Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

The amorphous indium tin oxide (ITO) films were grown by reactive pulsed middle frequency dual magnetron sputtering on the Si substrates covered with SiO₂. The real-time evolution of the ITO film structure during annealing was continuously investigated by synchrotron X-ray diffraction at ROssendorf Beam Line (ROBL), which is located at the European Synchrotron Radiation Facility in Grenoble, France. The annealing experiments were carried out in UHV annealing chamber equipped with beryllium dome at constant temperature of 310 C. In situ four-point probe technique was applied to characterize film resistivity. Three series of samples with thicknesses of ~50, 170 – 180, and 310-320 nm have been investigated.
The time dependence of the integral intensity exhibits an s-like shape, which is typical of the crystallization process. The XRD data were analyzed in a frame of the Kolmogorov – Johnson – Mehl – Avrami (KJMA) model. The kinetic parameters of crystallization process were determined. The range of KJMA exponent of approximately 2 obtained for all ITO films points to two-dimensional grain growth in the site saturated mode. However, detailed TEM studies of partly crystallized ITO samples display two-dimensional grain growth only during crystallization of 50 nm ITO films. In contrast, the thicker films show two stages of crystallization process. During the first stage, the single grains grew from a film surface to interface, after that, lateral growth of the grain was observed. The KJMA model can not predict the difference in the crystallization kinetics for the films with diverse thicknesses. The essential anisotropy and limited thickness of the films are discussed as the main reasons for the change of the crystallization kinetics.

Surface alloying of Ti–6Al–4V with Zr aiming at depletion of Al and V was realized by liquid-phase intermixing of multi-layer film [Zr(20 nm)/Ti(20 nm)]₁₂ of total thickness of 480 nm with substrate (Ti–6Al–4V) using a low energy (~20 keV), high-current electron beam (2.5 μs, 3.5 J/cm²). Scanning electron microscopy and energy dispersive X-ray spectroscopy, Auger electron spectroscopy, X-ray diffraction, and atomic force microscopy were used to study the surface morphology, microstructure, elemental and phase compositions of the alloyed layer. It was found that a homogeneous intermixing of all Zr/Ti nanolayers and diffusion of Zr into the substrate up to a depth of ≈1 μm take place after a singleshot pulsed melting. The surface layer of depth ≈ 0.5 μm is free of Al and V and contain ≈ 30 at % Zr. The near-surface layer, due to quenching from the melt, has a sub-microcrystalline grain structure with an average grain size of 112 nm. The post-irradiation vacuum annealing (500 °С, 2 h) leads to enrichment of alloyed layer with O and С impurities, and decrease of grain size up to 90 nm. As a result, the nanohardness of the alloyed layer was increased in comparison with the substrate.

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 µm. 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 µm). 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.

There is growing evidence showing that bacteria have contributed to the formation of minerals since the advent of life on Earth. Bacterial biomineralization thus appears to be crucial for the complex interactions of biological, chemical, and physical processes which drive modern and ancient biogeochemical cycles. On the other hand, the technological and environmental applications of bacterial mineralization are now being demonstrated to be far reaching. Despite the numerous efforts to better understand how bacteria induce/mediate or control mineralization, our current knowledge is far from complete. Considering that the number of recent publications on bacterial biomineralization has been overwhelming, here we attempt to show the importance of bacteria-mineral interactions by focusing in a single bacterial genus, Myxococcus, which displays an unusual capacity of producing mineral precipitates of varying compositions and morphologies. The first part of this review presents an overview of the recent history of bacterial mineralization, and briefly describes the most common bacteriogenic minerals as well as current models on bacterial biomineralization. The second part presents a description of myxobacteria. Myxococcus induced precipitation of a number of phosphates, carbonates, sulphates, chlorides, oxalates, and silicates is described and discussed in lieu of the information presented in the first part. Finally, implications of bacterial mineralization and perspectives for future research are outlined. This review strives to show that the mechanisms which control bacterial biomineralization are not mineral- or bacterial-specific. On the contrary, they appear to be universal and depend on the environment in which bacteria dwell.

High-quality Sn-doped In2O3 (ITO) films were grown epitaxially on yttria stabilized zirconia (111) with oxygen-plasma assisted molecular beam epitaxy (MBE). The 12 nm thick films, containing 2–6% Sn, are fully oxidized. Angle-resolved x-ray photoelectron spectroscopy (ARXPS) confirms that the Sn dopant substitutes In atoms in the bixbyite lattice. From XPS peak shape analysis and spectroscopic ellipsometry measurements it is estimated that, in a film with 6 at.% Sn, ~1/3 of the Sn atoms are electrically active. Reflection high energy electron diffraction (RHEED) shows a flat surface morphology and scanning tunneling microscopy (STM) shows terraces several hundred nanometers in width. The terraces consist of 10 nm wide orientational domains, which are attributed to the initial nucleation of the film. Low energy electron diffraction (LEED) and STM results show a bulk-terminated (1×1) surface, which is supported by first-principles density functional theory (DFT) calculations. Atomically resolved STM images are consistent with Tersoff–Hamann calculations that show that surface In atoms are imaged bright or dark, depending on the configuration of their O neighbors. The coordination of surface atoms on the In2O3(111)–1×1 surface is analyzed in terms of their possible role in surface chemical reactions.

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Self-assembled Si(Ge) quantum dots (QDs) may prove useful in future nanoelectronic device architectures. For this, they must be spatially arranged in specific patterns and possess the electronic or magnetic properties required for device operation. Previous work demonstrated Ga+ focused ion beam (FIB) templating of Si surfaces prior to epitaxial growth for fabricating patterned QDs with any desired complexity.1, 2
Our current research employs a mass-selecting FIB to template QD structures and to individually dope them. Ion species can be selected according to isotope mass and charge state using a Wien filter. Working with suitable liquid metal alloy ion sources (LMISs) provides the means to template with electrically non-invasive ions (e.g. Si+ from AuSi), then implant dopant ions for electronic or magnetic activation (e.g. with B+ from AsPdB or Mn+ from GeMn), with resolution of tens of nm and doses down to a few ions per dot.

Using synchrotron in-situ X-ray diffraction and grazing incidence small angle scattering, we studied the growth of FePt islands incorporated into a Ag matrix. The deposition, by dual DC magnetron sputtering on amorphous Si/SiO2 substrate, was carried out at 400°C to support the formation of the hard ferromagnetic L10-FePt phase during growth. Thin Ag interspacing layers provide a granular FePt film but, by depositing 6 nm Ag layer directly on the SiO2 substrate, we obtained well defined FePt clusters. FePt nanoislands have been achieved without degradation of the magnetic properties. We obtained a magnetic asymmetry with magnetic moments preferentially oriented parallel to the layer surface.

For the first time the hydrolysis reactions of submillimoar aqueous Np(VI) and U(VI) solutions was investigated comparatively, applying Attenuated Total Reflection Fourier-transform Infrared (ATR FT-IR) and NIR absorption spectroscopy. Actinyl(VI) concentration was set to 500 µM in a pH range from 2 to 5.3 under ambient conditions. Both spectroscopic methods give evidence that three different Np(VI) species contribute to the speciation. Using the structural information obtained from the FT-IR spectra these species are the fully hydrated NpO₂ ²⁺ ion at pH ≤ 3, a monomeric complex probably formed by hydrolysis reactions at pH 3 − 5 and a carbonate containing complex at pH ≥ 5. A comparison of the obtained results to the updated NEA thermodynamic data confirms the presence of the free neptunyl(VI) ion and neptunyl(VI) carbonate complexation within the investigated pH range. The monomeric hydrolysis complex is not predicted to be relevant by the current thermodynamic data. In comparison to U(VI) forms structural similar species at pH ≤ 4, namely the fully hydrated AnO₂ ²⁺ and monomeric hydroxo species. In contrast to Np(VI), the infrared spectroscopic data of U(VI) does not evidence carbonate complexation at higher pH.

The flow distribution in the rectangular channel of a pilot-scale electrolytic cell was observed using an imperfect tracer-pulse injection technique in order to study residence time distribution (RTD), back-mixing effects and velocity profiles of the electrolyte liquids within the cathode compartment. The electrolytic cell was operated under cold flow and at process conditions. While residence time studies performed in a vast number of applications are mainly based on classical conductometric measurements, alternative measurement methods have to be applied in electrochemical systems. Hence, the paper deals with the investigation of the residence time distribution (RTD) using laser induced fluorescence (LIF) visualization. Back-mixing effects of the electrolyte are quantified using the axial dispersion flow model. RTD studies for the overall cell as well as velocity profile analysis for selected individual regions are conducted. Additionally, the effects of the non-stabilized membrane and of the membrane stabilizing spacer grid are investigated.

Ultra-sensitive in-beam gamma-ray spectroscopy studies for nuclear astrophysics are performed at the LUNA (Laboratory for Underground Nuclear Astrophysics) 400 kV accelerator, deep underground in Italy's Gran Sasso laboratory. By virtue of a specially constructed passive shield, the laboratory gamma-ray background for E_\gamma < 3 MeV at LUNA has been reduced to levels comparable to those experienced in dedicated offline underground gamma-counting setups. The gamma-ray background induced by an incident alpha-beam has been studied. The data are used to evaluate the feasibility of sensitive in-beam experiments at LUNA and, by extension, at similar proposed facilities.

O podoru v Velikem vrhu (Košuta, Karavanke) ni zanesljivih zgodovinskih zapisov, vendar pa le ti obstajajo o podoru na Dobraču (25.1.1348), ki je od Velikega vrha oddaljen 46 km. Podor je povzročil potres in naša hipoteza je bila, da je tudi podor v Velikem vrhu posledica istega dogodka. Tako smo s pomočjo metode datiranja površinske izpostavljenosti analizirali vzorce matične kamnine v steni Velikega vrha ter vzorce s površine podornih blokov. Ugotavljali smo vsebnost ³⁶Cl, ki se je začel tvoriti po podoru. Na podlagi poznavanja števila atomov ³⁶Cl na gram Ca na leto izpostavljenosti, čas dogodka (podor) izračunamo iz koncentracij ³⁶Cl izmerjenih s pomočjo pospeševalnika (AMS). Prvi rezultati kažejo, da sta se podora na Dobraču in Velikem vrhu posledica potresa v Furlaniji Julijski krajini. Starost podora v Velikem vrhu je (740 +- 71) a. Za bolj zanesljive rezultate bo potrebno izboljšati sam model in upoštevati več dejavnikov - predvsem nadmorsko višino in klimatske razmere.

There are no reliable historical data about the Veliki vrh rockfall (Košuta, Karavanke Mountains) but the records exist about the Dobrach rockfall (25^th of January 1348) 46 km to the West of Veliki vrh. The rockfall was triggered by the earthquake and our hypothesis was that Veliki vrh rockfall was induced by the same event. Therefore we used the surface exposure dating method to analyze the bedrock samples in the Veliki vrh rock face as well as the samples of the boulder surface. The content of ³⁶Cl, that started to produce after the earthquake, was measured. On the basis of the known number of ³⁶Cl per gram Ca per year of exposure, the time of the rockfall can be calculated from the concentration of ³⁶Cl measured by accelerator mass spectrometry (AMS). Preliminary results show the correlation of the two rockfalls (Dobrach and Veliki vrh) as the consequence of the earthquake in Friuli Venezia Julia. The age of Veliki vrh rockfall is (740 +- 71) a. For more reliable results further research should be conducted using also the data on altitude and climate.

The chemical potential μ of a many-body system is valuable since it carries fingerprints of phase changes. Here, we summarize results for μ
for a three-dimensional electron liquid in terms of average kinetic and potential energies per particle. The difference between μ and the energy
per particle is found to be exactly the electrostatic potential step at the surface. We also present calculations for an integrable one-dimensional
many-body system with delta function interactions, exhibiting a BCS–BEC crossover. It is shown that in the BCS regime the chemical potential
can be expressed solely in terms of the ground-state energy per particle. A brief discussion is also included of the strong coupling BEC limit.

Theorieseminar TU Chemnitz

We investigate an interacting Bose gas using a scheme to eliminate successive collisions. For attractive interaction we find a two-particle bound state. When the binding energy of this bound state becomes twice the chemical potential there is a second order phase transition and a gap appears in the dispersion relation. The gap decreases with increasing density. At the critical point the gap vanishes, the dispersion becomes linear for small momenta and a Bose condensate appears. We interpret the appearance of the gap as a sign of structure formation.

International Workshop on Physics and Chemistry of FeAs-based Superconductors,
Dresden: Enhancement of Tc due to charge transfer

International Center for Condensed Matter Physics, Bras´ılia:

Talk for Professorship at University College Dublin

In order to investigate the two-phase flow behaviour in a complex reactor-typical geometry and to supply suitable data for CFD code validation, a model of the hot leg of a pressurised water reactor was built at Forschungszentrum Dresden-Rossendorf (FZD). The hot leg model is devoted to optical measurement techniques, therefore, a flat test section design was chosen and equipped with large windows. In order to enable the operation at high pressures, the test section is installed in the pressure chamber of the TOPFLOW test facility of FZD, which is used to perform the experiments under pressure equilibrium with the inside atmosphere. Counter-current flow limitation (CCFL) experiments were performed, simulating the refluxcondenser cooling mode appearing in small break LOCA scenarios. The fluids used were air and water at room temperature and pressures of up to 3.0 bar, as well as steam and water at pressures of up to 50 bar and the corresponding saturation temperature of 264°C. One selected 50 bar experiment is presented in detail: the observed behaviour is analysed and illustrated by typical high-speed camera images of the flow.

Furthermore, the flooding characteristic obtained from the different experimental runs are presented in terms of the Wallis parameter and Kutateladze number, which are commonly used in the literature. However, both parameters fail to properly correlate the data: a discrepancy is observed between the air/water and steam/water series. Therefore, a modified Wallis parameter is proposed, which takes into account the effect of the fluid viscosities on the CCFL. The new parameter is validated against comparable data found in the literature, even though no data was found with such a large range of viscosities. This analysis points out that the effect of the dynamic viscosity on flooding has already been observed, but not identified. Furthermore, it is shown that the proposed modification of the Wallis parameter allows a significant improvement for experimental series with variation of the viscosities.

In diesem Bericht werden die offenen Punkte OP1 bis OP3 aus der TÜV-Stellungnahme A-Nr.: 5576 vom 24.09.2008 durch ergänzende Nachweise zur von uns vorgelegten Borverdünnungsanalyse abgearbeitet.

Positron emission tomography (PET) is used for independent monitoring of dose delivery in ion therapy. An in-beam PET scanner registers the annihilation γ-rays following the decay of β+-radioactive nuclei produced via nuclear reactions between the ions of the therapeutic beam and the irradiated tissue. From a comparison of the reconstructed activity distributions with those predicted from the treatment plan, deviations between the prescribed and the applied dose can be detected. In-beam PET, therefore, allows to verify the physical beam model of the treatment planning, to detect patient dislocations and density changes in the irradiated tissue. Issues related to the image quality and evaluation of a whole PET imaging system are discussed in this paper.

To study the performance of the ELBE RF-couplers until their limits a 1.3 GHz resonant ring driven by a 10 kW CW klystron (VKL7811St, CPI) has been built to operate a RF-coupler and RF-window test bench. The ring allows tests with RF power up to 100 kW in continuous wave (CW) mode and 200 kW in pulsed mode.
Details of the resonant ring, the test bench design and results of the ELBE RF-coupler tests will be presented.

Using high plasma ion density influence during the reactive pulsed magnetron sputtering allowed formation of the dense amorphous Nb2O5 layers with controlled nano-porosity for optical applications. The films possess unique combination of high refractive index (n> 2.5 at wavelength 400 nm), low optical extinction (k<5x10^-4 at 400 nm), low mechanical stress and negligible thermal shift.

Al-doped ZnO (ZnO:Al) is a cost-effective alternative to the Sn-doped indium oxide which is widely used as a material for transparent electrodes. To further reduce the production costs, it would be preferable to sputter reactively from a metal alloy target at sufficiently high partial pressure of oxygen. However, under this condition, a sufficiently low resistivity of the films can not readily be obtained, so that a deposition on heated substrates is necessary. The mechanisms of incorporation and electrical activation of the Al doping impurity into ZnO at elevated temperatures are not well understood that makes difficult improvement of reproducibility and long-term stability of the film electrical properties.
In order to have a deeper insight into these processes, polycrystalline ZnO and ZnO:Al films were grown by reactive pulsed magnetron sputtering of Zn and Zn:Al (4.7 and 8.7 at. % of Al) targets, respectively. The O2 partial pressure in Ar-O2 sputtering gas mixture was precisely monitored during deposition using a capacitance gauge. The substrate temperatures (Ts) were spanning in the 40-580 °C range. The films were characterized by Hall effect measurements, spectroscopic ellipsometry (SE), X-ray diffraction (XRD) and X-ray absorption near edge spectroscopy (XANES). At fixed oxygen partial pressure, the electrical resistivity of ZnO:Al films shows a clear minimum at Ts=300 and 350 °C for the films grown using magnetron targets with 8.7 and 4.7 at. % of Al, respectively. The lower resistivity is due to the increased density and mobility of free electrons according to the Hall effect measurements. A maximum of the film crystallinity (grain size) is observed at temperatures of 250 and 300 °C for higher and lower Al concentration, respectively. At Ts>350 °C, the ZnO:Al film crystallinity significantly deteriorates with even stronger temperature effect at higher Al content, leading to formation of nanocrystalline ZnO:Al films at Ts>400 °C in the latter case. It is in contrast to undoped ZnO films grown at identical conditions whose crystallinity always improves by increasing Ts. XANES results show that the poorer film crystallinity and higher resistivity at high Ts can be related to a new homologous phase (ZnO)3(Al2O3) . Further, the electrical properties correlate with changes in the O(1s) absorption edge, whereas the Zn(2p) edge shows no modification with respect to undoped ZnO films.
The authors gratefully acknowledge the financial support of SMWA/SAB under the Project #11815/1854.

Intense excimer laser pulses, flash lamp annealing and rapid thermal annealing were used to form Si nanocrystals in thin SiO2 layers implanted with high doses of Si ions. The pulse durations were 20 ns, 20 ms and 1 s, respectively. Laser annealing produced light sources luminescing in the wavelength range of 400-600 nm. They were attributed to the Si clusters formed as a result of the fast segregation of Si atoms from the SiO2 network. There were no indications of nanocrystal formation in the as-implanted layers after 20 ns laser pulses; however, nanocrystals formed when, before the laser annealing, the amorphous Si nanoprecipitates were prepared in the oxide layers. Evaluations show that the crystallization may proceed via melting. A photoluminescence band near 800 nm, typical of Si nanocrystals, was found after 20 ms and 1 s anneals. Calculations revealed that the annealing times in both cases were too short to provide the diffusion-limited crystal growth if one uses the values of stationary Si diffusivity in SiO2. This points toward the existence of a transient rapid growth process at the very beginning of the anneals.

The insufficient oxidation resistance of TiAl-alloys above roughly 800°C is a major disadvantage for their use in several high temperature applications. This problem can be overcome by small amounts of halogens in the surface zone of the TiAl-material. Especially fluorine has proven to be a beneficial doping element. The fluorine effect is stable at least for one year at temperatures up to 900°C under thermocyclic conditions. Results of fluorine treated and untreated TiAl-coupons, more complex samples and real components are shown in this paper. The specimens were exposed isothermally and thermocyclically in the temperature range from 700 – 1050°C in laboratory air or synthetic air. The specimens can be treated with fluorine in several ways e.g. spraying or ion implantation. Without any treatment some samples were heavily destroyed during high temperature oxidation but after fluorine treatment the components were still intact. Post oxidation investigations revealed the formation of a thin and protective Al2O3-scale confirming the fluorine effect.

This work deals with surface structuring of gamma-titanium aluminide engineering alloy samples by using the oxidation behaviour of the material between 800 and 1000°C. Previous experiments showed that if the pattern of the structures is similar to that of a shark skin, aerodynamical losses due to shear strain at the surface of a solid moving in a flow are decreased. Tiny riblets arranged parallel to the flow direction hamper the cross flow which causes the most of the wall shear stress. Such improvement in the flow conditions is foreseen to be applied to compressor and turbine blades of aero engines for example. These components operate at temperatures between 500 and 1000°C and in strong temperature gradients. Therefore the surface structuring process should not prevent the growth of a protective oxide layer, necessary to protect the material against high temperature corrosion. Several structuring methods leading to self-growing of a shark skin pattern via partial microalloying of halogens (e.g. Fluor) in the first 100 nm of the sample surface were investigated. On the halogen- rich domains a thin protective Al-oxide layer grows slowly at temperatures up to 1000°C (so called “halogen effect”). On the domains free of halogen a thick mixed Al- and Ti-oxide layer grows fastly and builds the riblets. The growth of the non- protective mixed oxide layer should be stopped by lateral diffusion of the halogen from the halogen-rich to the halogen-free domains in order to prevent the spallation of the riblets. The principle and results on the development of such structures including the nucleation process depending on the halogen distribution over the sample surface are shown and discussed for isothermal oxidation at temperatures between 800 and 1000°C.

Surface modification of TiAl alloys by introducing fluorine is known to improve significantly their oxidation behaviour at high temperatures. The effect is based on the preferential reaction of the aluminium with the applied fluorine at the oxide/alloy interface, and is associated with the formation of an adherent and stable protective alumina layer.
Well-defined fluorine profiles beneath the surface of the material can be formed by either fluorine beam line ion implantation (BLI²) or plasma immersion ion implantation (PI3). As an alternative to the implantation-based approach, chemical fluorination techniques such as gas-phase treatment and dipping in F-based solutions have also been investigated in the present work. The fluorine depth profiles have been measured before and after oxidation at 900 °C using non-destructive ion beam analyses (Proton Induced Gamma-ray Emission, Rutherford Backscattering Spectroscopy and Elastic Recoil Detection Analysis). The results of this study enable one to optimize the fluorination conditions and to produce surface-modified technical TiAl alloys suitable for industrial applications.

Titanium aluminides are being used in an expansive variety of advanced applications requiring lightness in weight and retention of strength at elevated temperatures. To date, the upper temperature limit has been at about 700°C because of the poor oxidation resistance of the TiAl alloys at higher temperatures. It has recently been established that the TiAl alloys can be rendered highly resistant to environmental degradation by ion implanting halogens, notably fluorine. In modifying the oxidation properties of the TiAl alloys, strong preference has been given to the technique of plasma immersion ion implantation (PIII) because of the possibility to shorten substantially the implant times as well as to avoid the line-of-sight limitation inherent to the standard beamline implantation process. In this work, various TiAl alloys in the form of either rectangular coupons or actual components have been treated by PIII of fluorine. A cheap, easy-to-handle and environmentally-friendly hydrofluorocarbon (difluoromethane) mixed with argon has been used as the precursor gas to implant fluorine into the TiAl alloys. Energy Dispersive X-ray Analysis, Scanning Electron Microscopy, Rutherford Backscattering Spectrometry and Elastic Recoil Detection Analysis have been undertaken for sample characterization. Optimum processing windows and conditions have been identified under which the modified TiAl alloys acquire a stable, adherent and highly protective alumina scale against high-temperature oxidation. The degree of oxidation protection has been evaluated by testing F-implanted samples under conditions of both isothermal oxidation and thermal cyclic oxidation in air at temperatures up to 1050°C and for times as long as 6000 hours.

Titanium aluminides are promising lightweight materials for novel aerospace, automobile and power generation applications. However, because of their insufficient oxidation resistance at temperatures above 700°C, they cannot yet find broader use. The oxidation behaviour of TiAl-alloys can be improved considerably by adding small amounts of fluorine into the subsurface zone of the components (microalloying). The use of TiAl-components after fluorine treatment is feasible up to 1050°C in oxidising environments. One possibility to introduce fluorine into the near-surface of complex TiAl-components is the plasma immersion ion implantation (PI³) technique. The use of an Ar/CH2F2-plasma for the F-PI³ processing leads to a positive halogen effect. The main characterization techniques used in this study have been ERDA and RBS. The oxidation behavior of TiAl samples treated by PI³ of F has been examined using thermogravimetric analysis, and oxidation kinetics curves have been derived. Post-oxidation analyses by SEM reveal a thin protective alumina scale at the surface of the implanted samples as opposed to the thick mixed oxide scale (TiO2/Al2O3) that unavoidably grows upon high temperature oxidation in air.

Due to their light weight and high strength, TiAl alloys are of great interest for advanced automobile, aerospace and power generation applications. However, destructive oxidation occurring in these materials in air at temperatures above 700°C has to date hindered their widespread use. In the present study, the high-temperature oxidation behaviour of TiAl has been examined with consideration of the role of ion-implanted fluorine in providing practically useful oxidation protection. Samples of technical TiAl alloys have been treated by either beamline ion implantation of F or plasma immersion ion implantation (PIII) of F using various F-containing precursor gases. The degree of oxidation protection has been evaluated by testing ion implanted samples under conditions of isothermal and thermal cyclic oxidation at temperatures as high as 1050°C. Optimized ion implantation processing has been found to produce marked improvement in the oxidation behaviour of the TiAl alloys. After PIII of F at a bias voltage of 30 kV, broad fluorine implant profiles extending to depths well beyond those predicted by the theory have been identified by elastic recoil detection analysis (ERDA) in the as-implanted samples. The alloys modified in this way acquire a stable, adherent and highly protective alumina scale against environmental oxidation while retaining the bulk mechanical properties of the starting material. Some of the most important process parameters that enable the formation of an efficient protective scale are considered and assessed. PIII of F using suitable F-containing precursor gases has been successfully applied in protecting machine components made of gamma-TiAl and TNB such as turbine blades and turbochargers.

We report on laser-driven electron acceleration with 8 fs -three optical cycle- long pulses and only 40 mJ energy. This situation constitutes an ideal parameter range for laser wake-¯eld acceleration as prophesied by theory and numerical simulations, which was experimentally not explored before. The produced electron spectra are monoenergetic with a peak in the tens-of-MeV range -up to 50 MeV- and free of low-energy electrons with thermal spectrum. The electron beam has a typical divergence of 5 - 10 mrad. The accelerator is routinely operated at 10 Hz and correspondingly it is a promising source for several applications.

The MLLTRAP, presently under construction at the Maier---Leibnitz Laboratory (Garching), is a Penning trap system designed to decelerate, purify, charge-breed and cool the radioactive ions with the aim to perform the high-accuracy nuclear mass measurements. It involves novel techniques, like sympathetic cooling of highly-charged ions of interest with laser-cooled Mg+ ions. The goal is to reach an accuracy of 10-10, which is required for high precision fundamental physics studies like the determination of fundamental constants and measurement of electron binding energies for QED at strong fields.

Nanowires and chains of nanoparticles are of emerging interest in nanoelectronics, nanophotonics and plasmonics. One possible method is the use of a focused ion beam (FIB) that allows to write any pattern directly into the sample to create a particular nano structure. FIB implantation opens the way to manipulate the device structure locally on a length scale of a few 10 nm only and therefore as well as its electronic and optical properties. FIB combines implantation and lithography. The big advantage over other techniques is the reduction of technological steps necessary to make a particular device structure.
We report on strain and defect analysis of lateral nanostructures created in GaAs and Si substrates. The investigations were performed utilizing the method of high resolution X-ray diffraction using grazing incidence and micro focusing techniques at ESRF beamlines.
We have used two different ways to create lateral nanostructures in Si and GaAs, either by creating one or two dimensional patterns by FIB implantation using a liquid metal ion source of Ga or AuGeSi or by focused ion beam synthesis that allows the fabrication of epitaxial CoSi2 layers embedded in silicon. The spot size of the focused ion beam was in the order of 50nm; an ion beam energy in the order of 25keV was used. The whole implanted area had a size of less than 0.2 mm2.

The coupling effectiveness of the exchange spring effect can be influenced by the interface structure between hard and soft magnetic films. We have investigated the structural and magnetic properties of an Fe/Sm2Co7 exchange spring bilayer system after annealing and after low-energy ion irradiation. To study the interface properties X-ray reflectivity (XRR) measurements were carried out. From the XRR measurements it was shown that annealing influences the roughness of the layer surfaces causing an almost symmetrical broadening of the interfacial layers. Irradiation induces changes in the top three layers and the most pronounced effect upon irradiation is a change in electron density of the first two top layers and an interface broadening between Fe and Sm2Co7. In contrast annealing after irradiation triggers a material flow influencing the whole irradiated layers. The annealing afterwards therefore influences mainly the Fe layer by creating an Fe/Sm2Co7 intermixed region.

The MLLTRAP, presently under construction at the Maier-Leibnitz-Laboratory (Garching), is a Penning trap facility designed to
combine several novel techniques to decelerate, purify, charge breed and cool the reaction products and perform high-accuracy nuclear mass measurements of highly charged, laser-cooled ions.

We report on the influence of the initial roughness and crystallography of the substrate on the formation of self-organized ripple structures on semiconductors surfaces by noble gas ion bombardment. The Bradley-Harper theory predicts that an initial roughness is most important for starting the sputtering process which in the ends leads to the evolution of regular patterns. We produced regular patterns with intermediate Xe+ ion energies (5-70 keV) at different incidence and azimuthal angles which lead to the assumption that also crystallography plays a role at the beginning of ripple evolution. Most of the previous investigations started from the original roughness of a polished silicon wafer. We used (001) silicon wafers with a miscut angle of 1°, 5° and 10° towards [110]. We studied the ripple formation keeping the ion beam parallel to the [111], [-1-11] or [-111] direction, i.e. parallel, antiparallel or prependicular to the miscut direction [110]. The parallel and antiparallel case implies a variation of the incidence angle with increased roughness over the surface step terraces. The perpendicular orientation means almost no roughness. The results were compared to normal Si(001) and Si(111) wafers.

During ion sputtering of GaSb(100) surfaces a transient behavior from initial smoothing to roughening accompanied by self-organized pattern formation has been observed using in-situ x-ray reflectivity and grazing incidence small angle scattering. A characteristic hexagonal dot pattern with a spatial periodicity of 30 nm is observed and the correlation length increases with ion fluence. In the framework of the Bradley-Harper model, where the dot pattern formation results from an interplay of surface roughening due to sputtering and surface smoothing due to diffusion, the initial smoothing behavior results from the same diffusion processes as the pattern formation.

We report on the formation of ion beam induced ripples in Si(001) wafers when bombarded with Ar+ ions at an energy of 60keV. A set of samples varying incidence and azimuthal angles of the ion beam with respect to the crystalline surface orientation was studied by two complementary near surface sensitive techniques, namely atomic force microscopy and depth-resolved X-ray grazing incidence diffraction. Additionally cross-section TEM investigations were carried out. The ripple-like structures are formed at the sample surface as well as at the buried amorphous-crystalline interface. Best quality of the ripple pattern was found when the irradiating ion beam was aligned parallel to the <111> planes. The quality decreases rapidly if the direction of ion beam deviates from <111>.

We present spatially resolved X-ray diffraction pattern of individual GaAs nanorods grown catalyst-free throughout a pre-patterned amorphous SiNx mask onto germanium [111] surfaces. The experiment has been performed using synchrotron radiation using a micro-sized beam prepared by compound refractive lenses. Due to the non-polar character of the substrate the shapes of NR´s appear not uniform and vary between deformed hexagonal and trigonal in symmetry. Because the average diameter of NR´s equals the experimental resolution certain cuts through slightly inclined edges or corners of individual nanorods with lateral size of about 225 nm could be selected using spatially resolved reciprocal space mapping.

The Institute of Safety Research (ISR) is one of the six Research Institutes of Forschungszentrum Dresden-Rossendorf e.V. (FZD e.V.), which is a member institution of the Wissenschaftsgemeinschaft Gottfried Wilhelm Leibniz (Leibniz Association). Together with the Institute of Radiochemistry, ISR implements the research programme „Safety and Environment“, which is one of the three scientific programmes of FZD. In the framework of this research programme, the institute is responsible for the programme areas “Plant and Reactor Safety” and “Thermal Fluid Dynamics”, respectively (see Table 1). By participating in the development and operation of a pulsed photo-neutron source at the radiation source ELBE (Electron linear accelerator for beams of high brilliance and low emittance), we also contribute to the project “Neutron Induced Processes”, which is part of the FZD programme dedicated to the structure of matter.

The high price and the complexity of commonly used titanium-sapphire lasers hinder a more widespread use of pulsed THz systems. Er-doped fiber lasers can be a promising alternative. Since the band gap of GaAs is larger than the photon energy at 1.55 µm, this material – while being the standard material for photoconductive THz antennas excited with titanium-sapphire lasers – is not suitable for systems driven by fiber lasers. Dipole antennas have been successfully demonstrated on low-temperature grown InGaAs [1] and ion-irradiated InGaAs [2]. However, the resistivity of these small-gap materials is too low for microstructured large-area emitters. Such emitters consist of an interdigitated finger structure on the semiconductor substrate and a second metallization preventing destructive interference from THz wavelets excited in regions with opposite direction of the bias field [3]. Since these microstructured large-area emitters can withstand large excitation power and high bias fields, they show improved efficiency and higher THz power as compared to conventional antennas.
A 1000 nm thick Ga0.89In0.11As0.96N0.04 layer was grown lattice matched on semi-insolating GaAs by molecular beam epitaxy. On top, additional layers of 60 nm AlGaAs and 5 nm GaAs were grown, resulting in a higher resistivity of the substrate. A THz emitter with an active area of 1 mm  1 mm with a microstructure similar to the one described in ref. 3 was produced using standard optical lithography. The emitter was excited with radiation from an optical parametric oscillator (OPO; tuning range: 1.1 - 1.5 µm). The THz radiation was detected by electro-optic sampling in a ZnTe crystal gated with an 800 nm beam from a titanium sapphire laser. Strong THz emission is observed for excitation wavelength below 1.35 µm [4]. No saturation effects occur within the available range of average power (up to 50 mW) of the OPO. Furthermore the GaInAsN emitter was compared with an emitter based on semi-insolating GaAs at an excitation wavelength of 800 nm. The THz field amplitude of the GaAs emitter is eight times higher as compared to the GaInAsN emitter. Again, in the available excitation power range (up to 0.5 W), no saturation effects are observed.
In summary an efficient and easy-to-align microstructured THz emitters based on GaInAsN has been demonstrated. While further improvement of the material is necessary for excitation at 1.55 µm, the material studied here is suitable for fiber lasers operating at 1.1 µm.

References

[1] M. Suzuki and M. Tounouchi, Appl. Phys. Lett. 86, 163504, (2005).
[2] A. Takazato, et al., Appl. Phys. Lett. 91, 011102, (2007).
[3] A. Dreyhaupt, et al., Appl. Phys. Lett. 86, 121114 (2005).
[4] F. Peter et al., Appl. Phys. Lett. 93, 101102 (2008).

Typical pulsed terahertz (THz) systems operate with freely propagating THz waves of linear polarization and Gaussian beam profile. In the range of visible light and near-infrared radiation, Bessel-Gauss modes have stimulated great interest over last years. The lowest order Bessel-Gauss modes have a donut-like intensity distribution and are either radially or azimuthally polarized. Radially polarized beams have interesting fundamental properties such as smaller beam waists in the focus as compared to Gaussian modes and strong longitudinal field components in the focus. In the THz range, superior coupling properties for radially polarized light to plasmonic guided modes on wires, so called Sommerfeld modes are predicted. While there are many investigations on these guided THz modes, only little work is done on freely propagating modes of radial polarization [1] and no publications exist for azimuthally polarized THz beams. Here we present a concept that allows creating emitters and detectors for any desired mode. The principle is demonstrated by showing results on radially and azimuthally polarized THz beams.
The photoconductive emitter and detector structures are based on a microstructured electrode pattern on a semiconductor substrate. The electrode pattern is inverse to the desired polarization pattern, i.e. emitters for radially polarized modes consist of equidistant concentric ring electrodes, while emitters for azimuthally polarized modes are a circular structure of metallized sectors. To avoid electric fields pointing in the opposite direction, a second metallization layer is used in a similar way as previously described for emitter of linearly polarized radiation [2]. While the emitters were fabricated on semi-insulating GaAs, detectors were prepared on GaAs substrates implanted with N+. A titanium sapphire laser was used for excitation of the emitters and gating of the detectors. Beam profiles were measured for the divergent beam 25 mm behind the emitters, and for the refocused beam after a path of ~0.5 m. In this experiment the detector was sensitive for a linearly polarized component of the radiation. The observed beam profiles agree well with the ones expected for Bessel-Gauss beams. Furthermore radiation with radial polarization was detected with an antenna optimized for radial polarization and one for azimuthal polarization. With the detector for azimuthal polarization no signal is expected. However, due to imperfections of the beam profile of the near-infrared laser, a residual signal of about 10 % compared to the signal measured with the detector for radial polarization is observed. This experiment serves as a proof of principle that the emitter-detector combination well suited for studying changes of the mode structure.

References:

[1] G. Chang, Ch.J. Divin, C.-H. Liu, S.L. Williamson, A. Galvanauskas, and T.B. Norris, Opt. Lett. 32, 433 (2006).
[2] A. Dreyhaupt, S. Winnerl, T. Dekorsy, and M. Helm, Appl. Phys. Lett. 86, 121114 (2005).

We present a concept for the design of photoconductive antennas which allows the generation and detection of terahertz radiation pulses of freely selectable modes. We demonstrate the principle by showing electric field distributions for vector beams, namely the lowest order Gaussian-Bessel beams. They have a donut-like intensity distribution and either radial or azimuthal polarization. For comparison, Gaussian beams are investigated.

We report on emission and detection of pulsed terahertz radiation of radial and azimuthal polarization by microstructured photoconductive antennas. To this end the electrode geometry of the emitter is inverse to the desired THz field pattern and a second periodic structure prevents destructive interference effects. Beam profiles of freely propagating THz waves are studied for divergent and refocused beams. They can be well described as the lowest order Bessel-Gauss modes with a divergence comparable to linearly polarized Gaussian beams. Additionally, polarization sensitive detection is demonstrated for radially polarized radiation.

Nanocrystals formation in a –Si/SiO2 layer by ion mixing and plasma immersion ion implantation in different energy ranges

The molecular beam epitaxial (MBE) fabrication of blocked-impurity-band detectors (BIB) has been a technologically complex and delicate matter ever since its demonstration in silicon, and has not been adapted for other material systems offering detection onsets at lower terahertz frequencies. We report the fabrication and characterization of a vertical Si:B BIB, circumventing the intrinsically troublesome MBE growth of an ultrapure blocking layer by employing ion implantation. We present a thorough characterization of our device, which exhibits highly competitive figures of merits. Our results not only increase the accessibility of BIB fabrication tools for ultrasensitive terahertz detection but also open a road to other material systems.

The interaction of the ionic lattice with the superconducting condensate is treated in terms of the electrostatic force in superconductors. It is shown that the surface dipole supplies the force responsible for the volume difference of the normal and superconducting states. Assuming this mechanism, we argue that the usual parametrization of the theory of deformable superconductors should be revisited.

This report summarizes the results of the NURESIM project for the work package 2.1 “Pressurized Thermal Shock (PTS)”. It includes summaries of the single tasks done by the partners involved in this work package. In the Introduction chapter some more general information on the PTS issue is given, which should help to clarify the integration of the single activities. Since the PTS scenario involves different flow situations, for which also different modelling approaches are necessary, the tasks are sorted according to these flow situations. The relation of the work done to the general aim of the NURESIM project, which is to establish a new code platform, is indicated by assigning the activities to 6 different types. The results achieved in the PTS work package are in agreement with the expectations to the NURESIM project. The conclusion drawn from the single investigations and recommendations for future work are discussed in a separate chapter. It was shown, that for further improvement of the CFD-code capabilities for the two-phase PTS case new well-instrumented experimental data are needed especially for condensation at the surface of a sub-cooled liquid jet in a steam environment as well as on free surfaces, turbulence production and bubble entrainment below the jet and mixing in a stratified flow. Integral experiments, which reflect the PTS flow situations, are important to test the interplay between all the sub-models. Some of the local flow situations can be already captured quite well by presently available CFD codes, for other still many open questions exist. In general more flexible models are required which allow switching between different approaches within one flow domain but for the different local flow situations. Examples for such model approaches are the Large Scale Simulation (LSS) which should allow the application of a two-fluid model for dispersed flows and Interface Tracking Methods for large surfaces and the Scale Adaptive Simulations (SAS) which allow the simulation of large eddies while modelling the turbulence at the unresolved scales. The work done leads to a clear improvement of the simulation capabilities regarding a two-phase PTS situation, but caused by the complexity of the issue it will still be a long way to enable predictive simulations for all the different phenomena that occur in this application. In the near term, one may envisage a simplified treatment of two-phase PTS transients by neglecting some effects which are not yet controlled.

Driven by the progress in AMS and its spreading application within geosciences, measurements of increasing numbers of samples with low isotopic ratios will be required in the future. Therefore, we have examined the linearity of ¹⁰Be/⁹Be as a function of isotope ratio by distributing 3 secondary standards (dilutions of NIST4325: 10^-12-10^-14) to 9 AMS labs. The problem of low ratio samples is even more crucial for ³⁶Cl mainly due to the high volatility of chlorine. Thus, we have prepared large quantities of 3 ³⁶Cl/Cl solutions from a certified ³⁶Cl activity (NIST4943) by dilution with NaCl. AgCl precipitated from these solutions (10^-11-10^-13) has been distributed to 9 AMS labs. Some measurements are still ongoing. First results from 6 labs for each nuclide show that these interlaboratory exercises are very valuable.

The InGaAs/AlAsSb heterostructure has been attractive in recent years due to its high conduction band offsets, which is necessary for intersubband transition (ISBT) based devices at short wavelength (< 3 microns) [1]. To achieve this goal, very thin QWs are required, leading to a particular band configuration where a X or L cross-over takes place. Nevertheless it has been shown that intervalley transfer is rather inefficient for ISBT wavelengths as short as 2.3 m [2], indicating that quantum cascade lasers (QCL) at such short wavelength are feasible. Indeed, QCL with wavelength around 3.0 μm [3], using the same material, were recently demonstrated. Besides the intersubband relaxation time, providing the nonradiative lifetime of the QCL, another important parameter for the design of QCL is the homogeneous linewidth, since it would affect the gain profile of the laser. Therefore the study of the dephasing time, being directly relevant for the homogeneous linewidth, is as important as the intersubband relaxation time.
In this work we present a study of the intersubband relaxation dynamics and dephasing time of QW doped InGaAs /AlAsSb samples grown on InP substrate, through, respectively, degenerate pump-probe and four-wave-mixing (FWM) measurements, at wavelengths around 2 m. The measured intersubband relaxation time constants, between 1 and 1.8 ps, increase with the decrease of the temperature due to the reduced LO phonon scattering. The measured FWM time constant increases by a factor of 4, inferring from the increase of the FWM signal intensity with the decrease of the temperature from 300K to13K.
[1] C. V-B. Grimm, M. Priegnitz, S. Winnerl, H. Schneider, M. Helm, Appl. Phys. Lett. 91, 191121(2007).
[2] C.V-B. Tribuzy, S. Ohser, S. Winnerl, J. Grenzer, H. Schneider, M. Helm, J. Neuhaus, T. Dekorsy, K. Biermann, H. Künzel, Appl. Phys. Lett. 89, 171104 (2006).
[3] D. G. Revin, J. W. Cockburn, M. J. Steer, R. J. Airey, M. Hopkinson, A. B. Krysa, L. R. Wilson, and S. Menzel, Appl. Phys. Lett. 90, 021108 (2007).

Deep underground in the Gran Sasso national laboratory in Italy, the 400 kV LUNA accelerator supplying up to 0.5 mA H+ and He+ beams has been installed. Due to the 3400 meters (water equivalent) overburden present at Gran Sasso, the cosmic-ray induced laboratory background in gamma-ray detectors is strongly reduced at LUNA. The accelerator is being used for the study of radiative-capture reactions of astrophysical interest, both by in-beam gamma-ray spectrometry and by activation. The features of the site and accelerator will be shown on the poster; some applications and a future outlook will also be presented.

Si nanocrystals (NCs) have widely been used as sensitizers in Er-doped SiO2 layers where following the recombination of excitons (electron-hole pairs) in optically excited Si NCs energy is transferred to the higher energy levels of the 4f shell of Er3+ ions and subsequently decay radiatively via 4I13/2 → 4I15/2 transition. This as a result emits light at ~1.53 m, which coincides well the silica-based optical fibers. However, the temperature and concentration quenching of Er3+ ions in Si-rich SiO2 matrices are the basic obstacles for practical application. In the quest for an alternative host material, Ge-rich SiO2 layers have presently attracted a considerable interest due to strong quantum confinement effect and a better control of the surface oxidation of Ge NCs with respect to Si-NCs. In fact, the growth of Er-doped Ge nanoperticles embedded in a SiO2 layer and the photoluminescence (PL) properties have recently been studied. Despite progress in PL response, electrically pumped light-emitting devices (LEDs) are highly required from the optoelectronic standpoints.
We have investigated Ge NCs embedded in a metal-oxide semiconductor (MOS) structure co-doped with Er3+ ions and the corresponding electroluminescence (EL). Here we follow the two step implantation approach: (i) First, 130 keV Ge ions have been implanted (dose of 2–6E16 ions/cm2) into a thermally grown 200 nm thick SiO2 layer followed by furnace annealing (FA) at 950 C for 60 min, which is followed by (ii) 250 keV Er implantation (dose of 1-5E15 ions/cm2) combined with FA in the range of 800-1100oC for 30 min in nitrogen ambience. The MOS structures have been fabricated by depositing indium-tin-oxide (ITO) and aluminium in the front and rare sides of the samples, respectively, and patterning the ITO layer using photolithography. Interestingly, instead of electrically driven pumping of Er3+ ions by Ge NCs, we found that an EL band ~407 nm associated to the Ge-related oxygen-deficiency centres (GeODCs) have been pumped by Er3+ ions and discuss the observed phenomenon on the ground of an inverse energy transfer process. Concentration and temperature dependent modification of microstructure and the subsequent impact on EL response will also be discussed.

The Er-doped SiO2 layers containing Si nanocrystals (Si-ncs) have attracted considerable interest for more than a decade in realizing efficient light sources at 1.54 um, owing to the coincidence of the luminescence wavelength to the absorption minimum of the silicon based optical fibres.1 Although about two orders of magnitude Er luminescence has been noticed in long time annealed sputtered deposited samples,2 observation of such high efficiency in ion implantation processed samples is scarce. Recently, we have succeeded in producing such a system by a combination of sequential Si and Er implantations and rapid thermal annealing (RTA). The processing conditions have been optimized for achieving maximum Er photoluminescence (PL) at 1533 nm at the expense of a broad luminescence band peaking at ~580 nm. Spectral analyses suggest that the appearance of such visible range PL band can be explained in the light of the interfacial state mediated recombination of carriers in the Si-ncs according to the model proposed by Wolkin et al.3. The energy migration from Si-ncs to the nearby Er ions has further been manifested using time-resolved PL measurements.
[1] A. Polman, J. Appl. Phys. 82, 1 (1997).
[2] M. Fujii, M. Yoshida, Y. Kanzawa, S. Hayashi, and K. Yamamoto, Appl. Phys. Lett. 71, 1198 (1997).
[3] M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, and Delerue, Phys. Rev. Lett. 82, 197 (1999).

The Er-doped SiO2 layers containing Si nanocrystals (Si-ncs) have attracted considerable interest for more than a decade in realizing efficient light sources at 1.54 um [1,2], owing to the coincidence of the luminescence wavelength to the absorption minimum of the silicon based optical fibers. Although about two orders of magnitude Er luminescence has been noticed in presence of Si-ncs in a long time annealed sputtering deposited samples [3], observation of such high efficiency in ion implantation processed samples is scarce. Recently, we have succeeded in producing such a system by a combination of sequential Si and Er implantations and rapid thermal annealing (RTA). The processing conditions have been optimized for achieving maximum Er photoluminescence (PL) at 1533 nm at the expense of a broad luminescence band peaking at ~580 nm. Spectral analyses suggest that the appearance of such visible range PL band can be explained in the light of the interfacial state mediated recombination of carriers in the Si-ncs according to the quantum-confinement model [4]. The energy migration from Si-ncs to the nearby Er ions has further been manifested using time-resolved PL measurements.

[1] A. Polman, J. Appl. Phys. 82, 1 (1997).
[2] G. Franzò, S. Boninelli, D. Pacifici, F. Priolo, F. Iacona, and C. Bongiorno, Appl. Phys. Lett. 82, 3871 (2003).
[3] M. Fujii, M. Yoshida, Y. Kanzawa, S. Hayashi, and K. Yamamoto, Appl. Phys. Lett. 71, 1198 (1997).
[4] M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, and Delerue, Phys. Rev. Lett. 82, 197 (1999).

Optically active Er3+ ions dispersed in a SiO2 layer are often used for amplifying signals at the wavelength of ~1.5 m that coincides with the maximum transparency window of silica-based optical fibers. In order to enhance the 1.5 m Er luminescence, a sufficient number of Er ions must be in the excited state, and it is only possible by employing a high-power pumped laser. Si nanocrystals (NCs) embedded in an Er-doped SiO2 layer show a new avenue to excite Er3+ ions indirectly, where Si-NC behaves like a sensitizer [1]. The basic mechanism of the above mentioned system relies on three facts: (i) optical excitation of Si-NC, (ii) nonradiative transfer of energy from Si-NC to the neighbouring Er3+, and (iii) subsequent relaxation from the first excited state to the ground state of Er3+ by emitting light at 1.5 m. However, to integrate these photonic systems into current microelectronics, it is essential to fabricate metal-oxide-semiconductor based light-emitting devices. Based on electroluminescence (EL) experiments, some groups have shown the importance of Si-NCs in enhancing the 1.5 µm Er luminescence.
Here we use an experimental approach for evaluating the influence of Ge NCs on the Er EL in Er-doped SiO2 layers. In particular, we demonstrate an increase in intensity of the 400 nm band, characteristic of the Ge-NC related oxygen-deficiency centres (GeODC), at the expense of the Er-related signals with maxima at 522, 550, 660 and 1532 nm during electrical pumping at room-temperature  indicating an energy transfer process from the excited Er ions to the confined carriers in the GeODC, which is just opposite to the concept commonly used for the Er-doped SiO2 layers containing Si-NCs. Formation of Ge-NCs was verified by transmission electron microscopy.

[1] Polman A, Nature Materials 2002;1:10.

It is well established that Si nanocrystals (NCs) can act as sensitizers in Er-doped SiO2 during optical pumping [1-4]. In fact, following the recombination of excitons (electron-hole pairs) in optically excited Si NCs transfer their energy to the higher energy levels of the nearest Er3+ ions and subsequently decay to the ground state by intra-4f transitions. Among those, the 4I13/2 → 4I15/2 radiative transition has attracted substantial interest since the respective luminescence at ~ 1.53 m corresponds to the maximum transparency of silica-based optical fibers [2]. Ge is another group-IV element with similar electronic properties to that of Si. We have examined the impact of Ge NCs in Er-doped SiO2 layers by investigating electroluminescence (EL) of the metal-oxide semiconductor (MOS) structures, where the Er-doped Ge-rich SiO2 layers have been prepared by ion implantation technique combined with rapid thermal annealing (RTA). The samples have been prepared by two steps: (i) 130 keV Ge ions have been implanted with a dose of 2  1016 ions/cm2 in a 200 nm thick thermally grown SiO2 layers followed by RTA at 1050 oC for 180 s, and subsequently (ii) 250 keV Er ions have been implanted with a dose of 1  1015 ions/cm2 followed by RTA in the range of 850-1050 oC for 6-150 s in nitrogen ambience. Transmission electron microscopy experiments reveal formation of randomly oriented Ge NCs with average size ~4 nm. The MOS structures have been fabricated by depositing indium-tin-oxide (ITO) and aluminium in the front and rare sides of the samples, respectively, and patterning the ITO layer using photolithography. During EL measurements, in absence of the visible range band correlated to the quantum confinement in Ge NCs a band appears at ~400 nm in Ge-rich SiO2 layer as a consequence of hot electron mediated impact excitation in Ge-related oxygen-deficiency centres (GeODCs) during electrical pumping [5]. We find an increase of the 400 nm EL intensity with a concomitant reduction of the Er-related emission, and discuss the observed phenomenon on the ground of an inverse energy transfer process [4] from excited Er3+ to the GeODCs.

[1] A. Polman, J. Appl. Phys. 82, 1 (1997).
[2] O. Savchyn, F. R. Ruhge, P. G. Kik, R. M. Todi, K. R. Coffey, H. Nukala, and H. Heinrich, Phys. Rev. B 76, 195419 (2007).
[3] M. Fujii, K. Imakita, K. Watanabe, and S. Hayashi, J. Appl. Phys. 95, 272 (2004).
[4] I. Izeddin, A. S. Moskalenko, I. N. Yassievich, M. Fujii, and T. Gregorkiewicz, Phys. Rev. Lett. 97, 207401 (2006).
[5] L. Rebohle, J. von Borany, R. A. Yankov, W. Skorupa, I. E. Tyschenko, H. Fröb, and K. Leo, Appl. Phys. Lett. 71, 2809 (1997).

We present an overview over research on higher-dimensional cosmological models carried out from 1997 till 2004. The basic starting point is the problem of stabilizing the sizes of the extra-dimensional factor spaces so that they are kept unobservable at low energies --- as indicated by current data from high-energy collider experiments and astrophysical observations. This problem is well known as the moduli stabilization problem in string theory and M-theory. Stabilizing the scale factors of the extra-dimensional factor spaces by some effective potentials (which are naturally induced via dimensional reduction of the higher-dimensional setups) fixes only the average sizes (moduli) but still allows for small (quantum) fluctuations around the minimum position (radion fluctuations). Once these fluctuations will depend on the coordinates of our external (uncompactified) space they will act as multicomponent scalar fields. Due to their Planck-scale-suppressed coupling to usual standard matter they may be considered as possible candidates for dark matter or UHECR (ultra-high energy cosmic rays).
In the talk, we comment on various dimensional reduction schemes, large extra dimensions (ADD-setups) as well as on orbifold compactification schemes of Randall-Sundrum type. We provide a brief summary about the obtained results published during the 8 years of active research in this field --- including discussions of effective static and dynamic perfect fluid scenarios, relations to reheating, transitions from radiation dominance to matter dominance, quintessence, UHECR, reaction channels with standard matter and possible creation of moduli excitations in ultra-strong background magnetic fields of magnetars. Finally, we give a brief overview about results on higher-dimensional f(R) theories as they are used in 4D as possible explanation for dynamically induced dark energy.

In the first part of the talk, the eigenvalue behavior λ(α,β) of the 2×2 matrix differential operator of the spherically symmetric α²-dynamo of magnetohydrodynamics is considered for constant α-profiles and boundary conditions which depend on a parameter β. Specifically, β∈[0,1] acts as parameter in the homotopic interpolation between idealized (Dirichlet) and physically realistic (Robin) boundary conditions (BCs). For the quasi-exactly solvable monopole setup (with spherical mode number l=0) the characteristic equation is derived explicitly. It is shown that the β-homotopy describes an interpolation between spectra of mesh type (idealized BCs) and a countably infinite set of parabolas (physically realistic Robin Bcs). Interestingly, the mesh nodes (semisimple twofold degenerate eigenvalues) are fixed points of the β-homotopy. An underlying ruled-surface structure of the spectrum is uncovered.

In the second part of the talk, we provide a brief summary of recent results on the spectral behavior of the PT-symmetric
Bose-Hubbard system as it is used for the description of quantum Bose-Einstein condensates with balanced gain-loss interactions. For an N-particle system the corresponding Fock-space Hamiltonian reduces to an N×N-matrix which is selfadjoint in an N-dimensional Pontryagin space. The unfolding of higher-order branch-points of the spectrum is considered under parameter perturbations. Numerical as well as analytical results are presented which demonstrate the relevance of the Hessenberg type of the Hamiltonian as defining matrix structure for the occurrence of specific Galois cycles in the eigenvalue rings of the unfolding branch points.
partially based on: J. Phys. A 41 (2008) 255206; arXiv:0802.3164 [math-ph].

A linear densely defined operator A acting in a Krein space with fundamental symmetry J and indefinite metric [.,.]_J =(J.,.) is called J-selfadjoint if A^*J = JA.  In contrast to self-adjoint operators in Hilbert spaces (which necessarily have a purely real spectrum), J-selfadjoint operators, in general, have a spectrum which is only symmetric with respect to the real axis. However, one can ensure the reality of the spectrum by imposing an extra condition of symmetry. In particular, a J-selfadjoint operator A has the property of C-symmetry if there exists a bounded linear operator C in H such that: (i) C² =I; (ii) JC > 0; (iii) AC = CA. 
The properties of C are nearly identical to those of the charge conjugation operator in quantum field theory and the existence of C provides an inner product (.,.)_C=[C.,.]_J whose associated norm is positive definite and the dynamics generated by A is therefore governed by a unitary time evolution. However, the operator C depends on the choice of A and its finding is a nontrivial problem. 
The report deals with the construction of C-symmetries for J-selfadjoint extensions of a symmetric operator A_sym with finite deficiency indices . We present a general method allowing us: (i) to describe the set of J-selfadjoint extensions A of A_sym with C-symmetries; (ii) to construct the corresponding C-symmetries in a simple explicit form which is closely related to Clifford algebra operator structures; (iii) to establish a Krein-type resolvent formula for J-self adjoint extensions A with C-symmetries. 
The results are exemplified on 1D pseudo-Hermitian Schrödinger and Dirac Hamiltonians with complex point-interaction potentials. 

Using a homotopic family of boundary eigenvalue problems for the meanfield α²-dynamo with helical turbulence parameter α(r)= α₀ + γΔα(r) and homotopy parameter β∈[0,1], we show that the underlying network of diabolical points for Dirichlet (idealized, β=0) boundary conditions substantially determines the choreography of eigenvalues and thus the character of the dynamo instability for Robin (physically realistic, β = 1) boundary conditions. In the (α₀,β,γ)-space the Arnold tongues of oscillatory solutions at β =1 end up at the diabolical points for β = 0. In the vicinity of the diabolical points the space orientation of the 3D tongues, which are cones in first-order approximation, is determined by the Krein signature of the modes involved in the diabolical crossings at the apexes of the cones. The Krein space induced geometry of the resonance zones explains the subtleties in finding α-profiles leading to spectral exceptional points, which are important ingredients in recent theories of polarity reversals of the geomagnetic field.

Up to now only occasionally polynuclear sorption complexes of uranyl have been reported. This is undoubtedly surprising, since polynuclear hydrolysis species dominate aqueous uranyl speciation across a wide pH range and at solution concentrations typically used for sorption studies (10-6 - 10-4 M). Using U-LIII EXAFS spectroscopy at 15 K, we demonstrate here that polynuclear sorption complexes occur consistently at the gibbsite surface. They were observed across a pH range of 5.5 to 7.5 in equilibrium with atmospheric CO2, and at pH 8.5 in absence of CO2.
At these conditions, thermodynamic calculations predict the trimeric (UO2)3(OH)5+ complex to be the predominant hydrolysis species in the aqueous phase. This complex has been investigated recently by EXAFS spectroscopy and DFT calculations [1]. Both methods showed a stoichiometry of (UO2)3O(OH)3+ for this complex with a central oxo bridging, and provided a relatively short U-U distance of 3.8 Å. In contrast, coordination numbers and radial distances found for the sorption complex are 2xOaxial @ 1.8 Å, 6xOequatorial @ 2.4 Å and 1xU @ 4.2 Å. Both EXAFS shell fitting and a newly developed approach for the calculation of the radial pair distribution function achieved the same result. The U-U distance of 4.2 Å together with the coordination number of six for equatorial oxygen are in line with formation of uranyl dimers, where the two uranyl units are linked in edge-sharing configuration. This unusual structure is currently further investigated by DFT calculations.

REFERENCES
1. S. Tsushima, A. Rossberg, A. Ikeda, K. Mueller, A. C. Scheinost, Inorganic Chemistry 46, 10819-10826 (2007).

X-ray absorption fine-structure (EXAFS) spectroscopy allows the study of a large number of elements and compounds in different physical and chemical states. The most prominent structural parameters derived from EXAFS are coordination numbers, interatomic distances, static and vibrational disorder expressed by the Debye-Waller factor. These parameters are used in the conventional shell fitting approach to approximate the true radial pair distribution function (PDF). This shell fitting approach has three critical limitations for which we present possible solutions.

a) Due to the approximation of the PDF with Gaussian peak shapes, the conventional shell fitting approaches fails for all systems which show anharmonic contributions to the vibrational or static displacement of atoms, caused for instance by temperature effects or week bonds in adsorbate-adsorbent interactions [1]. To overcome this limitation we present two techniques: (1) a Monte Carlo simulation technique for EXAFS spectra which takes not only the first order scattering, but also higher order scattering into account [2] and (2) a novel application designed for the direct calculation of the PDF from the EXAFS spectrum.

b) Due to the limited resolution in R, the shell fitting approach commonly fails, if the X-ray absorbing atom is present in different chemical environments at the same time, i.e. in case of chemical mixtures. For this case we present a way to isolate the pure spectral components by Iterative Factor Analysis [3] and/or by a mathematical combination of Factor Analysis and Monte Carlo simulation [2].

c) In order to find a unique structural solution, shell fitting tends to be a soft modelling approach, because many structural arrangements may lead to a similar description of the EXAFS signal and have to be tested. Especially for the investigation of the unknown structures of sorption complexes, a hard modelling approach, which can take the complete substrate structure into account, would be much better suited. We show that the Monte Carlo simulation technique can be used as a hard modelling approach to solve the structure of sorption complexes [4].

[1] Crozier, E. D. Impact of the Asymmetric Pair Distribution Function in the Analysis of Xafs. Physica B-Condensed Matter 209, 330-333 (1995).
[2] Rossberg, A. & Scheinost, A. C. Three-dimensional modeling of EXAFS spectral mixtures by combining Monte Carlo Simulations and Target Transformation Factor Analysis. Analytical and Bioanalytical Chemistry 383(1), 56-66 (2005).
[3] Rossberg, A., Reich, T. & Bernhard, G. Complexation of uranium(VI) with protocatechuic acid - application of iterative transformation factor analysis to EXAFS spectroscopy. Analytical and Bioanalytical Chemistry 376, 631-638 (2003).
[4] Ulrich, K.-U., Rossberg, A., Foerstendorf, H., Zänker, H. & Scheinost, A. C. Molecular characterization of uranium(VI) sorption complexes on iron(III)-rich acid mine water colloids. Geochimica Et Cosmochimica Acta 70, 5469-5487 (2006).

In diesem Beitrag werden zwei neuartige, bildgebende Sensoren zur Untersuchung von Mehrphasenströmungen beschrieben - der kapazitive Gittersensor und der kapazitive Flächensensor. Beide Sensoren basieren auf einer matrixförmigen Anordnung von Messelementen, mit denen die elektrische Kapazität eines umgebenden Fluides sehr schnell abgetastet wird. Dadurch sind diese Sensoren in der Lage, zeitlich und räumlich hoch aufgelöste Bilder der Phasenverteilung einer Mehrphasenströmung zu erzeugen. Die Sensoren und die zugehörige Messelektronik werden präsentiert. Darüber hinaus werden ausgewählte Ergebnisse von Strömungsvisualisierungen dargestellt und diskutiert.

In this article, two novel imaging sensors for the investigation of multiphase flows are introduced – the capacitive wire-mesh sensor and the capacitive planar array sensor. Both sensor systems are based on a matrix-type arrangement of sensing elements by which the electrical capacitance of a surrounding fluid is very fast scanned. Thus these sensors are able to produce high temporal and spatial resolution images of the phase distribution in a multiphase flow. The sensors and associated measuring electronics are presented. Furthermore some selected flow visualization results are represented and discussed.

BGCore reactor analysis system, recently developed at Ben-Gurion University for calculating in-core fuel composition and spent fuel emissions following discharge, couples the Monte Carlo neutronic code (MCNP4C) with an independently developed burnup and decay module SARAF. The BGCore utilizes multi-group approach for generation of one group cross-sections. According to this approach, only multi-group neutron spectrum is calculated by MCNP, while reaction rates are calculated in a separate subroutine using pre-generated multi-group cross-section set and the fine group neutron spectrum obtained from MCNP.
BGCore code system offers a number of advantages over similar MCNP-depletion codes. These include:

• Multi-group coupling approach significantly reduces the code execution time without compromising the accuracy of the results.
• Use of the most recent data based on JEFF-3.1 data files,
• Careful choice of about 1700 isotopes to cover all potentially significant aspects of fuel irradiation and decay. All nuclides are included in the calculation matrix with no asymptotic approximation,
• The fact that all of the isotopes, and not just the most neutronically important, are tracked in the BGCore throughout all depletion steps, allows calculations of post-irradiation fuel characteristics such as, activity, radiotoxicity, and decay heat with high degree of accuracy.

WWER-400 second generation (V-213) reactor pressure vessels (RPV) were produced by IZHORA in Russia by and SKODA the former Czechoslovakia. The surveillance Charpy-V and SE(B) specimens of both producers have different orientation. The main difference is the crack extension direction which is through the RPV thickness and circumferential for ISHORA and SKODA RPV, respectively. In particular for the investigation of weld metal from multilayer submerged welding seams is the crack extension direction of importance. Depending on the crack extension direction in the specimen there are different welding beads or a uniform structure along the crack front. This is especially important for the fracture toughness determined according to the Master Curve (MC) approach as standardised in the ASTM Standard Test Method E1921-05. This approach was applied on weld metal of the RPV beltline welding seam of Greifswald Unit 8 RPV. Charpy size SE(B) specimens from 13 locations equally spaced over the thickness of the welding seam were tested. The specimens are TL and TS orientation.
The fracture toughness values measured on the SE(B) specimens with both orientations follow the course of the MC. Nearly all values lie within the fracture toughness curves for 5% and 95% fracture probability. There is a strong variation of the reference temperature T0 though the thickness of the welding seam, which can be explained with structural differences. The scatter is more pronounced for the TS SE(B) specimens. It can be shown that specimens with TL and TS orientation in the welding seam have a differentiating and integrating behaviour, respectively. The statistical assumptions behind the MC approach are valid for both specimen orientation even if the structure is not uniform along the crack front.

Pulsed laser deposited ZnO films were imlpanted with vanadium ions using ion energies between 30 and 250 keV with different fluences yielding vanadium concentrations in the range between 0.8 and 5 at. %. After annealing under oxygen ambient at 800°C, a broad luminescence band observed by photoluminescence covers nearly the total visible spectral region. This luminescence is a superposition of different bands triggered by the incoporated V and remaining implantation defects. The visual impression of the bright whitish emission of the implanted ZnO has been quantified using the color space map of the Commission internationale de l'Eclairage. Furthermore, the intensity of the white emission strongly increases with increasing V concentration, whereas Ar-implanted reference sample shows only weak white emission.

Ein ultraschneller Elektronenstrahl-Tomograph visualisiert strömende Stoffgemische berührungsfrei mit Bildraten von bis zu 7000 Bildern pro Sekunde und einer räumlichen Auflösung von einem Millimeter. Das Gerät kann so Prozesse in Blasensäulenreaktoren sichtbar machen.

During a jet gaseous bubbles are entrained. These bubbles influence the liquid flow field. The liquid flow field transports fibers in a sump geometry. Dependent on the flow situation different flow characteristics might occur. In the paper results of CFD calculations are presented and compared to experiments.

We performed wire-mesh and X-ray micro-tomography studies of the two-phase flow in the mixing chamber of an effervescent atomizer. Flow patterns in the mixing chamber can be revealed by the miniature wire mesh sensor with a temporal resolution of 10 kHz whereas microtomography provides accurate and high-resolution axial-radial gas fraction profiles. The paper describes both measurement techniques and first results of experimental investigation.

The article deals with the description of two-phase flow in the mixing chamber of an effervescent atomizer. The first observation has been carried out with the use of high-speed records of the flow inside the mixing tube. The flow in the mixing chamber is very fast and inhomogeneous thus the need to use a high-sampling frequency device has arisen in order to describe changes in the flow. Therefore, an experimental technique has been found which is able to describe the liquid-air distributions in small channels. As a two-phase flow measurement instrument, a miniature wire-mesh conductivity sensor to deal with cross-sections of 8 mm in diameter was designed and built. The frame rate of this sensor is 10 000 images per second. In this study, a special model of a transparent nozzle spraying deionized water that makes use of air as the atomizing medium was used. The effervescent nozzle is of "inside-out gas injection" configuration with the internal diameter of the mixing channel 8 mm. During the experiment, the effervescent atomizer was operated at different air pressure levels ranging from 0.1 to 0.5 MPa and mass GLR (Gas-to-liquid- ratio) from 0.1 to 25%. Mass flow rates of water ranged from 5 up to 65 g·s-1. The results reveal the unstable behavior of two-phase flow in the mixing chamber.

This work concerns experiments as well as CFD simulations on the gas entrainment for the impinging jet configuration. Impinging jets may occur in different situations related to reactor safety analyses. Many experiments have been carried out on impinging jets. A comprehensive overview by Bin [1993] reveals that the results depend critically on the individual setup (e.g. the nozzle geometry) and it is difficult to draw general conclusions. For the qualification of Computational Fluid Dynamics (CFD) models for such flow situations data with high resolution in space and time are required. Therefore a new experiment with a very simple setup was designed and first CFD simulations were done for these experiments.

The electrical stability of Tb-implanted SiO2 light emitting devices was drastically improved by using a SiON dielectric buffer layer. For fabrication thermally grown oxide layers on Si were implanted with Tb followed by a thermal treatment and the deposition of a SiON protection layer by plasma-enhanced chemical vapor deposition. The structures were finally provided with an indium tin oxide front contact and an aluminum rear contact. The incorporation of the SiON layer increases the breakdown electric field from 7.5 to 10.5 MV/cm and enhances the operation time of the light emitters up to three orders of magnitude under constant injection currents. By varying the SiO2 and SiON layer thickness it was found that the largest stability enhancements can be achieved if the SiON layer thickness is more than twice the thickness of the SiO2 layer.
The beneficial role of the SiON layer is mainly explained by reducing the chance of destructive avalanche breakdowns in the oxide layer and by an efficient cooling process of hot electrons moving in the conduction band of SiO2. The latter effect is based on the lower electric fields in SiON as compared to SiO2 and the lower band offset of SiON relative to the gate electrode. In addition, the SiON layer acts as a diffusion barrier against moisture from the working ambient and broadens the operation range of the light emitters on the voltage scale.

In this work a comparative study of charge trapping, electroluminescence intensity (ELI) and clustering in SiO2 implanted by different rare-earth (RE) impurities (Eu, Tb, Gd, Er, Tm) with following high-temperature annealing is performed to clarify the connection between the electrical properties, the structure of the luminescent centers, the ELI and the EL spectra. RE impurities were implanted into the bulk of thermally grown SiO2 on n-type Si. The implanted doses were chosen in such a way that the maximum concentration corresponded to 0.1, 0.5, 1.5 and 3.0 at %. To activate the RE implanted impurities a post implantation furnace anneal in the temperature range of 800-1100 °C for 30 min and flash lamp annealing (FLA) for 20 ms at 1000 °C in a nitrogen ambient have been carried out. The ITO layer was used as a transparent electrode. Charge trapping was studied by the shifting of the high-frequency CV characteristics and the changing of the applied voltage during constant current electron injection from Si into SiO2. The EL signal was recorded at the same injection regime at room temperature in the wavelength range of 300 to 750 nm. Some control structures were studies by transmission electron microscopy with high resolution (XTEM).
It was shown that RE impurities such as Tb, Gd, Er and Tm implanted into SiO2 cause mainly net positive charge trapping in the range of the injected charge from 1x1015 to 2x1017 e/cm² and stable ELI of their main luminescence lines in green, UV, IR and blue spectral region of the EL spectra, respectively. Above 1x1018 e/cm² of the injected charge an electron trapping in the bulk of the oxide and a hole trapping at the SiO2-Si interface is observed for all types of the RE impurities. The electron trapping correlates with the EL quenching of the main EL lines for all studied RE implanted structures with the exception of the Eu implanted one. The Eu implanted oxide demonstrates effective electron trapping up to 1x1020 e/cm² without EL quenching of the main studied EL lines: in red spectral region with a maximum at 618 nm (5D0-7F2 transition for Eu3+ ions); in the blue-green spectral region around 460-470 nm and in blue-violet one at 410 nm (corresponding to a 4f6d-4f7 transition of the Eu2+ ion). The XTEM measurements discovered that the clustering in the Eu-implanted SiO2 is enhanced considerably in comparison with the Tb-implanted one. It is suggested that the enhanced electron trapping in the Eu implanted structures is associated with enhanced clustering, which is partly caused by low valency (2+) oxides existing for the Eu impurity such as EuO and Eu3O4. The use of FLA for the Eu implanted SiO2 results in a decrease of the nanocluster size and an increase of the ELI in the red region of the spectrum.

Electroluminescence (EL) spectra, charge trapping during operation of EL devices and clustering of rare earth oxides in SiO₂ have been investigated in Eu-implanted SiO₂-Si structures which demonstrate luminescence associated with the light-emitting transitions in Eu²⁺ and Eu³⁺. Strong electron trapping in all studied regions of the injected charge (from 1x10E¹⁴ to 1x10E¹⁸ e/cm²) during operation of the light-emitting devices has been found that considerably differed from the oxides implanted by other rare earth impurities (Ce, Tb, Gd, Er, Tm). It has been shown that the observed strong electron trapping and the low EL intensity in the Eu implanted structures were associated with enhanced clustering of the Eu oxides. The mechanisms of electron trapping in the SiO₂ containing a large cluster concentration is discussed, and flash lamp annealing is proposed to decrease the nanocluster size and to enhance the EL intensity.

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Radioaktive Kupferisotope nehmen sowohl für den Einsatz in der nuklearmedizinischen Diagnostik als auch in der Therapie einen besonderen Stellenwert ein. ⁶⁴Cu und ⁶⁷Cu sind dabei aufgrund günstiger kernphysikalischer Eigenschaften (Halbwertzeit, Art der Strahlung) von besonderem Interesse. Derivate des Bispidins (3,7-Diazabicyclo[3.3.1]nonan) bilden mit Übergangsmetall-kationen wie Cu²⁺, Co²⁺ oder Fe²⁺, Komplexe hoher Stabilität [1,2].
Für Bispidine ergeben sich drei unterschiedliche Konformationsisomere. Die abgeflachte Doppel-Sessel-Form repräsentiert die thermodynamisch stabilste Konformation. Darin sind die beiden Aminstickstoff-Donoratome N3 und N7 für die Koordination mit Metallionen optimal vororganisiert. Die Stellung von Substitutenten in den Positionen C2 und C4 bezüglich der Ringebene kann zur Bildung von Isomeren einer cis-trans-Konfigurationsisomerie führen. Sterisch anspruchsvolle Reste erzwingen die Bildung von trans-Isomeren. Das gilt in gleicher Weise für die Bindung voluminöser Donoreinheiten – wie Benzimidazolyl-Substituenten – in N3-Position des Bispidin-Gerüsts. Das entsprechende Piperidon (Fig.1) sowie das Bispidon (Fig. 2) zeigen die erwartete trans-Isomerie [3]. Dieser Befund wurde mit Hilfe der Kristallstrukturen nachgewiesen. Der Precursor (C₂₇H₂₅N₅O₅), a=8.1752(6) Å, b=11.1511(8) Å, c=15.8128(14) Å, V=1260.89(17) ų, Z=2, R1=0.043 kristallisiert in der Raumgruppe P-1. In der Kristallstruktur des Kupferkomplexes [Cu(C₃₅H₃₃N₇O₅)(NO₂)₂] • H₂O, P2₁/n, a=13.242(5) Å, b=17.971(7) Å, c=15.650(6) Å, V=3722.1(2) ų, Z=4, R1=0.0451 2) ist das Kupferatom verzerrt quadratisch pyramidal koordiniert (d_Cu−N = 1.947(5) - 2.308(7) Å) und durch den Bispidin-Liganden nahezu vollständig von der Umgebung abgeschirmt.
Erste radiopharmakologische Untersuchungen zeigen, dass die Entwicklung kopplungsfähiger Liganden und deren Anknüpfung an ausgewählte Biomoleküle einen weiteren Schritt in Richtung einer besseren Bioverfügbarkeit von radioaktiv markierten Kupferverbindungen darstellt.

Self-organized dot patterns have been fabricated on GaSb and Si surfaces by sputtering from an inductively coupled plasma. The dependence of the patterns on the ion energy has been investigated. In agreement with previous studies using plasmas or Kaufman sources, the dot wavelength on GaSb increases with energy. On Si surfaces, however, the dot wavelength increases with energy only in the range E < 800 eV. For 800 eV < E < 1500 eV, the dot wavelength is constant.

As the digital equipment to measure positron lifetimes gets cheaper and more widely used, it is decided that EPOS, the ELBE positron source will sample the signals from the photomultipliers directly and evaluate it online or offline by digital means. Still using isotope sources, the EPOS lifetime spectrometer results in a timing resolution of around 170 ps (with Co-60), which compares good to analog equipment. A distinct improvement is expected when a coincidence setup will be used at ELBE. However, also the software needs further improvement: while one of the goals is of course to achieve the best time resolution, there is also the aspect of runtime and expandability. Results of evaluations will be presented and compared with results from other groups.

For the first time, trepans from a decommissioned VVER-440 (Greifswald-1) reactor pressure vessel (RPV) have been examined. Activities of a trepan, taken at the RPV weld with the highest fast-neutron load, were measured and estimated on the basis of fluence calculations by the codes TRAMO and DORT. A maximum fluence of 4.05*1019 n/cm2 (E>0.5 MeV) was calculated. The average deviation between the two codes is 2.6 %. Activities resulting from the reaction 93Nb(n,n’)93mNb were measured, niobium being a trace element in the RPV steel. Unfortunately, 93mNb is also produced by neutron capture in the alloy component 92Mo, the built-up 93Mo decaying by electron capture. The ratios of calculated to measured (C/E) 93mNb gamma activities for several trepan samples are between 0.42 and 0.97. The fact that all C/E ratios are below unity suggests that the measured values may have been additionally heightened by activities from other nuclides.

We report the activity of a new palladium catalyst supported on fundamentally different Gram negative (Desulfovibrio) and Gram positive (Bacillus) bacterial surfaces (bio-Pd). Under H-2 (electron donor), cells of both strains reduced Pd(II) to Pd(0) as discrete nanoparticles located in the periplasmic space of D. desulfuricans or between the peptidoglycan and the proteinaceous surface layer (S-layer) of B. sphaericus. The catalytic activity of the preparations was similar in their ability to reduce Cr(VI) to Cr(III) (aq), and in the hydrogenation of itaconic acid (aq.) Bio-Pd on D. desulfuricans was also an effective catalyst in a range of reactions in methanol, performing comparably to a commercially available catalyst (10% Pd/C) in the conversion of 4-azidoaniline to 1,4-phenytenediamine. In the hydrogenation of 3-nitrostyrene, the bio-Pd showed selectivity for the partly reduced (dehalogenated) product (1-ethyl-3-nitrobenzene -74%, 1-ethyl-3-aminobenzene -7%) whereas the ! commercial catalyst produced only the fully reduced product (1-ethyl-3-aminobenzene -73%). In the case of 1-bromo-2-nitrobenzene, again bio-Pd was selective for the dehalogenated product, nitrobenzene, whereas the commercial catalyst produced the salt aniline hydrobromide.

To explore the potential of complementary mirror-image oligonucleotides as recognition systems for pretargeting, the present work deals with radiopharmacological evaluation of an ⁸⁶Y-labeled 17mer l-DNA and of a hybrid consisting of the radiolabeled l-DNA and a complementary l-RNA (Figure 1). The 17mer l-DNA was functionalized with DOTA-Nhydroxysuccinimide ester and radiolabeled with ⁸⁶Y (t_1/2 = 14.7 h) resulting in the HPLCpurified [⁸⁶Y-DOTA]-17mer (31 MBq; 2.0 GBq/μmol). Biodistribution studies with [⁸⁶Y-DOTA]-17mer in Wistar rats showed high renal excretion and moderate kidney uptake (SUV: 18 ± 2.4, 18 h p. i.). More than 87% of [⁸⁶Y-DOTA]-17mer were found intact in rat urine 60 min after injection. In thermal denaturation studies, a high melting point of 68°C (32 mM [Na⁺]) was determined for the hybrid. In conclusion, the high metabolic stability of the l-DNA and the high thermal stability of the l-DNA/l-RNA hybrid suggest the potential of this class of compounds as molecular probes for pretargeting.

We investigated the structure of uranyl sorption complexes on gibbsite (pH 5.6 - 9.7) by two independent methods, density functional theory (DFT) calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy at the U-LIII edge. To model the gibbsite surface with DFT, we tested two Al (hydr)oxide clusters, a dimer and a hexamer. Based on polarization, structure, and relaxation energies during geometry optimization, the hexamer cluster was found to be the more appropriate model. An additional advantage of the hexamer model is that it represents both edges and basal faces of gibbsite. The DFT calculations of (monomeric) uranyl sorption complexes show an energetic preference for the corner-sharing versus the edge-sharing configuration on gibbsite edges. The energy difference is so small, however, that possibly both surface species may coexist. In contrast to the edge sites, sorption to basal sites was energetically not favorable. EXAFS spectroscopy revealed in all investigated samples the same interatomic distances of the uranyl coordination environment (RU-Oax ≈ 1.80 Å, RU-Oeq ≈ 2.40 Å), and towards the gibbsite surface (RU-O ≈ 2.87 Å, RU-Al ≈ 3.38 Å). In addition, two U-U distances were observed, 3.92 Å at pH 9.7 and 4.30 Å at pH 5.6, both with coordination numbers of ~ 1. The short U-U distance is close to that of the aqueous uranyl hydroxo dimer, UO2(OH)2, reported as 3.875 Å in the literature, but significantly longer than that of aqueous trimers (3.81-3.82 Å), suggesting sorption of uranyl dimers at alkaline pH. The longer U-U distance (4.30 Å) at acidic pH, however, is not in line with known aqueous uranyl polymer complexes. Based on the EXAFS findings we further refined dimeric surface complexes with DFT. We propose two structural models: in the acidic region, the observed long U-U distance can be explained with a distortion of the uranyl dimer to form both a corner-sharing and an edge-sharing linkage to neighboring Al octahedra, leading to RU-U = 4.150 Å. In the alkaline region, a corner-sharing uranyl dimer complex is the most favorable. The U-O path at ~2.87 Å in the EXAFS spectra arises from the oxygen atom linking two Al cations in corner-sharing arrangement. The adsorption structures obtained by DFT calculations are in good agreement with the structural parameters from EXAFS analysis: U-Al (3.394 Å), U-U (3.949 Å), and U-O (2.823 Å) for the alkaline pH model, and U-Al (3.279 Å), U-U (4.150 Å), and U-O (2.743 Å) for the acidic pH model.

Encapsulated nanostructures formed by surface diffusion assisted phase separation during thin film growth are promising candidates for the multifunctional devices, high density magnetic storage media, multifunctional coatings, as large scale templates for nanowire fabrication. In this talk our activities concerning the investigation of the growth mechanisms, structure of the metallic ‘inclusion’ and carbon ‘tissue’ phase as well as their interface structure of carbon: transition metal (TM=V,Co,Ni,Cu) thin films will be summarized. It will be demonstrated that a combined use of laboratory and element selective synchrotron radiation based analytical tools allows identification of the morphology, nearest neighbour coordination as well as the electronic structure of the metallic nanoparticles and the carbon matrix individually and tracking of their changes as a function of the growth temperature, metal type and metal content.

Thesis For the purpose of fission-fragment detection a double time-of-flight (TOF) spectrometer has been developed. The key component of the spectrometer is a TOF detector consisting of multichannel-plate (MCP) detectors with a position-sensitive readout, a foil for secondary electron production and an electrostatic mirror. The spectrometer performance is tested at tandem and superconducting linear accelerator.

The magnetorotational instability (MRI) is widely believed to play an essential role in the formation of stars and black holes. Destabilizing hydrodynamically stable Keplerian flows, the MRI triggers turbulence and enables outward transport of angular momentum in accretion discs which is necessary to explain the mass accumulation rates of central objects. The Potsdam Rossendorf Magnetic InStability Experiment (PROMISE) is intended to study the helical version of MRI which works already at Reynolds numbers of the order of 1000 and Hartmann numbers of the order 10. We focus on the results of an improved experiment in which split end caps are used to minimize the effect of Ekman pumping.

In the talk the advantages of semiconductor-based spintronics are presented.

Nd and Mn were codoped into ZnO films which have been grown on a-plane sapphire substrates by pulsed laser deposition with the thickness ranging between 46 and 971 nm. The room temperature resistivity of the codoped films is independent of the film thickness. Large positive magnetoresistance and clear anomalous Hall effect were observed at 5 K. Ferromagnetism with clear hysteresis up to 290 K was observed. Codoping is suggested to be an efficient method to introduce energy levels in the ZnO band gap to mediate electron spins of the magnetic doping ions.

For ZnO, the optical dielectric functions for polarizations parallel and perpendicular to the optical axis were determined in the photon energy range from 4.0 to 9.5 eV by using generalized spectroscopic ellipsometry and the band structure was calculated by means of the empirical pseudopotential method. From the band structure, a theoretical dielectric function was derived. The dielectric functions reveal features that were identified as band-to-band transitions. The energies of those transitions were assigned to band-to-band transition energies of critical points of the calculated band structure.

Besides other one of the remarkable properties making wurtzite ZnO such an interesting material is its large exciton binding energy of about 60 meV, leading to stable excitons at room-temperature. Also, the Curie temperature of this wide-gap material has been predicted to lie above room temperature, making ZnO alloyed with magnetic ions a possible material for spintronics applications. One big challenge in the fabrication of the ZnO-based heterostructure devices is the lattice mismatch between the ZnO films and the substrates and the different thermal expansion coefficient inducing biaxial strain. This work reports on the electronic band structure of biaxially strained ZnO for strains along the a- or c-axis ranging from -1% to 1 %, as calculated by means of the empirical pseudopotential method. Thereby, we also account for relativistic effects in the form of the spin-orbit interaction, as well as for the energy dependence of the crystal potential through the use of nonlocal model potentials. Moreover, the application of a variable plane wave basis set allows us to directly obtain the strain-induced variations of the electronic and the optical properties of wurtzite ZnO.

Homoepitaxial ZnO films deposited on annealed hydrothermal O-face ZnO single crystals show superior structural quality. This is demonstrated by narrow ZnO(00.2) rocking curves with FWHM of typically 23 to 35 arcsec, and nearly dislocation-free TEM cross sections. Nominally undoped ZnO films indicate a minor in-plane strain of about 250 ppm and no out-of-plane strain. Target doping by 0.01% P2O5 or 0.5% Li3N results in pseudomorphic film growth without in-plane strain. Increasing doping concentration of 0.1 and 1% P2O5 results in both in-plane and out-of-plane strain up to 0.9 % indicating relaxed films. The O-face polarity of the homoepitaxial ZnO films is confirmed by convergent beam electron diffraction.

Ac THz electric fields which couple strongly with intraband excitations in semiconductors can lead to spectral sidebands when an interband excitation is present. In this nonlinear mixing process a near-infrared (NIR) laser beam is mixed with a THz beam to generate sidebands around the NIR frequency with a frequency spacing equal to the THz frequency or multiples of it. In the last years this effect has been investigated in various semiconductor systems (i.e. in bulk GaAs or in multi quantum wells).
We investigated the third-order nonlinear mixing process between a near-infrared laser and a free-electron laser in an undoped symmetric AlGaAs/GaAs multi quantum well. Differently from the literature where electronic intersubband transitions were used, we are using the transition between the heavy-hole and light-hole states. This transition around 73 µm is pumped close to normal incidence by FELBE, the free-electron laser (FEL) of the Forschungszentrum Dresden-Rossendorf. The picosecond NIR laser is transmitted through the sample where the GaAs substrate has been etched away. It is focused on the entrance slit of the spectrometer of a Streak-camera system. With the Streak-camera temporal and spectral measurements are possible.
The n=+2 sideband conversion efficiency is of the order of 0,005% with respect to the incoming NIR intensity. Among other things we present the power dependency on NIR and FEL intensity and the resonance behavior with respect to the NIR and FEL wavelengths.

The transport properties of phosphorous-doped ZnO thin films, grown by pulsed-laser deposition on thermally pretreated hydrothermally grown ZnO single-crystal substrates, are reported. The ZnO:P thin films show very good morphological and structural properties as confirmed by atomic force microscopy (AFM), high resolution x-ray diffraction, and Rutherford backscattering (RBS) channeling. Steps of height c/2 are visible in AFM investigations for all samples. For an oxygen partial pressure of 0.1 mbar, two-dimensional growth was found. RBS channeling of a ZnO:P film shows a minimum yield of 0.034 which is comparable to that of an annealed substrate (0.033). Hall effect measurements revealed that all films are n-type for the present growth conditions. Peak mobilities of 800 cm^2/Vs have been observed around 70 K, in line with the high structural quality of the samples. Room-temperature mobility in ZnO:P is up to 170 cm^2/Vs.

The formation of surface-nanostructures with a characteristic size ranging from several nanometer up to microns has attracted significant interest in the last decades in the context of fabrication of novel opto-electronic and storage devices. One kind of those nanostructures are wave-like patterns (ripples) produced by an interplay between a roughening process caused by ion beam erosion (sputtering) of the surface and smoothening processes caused by surface diffusion. In this contribution we report on investigations of patterned Si (001) surfaces after irradiation with Xe-ions using ion-energies up to 70keV. During the sputtering, an amorphous surface-layer is formed followed by a rather sharp interface towards crystalline material, showing the same morphology as the surface. The structures of the amorphous layer and the amorphous-crystalline interface were studied by means of grazing incidence- small angle scattering (GISAXS) and diffraction (GID) using synchrotron-radiation. We found that the crystal structure at the interface is expanded along the ripples, caused by the creation of defects inside the surface region, whereas this expansion is strongly reduced across the ripples, which can be explained by an anisotropic defect distribution close to the amorphous-to-crystalline interface.