Vorträge im SS 2017

Kolloquiumsvorträge (dienstags, 16.15 Hans-Geiger Hörsaal)

 

  • 18. April 2017

    Hans Böhringer (Max-Planck-Institut für Extraterrestrische Physik)

    Erforschung der Struktur unseres Universums mit Hilfe von Galaxienhaufen im Röntgenlicht

    Galaxienhaufen als größte klar definierte Objekte in unserem Universum sind ideale Testobjekte zur Vermessung der großräumigen, kosmischen
    Struktur und zum Test kosmologischer Modelle. Durch das mehrere 10 Millionen Grad heiße Plasma, das die Haufen ausfüllt, erscheinen die Haufen als besonders interessante Objekte in der Röntgenastronomie. Röntgenbeobachtungen bieten daher einen besonders guten Zugang zur Charakterisierung und zum Verständnis dieser Systeme.

    Aus dem Himmelsatlas im Röntgenbereich, der mit dem deutschen Röntgensatelliten ROSAT erstellt wurde, haben wir den größten Katalog röntgenleuchtender Galaxienhaufen ertellt und die Struktur von Stichproben dieser Haufen im Detail untersucht. Tests kosmologischer Modelle mit Hilfe dieser Daten liefern z.B. einen Wert für die mittlere Materiedichte unseres Universums.
    Das Ergebnis zeigt, dass wir zur Erklärung der Beobachtungen eine bisher unbekannte "Dunkle Materie" brauchen, und dass möglicherweise ein
    kleinerer Teil dieser Materie aus Neutrinos mit endlicher Restmasse besteht. Beim Studium der Materieverteilung im nahen Universum stellen wir fest, dass wir uns in einer Umgebungen mit weniger als mittlerer Materiedichte befinden, mit wichtiger Auswirkung auf die lokale Hubble Konstante im Vergleich zu ihrem globalen Wert.
     

  • 2. Mai 2017

    Thomas Frauenheim (Uni Bremen)
    Multi-scale modeling of nanostructured devices

    Presently electronic and optoelectronic devices scale down to nanometer sizes at which quantum atomistic modeling can be used to understand the fundamental mechanisms and optimize quality and performance. In this talk I am going to describe recent developments of DFTB-based atomistic and charge transport simulations addressing applications to ultra-scaled Silicon-on-Oxide electronic devices and nanostructured solar cells and light emitting diodes.

    (1) Ultimate scaling of Si MOSFETs leads to extremely thin and short channels, which are justifiably modeled at the atomic level. Currently, hydrogen-termination of the channel is used in device models, as a compromise between efficiency and accuracy. This work advances the state of the art by adopting a density-functional tight-binding (DFTB) Hamiltonian, permitting the inclusion of the confining oxide explicitly in the simulation domain in an ab initio fashion. Simulations of band-structure and electron transport in extremely thin SOI MOSFET are studied with this method, showing good agreement with experiment and reveal a large quantitative difference when compared to simulations with H-passivated channel. S. Markov et al. IEEE Transaction on Electron Devices 62 (2015) 696.

    (2) simulations of photovoltaic devices are based on classical models, which neglect the atomistic details and quantum-mechanical effects besides the dependence on many empirical parameters. Within the nonequilibrium Green’ s function formalism, we present a quantum-mechanical study of the performance of inorganic nanowire-based photovoltaic devices. On the basis of density-functional tight-binding theory, the method allows simulation of current− voltage characteristics and optical properties of photovoltaic devices without relying on empirical parameters. Numerical studies of silicon nanowire-based devices of realistic sizes with 10 000 atoms are performed, and the results indicate that atomistic details and nonequilibrium conditions have a clear impact on the photoresponse of the devices. Y. Zhang, et al. Journal of Ohys. Chem. Letters, 5 (2014) 1272.

    (3) Understanding of the electroluminescence (EL) mechanism in optoelectronic devices is imperative for further optimization of their efficiency and effectiveness. Here, a quantum mechanical approach is formulated for modeling the EL processes in nanoscale light emitting diodes (LED). Based on non-equilibrium Green’s function quantum transport equations, interactions with the electromagnetic vacuum environment are included to describe electrically driven light emission in the devices. The presented framework is illustrated by numerical simulations of a silicon nanowire LED device. EL spectra of the nanowire device under different bias voltages are obtained and, more importantly, the radiation pattern and polarization of optical emission can be determined using the current approach. This work is an important step forward towards atomistic quantum mechanical modeling of the electrically induced optical response in nanoscale systems. R. Wang, et al. Nanoscale ( (2016) 13168.

    If time allows, I will report on the time-dependant DFTB implementations and describe recent applications to nitric oxide degradation on titania surfaces and hot electron injection into TiO2 nanoparticles.
     

  • 16. Mai 2017

    Jürgen Blum (IGeP, TU Braunschweig)
    Die Entstehung der Planetesimale im jungen Sonnensystem
     

    Unser Wissen über die Entstehung der Planeten ist immer noch lückenhaft. Aus astronomischen Beaboachtungen können wir lernen, dass Planeten in so genannten protoplanetaren Scheiben entstehen, die zu 99% aus Gas und zu 1% aus mikroskopisch kleinen Staubpartikeln bestehen. In diesen Scheiben kollidieren die Staubteilchen, haften aneinander und bilden bis zu zentimetergroße Agglomerate. Dies konnte durch Modellierung, 
    Labormessungen und astronomische Untersuchungen bestätigt werden. Das weitere Wachstum hin zu Planetesimalen, den kleinsten gravitativ 
    gebundenen Körpern mit Durchmessern von einigen Kilometern, wird zurzeit noch kontrovers diskutiert. Die beiden favorisierten Modelle 
    unterscheiden sich darin, dass im einen das Wachstum aufgrund haftender Stöße fortschreitet, im anderen hingegen eine hydrodynamische 
    Instabilität in der protoplanetaren Scheibe, die "Streaming Instability", für eine so starke Konzentration der zentimetergroßen Staubklumpen sorgt, dass der gesamte Komplex gravitativ kollabiert.
    Mit der jüngst abgeschlossenen Mission Rosetta zum Kometen 67P/Tschurjumow-Gerassimenko konnte erstmals ein Körper aus den Frühzeiten des Sonnensystems im Detail untersucht werden. Die bislang vorliegenden Ergebnisse deuten darauf hin, dass Komet 67P durch einen gravitativen Kollaps in seiner heutigen Form entstanden ist. Wenn sich dies bestätigen sollte, liegt der Schluss nahe, dass Planetesimale nicht durch haftende Stöße, sondern durch Instabilitäten in protoplanetaren Scheiben entstehen.
     

  • 13. Juni 2017,
    Beginn 17.15Uhr (!)


    Tiberiu Minea (U Paris-Sud, Frankreich)
    Modeling of vacuum breakdown. Thermo-field, dynamics of microparticles and laser assisted electron emission


    Vacuum is often used as an isolator in numerous applications, such as X-ray tubes, particle accelerators, high voltage pass-through, etc. However, their performance is limited by the risk of unpredictable breakdown events between electrodes. Moreover, the breakdown usually leads to the formation of arc discharges, which can seriously damage the system.

    Since 2010 the LPGP develops a research program “High Voltage holding In Vacuum”, in collaboration with CEA and CentraleSupelec which aims to give a better description of the origin of the vacuum breakdown.


    Three numerical models have been developed to tackles three particular aspects of the problem.


    (i) The model OVIP (Orsay Vacuum Insulation Percolation) deals with the thermos-field electron emission from a surface microprotrusion and the results are in good agreement with the experimental results for breakdown. The operation with fast pulses allows to enhance the field emission avoiding the breakdown.

    (ii) The model OFEN (Orsay Field Emission Nanoparticles) describes the micro-particles (MP) transport in the inter-electrodes gap, in vacuum) and the interactions (heating and modification of the MP charge) between electrons and the MP. It is an extension of the Cranberg’s theory of clumps when the MP is exposed to an intense field ~1-5 MV/m and it is simultaneous bombarded by electrons released from the cathode micro-tips. The results clearly show four different regimes of MP trajectories obtained for different emission currents, MP sizes and inter-electrode distances and the effect of the MP crash of on the cathode, helping to understand the vacuum conditioning.

    (iii) The last model OFELIE (Orsay Field Emission and Light Emission) analysis the electron emission induced by picosecond laser from solid surfaces placed under an intense electric field. The results show an important difference between the electrons temperature (5500 K) and the phonons temperature (850 K). In these conditions, the Fermi-Dirac distribution depends of the electron temperature, while the thermo-field emission becomes effective for temperatures well below the fusion of the metal.

    In conclusion, the modeling of the related phenomena between solid and vacuum depends on the way the energy is transferred to the electrons and it helps to distinguish between different scenarios and to design performant systems.
     
  • 04. Juli 2017

    Eberhard Möbius (Space Science Center and Department of Physics, University of New Hampshire)

    Astronomy with Neutral Atoms - Imaging the Heliospheric Boundary and Catching the Interstellar Wind with the Interstellar Boundary Explorer

    400 years after Galileo pointed a telescope at celestial objects for the first time, neutral atoms were added to the astronomical toolbox with the Interstellar Boundary Explorer (IBEX), launched October 19, 2008. Since early 2009, two energetic neutral atom (ENA) cameras take global images of the solar system’s interaction with its galactic neighborhood. They have returned stunning images of the heliospheric boundary region, where the solar wind slows down in response to the surrounding interstellar medium, including the front and tail region of the heliosphere. Most unexpectedly, the images show a bright and persistent “Ribbon” across the sky, which provides a marker for the direction of the local interstellar magnetic field, but the processes leading to the bright ENA emission are still being investigated. Time variations in the ENA fluxes that vary with energy provide additional constraints and point to the neutral solar wind that penetrates beyond the boundary of the heliosphere. The IBEX-Lo camera catches the interstellar wind of neutral H, He, O, and Ne atoms that blows through the solar system with a speed of ≈26 km/s and arises from the motion of the Sun relative to the surrounding local interstellar gas cloud (LIC). This observed gas flow distribution is an excellent probe of the state of the LIC and shows clear signatures of the deflection of the interstellar plasma at the heliospheric boundary.


     
  • 11. Juli 2017

    Erwin O. Flückiger
    60 Jahre Neutronenmonitor

     
  • 18. Juli 2017, 15:00 Uhr

    Abschiedskolloquium von Prof. Piel