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)