Seminar on the Theory of Complex Systems  Summer Term 2018

To be confirmed:

• Mario Nicodemi, University of Nepals and Max Dellbruck Center Berlin

Confirmed:

• Thu 17.05.18   at 14 c.t.
  Pierre Gönczy   EPFL Lausanne
  Mechanisms of centriole assembly
  

  The centriole is a remarkable microtubule-based organelle that is essential for the formation of cilia, flagella and centrosomes. The centriole is organized around a nine-fold symmetrical cartwheel typically ~100 nm in height, which is critical for the onset of organelle biogenesis. The cartwheel comprises a stack of ring-containing entities that each accommodates nine homodimers of SAS-6 proteins. In contrast to the knowledge about the self-assembly properties of SAS-6 proteins, the mechanisms enabling ring stacking are poorly understood. Furthermore, the assembly dynamics of SAS-6 ring-containing entities remains elusive. After introducing the subject matter, I will report notably on our development of a cell-free assay to address this important open question using the Chlamydomonas reinhardtii SAS-6 protein CrSAS-6. Using high-speed atomic force microscopy (AFM)-based, we monitored the assembly dynamics of CrSAS-6 homodimers, and thus determined possible routes and kinetic rates for ring formation.

• Thu 14.06.18   at 14 c.t.
  Falko Ziebert   Uni Heidelberg
  Substrate-based self-propulsion in biology and soft matter systems
  

  Self-propulsion, i.e. self-organized motion in the absence of external forces, is an active research topic in non-equilibrium physics. Depending on the system, open questions span from the propulsive force generation and transfer, over guiding mechanisms to collective effects in ensembles of self-propellers. The living realm presents plenty of examples, from crawling cells and swimming bacteria up to animal herds, having to move to survive and/or to fulfill their function. As living systems are complex, several biomimetic physico-chemical systems have been proposed in the last decade. Most of these, however, are microswimmers while artificial substrate-based propellers remain scarce.
  After a general introduction I discuss two examples for substrate-based motion: first, I will give an introduction to the crawling motility of eukaryotic cells and survey our recent advances in its modeling. A modular approach, based on the phase field method to track the deformable and moving cells, allows us to describe, e.g., cell movement on structured substrates or in confinement, and collective cell migration. I will also discuss the example of cellular shape waves, where the computational approach allows for additional insight via semi-analytic methods.
  In the second part I present a novel mechanism for self-propulsion, developed in collaboration with experiments at the Institut Charles Sadron in Strasbourg: namely, a thermally induced instability of elastic polymeric fibers towards rolling motion. I explain the mechanism theoretically and demonstrate its versatility by employing it to develop a minimal motor and a simple energy storage device.

• Thu 05.07.18   at 14 c.t.
  Eckhard Hitzer   International Christian University
  Geometric algebra and physics: examples of special relativity and space group visualization
  

  This talk first reviews the integration of relativistic physics through the works of Hamilton, Grassmann, Maxwell, Clifford, Einstein, Hestenes and lately the Cambridge (UK) Geometric Algebra Research Group. We start with the geometric algebra (Clifford algebra) of spacetime (STA). We show how frames and trajectories are described and how Lorentz transformations acquire their fundamental rotor form. Spacetime dynamics deals with spacetime rotors, which have invariant and frame dependent splits. Spacetime rotor equations yield the proper acceleration (bivector) and the Fermi (vector) derivative. A first application is given with the relativistic STA formulation of the Lorentz force law, leading to the description of spin precession in magnetic fields and Thomas precession. Now the stage is ready for introducing the STA Maxwell equation, which combines all 4 equations in one single STA equation. STA has procedures to extract from the electromagnetic field strength bivector F, electric and magnetic fields (also for relative motion observers) and field invariants, field momentum and stress-energy tensor. The Liénard-Wiechert potential gives the retarded field of a point charge. In addition, we formulate the Dirac equation in STA, both massless and massive. From the Dirac equation we can derive STA expressions for Dirac observables. Plane wave states are described with the help of rotor decomposition. Secondly, we briefly explain how the geometric product of crystal cell vectors generates all point groups. In the conformal version of geometric algebra, rotors also generate translations by cell vectors, providing a unified versor description of all space group symmetry transformations. This description has proven ideal for creating an animated, interactive, explorative software visualization of all three-dimensional space groups.

• Thu 12.07.18   at 14 c.t.
  Hideo Aoki   Department of Physics, University of Tokyo and AIST, Tsukuba, Japan
  Superconductivity in single- and multi-band correlated systems: can we optimise them?
  

  Various unconventional superconductors can be captured either with single-band models or multi-band ones. We then theoretically explore how we can optimise them for higher Tc's. Merits and demerits of the two classes are compared from both quantum many-body algorithms and materials-science points of view. For the former, I shall introduce D$\Gamma$A (dynamical vertex approximation) and DMFT + FLEX to fathom the correlation between the electronic structure and the superconductivity. An important point is "multibands" should not be confused with "multiorbital" systems. For the materials design, I shall present various ideas that include "hidden ladder" compounds and "flat-band" superconductivity.