 . And, if this SiOx volume is bordered at two sides by an Si/SiOx interface, the Si dot becomes self-aligned (isolated) by an SiO2 layer (SiOx denuted by excess Si).
The tiny SiOx volume has been formed by top-down processes: From a bulk Si/7nm SiO2/a-Si layer stack nanopillars of <20nm diameter have been fabricated by EBL and RIE. Then, the SiO2 layer embedded in the nanopillar was transformed into SiOx by 50keV Si+ ion irradiation. During subsequent annealing the single Si dot is expected to form. Two nanoscale phenomena not observed so far have to be overcome to get a reliable RT-SET fabrication.
This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 688072.

Keywords: HIM; nanostructures

  • Lecture (Conference)
    E-MRS Fall meeting, 16.-20.09.2018, Warsaw, Poland

Publ.-Id: 28742

Manufacturability of Single Si Quantum Dots for Single Electron Transistors operating at Room Temperature

Heinig, K.-H.; von Borany, J.; Prüfer, T.; Xu, X.; Möller, W.; Gharbi, A.; Tiron, R.; Hlawacek, G.; Bischoff, L.; Engelmann, H.-J.; Facsko, S.

Single Electron Transistors (SETs) are an extremely low power consuming alternative to Field Effect Transistors (FETs). Their room temperature operation is based on two conditions:
(i) The Coulomb energy of charging the dot with an electron must exceed kT. That requires dot sizes <5 nm. (ii) The tunneling distance between dot and electrodes through SiO2 must be <1.5nm.
These requirements are beyond top-down approaches. Thus, we follow a bottom-up approach: (i) A single Si dot forms by self-organization during phase separation of a tiny metastable SiOx volume into a Si precipitate and a SiO2 matrix. (ii) If the tiny SiOx volume is sandwiched between Si, then the single dot becomes self-aligned, i.e. two tunnel barriers form due to condensation of excess Si of SiOx onto the Si/SiO2 interfaces.
Here, a CMOS compatible manufacturabilty of vertical-nanowire-based SETs will be presented. Regular arrays of Si nanowires with diameters down to 20nm are fabricated by top-down processes. A SiO2 layer of 7nm thickness is sandwiched between Si of the wire. This SiO2 is transformed to SiOx(x<2) by ion beam mixing. During subsequent thermal activation (RTA) the dot structure should evolve as described above. Experimental and computer simulation results will be presented, critical fundamental issues of the nanofabrication will be discussed.
This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 688072.

Keywords: HIM; nanostructures

  • Lecture (Conference)
    E-MRS Fall meeting, 16.-20.09.2018, Warsaw, Poland

Publ.-Id: 28741

Extraction separation of rare-earth elements using an acidic diamide-type ligand and structural analysis of the extracted complex

Shimojo, K.; Sasanuki, T.; Schöne, S.; Sugita, T.; Okamura, H.; Ikeda-Ohno, A.

An acidic diamine type ligand was synthesised and its capability for extraction and separation of rare-earth elements were studied. The results showed that the ligand has a remarkably high selectivity for Sc amongst the rare-earth elements. Possible extraction and separation mechanisms are further discussed based on structure information derived from single-crystal X-ray diffraction.

Keywords: Separation; solvent extraction; rare-earth elements; single-crystal X-ray diffraction; coordination

  • Lecture (Conference)
    79th meeting of the Japan Society for Analytical Chemistry, 18.05.2019, Kita-Kyushu, Japan

Publ.-Id: 28740

HeFIB 2018: Helium and emerging focused ion beams

Hlawacek, G.; Facsko, S.; Bischoff, L.; Klingner, N.; Xu, X.; Serralta, E.; Ghaderzadeh, S.

Gas field ion sources (GFIS) using helium and neon as ion species are new and rapidly growing ion beam techniques.
However, GFIS based focused ion beams (FIB) are not the only new ion beam techniques offering new capabilities that go
beyond what classic Ga based FIB can do. Based on the contributions to the recently held meeting on Helium and
Emerging Focused Ion Beams (HeFIB) I will report on the newest developments in this field.
I will try to highlight new technological developments in the field of GFIS based FIBs, but also present new and emerging
alternative FIB source techniques such as Laser cooled sources, liquid metal alloy source, or Xe plasma FIBs. However,
such new techniques also open up many new application fields. I will present selected examples of in which focused ion
beams have been used for imaging, localized materials modification as well as classical FIB based fabrication of nano-

Keywords: HIM

  • Invited lecture (Conferences)
    CAARI 2018 - The Conference on Application of Accelerators in Research and Industry, 13.-17.08.2018, Fort Worth, USA

Publ.-Id: 28739

Molecular Dynamics simulations of 30 keV He impacts on gold nano-clusters

Ghaderzadeh, S.; Hlawacek, G.; Krasheninnikov, A.

At the Helmholtz-Zentrum Dresden-Rossendorf, molecular dynamics computer simulations are employed to study the sputtering yield and channeling effects in Gold nano-clusters of different sizes. Primary ion energy and crystal orientation are varied to obtain a holistic image of the possible effects relevant for scanning transmission ion microscopy. Our results show that ion-channeling occurs not only in the principal low-index, but also in other directions in between. The strengths of different channels are specifed, and their correlations with sputtering yield and damage production is addressed. The figure below shows sputtering under 30 keV He ion irradiation of 5 nm Gold nano-clusters.

Keywords: HIM; nanostructures; simulations; channeling

Publ.-Id: 28738

Imaging and analytic possibilities in the Helium Ion Microscope

Hlawacek, G.; Klingner, N.; Heller, R.; Veligura, V.; van Gastel, R.; Poelsema, B.; von Borany, J.; Facsko, S.

Helium Ion Microscopy (HIM) utilizes a Gas Field Ion Source (GFIS) to create a Helium or Neon ion beam with a diameter better than 0.5 nm and 1.8 nm, respectively. The method is well known for its high resolution imaging and nano-fabrication capabilities which it is able to provide not only for conducting but also insulating samples without the need for a conductive coating. The latter specimens are typically found in the fields of biosciences, MEMS/NEMS technology, catalyst research and many others. The availability of He and Ne ions with either low or moderate sputter yields, allow direct write nano-structuring with a precision below 10 nm in the HIM [1, 2]. However, the existing GFIS based focused ion beam (FIB) tools suffer from the lack of a well integrated analytic method that can enrich the highly detailed morphological images with materials contrast. While HIM technology is relatively young several efforts have been made to add such an analytic capability to the technique. So far, ionoluminescence [1, 3], backscattering spectrometry (BS) [1, 4], and secondary ion mass spectrometry (SIMS) using a magnetic sector [5] or time of flight (TOF) setup have been demonstrated [4].
I will present results obtained using the above mentioned methods beginning with iono-luminescence and its application to various materials systems. The method is in particular suited for the analysis of various defects present in the sample and the behaviour of defects under ion beam irradiation. In the second part of the talk I will present our newly developed TOF-BS and TOF-SIMS setup which allow to obtain information on the composition of the sample. They both utilize the same cost efficient and minimal invasive pulsing scheme for the primary ion beam. The lateral resolution reached for TOF-BS is approximately 50 nm while for TOF-SIMS a value of 8 nm could be reached. First images will be presented and the performance of the TOF-SIMS spectrometer will be discussed.

Keywords: HIM; nanostructures; analytic; SIMS

  • Invited lecture (Conferences)
    Imaging 2020 Workshop, 06.04.2018, Sønderborg, Denmark
  • Lecture (others)
    PNNL Seminar, 19.10.2018, Richland, USA

Publ.-Id: 28737

Dispersion relation of interlayer exchange coupled tailored ferrimagnets

Sorokin, S.; Gallardo, R.; Fowley, C.; Atcheson, G.; Dennehy, G. S.; Rode, K.; Stamenov, P.; Lindner, J.; Fassbender, J.; Deac, A. M.

Different ferromagnetic resonance (FMR) modes in micron-sized antiferromagnetically interlayer exchange coupled SiO2/Ta(5nm)/Py(t)/Ru(0.85 nm)/Py(3nm)/Ru(3nm) structures, with t = 3, 6 and 9 nm, were studied by means of the electrically detected ferromagnetic resonance (ED-FMR).
The main magnetoresistance effect used in ED-FMR was anisotropic magnetoresistance (AMR). Bilinear and biquadratic coupling strengths for each sample were determined by fitting SQUID-VSM measurements on 4×4 mm² thin films, using equilibrium total energy minimization. The existence of two different resonance modes (in-phase (acoustic) and out-of-phase (optic)) is shown for asymmetric samples. For the symmetric sample only the acoustic mode was observed, due to the compensation of AMR response from Py layers for the out-of-phase mode. The obtained dispersion relations show a clear dependence of the acoustic mode frequency minimum on the bilinear coupling strength.

Keywords: FMR; ED-FMR; Synthetic Antiferromagnets; Dispersion relation

  • Poster
    The European School on Magnetism, 17.09.2018, Krakow, Poland

Publ.-Id: 28736

Dispersion relation of the interlayer exchange coupled tailored ferrimagnets

Sorokin, S.; Fowley, C.; Atcheson, G.; Dennehy, G.; Duan, J.; Khudorozhkov, A.; Rode, K.; Lindner, J.; Fassbender, J.; Deac, A.; Stamenov, P.

Here we present a study of different ferromagnetic resonance (FMR) modes in micron-sized antiferromagnetically interlayer exchange coupled SiO2/Ta(5nm)/Py(t)/Ru(0.85 nm)/Py(3nm)/Ru(3nm) structures, with t = 3, 6 and 9 nm, by means of the electrically detected ferromagnetic resonance (ED-FMR).
The main magnetoresistance effect used in ED-FMR was anisotropic magnetoresistance (AMR). Bilinear and biquadratic coupling strengths for each sample were determined by fitting SQUID-VSM measurements on 4×4 mm² thin films, using equilibrium total energy minimization. The existence of two different resonance modes (in-phase (acoustic) and out-of-phase (optic)) is shown for asymmetric samples. For the symmetric sample only the acoustic mode was observed, due to the compensation of AMR response from Py layers for the out-of-phase mode. The obtained dispersion relations show a clear dependence of the acoustic mode frequency minimum on the bilinear coupling strength. For asymmetric samples, mode intermixing occurs for certain resonance fields, accompanied by abrupt jumps in both mode frequencies. Such behavior is not observed for symmetric samples, in accordance with predictions based on VNA-FMR experiments and simulations performed elsewhere.

Keywords: FMR; ED-FMR; Synthetic antiferromagnets; Dispersion relation

  • Lecture (Conference)
    JEMS 2018 - The Joint European Magnetic Symposia, 03.09.2018, Mainz, Germany

Publ.-Id: 28735

Magneto-transport measurements in para- and ferromagnetic Fe60Al40 wires

Liersch, V.; Schmeink, A.; Eggert, B.; Warnatz, T.; Wintz, S.; Ehrler, J.; Böttger, R.; Hlawacek, G.; Potzger, K.; Lindner, J.; Faßbender, J.; Wende, H.; Bali, R.

Certain alloys (Fe60Al40, Fe50Rh50, Fe65V35) have stable, chemically ordered, B2 and metastable, chemically dis-ordered, A2 crystal structures with different magnetic properties.
B2 Fe60Al40 is paramagnetic (PM), A2 Fe60Al40 is ferromagnetic (FM).
Possible applications are in phase change memories and sensors.
We investigate change of resistivity 𝜌 during A2 ↔ B2 transitions in Fe60Al40 wires.

Keywords: HIM; magnetic structures; nanopatterning

  • Poster
    DPG-Frühjahrstagung 2018, 11.-16.03.2018, Berlin, Germany

Publ.-Id: 28734

Synthesis, Structural Characterization and Photodecarbonylation Study of a Dicarbonyl Ruthenium(II)-Bisquinoline Complex

Kubeil, M.; Joshi, T.; Wood, B. R.; Stephan, H.

A photoactivatable ruthenium(II) carbonyl complex [Ru(II)(BisQ)Cl(CO)2]PF6 2 was prepared using a tridentate bisquinoline ligand (BisQ = (2,6-diquinolin-2-yl)pyridin). Compound 2 was thoroughly characterized by standard analytical methods and single crystal X-ray diffraction. The crystal structure of the complex cation reveals a distorted octahedral geometry. The CO release upon exposure to UV light was monitored by UV/VIS absorbance and Fourier transform infrared spectroscopies in acetonitrile and 1% (v/v) DMSO in water, respectively. The photodecarbonylation follows a stepwise CO release. The first CO release occurs very quickly whereas the second decarbonylation step proceeds more slowly. Moreover, the photoreaction in acetonitrile is more distinguished and faster than in 1% aq. (v/v) DMSO.

Keywords: photoCORM; ruthenium; tridentate ligand; UV/VIS and FTIR; photodecarbonylation

Publ.-Id: 28733

Luminescence spectroscopy of uranium in environmental systems

Steudtner, R.; Drobot, B.; Zabelt, D.; Bader, M.; Hilpmann, S.; Großmann, K.

Luminescence spectroscopy is a powerful tool to study the chemistry of uranium in trace concentration. Manifold operating mode, e.g. steady state, time-resolved, laser-induced, site-selective, cryogenic, etc. were used to investigate the environmental behavior of uranium in various geological and biological systems.
Hydrolysis is the basis for more complex aquatic systems and thus a deep understanding of those systems is indispensable. In case of U(VI) we demonstrated that a combination of luminescence spectroscopic methods together with state of the art data analysis (parallel factor analysis – PARAFAC) and quantum chemical calculations is a powerful setup to gain information on that system. We were able to extract thermodynamic constants for the mononuclear hydrolysis species using optimized data processing. Furthermore, advanced deconvolution of individual luminescence spectra demonstrates the correlation of luminescence spectroscopy and vibrational spectroscopy.
For kinetic studies of geological or biological sorption phenomena, different microscopic or flow-through cell techniques are useable. For online monitoring and characterization of U(VI) sorption species we develop a new technical in situ luminescence spectroscopy setup in comparability to the well established in situ time resolved ATR FT-IR spectroscopy. For biological systems, we combined microscopy with luminescence spectroscopic measurements for localization, visualization and chemical characterization of uranium complexes. This approach enables us to distinguish between biosorption, intracellular uptake or biomineralization as dominant retention process for uranium in biological samples.
Under reducing conditions expected in the near field of nuclear waste repository, the tetravalent uranium should be the major oxidation state. We studied the U(IV) luminescence characteristics in presence of various inorganic ligands (ClO4–, Cl–, SO42–). By using cryo-TRLFS at 77 K the speciation analysis limit for U(IV) was determined with 5·10–6 M and this corresponds to uranium concentrations occurring in the environment.

  • Lecture (Conference)
    8. RCA-Workshop, 12.-14.06.2018, Dresden, Germany

Publ.-Id: 28732

Towards an understanding of U(VI) interaction with Boom Clay dissolved organic matter by TRLFS

Buchatskaya, Y.; Salah, S.; Durce, D.; Steudtner, R.; Devillers, M.

Uranium represents the most abundant radionuclide in nuclear waste and its behavior is a major concern for geological waste disposal. The speciation of Uranium in solution affects its mobility in porous media and must in consequence be well understood. The dissolved organic matter (DOM) present in pore waters of geological formations can change the speciation and the geochemical behavior of uranium [1]. DOM present in the potential host rock formation Boom Clay in Belgium differs in size (from hundreds to thousands of Da), solubility at different pH (humic and fulvic acids) and functionality. Due to DOM polydispersity and polyfunctionality, its interaction with U(VI) could involve various mechanism which need to be studied in details. Time-resolved laser-induced fluorescence spectroscopy (TRLFS) is a well-known technique used to characterize U(VI) speciation and complex formation with inorganic and organic ligands. This method was already successfully applied to derive complex stability constants of U(VI) and humic acids [2]. The objective of the present work is to perform a systematic and multi-parametric study on the U(VI) complexation with BC DOM fractions using TRLFS.
Two DOM fractions different in molecular size were separated, concentrated and purified from natural Boom Clay organic matter: one “colloidal” (100 kDa – 0.45 µm) and one “small” (< 1 kDa). The separation and concentration were performed using ultrafiltration and solid phase extraction methods. The size distribution and reactivity of final fractions were characterized using size-exclusion chromatography, X-ray photoelectron spectroscopy and potentiometric titrations. Complex formation of U(VI) with different DOM fractions was studied using TRLFS in presence and absence of carbonates in 0.1 M NaClO4 for a pH range 4-12.5.

  • Lecture (Conference)
    International conference Uranium biogeochemistry, 21.-26.10.2018, Ascona, Switzerland

Publ.-Id: 28731

Mechanistic understanding for biochemical and biological processes of uranium(VI) by time-resolved laser-induced fluorescence spectroscopy (TRLFS)

Steudtner, R.; Hilpmann, S.; Bader, M.; Jessat, J.; Sachs, S.; Cherkouk, A.

The transfer of radionuclides into the food chain is of central concern for the safety assessment of both nuclear waste repositories and radioactive contaminated areas, such as legacies of the former uranium mining. The interaction of radionuclides, here in this particular case uranium(VI), with microorganism or plants is mostly described by transfer factors without knowing the underlying processes. In two examples we want present that luminescence spectroscopy is a powerful tool to study these unknowing processes of uranium(VI) on a molecular level.
Rock salt formations are considered as potential host rocks for the long-term storage of highly radioactive waste in a deep geological repository. Extremely halophilic archaea, e.g. Halobacterium species, dominate this habitat. We studied and compared the interactions of different extremely halophilic Halobacterium species with uranium(VI) by classical chemical and biological, by multi-spectroscopic and microscopic and by molecular biological methods. Depending on the used initial uranium(VI) concentration the different Halobacterium species showed a different bioassociation behaviour of uranium(VI). By using TRLFS the formation of uranium(VI) phosphate minerals, such as meta-autunite, as well as the complexation with carboxylate groups was observed as a function of the uranium(VI) concentration and the Halobacterium species.
In a second example, we studied the interaction of uranium(VI) with canola cells (Brassica napus) focusing on the concentration dependent impact of uranium(VI) on the cell metabolism. Previous studies showed, for instance, a speciation dependent influence of radionuclide uptake and translocation in plants [1]. Heavy metal stress induces the synthesis of metal-binding metabolites, storage of metal chelates in vacuoles or the secretion into the rhizosphere [2], which changes the plant cell metabolism. To study the interaction of lanthanides with Brassica napus on a cellular level, callus and suspension cells were exposed to uranium(VI). Besides the kinetics of the bio-association, the amount of associated uranium(VI) and its effect on cell growth and viability was determined. TRLFS was used as direct speciation technique to determine the uranium(VI) species on callus cells and the supernatant. In combination with high performance liquid chromatography (HPLC) experiments the metabolic answer of the callus cells during the presence of uranium(VI) will be investigated.

  • Lecture (Conference)
    4th International Workshop on Advanced Techniques in Actinide Spectroscopy (ATAS), 06.-09.11.2018, Nice, France

Publ.-Id: 28730

Estimating mean residence times and flow velocities to quantify recharge to the Western Dead Sea aquifer system using multiple environmental tracers

Wilske, C.; Suckow, A.; Roediger, T.; Geyer, S.; Merchel, S.; Rugel, G.; Pavetich, S.; Merkel, B.; Siebert, C.

The geologically complex, partly karstic limestone aquifers located west of the Dead Sea provide vital freshwater sources for all populated areas from Ramallah to Hebron. Discharge from these aquifers also sustains sensitive ecosystems located along the western shore of the lake. Recharge to these aquifers is restricted to winter precipitation events, which only occur in Hebron and Jerusalem uplands. To inform future sustainable water management strategies, a detailed characterization of these aquifers, including the volume of sustainable extraction is needed. In this study, the time scales of groundwater recharge to the two main aquifers were estimated through the sampling and interpretation of a suite of young age environmental tracers including tritium (³H), chlorine-36 (³⁶Cl), chlorofluorocarbons (CFC-11, CFC-12 and CFC-113) and sulfur hexafluoride (SF₆).

Keywords: water management; AMS; tritium; groundwater

  • Lecture (Conference)
    International Symposium on Isotope Hydrology: Advancing the Understanding of Water Cycle Processes, 20.-24.05.2019, Wien, Österreich

Publ.-Id: 28729

¹⁰Be exposure dating on moraines in the Sayan Mountains, Siberia

Rauh, P.; Schweri, L.; Garcia Morabito, E.; Merchel, S.; Rugel, G.; Zech, R.

Mountain glaciers are very sensitive to (past) climate changes. Paleoglaciation of the Tibetan Plateau and the Himalaya received plenty of attention for the reconstruction of past climate and environmental conditions, whereas few studies have been conducted so far in the adjacent Altai and Sayan Mountains in Siberia, although they promise a complex history.
The investigated Sayan Mountains in the Russian Altai indicate extensive glacial activity during the Pleistocene and a shifting of the local Last Glacial Maximum (LGM). Rich glacial residues in terms of moraine ridges and erratic boulders in the region provide excellent opportunities to establish a detailed age chronology using in-situ cosmogenic ¹⁰Be surface exposure dating and derive information about paleoclimatic conditions.
We present 28 ¹⁰Be surface exposure ages from the Ergaki Range and Tuva Uplands in the western Sayan Mountains. Distinct lateral moraines document the extents of former glaciation during a last glacial maximum. Several recessional moraines in the Ergaki Range additionally record stand stills or readvances during the last glacial termination.
The prominent lateral moraine in the Ergaki Range yields three exposure ages of ~19 ka, indicating a MIS 2 glaciation. The maximum advances correlate with the end of the global LGM. Concluding, glaciers in the Ergaki Range existed continuously during the late Pleistocene only varying in their extent. The well-clustered ages of the Ergaki Range provide a robust deglaciation chronology. The readvances/ stand stills consistently date to ~18 ka (three boulders), ~17.5 ka (two boulders), ~17 ka (three boulders), and ~16 ka (two boulders), with a deglaciation starting after 16 ka. The valley is not elevated enough to record younger, less extensive glaciation, e.g. during the Younger Dryas and the Holocene. Exposure ages from the Tuva Uplands are more difficult to interpret: The lateral moraine documenting the most extensive glaciation yields two exposure ages of ~22 ka. The immediately adjacent inner lateral moraine has two slightly younger and stratigraphically consistent ages of ~21 ka, yet three more boulders from that moraine are ~27 and 43 ka. At this point, we suspect these older boulders to have inheritance, but we cannot confidently exclude a much older deposition age for both moraines. Planned are analyzes of other cosmogenic nuclides, such as ²⁶Al and/or in situ ¹⁴C, which might help to more robustly identify inheritance and complex exposure histories.

Keywords: AMS; glaciation; moraine; LGM

  • Poster
    INQUA 2019 (International Union for Quaternary Research), 25.-31.07.2019, Dublin, Ireland

Publ.-Id: 28728

Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions

Kozubek, R.; Tripathi, M.; Ghorbani-Asl, M.; Kretschmer, S.; Madauß, L.; Pollmann, E.; O'Brien, M.; Mcevoy, N.; Ludacka, U.; Susi, T.; Duesberg, G. S.; Wilhelm, R. A.; Krasheninnikov, A. V.; Kotakoski, J.; Schleberger, M. Y.

Porous single-layer molybdenum disulfide (MoS2) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradiation with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS2. Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies. Comparison to atomistic simulations reveals the critical role of energy deposition from the ion to the material through electronic excitation in the defect creation process, and suggests an enrichment in molybdenum in the vicinity of the pore edges at least for ions with low potential energies. Analysis of the irradiated samples with atomic resolution scanning transmission electron microscopy reveals a clear dependence of the pore size on the potential energy of the projectiles, establishing irradiation with highly charged ions as an effective method to create pores with narrow size distributions and radii between ca. 0.3 and 3 nm.

Keywords: ion irradiation; highly charged ions; molybdenum disulfide; 2D material; STEM; MD simulation; perforation


Publ.-Id: 28727

Study of the influence of water gaps between fuel assemblies on the activation of an aeroball measurement system (AMS)

Konheiser, J.; Mueller, S. E.; Seidl, M.

The aeroball measuring system (AMS) at Siemens/KWU built pressurized water reactors (PWR) is an important part of the in-core instrumentation to determine in detail the local power distribution. Simulations were carried out with the help of the MCNP6 Monte Carlo program to determine the possible impact of an additional the water gap between the fuel assemblies with regard to the 51V(n,γ)52V reaction rate in the AMS. A simplified geometric model in a 3x3 matrix of identical fuel assemblies was used and four AMS lances were simulated in the central fuel assembly. By shifting the outer 8 fuel assemblies, different water gaps were created and the effects on the reaction rate of the AMS were calculated for different burn-up values and boron contents in the cooling water. It was found that the change of reaction rates can reach up to 10% for an assumed gap maximum of 1cm. The changes are largest for burn-up values at 30 and 45 GWd/t and slowly increase with decreasing boron concentration. The results are an important piece of information to assess the possibility of detecting non-nominal water gaps during reactor operation.

Keywords: AMS; PWR; MCNP6; in-core instrumentation


Publ.-Id: 28726

Site-controlled Si Nanodot Formation for a RT-SET via Ion Beam Mixing and Phase Separation

Xu, X.; Prüfer, T.; Wolf, D.; Hübner, R.; Bischoff, L.; Engelmann, H.-J.; Gharbi, A.; Heinig, K.-H.; Hlawacek, G.; von Borany, J.

CMOS-compatible formation of Si nanodots (NDs) as Coulomb islands is a prerequisite for an RT Single Electron Transistor operation. In this work, Si NDs are formed via ion beam mixing and thermally stimulated phase separation. Broad-beam Si+ and Ne+ beams followed by a rapid thermal annealing treatment were utilized to create a layer of NDs and visualized by Energy-Filtered Transmission Electron Microscopy (EFTEM). The conditions for ND formation are optimized based on an extensive survey of the parameter space. The work is guided by TRIDYN simulations during the ion beam mixing and 3D Kinetic Monte-Carlo simulation for the phase separation during the thermal treatment. To tailor towards a single Si ND, the focused Ne+ beam from the Helium Ion Microscope (HIM) is utilized to create patterns of NDs in planar layer stacks. The formation of site-controlled single NDs with a diameter of 2.2 nm is confirmed by comparing the EFTEM Si plasmon-loss intensity with simulated intensity.

  • Lecture (Conference)
    HeFIB2018 Helium and emerging Focused Ion Beams, 11.06.2018, Dresden, Deutschland

Publ.-Id: 28724

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