arXiv subjects

Alessio Zaccone

Publications and source records attributed to Alessio Zaccone.

At least 73 records · Page 4Linked to original sources

arXiv2024

Random close packing of binary hard spheres favors the stability of neutron-rich atomic nuclei

In spite of the success of the Bethe-Weizsäcker mass formula in its modern numerical and predictive implementations, the common-knowledge principle that it is electrostatics which, ultimately, favors neutron-rich nuclei still presents unclear aspects. For example, while it is true that the Coulomb interaction promotes the tendency towards neutron-rich nuclei, the opposite effects of Majorana exchange forces and Pauli exclusion are known to counteract this tendency. We show that a recent analytical progress in the mathematical description of random close packing of spheres with different sizes provides a missing contribution to the theoretical description of the $Z$ versus $N$ slope in the nuclides chart. In particular, the theory suggests, on geometric grounds and with a physically-reasoned assumption that the excluded-volume size of neutrons is 20\% larger than that of protons, that the most stable nuclei are those with ratio $Z/N\approx 0.75$. This new ``geometric'' random-packing contribution to the semi-empirical mass formula may be the missing aspect of nuclear structure that tilts the balance towards neutron-rich nuclei in the Segrè stability chart.

nucl-th↗

arXiv2024

Theory of elastic constants of athermal amorphous solids with internal stresses

A new microscopic derivation of the elastic constants of amorphous solids is presented within the framework of nonaffine lattice dynamics, which makes use of a perturbative form of the low-frequency eigenvectors of the dynamical matrix introduced in [V. Mazzacurati, G. Ruocco, M. Sampoli EPL 34, 681 (1996)]. The theory correctly recovers the shear modulus at jamming, $μ\sim (z-2d)$, including prefactors in quantitative agreement with simulations. Furthermore, this framework allows us, for the first time, to include the effect of internal stresses. The theory shows that the Maxwell rigidity criterion $z=2d$ is violated with internal stress. In particular, $μ\sim (z-2df)$ where $f<1$ if the bonds are, on average, stretched, and the solid is thus rigid below the Maxwell isostatic limit, while $f>1$ if the bonds are, on average, compressed. The coefficient $f$ is derived in analytical form and depends only on $d$ and on the average particle displacement from the interaction energy minimum.

cond-mat.soft↗

arXiv2024

Clustering of negative topological charge precedes plastic failure in 3D glasses

The deformation mechanism in amorphous solids subjected to external shear remains poorly understood because of the absence of well-defined topological defects mediating the plastic deformation. The notion of soft spots has emerged as a useful tool to characterize the onset of irreversible rearrangements and plastic flow, but these entities are not clearly defined in terms of geometry and topology. In this study, we unveil the phenomenology of recently discovered, precisely defined topological defects governing the microscopic mechanical and yielding behavior of a model 3D glass under shear deformation. We identify the existence of vortex-like and anti-vortex-like topological defects within the 3D non-affine displacement field. The number density of these defects exhibits a significant anti-correlation with the plastic events, with defect proliferation-annihilation cycles matching the alternation of elastic-like segments and catastrophic plastic drops, respectively. Furthermore, we observe collective annihilation of these point-like defects via plastic events, with large local topological charge fluctuations in the vicinity of regions that feature strong non-affine displacements. We reveal that plastic yielding is driven by several large sized clusters of net negative topological charge, the massive annihilation of which triggers the onset of plastic flow. These findings suggest a geometric and topological characterization of soft spots and pave the way for the mechanistic understanding of topological defects as mediators of plastic deformation in glassy materials.

cond-mat.soft↗

arXiv2024

Unveiling the Asymmetry in Density within the Shear Bands of Metallic Glasses

Plastic deformation in metallic glasses at room temperature leads to the development of shear bands due to shear localization. In many experiments, shear bands have shown local density variations along their path, with a distinct imbalance in magnitude between local densification and dilation. However, a comprehensive mechanistic understanding or theory to explain this asymmetry has been lacking until now. Here, we introduce a new model that consists of a sequential arrangement of alternating topological 'charges', generating a dipolar field. The resulting microscopic displacement field, when integrated into the deformation gradient tensor, provides an accurate analytical solution for the observed imbalances in the density variations. The implications of this method are discussed, highlighting the potential to elucidate a broader range of observations in shear bands.

cond-mat.dis-nn↗

arXiv2024

Timescale bridging in atomistic simulations of epoxy polymer mechanics using non-affine deformation theory

Developing a deep understanding of macroscopic mechanical properties of amorphous systems which lack structural periodicity, has posed a key challenge, not only at the level of theory but also in molecular simulations. Despite significant advancements in computational resources, there is a vast timescale disparity, more than 6 orders of magnitude, between mechanical properties probed in simulations compared to experiments. Using the theoretical framework of non-affine lattice dynamics (NALD), based on the instantaneous normal modes analysis determined through the dynamical matrix of the system, we study the viscoelastic response of a cross-linked epoxy system of diglycidyl ether of bisphenol A (DGEBA) and poly(oxypropylene) diamine, over many orders of magnitude in deformation frequency, below the glass transition temperature. Predictions of the elastic modulus are satisfactorily validated against the non-equilibrium molecular dynamics simulations in the high-frequency regime, and against experimental data from dynamic mechanical analysis at frequencies $ \sim 1 {\rm Hz}$, hence successfully bridging the timescale gap. The comparison shows that non-affine displacements at the atomic level account for nearly two orders of magnitude reduction in the low-frequency elastic modulus of the polymer glass, compared to affine elasticity estimates. The analysis also reveals the role of internal stresses (as reflected in the instantaneous normal modes), which act as to strengthen the mechanical response.

cond-mat.soft↗

arXiv2024

Quantum confinement theory of the heat capacity of thin films

A theory and mechanistic understanding of the thermal properties of solids under nanoscale confinement are currently missing. We develop a theoretical quantum confinement description of thin films which predicts a new physical law for the heat capacity. In particular, due to the suppression of vibrational modes caused by the thin film confinement, the vibrational density of states (VDOS) deviates from the Debye quadratic law in frequency and is, instead, cubic in frequency. This leads to a temperature dependence of the heat capacity which is $\sim T^4$ instead of Debye's $\sim T^3$ law. Furthermore, the new theory predicts a linear increase of the heat capacity upon increasing the nanometric film thickness. Both dependencies are found in excellent agreement with recent experimental data on NbTiN thin films.

cond-mat.mes-hall↗

arXiv2024

Theory of superconductivity in thin films under an external electric field

The supercurrent field effect is experimentally realized in various nano-scale devices, based on the superconductivity suppression by external electric fields being effective for confined systems. In spite of intense research, a microscopic theory and explanation of this effect is missing. Here, a microscopic theory of phonon-mediated superconductivity in thin films is presented, which accounts for the effect of quantum confinement on the electronic density of states, on the Fermi energy, and on the topology of allowed states in momentum space. By further accounting for the interplay between quantum confinement, the external static electric field, the Thomas-Fermi screening in the electron-phonon matrix element, and the effect of confinement on the Coulomb repulsion parameter, the theory predicts the critical value of the external electric field as a function of the film thickness, above which superconductivity is suppressed. In particular, this critical value of the electric field is the lower the thinner the film, in agreement with recent experimental observations. Crucially, this effect is predicted by the theory when both Thomas-Fermi screening and the Coulomb pseudopotential are taken into account, along with the respective dependence on thin film thickness. This microscopic theory of the supercurrent field-effect opens up new possibilities for electric-field gated quantum materials.

cond-mat.supr-con↗

arXiv2024

General theory of the viscosity of liquids and solids from nonaffine particle motions

A new microscopic formula for the viscosity of liquids and solids is derived rigorously from a first-principles (microscopically reversible) Hamiltonian for particle-bath atomistic motion. The derivation is done within the framework of nonaffine linear response theory. The new formula may lead to a valid alternative to the Green-Kubo approach to describe the viscosity of condensed matter systems from molecular simulations without having to fit long-time tails. Furthermore, it provides a direct link between the viscosity, the vibrational density of states of the system, and the zero-frequency limit of the memory kernel.

cond-mat.soft↗

arXiv2024

Explaining the thickness-dependent dielectric permittivity of thin films

The dielectric properties of thin films are of paramount important in a variety of technological applications, and of fundamental importance for solid state research. In spite of this, there is currently no theoretical understanding of the dependence of the dielectric permittivity on the thickness of thin films. We develop a confinement model within the Lorentz-field framework for the microscopic Langevin-equation description of dielectric response in terms of the atomic-scale vibrational modes of the solid. Based on this, we derive analytical expressions for the dielectric permittivity as a function of thin film thickness, in excellent agreement with experimental data of Barium-Strontium-Titanate (BST) thin films of different stoichiometry. The theory shows that the decrease of dielectric permittivity with decreasing thickness is directly caused by the restriction in $k$-space of the available eigenmodes for field-induced alignment of ions and charged groups.

cond-mat.mtrl-sci↗

arXiv2024

Theory of heavy-quarks contribution to the quark-gluon plasma viscosity

The shear viscosity of quark gluon plasma is customarily estimated in the literature using kinetic theory, which, however, is well known to break down for dense interacting systems. Here we propose an alternative theoretical approach based on recent advances in the physics of dense interacting liquid-like systems, which is valid for strongly-interacting and arbitrarily dense relativistic systems. With this approach, the viscosity of strongly interacting dense heavy-quarks plasma is evaluated analytically, at the level of special relativity. For QGP well above the confinement temperature, the theory predicts that the viscosity increases with the cube of temperature, in agreement with evidence.

hep-ph↗

arXiv2024

Anharmonic theory of superconductivity and its applications to emerging quantum materials

The role of anharmonicity on superconductivity has often been disregarded in the past. Recently, it has been recognized that anharmonic decoherence could play a fundamental role in determining the superconducting properties (electron-phonon coupling, critical temperature, etc) of a large class of materials, including systems close to structural soft-mode instabilities, amorphous solids and metals under extreme high-pressure conditions. Here, we review recent theoretical progress on the role of anharmonic effects, and in particular certain universal properties of anharmonic damping, on superconductivity. Our focus regards the combination of microscopic-agnostic effective theories for bosonic mediators with the well-established BCS theory and Migdal-Eliashberg theory for superconductivity. We discuss in detail the theoretical frameworks, their possible implementation within first-principles methods, and the experimental probes for anharmonic decoherence. Finally, we present several concrete applications to emerging quantum materials, including hydrides, ferroelectrics and systems with charge density wave instabilities.

cond-mat.supr-con↗

arXiv2023

Unified physical framework for stretched-exponential, compressed-exponential, and logarithmic relaxation phenomena in glassy polymers

We develop a simple yet comprehensive nonlinear model to describe relaxation phenomena in amorphous glass-formers near the glass transition temperature. The model is based on the two-state, two-(time)scale (TS2) framework, and describes the isothermal relaxation of specific volume, enthalpy, or shear stress via a simple first-order nonlinear differential equation (the Trachenko-Zaccone [TZ] equation) for local cooperative events. These nonlinear dynamics of cooperatively rearranging regions (CRR) naturally arise from the TS2 framework. We demonstrate that the solutions of the TZ equation comprehensively encompass the Debye exponential relaxation, the Kohlrausch-Williams-Watts (KWW) stretched and compressed relaxations, and the Guiu-Pratt logarithmic relaxation. Furthermore, for the case of stress relaxation modeling, our model recovers, as one of its limits, the Eyring law for plastic flow, where the Eyring activation volume is related to thermodynamic parameters of the material. Using the example of polystyrene (PS), we demonstrate how our model successfully describes the Kovacs "asymmetry of approach" specific volume and enthalpy experiments, as well as the stress relaxation. Other potential applications of the model, including the dielectric relaxation, are also discussed. The presented approach disentangles the physical origins of different relaxation laws within a single general framework based on the underlying physics.

cond-mat.soft↗

arXiv2023

Translation-invariant relativistic Langevin equation derived from first principles

The relativistic Langevin equation poses a number of technical and conceptual problems related to its derivation and underlying physical assumptions. Recently, a method has been proposed in [A. Petrosyan and A. Zaccone, J. Phys. A: Math. Theor. 55 015001 (2022)] to derive the relativistic Langevin equation from a first-principles particle-bath Lagrangian. As a result of the particle-bath coupling, a new ``restoring force'' term appeared, which breaks translation symmetry. Here we revisit this problem aiming at deriving a fully translation-invariant relativistic Langevin equation. We successfully do this by adopting the renormalization potential protocol originally suggested by Caldeira and Leggett. The relativistic renormalization potential is derived here and shown to reduce to Caldeira and Leggett's form in the non-relativistic limit. The introduction of this renormalization potential successfully removes the restoring force and a fully translation-invariant relativistic Langevin equation is derived for the first time. The physically necessary character of the renormalization potential is discussed in analogy with non-relativistic systems, where it emerges due to the renormalization of the tagged particle dynamics due to its interaction with the bath oscillators (a phenomenon akin to level-repulsion or avoided-crossing in condensed matter). We discuss the properties that the corresponding non-Markovian friction kernel has to satisfy, with implications ranging from transport models of the quark-gluon plasma, to relativistic viscous hydrodynamic simulations, and to electrons in graphene.

cond-mat.stat-mech↗

arXiv2023

Glassy heat capacity from overdamped phasons and a hypothetical phason-induced superconductivity in incommensurate structures

Phasons are collective low-energy modes that appear in disparate condensed matter systems such as quasicrystals, incommensurate structures, fluctuating charge density waves, and Moiré superlattices. They share several similarities with acoustic phonon modes, but they are not protected by any exact translational symmetry. As a consequence, they are subject to a wavevector independent damping, and they develop a finite pinning frequency, which destroy their acoustic linearly propagating dispersion. Under a few and simple well-motivated assumptions, we compute the phason density of states, and we derive the phason heat capacity as a function of the temperature. Finally, imagining a hypothetical s-wave pairing channel with electrons, we compute the critical temperature $T_c$ of the corresponding superconducting state as a function of phason damping using the Eliashberg formalism. We find that for large phason damping, the heat capacity is linear in temperature, showing a distinctive glass-like behavior. Additionally, we observe that the phason damping can strongly enhance the effective Eliashberg coupling, and we reveal a sharp non-monotonic dependence of the superconducting temperature $T_c$ on the phason damping, with a maximum located at the underdamped to overdamped crossover scale. Our simple computations confirm the potential role of overdamped modes in explaining the glassy properties of incommensurate structures, but also in possibly inducing strongly-coupled superconductivity therein, and enhancing the corresponding $T_c$.

cond-mat.supr-con↗

arXiv2023

Constrains on s and d components of electron boson coupling constants in one band d wave Eliashberg theory for high Tc superconductors

The phenomenology of overdoped high Tc superconductors can be described by a one band d wave Eliashberg theory where the mechanism of superconducting coupling is mediated by antiferromagnetic spin fluctuations and whose characteristic energy Omega0 scales with Tc according to the empirical law Omega0 = 5.8 kBTc. This model presents universal characteristics that are independent of the critical temperature such as the link between the s and d components of electron boson coupling constants and the invariance of the ratio 2Delta/kBTc.

cond-mat.supr-con↗

arXiv2023

Possible enhancement of the superconducting $T_c$ due to sharp Kohn-like soft phonon anomalies

Phonon softening is a ubiquitous phenomenon in condensed matter systems which is often associated with charge density wave (CDW) instabilities and anharmonicity. The interplay between phonon softening, CDW and superconductivity is a topic of intense debate. In this work, the effects of anomalous soft phonon instabilities on superconductivity are studied based on a recently developed theoretical framework that accounts for phonon damping and softening within the Migdal-Eliashberg theory. Model calculations show that the phonon softening in the form of a sharp dip in the phonon dispersion relation, either acoustic or optical (including the case of Kohn-type anomalies typically associated with CDW), can cause a manifold increase of the electron-phonon coupling constant $λ$. This, under certain conditions, which are consistent with the concept of optimal frequency introduced by Bergmann and Rainer, can produce a large increase of the superconducting transition temperature $T_c$. In summary, our results suggest the possibility of reaching high-temperature superconductivity by exploiting soft phonon anomalies restricted in momentum space.

cond-mat.supr-con↗

arXiv2023

Generalized Langevin equation with shear flow and its fluctuation-dissipation theorems derived from a Caldeira-Leggett Hamiltonian

We provide a first-principles derivation of the Langevin equation with shear flow and its corresponding fluctuation-dissipation theorems. Shear flow of simple fluids has been widely investigated by numerical simulations. Most studies postulate a Markovian Langevin equation with a simple shear drag term à la Stokes. However, this choice has never been justified from first principles. We start from a particle-bath system described by a classical Caldeira-Leggett Hamiltonian modified by adding a term proportional to the strain-rate tensor according to Hoover's DOLLS method, and we derive a generalized Langevin equation for the sheared system. We then compute, analytically, the noise time-correlation functions in different regimes. Based on the intensity of the shear-rate, we can distinguish between close-to-equilibrium and far-from-equilibrium states. According to the results presented here, the standard, simple and Markovian form of the Langevin equation with shear flow postulated in the literature is valid only in the limit of extremely weak shear rates compared to the effective vibrational temperature of the bath. For even marginally higher shear rates, the (generalized) Langevin equation is strongly non-Markovian and non-trivial fluctuation-dissipation theorems are derived.

cond-mat.soft↗

arXiv2023

Estimating random close packing in polydisperse and bidisperse hard spheres via an equilibrium model of crowding

We show that an analogy between crowding in fluid and jammed phases of hard spheres captures the density dependence of the kissing number for a family of numerically generated jammed states. We extend this analogy to jams of mixtures of hard spheres in $d=3$ dimensions, and thus obtain an estimate of the random close packing (RCP) volume fraction, $ϕ_{\textrm{RCP}}$, as a function of size polydispersity. We first consider mixtures of particle sizes with discrete distributions. For binary systems, we show agreement between our predictions and simulations, using both our own and results reported in previous works, as well as agreement with recent experiments from the literature. We then apply our approach to systems with continuous polydispersity, using three different particle size distributions, namely the log-normal, Gamma, and truncated power-law distributions. In all cases, we observe agreement between our theoretical findings and numerical results up to rather large polydispersities for all particle size distributions, when using as reference our own simulations and results from the literature. In particular, we find $ϕ_{\textrm{RCP}}$ to increase monotonically with the relative standard deviation, $s_σ$, of the distribution, and to saturate at a value that always remains below 1. A perturbative expansion yields a closed-form expression for $ϕ_{\textrm{RCP}}$ that quantitatively captures a distribution-independent regime for $s_σ < 0.5$. Beyond that regime, we show that the gradual loss in agreement is tied to the growth of the skewness of size distributions.

cond-mat.soft↗