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John Russo

Publications and source records attributed to John Russo.

At least 19 recordsLinked to original sources

Computational Methods toward Ultrastable Glasses

Ultrastable glasses, amorphous solids with exceptionally low-energy states and enhanced kinetic, thermodynamic, and mechanical stability, have long been a subject of intense experimental interest. Over the past decade, their computational realization has emerged as a major goal in condensed matter physics, as numerical methods can exploit unphysical moves to access deeply supercooled and nonequilibrium glassy states far beyond the reach of conventional cooling protocols, thereby providing key insights into the nature of the glass transition and amorphous states and enabling the design of mechanically robust glassy materials. In this review, we outline the key steps underlying the most effective algorithms developed across the field. For each approach, we discuss its efficiency, limitations, and physical interpretation. We finally present a comparative analysis of the stability achieved across these methods, with the aim of equipping both newcomers and experts with an intuitive and comprehensive understanding of the field's current state and the opportunities it presents.

cond-mat.dis-nn

Sampling the Liquid-Gas Critical Point with Boltzmann Generators

Generative models based on invertible transformations provide a physics-aware route to sample equilibrium configurations directly from the Boltzmann distribution, enabling efficient exploration of complex thermodynamic landscapes. Here, we evaluate their applicability in regions where conventional simulations suffer from severe dynamical bottlenecks, focusing on the liquid-gas critical point of a Lennard-Jones fluid. We show that Boltzmann Generators capture essential signatures of critical behavior, retain reliable performance when trained at or near criticality, and extrapolate across neighboring states of the phase diagram. An intriguing observation is that the model's efficiency metric closely traces the underlying phase boundaries, hinting at a connection between generative performance and thermodynamics. However, the approach remains limited by the small system sizes currently accessible, which suppress the large fluctuations that characterize critical phenomena. Our results delineate the current capabilities and boundaries of Boltzmann Generators in challenging regions of phase space, while pointing toward future applications in problems dominated by slow dynamics, such as glass formation and nucleation.

cond-mat.stat-mech

Liquid anomalies and Fragility of Supercooled Antimony

Phase-change materials (PCMs) based on group IV, V, and VI elements, such as Ge, Sb, and Te, exhibit distinctive liquid-state features, including thermodynamic anomalies and unusual dynamical properties, which are believed to play a key role in their fast and reversible crystallization behavior. Antimony (Sb), a monoatomic PCM with ultrafast switching capabilities, stands out as the only elemental member of this group for which the properties of the liquid and supercooled states have so far remained unknown. In this work, we use large-scale molecular dynamics simulations with a neural network potential trained on first-principles data to investigate the liquid, supercooled, and amorphous phases of Sb across a broad pressure-temperature range. We uncover clear signatures of anomalous behavior, including a density maximum and non-monotonic thermodynamic response functions, and introduce a novel octahedral order parameter that captures the structural evolution of the liquid. Moreover, extrapolation of the viscosity to the glass transition, based on configurational and excess entropies, indicates that Sb is a highly fragile material. Our results present a compelling new case for the connection between the liquid-state properties of phase-change materials and their unique ability to combine high amorphous-phase stability with ultrafast crystallization.

cond-mat.mtrl-sci

Collective filament wrapping and nested spiral formation in active polydisperse systems

We investigate a two-dimensional polydisperse suspension of self-propelled semiflexible filaments and reveal a collective wrapping mechanism that is absent in monodisperse systems. At intermediate activity levels, long filaments coil around shorter ones, forming nested spiral structures stabilized by filament length disparity. These assemblies generalize the single-filament spiraling seen in active systems into cooperative, multi-filament configurations. As activity increases, the nested spirals undergo structural transitions: medium-length filaments unwind, longer filaments encapsulate shorter ones, and eventually all spiral structures dissolve. This reorganization is reflected in the dynamics, where van Hove distributions uncover coexisting confined and motile filament populations. Our findings identify filament length as a key control parameter for nonequilibrium self-assembly and establish inter-filament wrapping as a minimal mechanism for hierarchical organization in active matter. This mechanism provides a simple model for the cooperative confinement and structural hierarchy observed in both biological and synthetic active systems.

cond-mat.soft

Devitrification and Melting Dynamics in Vapor Deposited Water Ice

The equilibration dynamics of ultrastable glasses subjected to heating protocols has attracted recent experimental and theoretical interest. With simulations of the mW water model, we investigate the devitrification and melting dynamics of both conventional quenched (QG) and vapor deposited (DG) amorphous ices under controlled heating ramps. By developing an algorithm to reconstruct hydrogen-bond networks, we show that bond ring statistics correlates with the structural stability of the glasses and allows tracking crystalline and liquid clusters during devitrification and melting. We find that QG melts in the bulk, whereas melting in DG preferentially begins near the free surface. During devitrification, the DG shows an excess of 5-membered rings near the free surface, which is consistent with its tendency to nucleate the crystal phase in this region. Additionally, the DG shows an Avrami exponent exceeding the standard 1+d behavior, while both glasses display the same sub-3d growth of liquid clusters across heating rates, indicating that the DG enhanced exponent stems from its higher kinetic stability.

cond-mat.soft

Inverse Thermodynamics: Designing Interactions for Targeted Phase Behavior

The traditional goal of inverse self-assembly is to design interactions that drive particles toward a desired target structure. However, achieving successful self-assembly also requires tuning the thermodynamic conditions under which the structure is stable. In this work, we extend the inverse design paradigm to explicitly address this challenge by developing a framework for inverse thermodynamics, i.e. the design of interaction potentials that realize specific thermodynamic behavior. As a step in this direction, using patchy particle mixtures as a model system, we demonstrate how precise control over both bonding topology and bond energetics enables the programming of targeted phase behavior. In particular, we establish design principles for azeotropic demixing and show how to create mixtures that exhibit azeotropy at any prescribed composition. Our predictions are validated through Gibbs-ensemble simulations [A.Z. Panagiotopoulos,Molecular Physics 61, 813-826 (1987)]. These results highlight the necessity of coupling structural design with thermodynamic engineering, and provide a blueprint for controlling complex phase behavior in multi-component systems.

cond-mat.soft

A Falsifiability Test for Classical Nucleation Theory

Classical nucleation theory (CNT) is built upon the capillarity approximation, i.e., the assumption that the nucleation properties can be inferred from the bulk properties of the melt and the crystal. Although CNT's simplicity and usefulness cannot be overstated, experiments and simulations regularly uncover significant deviations from its predictions, which are often reconciled through phenomenological extensions of the CNT, fueling the debate over the general validity of the theory. In this study, we present a falsifiability test for any nucleation theory grounded in the capillarity approximation. We focus on cases where the theory predicts no differences in nucleation rates between different crystal polymorphs. We then introduce a system in which all polymorphs have the same free energy (both bulk and interfacial) across all state points. Through extensive molecular simulations, we show that the polymorphs exhibit remarkably different nucleation properties, directly contradicting CNT's predictions. We argue that CNT's primary limitation lies in its neglect of structural fluctuations within the liquid phase.

cond-mat.soft

Automating Blueprints for Colloidal Quasicrystal Assembly

One of the frontiers of nanotechnology is advancing beyond the periodic self-assembly of materials. Icosahedral quasicrystals, aperiodic in all directions, represent one of the most challenging targets that have yet to be experimentally realized at the colloidal scale. Previous attempts have required meticulous human-designed building blocks and often resulted in interactions beyond current experimental capabilities. In this work, we introduce a framework for generating experimentally accessible designs that self-assemble into quasicrystalline arrangements. We present a design for icosahedral DNA-origami building blocks and demonstrate, through molecular simulations, their successful assembly into the target quasicrystalline structure. Our results highlight the feasibility of using automated design protocols to achieve complex quasicrystalline patterns, paving the way for new applications in material science and nanotechnology.

cond-mat.soft

Correlating ultrastability with fragility and surface mobility in vapor deposited tetrahedral glasses

Several experiments on molecular and metallic glasses have shown that the ability of vapor deposition to produce ultrastable glasses is correlated with their structural and thermodynamic properties. Here we investigate the vapor deposition of a class of tetrahedral materials (including silicon and water) via molecular dynamics simulations of the generalized Stillinger-Weber potential. By changing a single parameter that controls the local tetrahedrality, we show that the emergence of ultrastable behavior is correlated with an increase in the fragility of the model. At the same time, while the mobility of the surface compared to the bulk shows only slight changes at low temperature, with increasing the tetrahedrality, it displays a significant enhancement towards the glass transition temperature. Our results point towards a strong connection between bulk dynamics, surface dynamics and glass-ultrastability ability in this class of materials.

cond-mat.soft

Self-Assembly of Active Bivalent Patchy Particles

In the present work, with the intent of exploring the out-of-equilibrium polymerization of active patchy particles in linear chains, we study a suspension of active bivalent Brownian particles (ABBPs). At all studied temperatures and densities, ABBPs self-assemble in aggregating chains, as opposed to the uniformly space-distributed chains observed in the corresponding passive systems. The main effect of activity, other than inducing chain aggregation, is to reduce the chain length and favor alignment of the propulsion vectors in the bonding process. At low activities, attraction dominates over activity in the bonding process, leading self-assembly to occur randomly regardless of the particle orientations.Interestingly, we find that at the lowest temperature, as density increases, chains aggregate forming a novel state: MISP, i.e., Motility-Induced Spirals, where spirals are characterised by a finite angular velocity. On the contrary, at the highest temperature, density and activity, chains aggregate forming a different novel state (a spinning crystalline cluster) characterized by a compact and hexagonal ordered structure, both translating and rotating. The rotation arises from an effective torque generated by the presence of competing domains where particles self-propel in the same direction.

cond-mat.soft

The interplay between liquid-liquid and ferroelectric phase transitions in supercooled water

The distinctive characteristics of water, evident in its thermodynamic anomalies, have implications across disciplines from biology to geophysics. Considered a valid hypothesis to rationalize its unique properties, a liquid-liquid phase transition in water's supercooled regime has nowadays been observed in several molecular dynamics simulations and is being actively researched experimentally. Here, we highlight intriguing and far-reaching implications of water: the ferroelectric and liquid-liquid phase transitions can be designed as two facets of the same underlying phenomenon. Our results are based on the analysis of extensive molecular dynamics simulations and are explained in the context of a classical density functional theory in mean-field approximation valid for a polar liquid. The theory underpins the potential role of ferroelectricity in promoting the liquid-liquid phase transition. The existence of ferroelectric order in supercooled low-density liquid water is confirmed by the observation in molecular dynamics simulations of collective modes in polarization fluctuations dynamics, traceable to spontaneous breaking of continuous rotational symmetry. Our work opens the door to new experimental investigations of the static and dynamic behavior of water polarization.

cond-mat.soft

Generating ultrastable glasses by homogenizing the local virial stress

In recent years, the possibility of algorithmically preparing ultra-stable glasses (UG), i.e., states that lie very deep in the potential energy landscape, has considerably expanded our understanding of the glassy state. In this work, we report on a new protocol for ultrastable glass preparation that iteratively modifies the particle diameters to reduce local virial stress fluctuations. We apply the algorithm to an additive Lennard-Jones mixture and show that, compared to the states obtained via thermal annealing, virial homogenized glasses (VHG) are characterized by a considerable increase in both kinetic stability and the number of locally favored structures (icosahedra). We also consider the melting dynamics during heating ramps and show that it occurs via an accumulation of localized events. Our results highlight the connection between the thermodynamic and mechanical stability of ultra-stable glassy states.

cond-mat.soft

Dominating Lengthscales of Zebrafish Collective Behaviour

Collective behaviour in living systems is observed across many scales, from bacteria to insects, to fish shoals. Zebrafish have emerged as a model system amenable to laboratory study. Here we report a three-dimensional study of the collective dynamics of fifty zebrafish. We observed the emergence of collective behaviour changing between \yy{ordered} to randomised, upon \yy{adaptation} to new environmental conditions. We quantify the spatial and temporal correlation functions of the fish and identify two length scales, the persistence length and the nearest neighbour distance, that capture the essence of the behavioural changes. The ratio of the two length scales correlates robustly with the polarisation of collective motion that we explain with a reductionist model of self--propelled particles with alignment interactions.

cond-mat.stat-mech

Inverse design of a pyrochlore lattice of DNA origami through model-driven experiments

Sophisticated statistical mechanics approaches and human intuition have demonstrated the possibility to self-assemble complex lattices or finite size constructs, but have mostly only been successful in silico. The proposed strategies quite often fail in experiment due to unpredicted traps associated to kinetic slowing down (gelation, glass transition), as well as to competing ordered structures. An additional challenge that theoretical predictions face is the difficulty to encode the desired inter-particle interaction potential with the currently available library of nano- and micron-sized particles. To overcome these issues, we conjugate here SAT-assembly -- a patchy-particle interaction design algorithm based on constrained optimization solvers -- with coarse-grained simulations of DNA nanotechnology to experimentally realize trap-free self-assembly pathways. As a proof of concept we investigate the assembly of the pyrochlore (also known as tetrastack) lattice, a highly coveted 3D crystal lattice due to its promise in construction of optical metamaterials. We confirm the successful assembly with two different patchy DNA origami designs via SAXS as well as SEM visualization of the silica-coated lattice. Our approach offers a versatile modeling pipeline that starts from patchy particles designed in silico and ends with wireframe DNA origami that self-assemble into the desired structure.

cond-mat.soft

Inverse design of self-folding 3D shells

Self-folding is an emerging paradigm for the inverse design of three-dimensional structures. While most efforts have concentrated on the shape of the net, our approach introduces a new design dimension-bond specificity between the edges. We transform this design process into a Boolean Satisfiability problem to derive solutions for various target structures. This method significantly enhances the yield of the folding process. Furthermore, by linearly combining independent solutions, we achieve designs for shape-shifting nets wherein the dominant structure evolves with varying external conditions. This approach is demonstrated through coarse-grained simulations on two examples of triangular and square nets capable of folding into multiple target shapes.

cond-mat.soft

Crystal nucleation in a vapor deposited Lennard-Jones mixture

Understanding the pathways to crystallization during the deposition of a vapor phase on a cold solid substrate is of great interest in industry, e.g., for the realization of electronic devices made of crystallites-free glassy materials, as well as in the atmospheric science in relation to ice nucleation and growth in clouds. Here we numerically investigate the nucleation process during the deposition of a glassformer by using a Lennard-Jones mixture, and compare the properties of this nucleation process with both its quenched counterpart and the bulk system. We find that all three systems homogeneously nucleate crystals in a narrow range of temperatures. However, the deposited layer shows a peculiar formation of ordered domains, promoted by the faster relaxation dynamics toward the free surface even in an as-deposited state. In contrast, the formation of such domains in the other systems occurs only when the structures are fully relaxed by quenching. Furthermore, the nucleus initially grows in an isotropic symmetrical manner, but eventually shows sub-3D growth due to its preference to grow along the basal plane, irrespective of the layer production procedure.

cond-mat.soft

Colloidal Hard Spheres: Triumphs, Challenges and Mysteries

The simplicity of hard spheres as a model system is deceptive. Although the particles interact solely through volume exclusion, that nevertheless suffices for a wealth of static and dynamical phenomena to emerge, making the model an important target for achieving a comprehensive understanding of matter. In addition, while real colloidal suspensions are typically governed by complex interactions, Pusey and Van Megen [Nature 320 340--342 (1986)] demonstrated that suitably tuned suspensions result in hard-sphere like behavior, thus bringing a valuable experimental complement to the celebrated theoretical model. Colloidal hard spheres are thus both a material in their own right and a platform upon which phenomena exhibited by simple materials can be explored in great detail. These various purposes enable a particular synergy between experiment, theory and computer simulation. Here we review the extensive body of work on hard spheres, ranging from their equilibrium properties such as phase behavior, interfaces and confinement to some of the non--equilibrium phenomena they exhibit such as sedimentation, glass formation and nucleation.

cond-mat.soft

From ultra-fast growth to avalanche growth in devitrifying glasses

During devitrification, pre-existing crystallites grow by adding particles to their surface via a process that is either thermally activated (diffusive mode) or happens without kinetic barriers (fast crystal growth mode). It is yet unclear what factors determine the crystal growth mode and how to predict it. With simulations of repulsive hard-sphere-like (Weeks-Chandler-Andersen) glasses, we show for the first time that the same system at the same volume fraction and temperature can devitrify via both modes depending on the preparation protocol of the glass. We prepare two types of glass, a conventional glass (CG) via fast quenching and a uniform glass (UG) via density homogenization. Firstly, we bring either glass into contact with a crystal (X) and find the inherent structure (CGX/UGX). During energy minimization, the crystal front grows deep into the CG interface, while the growth is minimal for UG. When thermal noise is added, this behavior is reflected in different crystallization dynamics. CGX exhibits a density drop at the crystal growth front which correlates with enhanced dynamics at the interface and a fast growth mode. This mechanism may explain the faster crystal growth observed below the glass transition experimentally. In contrast, UGX grows via intermittent avalanche-like dynamics localized at the interface, a combination of localized mechanical defects and the exceptional mechanical stability imposed by the UG glass phase.

cond-mat.soft