arXiv subjects

Alessio Zaccone

Publications and source records attributed to Alessio Zaccone.

At least 55 records · Page 3Linked to original sources

arXiv2025

Open quantum systems with particle and bath driven by time-dependent fields

We derive a generalized quantum Langevin equation and its fluctuation-dissipation relation describing the quantum dynamics of a tagged particle interacting with a medium (environment), where both the particle and the environment are driven by an external time-dependent (e.g. oscillating) field. We specialize on the case of a charged tagged particle interacting with a bath of charged oscillators, under an external AC electric field, although the results are much more general and can be applied to any type of external time-dependent fields. We derive the corresponding quantum Langevin equation, which obeys a modified fluctuation-dissipation relation (FDR) where the AC field plays an explicit role. The modified FDR is non-Markovian even if the undriven particle-bath system is Markovian without the external field. We provide an illustration of the usefulness of these results and derive a new form of the quantum Nyquist noise for the voltage fluctuations in electrical circuits under AC conditions (finite frequency), which is the most general since it also accounts for the response of the heat bath (e.g. lattice ions) to the applied AC electric field in the GHz-THz region, of relevance for 5G/6G wireless technologies. This generalized quantum fluctuation-dissipation relation for driven systems can also find other applications ranging from quantum noise in quantum optics to quantum computing with trapped ions.

quant-ph↗

arXiv2025

Emergent rigidity percolation of five-fold aggregates enables controllable glass properties

Metallic glasses possess outstanding mechanical and physical properties, making them promising candidates for advanced structural and functional applications; however, the lack of understanding and control over their glass transition and solidification processes remains a significant barrier to practical design. The glass transition from liquid to amorphous solid has remained an open problem in physics despite many theories and recent advances in computational efforts. The question of identifying a clear and well-defined diverging length scale accompanying the glass transition has remained unanswered, as has the nature of the transition and, indeed, the presence of a transition at all, as opposed to a mere dynamical crossover. Here we answer these questions using numerical results and theoretical analysis showing that, in atomic (metallic) glass formers, the glass transition coincides with, and is caused by, a continuous rigidity percolation transition from a liquid-like to a solid-like material. The transition occurs as five-fold symmetric atomic clusters progressively aggregate, forming a system-spanning rigid network that marks the onset of mechanical stability. This percolation-driven rigidity growth is accompanied by a sharp increase in the shear modulus G', indicating the emergence of macroscopic solid-like behavior. Beyond this point, which coincides with the Maxwell isostatic point of the percolating structure, dynamical arrest or "freezing-in" prevents further evolution. The long-sought diverging length scale is thus identified as the percolation-driven growth of rigid five-fold clusters, providing a direct link between local structural motifs and macroscopic mechanical properties at the glass transition. These insights offer practical routes to rationally engineer metallic glasses with targeted mechanical stiffness, hardness, and toughness.

cond-mat.soft↗

arXiv2025

Quantum confinement theory of ultra-thin films: electronic, thermal and superconducting properties

The miniaturization of electronic devices has led to the prominence, in technological applications, of ultra-thin films with a thickness ranging from a few tens of nanometers to just about 1-2 nanometers. While these materials are still effectively 3D in many respects, traditional theories as well as ab initio methods struggle to describe their properties as measured in experiments. In particular, standard approaches to quantum confinement rely on hard-wall boundary conditions, which neglect the unavoidable, ubiquitous, atomic-scale irregularities of the interface. Recently, a unified theoretical approach to quantum confinement has been proposed which is able to effectively take the real nature of the interface into account, and can efficiently be implemented in synergy with microscopic theories. Its predictions for the electronic properties such as electrical conductivity of semiconductor thin films or critical temperature of superconducting thin films, have been successfully verified in comparison with experimental data. The same confinement principles lead to new laws for the phonon density of states and for the heat capacity of thin films, again in agreement with the available experimental data.

cond-mat.supr-con↗

arXiv2025

Universality of Dynamic and Thermodynamic Behavior of Polymers near their Glass Transition

Describing the dynamics and thermodynamics of amorphous materials near the glass transition is a major challenge in soft-matter physics and polymer engineering. Here, we show that the dependence of the dielectric alpha-relaxation time on temperature can be captured by a universal equation with only two independent parameters, Tg and fragility, m. This is similar in spirit to the ideas of van Krevelen and Bicerano, and can be related to the Boyer-Spencer and Simha-Boyer rules for the volumetric thermal expansion via a modified free-volume approach such as Sanchez-Lacombe "Two-state, two-(time)scale" (SL-TS2) theory. The model is compared to experimental data for nine amorphous polymers with varying values of Tg and m, and a good qualitative and quantitative agreement is found. We also derive the relationship between the experimental and computational (high-cooling-rate) Tg, and compare our model prediction for the Tg-shift between the two with the simulation results of Afzal et al., finding a good qualitative and semi-quantitative agreement. The results could serve as a guidance for future simulation studies.

cond-mat.soft↗

arXiv2025

Thickness-dependent conductivity of nanometric semiconductor thin films

The miniaturization of electronic devices has led to the prominence, in technological applications, of semiconductor thin films that are only a few nanometers thick. In spite of intense research, the thickness-dependent resistivity or conductivity of semiconductor thin films is not understood at a fundamental physical level. We develop a theory based on quantum confinement which yields the dependence of the concentration of intrinsic carriers on the film thickness. The theory predicts that the resistivity $ρ$, in the 1-10 nm thickness range, increases exponentially as $ρ\sim \exp(const/L^{1/2})$ upon decreasing the film thickness $L$. This law is able to reproduce the remarkable increase in resistivity observed experimentally in Si thin films, whereas the effect of surface scattering (Fuchs-Sondheimer theory) alone cannot explain the data when the film thickness is lower than 10 nm.

cond-mat.mes-hall↗

arXiv2025

Fragility and thermal expansion control crystal melting and the glass transition

Analytical relations for the glass transition temperature, $T_g$, and the crystal melting temperature, $T_m$, are developed on the basis of nonaffine lattice dynamics. The proposed relations explain: (i) the seemingly universal factor of $\approx 2/3$ difference between glass transition temperature and melting temperature of the corresponding crystal, and (ii) the recent empirical discovery that both $T_g$ and $T_m$ are proportional to the liquid fragility $m$ divided by the thermal expansion coefficient $α$ of the solid.

cond-mat.soft↗

arXiv2025

Hedgehog topological defects in 3D amorphous solids

The underlying structural disorder renders the concept of topological defects in amorphous solids difficult to apply and hinders a first-principle identification of the microscopic carriers of plasticity and of the regions more prone to structural rearrangements (``soft spots''). Recently, it has been proposed that well-defined topological defects can still be identified in glasses, and correlated to local and global plasticity, by looking at the eigenvector field or the particle displacement field. Nevertheless, all the existing proposals and analyses are only valid in two spatial dimensions. In this work, we propose the idea of using hedgehog topological defects to characterize the plasticity of 3D glasses and to geometrically predict the location of their soft spots. We corroborate our proposal by simulating a Kremer-Grest 3D polymer glass, and by using both the normal mode eigenvector field and the displacement field around large plastic events. Contrary to the 2D case, the sign of the topological charge defined from the eigenvector field is ambiguous and the geometry of the topological defects, whether radial or hyperbolic, plays a fundamental role in 3D. In fact, we find that the topological hedgehog defects relevant for plasticity are those exhibiting hyperbolic geometry, resembling the saddle-point structure of 2D topological defects with negative winding number (anti-vortices). Our results confirm that a topological characterization of plasticity in glasses is feasible and provide a concrete realization of this program in 3D amorphous systems.

cond-mat.soft↗

arXiv2025

Possible superconductivity in very thin magnesium films

It is known that noble metals such as gold, silver and copper are not supercon- 1 ductors, so as magnesium. This is due to the weakness of the electron-phonon interaction 2 which makes them excellent conductors but not superconductors. As it has recently been 3 shown for gold, silver and copper, even for magnesium it is possible that in very particular 4 situations superconductivity may occur. Quantum confinement in thin films has been 5 consistently shown to induce a significant enhancement of the superconducting critical 6 temperature in several superconductors. It is therefore an important fundamental question 7 whether ultra-thin film confinement may induce observable superconductivity in non- 8 superconducting metals such as magnesium. We study this problem using a generalization, 9 in the Eliashberg framework, of a BCS theory of superconductivity in good metals under 10 thin-film confinement. By numerically solving these new Eliashberg-type equations, we 11 find the dependence of the superconducting critical temperature on the film thickness, L. 12 This parameter-free theory predicts superconductivity in very thin magnesium films. We 13 demonstrate that this is a fine-tuning problem where the thickness must assume a very 14 precise value, close to half a nanometer.

cond-mat.supr-con↗

arXiv2025

Microscopic mechanism of electric field-induced superconductivity suppression in metallic thin films

Supercurrent field-effect transistors made from thin metallic films are a promising option for next-generation high-performance computation platforms. Despite extensive research, there is still no complete quantitative microscopic explanation for how an external DC electric field suppresses superconductivity in thin films. This study aims to provide a quantitative description of superconductivity as a function of film thickness based on Eliashberg's theory. The calculation considers the electrostatics of the electric field, its realistic penetration depth in the film, and its effect on the Cooper pair, which is described as a standard s-wave bound state according to BCS theory. The estimation suggests that an external electric field of approximately $10^8$ V/m is required to suppress superconductivity in 10-30-nm-thick films, which aligns with experimental observations. Ultimately, the study offers "materials by design" guidelines for suppressing supercurrent when an external electric field is applied to the film surface. Furthermore, the proposed framework can be easily extended to investigate the same effects for ultrathin films.

cond-mat.supr-con↗

arXiv2025

Can the noble metals (Au, Ag and Cu) be superconductors?

It is common knowledge that noble metals are excellent conductors but do not exhibit superconductivity. On the other hand, quantum confinement in thin films has been consistently shown to induce a significant enhancement of the superconducting critical temperature in several superconductors. It is therefore an important fundamental question whether ultra-thin film confinement may induce observable superconductivity in non-superconducting metals. We present a generalization, in the Eliashberg framework, of a BCS theory of superconductivity in good metals under thin-film confinement. By numerically solving these new Eliashberg-type equations, we find the dependence of the superconducting critical temperature on the film thickness $L$. This parameter-free theory predicts a maximum increase in the critical temperature for a specific value of the film thickness, which is a function of the number of free carriers in the material. Exploiting this fact, we predict that ultra-thin films of gold, silver and copper of suitable thickness could be superconductors at low but experimentally accessible temperatures. We demonstrate that this is a fine-tuning problem where the thickness must assume a very precise value, close to half a nanometer.

cond-mat.supr-con↗

arXiv2025

Analytical solution for the polydisperse random close packing problem in 2D

An analytical theory for the random close packing density, $ϕ_\textrm{RCP}$, of polydisperse hard disks is provided using an equilibrium model of crowding [A. Zaccone, Phys. Rev. Lett. 128, 028002 (2022)] which has been justified on the basis of extensive numerical analysis of the maximally random jammed (MRJ) line in the phase diagram of hard spheres [Anzivino et al., J. Chem. Phys. 158, 044901 (2023)]. The solution relies on the equations of state for the hard disk fluid and provides predictions for $ϕ_\textrm{RCP}$ as a function of the ratio, $s$, of the standard deviation of the distribution of disk diameters to its mean. For a power-law size distribution with $s=0.246$, the theory yields $ϕ_\textrm{RCP} =0.892$, which compares well with the most recent numerical estimate $ϕ_\textrm{RCP} =0.905$ based on the Monte-Carlo swap algorithms [Ghimenti, Berthier, van Wijland, Phys. Rev. Lett. 133, 028202 (2024)].

cond-mat.soft↗

arXiv2025

Topological Bardeen-Cooper-Schrieffer theory of superconducting quantum rings

Quantum rings have emerged as a playground for quantum mechanics and topological physics, with promising technological applications. Experimentally realizable quantum rings, albeit at the scale of a few nanometers, are 3D nanostructures. Surprisingly, no theories exist for the topology of the Fermi sea of quantum rings, and a microscopic theory of superconductivity in nanorings is also missing. In this paper, we remedy this situation by developing a mathematical model for the topology of the Fermi sea and Fermi surface, which features non-trivial hole pockets of electronic states forbidden by quantum confinement, as a function of the geometric parameters of the nanoring. The exactly solvable mathematical model features two topological transitions in the Fermi surface upon shrinking the nanoring size either, first, vertically (along its axis of revolution) and, then, in the plane orthogonal to it, or the other way round. These two topological transitions are reflected in a kink and in a characteristic discontinuity, respectively, in the electronic density of states (DOS) of the quantum ring, which is also computed. Also, closed-form expressions for the Fermi energy as a function of the geometric parameters of the ring are provided. These, along with the DOS, are then used to derive BCS equations for the superconducting critical temperature of nanorings as a function of the geometric parameters of the ring. The $T_c$ varies non-monotonically with the dominant confinement size and exhibits a prominent maximum, whereas it is a monotonically increasing function of the other, non-dominant, length scale. For the special case of a perfect square toroid (where the two length-scales coincide), the $T_c$ increases monotonically with increasing the confinement size, and in this case, there is just one topological transition.

cond-mat.supr-con↗

arXiv2025

Complete mathematical theory of the jamming transition: A perspective

The jamming transition of frictionless athermal particles is a paradigm to understand the mechanics of amorphous materials at the atomic scale. Concepts related to the jamming transition and the mechanical response of jammed packings have cross-fertilized into other areas such as atomistic descriptions of the elasticity and plasticity of glasses. In this perspective article, the microscopic mathematical theory of the jamming transition is reviewed from first-principles. The starting point of the derivation is a microscopically-reversible particle-bath Hamiltonian from which the governing equation of motion for the grains under an external deformation is derived. From this equation of motion, microscopic expressions are obtained for both the shear modulus and the viscosity as a function of the distance from the jamming transition (respectively, above and below the transition). Regarding the vanishing of the shear modulus at the unjamming transition, this theory, as originally demonstrated in [Zaccone & Scossa-Romano, Phys. Rev. B 83, 184205 (2011)], is currently the only quantitative microscopic theory in parameter-free agreement with numerical simulations of [O'Hern et al. Phys. Rev. E 68, 011306 (2003)] for jammed packings. The divergence of the viscosity upon approaching the jamming transition from below is derived here, for the first time, from the same microscopic Hamiltonian. The quantitative microscopic prediction of the diverging viscosity is shown to be in fair agreement with numerical results of sheared 2D soft disks from [Olsson & Teitel, Phys. Rev. Lett. 99, 178001 (2007)].

cond-mat.soft↗

arXiv2025

Hyperballistic transport in dense systems of charged particles under ac electric fields

The Langevin equation is ubiquitously employed to numerically simulate plasmas, colloids and electrolytes. However, the usual assumption of white noise becomes untenable when the system is subject to an external AC electric field. This is because the charged particles in the system, which provide the thermal bath for the particle transport, become themselves responsive to the AC field and the thermal noise is field-dependent and non-Markovian. We theoretically study the particle diffusivity in a Langevin transport model for a tagged charged particle immersed in a dense system of charged particles (plus also, possibly, other neutral particles) that act as the thermal bath, under an external AC electric field. This is done by properly accounting for the effects of the AC field on the thermal bath statistics. We analytically derive the time-dependent generalized diffusivity $D(t)$ for different initial conditions. The generalized diffusivity exhibits damped oscillatory-like behaviour with initial very large peaks, where the generalized diffusion coefficient is enhanced by orders of magnitude with respect to the infinite-time steady-state value. The latter coincides with the Stokes-Einstein diffusivity in the absence of external field. For initial conditions where the external field is already on at $t=0$ and the system is thermalized under DC conditions for $t \leq 0$, the short-time behaviour is hyperballistic, $MSD \sim t^4$ (where MSD is the mean-squared displacement), leading to giant enhancement of the particle transport. Finally, the theory elucidates the role of medium polarization on the local Lorentz field, and allows for estimates of the effective electric charge due to polarization by the surrounding charges.

physics.plasm-ph↗

arXiv2025

Experimental identification of topological defects in 2D colloidal glass

Topological defects are singularities within a field that cannot be removed by continuous transformations. The definition of these irregularities requires an ordered reference configuration, calling into question whether they exist in disordered materials, such as glasses. However, recent work suggests that well-defined topological defects emerge in the dynamics of glasses, even if they are not evident in the static configuration. In this study, we reveal the presence of topological defects in the vibrational eigenspace of a two-dimensional experimental colloidal glass. These defects strongly correlate with the vibrational features and spatially correlate with each other and structural "soft spots", more prone to plastic flow. This work experimentally confirms the existence of topological defects in disordered systems revealing the complex interplay between topology, disorder, and dynamics.

cond-mat.soft↗

arXiv2025

New Insights into the Dependence of Glass Transition Temperature and Dynamic Fragility on Molecular Weight in Oligomers and Polymers

In an earlier preprint (V. V. Ginzburg, O. V. Gendelman, R. Casalini, and A. Zaccone, arxiv:2409.17291), we demonstrated that the dynamic (relaxation time) and volume equations of state for many amorphous polymers are near-universal -- each material is characterized by two governing parameters -- the glass transition temperature, T_{g}, and the elementary relaxation time, /tau_{el}. The dynamic fragility, m, is uniquely related to /tau_{el}. The earlier analysis was based on the data for high polymers (degree of polymerization, N > 200, and molecular weight, Mw > 20,000 g/mol). Here, we investigate the dependence of T_{g} and /tau_{el} on N (or, equivalently, on the molecular weight, M). We consider the dielectric spectroscopy data for homologous series of four polymers: polystyrene (PS), poly-(2-vinylpyridine) (P2VP), poly(dimethylsiloxane) (PDMS), and polybutadiene (PB). It is shown that the relaxation time curves for various oligomers can be successfully collapsed onto the same master curve as the high polymers. The glass transition temperature and the dynamic fragility are found to be linear functions of 1/M, in agreement with the Fox-Flory equation. The scaling results are interpreted based on the SL-TS2 theory, ensuring that the model description is consistent with the Boyer rules for the coefficients of thermal expansion above and below T_{g}, as well as the thermodynamical scaling for the pressure dependence of the relaxation time.

cond-mat.soft↗

arXiv2024

Relativistic theory of the viscosity of fluids across the entire energy spectrum

The shear viscosity is a fundamental transport property of matter. Here we derive a general theory of the viscosity of gases based on the relativistic Langevin equation (deduced from a relativistic Lagrangian) and nonaffine linear response theory. The proposed relativistic theory is able to recover the viscosity of non-relativistic classical gases, with all its key dependencies on mass, temperature, particle diameter and Boltzmann constant, in the limit of Lorentz factor $γ=1$. It also unveils the relativistic enhancement mechanism of viscosity. In the limit of ultrarelativistic fluids, the theory provides a new analytical formula which reproduces the cubic increase of viscosity with temperature in agreement with various estimates for hot dense matter and the QGP-type fluid.

hep-ph↗

arXiv2024

Convective Instability Driven by Diffusiophoresis of Colloids in Binary Liquid Mixtures

In a binary fluid mixture, the concentration gradient of a heavier molecular solute leads to a diffusive flux of solvent and solute to achieve thermodynamic equilibrium. If the solute concentration decreases with height, the system is always in a condition of stable mechanical equilibrium against gravity. We show experimentally that this mechanical equilibrium becomes unstable in case colloidal particles are dispersed uniformly within the mixture, and that the resulting colloidal suspension undergoes a transient convective instability with the onset of convection patterns. By means of a numerical analysis, we clarify the microscopic mechanism from which the observed destabilisation process originates. The solute concentration gradient drives an upward diffusiophoretic migration of colloids, in turn causing the development of a mechanically unstable layer within the sample, where the density of the suspension increases with height. Convective motions arise to minimize this localized rise in gravitational potential energy.

cond-mat.soft↗