Recently, topological phases of matter have attracted considerable attention due to their unique and robust properties. In particular, the possibility of finding or inducing superconductivity in a topological phase (known as topological superconductivity) has been an active area of research in recent years, for instance in quantum engineering where such a phase would be useful for quantum computing.

In this seminar, I will present a charge transport study of nanostructures of trigonal-PtBi2, a noncentrosymmetric
crystal with very strong spin-orbit coupling. In recent works (1, 2), we evidenced the superconducting properties of this material and we predict it to be a Weyl- and nodal-linesemimetal. I will focus on two main results: the discovery of 2-dimensional superconductivity at sub-kelvin temperatures, and the discovery of an anomalous planar Hall effect (APHE) in the normal phase, robust up to room temperature.

More details are given in the pdf file.

1. A. Veyrat et al., Nano Letters, ISSN: 1530-6984 (Jan. 2023).
2. A. Veyrat et al., submitted to arXiv (2023).

In this talk, I discuss recent progress in constructing explicit examples of de Sitter vacua at leading order in the \alpha' and g_s expansions as envisioned by Kachru, Kallosh, Linde and Trivedi 20 years ago. Initially, I will specify explicit Calabi-Yau orientifolds, and choices of quantised fluxes leading to exponentially small flux superpotential. I derive the four-dimensional effective supergravity theories, incorporating the leading non-perturbative superpotential terms from Euclidean D3-branes, and the leading corrections to the Kähler potential that are inherited from N=2 supersymmetry. Subsequently, I will demonstrate how warped Randall-Sundrum throats and SUSY breaking uplifts from anti-D3 branes can be incorporated in this construction. Lastly, I will briefly comment on the robustness of the construction against genuinely N=1 corrections in the \alpha’ and g_s expansions which are however not fully known.

Cells and tissues in many contexts are electrically polarized.
In this work, we develop a three-dimensional electrohydraulic model that
simultaneously solves for the electric potential, ion concentration, and
hydrodynamic flows and stresses inside and outside a spherical
water-filled cavity enclosed by an epithelium. We analyze two sources of
electric asymmetry in the system: (i) heterogeneous ion transporter
along the surface of the sphere and (ii) organoid in an external
electric field. We show that inhomogeneous ion transport leads to an
organ-scale electric current and water flux. Consistent with recent
experimental observation we find that a constant electric field leads to
isotropic swelling of the organ.

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Many natural ecosystems are composed of a large number of coexisting and interacting species. Thus, detailed modelling and characterization of the dynamics is unfeasible. Nonetheless, recent works have shown that insights on their phenomenology can be gained from studying canonical ecological models with many species and random interactions between them. In this talk, I will show that unexplained commonly observed qualitative features of species-rich ecosystems, such as species turnover and broad distributions of abundances, are robust emerging properties of a chaotic phase of these high-dimensional systems. Importantly, this requires migration from an outside "pool of species". In fact, for isolated well-mixed systems, fluctuations in population sizes are so large that they drive species to extinction. This opens the door for a discussion on the interplay between spatial heterogeneity and complex dynamics in diverse ecosystems.

Nonequilibrium processes within living systems exact a metabolic price: the constant maintenance of fuel molecules and raw materials at sufficient concentrations to provide thermodynamic driving potentials for biological function. Whether/how the metabolic costs constrain biological processes and natural evolution remains elusive as the fitness of organisms is subject to many trade offs from bioenergetic costs to speed and accuracy of cellular processes. In this talk, I will present recent efforts in establishing a quantitative framework to study the role of metabolism in evolution of cellular processes and development of organisms.

In the first part, I will discuss the relation between the bioenergetics and evolution of cellular processes, and demonstrate the role of metabolic costs in shaping interaction affinities in gene regulatory (microRNA-mRNA) networks. In the second part, I will talk about how metabolism is linked to the body plan development of multicellular organisms in the light of our recent work on gut morphogenesis in planarians. Finally, I will present ideas to bridge theory of chemical reactions and soft-matter to elucidate spatial organization of metabolism in living systems allowing to drive biological processes across the entire organism.

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Many important processes in chemistry and biology occur at interfaces where the molecular interactions and dynamics significantly differ from those observed in the bulk. Proper characterization of the behavior of peptides and proteins at macromolecular interfaces, including their reversible adsorption to surfaces and their folding into the membrane, is a prerequisite for understanding fundamental physiological processes stemming from intracellular reactions. A powerful noninvasive, optical, surface analytical tool is vibrational sum frequency generation (VSFG) spectroscopy, which often enables direct observation of the structural and dynamical properties of interfacial molecules. Nevertheless, the acquisition times and spectral resolution of traditional broadband VSFG spectrometers are far from ideal to follow the interaction of macromolecules at bio-interfaces in real-time and in situ.

In my talk, I will provide an overview of our mid-IR light sources integrated into our unique high-resolution 100-kHz VSFG spectrometer. By employing such a spectrometer, the collection of vibrational spectra of biologically relevant macromolecules at air-solid, air-liquid, and buried interfaces has become possible at an unprecedented combination of signal-to-noise ratios, acquisition times, and spectral resolution in the spectral range of 770-3800 cm-1. I will present examples of in situ peptide and protein folding at model cell membranes and a model system for studying protein-protein interactions or even enzymatic reactions.

Approche in silico au sein des matériaux organiques
Armand Soldera

Était-il concevable, il y a à peine quelques années, de voir émerger, au côté de la théorie et de l’expérience, une troisième voie pour faire de la science? La simulation moléculaire est en effet devenue une technique à part entière dans le domaine de la recherche. En ce sens, elle n’est plus l’apanage de théoriciens qui l’utilisaient pour vérifier la conformité de leurs modèles théoriques. Une croissance de la puissance des ordinateurs, une amélioration des algorithmes, et un environnement d’utilisation accueillant, amènent de plus en plus de personnes à l’utiliser. En somme un nouveau type de scientifique, que l’on peut qualifier de numériste expérimental, est en train de prendre forme. Son but est de connaître les fondements même des codes tout en étant susceptible de manipuler des équations ou de comprendre la nature des phénomènes physiques, afin d’apporter des résultats probants issus des simulations. C’est dans ce cadre que se situe cet exposé. Il sera alors présenté les potentialités qu’offre cette recherche in silico dans le domaine des matériaux organiques, permettant d’apporter une vision moléculaire de phénomènes physiques. C’est ainsi que des industriels sont de plus intéressés par cette approche les amenant à développer de nouvelles voies de développement dans leur recherche plus appliquée. L’exposé se terminera par l’utilisation de cette technique pour donner une vision plus atomistique à la transition vitreuse au sein des polymères.

Massive crowd gatherings form some of the most dangerous and unpredictable environments [1]. However, we lack quantitative characterizations of their dynamics and the heuristic principles used to explain and predict their motion remain elusive. In this talk, I will present our analysis of the dynamics of thousands of packed individuals in a model system, namely, the opening ceremony of the festival of San Fermín in Pamplona, Spain [2]. This analysis reveals that at extreme densities crowds experience self-sustained oscillatory flows, which echo the correlated orbital motion of hundreds of individuals in the absence of any external guidance. I will then detail how the combination of mechanics and symmetry principles has allowed us to establish a robust predictive model of dense crowds inferred from our measurements to elucidate this emergent chiral dynamics. In particular, we establish that the self-organization of crowds into macroscopic oscillators originates from transverse frictional forces between the crowd and the ground.

[1] D. Helbing, A. Johansson, and H. Z. Al-Abideen. ``Dynamics of crowd disasters: An empirical study''. Physical Review E, 75(4), 046109 (2007).

[2] F. Gu*, B. Guiselin*, N. Bain, I. Zuriguel, and D. Bartolo. Submitted (2024).