Ralf Blossey: „Chromatin remodeling for physicists: gene regulation by active processes”
University Lille, France - Seminar@HHUD: 28.9.2020 14:00 s.t., Seminarroom 25.33 O0.61

Chromatin is the DNA-protein complex that makes out the eukaryotic cell nucleus. DNA in chromatin is spooled around protein complexes, the histone octamers, thus creating its basic structural unit: the nucleosomes. In essentially all gene regulatory and chromatin structural processes, nucleosomes need to be displaced along DNA or removed from it. This task is performed by molecular motors, the chromatin remodelers, which convert chemical energy in form of ATP into the displacement of DNA. The first half of this talk gives an introduction into chromatin and chromatin remodelers. The second half will be devoted to explain, based on recent insights from structural biology, that the displacement mechanism of nucleosomes can be mapped to an active Brownian dimer model. The talk ends with a discussion of further directions of study.

Ben Goddard: „Mathematical Foundations of DDFT”
University of Edinburgh, Scotland, United Kingdom - Seminar@HHUD: 18.6.2020 14:30 s.t., Seminarroom Online

I will present mathematical results for a class of DDFTs, posed in the overdamped (high friction) regime, including hydrodynamic interactions, with no-flux boundary conditions. The system of interest consists of a continuity equation, coupled to a self-consistency equation for the flux. After explaining the derivation of the system from the inertial dynamics, I will present conditions that ensure the existence of unique solutions to the non-local, non-linear PDE system. I will also show that the solution is strictly positive for all positive time, and give a prori estimates for the rate of convergence to equilibrium, both in norm and relative entropy. Whilst the results are necessarily of a technical nature, I will try to present the intuition behind the results, rather than the mathematical details.

Joint work with Serafim Kalliadasis, Rory Mills-Williams, and Greg Pavliotis.

Dr. Selym Villalba-Chávez: „Probing quantum vacuum-like scenarios with high-intensity laser pulses”
Institut für Theoretische Physik I, Heinrich-Heine-Universität Düsseldorf - Seminar@HHUD: 9.7.2020 16:30 s.t., Seminarroom Online

Gaining comprehensive insights into the properties of the quantum vacuum is central in our continuous quest for obtaining an in-depth and clear understanding of the fundamental laws of our Universe. While the vacuum in classical physics simply describes a region devoid of matter, the framework of quantum field theory reveals that it is characterized by fluctuations of all plausible and realizable fields in Nature. The talk focuses first on the hitherto unobserved effect of the vacuum decay into electron-positron pairs, paying particular attention to a novel way of amplifying this phenomenon. It will be shown that the principle on which this enhancement relies can be tested via gapped graphene monolayers, and revealed that – although this material resembles the QED vacuum – in some processes its two dimensional structure causes some striking differences between both scenarios. The final part of the talk is oriented to show how experiments involving high-intensity lasers can become a powerful tool to limit the parameter spaces of some dark matter candidates such as axions, hidden-photons and minicharged particles.

The left panel depicts the electron/positron spectrum produced by superimposing a strong time dependent electric field and a fast oscillating wave. A scheme for testing the Breit-Wheeler-like production of massive Dirac pairs in a gapped graphene monolayer is shown in the right panel.

Prof. Dr. Jörg Rottler: „The nonequilibrium physics of driven amorphous soft matter: plasticity and collective effects”
University of British Columbia, Vancouver, Canada - Seminar@HHUD: 23.1.2020 16:30 s.t., Lecture hall 25.31 O0.5J

This talk will describe the yielding transition in disordered solids, a nonequilibrium phase transition between arrested and flowing states of matter. We use computer simulations at the particle scale to characterize the elementary shear transformations responsible for plastic flow, determine how they interact elastically and illustrate the resulting scale dependent shear strain correlations in (active and passive) amorphous packings. We then show how the statistics of the intermittent stick-slip motion that is typically observed macroscopically in these materials at slow mechanical driving can be related to the microscopic distribution of residual stresses that we measure in the simulations. Despite strong correlations, the statistical properties of steady state flow can then be captured by a mean field description that takes into account the broadly distributed mechanical excitations.

Prof. Dr.-Ing. Rupert Klein: „Mathematical Colloquium: How Mathematics helps structuring climate discussions”
Freie Universität Berlin - Seminar@HHUD: 17.1.2020 16:45 s.t., Lecture hall 25.22 O0.5H

Mathematics in climate research is often thought to be mainly a provider of techniques for solving the continuum mechanical equations for the flows of the atmosphere and oceans, for the motion and evolution of Earth´s ice masses, and the like. Three examples will elucidate that there is a much wider range of opportunities.

Climate modellers often employ reduced forms of "the continuum mechanical equations" to efficiently address their research questions of interest. The first example discusses how mathematical analysis can provide systematic guidelines for the regime of applicability of such reduced model equations.

Meteorologists define "climate", in a narrow sense, as "the statistical description in terms of the mean and variability of relevant quantities over a period of time" (World Meteorological Society; see the website for a broader sense definition). Now, climate researchers are most interested in changes of the climate over time, and yet there is no unique, well-defined notion of "time dependent statistics". In fact, there are restrictive conditions which data from time series need to satisfy for classical statistical methods to be applicable. The second example describes recent developments of analysis techniques for time series with non-trivial temporal trends.

Modern climate research has joined forces with economy and the social sciences to generate a scientific basis for informed political decisions in the face of global climate change. One major type of problems hampering progress of the related interdisciplinary research consists of often subtle language barriers. The third example describes how mathematical formalization of the notion of "vulnerability" has helped structuring related interdisciplinary research efforts.

Prof. Dr. Johann Rafelski: „Special Relativity and Strong Fields”
University of Arizona, Tucson, USA - Seminar@HHUD: 9.1.2020 16:30 s.t., Lecture hall 25.31 O0.5J

Special Relativity (SR), the foundation of modern physics, experiences a renaissance as a discipline and is rapidly evolving: We are probing the acceleration/strong electromagnetic field frontier in relativistic heavy ion experiments, and thinking ahead to the very high intensity laser-particle interaction. We are facing a challenge: Teaching of SR to future researchers in these field. SR remains poorly represented in many introductory text books. No-expert lecturers do not correctly understand SR and related elementary physics phenomena. The unfinished formulation of SR when forces are not gravity will be explained. Strong EM fields will be introduced.

Prof. Dr. Ralf Metzler: „Brownian motion and beyond”
Universität Potsdam, Institut für Physik und Astronomie - Seminar@HHUD: 12.12.19 16:30 s.t., Lecture hall 25.31 O0.5J

Roughly 190 years ago Robert Brown reported the "rapid oscillatory motion" of microscopic particles, the first systematic study of what we now call Brownian motion. At the beginning of the 20th century Albert Einstein, Marian Smoluchowski and Pierre Langevin formulated the mathematical laws of diffusion. Jean Perrin‘s experiments 110 years ago then prompted a very active field of ever refined diffusion experiments.

Despite the long-standing history of Brownian motion, after an historic introduction I will report several new developments in the field of diffusion and stochastic processes. This new research has been fuelled mainly by novel insights into complex microscopic systems such as living biological cells, made possible by Nobel-Prize winning techniques in laser physics, superresolution microscopy, or through supercomputing studies. Topics covered include Brownian yet non-Gaussian diffusion, the geometry-control of chemical reactions and anomalous diffusion with a power-law time dependence of the mean squared displacement. For the latter, questions of ergodicity and ageing will be discussed.

Dr. Bastian Aurand: „Novel target schemes for laser-driven proton acceleration”
Institut für Laser- und Plasmaphysik, Heinrich-Heine-Universität Düsseldorf - Seminar@HHUD: 5.12.19 16:30 s.t., Lecture hall 25.31 O0.5J

The acceleration of charged particles by super-intense laser-matter interactions has gained a high interest within the last decades. The unique features of the so-called plasma-accelerators, like their compact design, ultra-short particle bursts or high particle-flux compared to conventional accelerators can be a milestone, for example as a diagnostic tool in physics or to simplify applications like hadron therapy. Besides the fundamental research on the different processes used to accelerate particles, more and more effort is taken in the last years to pave the way towards applications using the unique features of those beams.

In this talk, I will present different approaches to improve laser-driven proton acceleration, e.g. towards a higher repetition-rate, better energy stability, less debris production or the shaping of the spatial beam profile. I will start with a brief introduction into the field by comparing conventional particle accelerators to plasma-accelerators and highlight today’s state of the art. Discussing the best-studied mechanism, the so-called Target-Normal-Sheath-Acceleration (TNSA) is the base to introduce modifications, like a double laser focus or the effect of improved target geometry. While both schemes are a variation of TNSA, I will identify the general drawbacks of this method and show two alternative approaches using isolated, mass-limited targets. In the first experiment, a controlled droplet formation from a liquid stream is used, resulting in targets which are larger than the laser focus, leading to a hybrid mechanism. Finally, a hydrogen cluster source will be presented, delivering targets which are Coulomb-exploded by the laser. This mechanism is inherently more stable against fluctuations during the laser-matter interaction and allows at the same time a fine-tuning of the particle energy within a wide range.

A comparison of the approaches will summarize the talk along with identifying future tasks and work-packages towards the goal of applicable laser-driven particle accelerators.

Prof. Dr. Hari Srikanth: „Tuning magnetic anisotropy in nanostructures for biomedical applications”
University of South Florida, USA - Seminar@HHUD: 1.10.2019 16:30 s.t., Lecture hall 6A

Magnetic nanoparticles have been building blocks in applications ranging from high density recording to spintronics and nanomedicine. Magnetic anisotropies in nanoparticles arising from surfaces, shapes and interfaces in hybrid structures are important in determining the functional response in various applications.

In this talk I will first introduce the basic aspects of anisotropy, how to tune it in nanostructures and ways to measure it. I will discuss resonant RF transverse susceptibility, that we have used extensively, as a powerful method to probe the effective anisotropy in magnetic materials. Tuning anisotropy has a direct impact on the performance of functional magnetic nanoparticles in biomedical applications such as contrast enhancement in MRI and magnetic hyperthermia cancer therapy. There is a need to improve the specific absorption rate (SAR) or heating efficiency of nanoparticles for hyperthermia and I will focus on the role of tuning surface and interfacial anisotropy with a goal to enhance SAR.

Strategies going beyond simple spherical structures such as exchange coupled core-shell nanoparticles, nanowire, nanotube geometries can be exploited to increase saturation magnetization, effective anisotropy and heating efficiency in magnetic hyperthermia. This lecture will combine insights into fundamental physics of magnetic nanostructures along with recent research advances in their application in nanomedicine.