Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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34122 Publications

Superconductivity of Mg-B layers prepared by a multi-energy implantation of boron into magnesium and magnesium into boron bulk substrates followed by the furnace and pulsed plasma annealing

Piekoszewski, J.; Kempinski, W.; Barlak, M.; Werner, Z.; Los, S.; Andrzejewski, B.; Stankowski, J.; Piekara-Sady, L.; Skladnik-Sadowska, E.; Szymczyk, W.; Kolitsch, A.; Groetzschel, R.; Starosta, W.; Sartowska, B.

B ions into Mg and Mg ions into B substrates were implanted in triple mode, i.e. each sample was sequentially implanted at three different energies starting from the highest one (80-150 keV range) to the lowest one (40-70 keV range). The energies and fluencies in each particular batch were simulated to yield a possibly large region in which the Mg:B ratio corresponds to stoichiometric MgB2 compound. These structures were next annealed using high intensity hydrogen plasma pulses of energy densities between 1.5 and 4.0 J/cm(2), or furnace annealed at 350-600 degrees C in a stream of flowing Ar-4%H-2 mixture. The simulated profiles were in fair agreement with those derived from the RBS measurements. Magnetically modulated microwave absorption (MMMA), magnetization and resistance measurements showed that the superconducting transition onset temperature T-c(onset) shifted from about 13 K in the best magnesium sample implanted with single-energy B ions, to 22-28 K for multi-energy!
implantation treatments. Respective shift in Mg-implanted boron samples was from about 33.3 K to 36.5 K. However, broadening of the transition to the superconducting state is observed for the multi-energy treatment in both cases. Possible reasons for these effects and proposed means to improve the method are discussed. (C) 2009 Elsevier B.V. All rights

  • Surface & Coatings Technology 203(2009)17-18, 2694-2699

Publ.-Id: 13093

Wettability improvement of carbon ceramic materials by mono and multi energy plasma pulses

Barlak, M.; Piekoszewski, J.; Werner, Z.; Stanislawski, J.; Skladnik-Sadowska, E.; Borkowska, K.; Miskiewicz, M.; Kolitsch, A.; Groetzschel, R.; Starosta, W.; Sartowska, B.; Kierzek, J.

joining of carbon type ceramics with other materials presents an important challenge, especially in view of ITER technology. The crucial problem in joining is wettability of such ceramics with copper. In our previous works we succeeded in improving wettability using two-steps procedure, i.e.: pre-treatment of the ceramics surface with Ti ions implantation and/or deposition of metallic Ti using high intensity pulse plasma treatment followed in both cases by the deposition of additional, thicker Ti layers using the Arc PVD technique. In the present work we performed successful experiments in which two-steps procedure is reduced to the one step using exclusively the pulse plasma pre-treatment in various combinations of the number of pulses and their energy density. (C) 2009 Elsevier B.V. All rights reserved.

  • Surface & Coatings Technology 203(2009)17-18, 2536-2540

Publ.-Id: 13092

Plasma-Immersion-Ion-Implantation of Fluorine to Protect TiAl-Components against High-Temperature Oxidation

Donchev, A.; Kolitsch, A.; Schuetze, M.; Yankov, R.

Titanium aluminides are promising light weight materials for several high-temperature applications, e.g., in aero engines but due to their insufficient oxidation resistance at temperatures above roughly 800 degrees C they cannot be used yet despite of their good mechanical high-temperature properties. The oxidation behavior of TiAl-alloys can be improved significantly by adding small amounts of fluorine into the subsurface zone of the components (microalloying). One possibility to apply fluorine to the surface of complex TiAl-components is the PI3-technique (plasma-immersion-ion-implantation). The use of an Ar/CH2F2-plasma for the F-PI3 into small coupons led to a positive effect which was as good as the beamline implantation of elemental fluorine gas into samples of the same geometry. Turbine blades, as examples for real TiAl-components, were implanted with an optimum set of parameters. Post-exposure investigations like scanning electron microscopy revealed a thin protectiv!
e alumina scale on the surface of the implanted samples in contrast to a thick mixed oxide scale (TiO2/Al2O3) growing on untreated samples during high-temperature oxidation in air. The high-temperature oxidation resistance of several TiAl-alloys was improved by plasma-immersion-ion-implantation of fluorine. Small coupons showed a much lesser oxidation during high-temperature exposure after fluorine treatment than untreated samples. The performance of turbine blades for the low pressure compressor of a new generation of jet engines was also improved. Fluorine treated samples are covered with a thin, protective alumina scale after high-temperature exposure in air instead of a fast growing, nonprotective mixed oxide (TiO2/Al2O3) scale which is found on untreated samples.

  • Plasma Processes and Polymers 6(2009)6, 434-439

Publ.-Id: 13091

Quantitative dopant profiling in semiconductors: A Kelvin probe force microscopy model

Baumgart, C.; Helm, M.; Schmidt, H.

Kelvin probe force microscopy (KPFM) is used to investigate the electrostatic force between a conductive probe and nanostructured Si with shallow or buried selectively doped regions under ambient conditions. A unique KPFM model correlates the measured Kelvin bias with the calculated Fermi energy, and thus allows quantitative dopant profiling. We show that due to an asymmetric electric-dipole formation at the semiconductor surface the measured Kelvin bias is related with the difference between Fermi energy and respective band edge, and independent of the probe potential.

Publ.-Id: 13090

Einführung eines Dokumentenmanagementsystems für die Unterstützung des Beschaffungsprozesses des Forschungszentrum Dresden-Rossendorf

Zander, R.

In dieser Arbeit wird ein Lösungsvorschlag für die Einbindung eines Dokumentenmanagementsystems in den Beschaffungsprozess des Forschungszentrum Dresden-Rossendorf vorgestellt. Nach einer eingehenden Analyse der Prozessabläufe bei der Beschaffung und der am Standort vorhandenen IT-Landschaft wurde in Zusammenarbeit mit den zukünftigen Anwendern und einer beratenden externen Firma ein Konzept erarbeitet, durch dessen Umsetzung das Forschungszentrum Dresden-Rossendorf in der Lage ist, seinen Beschaffungsprozess zu optimieren.

Keywords: Dokumentenmanagementsystem; DMS; Beschaffungsprozess

  • Article, self-published (no contribution to HZDR-Annual report)
    Forschungszentrum Rossendorf 2009
    64 Seiten
    ISSN: 1437-322X


Publ.-Id: 13089

Qualitative und quantitative Analysen dünner Funktionsschichten für das Kleben durch XPS-, AES-, REM- und TEM-Untersuchungen

Lebelt, P.; Gehrke, J.; Hallmeier, K.-H.; Reuther, H.; Dörfel, I.

Die Schwachstelle aller Klebverbindungen ist die Grenzschicht zwischen Klebstoff und Substrat. Aus diesem Grund ist eine Optimierung hinsichtlich Klebstoffauswahl und Oberflächenvorbereitung/-vorbehandlung des Substrats unerlässlich, um definiert und sicher zu kleben.

  • Galvanotechnik 100(2009)4, 930-937

Publ.-Id: 13088

Spin-dependent transport in nanocomposite C:Co films

Zhou, S.; Berndt, M.; Bürger, D.; Heera, V.; Potzger, K.; Abrasonis, G.; Radnóczi, G.; Kovács, G. J.; Kolitsch, A.; Helm, M.; Fassbender, J.; Möller, W.; Schmidt, H.

The magneto-transport properties of nanocomposite C:Co (15 and 40 at.% Co) thin films are investigated. The films were grown by ion beam co-sputtering on thermally oxidized silicon substrates in the temperature range from 200 to 500 °C. Two major effects are reported: (i) a large anomalous Hall effect amounting to 2 μΩ cm, and (ii) a negative magnetoresistance. Both the field-dependent resistivity and Hall resistivity curves coincide with the rescaled magnetization curves, a finding that is consistent with spin-dependent transport. These findings suggest that C:Co nanocomposites are promising candidates for carbon-based Hall sensors and spintronic devices.

Keywords: Carbon:cobalt nanocomposite; TEM; Magneto-transport; Anomalous hall effect; Magnetic nanoparticles

Publ.-Id: 13087

Wire-mesh sensors for high-resolution gas-liquid multiphase flow visualization

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

Wire-mesh sensors are flow imaging devices and allow the investigation of multiphase flows with high spatial and temporal resolution. This type of sensor was introduced about ten years ago at Research Center Dresden-Rossendorf, Germany and since then it has been successfully employed to investigate several single phase and two-phase flow phenomena. The sensor can be seen as a hybrid solution in between intrusive local measurement of phase fraction and tomographic cross-sectional imaging. It comprises of two set of wires stretched over the cross-section of a vessel or pipe. The planes of wires are perpendicular to each other with a small axial separation between them, thus forming a grid of electrodes. The associated electronics measures an electrical property of the flowing media at each crossing point in a very fast and multiplexed manner. The wire mesh subdivides the flow channel cross section into a number of independent sub-regions, whereas each crossing point represents one sub-region. Each of the measured signals reflects the constitution of the flow within its associated sub-region, i.e. each crossing point acts as local phase indicator. Hence, the set of data obtained from the sensor directly represents the phase distribution over the cross-section and no reconstruction procedure, e.g. by solving an inverse problem as usual in tomography, is needed in order to determine cross-sectional phase distributions. The first generation of wire-mesh sensors is based on conductivity measurements, thus being able to investigate electrically conducting fluids only. Typically air-water and steam-water flows have been investigated. Recently, the capacitance wire-mesh sensor has been developed and tested, which allow the investigation of non-conducting fluids such as oil or organic liquids. The newest systems are able to produce up to 10,000 images per second. Sensors can be constructed to operate under temperatures up to 286 °C and pressures up to 7 MPa. First this article reviews the measuring principle of wire-mesh sensors. In addition, measurement results of the application of a new-developed capacitance wire-mesh sensor to investigate two-phase gas-oil flow are presented. Furthermore, the use of a wire-mesh sensor for the investigation of a simulated three-phase flow in a laboratory setup is described and discussed. Thus, the wire-mesh sensor can considered as a simpler and inexpensive alternative to investigate either two-phase gas-liquid or three-phase gas-liquid-liquid flows. The good accuracy achieved in the permittivity measurement allows the wire-mesh system to securely distinguish even each of the three phases of a gas-oil-water flow.

Keywords: multiphase flow; flow visualization; phase fraction; wire-mesh sensor; imaging system

  • Contribution to proceedings
    20th International Congress of Mechanical Engineering, 15.-20.11.2009, Gramado, Brazil
    Proceedings of COBEM 2009, Paper No. COB09-2457, 2176-5480
  • Lecture (Conference)
    20th International Congress of Mechanical Engineering, 15.-20.11.2009, Gramado, Brazil

Publ.-Id: 13086

Investigation on drag reduction phenomenon in oil-water dispersed pipe flow via wire-mesh sensor

Rodriguez, I. H.; Yamaguti, H. K. B.; de Castro, M. S.; Da Silva, M. J.; Rodriguez, O. M. H.

Liquid-liquid flows are present in a wide range of industrial processes; however they have not been studied as intensively as gas-liquid flows. The interest in two-phase liquid-liquid flows have increased recently mainly due to the petroleum industry where oil and water are often produced and transported together for long distances. Nevertheless, the frictional pressure gradient in oil-water pipe flow not rare cannot be predicted by correlations developed for gas-liquid flow. The dispersed flow pattern is common in crude oil transmission pipelines and offshore pipelines, with either oil or water as the dominant phase. An interesting feature of dispersed flow is that it can behave as a non-Newtonian fluid. There are several works on drag reduction in single and gas-liquid two-phase flows, but only few on liquid-liquid flow. Drag reduction phenomenon in oil-water flows without the addition of any drag reduction agent has been detected in previous works, but the physics behind the phenomenon is yet not well understood. This work’s main goal is the experimental study of the drag reduction phenomenon in dispersed oil-water flow. Pressure gradients were measured during the flow of oil (860 kg/m3 density and 100 mPa.s viscosity) and water. The experimental work was performed in a 12-m-long 2.62-cm-i.d. horizontal glass pipe. A new wire-mesh sensor based on capacitance (permittivity) has been employed in this study. The sensor consists of two layers made of 8 steel wires each separated 1 mm from each other. It is able to discriminate fluids having different relative permittivity values in a multiphase flow and was used to measure local, transient and time-and-space averaged phase fraction distributions in the flow cross-section. A high-speed video camera and the Quick Closing Valves technique were used to compare and validate the signals of the wire-mesh sensor.

Keywords: liquid-liquid flow; drag reduction; dispersed flow; pressure gradient; wire-mesh sensor; liquid holdup

  • Contribution to proceedings
    20th International Congress of Mechanical Engineering, 15.-20.11.2009, Gramado, Brazil
    Proceedings of COBEM 2009, Paper No. COB09-0476
  • Lecture (Conference)
    20th International Congress of Mechanical Engineering, 15.-20.11.2009, Gramado, Brazil

Publ.-Id: 13085

Kapazitäts-Gittersensor: Prinzip und Anwendung

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

Gittersensoren ermöglichen räumlich und zeitlich hochaufgelöste Untersuchungen von Mehrphasenströmungen. Solche Sensoren können damit für viele strö-mungsdiagnostische Probleme in der Forschung und Industrie eingesetzt werden. Dieser Beitrag beschreibt den neu entwickelten Kapazitäts-Gittersensor, welcher auf der Messung der elektrischen Permittivität (Kapazität) basiert. Neben der Beschreibung des Messprinzips und der dazugehörigen Messelektronik wird auch ein ausgewähltes Anwendungsbeispiel der Visualisierung einer Gas-Öl-Zweiphasenströmung präsentiert.

Keywords: wire-mesh sensor; multiphase flow; flow visualization; void fraction; electrical capacitance

  • Contribution to proceedings
    XXIII. Messtechnisches Symposium des Arbeitskreises der Hochschullehrer für Messtechnik e.V., 17.-19.09.2009, Bremen, Deutschland
    Tagungsband XXIII. Messtechnisches Symposium des AHMT, Gert Goch (Hrsg.), Aachen: Shaker Verlag, 978-3-8322-8491-6, 139-148
  • Lecture (Conference)
    XXIII. Messtechnisches Symposium des Arbeitskreises der Hochschullehrer für Messtechnik e.V., 17.-19.09.2009, Bremen, Deutschland
  • Technisches Messen 77(2010), 209-214
    DOI: 10.1524/teme.2010.0055

Publ.-Id: 13084

Thermoelectric currents in laser induced melts pools

Lange, A.; Cramer, A.; Beyer, E.

Applying an external static magnetic field to laser beam welding may lead to a change in geometry of the weld seam. Since the most common influence of a magnetic field acting on an electrically high conducting fluid is the generation of a Lorentz force, the outcome of such experiments is a strong indication of the presence of electric current in the melt pool. Owing to the static nature of the magnetic field, induction is restricted to the movement of the liquid metal and therefore negligible, leaving thermoelectricity as the sole potential source of current. The present work discloses analytically that the current may indeed originate from a gradient of thermoelectric power. Based on the examples iron and aluminum, key features of the current distribution are determined numerically together with an investigation of the dependence of the distribution on material properties. By carrying out welding experiments in the heat conduction mode, validation is achieved of the thermoelectric source of the current and of basic properties of its contribution.

Keywords: thermoelectric current; laser welding; geometry of weld seam

  • Journal of Laser Applications 21(2009)2, 82-87

Publ.-Id: 13083

Voigt effect measurement on PLD grown NiO thin films

Scarlat, C.; Mok, K. M.; Zhou, S.; Vinnichenko, M.; Lorenz, M.; Grundmann, M.; Helm, M.; Schubert, M.; Schmidt, H.

NiO has great potential applications in spin valves, magnetooptical sensors, optical fibers, solar thermal absorbers, and in nonvolatile resistive random access memory devices. In our study NiMnO and NiMnLiO films have been grown on double-side polished r-plane sapphire substrates by pulsed laser deposition (PLD). We measured the complex Voigt angle using the polarized light from a HeCd laser, a Glan Taylor polarizer, a Hinds PEM-100 [1] and two Lock-Ins. The Voigt effect is a second order magnetooptic effect [2]. The polarization state of light after transmission through a sample consisting of ca. 1 μm thick, weak ferromagnetic NiO thin on purely diamagnetic r-plane sapphire substrates has been modelled using the 4x4 matrix formalism [3] in dependence of an external magnetic field applied in-plane, i.e. in Voigt configuration. The modelling results revealed that for the diamagnetic sapphire substrate the Voigt angle depends parabolically on the external magnetic field and that the weak ferromagnetic NiO thin films change the parabolic dependence of the Voigt angle in the range of ±0.1 T to a flat-top shape in agreement with the experimentally determined Voigt angle.

[2] R. Carey and B.W.J. Thomas, J. Phys. D: Appl. Phys. 7,
2362-2368 (1974).
[3] M. Schubert, Phys. Rev. B 53, 4265 (1996).

Keywords: Magnetooptical effects; Magnetic semiconductors

  • Contribution to proceedings
    12th International Conference on the Formation of Semiconductor Interfaces (ICFSI-12), 05.-10.07.2009, Weimar, Germany
  • Physica Status Solidi (C) 7(2010)2, 334-337

Publ.-Id: 13082

Paramagnetic Mn-implanted amorphous Si

Xu, Q.; Zhou, S.; Schmidt, B.; Mücklich, A.; Schmidt, H.

Different fluences of Mn ions have been implanted into 200 nm thick, n-type conducting amorphous Si films on a cold stage cooled by liquid N2 with a Mn concentration in MnxSi1-x ranging from x=0.0006 to 0.088. Magnetic measurements reveal paramagnetism in all the films at temperatures down to 2 K. The field dependent magnetization curves were well fitted by Brillouin functions, indicating that the magnetically active Mn ions are on MnI2+ interstitial sites with J=5/2. Only a small percentage of the implanted Mn ions contribute to the magnetization, indicating that the magnetic moments are quenched for most of the Mn ions, which was attributed to the nonuniform distribution of Mn ions in amorphous Si. Positive magnetoresistance due to ordinary magnetoresistance was observed at 5 K in the highest fluence Mn implanted amorphous Si film, indicating the lack of magnetic scattering of the conducting electrons by the implanted Mn2+ ions.

Keywords: diluted magnetic semiconductor; implantation; paramagnetism; magnetoresistance

Publ.-Id: 13081

Micro-Hall effect probes for applications at pulsed magnetic fields above 75 T

Mironov, O. A.; Palewski, T.; Zherlitsyn, S.; Uhlarz, M.; Skourski, Y.; Wosnitza, J.

Microminiature Hall probes (MHP) may be used as magnetic field transducers, with virtually no change of sensitivity with temperature, for applications at room and cryogenic temperatures. The probes have a nominal active sensing area of 90x90 µm and are based on Sn-doped n-InSb/i-GaAs MBE-grown heterostructures. Previously MHPs were intensively tested [1] and shown to be appropriate for various applications in the temperature range 50 mK- 300 K and in pulsed magnetic fields up to 52 T (of 120 ms pulse duration). The latest probes, with overall dimensions of 150x750 µm, are the smallest encapsulated Hall probes currently available and can be placed in areas not previously accessible to commercial packaged sensors. The new MHP modification has recently been tested in pulsed magnetic fields above 75 T using a new 9 MJ dual-coil magnet designed and fabricated at the Dresden High Magnetic Field Laboratory. The inner coil, having a bore of 20 mm, produced a 10 ms pulse on top of the 30 T background field from the outer coil of 100 ms duration. Tests over the range 300 K to 78 K yielded a remarkably linear Hall voltage up to 76.7 T, with a temperature independent sensitivity of 2.3 µV/mT (at an input current of 5 mA). The Hall-device input resistance was less than 0.4 ohms and < 1.5 ohms including the 80 mm long MHP copper wires.

  • Poster
    RHMF 09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13080

Optimization of large multiple coil systems for pulsed magnets

Perenboom, J. A. A. J.; Frings, P.; Béard, J.; Bansal, B.; Herlach, F.; Peng, T.; Zherlitsyn, S.

The generation of pulsed magnetic fields is limited by the Lorentz force on the conductor and by ohmic heating. A large coil can give higher field (and/or longer pulse duration), but for a single coil this may require high power that cannot be supplied by a feasible source. Power can be kept within acceptable limits by using a system of multiple coils. Pulsed field coils can be energized either by a pulsed power supply or a pulsed energy source. The energy or power supplies tend to be the most expensive part of the installation and a combination of high power and high energy is extremely expensive. In a multi-coil system , the increased design freedom allows to optimize the strength, pulse duration and heating of the coil, and to optimize the selection of materials and power supplies; one can so also minimize damage in case of coil failure. Because of the increased number of parameters, systematic insight into their mutual dependence is helpful in order to converge to an optimized design. Since the local optima are relatively weak, for the final design any standard method of pulsed coil design can be used. In this paper we will discuss strategies to determine the optimum choice for the design of inner- and outer-coil and how to optimize their design in relation to the supply type used. In particular, we will consider energy-limited capacitor banks and power-limited supplies. The approach will use scaling arguments and modeling tools as the PMDS package originally developed in Leuven. Optimization of coil systems is demonstrated with practical examples, such as the successful 87 T pulsed dual coil system in Dresden, and the design of the future ‘ARMS’ successor in Toulouse

  • Poster
    RHMF 09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13079

Temperatur Dependence of the Upper Critical Field of Superconducting Eu0.5K0.5Fe2As2

Weickert, F.; Hirale, J. S.; Gegenwart, P.

The discovery of superconductivity (SC) in FeAs-layered systems has triggered enormous activities on this field of solid state physics. Up to now a variety of compounds based on the two families RFeOAs (R = rare-earth element) and AFe2As2 (A = Ba, Sr, ...) were found with transition temperatures up to 57 K. A main feature of these compounds is, that SC occurs in close proximity to magnetic order, which suggests, that magnetic fluctuations play a crucial role in the formation of the superconducting ground state. In the system EuFe2As2 even two different types of magnetic order have been found. At 190 K a Fe spin density wave (SDW) forms, which is accompanied by a structural phase-transition. Below 19 K a second magnetic phase transition is observed, where Eu2+-irons order antiferromagnetically. Band structure calculation suggest, that both magnetic lattices are predominantly decoupled. A detailed study of Eu1-xKxFe2As2 indicates the complete suppression of the SDW order for x = 0.5 and the simultaneous development of SC below 32 K. In the contribution, we present magnetoresistance measurements carried out at the Hochfeldmagnetlabor Dresden on polycrystalline samples of Eu0.5K0.5Fe2As2 in pulsed magnetic fields up to 60 T. We could determine the complete temperature dependence of the upper critical field Hc2(T), at temperatures down to 1.5 K. The initial slope µ0dHc2/dT in the proximity of Tc(H = 0) varies between -1.60 T/K (0.1 Rn) and -1.75 T/K (0.9 Rn), dependent on the used criterion, and is rather small compared to other pnictides of the same family. A comparison of our results with the Werthamer-Helfand-Hohenberg (WHH) model indicates that the upper critical field in Eu0.5K0.5Fe2As2 is not Pauli limited. Moreover, the experimentally estimated critical field µ0Hc2(T → 0) of 45 T is significant higher than the value of 38 T expected for orbital limiting in the WHH model.

  • Poster
    RHMF 09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13078

Probing magneto-transport properties of carbon nanostructures in pulsed magnetic fields

Jhang, S.; Marganska, M.; Schmidmeier, S.; Skourski, Y.; Preusche, D.; Eroms, J.; Grifoni, M.; Strunk, C.

We present the magnetoconductance of carbon nanostructures (including carbon nanotubes, graphene and graphene nanoribbons) in pulsed magnetic fields up to 60 Tesla. Applying a magnetic field to such systems provides an efficient tool to investigate the band structure near the charge neutrality point. From the study of magnetoconductance in carbon nanotubes, we show how axial magnetic field as well as curvature, spin-orbint coupling, and Zeeman splitting modify the bandstructure of carbon nanotubes. Preliminary results on graphene and graphene nanoribbons are also reported, and technical issues to measure magnetoconductance of nanostructures in pulsed magnet facility are discussed.

  • Poster
    RHMF 09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13077

Evidence for a metamagnetic transition in the heavy fermion system CeTiGe

Deppe, M.; Skourski, Y.; Caroca-Canales, N.; Geibel, C.; Sereni, J. G.

Systems located close to a quantum critical point are of high interest, because low lying quantum fluctuations (QF’s) can lead to new physical phenomena. The compound CeTiGe crystallizes in the CeFeSi structure type, which presents some (2D) character enhancing QF’s. Our first detailed investigations of Cp(T), Chi(T), and rho(T) at low temperatures for CeTiGe indicate that it is a new paramagnetic heavy fermion system with an enhanced Sommerfeld - coefficient gamma0 ≈ 0.3 J/(K2mol). Further analyses using the Coqblin-Schrieffer (CS) model showed that the whole J = 5/2 multiplet is involved in the formation of the ground state. The CS - model predicts a broad s-shaped magnetization at high fields and to study this we extended our investigation to measurements under high magnetic field. Here we present the results of magnetization measurements M vs. B in fields up to 14 T performed in a commercial Physical Property Measurement System (PPMS®) and additional data obtained in pulsed high fields up to 60 T performed at the Dresden high magnetic field laboratory (HLD). Instead of the expected behavior these measurements revealed a steplike metamagnetic transition (MMT) at a critical field of μ0Hc ≈ 12.5 T of the size ΔM ~ 0.74 μB/Ce for T = 1.4 K. Thus CeTiGe is the second paramagnetic Ce - based Kondo lattice besides CeRu2Si2 showing a clear MMT below 20 T. In contrast to CeRu2Si2, CeTiGe shows a hysteresis at the MMT. We shall further present the effect of substitution La for Ce on this metamagnetic transition.

  • Poster
    RHMF 09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13076

Nitrocarburizing of AISI-304 stainless steel using high-voltage plasma immersion ion implantation

Abd El-Rahman, A. M.; Mohamed, S. H.; Ahmed, M. R.; Richter, E.; Prokert, F.

AISI-304 austenitic stainless steel has been nitrocarburized in N-2 and C2H2 ambient using high-voltage plasma immersion ion implantation (PIII) technology. The use of different PIII treatment times revealed important hints with respect to the microstructural, mechanical and corrosion properties of the nitrocarburized layer. Grazing incidence X-ray diffraction (GIXRD) shows the presence of nitride (gamma(N) and CrN) and carbide (gamma(c) and Fe3C) phases. Glow discharge optical spectroscopy (GDOS) has been used to characterize the elemental depth profiles in which the thickness of the modified layers is derived. Dynamic microindentation method is used for the study of mechanical performance of the nitrocarburized layer as well as the untreated material. The microhardness has been increased to a maximum value of more than nine times compared to that of the untreated one. The corrosion performance is characterized by potentiodynamic polarization technique and was found to be t! reatment time dependent.

Publ.-Id: 13075

Stoichiometric MgB2 layers produced by multi-energy implantation of boron into magnesium

Werner, Z.; Szymczyk, W.; Piekoszewski, J.; Seah, M. P.; Ratajczak, R.; Nowicki, L.; Barlak, M.; Richter, E.

Ion implantation manufacture of superconducting magnesium diboride films of the MgB2 stoichiometry (B: Mg = 2:1 composition) by boron implantation in Mg wafers requires a precise knowledge of the implantation process properties, in particular of the partial sputtering yields of Mg atoms by B ions. To verify these yields experimentally we deposited thin Mg films on glassy carbon platelets and implanted them with high fluences of 40, 60, and 80 keV B+ ions. He-backscattering (RBS) spectrometry was used to determine before- and after-implantation depth profiles of Mg and B. The sputtering yields turned out to be small enough (<0.1 atoms per ion) to neglect sputtering in simulations of the implanted profiles. The results of the simulations have been compared to RBS spectra recorded on samples treated with 3 energies/fluencies optimised for a wide plateau of the B:Mg = 2:1 stoichiometric composition.

Publ.-Id: 13074

Upper critical field measurements up to 60 T in arsenic-deficient LaO0.9F0.1 Fe As: Pauli limiting at high fields vs. improved superconductivity at low fields

Fuchs, G.; Drechsler, S.-L.; Kozlova, N.; Bartkowiak, M.; Wosnitza, J.; Behr, G.; Nenkov, K.; Klaus, H.-H.; Büchner, B.; Schultz, L.

We report resistivity, Hall and upper critical field Bc2(T) data for arsenic deficient LaO0.9F0.1 FeAs1-δ in a wide temperature and high field range up to 60 T. These disordered samples exhibit a slightly enhanced transition temperature of Tc = 29.0 K and a significantly enlarge slope dBc2/dT = –5.4 T/K near Tc. The high-field Bc2(T) data obtained from resistance measurements in pulsed magnetic fields follow up to about 30 T the WHH (Werthamer-Helfand-Hohenberg) curve for the orbital limited upper critical field, but show a clear flattening above 30 T. This flattening is interpreted as the onset of Pauli-limited behavior with Bc2(0) ~ 63 to 68 T. We compare our results with Bc2(T) data reported in the literature for clean and disordered samples. Whereas clean samples show no Pauli-limiting behavior for fields below 60 to 70 T as measured so far, the hitherto unexplained flattening of Bc2(T) for applied fields H||ab observed for several disordered closely related systems as Co-doped BaFe2As2 or (Ba,K)Fe2As2 (obtained from a Sn-flux) is interpreted as a manifestation of Pauli-limiting behavior. The corresponding enhanced Maki parameters point to significant paramagnetic effects in these disordered FeAs-based superconductors. Consequences of our results are discussed in terms of disorder effects within the frame of conventional and unconventional superconductivity. The enhancement of the upper critical field slope near Tc by strong disorder provides evidence for an important attractive intraband contribution to s-wave pairing of Cooper pairs in disordered Fe pnictides at relatively high transition temperatures.

  • Poster
    RHMF 09 (Research on High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13073

Magnetic properties of ZnO nanopowders

Xu, Q.; Zhou, S.; Schmidt, H.

We report the magnetic characterization of ZnO nanopowders with an average grain size of about 50 nm prepared by sol-gel method. At room temperature, ZnO nanopowders exhibit diamagnetism like bulk ZnO. However, strong paramagnetism was observed at 4 K. With increasing temperature, ferromagnetic to paramagnetic transition happens at around 90 K. Paramagnetism changes to diamagnetism at around 100 K, which is due to the competition between the paramagnetism from defects and diamagnetism from the bulk ZnO. Our work clearly demonstrates that the interaction betweeen the defects is short-range. Thus the interaction between the defects is weak due to low defect concentration. It is suggested that a high defect concentration is needed for room temperature ferromagnetism.

Keywords: Diluted magnetic semiconductor; ZnO; Magnetism; Defect

Publ.-Id: 13072

Charge Transport and Magnetotransport Properties of Polyimide Irradiated by 80 keV Co Ions

Chen, T.; Yao, S.; Hua, W.; Fa, T.; Li, L.; Zhou, S.

Charge transport properties of polyimide films implanted with 80 keV Co ions at two different fluences (series I: 1.25 x 10(17) ions/cm(2), series II: 1.75 x 10(17) ions/cm(2)) are studied in detail. For series I, the temperature dependence of surface resistivity fits Mott's equation very well. It is on the insulating side of the insulator-metal transition (IMT). However, for series II, the temperature dependence of surface resistivity is not in agreement with Mott's equation. It is on the metallic side of IMT. The magnetotransport properties of these two series are also studied. No significant magnetoresistive effect is observed for series I at both 5K and 300 K. For series II, an obvious magnetoresistive effect is observed at 5K, while there is no magnetoresistive effect at 300 K. Rutherford backscattering spectrometry (RBS) is applied to confirm the actual fluence for these two series.

  • Chinese Physics Letters 26(2009), 087201

Publ.-Id: 13071

Modification of the electrical properties of polyimide by irradiation with 80 keV Xe ions

Chen, T.; Yao, S.; Wang, K.; Wang, H.; Zhou, S.

We modify the electrical properties of polyimide (PI) films by irradiation with 80 keV Xe ions. The surface resistivity of irradiated PI film at room temperature decreases remarkably from 1.2 x 10(14) Omega/square for virgin PI film to 3.15 x 10(6) Omega/square for PI film irradiated by 5.0 x 10(16) ions/cm(2), and the temperature dependence of the resistivity of the treated films is well-fit using Mott's Equation. The irradiated PI film structure is studied using Raman spectroscopy, X-ray diffraction, and Rutherford Backscattering Spectrometry. The concentration of O in the irradiated layer decreases with increasing fluence, while the variation of N concentration is negligible. Graphite-like carbon-rich phases are created in the irradiated layers, leading to the modification of the electrical properties. (C) 2009 Elsevier B.V. All rights reserved.

Keywords: Ion implantation; Polyimide; Electrical properties

Publ.-Id: 13070

Dose effect curve for in vitro cell irradiations with laser accelerated electrons

Karsch, L.; Beyreuther, E.; Enghardt, W.; Kaluza, M.; Laschinsky, L.; Lessmann, E.; Naumburger, D.; Nicolai, M.; Pawelke, J.; Richter, C.; Sauerbrey, R.; Schlenvoigt, H.; Baumann, M.

Fragestellungen: Die neue Technologie der Laserteilchenbeschleunigung verspricht die Realisierung von kompakten und ökonomisch effektiven Therapieanlagen. Die dabei zur Anwendung kommenden Hochintensitätslaser führen, verglichen mit den heute in der Strahlentherapie eingesetzten Beschleunigern, zu viel kürzeren Teilchenpulsen mit geringerer Wiederholfrequenz, aber viel größerer Pulsdosisleistung. Vor einem klinischen Einsatz der Laserteilchenbeschleunigung müssen deshalb die Konsequenzen bezüglich dosimetrischer Erfassung und Strahlenwirkung auf menschliches Gewebe untersucht werden. Im Beitrag werden die weltweit ersten systematischen Zellbestrahlungen mit Laser beschleunigten Elektronen vorgestellt und die Ergebnisse diskutiert.
Methodik: Die Experimente wurden am Lasersystem JETI in Jena durchgeführt.
Das Lasersystem, bisher nur für physikalische Einzelschussexperimente benutzt, wurde für die Durchführung von Routine-Zellbestrahlungen angepasst und ein geeignetes Dosimetriesystem für die ultrakurz gepulsten Elektronen mit einer Energie bis zu 20 MeV entwickelt. Über einen Zeitraum von mehreren Monaten wurden Zellen von je zwei humanen Normal- und Tumorgeweben bestrahlt. Zur Erfassung der Strahlenwirkung wurden die residuellen Doppelstrangbrüche (24 h nach Bestrahlung) und der Anteil des klonogenen Zellüberlebens bestimmt. Der Effekt jeder applizierten Dosis wurde zunächst durch Bestrahlung mehrerer Zellproben an einem Tag doppelt bestimmt und jede Dosis-Effekt-Kurve an zwei weiteren Tagen wiederholt. Parallel dazu wurden Referenzbestrahlungen mit kontinuierlicher 200 kV Röntgenstrahlung durchgeführt.
Ergebnisse: Auf dem Weg zur klinischen Anwendung der Laserteilchenbeschleunigung sind die erfolgreichen systematischen in vitro Zellbestrahlungen ein erster wichtiger Schritt. Obwohl die hohen Anforderungen für Patientenbestrahlung noch nicht vollständig erfüllt sind, wurde über mehrere Wochen ein für Zellbestrahlungen ausreichend stabiler, reproduzierbarer Elektronenstrahl erzeugt. Die dosimetrische Überwachung erlaubte eine kontrollierte Bestrahlung der Zellen mit einer vorgegebenen Dosis. Für alle vier Zelllinien wird eine deutlich niedrigere biologische Wirksamkeit der ultrakurz gepulsten, Laser beschleunigten Elektronenstrahlen nachgewiesen. Die Ergebnisse der beiden biologischen Endpunkte sind dabei konsistent.
Schlussfolgerungen: Gegenwärtige Experimente untersuchen die Ursache der reduzierten Strahlenwirkung für Laser beschleunigte Elektronen. Erste Ergebnisse zum Einfluss der Pulsdosisleistung, der mittleren Dosisleistung und dem Energiespektrum werden vorgestellt.
Die Arbeit wird gefördert durch das BMBF 03ZIK445.

  • Strahlentherapie und Onkologie 185(2009)(Suppl. 1), 158
  • Poster
    DEGRO 2009 15. Jahreskongress der Deutschen Gesellschaft für Radioonkologie, 11.-14.06.2009, Bremen, Deutschland

Publ.-Id: 13069

Magnetic-field dependence of the T*-anomaly in quasi-2D organic superconductors

Brandenburg, J.; Müller, J.; Das, P.; Lang, M.; Weickert, F.; Bartkowiak, M.; Wosnitza, J.

The family of quasi-2D superconductors kappa–(BEDT–TTF)2X are model systems for strongly correlated low–dimensional metals. Recently, the unusual normal–conducting state — characterized by a line of anomalies T* (in the order of 40 K) — has attracted considerable attention: a ”pseudo-gap”behavior in analogy to the high-Tc cuprates, a crossover from an incoherent ”bad” metal to a coherent Fermi–liquid regime, and a density–wave–type phase transition have been suggested as possible scenarios. To investigate the possibility of a magnetic origin we carried out detailed transport measurements in pulsed magnetic fields up to 60 T. For two different compounds, X = Cu[N(CN)2 ]Br and Cu(NCS)2 , we observed a maximum in the relative magnetoresistance change right around T* . This indicates the significance of magnetic degrees of freedom which are coupled to the transport properties. Also, for the first time we were able to determine the magnetic–field dependence of T* showing a small negative shift with increasing field. We discuss the implications of our experimental data for possible models explaining the anomalous normal–conducting state.

  • Poster
    RHMF 09 (Research on High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13068

Theory of Unusual Superconducting Phase Transitions in Heavy Fermion Metals at High Magnetic Fields

Maniv, T.; Zhuravlev, V.; Wosnitza, J.

The large effective masses characterizing heavy-fermion metals enable, at sufficiently high magnetic fields and low temperatures, paramagnetically-driven first-order superconducting (SC) phase transitions, as well as phase transitions to non-uniform SC states with spatially modulated order parameters along the field direction. Here, we present a non-perturbative theory of these phase transitions, which reliably determines the stable SC phases, treats properly the corresponding finite jumps of the order parameter, and can account for various unusual features reported recently for some heavy-fermion superconductors. It is found that for quasi-2D heavy-fermion metals, such as CeCoIn5, at high magnetic fields oriented perpendicular to the highly conducting planes, the compensation effect of the Fulde-Ferrel (FF) modulation is too weak to prevent a first-order phase transition from the normal to the uniform SC state. No modulated FF phase can be therefore stabilized at fields below Hc2 in this material. The calculated thermodynamic properties are in good quantitative agreement with the experimentally derived phase diagram [1] and the sharp additional damping of the de Haas-van Alphen (dHvA) oscillations observed at Hc2 in CeCoIn5 [2]. For 3D heavy-fermion metals, such as URu2Si2, the FF modulation stabilizes, under a decreasing magnetic field, a non-uniform SC state via a second-order phase transition from the normal state. However, at a slightly lower field the modulated phase becomes unstable, transforming to a uniform SC state via a first-order transition. The sharp onset of the SC order parameter calculated for this double-stage scenario of the SC transition, including fluctuation effect, is found to be in good agreement with dHvA results in the SC state of URu2Si2 [3].

  • Lecture (Conference)
    RHMF 09 (Research on High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13067

Violation of critical universality at the antiferromagnetic phase transition of YbRh2Si2

Krellner, C.; Hartman, S.; Pikul, A. P.; Oeschler, N.; Klingner, C.; Donath, J. G.; Geibel, C.; Steglich, F.; Wosnitza, J.

In recent years,....

  • Poster
    Quantum Critically & Novel Phases 2009, 02.-05.08.2009, Dresden, Deutschland

Publ.-Id: 13066

Stand der experimentellen Arbeiten im Rahmen des TOPFLOW-PTS Projektes

Beyer, M.; Schleicher, E.; Pietruske, H.; Hampel, U.; Weiß, F.-P.

Ausgehend von einer Beschreibung der Aufgabenstellung und eines kompakten Überblicks über die TOPFLOW Versuchsanlage wird im Vortrag auf die in den Jahren 2006 bis 2009 realisierten Arbeitsschwerpunkte eingegangen. Detailliert werden die notwendigen anlagentechnischen Erweiterungen im Bereich des TOPFLOW Drucktankes sowie die Konstruktion und der Aufbau der Testsektion beschrieben. Des Weiteren enthält der Vortrag ausführliche Informationen zur Spezialmesstechnik und zur Versuchsmatrix. Eine Zusammenfassung der gegenwärtigen Arbeitsaufgaben und ein Ausblick schließen den Vortrag ab.

Keywords: PTS; pressurized thermal shock; TOPFLOW facility; pressure equilibrium; wire mesh sensor; heat flux prob

  • Lecture (others)
    Steering Committee Meeting on R&D Cooperation between FZD and AREVA NP GmbH, 07.09.2009, Dresden, Deutschland

Publ.-Id: 13065

Stand der experimentellen Arbeiten im Rahmen des TOPFLOW-II Projektes

Beyer, M.; Fischer, F.; Danciu, D. V.; Seidel, T.; Schütz, P.; Pietruske, H.; Lucas, D.; Weiß, F.-P.

Ausgehend von einer Darstellung der versuchstechnischen Aufgaben und einem kurzen Überblick über die TOPFLOW Anlage wird der aktuelle Stand der Arbeiten zum Projekt erläutert. Insbesondere in den Bereichen Kondensations- und Verdampfungsexperimente sowie zur schnellen Röntgentomographie werden die 2008 und 2009 durchgeführten Arbeiten detailliert beschrieben. Zu den Experimenten mit Phasenübergang an freien Oberflächen wird auf die Designphase des Testbassins und die in den vergangenen Jahren durchgeführten kleinskaligen Versuche zum impinging jet eingegangen. Eine Zusammenfassung und ein Ausblick auf die zukünftigen Arbeitsaufgaben schließen den Vortrag ab.

Keywords: condensation; evaporation; vertical tube; condensation on free surfaces; impinging jet; bubble entrainment; wire mesh sensor; CFD validation; TOPFLOW facility

  • Lecture (others)
    CFD im Containment / CFD für Zweiphasenströmungen (Meilstein-Workshop Sommer 2009), 29.07.2009, Dresden, Deutschland

Publ.-Id: 13064

Spraying spin coating silanization at room temperature of a SiO2 surface for silicon-based integrated light emitters

Cherkouk, C.; Rebohle, L.; Skorupa, W.; Strache, T.; Reuther, H.; Helm, M.

A new silanization method for SiO2 surfaces has been developed for Si-based light emitters which are intended to serve as light sources in smart biosensors relying on fluorescence analysis. This method uses a special silanization chamber and is based on spraying and spin coating (SSC) in nitrogen atmosphere at room temperature for 10 min. It avoids processes like sonication and the use of certain chemicals being harmful to integrated light emitters. The surface of a SiO2 layer serving as a passivation layer for the light emitters was hydrolyzed to silanols using an in situ-hybridization chamber and catalyzed with MES (2-(N-morpholino)ethanesulfone acid hydrate) buffer solution. Subsequently, the substrates were silanized with the SSC method using two coupling agents as (3-Aminopropyl)trimethoxysilane (APMS), and N0-(3-(trimethoxysilyl)-propyl)-diethylenetriamine (triamino-APMS). The structure of the SiO2 surface, the APMS and the triamino-APMS layers was controlled and characterized
by Infrared spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The results show a covalent binding of the silane coupling agents on the surface. Atomic force microscopy was used to investigate the roughness of the surface. The silanized samples exhibit smooth and densely covered surfaces. Finally, the suitability of the SSC method was verified on real light emitters.

Keywords: Si-based light emission; Estrogen; Biosensors; X-ray photoelectron spectroscopy; Atomic force microscopy; APMS; Silanol; Silanization

Publ.-Id: 13063

Physical limitations of the electroluminescence mechanism in Terbium-based light emitters: Matrix and layer thickness dependence

Rebohle, L.; Lehmann, J.; Prucnal, S.; Sunb, J. M.; Helm, M.; Skorupa, W.

The physical limits of downscaling the SiO2 thickness of rare earth implanted metal-oxynitride-oxide-semiconductor based light-emitters are explored by investigating the drop down of the electroluminescence power efficiency with decreasing SiO2 thickness of Tb-implanted devices. It will be experimentally shown that there is a dark zone with an extension of about 20 nm behind the injecting interface in which the hot electrons have not yet gained enough kinetic energy in order to excite the Tb3+ luminescence centers. In addition, replacing the host matrix SiO2 by SiON results in a decrease of power efficiency by two orders of magnitude which is consistent with experimental data about the hot energy distribution in these media.

Keywords: electroluminescence; rare earth; SiO2; MOS; Terbium

  • Applied Physics B 98(2010)2, 439

Publ.-Id: 13061

In situ Spectroelectrochemical Investigation of Pt(II/IV) Oxidation in Aqueous Solution Using X-ray Absorption Spectroscopy

Takao, K.; Takao, S.; Scheinost, A. C.; Bernhard, G.; Hennig, C.

The oxidation from PtIICl42− to PtIVCl62− in HCl aq. was studied in situ by combining electrochemistry with XAFS spectroscopy. During the oxidation of PtIICl42−, isosbestic points were observed in Pt LIII and LII XANES spectra as a function of time, indicating that the Pt(II/IV) redox equilibrium is the only reaction in the system. The Pt LIII and LII X-ray absorption edge energies of the initial PtIICl42− are 11562.9 and 13271.8 eV, respectively, while those of the electrolyzed species are 11564.6 and 13273.7 eV which are identical with those of a PtIVCl62– reference sample. The coordination of the electrolyzed species was characterized by structural parameters derived from the EXAFS curve fit, and identified to PtIVCl62–.

Keywords: XAFS; Spectroelectrochemistry; Platinum(II/IV); Redox; Molecular Structure

Publ.-Id: 13060

Vanadium:Silicon - an ion-beam generated diluted magnetic semiconductor? Vanadium diffusion and concentration predictions in Silicon

Thieme, M. B.; Gemming, S.; Potzger, K.

The generation of dilute magnetic semiconductors (DMS) by ion-beam implantation of magnetic centres into semiconducting materials has experienced renewed interest since the generation of magnetic thin films from the Cobalt-doped wide-gap semiconductor TiO2. Since the magnitude of the magnetic moment in such films is strongly varying and since the implementation in a standard, Silicon-based semiconductor device is challenging, we have concentrated on the binary and fully integrable system Vanadium:Silicon. Vanadium form several binary compounds in Silicon at higher doping concentrations; the most well characterised structures have the compositions V:Si= 3:1, 5:3, 6:5, 1:2, and bear the potential to exhibit magnetism. At higher dilution, Vanadium may form point defects in the crystalline Silicon host matrix. Here, we investigate different combinations of substitutional and interstitial vanadium atoms in crystal matrix as well as its diffusion. Spin dependent density functional band-structure calculations with the Projector Augmented Wave (PAW)-method (with Abinit, VASP) in LDA and GGA have been carried out to investigate magnetism for all configurations. For special structures also the all-electron full-potential local-orbitals (FPLO)-method (with FPLO) have been used, to confirm the magnetic properties. At the same time first experiments (RBS, XRD, SQUID) have been arranged to support the simulations.

  • Lecture (Conference)
    23rd Nordic Semiconductor meeting, 14.-17.06.2009, Reykjavik, Iceland

Publ.-Id: 13059

Magnetostriction and Thermal Expansion on the quantum spin system Azurite

Wolff-Fabris, F.; Francoual, S.; Zapf, V.; Jaime, M.; Lacerda, A.

Azurite is a natural mineral with a chemical structure Cu3(OH)2(CO3)2. This compound has been proposed as a model substance for the 1D distorted diamond chain where a frustrated triangular quantum magnet consisting of Cu S=1/2 atoms arranged alternately to form infinite chains along the b axis. The magnetic behavior of this compound reflects the existence of both monomers and dimers made of S = 1/2 Cu. A magnetization plateau at 1/3 of the saturization magnetization is observed in M vs H measurements between 11 and 30 T due to saturization of the monomers. For fields above the plateau, the magnetic field energy exceeds the dimer bonding and thus the dimers cant and then align with the field. The magnetic structure and the detailed phase diagram in temperature and field are largely unknown or controversial. A recent report [1] in the specific heat behavior suggests a more complicated structure than previously thought. In addition, recent ultrasound measurements [2] indicate significant magnetoelastic coupling must be taken into account. We have acquired interesting results on magnetic torque, magnetostriction and thermal expansion. We have demonstrated that significant, anisotropic magnetostriction is evidenced in azurite, giving us an indication of the magnetically induced structural distortions.

  • Poster
    RHMF09 (Research on High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland
  • Poster
    Perspectives in Highly Frustrated Magnetism (PHMF10), 19.-23.04.2010, Dresden, Deutschland

Publ.-Id: 13058

Spin Dynamics in the Model Spin-ladder Compound (C5H12N)2CuBr4 Probed with ESR

Zvyagin, S.

In first part of my presentation I will talk about recent development of the high-fi eld electron spin resonance (ESR) facility at the Dresden High Magnetic Field Laboratory. The unique feature of the facility is a combination of the free electron laser (1.2 – 75 THz) and pulsed (up to 65 T) magnetic fields. In the low-frequency range the FEL facility is complemented by conventional radiation sources allowing for ESR experiments down to 30 GHz. In second part of my talk I will present results of ESR studies of the spin-ladder compound (C5H12N)2CuBr4 (known as BPCB). Angular, temperature dependences, as well as the frequency-fi eld diagram of magnetic excitations have been studied in detail. The energy gap in center of the Brillouin zone (16.46 K), was observed directly, confirming the presence of the Dzyaloshinskii-Moriya interaction. A small (~140 mK) splitting between the gapped modes can be accounted by a weak interladder coupling. In addition, the temperature evolution of the ESR excitation spectrum revealed a highly unusual behavior, whose peculiarities are discussed.

  • Invited lecture (Conferences)
    RHMF09 (Research on High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13057

High-field magnetization of Er2(Co0.4Fe0.6)17

Skourski, Y.; Andreev, A. V.; Kuz´Min, M. D.; Wosnitza, J.

The quasibinary intermetallic compound Er2(Co0.4Fe0.6)17 (hexagonal Th2Ni17-type structure) is a ferrimagnet with TC = 1170 K and Ms = 21 μB/f.u. The magnetic anisotropy is of easy-axis type at low temperatures, changing to an easy-plane anisotropy near room temperature. In the present work, the magnetization was measured in fields up to 60 T along the easy c axis. A first-order transition was observed at 47 T with a magnetization jump of 9 μB/f.u. Er2(Co0.4Fe0.6)17 is an ideal model system for high-field experiments since the observed transition admits an unambiguous quantitative interpretation. This is because the compound also undergoes a first-order magnetization process at 1.5 T enabling an independent direct determination of the anisotropy constants. A further advantage is the smallness of the 3d contribution to the anisotropy, which can be neglected altogether. With these parameters the observed high-field magnetization curve was successfully modeled.

  • Poster
    ICM09 (International Conference on Magnetism 2009), 26.-31.07.2009, Karlsruhe, Deutschland

Publ.-Id: 13056

Ultrasonic investigation of the quasi-2D quantum antiferromagnet Cs2CuCl4

Zherlitsyn, S.; Sytcheva, A.; Wosnitza, J.; Zvyagin, A. A.; Chiatti, O.; Coldea, R.

We report results of sound-velocity and sound-attenuation measurements in the triangularlattice quasi-two-dimensional spin-1/2 antiferromagnet (AFM) Cs2CuCl4 (TN = 0.6 K) in magnetic fields up to 18 T applied along the a axis. The possibility for a Bose-Einstein condensation of magnons in the AFM phase and a proximity to the spin liquid state at low temperatures beyond the AFM phase has been suggested for this compound. The longitudinal acoustic c11 mode shows pronounced anomalies in sound velocity and sound attenuation at low temperatures and in applied magnetic field. Below 1.5 K, this mode demonstrates a softening with increasing field, followed by an increase of the sound velocity close to the saturation field, Bs approx 8.5 T. The ultrasonic results are analyzed with a theory based on exchange-striction coupling. There is good qualitative agreement between theoretical results and experiment.

  • Poster
    ICM 09 (International Conference on Magnetism), 26.-31.07.2009, Karlsruhe, Deutschland

Publ.-Id: 13055

Multiband superconductivity in YNi2B2C single crystals studied by use of specific-heat measurements

Bekkali, A.; Wosnitza, J.; Uhlarz, M.; Beyer, R.; Schneider, M.; Behr, G.; Drechsler, S.-L.; Fuchs, G.

We present new specific-heat data for two different YNi2B2C single crystals grown by a zone-melting method. The two samples (Tc,A = 15.26(4)K, Tc,B = 15.6(1) K) were studied in magnetic fields up to B = 9 T in the temperature range from T = 0.35 . . . 20 K, using both a relaxation and a heat-pulse method. In the superconducting state (B = 0) we find an uncommon dependence of the electronic contribution to the specific heat, Cel(T), strengthening the assumption of a multiband nature of the superconducting state of YNi2B2C. A quantitative analysis of Cel(T) evidences multiple electronic contributions from electrons with very different electron-phonon coupling strengths, thus exhibiting several different superconducting energy gaps Δ(T,B = 0). This feature is in agreement with recent de Haas–van Alphen results [1] and point-contact spectroscopy data [2].

  • Poster
    RHMF 09 (Research on High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13054

High-field properties of spin-1/2 frustrated square lattice compounds

Tsirlin, A.; Schmidt, B.; Skourski, Y.; Nath, R.; Weickert, F.; Geibel, C.; Rosner, H.

Quantum spin systems present a broad range of exotic phenomena upon the application of external magnetic field. High-field properties of numerous low-dimensional Heisenberg models are actively studied theoretically, but still poorly understood experimentally due to the lack of proper materials with feasible saturation fields. Recently, we proposed a bundle of vanadium compounds that show the physics of spin-1/2 frustrated square lattice model at the energy scale of 5–10 K. In this contribution, we present experimental results for high-field properties of these materials.
Magnetization curves of five frustrated square lattice compounds were recorded in either static or pulsed magnetic fields. Most of the compounds show the saturation at 15–25 T and present the ideal energy scale for high-field studies. The values of the saturation fields are in remarkable agreement with the previous estimates of individual exchange couplings, based on thermodynamic measurements in low fields. The consistency of the low-field and high-field results confirms the interpretation of the materials within the frustrated square lattice model, despite the actual crystal symmetry is low, and numerous non-equivalent exchange couplings are possible. The change of the frustration ratio within the same compound family enables to study the change of the magnetization curve as the frustration is enhanced. We show that the increase of the frustration ratio leads to the enhanced bending of the magnetization curve. This result agrees with the recent theoretical study of the model [1] and is further supported by the direct comparison of the experimental magnetization curves and simulations for finite-size clusters.

  • Poster
    RHMF 09 (Research on Hihg Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13053

Deviations from the conventional BCS behavior in the penetration depth of LuNi2B2C films at terahertz frequencies

Fischer, T.; Pronin, A. V.; Wosnitza, J.; Lobo, R. P. S. M.; Niemeyer, T.; Holzapfel, B.

We have measured the temperature and frequency-dependant transmission and phase shift through LuNi2B2C thin films on MgO substrates at terahertz frequencies. From the measured data, we could accurately determine the complex dielectric constant, epsilon, the complex optical conductivity, sigma, and the penetration depth, lambda. Comparing our measured results with theory, we find strong deviations from the standard one-band BCS predictions. These deviations can be attributed to the multiband nature of the superconducting state in LuNi2B2C.

  • Poster
    RHMF 09 (Research on High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13052

NMR in pulsed magnetic fields

Meier, B.; Haase, J.; Braun, M.; Bartkowiak, M.; Herrmannsdörfer, T.; Wosnitza, J.

The first observation of NMR in the pulsed high magnetic field at the Hochfeld-Magnetlabor Dresden (HLD), Forschungszentrum Dresden-Rossendorf (FZD) is reported. The new spectrometer that operates at up to 3:0 GHz is described, as well as its implementation in the pulsed field facility. Free induction decays and spin echo experiments on 1H and 63,65Cu will be described and discussed in terms of sensitivity and resolution.

  • Poster
    RHMF 09 (Research in High Magnetic Fields), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13051

Field induced phases in UPtSi2

Schulze Grachtrup, D.; Süllow, S.; Bleckmann, M.; Willenberg, B.; Mydosh, J.; Skourski, Y.; Rakoto, H.; Bartkowiak, M.; Sheikin, I.

Tetragonal UPtSi2 has recently been characterized as moderately mass enhanced antiferromagnetic compound, which in various physical properties and in low magnetic fields reveals a resemblance to URuSi2 [1-3]. In contrast to URuSi2 with the order parameter still unknown, UPtSi2 was shown to order antiferromagnetically below TN = 32 K in zero magnetic field [1]. Here, we present an extensive study of the behaviour of UPtSi2 in high magnetic fields. We have examined single crystalline UPtSi2 with resistivity and magnetization measurements in pulsed and static magnetic fields up to 50 T. Initially, and as it is typical for antiferromagnets, in these measurements we find the antiferromagnetic phase transition at TN = 32 K in zero field to shift downwards with increasing field for both crystallographic axes. However, below ~15 K and for magnetic fields B parallel to the a axis we see this transition to split up, forming a new and distinct high field phase between ~38 T and 45 T. Similarly, with the magnetic field applied parallel to the c axis we also find a splitting resulting in a new high field phase between 24 T and 31 T. Moreover, below 5 K our measurements along c axis show even more structure, indicating the presence of at least one more high field phase between 29 T and 33 T. Altogether, our data reveal a rich high field magnetic phase diagram, which closely resembles the observations made for URuSi2 [4]

  • Poster
    RHMF 09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13050

Porous steel surfaces produced by plasma immersion ion implantation

Shevchenko, N.; Kolitsch, A.

Plasma immersion ion implantation (PIII) using helium or argon plasmas has been employed for porous layers creation on metal surfaces. These void structures may show unique characteristics which offer potential for medical applications such as metal-based drug-eluting stents. The paper addresses the influence of implantation parameters on surface morphology, cavity characteristics and mechanical properties of stainless steel stents. Argon and/or helium PIII processing of stainless steel samples has been performed at ion energies ranging from 5 to 35 keV, ion fluence of more than 1e17 cm-2, and substrate temperature in the range 50 – 400°C. Scanning electron microscopy, grazing incidence X-ray diffraction analysis, X-ray photoelectron spectroscopy, and elastic recoil detection analysis have been employed for sample characterization. Argon PIII treatment at elevated temperatures of 200 – 350°C leads to spongy structure formation of a size of 1-2 micron. Helium implantation results in a surface roughening and creation of voids in high concentration with size in the range 300 – 500 nm as well as nano-scale cavities (5-50 nm). So, varying the ion species (helium or argon), ion energy and fluence, and substrate temperature has been found to produce either void or sponge like structures at the nano- (~10 nm) to micro-scale (~1 micron). The best mechanical properties have been obtained in the stainless steel samples implanted at elevated temperature (higher than 250°C). Surface flaking and cracks formation have been greatly reduced by subsequent post-implantation annealing at temperatures of 600 – 800°C.

Keywords: Plasma immersion ion implantation; porous layers; stainless steel; structure

  • Poster
    International Conference on Physical Mesomechanics and Computer-Aided Design and Development of Advanced Materials, 08.-11.09.2009, Tomsk, Russia

Publ.-Id: 13049

Aluminum incorporation in Ti1-xAlxN films studied by x-ray absorption near-edge structure

Gago, R.; Redondo-Cubero, A.; Endrino, J. L.; Jimenez, I.; Shevchenko, N.

The local bonding structure of titanium aluminum nitride (Ti1-xAlxN) films grown by dc magnetron cosputtering with different AlN molar fractions (x) has been studied by x-ray absorption near-edge structure (XANES) recorded in total electron yield mode. Grazing incidence x-ray diffraction (GIXRD) shows the formation of a ternary solid solution with cubic structure (c-Ti1-xAlxN) that shrinks with the incorporation of Al and that, above a solubility limit of x similar to 0.7, segregation of w-AlN and c-Ti1-xAlxN phases occurs. The Al incorporation in the cubic structure and lattice shrinkage can also be observed using XANES spectral features. However, contrary to GIXRD, direct evidence of w-AlN formation is not observed, suggesting a dominance and surface enrichment of cubic environments. For x>0.7, XANES shows the formation of Ti-Al bonds, which could be related to the segregation of w-AlN. This study shows the relevance of local-order information to assess the atomic structure of Ti1-xAlxN solutions.

Keywords: local bonding structure; Ti1-xAlxN; XANES; GIXRD

Publ.-Id: 13048

[Ni(phen)(fum)]: an S = 1 two-dimensional quantum magnet

Orendacova, A.; Cizmar, E.; Pavlova, A.; Kajnakova, M.; Uhlarz, M.; Wosnitza, J.; Orendac, M.; Cernak, J.; Feher, A.

[Ni(phen)(fum)] comprises of weakly bound covalent-layers containing Ni(II) ions forming a spatially anisotropic square lattice with pronounced intradimer exchange coupling. Susceptibility studies performed from 1.8 to 300 K revealed significant differences between field and zero-field cooling below 10 K suggesting the presence of easy-axis anisotropy. Specific heat, investigated in zero magnetic field from 0.3 to 20 K, did not reveal any lambda-like anomaly only a tiny hump was observed at about 1.2 K. The amount of magnetic entropy removed down to 1.2 K supports the notion of magnetic low dimensionality. The specific-heat data are dominated by a round anomaly at 4.2 K which was analyzed within limiting models of a paramagnet with E/D = 0.27 and D/kB = -13.4 K and antiferromagnetic S = 1 dimers with D/J = -0.8 and D/kB = -5.3 K. The analysis indicates the importance of introducing an interdimer coupling responsible for the formation of magnetic order on the square lattice.

  • Poster
    ICM 2009 (The International Conference on Magnetism), 26.-31.07.2009, Karlsruhe, Deutschland

Publ.-Id: 13047

Slow spin relaxation in the Heisenberg antiferromagnet Gd2(fum)3(H2O)4·3H2O induced by magnetic field

Sedlakova, L.; Orendac, M.; Cizmar, E.; Orendacova, A.; Zvyagin, S. A.; Wosnitza, J.; Gao, S.

The spin relaxation of Gd2(fum)3(H2O)4·3H2O, an S = 7/2 Heisenberg antiferromagnet was investigated by ac-susceptibility measurements at temperatures from 2 to 30 K, frequencies from 100 Hz to 10 kHz, and magnetic fields up to 3 T. It was found, that the magnetic field induces thermally activated relaxation over an energy barrier EB/kB approx 55 K. Subsequent investigation of the specific heat, magnetic entropy, magnetization, and shape of the electron-spin-resonance line yielded consistent values of the g-factor g = 2.0, single-ion anisotropy D/kB approx 0.23 K, and exchange interaction J/kB approx 0.12 K. The knowledge of these characteristic parameters allows to discuss the origin of the observed relaxation behavior. In particular, resonance trapping of low-energy phonons, recently proposed in Ni10 magnetic molecules [1] will be considered.

  • Poster
    ICM 2009 (The International Conference on Magnetism), 26.-31.07.2009, Karlsruhe, Deutschland

Publ.-Id: 13046

ESR Study of the Spin-Ladder Compound (C5H12N)2CuBr4

Cizmar, E.; Ozerov, M.; Wosnitza, J.; Zvyagin, S. A.; Thielemann, B.; Rüegg, C.; Klanjsek, M.; Horvatic, M.; Berthier, C.

A systematic multi-frequency electron spin resonance (ESR) study of the spin-ladder material (C5H12N)2CuBr4 (known as BPCB) and its deuterated analog have been performed at temperatures down to 1.3 K in magnetic fields up to 16 T. The energy gap in the magnetic excitation spectrum at k = 0, Delta/kB = 16.45 K, was observed directly, confirming the existence of Dzyaloshinskii-Moriya interaction in this compound. A small (» 140 mK) splitting between the levels of the excited spin-triplet confirmed the presence of a weak interladder coupling. The temperature evolution of the ESR excitation spectrum in BPCB revealed its very unusual behavior, whose origin is discussed.

  • Poster
    ICM 2009 (The International Conference on Magnetism), 26.-31.07.2009, Karlsruhe, Deutschland

Publ.-Id: 13045

The Dresden High Magnetic Field Laboratory (HLD), a User Facility for Advanced Pulsed-Field Experiments

Herrmannsdörfer, T.; Wosnitza, J.; Zherlitsyn, S.; Zvyagin, S.

Since 2007, the Dresden High Magnetic Field Laboratory operates as a user facility, providing unique experimental capabilities in pulsed fields. The HLD offers a variety of measurement techniques, such as electrical transport, magnetization, ultrasound, and magnetic resonance. A particular feature of the laboratory is the next-door free-electron-laser installation used for high-field infrared spectroscopy, cyclotron and electron spin resonance (ESR) in pulsed fields. Additionally, experimental techniques such as nuclear magnetic resonance (NMR) and calorimetry are being adapted for their use in pulsed magnetic fields. The HLD maintains design programs for pulsed-power supplies and pulsed magnets focusing on the development of benchmark equipment for scientific and industrial use as well. Recently, the HLD has reached magnetic fields of up to 87 T. This field range is accessible now for experiments in advanced materials research. Several 60 T and 70 T magnets are regularly used by in-house and external users as well. A two-coil 100 T prototype and a long-pulse (1000 ms) 60 T magnet are ready for their first tests. The HLD participates in the EuroMagNET II program, a coordinated research initiative which supports users of European high-magnetic field installations. The in-house research program of the FZD is dedicated to electronically correlated systems, comprising material classes such as heavy-fermion compounds, novel and high-Tc superconductors, confined metallic nanostructures and low-dimensional spin systems as well.

  • Poster
    RHMF 09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13044

Quantum oscillations in the superconducting state of LuNi2B2C

Bergk, B.; Ignatchik, O.; Maniv, T.; Zhuravlev, V.; Canfield, P.; Wosnitza, J.

We have studied the de Haas-van Alphen (dHvA) effect of the borocarbide superconductor LuNi2B2C both in the normal and in the superconducting state by use of the field-modulation method at high magnetic fields up to 15 T and at low temperatures down to 0.5 K. Starting in the normal state we were able to observe dHvA oscillations deep inside the superconducting state with only a minor additional damping of the oscillation amplitudes. Only close to the upper critical field we find a slightly stronger damping. However, in this region we also observe a strong peak effect which hampers the analysis and complicates the interpretation. We compare our results with recent theories and discuss the possibilities of determining the magnetic-field-dependent gap for different bands with this method. Nevertheless, the apparent correlation between the occurence of the peak effect and the extra damping might be attributet to suppression of vortex-lattice order associated with the enhanced flux-line pinning in this region.

  • Poster
    RHMF09 (Research in High Magnetic Fields), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13043

Sound-Wave Anomalies in UCuGe

Yasin, S.; Sytcheva, A.; Wosnitza, J.; Zherlitsyn, S.; Andreev, A.

The hexagonal compound UCuGe is an antiferromagnet (AFM) with TN = 48 K. In this study we report on results of sound-velocity and sound-attenuation measurements performed on a single crystal of UCuGe at different frequencies, in the temperature range 1.3 K > T > 295 K, and in a magnetic fields up to 18 T applied along the c axis (hard axis). The temperature dependences of the sound velocity and attenuation display a pronounced anomaly at TN suggesting a large magnetoelastic coupling. A double-peak structure is observed in the sound velocity at TN which might be due to frustration. In the paramagnetic state (T > TN), both acoustic characteristics show large frequency-dependent changes revealing the presence of unusual relaxation processes. No hysteresis in the field dependence of the sound velocity in the AFM state is observed confirming that the compound UCuGe has a single-domain state.

  • Poster
    ICM 09 (International Conference On Magnetism 2009), 26.-31.07.2009, Karlsruhe, Deutschland

Publ.-Id: 13041

Infrared spectroscopy of La2-xEuxCuO4

Pronin, A. V.; Lobo, R. P. S. M.; Wosnitza, J.; Tsukada, A.; Naito, M.

We report on measurements of the complex conductivity of LaEuCu-oxide super-conductors in a wide frequency range (10 - 50000 cm-1) and for temperatures 4 K < T < 300 K. High-quality films on transparent substrates have been used for the investigations. We discuss the temperature and frequency behavior of the complex conductivity for several samples with different Eu concentrations.

  • Poster
    ICM 09 (International Conference On Magnetism 2009), 26.-31.07.2009, Karlsruhe, Deutschland

Publ.-Id: 13040

Fractional chain-end effects in quasi-one-dimensional S = 1 material NTNB: electron spin resonance studies

Ozerov, M.; Kamenskyi, D.; Cizmar, E.; Wosnitza, J.; Zvyagin, S. A.; Long, V. C.; Willet, R. D.

We report on X-band electron spin resonance studies (ESR) of the quasi-one-dimensional anisotropic S = 1 Heisenberg antiferromagnet compound Ni(tn)2NO2BF4 (or NTNB for short). The presence of interacting S = 1/2 chain-end states is clearly observed in the ESR spectra even in nominally pure samples, visible as a g ~ 2 main peak with several satellites. The data are analyzed in the frame of the theory proposed by Batista et al. [Phys. Rev. B 60, R12533 (1999)]. The ESR angular dependence revealed a pronounced anisotropy, whose origin can be explained assuming the presence of the field-induced staggered magnetization.

  • Poster
    ICM 09 (International Conference on Magnetism 2009), 26.-31.07.2009, Karlsruhe, Deutschland

Publ.-Id: 13039

High-field ESR and magnetization study of [Cu(pyz)2(HF2)]PF6: an S = 1/2 quasi-two-dimensional Heisenberg magnet

Cizmar, E.; Ozerov, M.; Skourski, Y.; Beyer, R.; Uhlarz, M.; Zvyagin, S. A.; Schlueter, J. A.; Manson, J. L.; Wosnitza, J.

Electron spin resonance and magnetization studies of the quasi-two-dimensional spin system [Cu(pyz)2(HF2)]PF6 have been performed in static and pulsed magnetic fields up to 50 T. It is argued that the magnetization is governed by the two-dimensional nature of spin correlations due to the large anisotropy of the exchange couplings (J_perp/J = 0.01, where J is the in-plane and J_perp is the interlayer exchange parameters). The magnetization saturates at the critical fields 37.1 T and 34.3 T for magnetic field applied perpendicular and parallel to the direction, respectively. The frequency-field diagram of the magnetic excitations changes dramatically below the Néel temperature (TN = 4.38 K). Our observation reveals an easy-plane type of the magnetic anisotropy. The exchange field (HE = 18.5 T) and out-of-plane anisotropy field (HA = 0.05 T) were calculated using a mean-field-theory approximation.

  • Poster
    RHMF09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13038

Superconducting Phase Diagram of Rh17S15

Uhlarz, M.; Ignatchik, O.; Wosnitza, J.; Naren, H. R.; Thamizhavel, A.; Ramakrishnan, S.

Rh17S15 (Miasite) is a 4d-electron metal which displays superconductivity below Tc = 5.4 K at zero field, with an upper critical field value Bc2 = 19.2 T at T = 0.07 K. Above Tc, Rh17S15 is a paramagnet. The crystallographic structure (Pm3m) of Rh17S15 features a nearest-neighbor Rh-Rh distance even less than in elementary (fcc) Rh, resulting in a high density of 4d-electron states at the Fermi level [1]. In our contribution, we report on measurements of the specific-heat and resistivity in fields up to 14 T and of the magnetic susceptibility in fields up to 20 T of a polycrystalline sample of Rh17S15. Our data allow us to map out the complete superconducting phase diagram. The existence of narrow 4d band states (and thus of strong electronic correlations that seem not to provide magnetic correlations) is supported by the found moderately high electronic contribution to the specific-heat of about 100 mJ/molK2 and favors the existence of a strong superconducting interaction. Together with the remarkably high upper critical field (exceeding the Pauli limit by a factor of two), these facts make Rh17S15 a likely candidate for unconventional superconductivity.

  • Poster
    RHMF09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13037

Superconductivity in Ga-doped Germanium

Skrotzki, R.; Herrmannsdörfer, T.; Heera, V.; Ignatchik, O.; Uhlarz, M.; Mücklich, A.; Posselt, M.; Reuther, H.; Schmidt, B.; Heinig, K.-H.; Skorupa, W.; Voelskow, M.; Wündisch, C.; Helm, M.; Wosnitza, J.

We report the first observation of superconductivity in heavily p-type doped germanium at ambient pressure conditions. Using Ga as dopant, we have produced a series of Ge:Ga samples by ion-beam implantation and subsequent short-term (msec) flash-lamp annealing. The combination of these techniques allows for Ga concentrations up to 6 %, i.e., a doping level which is clearly larger than the solubility limit and not accessible to any other method so far. Transport measurements reveal superconducting transitions with Tc up to 0.5 K. In more detail, we observe a strong dependence of the superconducting critical parameters on the annealing conditions. Further, we find a strong anisotropy of the superconducting critical field reflecting the two-dimensional character of the superconducting state in the ~ 60 nm thin Ge:Ga layer. We find critical magnetic in-plane fields following a linear temperature dependence up to a maximum field which is even larger than the Pauli-Clogston limit. Ge:Ga appears to be a superconductor in the extreme type-II limit with a very small Cooper-pair density. After its finding in Si [1] and diamond [2], our work reports another unexpected observation of superconductivity in doped elemental semiconductors.

  • Poster
    RHMF09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13036

de Haas-van Alphen study of the mixed-valence compound YbCoIn5

Polyakov, A.; Ignatchik, O.; Bianchi, A.; Prevost, B.; Seyfarth, G.; Fisk, Z.; Hurt, D.; Goodrich, R. G.; Wosnitza, J.

The interplay between conduction and localized electrons in mixed-valence materials has received great attention and remain to be a hot topic. The de Haas-van Alphen (dHvA) technique in combination with band-structure calculations is one of the best methods to investigate the multiband Fermi surfaces inherently present in these compounds. Here, we present a dHvA study of the nonmagnetic mixed-valence compound YbCoIn5. This material has the same crystal structure as the heavy-fermion superconductors CeCoIn5 and CeIrIn5 [1,2]. Our dHvA measurements thus allow a comparative investigation of the electronic properties of these highly interesting isostructural compounds. The dHvA signal was measured by use of a capacitive torque cantilever in a He3 cryostat in fields up to 13 T as well as in a dilution refrigerator down to 20 mK and up to 18 T. A number of different dHvA branches could be resolved. Besides their angular dependence, we as well determined the effective masses of the different bands by following the temperature-dependent amplitude change of the dHvA oscillations. In contrast to the heavy fermions CeCoIn5 and CeIrIn5, the effective masses for YbCoIn5 are in the range from 0.7 to 2.0 m0.

  • Poster
    RHMF09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13035

Specific heat of the highly anisotropic antiferromagnet [Cu(pyz)2(HF2)]PF6

Beyer, R.; Uhlarz, M.; Schlueter, J. A.; Manson, J.; Wosnitza, J.

The metal-organic compound [Cu(pyz)2(HF2)]X with X = PF6 exhibits a quasi-cubic lattice of copper ions (S = ½), but the magnetic properties show a predominantly two-dimensional (2D) nature due to a large anisotropy in the exchange couplings. The magnetic entropy and the antiferromagnetic ordering, eventually occurring at about 4 K, were investigated by specific-heat measurements. For this we established a continuous relaxation-time technique, using a single relaxation process to get specific-heat data over a wide temperature range. The calorimetric investigations, performed between 2 and 100 K and in magnetic fields up to 14 T, have revealed a non-monotonic field dependence of the ordering temperature. The results are as expected from the model for an S = ½ two-dimensional square-lattice quantum Heisenberg antiferromagnet with an additional weak interlayer exchange (via Cu-F-H-F-Cu bonds). In comparison to the X = BF4 compound, the antiferromagnetically ordered phase extends into much higher temperatures. In a more detailed analysis, we can extract all exchange interactions with an interlayer coupling ten times higher than in X = BF4. Thus, the 2D character is significantly reduced in X = PF6.

  • Poster
    RHMF09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13034

Upper critical fields and magnetic phase diagram of F-doped LaFeAsO

Jun Sung, K.; Kremer, R.; Bartkowiak, M.; Wosnitza, J.

The high-Tc superconductivity in ReFeAsO1−xFx (Re=Rare earths) with a maximum Tc of about 55 K has attracted great interest. LaFeAsO1−xFx is the first-discovered superconducting compound in this class of materials, and has been intensively studied so far. We have performed resistivity measurements on polycrystalline samples of LaFeAsO1−xFx with different Tc’s and obtained the superconducting-to-normal state transition at various temperatures with 60 T pulsed magnetic fields. At the normal state, the under-doped samples show a positive magnetoresistivity ~ 30% at 60 T while for the optimally-doped sample, the magnetoresitivity effect is almost negligible. In the superconducting state, the magnetoresistance shows a broad transition, which is often observed in polycrystalline samples consisting of randomly oriented anisotropic grains. The temperature dependence of Hc2 (T) for all LaFeAsO1−xFx samples with different Tc’s do not follow the conventional Werthammer-Helfand-Hohenberg (WHH) behavior, and shows an almost linear temperature dependence down to ~ 0.1 Tc. A strong deviation of Hc2 (T) from the WHH prediction will be discussed in terms of a multiband effect and a Pauli paramagnetic limiting effect.

  • Poster
    RHMF09 (Research in High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13033

Untersuchungen zu Redoxprozessen von Uran in Mischkulturbiofilmen und spektroskopische Verifizierung von ultradünnen Mineralüberzügen auf „Depleted Uranium“ mit Laser- Fluoreszenz- Techniken

Arnold, T.; Großmann, K.; Baumann, N.

Particles of U(V) and U(VI) were observed in vivo in a living multispecies biofilm by a combined laser-induced fluorescence spectroscopy (LIFS) and confocal laser scanning microscopy (CLSM) approach. The fluorescent uranium particles were located at the bottom and at the edges of biofilms colonies and ranged in size from up to 20 µm in length to 7 µm in width. Laser fluorescence spectroscopy was applied to identify these uranium particles. They showed either a characteristic fluorescence spectrum in the wavelength range of 415-475 nm, indicative for U(V), or in the range of 480-560 nm, which is typical for U(VI). The particles of U(V) as well as U(VI) were simultaneously observed in the biofilms. For U(VI) the particles were attributed to biologically mediated precipitation and for U(V) to redox processes taking place within the biofilm. The detection of U(V) in a living multispecies biofilm was interpreted as a short-lived intermediate of the U(VI) to U(IV) redox reaction. The presence of U(V) clearly shows that the U(VI) reduction is not a two electron step but that only one electron is involved.
LIFS was also applied to study the alteration of a depleted uranium (DU) disk in contact with synthetic pore water. The pore water used was a calcium phosphate solution (2.5 × 10-3 M Ca, 1 × 10-3 M P) and should mimic fertilized agricultural soil. The storage of the DU disk in calcium phosphate solution for 12 month led to the formation of a thin film of a secondary uranium mineral on the metallic DU. Time-resolved laser-induced fluorescence spectroscopy (TRLFS) was applied to spectroscopically characterise the reaction product. TRLFS provided its unequivocal identification as meta-autunite based on the positions of the fluorescence emission maxima at 487.8, 502.0, 523.6, 547.0, 572.1, and 600.6 nm and fluorescence lifetimes of 410 ± 15 and 3300 ± 310 ns. These results highlight the enhanced performance and sensitivity of the TRLFS technique for mineralogical characterization of thin surface films. Furthermore, they demonstrate that the dissolution of uranium from DU projectiles under the conditions described here is limited by the development and solubility of a meta-autunite secondary phase.

  • Invited lecture (Conferences)
    15. Tagung Festkoerperanalytik, 12.-16.07.09, Chemnitz, Deutschland

Publ.-Id: 13031

Wear-resistant Ti–B–N nanocomposite coatings synthesized by reactive cathodic arc evaporation

Neidhardt, J.; Czigány, Z.; Sartory, B.; Tessadri, R.; Mitterer, C.

Wear-resistant Ti–B–N coatings have been deposited by reactive arc evaporation of Ti–TiB2 compound cathodes in a commercial Oerlikon Balzers Rapid Coating System. Owing to the strong non-equilibrium conditions of the deposition method, a TiN–TiBx phase mixture is observed at low N2 partial pressures, as determined by elastic recoil detection analysis, X-ray diffraction, X-ray spectroscopy, transmission electron microscopy and selected area electron diffraction. The indicated formation of a metastable solid solution of B in face-centered cubic TiN gives rise to a maximum in hardness (>40 GPa) and wear resistance on the expense of increased compressive stresses. A further saturation of the nitrogen content results in the formation of a TiN–BN nanocomposite, where the BN phase fraction was tailored by the target composition (Ti/B ratio of 5/3 and 5/1). However, the amorphous nature of the BN phase does not support self-lubricious properties, showing friction coefficients of 0.7 ± 0.1 against alumina. The effect of an increased bias voltage on structure and morphology was investigated from -20 to -140 V and the thermal stability assessed in Ar and air by simultaneous thermal analysis up to 1400 °C.

Publ.-Id: 13030

First Hexanuclear U(IV) and Th(IV) Formate Complexes - Structure and Stability Range in Aqueous Solution

Takao, S.; Takao, K.; Kraus, W.; Emmerling, F.; Scheinost, A. C.; Bernhard, G.; Hennig, C.

The actinide(IV) hexanuclear M6(mu3-O(H))8(HCOO)12(LT)6 complexes were prepared (LT = H2O or CH3OH). HCOO- acts as a bridging ligand, which prevents formation of polynuclear hydrolysis species of U(IV) hydrous oxide colloids at least until pH 3.25, and stabilizes the nano-sized clusters in solution. The charge of the hexamer is balanced by the O/OH ratio of the mu3-bridges.

Keywords: actinides; polynuclear species; formic acid; solution; XAFS

Publ.-Id: 13029

Superconducting phase diagram of Sm-based oxy-pnictide single crystals at high magnetic fields

Bartkowiak, M.; Lee, H.-S.; Park, J.-H.; Lee, J.-Y.; Kim, J.-Y.; Sung, N.-H.; Cho, B. K.; Kim, J. S.; Lee, H.-J.

The recently discovered class of high Tc superconducting pnictides has led to very intense research efforts over the last year. Despite the efforts, there is still no consensus about such fundamental problems as the gap symmetry or the pairing mechanism. Measurements of the critical field Bc2 can help to find an answer. In practice, accurate measurements of Bc2 near the zero temperature limit are often hampered by the large critical fields, which promotes these systems as good candidates for high field experiments. One method to obtain Bc2 is by measuring electric transport in magnetic fields. So far, most studies of this kind are performed on polycrystaline samples where the interpretation of the resistance curves is not straightforward and assumptions about the anisotropy are based on the width of the transition. We present transport measurements of Bc2 in fields up to 60 T on single crystals of F-doped SmFeAs(O,F) and oxygen-deficient SmFeAsO1-δ. From the measurements for different orientations we were able to extract the anisotropy in the upper critical field, which show a significant temperature-dependence below Tc. We compare our results with data obtained on polycrystaline samples and discuss our data in terms of superconducting gap symmetries

  • Poster
    RHMF09 (Research In High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13028

High magnetic field elastic properties of CdCr2O4

Zherlitsyn, S.; Chiatti, O.; Sytcheva, A.; Wosnitza, J.; Bhattacharjee, S.; Moessner, R.; Zhitomirsky, M. E.; Lemmens, P.; Tsurkan, V.; Loidl, A.

CdCr2O4 is a geometrically frustrated Heisenberg "pyrochlore" magnet with an incommensurate antiferromagnetic (AF) ordering below TN = 7.8 K. At low temperatures a metamagnetic phase transition at 28 T in this compound is followed by a very wide magnetization plateau with one half of the full moment of S = 3/2 Cr3+ (three spins up and one spin down); the fully polarized state appears above approximately 90 T. Because of the strong spin-lattice coupling some magnetic phase transitions in CdCr2O4 are accompanied by structural distortions. Such spin-lattice coupling can be especially well investigated by ultrasound techniques. We have performed ultrasonic experiments on a high-quality CdCr2O4 single crystal at low temperatures and in pulsed magnetic fields up to 64 T. In our experiments, a longitudinal acoustic mode propagates along the [111] crystallographic direction and the magnetic field is applied in the same direction. A hysteretic minimum followed by a jump in the sound velocity and a peak in the attenuation are observed at 28 T for temperatures below the AF phase transition. An abrupt softening of the acoustic mode is detected at 59 T, where the magnetization plateau terminates. The above-mentioned anomalies get smoother at higher temperatures. The observed behavior of the acoustic characteristics demonstrates a crucial role of the spin-lattice coupling, which leads to lattice instabilities in this geometrically frustrated antiferromagnet in applied magnetic field. Our experimental results will be discussed in the framework of recent theories. Specifically, we construct a simple model, which gives quantitative description of the temperature
variations of the sound velocity in the vicinity of zero-field transition.

  • Poster
    RHMF09 (Research In High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13027

Magneto-acoustic properties of UCuGe

Yasin, S.; Andreev, A. V.; Sytcheva, A.; Wosnitza, J.; Zherlitsyn, S.

The hexagonal intermetallic compound UCuGe is an antiferromagnet with TN = 48 K. In magnetic fields of 38 – 45 T, applied along the hexagonal c axis, a spin-flop phase transition in this compound has been observed at 4.2 K [1]. We report on sound-velocity and sound-attenuation results obtained on a UCuGe single crystal at different frequencies. Thereby, a longitudinal ultrasonic wave was propagated along the [001] direction with static (up to 18 T) and pulsed (up to 57 T) magnetic fields applied along the same direction. The temperature dependences of the sound velocity and of the attenuation display a pronounced anomaly at TN, which is evidence for a strong magneto-elastic interaction. The pulse-field measurements at 4.2 K show a minimum in the sound velocity followed by a jump-like anomaly at 37 T, and another kink-like anomaly at 45-46 T. These anomalies are clearly connected with known field-induced spin re-arrangements and can be attributed to the symmetry changes at the start and end of the spin-flop transition. In the paramagnetic state (T > TN), the temperature dependences for both acoustic characteristics show large frequency-dependent changes. The observed linear frequency dependence of these changes reveals the presence of unusual relaxation processes. The origin of these anomalies will be discussed.

  • Poster
    RHMF09 (Research In High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13026

Terahertz spectroscopy of superconductors in high magnetic fields

Pronin, A. V.; Fischer, T.; Wosnitza, J.; Pimenov, A.; Loidl, A.; Tsukada, A.; Naito, M.

We report on measurements of the complex conductivity of cuprate superconductors at terahertz frequencies in fields up to 10 T. High-quality films of La2-xCexCuO4 and La2-xEuxCuO4 with different doping levels on transparent substrates have been used for our investigations. The measurements have been performed in the transmission mode. We discuss the behavior of the complex conductivity as a function of temperature, frequency, and magnetic field. We have found a small but distinctive optical magnetoresistance. We also realized some signatures of the pseudogap in the terahertz spectral range in magnetic field.

  • Poster
    RHMF09 (Research In High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13025

Magneto-acoustic Faraday rotation in Tb3Ga5O12

Sytcheva, A.; Yasin, S.; Wosnitza, J.; Zherlitsyn, S.; Lüthi, B.; Löw, U.; Goto, T.; Wyder, P.

We report on observation of the magneto-acoustic Faraday effect in paramagnetic Tb3Ga5O12 (TGG). This effect is the acoustic analog of the magneto-optical Faraday rotation. In cubic crystals, such as TGG, the two transverse sound waves propagating along the [100] direction are doubly degenerated. The magnetic field applied along the propagation direction breaks the time-reversal invariance and, therefore, lifts the degeneracy and induces a polarization rotation. This can be detected in the sound attenuation as oscillations as function of magnetic field, where each period corresponds to a polarization rotation of pi. Here, we present magneto-acoustic measurements performed at T = 1.4 K in fields up to 20 T in the frequency, f, range from 30 to 330 MHz. Indeed, we observed oscillations in the sound attenuation due to the Faraday effect. Theoretical studies predict an f2 dependence of the Faraday rotation [1], whereas we observe a linear dependence. Such behavior has also been reported for CeAl2 in the paramagnetic phase [2]. We observed a softening of the sound velocity with increasing field with a minimum at about 19 T. At the same field the attenuation oscillations cease. These results can be attributed to a crystal-field level crossing. With the appropriate choice of crystal-field parameters an effective theory predicts this level crossing to occur at ~20 T. Taking the magneto-elastic interaction into account, the theory reproduces the qualitative features of our data.

  • Poster
    RHMF09 (Research In High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13024

Acoustic study of the low-dimensional quantum antiferromagnet Cs2CuCl4

Sytcheva, A.; Wosnitza, J.; Zherlitsyn, S.; Zvyagin, A. A.; Chiatti, O.; Coldea, R.

We report on results of acoustic measurements in the triangular-lattice low-dimensional spin-1/2 antiferromagnet (AFM) Cs2CuCl4 (TN = 0.6 K) at high magnetic fields. At low temperature, the magnetic ordering in this compound has been the subject of intensive experimental and theoretical studies. This is caused by the possible occurrence of a Bose-Einstein condensation of magnons in the AFM phase and the proximity to a spin-liquid state beyond it. The Cu2+ in Cs2CuCl4 are arranged in planar triangular lattices within the bc plane. The magnetic inter-plane interaction along the a axis is much weaker than the in-plane interaction. Therefore, the material can be regarded as magnetically low dimensional. We measured the sound velocity and attenuation of the longitudinal acoustic c22 mode, i.e., the sound wave propagating along the b axis, in fields up to 15 T applied along the same direction and for temperatures between 0.3 and 7 K. We found that below 1.5 K this mode softens with increasing magnetic field, whereas in the vicinity of the critical field, at about 8.9 T, the sound velocity increases again. Near this field, the sound attenuation attains a maximum and both acoustic characteristics depend strongly on frequency. We analyzed our results in the framework of an effective quantum theory based on the exchange-striction coupling. The theoretical results agree qualitatively with our data.

  • Poster
    RHMF09 (Research In High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13023

High-field magnetization of the Ho2Fe17 single crystals

Skourski, Y.; Kuzmin, M. D.; Skokov, K.; Andreev, A. V.; Wosnitza, J.

Most of the modern permanent magnet materials are based on the combination of a 3d transition metal (T) and a rare earth (R). The inter-sublattice exchange field binds the high magnetic moment of the former with the high anisotropy of the latter, either ferri- or ferromagnetically. Applying high external magnetic field one can break down the ferrimagnetic ground state and drive the system towards ferromagnetic order via a non-collinear intermediate phases. This manifests itself in peculiarities of the magnetization curve. In some cases the value of the exchange field can be extracted form those experiments [1, 2]. Here we report on a magnetization measurement on single crystals of Ho2Fe17 in the fields up to 60 Tesla. Magnetization measured in the basal plane shows jumps. The highest one, measured along [100] is observed at about 55 Tesla . Considering that both R and T moments rotation is constrained within the basal plane we describe observed curve shape.

  • Poster
    RHMF09 (Research In High Magnetic Fields 2009), 22.-25.07.2009, Dresden, Deutschland

Publ.-Id: 13022

Influence of air entrainment on the liquid flow field caused by a plunging jet

Krepper, E.; Alt, S.; Renger, S.

Plunging jets play an important role in nuclear reactor safety research. In the actual paper the case of the strainer clogging issue is considered. During a loss of coolant accident (LOCA) insulation debris might be released near the break. Fractions of the released insulation debris can be transported into the reactor sump, where it may perturb/impinge on the emergency core cooling systems. According to the considered scenario, the water ejected by the anticipated break falls several meters on to the sump water surface. On its way, the jet is mixed with air. Furthermore, air bubbles are entrained by the impinging jet. The entrained gaseous bubbles will rise and have an additional influence on the flow field. The jet-induced flow into the sump will sensitively influence the fiber transport.
In the CFD calculations presented in the paper the consequences of the entrained air on the liquid flow field, on the fibre deposition and on the temperature mixing are investigated and compared to experiments.

Keywords: fibres; experiments; cfd; multiphase flow

  • Contribution to proceedings
    Nuclear Energy for New Europe 2009, 14.-17.09.2009, Bled, Slovenia
  • Lecture (Conference)
    Nuclear Energy for New Europe 2009, 14.-17.09.2009, Bled, Slovenia

Publ.-Id: 13021

Investigation of defects in nitrogen implanted n-type ZnO by capacitance spectroscopy and simultaneous optical excitation

Schmidt, M.; Ellguth, M.; Lüder, T.; von Wenckstern, H.; Pickenhain, R.; Grundmann, M.; Brauer, G.; Skorupa, W.; Helm, M.

  • Poster
    25th International Conference on Defects in Semiconductors (ICDS-25), 20.-24.07.2009, Petersburg, Russia

Publ.-Id: 13020

Characterization of single ZnO nanorods by conductive atomic force microscopy

Beinik, I.; Teichert, C.; Brauer, G.; Anwand, W.; Chen, X.; Hsu, Y. F.; Djurisic, A. B.

  • Poster
    International Symposium Piezoresponse Force Microscopy and Nanoscale Phenomena in Polar Materials (PFM-2009), 23.-27.06.2009, Aveíro, Portugal

Publ.-Id: 13019

Electroluminescence, charge trapping and quenching in Eu implantes SiO2-Si structures

Tyagulskiy, S.; Tyagulskyy, I.; Nazarov, A.; Lysenko, V.; Rebohle, L.; Lehmann, J.; Skorupa, W.

This paper reports an analysis of the electroluminescence (EL) spectra changing, charge trapping and EL quenching during operation of the multicolor Eu implanted metal-oxide-silicon light-emitting devices (MOSLEDs). The nature of the changing of the light-emitting color in the MOSLED is discussed. It is shown that the EL life time of the Eu implanted MOSLED is considerably longer than that of high-efficiency MOSLEDs with Tb and Ge implanted oxide. It is demonstrated that a reduced EL intensity, enhanced negative charge trapping and a good EL stability are associated with enhanced clustering in the Eu implanted oxide during high-temperature furnace annealing. The comparison with the operation of the MOSLED fabricated by using the flash lamp annealing technique is performed.

Keywords: Silicon dioxide; Europium; Electroluminescence

  • Microelectronic Engineering 86(2009)7-9, 1954-1956

Publ.-Id: 13018

Pulsed field facility in Dresden: Magnet technology and some recent ultrasonic results

Zherlitsyn, S.

The Dresden High Magnetic Field Laboratory (HLD) is a user facility which provides external users with the possibility of performing diverse experiments in pulsed magnetic field. Various experimental techniques, such as electrical transport, magnetization, ultrasound and magnetic-resonance measurements are available or are under installation at the HLD [1]. A particular feature of the laboratory is a nearby free-electron laser facility which enables high-field infrared spectroscopy in pulsed magnetic fields.
A 50 MJ modular capacitor bank with a maximum charging voltage of 24 kV is used to energize the pulsed magnets at the HLD. At present, a variety of pulsed magnets are in operation in the laboratory. I report on current status of the pulsed magnet program and recent progress at the HLD. I consider various issues of design, fabrication, and performance of the non-destructive pulsed magnets. Further magnet-technology developments and the route to 100 T are discussed.
In the second part of my talk I will present some ultrasonic results obtained at the HLD. This includes a magneto-acoustic study of the quantum S = 1 spin-chain magnet NiCl2-4SC(NH2)2 (DTN) near the quantum critical points and some recent ultrasonic results for CdCr2O4, a geometrically frustrated magnet with a metamagnetic phase transition at 28 T followed by a very wide magnetization plateau with one half of the full moment of S = 3/2 Cr3+. I discuss a possible application of the infrared radiation produced by next-door free electron lasers for photoacoustic spectroscopy.

  • Lecture (others)
    Einladung an das NHMFL Los Alamos, 22.-26.04.2009, Los Alamos, USA

Publ.-Id: 13017

Influence of Annealing on Mn Implanted GaAs Films

Bürger, D.; Zhou, S.; Grenzer, J.; Reuther, H.; Anwand, W.; Pandey, M.; Gottschalch, V.; Helm, M.; Schmidt, H.

Magnetic semiconductors with high Curie temperatures are very promising materials for spintronic applications. An approach to fabricate GaMnAs is the Mn-implantation of GaAs followed by pulsed laser annealing (PLA) [1,2]. We investigated the influence of the Mn concentration and PLA conditions, e.g. number of laser pulses, on the structural and magnetic properties of (001)-oriented GaMnAs. Results from heatflow calculations helped us to understand the PLA process. Using SQUID magnetometry, we reveal a strong decrease of the saturation magnetization with increasing number of laser pulses. Zero field cooled/ field cooled measurements were performed to investigate the magnetization of the annealed GaMnAs layer. We found a spontaneous magnetization below the Tc and a large out-of- plane anisotropy. HR-XRD measurements revealed a lattice expansion normal to the surface after implantation. In dependence on the number of pulses, PLA decreases the strain (1 pulse) or overcompensates the strain (10 and 100 pulses). We conclude that Mn-implantation into GaAs followed by 1 laser pulse allows for the fabrication of strongly anisotropic, diluted magnetic GaMnAs. The drawback of the Mn-implantation is the loss of As from the GaAs surface as detected by means of Auger electron spectroscopy. Co-implantation with suitable elements is a possible approach to countervail the magnetic properties of annealed GaMnAs.

[1] M. A. Scarpulla, O. D. Dubon, K. M. Yu, O. Monteiro, M. R. Pillai, M. J. Aziz, and
M. C. Ridgway, Appl. Phys. Lett. 82, 1251 (2003).

[2] M. A. Scarpulla, R. Farshchi, P. R. Stone, R. V. Chopdekar, K. M. Yu, Y. Suzuki, and
O. D. Dubon, J. Appl. Phys. 103, 073913 (2008).

Keywords: diluted magnetic semiconductor; ferromagnetism; pulsed laser annealing

  • Poster
    Fifth International School and Conference on Spintronics and Quantum Information Technology, 07.-11.07.2009, Kraków, Poland

Publ.-Id: 13016

CFD Modellierung einer partikelbelasteten Kühlmittelströmung im Sumpf und in der Kondensationskammer

Krepper, E.; Cartland-Glover, G.; Grahn, A.

Der Bericht beschreibt die Arbeiten zur CFD-Modellentwicklung zur Beschreibung des Fasertransportes in einer Wasserströmung, die im Unterauftrag der Hochschule Zittau/Görlitz erfolgten. Während die experimentellen Arbeiten zu dieser Thematik in Zittau durchgeführt wurden, lag der Schwerpunkt der theoretischen Arbeiten in Rossendorf.
Im Arbeitspunkt EZ 1 des Projektantrages ist die Erweiterung der Einzeleffektuntersuchungen vorgesehen. Die entsprechenden Modellansätze zum Partikeltransport sind im Kapitel 3.1. beschrieben. Die Modellanpassung und Validierung ist in 3.2 und 3.3 dargestellt.
Der Fasertransport in einer Wasserströmung wird durch Jet-Phänomene bestimmt. Untersuchungen dazu sind im EZ3.1 des Projektantrages: „3D-Phänomene infolge Blasenmitriss“ vorgesehen und die Modellansätze und der Vergleich zu Experimenten in den Kapiteln 4.1 bis 4.3 dargestellt. Des Weiteren wird der Einfluss auf den Ausgleich der Temperatur für den Fall untersucht, dass der Jet kälter als die Wasservorlage im Tank ist. Dieser Abschnitt entspricht damit der EZ3.2 des Antrages: „3D-Phänomene infolge Temperaturdifferenzen“. Im Kapitel 4.4 wird auf die Strömungsvorgänge in der Zittauer Strömungswanne eingegangen und damit der Punkt EZ4 des Antrages: „Integraluntersuchungen“ bearbeitet.
Kapitel 5 beschreibt die Entwicklung eines Sieb-Modells, das die Faser-Kompaktierung berücksichtigt und auf der Darcy-Gleichung basiert. Die Modellparameter werden an Experimenten in Zittau justiert. Diese Experimente wurden für verschiedene Materialien durchgeführt und mit deren Hilfe ein Koeffizientenkatalog erstellt. Das Modell wurde in den CFD-Code CFX implementiert und anhand einiger Anwendungsbeispiele demonstriert.

Keywords: CFD; fibres; water flow; sedimentation; agglomeration; strainer

  • Open Access Logo Wissenschaftlich-Technische Berichte / Helmholtz-Zentrum Dresden-Rossendorf; FZD-521 2009
    ISSN: 1437-322X


Publ.-Id: 13015

Dilution of Mn in Ge: the evidence from samples’ electrical and magneto-transport properties

Zhou, S.; Buerger, D.; Heera, V.; Potzger, K.; Fassbender, J.; Helm, M.; Schmidt, H.

The investigation of Mn in Ge was motivated by its potential application as spintronics material. Recently various groups realized that two phases of Mn in Ge:Mn, prepared by ion implantation and MBE [1], coexist: diluted Mn ions and a Mn-rich secondary phase. Usually the Mn-rich secondary phase is believed to be responsible for the observed ferromagnetism. In this contribution, we provide direct evidence for the dilution of Mn in Ge by electrical and magneto-transport investigation. Mn ions were implanted into semi-insulating Ge wafers. We observed p-type conductivity in the implanted surface layer with the thermal activation energy similar to that of heavily doped Ge [2]. The observed anomalous Hall effect (AHE) cannot be explained by the presence of Mn5Ge3 clusters, since the AHE does not mimic the hysteresis of magnetic moments probed by SQUID. We propose that the dilution of Mn in Ge results in a large spin-splitting of the valence band, however the concentration of diluted Mn is not large enough to develop ferromagnetic coupling. [1] Appl. Phys. Lett. 88, 061907 (2006); Phys. Rev. B 77, 045203 (2008). [2] Phys. Rev. 100, 659 (1955), Phys. Rev. Lett. 91, 177203 (2003).

  • Poster
    International Conference on Magnetism, 26.-31.07.2009, Karlsruhe, Germany

Publ.-Id: 13014

The Mobilization of Actinides by Microbial Ligands Taking into Consideration the Final Storage of Nuclear Waste - Interactions of Selected Actinides U(VI), Cm(III), and Np(V) with Pyoverdins Secreted by Pseudomonas fluorescens and Related Model Compounds (Final Report BMBF Project No.: 02E9985)

Moll, H.; Glorius, M.; Barkleit, A.; Roßberg, A.; Bernhard, G.

The groundwater bacterium Pseudomonas fluorescens (CCUG 32456) isolated at a depth of 70 m in the Äspö Hard Rock Laboratory secretes a pyoverdin-mixture with four main components (two pyoverdins and two ferribactins). The dominant influence of the pyoverdins of this mixture could be demonstrated by an absorption spectroscopy study.
The comparison of the stability constants of U(VI), Cm(III), and Np(V) species with ligands simulating the functional groups of the pyoverdins results in the following order of complex strength: pyoverdins (PYO) > trihydroxamate (DFO) > catecholates (NAP, 6­HQ) > simple hydroxamates (SHA, BHA). The pyoverdin chromophore functionality shows a large affinity to bind actinides.
As a result, pyoverdins are also able to complex and to mobilize elements other than Fe(III) at a considerably high efficiency. It is known that EDTA may form the strongest actinide complexes among the various organic components in nuclear wastes. The stability constants of 1:1 species formed between Cm(III) and U(VI) and pyoverdins are by a factor of 1.05 and 1.3, respectively, larger compared to the corresponding EDTA stability constants. The Np(V)-PYO stability constant is even by a factor of 1.83 greater than the EDTA stability constant. The identified Np(V)-PYO species belong to the strongest Np(V) species with organic material reported so far. All identified species influence the actinide speciation within the biologically relevant pH range.
The metal binding properties of microbes are mainly determined by functional groups of their cell wall (LPS: Gram-negative bacteria and PG: Gram-positive bacteria). On the basis of the determined stability constants raw estimates are possible, if actinides prefer to interact with the microbial cell wall components or with the secreted pyoverdin bioligands. By taking pH 5 as an example, U(VI)-PYO interactions are slightly stronger than those observed with LPS and PG. For Cm(III) we found a much stronger affinity to aqueous pyoverdin species than to functional groups of the cell wall compartments. A similar behavior was observed for Np(V). This shows the importance of indirect interaction processes between actinides and bioligands secreted by resident microbes.

Keywords: Uran; Neptunium; Curium; Pseudomonas fluorescens; Bioligands; Hydroxamate; Pyoverdins; Spectroscopy; TRLFS; fs-TRLFS; UV-vis-NIR; XAS; Complexation; Repository

  • Other report
    Dresden: Forschungszentrum Dresden-Rossendorf, 2009
    119 Seiten
  • Open Access Logo Wissenschaftlich-Technische Berichte / Helmholtz-Zentrum Dresden-Rossendorf; FZD-522 2009
    ISSN: 1437-322X


Publ.-Id: 13013

Electromagnetic flow control in the process of aluminium investment casting

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

The mould filling process of aluminium investment casting consists basically of the flow in a U-bend showing a high pouring velocity at the beginning and decreasing velocity values during the course of the process. The high velocities during the starting phase are supposed to cause distinct problems like bubble or inclusion entrapment.
We present results on the design and application of a DC magnetic field to control the pouring velocity. Numerical 3d transient calculations were performed to simulate the filling process and the effect of the magnetic field. In parallel, model experiments with a plexiglas model have been performed using the low melting eutectic GaInSn. Ultrasonic Doppler velocimetry was applied to carry out detailed velocity measurements in the model. These measurements served for the validation of the numerical calculations, thus allowing to scale up the simulations to the realistic aluminium casting process.
The developed DC field system has been tested under industrial conditions. The amplitude of the DC field was tuned during the process as the braking action is only needed during the first part of the process. In this way, a clear reduction of the peak velocities is obtained without a significant prolongation of the overall filling time. A multitude of investment casting units have been produced showing a significant diminishment of defects due to the magnetic field control of the pouring process.

  • Lecture (Conference)
    Numerical simulation of MHD-problems in industrial applications with liquid metals, 11.-12.12.2008, Linz, Austria

Publ.-Id: 13010

Numerical simulation of two-phase flow in a casting steel mold

Miao, X.; Galindo, V.; Gerbeth, G.; Lucas, D.; Ren, Z.

In this work, we will concentrate on the impact of the argon bubbles in the liquid metal concast mould with and without employed static magnetic field. An inhomogeneous Eulerian–Eulerian multi-phase model built-in commercial CFX software was adopted, taking into account the effect of different bubble sizes. The calculations show that argon gas bubbling increases the probability of an asymmetric instability and even unbalances the two-roll flow pattern in the slab mould with increasing mass flow rate of the gas. On the other hand, magnetic fields are an attractive contactless possibility in order to influence the liquid steel flow in the mould of the continuous casting process. For the magnetic field control, a steady magnetic field is applied either over the mould span or just focussed to the nozzle region. The simulations show that the magnetic field primarily damps the local velocities in the mould with complex consequences on the local flow structure.

  • Lecture (Conference)
    Numerical simulation of MHD-problems in industrial applications with liquid metals, 11.-12.12.2008, Linz, Austria

Publ.-Id: 13009

Liquid metal Taylor-Couette experiment on the magnetoreotational instability

Gundrum, T.; Stefani, F.; Gerbeth, G.; Szklarski, J.; Rüdiger, G.; Hollerbach, R.

The magnetorotational instability (MRI) plays an essential role in the formation of stars and black holes. By destabilizing hydrodynamically stable Keplerian flows, the MRI triggers turbulence and enables outward transport of angular momentum in accretion discs. We present the results of a liquid metal Taylor-Couette experiment under the influence of helical magnetic fields that show typical features of MRI at Reynolds numbers of the order 1000 and Hartmann numbers of the order 10. Particular focus is laid on an improved experiment in which split end caps are used to minimize the Ekman pumping.

  • Lecture (Conference)
    6th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering, 09.-11.09.2008, Prague, Czech Republic

Publ.-Id: 13008

Numerical and experimental modelling of the melt flow in a travelling magnetic field for Vertical Gradient Freeze crystal growth

Galindo, V.; Lantzsch, R.; Pätzold, O.; Gerbeth, G.

A traveling magnetic field (TMF) driven convection and its transition from a laminar to a time-dependent flow is studied by means of ultrasonic Doppler velocimetry and numerical simulations.
The experimental setup comprises a cylindrical cavity containing the electrically conducting model fluid GaInSn and a system of six equidistant coils, which are fed by an out-of-phase current to create an up- or downward directed TMF. Hence, a Lorentz force is induced in the melt which leads to meridional flow patterns. For numerical simulations commercial codes (Opera/Fidap) and a spectral code are used. The characteristic parameters of the magnetohydrodynamic model system are chosen close to the conditions used for Vertical Gradient Freeze (VGF) crystal growth.
The axisymmetric basic flow and its dependence on the dimensionless shielding parameter S are examined. It is shown that, for S>10, the flow velocity decreases significantly, whereas almost no influence is found for a smaller shielding parameter. The critical Reynolds number for the onset of instability is found in the range of 300-450. Good agreement between experimental results and the numerical simulations is achieved.

  • Lecture (Conference)
    Annual Meeting GAMM2008, 02.-04.04.2008, Bremen, Germany

Publ.-Id: 13007

The liquid lead target at nELBE

Galindo, V.; Erlebach, S.; Gerbeth, G.; Weiss, F.-P.

We report on the development of a lead target fort he pulsed neutron source at ELBE.

  • Lecture (Conference)
    Workshop on modern methods using fast neutrons, 13.-15.02.2008, Dresden, Germany

Publ.-Id: 13006

Cyclotron resonance absorption of 2D holes in strained InGaAs/GaAs quantum wells under high magnetic fields

Drachenko, O.; Winnerl, S.; Schneider, H.; Helm, M.; Wosnitza, J.; Kozlov, D.; Maremyanin, K.; Ikonnikov, A.; Gavrilenko, V.; Zvonkov, B.; Goiran, M.; Leotin, J.; Fasching, G.

We report a systematic study of the cyclotron resonance absorption of two-dimensional holes in strained InGaAs/GaAs quantum wells under high magnetic fields up to 60 Tesla. The energies of the CR transitions are traced as a function of magnetic field. A remarkable CR line splitting was evidenced when the resonant field exceeds 20 T. We analyze our data with a 4x4 Luttinger Hamiltonian including strain and QW potentials using two different methods to calculate Luttinger parameters for ternary alloys.

Keywords: Cyclotron resonance; effective mass; strain; quantum wells; InGaAs

  • Poster
    International Conference on Research in High Magnetic Fields, 22.-25.07.2009, Dresden, Germany

Publ.-Id: 13005

Quantum transport and cyclotron resonance study of Ge/SiGe quantum wells in high magnetic fields

Miura, N.; Kozlova, N.; Dörr, K.; Freudenberger, J.; Schultz, L.; Drachenko, O.; Sawano, K.; Shiraki, Y.

Shubnikov-de Haas oscillation and cyclotron resonance were studied for high mobility p-type Ge channels in strained Ge/Si1-xGex quantum wells (well width was 7.5 nm). The samples were grown on Si (001) substrates by gas source MBE. The width of the quantum wells was 7.5 nm. Pulsed high magnetic fields up to 50 T were employed for the measurement. Fine quantum oscillations were observed in rxx, although the quantum Hall plateaux in rxy were slightly deformed due to the pulsed field measurements. Reflecting the quantum effect in the nearly degenerate valence bands, the Fourier transform of the oscillatory spectra consist of several peaks. From the temperature dependence of the oscillation amplitude of a peak at B = 11 T, the effective mass was obtained as 0.17 m0. Cyclotron resonance was measured for two samples with different Ge concentration x which provides different strain. Sample A is the same as the one studied in the transport experiment (x = 0.41), and the sample B has the larger Ge content (x = 0.53). Photon energy was varied in the range between 10 – 17 meV using a FEL of ELBE. Two well-defined resonance peaks were observed in both samples. The effective mass obtained from the peak at the low field side at the lowest photon energy (10.5 meV) is 0.175 m0 and 0.154 m0 for samples A and B, close to the value obtained from the Shubnikov de-Haas experiment. However, the photon energy vs. resonance field Br curves are strongly non-linear in the higher energy. They are super-linear below photon energy of 14.1 meV and a sudden jump of the Br is observed at 16.5 meV, implying the existence of a prominent break of the curves. These results should be analyzed by the calculation of Landau levels in the degenerate valence bands of the quantum wells with strain.

Keywords: cyclotron resonance; SiGe; strain; magnetotransport; effective mass

  • Contribution to proceedings
    9th International Conference on Research in High Magnetic Fields, 22.-25.07.2009, Dresden, Germany
    Journal of low temperature physics


Publ.-Id: 13004

Cyclotron resonance of extremely conductive 2D holes in high Ge content strained heterostructures

Mironov, O. A.; Goiran, M.; Galibert, J.; Drachenko, O.; Helm, M.; Wosnitza, J.; Kozlov, D. V.; Ikonnikov, A. V.; Gavrilenko, V. I.; Kummer, M.; von Känel, H.; Morris, R. J. H.; Leadley, D. R.

Cyclotron resonance (CR) and magneto-transport have been studied in pulsed (40 T) and quasi-static (15 T) magnetic fields, for holes in highly conductive “pure Ge” strained channels. Modulation doped structures with 10-20 nm thick “Ge” quantum wells (which in fact contained 2-5% Si) were grown (i) by hybrid-epitaxy, combining UHV-CVD for the Si0.4Ge0.6 strain-tuning buffer and low temperature SS-MBE for the “Ge channel” [1], with a hole mobility of 27,000 cm2/Vs at a density of 1.8 ×1012cm−2 that represents a higher conductance than other “high-Ge” heterostructures and (ii) by LEPE-CVD [2] on a Si0.3Ge0.7 buffer, having a mobility of 47,000 cm2/Vs at 0.61012 cm-2.

Very good CR lines were obtained for all samples tested across the wavelength range 70 μm< λ <871 μm and for temperatures 4-200 K. These CR results confirmed the presence of 2DHGs with extremely high mobility, above 55,000 cm2/Vs at 4K, and the lowest cyclotron effective mass mCR= 0.11 m0 agrees well with that obtained from Shubnikov-de Haas oscillations (SdHO). At the highest excitation energies (70-96μm), a splitting of the CR line appeared. This is consistent with beating seen in pulsed field SdHO that provides evidence for the, unexpected, occupation of two hole subbands.

The results will be compared to calculations of the hole Landau levels and CR transition energies as a function of magnetic field using a 4x4 Luttinger Hamiltonian that also includes strain and the real triangular profile. A full interpretation requires the effects of hole-hole interactions (HHI) on both CR and SdHO. These HHIs are extremely important for such high mobility holes, as we previously demonstrated by considering transport in the diffusive and ballistic regimes [3].

1. R J H Morris et al 2004 Semicond. Sci. Technol. 19 L106-L109.
2. Benjamin Rößner, Giovanni Isella and Hans von Känel 2003 Appl. Phys. Lett. 82, 754-756.
3. I.B. Berkutov et al 2006 Low Temp. Phys. 32 No 7 683-688.

Keywords: Cyclotron resonance; Strain; quantum well; SiGe; magnetic field; effective mass

  • Contribution to proceedings
    9th International Conference on Research in High Magnetic Fields, 22.-25.07.2009, Dresden, Germany
    Low temperature physics


Publ.-Id: 13003

Anwendung von CFD-Methoden für den Kern sowie den Primärkreislauf von LWR

Höhne, T.

Sektion 2 Thermo- und Fluiddynamik / Thermodynamics and Fluid Dynamics
Sitzung: Anwendung von CFD-Methoden für den Kern sowie den Primärkreislauf von LWR
Die Leitung der Sitzung hatte Herr Dr. Th. Höhne vom Forschungszentrum Dresden- Rossendorf e.V., Dresden inne.

Keywords: Jahrestagung; Sektion 2; Technische Sitzung

  • atw - International Journal for Nuclear Power 8/9(2009), 546-548

Publ.-Id: 13002

Low-temperature modeling for degenerate and frustrated Heisenberg systems with anisotropy

Kunze, T.; Gemming, S.; Numazawa, S.; Schreiber, M.

Thermodynamic equilibration of complex systems like spin glasses or (degenerate) anisotropic spin crystals by numerical methods can be challenging due to the presence of multiple minima on the potential energy surface. This problem becomes pronounced especially at low temperature, where the system remains mainly in few states. We employ the anisotropic Heisenberg model in two dimensions to simulate and analyze the domain formation and the domain structure of multiferroic oxides. In particular, we discuss various techniques to improve the low-temperature equilibration behavior by means of a trigonal antiferromagnet with single-ion anisotropy. Furthermore, we present a localized sampling method for the Metropolis algorithm, which increases the acceptance ratio significantly.

Publ.-Id: 13001

Modification of the magnetic and the structural properties of Pt/Cr/Co multilayers by He+-ion irradiation

Tripathi, J. K.; Kanjilal, A.; Rajput, P.; Gupta, A.; Som, T.

We report on the effects of 2 MeV He+-ion irradiation on the magnetic and structural properties of Pt/Cr/Co multilayers. We observe He+ ion irradiation leads to mixing across the interfaces [Pt (2.5 nm)/Cr (0.8 nm)/Co (3.0 nm)] x 6/Si multilayers. In addition, we observe Co–Cr–Pt phase formation at the highest fluence of 5.5 x 1016 ions cm2. This is accompanied by an enhancement in the coercivity. Such enhancement in the coercivity is attributed to inhomogeneous alloying and a possible mixing-induced strain. High-resolution transmission electron microscopy confirms the formation of CoCrPt ternary alloy phase. These findings are explained in the light of ion beam induced recoil mixing and ionization events.

Keywords: multilayers; TEM; magnetic properties

  • Nuclear Instruments and Methods in Physics Research B 267(2009), 1608-1611

Publ.-Id: 13000

Structural and optical properties of Mn-doped CdS thin films prepared by ion implantation

Chandramohan, S.; Kanjilal, A.; Tripathi, J. K.; Sarangi, S. N.; Sathyamoorthy, R.; Som, T.

We report on structural and optical properties of Mn-doped CdS thin films prepared by 190 keV Mn-ion implantation at different temperatures. Mn-ion implantation in the fluence range of 1x 1013-1x1016 ions/cm2 does not lead to the formation of any secondary phase. However, it induces structural disorder, causing a decrease in the optical band gap. This is addressed on the basis of band tailing due to creation of localized energy states and Urbach energy calculations. Mn-doped samples exhibit a new band in their photoluminescence spectra at 2.22 eV, which originates from the d-d 4T16AT1 transition of tetrahedrally coordinated Mn2+ ions.

Keywords: CdS; implantation; optical properties

Publ.-Id: 12999

Investigating the role of hydrogen in indium oxide tubular nanostructures as a donor or oxygen vacancy passivation center

Kumar, M.; Chatterjee, R.; Milikisiyants, S.; Kanjilal, A.; Voelskow, M.; Grambole, D.; Lakshmi, K. V.; Singh, J. P.

Electron paramagnetic resonance (EPR) spectroscopy of hydrogen-doped indium oxide (IO) tubular nanostructures shows presence of paramagnetic oxygen vacancies (Vo) at room temperature. For temperatures below 80 K, the EPR spectra exhibit two distinct split resonances correspond to S = 1/2 hydrogen electron spin. Interestingly, presence of hydrogen EPR resonances is accompanied by absence of EPR signal of Vo, which is restored above 80 K with the concomitant disappearance of signature resonances from hydrogen. The temperature dependent donor and passivation behavior of hydrogen has been directly observed in metal oxide. This could provide valuable explanations of various Vo induced controversial properties of IO nanostructures.

Keywords: EPR; Hydrogen; indium oxide; nanostructures; vacancy

  • Applied Physics Letters 95(2009), 013102-1-013102-3

Publ.-Id: 12998

Defect-engineered blue-violet electroluminescence from Ge nanocrystal rich SiO2 layers by Er doping

Kanjilal, A.; Rebohle, L.; Voelskow, M.; Skorupa, W.; Helm, M.

Using combined microstructural and electroluminescence (EL) investigations of the Er-doped Ge-rich SiO2 layers, it is established that the Ge-related oxygen-deficiency centers GeODCs, which are associated with the 407 nm light emission, are situated at the Ge nanocrystal/SiO2 interface. Electrically driven energy transfer from the Er3+ to GeODCs causes an increase in the 407 nm EL intensity. It reaches a maximum before quenching with increasing Er concentration due to the crystalline-to-amorphous transition of Ge nanocrystals. Ge concentration dependent quenching of the maximum EL intensity and the peak shifting toward higher Er concentration are discussed in terms of the reduction of the surface-to-volume ratio with increasing nanocrystal size.

Keywords: Ge nanocrystals; Er; electroluminescence; microstructure

  • Journal of Applied Physics 106(2009), 026104-1-026104-3

Publ.-Id: 12997

The 0.987MeV resonance in the 14N(p,gamma)15O reaction.

Marta, M.; Bemmerer, D.; Beyer, R.; Broggini, C.; Caciolli, A.; Erhard, M.; Fülöp, Z.; Grosse, E.; Gyürky, G.; Hannaske, R.; Junghans, A. R.; Menegazzo, R.; Nair, C.; Schwengner, R.; Szücs, T.; Trompler, E.; Wagner, A.; Yakorev, D.

The 14N(p,gamma)15O reaction is the bottleneck of the hydrogen burning CNO cycle. Recent studies of this reaction at E < 500 keV have led to a revision of the S-factor value. However, also data at higher energy are necessary to predict the extrapolated S-factor at energies of astrophysical interest. Here we report on a new study of the E = 0.987MeV (Ex = 8.284MeV) resonance carried out at the high-current FZD Tandetron in Dresden. Solid TiN targets and four escape-suppressed HPGe detectors have been used.

Keywords: Nuclear astrophysics; CNO cycle; Tandetron; TiN solid target

  • Poster
    Nuclear Physics in Astrophysics 4, 08.-12.06.2009, Frascati (LNF), Italy

Publ.-Id: 12996

A proposal for an unusually stiff and moderately ductile hard coating material: Mo2BC

Emmerlich, J.; Music, D.; Braun, M.; Fayek, P.; Munnik, F.; Schneider, J. M.

The elastic properties of Mo2BC were studied using ab initio calculations. The calculated bulk modulus of 324 GPa is by 45% larger than that of Ti0.25Al0.75N and 14% smaller than that of c-BN indicating a highly stiff material. The bulk modulus (B) to shear modulus (G) ratio is with 1.72 at the transition from brittle to ductile behavior. This, in combination with a positive Cauchy pressure (c12-c44), suggests moderate ductility. When compared to a typical hard protective coating such as Ti0.25Al0.75N (B = 178 GPa; B/G = 1.44; negative Cauchy pressure) Mo2BC displays great potential as protective coating for metal cutting applications. In order to test this proposal, Mo2BC thin films were synthesized using DC magnetron sputtering from three plasma sources on Al2O3(0001) at a substrate temperature of ~900°C. The calculated lattice parameters are in good agreement with values determined from xray diffraction. Measured Young’s modulus values of ~460±21 GPa are in excellent agreement with the 470 GPa value obtained by calculations. Scanning probe microscopy imaging of the residual indent revealed no evidence for crack formation as well as significant pile-up, which is consistent with the moderate plasticity predicted. The apparent contradiction between moderate ductility on one hand and indentation hardness values of 29 GPa can be understood by considering the electronic structure particularly the extreme anisotropy. The presence of stiff Mo-C and Mo-B layers with metallic interlayer bonding enables this intriguing and unexpected property combination.

Publ.-Id: 12995

Atomic Layer Deposition Growth of BaB2O4 Thin Films from an Exceptionally Thermally Stable Tris(pyrazolyl)borate-Based Precursor

Saly, M. J.; Munnik, F.; Baird, R. J.; Winter, C. H.

The atomic layer deposition growth of BaB2O4 thin films was investigated using Ba(TpEt2)2 and water as precursors between 240 and 400 °C. The process provided uniform films and exhibited a large ALD window between 250 and 375 °C, in which a constant growth rate of 0.23 Å/cycle was observed.

Publ.-Id: 12994

The Dresden High Magnetic Field Laboraory (HLD), a User Facility for Advanced Pulsed-Field Experiments

Herrmannsdörfer, T.; Wosnitza, J.; Zherlitsyn, S.; Zvyagin, S.

Since 2007, the Dresden High Magnetic Field Laboratory (Hochfeld-Magnetlabor Dresden, HLD) operates as a user facility, providing unique experimental possibilities in pulsed fields. The HLD offers various measurement techniques, such as electrical transport, magnetization, magnetostriction, ultrasound, and magnetic resonance. A particular feature of the laboratory is the next-door free-electron-laser installation used for high-field infrared spectroscopy cyclotron resonance, as well as electron spin resonance (ESR) in pulsed fields. Additionally nuclear magnetic resonance (NMR) and specific-heat measurement techniques are being developed in pulsed magnetic field. As the only laboratory in Europe, the HLD has reached magnetic fields of about 87 T. This field range is accessible now for experiments in modern materials research. Several 60 T and 70 T magnets are regularly used by in-house and external users as well. A two-coil 100 T prototype and a long-pulse (1000ms) 60 T magnet are ready for their first tests. Some recent scientific results on strongly correlated electron systems, nanostructures, low dimensional spin system, and high-Tc superconductors will be highlighted. Novel experimental techniques such as pulsed NMR in pulsed magnetic fields and technical applications for pulsed magnets will be demonstrated.

  • Invited lecture (Conferences)
    Helmholtz Zentrum Berlin, 28.04.2009, Berlin, Deutschland

Publ.-Id: 12993

High-field ESR in low-dimensional spin systems

Zvyagin, S. A.

es hat kein Abstract vorgelegen.

  • Lecture (others)
    Einladung an die P.J. Safarik University and Slovak Academy of Sciences, 18.-26.04.2009, Kosice, Slovakia

Publ.-Id: 12992

Antiferromagnetic resonance in the hexagonal YMnO3

Zvyagin, S. A.; Ozerov, M.; Kamenskii, D.; Wosnitza, J.; Ciznar, E.; Kolzhuk, A. K.; Smirnov, D.; Zhou, H. D.; Wiebe, C. R.

Multiferroic rare-earth manganites have attracted much attention because of the coexistence of ferroelectric and magnetic orders. Geometrical frustrations appear to be one of the most important factors contributing to the multiferroicity. Combining conventional far-infrared Fourier-transform and multi-frequency electron spin resonance (ESR) spectroscopy techniques, magnetic excitations in hexagonal multiferroic YMnO3 in the antiferromagnetically (AFM) ordered phase have been studied. The gap in the excitation spectrum, about 42 cm-1, has been observed directly. A fine structure of AFM resonance absorption was revealed by means of ESR, which can be explained taking into account a finite interaction between the neighboring Mn3+ layers. Our observations has allowed us to refine spin-Hamiltonian parameters, emphasizing the role of interlayer interactions in this frustrated compound.

  • Invited lecture (Conferences)
    2. Estonian-German Workshop, 20.-23.05.2009, Cottbus, Deutschland

Publ.-Id: 12991

FEL-based pulsed-field ESR technique: recent developments at the Dresden High Magnetic Field Laboratory

Ozerov, M.; Cizmar, E.; Kamenskyi, D.; Wosnitza, J.; Zvyagin, S. A.

The successful use of picosecond-pulse free-electron-laser (FEL) radiation for the continuous-wave THz-range electron spin resonance (ESR) spectroscopy has been demonstrated. The unique combination of two linac-based FELs [1] (covering the wavelength range of 4 - 250 μm) with high magnetic fields at the Research Center Dresden-Rossendorf (FZD) allows for tunable-frequency ESR spectroscopy in a frequency range of 1.2 - 75 THz in magnetic fields up to ~ 70 T with a spectral resolution better than 1% [2]. The new approach is of particular importance for studying magnetic excitations in spin systems with a large zero-field splitting and materials exhibiting field-induced phenomena. The performance of the spectrometer is illustrated with ESR spectra obtained in the low-dimensional organic material (C6H9N2)CuCl3 and the multiferroic compound YMnO3††.

  • Lecture (Conference)
    XXIII International EPR Seminar, 23.-25.04.2009, Bad Gottleuba, Deutschland

Publ.-Id: 12990

Antiferromagnetic resonance in multiferroic HoMnO3

Kamenskyi, D.; Ozerov, M.; Wosnitza, J.; Zvyagin, S. A.; Cizmar, E.; Zhou, H. D.; Wiebe, C. R.

Multiferroic rare-earth manganites have attracted much attention because of the coexistence of ferroelectric and magnetic orders. Among hexagonal manganites, the largest magnetoelectric effect was found in HoMnO3, which is ferroelectric below Tc = 875 K and antiferromagnetically ordered below TNMn = 75 K (Mn subsystem). In addition, at TNHo ~ 5 K the Ho subsystem undergoes a transition into an antiferromagnetically ordered state. The coupling between the ferroelectric and antiferromagnetic order parameters in HoMnO3 has not been yet unambiguously explained, revealing the importance of geometrical frustrations and a complex interplay between Ho and Mn magnetic subsystems [1, 2]. Here, the magnetic excitation spectrum in high-quality single-crystalline samples of HoMnO3 was probed by means of X-band (9.3 GHz) electron spin resonance spectroscopy. A relatively strong absorption was found below 5 K, which corresponds to the temperature of the magnetic ordering of the Ho subsystem. The observed mode exhibits a very pronounced anisotropic behavior and can be interpreted as excitations within the antiferromagnetically ordered Ho subsystem.

  • Lecture (Conference)
    XXIII International EPR Seminar, 23.-25.04,2009, Bad Gottleuba, Deutschland

Publ.-Id: 12989

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