Suvendu Mandal: „Optimal spreading of active polymers in porous media”
Universität Darmstadt - Seminar@HHUD: 5.5.22 12:30 s.t., Seminarroom 25.32 O3.51

We perform Brownian dynamics simulations of active stiff polymers undergoing run-reverse dynamics, and so mimic bacterial swimming, in porous media. In accord with recent experiments of Escherichia coli, the polymer dynamics are characterized by trapping phases interrupted by directed hopping motion through the pores. We find that the effective translational diffusivities of run-reverse agents can be enhanced up to two orders in magnitude, compared to their non-reversing counterparts, and exhibit a non-monotonic behavior as a function of the reversal rate, which we rationalize using a coarse-grained model. We discover a geometric criterion for the optimal spreading, which emerges when their run lengths are comparable to the longest straight path available in the porous medium. More significantly, our criterion unifies results for porous media with disparate pore sizes and shapes and thus provides a fundamental principle for optimal transport of microorganisms and cargo-carriers in densely-packed biological and environmental settings.

Tanja Schilling: „The interplay between memory and the potential of mean force”
Universität Freiburg - Seminar@HHUD: 24.3.2022 14:30 s.t., Lecture hall 25.31 5J

Active matter, responsive materials and materials under time-dependent load are systems out of thermal equilibrium. To construct coarse-grained models for such systems, one needs to integrate out a distribution of microstates that evolves in time. This is a challenging task. In this talk, we review recent developments in theoretical approaches to the non-equilibrium coarse-graining problem, in particular, projection operator formalisms. We discuss the role of the potential of mean force in non-linear versions of the Langevin equation and its relation to the second fluctuation dissipation theorem.

Prof. Dr. Mathias Kläui: „Antiferromagnetic Insulatronics: Spintronics without magnetic fields”
Johannes Gutenberg-University Mainz - Seminar@HHUD: 02.06.2022 16:30 s.t., Lecture hall 25.31 O0.5J

While known for a long time, antiferromagnetically ordered systems have previously been considered, as "interesting but useless". However, since antiferromagnets potentially promises faster operation, enhanced stability and higher integration densities, they could potentially become a game changer for new spintronic devices. Here I will show how antiferromagnets can be used as active spintronics devices by demonstrating the key operations of "reading"[1], "writing"[2], and "transporting information"[3] in antiferromagnets.

References

1. S. Bodnar et al., Nature Commun. 9, 348 (2018); L. Baldrati et al., Phys. Rev. Lett. 125, 077201 (2020)
2. L. Baldrati et al., Phys. Rev. Lett. 123, 177201 (2019); H. Meer et al., Nano Lett. 21, 114 (2020); S. P. Bommanaboyena et al., Nature Commun. 12, 6539 (2021);
3. R. Lebrun et al., Nature 561, 222 (2018). R. Lebrun et al., Nature Commun. 11, 6332 (2020)

Dr. Selym Villalba-Chavez: „Gravitational waves and binary black holes”
Institut für Theoretische Physik I, Heinrich-Heine-Universität Düsseldorf - Seminar@HHUD: 19.5.2022 16:30 s.t., Lecture hall 25.31 O0.5J

Gravitational waves are space-time ripples propagating with the speed of light somewhat similar to electromagnetic waves. In 2015, the gravitational-wave observatory LIGO detected, for the first time, the passing of gravitational waves through Earth. This event was triggered by waves produced by two black holes that were orbiting on quasi-circular orbits. The talk focuses first on verifying that the existence of gravitational waves is unavoidable within the framework of General Relativity. The final part of the presentation is oriented to show how one can estimate some physical parameters of the binary black hole system, from the properties of the detected signal.

The left panel sketches two orbiting black holes while losing energy in the form of gravitational waves. The signal detected at LIGO on September 14, 2015 is shown in the right.

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.