In theories with compact dimensions, Bubbles of Nothing (BoN) are vacuum decays in which the compactification manifold C shrinks to a point, with the instability expanding at the speed of light and leaving nothing (not even spacetime) behind. The Swampland Cobordism Conjecture predicts spacetime-ending configurations (known as bordisms to nothing) such as these being commonplace in a consistent theory of Quantum Gravity. Most constructions of BoN or more general bordisms to nothing found in the literature feature simple instances of C or singular bordisms, which cannot be approached from the effective field theory. Assuming the solution mediating such decay to nothing is homeomorphic to a smooth description, one can go a step further, and obtain topological properties from those of C. Through the use of Morse-Bott theory, these can be translated into information on the number and types of topology changes the compact manifold experiences as we move towards the tip of the bordism, as well as the location of possible cobordism defects. I will illustrate these results with different examples coming from String Theory and, time permitting, study more complicated arrangements such as BoN collisions. The talk is based on the paper https://arxiv.org/abs/24LF.2CR72.

Granular materials can be defined as large assemblies of macroscopic particles (i.e. the grains). When external energy is applied to these systems, they can reach a non-equilibrium steady state (NESS), where the statistical properties of the dynamics remain constant, but the time reversal symmetry is broken by forcing and dissipative mechanisms. The main idea of my talk will be to show how granular materials subjected to mechanical vibration can realise NESSs where equilibrium-like and nonequilibrium phenomena are deeply intertwined. First, I will report on our experimental study of the self-assembly processes of binary mixtures of grains vibrated on a substrate. Starting from a disordered configuration, and depending on the size ratio and number proportions of the two grain species, we observed the spontaneous self-assembly of quasi-crystalline structures [1] and periodic square crystals [2]. Remarkably, this follows predictions from the equilibrium phase diagram of a hard disk model following conservative dynamics. We have also obtained experimental conditions that realise a stable coexistence between the square crystal phase and a granular amorphous fluid. Using numerical simulations, we showed that the observed phenomenology resembles that of a first-order equilibrium phase transition, but with the two phases coexisting at different effective temperatures, which is a striking nonequilibrium effect. Unexpectedly, the denser crystalline phase is hotter than the less dense liquid phase. Through qualitative arguments and supported by kinetic theory, we elucidate the role of the coupling between local structure and energy transfer mechanisms in maintaining kinetic temperature gradients across the fluid-crystalline interface. In the last part of my talk I will discuss how, by changing the external driving parameters and considering dilute monodisperse systems, our setup can also be used as a playground for the study of absorbing phase transitions and hyperuniform fluids. I will summarise theoretical and numerical results [3,4] and present some preliminary experimental attempts. [1]: A. Plati, R. Maire, E. Fayen, F. Boulogne, F. Restagno, F. Smallenburg, and G. Foffi, Nat. Phys. 20, 465–471 (2024) [2]: Self-assembly and non-equilibrium phase coexistence in a binary granular mixture arXiv:24LF.21576 [3]: R. Maire, A. Plati, M. Stockinger, E. Trizac, F. Smallenburg, G. Foffi, Phys. Rev. Lett. CR2, 238202 (2024) [4]: R. Maire, A. Plati, J. Chem. Phys. 161, 054902 (2024)

Upon approaching a magnet, any nonmagnetic material feels a force. This force finds its origin in the spin and charge (orbital) motions of the electrons of the materials under the influence of the magnet field. For most materials this force is attractive because it is dominated by the spin magnetic susceptibility which is always paramagnetic. When the force appears repulsive the material is said to be diamagnetic and its magnetic response is then determined by the orbital motion of electrons. A well known example of diamagnetic material is water. In this talk I will review the microscopic theory of "diamagnetism" encoded by the orbital magnetic susceptibility of electrons in metals and insulators. First, I will remind why orbital magnetism is a genuine quantum effect related to the quantum motion of a charge in magnetic field. I will then show how the early theory of Landau and Peierls succeeds in describing the orbital response of single band metals but fail in describing multiband systems. Then I will present our work highlighting the key role of interband effects encoded in the quantum geometry of Bloch states (Berry curvature and quantum metric) in order to correctly describe the orbital magnetic susceptibility of multiband systems metal and insulators.

Computational modeling techniques have become a powerful tool to interpret and predict the properties of matter at different scales. After briefly summarizing my experience with some of these techniques, I will present recent collaborative studies with several L2C teams on atomistic modeling of materials and biomolecules at the quantum and classical level. This includes results unveiling the mechanism of adsorption of polar anions on carbonaceous nanostructures [1], and the origin of different anomalies associated with amorphous-amorphous transitions in SiO2 glasses [2] and in supercooled, supercritical and glassy water [3]. We have been applying also combined quantum-classical methods such as QM/MM to capture the crucial quantum interactions that affect the properties of matter. In particular, we are implementing this hybrid technique for the study of small biomolecules in water, for their interactions with carbon nanotubes and protein complexes in order to help improving medical therapies. I will conclude by discussing some future prospects of AI tools for atomic structure optimization and multiscale modeling. The latter is a promising field that seeks to elucidate how the atomic scale influences the properties of matter at larger scales. [1] A. H., L. Rincón, A. Noury, F. Henn & V. Jourdain. “Physical interactions tune the chemisorption of polar anions on carbon nanostructures”. J. Phys. Chem. C 126, CR349-CR357 (2022). [2] A. H., S. Ispas & B. Hehlen. “Percolation transitions in compressed SiO2 glasses”. Nature 599, pp. 63-66 (2021). [3] A. H., B. Hehlen & R. Paredes. “Unravelling water anomalies using a percolation approach”. https://doi.org/LF.21203/rs.3.rs-5235414/v1 (submitted).

Carbon nanostructures as functional and sustainable device components