Seminar on the Theory of Complex Systems  Winter Term 2015/2016

The harvest of solar photon energy is at the heart of photosynthetic systems realized by nature in large biomolecular complexes or in man made solar cell devices. After a photon has transferred its energy to form an exciton, the nonequilibrium quantum dynamics of this quasiparticle delivers the stored energy to a reaction center where chemical reactions are triggered. These first steps of photosynthesis have moved again into the focus of research, since recent advances in ultrafast optical spectroscopy reveal femtosecond time scales. This also challenged the traditional picture of an incoherent, hopping-like Förster transfer of the excitons between different molecular sites and a supportive role of quantum coherence for the transfer efficiency has been suggested, despite the ubiquitous “hot and wet” disturbing environment. I will show how quantum coherence of biomolecular excitons is influenced by environmental noise provided by polar solvents or the molecular backbone [1-3]. Using numerically exact model simulations, we address the exciton dynamics in the particularly well characterized Fenna-Matthews Olsen complex. It is shown how nonequilibrium molecular vibrational modes can enhance the exciton transfer efficiency considerably. Moreover, I discuss a simpler dimer of an artifical dye molecule [4]. We formulate an accurate vibronic exciton model with parameters directly taken from experimental data. Electronic dephasing and relaxation induce rapidly decaying large-amplitude oscillations of the quantum coherent cross peaks. Their long-time dynamics is governed by vibrational coherence and does not help to enhance electronic coherence channels beyond the electronic coherence times. Our findings could be useful to enhance functionality of artificial excitonic systems [5,6]. References

1. [1] P. Nalbach, C. A. Mujica-Martinez, and M. Thorwart, Vibronic speed-up of the excitation energy transfer in the Fenna-Matthews-Olson complex , Phys. Rev. E 91, 022706 (2015)
2. [2] C. Mujica-Martinez, P. Nalbach, and M. Thorwart, Quantification of non-Markovian effects in the Fenna- Matthews-Olson complex , Phys. Rev. E 88, 062719 (2013)
3. [3] P.Nalbach,D.Braun,andM.Thorwart, Phys. Rev. E 84, 041926 (2011)
4. [4] H.-G. Duan, P. Nalbach, V.I. Prokhorenko, S. Mukamel, and M. Thorwart, On the nature of oscillations in two-dimensional spectra of excitonically-coupled molecular systems, New J. Phys. 17, 072002 (2015)
5. [5] C.A. Mujica-Martinez, P. Nalbach and M. Thorwart, Organic pi-conjugated copolymers as molecular charge qubits, Phys. Rev. Lett. 111, 016802 (2013)
6. [6] P. Nalbach, I. Pugliesi, H. Langhals, and M. Thorwart , Noise-induced Förster resonant energy transfer between orthogonal dipoles in photoexcited molecules, Phys. Rev. Lett. 108, 218302 (2012)

When elastic systems like contact lines on a rough substrate, domain walls in disordered magnets, or tectonic plates are driven slowly, they remain immobile most of the time, before responding with strong intermittent motion, termed avalanche. I will describe the field theory behind these phenomena, explain why its effective action has a cusp, and how such intricate objects as the temporal shape of an avalanche can be obtained.

The motion of living cells is in large part due to the interaction of semi-flexible actin filaments (F-actin) and myosin molecular motors, which induce the relative sliding of F-actin. It is often assumed that this simple sliding is sufficient to account for all actomyosin-based motion. While this is correct in our highly organized striated muscle, we question the application of this dogma to less ordered actomyosin systems, thus reexamining a cornerstone of our understanding of cellular motion.