Seminar on the Theory of Complex Systems  Summer Term 2017

To be confirmed:

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

Winter Term

• Prof. Carsten Rockstuhl, KIT, Design of metamaterials: form follows function, 14.12.2017

Confirmed:

• Thu 18.05.17   at 14 c.t.
  Johannes Bausch   Cambridge University
  Size-Driven Quantum Phase Transitions
  

  Can the properties of the thermodynamic limit of a many-body quantum system be extrapolated by analysing a sequence of finite-size cases? We present a model for which such an approach gives completely misleading results: a translationally invariant, local Hamiltonian on a square lattice with open boundary conditions and constant spectral gap, which has a classical product ground state for all system sizes smaller than a particular threshold size, but a ground state with topological degeneracy for all system sizes larger than this threshold. Starting from a minimal case with spins of dimension 6 and threshold lattice size 15 × 15, we show that the latter grows faster than any computable function with increasing local spin dimension. The resulting effect may be viewed as a new type of quantum phase transition that is driven by the size of the system rather than by an external field or coupling strength. We prove that the construction is thermally robust, opening the possibility that these effects are accessible to experimental observation.

• Thu 22.06.17   at 14 c.t.
  Stefan Klumpp   Göttingen
  Population Dynamics of Bacterial Persistence
  

  Phenotypic bacterial tolerance to antibiotics provides a prime example of bet-hedging, a strategy to optimize survival under changing conditions: normally growing cells stochastically switch to a slow-growing, but antibiotic-tolerant persister state to survive through periods of exposure to antibiotics. The long-term fitness of such a population can be maximized by adjusting the rates of phenotype switching to characteristic environmental time scales. Based on an exact solution for the case of a periodic variation of the environment, I will discuss how phenotype switching emerges and under what conditions switching is or is not beneficial for long-time growth. I will show that there is a critical duration of stress (antibiotic exposure), at which a bifurcation occurs and beyond which switching is indeed beneficial. Time permitting, I will also discuss the relation of phenotypic tolerance to genetic resistance.

• Thu 29.06.17   at 14 c.t.
  Johannes Nübler   Massachusetts Institute of Technology
  Active polymer models for the 3D organization of chromosomes
  

  Eukaryotic chromatin, DNA together with associated proteins, is far from being simply a randomly arranged polymer in the cell nucleus. Rather, it has a high degree of spatial organization on length scales ranging from individual nucleosomes up to entire chromosomes of hundreds of millions of base pairs. Furthermore, chromatin is dynamic: for example, enhancers find promoters to activate genes and chromosomes segregate and compactify for cell division. We build polymer models in order to identify mechanisms behind the 3D organization of chromatin. The interplay of its polymeric nature with active formation of growing chromatin loops by molecular motors emerges as a general organizational principle throughout the cell cycle. Additionally, loop extrusion competes with spatial segregation of active and inactive chromatin. This interplay highlights that chromatin is complex, active matter. We address the physics of this interplay and it’s relation to nuclear organization.