Physikalisches Kolloquium

Vorträge im SS 2014

Physikalisches Kolloquium der Universität Kiel

 

Dienstag, 29. Apr. 2014 - Prof. K. Meier, KIP Universität Heidelberg

From Ions to Electrons - Physical Models of Brain Circuits

Dienstag, 06. Mai 2014 - Prof. B. Abel, IOM Leipzig / U Leipzig

Ultrafast spectroscopy near liquid water interfaces employing high-harmonics radiation

Dienstag, 13. Mai 2014 - Frau Dr. Tanja Mehlstäubler, Physik. Techn. Bundesanstalt

Relativistic Geodesy with Optical Clock

Dienstag, 27. Mai 2014 - Dr. Julia Stähler, F.-Haber-Institut Berlin

Ultrafast dynamics of exciton formation at the ZnO (10-10) surface

Dienstag, 03. Jun. 2014 - Prof. G. Brezesinski, MPI Potsdam

Amphilic molecules confined at the air / liquid interface

Montag, 16. Jun. 2014 (Sonderkolloqium) - Dr. Dieter Bilitza, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA und George Mason University, Space Weather Laboratory, Fairfax, Virginia, USA

Die Ionosphäre - Einführung und neueste Ergebnisse

Dienstag, 24. Jun. 2014 - Dr. D. Lundin

Thin film deposition by plasma-based processes

Dienstag, 01. Jul. 2014 - Dr. Birger Buttenschön, Max-Planck-Institut für Plasmaphysik, Greifswald

Plasma Wakefield Acceleration - Ein neues Beschleunigerkonzept zur Erzeugung hochenergetischer Elektronen- und Positronenstrahlen

Dienstag, 08. Jul. 2014 - Dr. Klaus Ellmer, Helmholtz-Zentrum Berlin

Großflächiges Magnetronsputtern von Solarzellen und Wasserspaltungselektroden für zukünftige regenerative Energieversorgung

Vorträge im WS 2016/2017

Kolloquiumsvorträge (dienstags, 12.15 Max-Planck Hörsaal)

  • 18. Oktober

    Bridget Murphy (IEAP)  Murphy.pdf
    When (two) surfaces meet
     
  • 25. Oktober 2016

    Rodger Thomson (Tucson)  Thompson.pdf
    Testing Dark Energy and New Physics with Fundamental Constants
     
  • 15. November 2016

    Hanno Kählert (ITAP) Kählert.pdf
    Challenges for theory in strongly coupled (complex) plasmas

     
  • 13. Dezember 2016

    Alexey Chernikov (Univ. Regensburg)
    Excitons in 2D materials

    Since the discovery of graphene, a single sheet of carbon atoms, research focused on two-dimensional (2D) van der Waals materials evolved rapidly due the availability of atomically thin, thermally stable crystals with intriguing physical properties. The 2D materials naturally inherit major traits associated with systems of reduced dimensionality: strongly enhanced interactions, efficient light-matter coupling, and sensitivity to the environment. In particular, the considerable strength of the Coulomb forces, i.e., electrical attraction and repulsion between the charge carriers, introduces a rich variety of many-body phenomena. It leads to the emergence of atom-like electron-hole quasi-particles, such as excitons, trions, and biexcitons, with unusually high binding energies and efficient light absorption.

     

    In this talk, I will focus on the physics of excitons in semiconducting 2D materials, largely determining the optical response of these ultra-thin layers. The observation of exciton binding energies on the order of many 100’s of meV and the marked deviation of the electron-hole attraction from the conventional Coulomb law will be discussed. The results reflect both strong carrier confinement and the distinctive nature of the dielectric screening in atomically thin systems. I will further describe how some of the more extreme non-equilibrium conditions such as strong photo-excitation and high electrical doping can profoundly alter the many-body interactions.



  • 17. Januar 2017

    Karl Jakobs (Uni Freiburg)

    From the Discovery of the Higgs Boson to the Search for Dark Matter

    -New results from the LHC-

     

    With the discovery of the Higgs boson by the two large experiments ATLAS and CMS at the Large Hadron Collider (LHC) at the European laboratory for particle physics CERN in Geneva, an important milestone in the investigation of the fundamental interactions was reached. Despite this discovery important questions remain open: does the discovered particle have the properties as predicted by the Brout-Englert-Higgs mechanism in the Standard Model or does it show devi­ations pointing to new physics? Are there new symmetries and -linked to them- new particles that could explain the Dark Matter in the Universe?

     

    In June 2015 a new data-taking period has started and proton-proton collisions are investi­gated with an energy of 13 TeV –nearly twice the collision energy used in the previous years. A new energy window opens up where the above-mentioned questions can be addressed. In the talk, the current status of the measurements at the LHC is presented. In particular, the properties of the discovered Higgs boson and the search for candidate Dark Matter particles are discussed.

     


     
  • 24. Januar 2017

    Angel Rubio (Max Planck Institute for the Structure and Dynamics of Matter, Hamburg)

    Modeling Light-Matter interaction: From Weak to Strong Coupling in QED Chemistry and Materials

    Computer simulations that predict the light-induced change in the physical and chemical properties of complex systems, molecules, nanostructures and solids usually ignore the quantum nature of light.  We have recently shown how the effects of the photons can be properly included in such calculations.  The basic idea is to treat the full QED system of particles and photons as a quantum fluid. Here the particles are represented by a charge current, and the photons by a classical electromagnetic field that acts on the current in a very complex manner.  This study opens up the possibility to predict and control the change of material properties due to the interaction with light particles from first principles .
     
    Here we will review the recent advances within density-functional a schemes to describe spectroscopic properties of complex systems with special emphasis to modeling  time and spatially resolved electron spectroscopies   We will discuss the theoretical approaches developed in the group  for  the characterization of matter out of equilibrium, the control material processes at the electronic level and tailor material properties, and  master energy and information on the nanoscale to propose new devices with capabilities. We will focus on examples  linked to the efficient conversion of light into electricity or chemical fuels ("artificial photosynthesis") and the design on new nanostructure based optoelectronic devices, among others.  
     
    Our  goal is to provide a detailed, efficient, and at the same time accurate microscopic approach for the ab-initio description and control of the dynamics of decoherence and dissipation in quantum many-body systems. This theoretical framework provides a new way to control and alter chemical reactions in complex systems,  direct the movement of electrons, selectively trigger physico-chemical processes, and create new state of matter.

 

  • 31. Januar 2017

    Axel Gross (Uni Ulm)
    Challenges in the theoretical description of electrochemical energy storage and conversion

    In spite of its technological relevance in the energy conversion and storage, our knowledge about the microscopic structure of electrochemical electrode-electrolyte interfaces and electrical double layers is still rather limited. The theoretical description of these interfaces from first principles is hampered by three facts. i) In electrochemistry, structures and properties of the electrodeelectrolyteinterfaces are governed by the electrode potential which adds considerable complexity to the theoretical treatment since charged surfaces have to be considered. ii) The theoretical
    treatment of processes at solid-liquid interfaces includes a proper description of the liquid which requires to determine free energies instead of just total energies. This means that computationally expensive statistical averages have to be performed. iii) Electronic structure methods based on
    density functional theory (DFT) combine numerical efficiency with a satisfactory accuracy. However, there are severe shortcomings of the DFT description of liquids, in particular water, using current functionals.

    Despite these obstacles, there has already significant progress been made in the first-principles modeling of electrochemical electrode-electrolyte interfaces. In this contribution, I will present our attempts to contribute to this progress by systematically increasing the complexity of the considered systems [1]. Different approaches to describe aqueous electrolytes at electrodes using first-principles calculations will be compared: the electrolyte can be described either as a thermodynamic reservoir or using implicit of explicit solvent models [2,3]. The equilibrium coverage of specifically adsorbed anions such as halides will be addressed which is an integral part of the realistic modeling of electrochemical double layers [2]. Furthermore, the modelling of electrocatalytic reactions occurring in fuel cells [3] will be presented. Finally, first attempts to model
    structures and processes in batteries using electronic structure calculations will be presented.

    [1] N. Hörmann, M. Jaeckle, F. Gossenberger, T. Roman, K. Forster-Tonigold, M. Naderian, S. Sakong, and A. Groß, Some challenges in the first-principles modeling of structures and processes in electrochemical energy storage and transfer, J. Power Sources 275, 531-538 (2015).

    [2] F. Gossenberger, T. Roman and A. Groß , Hydrogen and halide co-adsorption on Pt(111) in an electrochemical environment: a computational perspective, Electrochim. Acta 216, 152-159 (2016).

    [3] S. Sakong and Axel Groß, The importance of the electrochemical environment in the electrooxidation of methanol on Pt(111), ACS Catal. 6, 5575 (2016).
     
  • 07. Februar 2017

    Ronald Redmer (Universität Rostock)
    Warm Dense Matter - Probing Planetary Interiors

    The behaviour of warm dense matter (pressures up to the TPa region and temperatures up to several eV) is of paramount importance for understanding the interior, evolution and magnetic field of solar and extrasolar planets. While the light elements H and He are the main components of gas giants like Jupiter, mixtures of H-He-C-N-O are relevant for Neptune-like planets, and minerals of the MgO-FeO-SiO2 complex are the building blocks of rocky planets (Earth, super-Earths). The high-pressure phase diagram of these elements and mixtures has to be known in order to develop corresponding models. Of special interest in this context is the location of the melting line, of potential regions of phase separation and metal-insulator transitions. These high-pressure phenomena have a strong impact on interior, evolution, and dynamo models for planets and, simultaneously, constitute a major challenge to plasma and computational physics. 
    Molecular dynamics simulations based on finite-temperature density functional theory are used to predict the equation of state, the high-pressure phase diagram, and the transport properties of warm dense matter for a wide range of densities and temperatures as typical for the interior of planets. These data are benchmarked against diamond-anvil-cell and shock-wave experiments and then applied to construct interior and evolution models for solar and extrasolar planets.

     
  • 25. April 2017

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

     
  • 16. Mai 2017

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

     
  • 30. Mai 2017

    Claus Lämmerzahl (ZARM, Univ. Bremen)
    Quantenmechanik und Gravitation
     
  • 18. Juli 2017

    Reserviert (A. Piel)

     

 

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