Pawat Akarapipattana (LPTMS, Université Paris Saclay) — 1-d self-assembly of complex elastic molecules
In biological systems, many different proteins can self-assemble into fiber-like aggregates. This process depends on both the complexity and elastic properties of the protein. The universality of fiber formation suggests the presence of underlying mechanisms beyond specific details of a given protein. The aim of this work is to develop and investigate a toy model for the self-assembly of generic proteins. Generally speaking, an aggregate of frustrated particles limits its growth in space to mitigate its frustration. We study this phenomenon in a system of one dimensional complex elastic particles, where each subunit is a fully connected random spring network of an arbitrary shape. Thanks to the low dimensionality of this system, we are able to provide detailed analytical descriptions of deformations in the aggregate, as well as how these deformations control the aggregate size.
Kévin Bilai Biloa (ILM, Université Lyon 1) — Optimal control of the navigation of Brownian particles in disordered environments
At the nanoscale, the navigation of nano-robots [1] and living organisms [2] are hampered both by thermal fluctuations and by the disorder of the environment. We investigate the optimal control strategy of diffusion on a lattice biased by a force. This model could account for diffusion of nano-particles crystalline surfaces with defects, and could serve as a simple representation of Brownian motion in crowded biological systems. By applying Dynamic Programming, we explore the optimal navigation policies in uniform and disordered environments. Through a combination of numerical and analytical methods, we examine how disorder influences the optimal choice of force.
[1] F. Novotny, et al, Chem. 6, 867 (2020). [2] G. Li et al., PNAS 105, 18355 (2008).
Oscar Bouverot-Dupuis (LPTMS, Université Paris Saclay) — Antiferromagnetic order enhanced by local dissipation
We study an XXZ spin chain at zero magnetization coupled to a collection of local harmonic baths at zero temperature. We map this system on a (1+1)D effective field theory using bosonization, where the effect of the bath is taken care of in an exact manner. We provide analytical and numerical evidence of the existence of two phases at zero temperature: a Luttinger liquid (LL) and an antiferromagnetic phase (AFM), separated by a phase transition akin to the Berezinsky– Kosterlitz–Thouless (BKT) type. While the bath is responsible for the LL-AFM transition for subohmic baths, the LL-AFM transition for superohmic baths is due to the interactions within the spin chain.
Benoît Ferté (LPENS Paris) — Quantum-classical phase transitions on expanding lattices
A generic many-body system is expected to scramble quantum information and render it inaccessible in practice. On the contrary, measuring a system will "classicalize" its information and make it easily accessible to observers. We study toy models of expanding lattices displaying phase transitions between these two behaviors.
Ibrahim Ghanem (Department of Microsystems Engineering, University of Freiburg) — Plasticity in amorphous carbon as a stress-assisted chemical reaction
We study the plasticity of a-C networks within the framework of shear transformation zones which are the fundamental carriers of plasticity in amorphous solids. We use atomic-scale simulations to show that for amorphous carbon, a shear transformation can be reduced to a simple chemical reaction: the stress-induced breaking or forming of a single covalent bond. By forcing the breaking/forming of individual bonds in auxiliary calculations, we extract the potential energy landscapes of single bonds as a function of bond length. We find a good correlation between the energy barriers associated with a bond and the bond jump distance identified as the change in bond length due to the plastic event. Additionally, we apply a simple shear to an ensemble of a-C networks, all prepared by the same liquid-quench protocols, in the athermal quasistatic limit. By identifying plastic events that occur and using the correlation previously derived, we are able to calculate the rate of energy dissipation in the plastic flow regime. We use this to predict the yield stress of the amorphous carbon systems at hand. Our results help in interpreting some of the plastic properties of a-C networks and allow the parametrization of mesoscopic models of their plastic flow.
Liubov Gosteva (LPMMC, Grenoble-Alpes & CNRS) — Functional RG to unveil the z=1 scaling in the multidimensional KPZ and Burgers equations
Recent numerical simulations of the 1D KPZ equation uncovered a new scaling regime with the dynamical exponent z=1 in the tensionless limit. The same scaling is observed in the inviscid limit for the Burgers equation, which maps to the KPZ equation [1]. This new scaling regime was confirmed in 1D by the functional renormalisation group (FRG) analysis, which showed the existence of a corresponding UV-stable fixed point [2]. In this poster, we obtain the z=1 scaling in the multidimensional Burgers equation.
[1] C. Cartes, E. Tirapegui, R. Pandit, M. Brachet, "The Galerkin-truncated Burgers equation: crossover from inviscid-thermalized to Kardar–Parisi–Zhang scaling", Phil. Trans. R. Soc. A 380, 20210090 (2022).
[2] C. Fontaine, F. Vercesi, M. Brachet, L. Canet, "The unpredicted scaling of the one-dimensional Kardar-Parisi-Zhang equation", Phys. Rev. Lett. 131, 247101 (2023).
Hoa Le (Department of Microsystems Engineering, University of Freiburg) — Dislocation structure in High entropy alloys
High-entropy alloys (HEAs) represent a class of complex concentrated alloys containing five or more elements in high concentrations, known for their exceptional mechanical properties, including high strength, toughness, and wear resistance. Understanding dislocation behaviors plays a fundamental role in understanding the strength of these materials. However, studying dislocations in HEAs presents significant challenges due to the heterogeneity of local chemical disorders (quenched disorder) resulting from the random distribution of different elements and thermal fluctuation in which dislocations are embedded. In this study, we use large-scale molecular dynamics simulations to revisit the relationship between depinning and structure of dislocation in equicomposition Fe-Ni-Cr-Co-Cu high-entropy alloy (HEA). We focus on the combined effect of temperature and compositional disorder. To disentangle the effects of the quenched and thermal disorder, we compare the dislocation structure in HEA to a unary crystal interacting via a mean-field representation of the alloy, also called an “average alloy” (AA) mode. This AA model has identical elastic and thermodynamic properties but lacks the quenched disorder of the full HEA. We confirm that under the effect of quenched disorder, the profile of dislocations exhibits a power-law correlation with a Hurst exponent of 2/3. As the temperature increases, the exponent crosses over to 1/2, indicating a random walk. We also demonstrate that the pinning of dislocations is stronger at lower temperatures by investigating hysteresis in the glide of dislocations.
Kirsten Martens (LIPhy, Grenoble-Alpes) — Protocol dependence for avalanches under constant stress in elastoplastic models
Close to the yielding transition, amorphous solids exhibit a jerky dynamics characterized by plastic avalanches. The statistics of these avalanches have been measured experimentally and numerically using a variety of different protocols, assuming that all of them were equivalent for this purpose. In particular two main classes of protocols have been studied, deformation under controlled strain or under controlled stress. In this work, we investigate different protocols to generate plasticity avalanches and conduct 2D simulations of an elastoplastic model to examine the protocol dependence of avalanche statistics in yield-stress fluids. We demonstrate that when stress is controlled, the value and even the existence of the exponent governing the probability distribution function of avalanche sizes greatly depend on the choices one has to make to initiate avalanches. We identify a consistent stress-controlled protocol whose associated avalanches differ from the quasi-static ones in their fractal dimension and dynamical exponent. Remarkably, this new protocol also seems to verify the scaling relations among exponents proposed previously. This confirms in finite dimensions the scenario presented in a previous mean-field analysis and underscores the necessity for a cautious interpretation of avalanche universality within elastoplastic models.
Takumi Nagasawa (LIPhy, Grenoble-Alpes) — Relaxation dynamics of supercooled liquids by Xray irradiation
It has been widely acknowledged that plastic events in supercooled liquids can be induced by applying shear, a phenomenon extensively studied in plenty of literature. The application of shear, however, results in a departure from isotropy within the system. Recently, new types of relaxation induced by X-ray irradiation have been proposed in X-ray Photon Correlation Spectroscopy (XPCS) experiments. The dynamics induced by X-ray irradiation are regarded as originating from defects randomly distributed through the interaction between particles and X-ray photons. Notable advantages of this experiment are its ability to observe plastic events while maintaining system isotropy and enabling manipulation of photon dosage. We have conducted Molecular Dynamics simulations inspired by XPCS experiments to elucidate the nature of dynamics induced by X-ray irradiation. Our primary objective is to provide a detailed microscopic perspective behind this phenomenon.
Alessandro Pacco (LPTMS, Université Paris-Saclay) — Correlation and clustering of the stationary points in the p-spin model
In this work we aim at making progress in the understanding of the energy landscape of prototypical high-dimensional random functions, namely spherical spin glasses. Our goal is to infer important insights on the activated dynamics of such models, by computing the three-point complexity. Starting from the assumption that the optimal energy barrier between stationary points of the landscape grows with the distance between them in configuration space, we identify two possible scenarios. We observe that jumps at equal energy are "memoryless", and characterized by the same typical barrier, whereas jumps at higher energy are correlated, and large barriers are followed by smaller ones. This second scenario is observed in the form of clustering, meaning that close to a deep local minimum, higher energy stationary points are more densely packed than usual. This makes a link with previously observed simulations in finite dimensional glasses and driven interfaces, where large activated jumps are followed by smaller activations.
Liviu Iulian Palade (Institut Camille Jordan, Lyon) — Modeling soft matter rheological behavior
We discuss two types of models: one based on continuum physics capitalizing on fractional derivative concept to obtain frame invariant constitutive equations exhibiting all necessary stability properties, and the De Gennes-Doi-Edwards reptation model in general shear flows, for which we proved existence and uniqueness of solutions.
Lila Sarfati (MSC, Université Paris Cité) — From local to global polarisation in sheared amorphous materials
Under shear, dense amorphous materials build a directional memory of the driving protocol, displayed for instance in oscillatory-shear or flow-cessation experiments. Here we relate this global polarisation to the statistical features of the built-in structural disorder. We consider a mean-field coarse-grained model which allows for a distribution of local yield stresses with a local polarisation, i.e. with distinct values of these thresholds for yielding in either the direction of the driving or the opposite one. A global polarisation naturally emerges from the symmetry interplay of the mechanical noise and local disorder, both driving-dependent. We propose a set of predictions to be tested in experimental rheological setups, and discuss their robustness to finite-statistics symmetry breaking.
Vincenzo Maria Schimmenti (Max Planck Institute for the Physics of Complex Systems) — Earthquake-like dynamics in ultrathin magnetic film
We study the motion of a domain wall on an ultrathin magnetic film using the magneto-optical Kerr effect (MOKE). At tiny magnetic fields, the wall creeps only via thermal activation over the pinning centers present in the sample. Our results show that this creep dynamics is highly intermittent and correlated. A localized instability triggers a cascade, akin to aftershocks following a large earthquake, where the pinned wall undergoes large reorganizations in a compact active region for a few seconds. Surprisingly, the size and shape of these reorganizations display the same scale-free statistics of the depinning avalanches in agreement with the quenched Kardar-Parisi-Zhang universality class.
Anand Sharma (LIPhy, Grenoble-Alpes) — Structure-Dynamics Relationship of Swap Monte-Carlo Revealed by Machine Learning
Swap Monte-Carlo simulation is a power sampling algorithm for glass-forming liquids. We study its dynamical aspect. In particular, we observe heterogeneous dynamics, similar to those observed in standard molecular dynamics simulations. Yet the magnitude of dynamical heterogeneity is quite suppressed. We then try to predict dynamical patterns quantified by propensity maps using various machine learning techniques.
Gaël Tejedor (LSPM, CNRS, Villetaneuse) — Anatomy of Dislocation Avalanches
Dislocation avalanches in crystals are sudden, collective movements of dislocations, which are line defects within the crystal structure. These events occur when an applied load to a crystal reaches a critical level, causing a rapid and large-scale rearrangement of dislocations. The occurrence of dislocation avalanches highlights the complex, non-linear behavior of crystals under external stimulus and is a critical factor in understanding the plastic deformation of materials.
Detailed mathematical modelling of dislocation avalanches remains a major challenge for materials science. In this work, we use a mesoscopic post-DDD approach that involves constructing a coarse-grained non-convex energy which poses GL(2, Z) group symmetry to describe crystal plasticity [1, 2]. This model has the advantage of capturing the fast topological changes in dislocation configurations such as nucleation, annihilation, interaction with obstacles in a self-consistent manner, only requiring initial assumptions about the crystalline symmetry and inter-atomic potential.
In this work, we will show that dislocation avalanches are often preceded by localized eigen-modes of the stiffness tensor, which is akin to the plastic rearrangements observed in amorphous materials. These eigen-modes repre- sent specific, highly localized deformations within the crystal structure that act as precursors to the sudden and collective movement of dislocations. By identifying these precursor modes, we can gain deeper insights into the specific pathways through which dislocation avalanches are triggered.
[1] S. Conti and G. Zanzotto, “A variational model for reconstructive phase transformations in crystals, and their relation to dislocations and plasticity,” Archive for Rational Mechanics and Analysis, 173, 69 (2004).
[2] O. U. Salman, R. Baggio, B. Bacroix, G. Zanzotto, N. Gorbushin, and L. Truskinovsky, “Discontinuous yielding of pristine micro-crystals”, Comptes Rendus. Physique 22, 201 (2021).
Saptarshi Majumdar (Laboratoire de Physique Théorique, Toulouse) — Localization in Open Quantum Systems
We investigate the zero-temperature phase diagram of a one-dimensional XXZ spin chain coupled with local dissipative baths composed of simple harmonic oscillators. In a finite magnetization sector, we map this system onto a two-dimensional classical action using bosonization. From this classical field theory, we find the existence of a BKT phase transition between the pre-existing Luttinger liquid phase and a new dissipative phase at zero temperature. This new phase is a gapless spin density wave with unaltered susceptibility and vanishing spin stiffness. These analytical predictions are verified against numerical Langevin dynamics simulations of the action. The local baths in the spin chain can also be interpreted as annealed disorder and they affect the transport properties. Particularly for subohmic baths, the static conductivity vanishes, which can be interpreted as a localization effect induced by the presence of dynamical disorder. We also solve the model at zero magnetization and show that in that case, the gapless spin density wave is replaced by a gapped antiferromagnetic phase.
