Jean-Louis Barrat

The calculation of thermal conductivity in insulating solids at temperatures below the Debye temperature is problematic, due to the breakdown of classical and semiclassical approaches. In this Letter, we present a fully nonperturbative quantum methodology to compute thermal conductivity based on path integral Monte Carlo (PIMC) simulations combined with the Green-Kubo linear response theory. The method is applied to rare gas solids modeled by a Lennard-Jones potential, paradigmatic systems where quantum effects strongly affect both thermodynamic and transport properties. From PIMC simulations, we obtain the temperature-dependent phonon frequencies, lifetimes, and specific heat. From the imaginary time correlations of the energy current, we extract the thermal transport coefficients based on a physically motivated prior. We show that the experimentally observed increase of the thermal conductivity of argon and neon at low temperatures cannot be explained within a Peierls-Boltzmann framework using phonon linewidths at equilibrium. In contrast, a distinct transport lifetime emerges from the analysis of heat-current correlations. Our results demonstrate that quantum Monte Carlo methods provide a robust, nonperturbative framework to investigate heat transport in insulating solids, beyond the limits of classical molecular dynamics without relying on perturbative or semiclassical approximations.

We show that all existing methods quantifying rotational motion in molecular fluids eventually have severe limitations in systems undergoing complex rotational motion characterized by slow, heterogeneous, or intermittent dynamics. This impacts, in particular, the study of rotational dynamics in molecular supercooled liquids near their glass transition, as well as discussions of the decoupling between rotational and translational motion and violations of the Debye–Stokes–Einstein relation. We present a brief overview of existing methods and explain why none of them can accurately capture the evolution of rotational dynamics from a diffusive fluid to an arrested solid, thus resolving inconsistent literature results. We then introduce an empirical method that efficiently solves all issues. We benchmark our method by devising a family of continuous-time random walk models for rotational dynamics. Our method correctly quantifies the statistics of free and caged rotational motion, as well as non-Gaussian and non-Fickian rotational dynamics, and should allow a better characterization of dynamic heterogeneity in the rotational motion of supercooled molecular fluids.

The molecular mechanisms governing internal fluctuations in intrinsically disordered protein (IDP) assemblies are crucial to the stability and dynamics of both regulated and aberrant toxic cellular aggregates, but remain poorly understood. By comprehensively combining high-resolution quasi-elastic neutron scattering with all-atom molecular dynamics simulations, we probe the motions of β -casein, a model IDP, inside its assemblies. We uncover a previously unresolved slow relaxation process with phenomenological characteristics of anomalous non-Fickian diffusion. This anomalous signature emerges from a continuous mobility gradient governed by density and crowding within the assemblies; the core is denser and more compact, and mobility increases progressively toward the exterior. This dynamical heterogeneity underlies the non-Gaussian behavior and accounts for the observed spectral broadening. Our findings provide insight into how disorder and extreme local crowding within IDP assemblies can result in a fundamentally different behavior compared to, e.g., clusters of well-folded proteins. The deviations from Fickian diffusion arise from dynamic heterogeneity and can be captured within the framework by a model typically used for the jump diffusion observed in liquids, thereby extending its applicability.

Multiple polymer networks, such as double-network elastomers comprising a sacrificial and a matrix network, exhibit exceptional mechanical resilience, commonly attributed to the formation of an extended damage zone before a crack can grow. However, the microscopic mechanisms underlying their toughness remain poorly understood. Here, we combine advanced light scattering methods and molecular dynamics simulations to explore the microscopic relaxation dynamics and stress redistribution at the polymer strand scale of single-network and double-network elastomers under uniaxial loading. Dynamic light scattering experiments show that microscopic rearrangements and bond breaking events are localized near the crack tip in single networks, readily causing the crack to advance. In contrast, double networks exhibit delocalized microscopic rearrangements well ahead of and not directly correlated with crack propagation, enabling the dissipation of energy over broader regions and timescales. Numerical simulations of the damage zone show that bond breaking in the matrix network of double networks leads to widespread stress redistribution, mitigating catastrophic damage localization. This enhanced ability to redistribute stress in a nonlocal manner allows a much larger extension before localized macroscopic failure occurs, explaining the superior toughness of double networks. Our findings identify early, delocalized bond breaking events combined with more efficient dissipation pathways through enhanced microscopic rearrangements as the key microscopic mechanisms responsible for the outstanding toughness and extensibility of multiple elastomer networks.

We propose a novel computational strategy to study the glass transition of molecular fluids. Our approach combines the construction of simple yet realistic models with the development of Monte Carlo algorithms to accelerate equilibration and sampling. Inspired by the well-studied orthoterphenyl glass former, we construct a molecular model with an analogous triangular geometry and construct a “flip” Monte Carlo algorithm. We demonstrate that the flip Monte Carlo algorithm achieves a sampling speedup of about 10 9 at the experimental glass transition temperature T g . This allows us to systematically analyze the equilibrium structure and molecular dynamics of the model over a temperature regime previously inaccessible. We carefully compare the observed physical behavior to earlier studies that used atomistic models. In particular, we find that the glass fragility and the departure from the Stokes-Einstein relation are much closer to experimental observations. We characterize the development and temperature evolution of spatial correlations in the relaxation dynamics using both orientational and translational degrees of freedom. Excess wings emerge at intermediate frequencies in dynamic rotational spectra, and we directly visualize the corresponding molecular motion near T g . Our approach can be generalized to a broad range of molecular geometries and paves the way to a deeper understanding of how molecular details may affect more universal physical aspects characterizing molecular liquids approaching their glass transition.

Recent studies have revealed a series of connections between the topological features of structural glasses and their material properties. These findings show a striking resemblance to results observed in quantum physics that underscore the significance of the nature of the wavefunction. However, so far the structural arrangement of the topological defects in glasses has remained elusive. Here, we investigate numerically the geometry and statistical properties of the topological defects related to the vibrational eigenmodes of a prototypical three-dimensional glass. We find that at low frequencies these defects form scale-invariant, quasi-linear structures and dictate the morphology of plastic events when the system is subjected to a quasi-static shear, i.e., the eigenmode geometry shapes the plastic behavior in 3D glasses. Our results indicate the presence of a profound connection between the topology of eigenmodes and plastic energy dissipation in disordered materials, thus generalizing the known link identified in crystalline materials.

Using extensive numerical simulations, we develop a model to explain energy dissipation under ballistic impact based on geometrical factors.

Recent studies have revealed a series of connections between the topological features of structural glasses and their material properties. These findings bear a striking resemblance to results observed in quantum physics, underscoring the significance of the nature of the wavefunction. However, so far the structural arrangement of the topological defects in glasses has remained elusive. Here, we investigate numerically the geometry and statistical properties of the topological defects related to the vibrational eigenmodes of a prototypical three-dimensional glass. We find that at low frequencies these defects form scale-invariant, quasi-linear structures and dictate the morphology of plastic events when the system is subjected to a quasi-static shear, i.e., the eigenmode geometry shapes the plastic behavior in 3 D glasses. Our results indicate the presence of a profound connection between the topology of eigenmodes and plastic energy dissipation in disordered materials, thus generalizing the known link identified in crystalline materials. It is anticipated that this link will also have consequences for the relaxation dynamics in the liquid state, thus opening the door for a novel approach to describe glassy dynamics.