Orbital Magnetic Ordering in Disordered Mesoscopic Systems

We present some model calculations of persistent currents in disordered one- and two-dimensional mesoscopic systems. We use the tight-binding model and calculate numerically the currents in small systems for several values of disorder. Next we fit appropriate analytical formulae, and using them we find self- -sustaining currents and critical fields in larger, more realistic systems with different shapes of the Fermi surfaces.Comment: 16 pages, LaTeX, 8 figures, in print in J. Magn. Magn. Ma

An Atom Faucet

We have constructed and modeled a simple and efficient source of slow atoms. From a background vapour loaded magneto-optical trap, a thin laser beam extracts a continuous jet of cold rubidium atoms. In this setup, the extraction column that is typical to leaking MOT systems is created without any optical parts placed inside the vacuum chamber. For detailed analysis, we present a simple 3D numerical simulation of the atomic motion in the presence of multiple saturating laser fields combined with an inhomogeneous magnetic field. At a pressure of $P_{\rm Rb87}=1 \times 10^{-8}$ mbar, the moderate laser power of 10 mW per beam generates a jet of flux $\Phi =1.3\times 10^8$ atoms/s with a mean velocity of 14 m/s and a divergence of $<20$ mrad.Comment: Submitted to EPJD. 1 TeX file (EPJ format), 7 picture

Vortex formation in a stirred Bose-Einstein condensate

Using a focused laser beam we stir a Bose-Einstein condensate of 87Rb confined in a magnetic trap and observe the formation of a vortex for a stirring frequency exceeding a critical value. At larger rotation frequencies we produce states of the condensate for which up to four vortices are simultaneously present. We have also measured the lifetime of the single vortex state after turning off the stirring laser beam.Comment: 4 pages, 3 figure

Femtosekunden-Spektroskopie biologischer Systeme mittels kohärenter Kontrolle

This doctoral thesis presents new approaches for the characterisation of ultrafast energy flow in complex systems, based on concepts of coherent control. By initiating a photoreaction with femtosecond pulses whose temporal phase and amplitude are shaped in such a manner that specific molecular vibrations and states are addressed, the energy flow can be steered at will. The comparison between the ensuing energy flow patterns following shaped and unshaped excitation pulses constitutes a differential measurement of the function of the controlled vibrations and states within the photoreaction. Coherent control as a spectroscopic tool is first applied to biological systems, specifically the light harvesting complex LH2 from the photosynthetic purple bacterium Rhodopseudomonas acidophila, and the isolated carotenoid donor of the same complex. The pump-probe method using shaped excitation pulses is shown to be successful for the first time in controlling the natural function of a biological system, namely the flow of excitation energy in the complex network of states in LH2. By means of a closed-loop optimisation of parametrised excitations, a bending mode in the carotenoid donor can be identified as being responsible for steering the energy flow. This bu vibrational mode couples the carotenoid S2-S1 states; its frequency is determined to be 160±25cm-1. Furthermore the deactivation of the carotenoid S2 state in LH2 and in solution is studied with pump-probe and pump-deplete-probe spectroscopy. Here it is shown that there exists an alternative singlet state S*T (1Bu-) involved in the deactivation process, though only in LH2. Its function as a precursor of ultrafast triplet population and as a donor for photosynthetic energy transfer is characterised with a novel evolutionary target analysis of conventional pump-probe spectra. Secondly, coherent control as a measurement technique is applied to another extremely complex system, in this case a material dominated by non-linear interactions with instantaneous dynamics: Propagation of femtosecond pulses in optical fibres that are only a few micrometers in diameter to generate a supercontinuum of optical frequencies. Here shaped pump pulses succeed in resolving for the first time the sequential steps leading to the enormous spectral broadening. Open-loop variations of precompression allows the evolution and fission of optical solitons to be followed, while closed-loop optimisations render observable the coupling of solitons with phase-matched visible frequencies. On atoms, finally, open-loop control of interfering pathways from the ground to the excited state by application of strongly modulated spectra seeks to establish a direct link between coherent control experiments and theory. The novel phenomenon of a Fresnel zone plate in the time domain is first developed in theory and then successfully realised in experiment.Die vorliegende Dissertation beschreitet neue Wege zur Charakterisierung des ultraschnellen Energieflusses in komplexen Systemen, indem Konzepte der kohärenten Kontrolle als neuartige Untersuchungsmethode in die Femtosekunden-Spektroskopie eingeführt werden. Die Anregung einer Photoreaktion mit Femtosekunden-Impulsen, deren zeitliche Phase und Amplitude derart geformt ist, daß spezifische Schwingungen und Zustände angesprochen werden, bewirkt die gezielte Steuerung des Energieflusses. Die Unterschiede des Energieflusses nach geformter oder ungeformter Anregung erlauben eine differentielle Messung der Funktion der kontrollierten Schwingungen und Zustände innerhalb der Photoreaktion. Die kohärente Kontrolle als Werkzeug der Spektroskopie wird einerseits an biologischen Systemen angewandt, dem Lichtsammelkomplex LH2 aus Rhodopseudomonas acidophila, einem photosynthetischen Purpurbakterium, und am Carotinoid als isolierten Donor des Komplexes. Es gelingt mit der spektroskopischen Methode geformter-Pump-Probe erstmals, die natürliche Funktion eines biologischen Systems, den Fluß der Anregungsenergie im komplexen Netzwerk von Zuständen in LH2, zu kontrollieren. Mit Hilfe der closed-loop Optimierung parametrisierter Anregungen kann eine Biegeschwingung im Carotinoid als verantwortlich für die Steuerung des Energieflusses identifiziert werden. Diese Schwingung koppelt durch ihre bu Symmetrie die Carotinoid S2-S1 Zustände und konnte zu 160±25cm-1 bestimmt werden. Mit Pump-Probe und Pump-deplete-probe Spektroskopie an der Desaktivierung von Carotinoid S2 in LH2 und in Lösung wird die Existenz eines alternativen Singulettzustands S*T (1Bu-) bewiesen, der nur in LH2 aktiv an der Desaktivierung beteiligt ist. Seine Funktion als Zwischenzustand ultraschneller Besetzung von Tripletts und als Donor des photosynthetischen Energietransfers kann mit einer neuartigen evolutionären Target-Analyse von konventionellen Pump-Probe Spektren charakterisiert werden. Die kohärente Steuerung als Meßmethode wird andererseits am gegensätzlichen Extrem eines komplexen Systems angewandt, das durch eine nichtlineare statt linearer Wechselwirkung beherrscht wird, aber dafür eine instantane und nicht molekulare Dynamik zeigt: Die Erzeugung eines Superkontinuums aus Femtosekunden-Impulsen in optischen Fasern mit wenigen Mikrometern Durchmesser. Hier können mittels geformter Pump-Impulse erstmals die einzelnen Schritte der extremen spektralen Verbreiterung aufgelöst werden: Mit der open-loop Variation der Vorkompression läßt sich die Bildung und Spaltung optischer Solitonen verfolgen, und mit closed-loop Optimierungen wird deren Kopplung mit phasenangepaßten sichtbaren Frequenzen direkt beobachtbar. An Atomen wird schließlich eine mögliche direkte Verbindung der kohärenten Kontrolle zu theoretischen Rechnungen gesucht, indem die open-loop Kontrolle des Weges vom Grund- zum angeregten Zustand durch Anwendung komplex modulierter Spektren demonstriert wird. Es wird das neuartige Phänomen einer Fresnel-Zonenplatte in der Zeit zunächst theoretisch abgeleitet und darauf experimentell erfolgreich realisiert

Regulierung der Krautfäule(Phytophthora infestans) mit reduzierten Kupfer-Aufwandmengen im Kartoffelanbau

Four field experiments were conducted in 2004 to analyse the efficacy of reduced rates of copper fungicides against Phytophthora infestans (3 x organic and 1 x conventional growing). Copper (Cu-hydroxide, Cu-oxychloride or Cu-octanoate) was sprayed in a weekly routine or according to the decision support system SIMPHYT with total dosages ranging from 0.57 kg/ha to 4.0 kg/ha. The results revealed in almost all treatments a significant reduction of late blight, except for the lowest rates (e.g. 4 x 0.14 kg/ha Cu as Cu-octanoate or 6 x 0.15 kg/ha Cu as Cu-hydroxide). There was a clear dose-response relation between increasing rates of copper and control of late blight in each experiment. Marketable yields varied from 132 dt/ha to 238 dt/ha in organic field trials depending on trial site and copper treatment. Generally, an effect of copper treatment and late blight on yield could be shown. Even a slight reduction of P. infestans increased yield by 10 % to 20 %. The influence of copper dosage on late blight was more obvious than influence of copper spraying on yield. A dropleg spraying system (copper application within the crop canopy) which was tested on one trial site did not improve blight control compared to conventional application

Athermal Jamming vs. glassy dynamics for particles with exponentially decaying repulsive pair interaction potentials with a cutoff

We study athermal jamming as well as the thermal glassy dynamics in systems composed of spheres that interact according to repulsive interactions that exponentially decay as a function of distance. As usual, a cutoff is employed in the simulations. While the athermal jamming transition that is determined by trying to remove overlaps is found to depend on the arbitrary and therefore unphysical choice of the cutoff, we do not find any athermal jamming transition or crossover that only relies on the physical decay length. In contrast, the glassy dynamics mainly depends on the decay length. Our findings constitute another demonstration of the fact that the athermal jamming transition is not related to thermal glassy dynamics. In addition, we argue that interactions without sharp physical cutoff should be considered more often as a model system in jamming. By exploring how widely-used theoretical approaches or methods of analysis in the field of jamming have to be changed in order to not depend on unphysical cutoffs will lead to deeper insights into the nature of athermal and thermal jamming.Comment: 6 pages, 3 figure

Mixture of ultracold lithium and cesium atoms in an optical dipole trap

We present the first simultaneous trapping of two different ultracold atomic species in a conservative trap. Lithium and cesium atoms are stored in an optical dipole trap formed by the focus of a CO$_2$ laser. Techniques for loading both species of atoms are discussed and observations of elastic and inelastic collisions between the two species are presented. A model for sympathetic cooling of two species with strongly different mass in the presence of slow evaporation is developed. From the observed Cs-induced evaporation of Li atoms we estimate a cross section for cold elastic Li-Cs collisions.Comment: 10 pages 9 figures, submitted to Appl. Phys. B; v2: Corrected evaporation formulas and some postscript problem

Lessons Learned from the Grouping of Chemicals to Assess Risks to Human Health

In analogy to the periodic system that groups elements by their similarity in structure and chemical properties, the hazard of chemicals can be assessed in groups of similar structures and similar toxicological properties. Here we review case studies of grouping strategies that supported the assessment of hazard, exposure, and risk to human health. By the EU-REACh and the US-TSCA New Chemicals Program, structural similarity is commonly used as the basis for grouping, but that criterion is not always adequate and sufficient. Based on the lessons learned, we derive ten principles for grouping, including: transparency of the purpose, criteria and boundaries of the group; adequacy of methods used to justify the group; inclusion or exclusion of substances in the group by toxicological properties. These principles apply to initial grouping to prioritize further actions as well as to definitive grouping to generate data for risk assessment. Both can expedite effective risk management

Gene disruption and gene replacement in Streptomyces via single stranded DNA transformation of integration vectors

Hillemann D, Pühler A, Wohlleben W. Gene disruption and gene replacement in Streptomyces via single stranded DNA transformation of integration vectors. Nucleic Acids Research. 1991;19(4):727-731.For the isolation of single stranded plasmid DNA, various E. coli and E. coli- Streptomyces shuttle plasmids were equipped with the f1 replication origin. The transformation of some representative Streptomyces species with plasmid vectors occurred irrespective of whether single or double stranded DNA was used. In contrast, the transformation of Streptomyces was 10 to 100 times more efficient when an integration vector was in the single stranded form as opposed to the double stranded form. Streptomyces viridochromogenes was transformed by single stranded DNA integration vectors in order to replace the pat by the tsr gene and generate mutants unable to synthesize phosphinothricin - tripeptide (PTT)