Stephanie S.M.H. Höhn1, Pierre A. Haas2, Raymond E. Goldstein1, Moritz Mercker3, Anna Marciniac-Czochra3 1: Department of Applied Mathematics and Theoretical Physics, University of Cambridge, UK 2: MPI-CBG, Dresden 3: Mathematikon, University of Heidelberg Events of cell sheet folding are essential during development, examples including gastrulation, neurulation, and organogenesis. The micro-alga Volvox is uniquely suited for studies on morphogenetic movements as it achieves cell sheet folding without cell divisions, intercalation or migration, which facilitates both experimental and computational approaches. Volvox embryos consist of a spherical cellular monolayer which inverts its curvature in order to expose its flagella. Volvox globator exhibits one of the most striking processes of cell sheet folding: Through inwards folding at the equator of the initially spherical cell sheet it adopts a mushroom shape and eventually turns itself entirely inside-out through an anterior opening . These global deformations are driven by several waves of active cell shape changes [2, 3]. A combination of advanced imaging and computational analyses is used to correlate cell shape changes, tissue contractility and the occurring tissue invagination and involution. The associated internal stresses are determined through laser ablation experiments which allow conclusions on the underlying forces as well as the elastic properties of dynamic cell sheets. This research is supported by a Wellcome Trust Investigator Award, 207510/Z/17/Z.  Höhn S and Hallmann A. BMC Biology 9, 89 (2011).  Höhn S, Honerkamp-Smith AR, Haas PA, Khuc Trong P, and Goldstein RE. Physical Review Letters 114, 178101 (2015).  Haas PA, Höhn S, Honerkamp-Smith AR, Kirkegaard JB, and Goldstein RE. PLOS Biology 16, e2005536 (2018).
In this talk, I will introduce the exceptional topological insulator (ETI), a non-Hermitian topological state of matter that features exotic non-Hermitian surface states which can only exist within the three-dimensional topological bulk embedding. I will show how this phase can evolve from a Weyl semimetal or Hermitian three-dimensional topological insulator close to criticality when quasiparticles acquire a finite lifetime. The ETI does not require any symmetry to be stabilized. It is characterized by a bulk energy point gap, and exhibits robust surface states that cover the bulk gap as a single sheet of complex eigenvalues or with a single exceptional point. The ETI can be induced universally in gapless solid-state systems, thereby setting a paradigm for non-Hermitian topological matter.
I did my postdoc in the biophysics group at MPI-PKS until March 2018, continued in an engineering company, and now work as a desk officer in the Saxon State Ministry for Energy, Climate Protection, Environment, and Agriculture. In my talk, I will briefly outline how I found my way and what are powerful and helpful techniques for my job, which I learned in academia. Afterward, I am looking forward discussing this topic with you.
Two-dimensional electron gases under a strong magnetic field have tremendously expanded our understanding of many-body physics, with the discovery of integer and fractional quantum Hall effects, together with chiral edge states, fractional excitations, anyons. Another striking effect is the strong coupling between charge and spin/valley degrees of freedom, which takes place near integer filling of the magnetic Landau levels. More precisely, because of the large energy gap associated to cyclotron motion, any slow spatial variation of the spin background induces a variation of the electronic density proportional to the topological density of the spin background. Minimizing Coulomb energy leads to an exotic class of two-dimensional crystals, which exhibit a periodic non-collinear spin texture called a Skyrmion lattice. I will review the history of these concepts, with an emphasis on the notions of Berry phases and Berry curvature, which play a prominent role in all aspects of topological condensed matter physics. A main feature of these Skyrmion crystals is that their theoretical description involves effective theories with local and possibly non-Abelian gauge symmetries. It is therefore a theoretical challenge to identify the actual physical degrees of freedom in such systems. I will show how this may be achieved using some ideas from complex geometry. The main outcome of such analysis is the existence of two regimes depending on whether the topological charge per unit cell is smaller (unfrustrated case) or larger (frustrated case) than the number of internal states (spin/valley) accessible to electrons.