Examinando por Autor "Davis, Sergio"
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Ítem A Bayesian interpretation of first-order phase transitions(Springer Science and Business Media, LLC, 2016-03) Davis, Sergio; Peralta, Joaquín; Navarrete, Yasmín; González, Diego; Gutiérrez, GonzaloIn this work we review the formalism used in describing the thermody namics of first-order phase transitions from the point of view of maximum entropy inference. We present the concepts of transition temperature, latent heat and entropy difference between phases as emergent from the more fundamental concept of internal energy, after a statistical inference analysis. We explicitly demonstrate this point of view by making inferences on a simple game, resulting in the same formalism as in thermodynamical phase transitions. We show that analogous quantities will inevitably arise in any problem of inferring the result of a yes/no question, given two different states of knowledge and information in the form of expectation values. This exposi tion may help to clarify the role of these thermodynamical quantities in the context of different first-order phase transitions such as the case of magnetic Hamiltonians (e.g. the Potts model).Ítem A classification of nonequilibrium steady states based on temperature correlations(Elsevier B.V., 2022-12-15) Davis, SergioAlthough generalized ensembles have now been in use in statistical mechanics for decades, including frameworks such as Tsallis’ nonextensive statistics and superstatistics, a classification of these generalized ensembles outlining the boundaries of validity of different families of models, is still lacking. In this work, such a classification is proposed in terms of supercanonical and subcanonical ensembles, according to a newly defined parameter, the inverse temperature covariance parameter U. This parameter is non-negative in superstatistics (and is equal to the variance of the inverse temperature) but can be negative for other families of statistical ensembles, acquiring then a broader meaning. It is shown that U is equal for every region of a composite system in a steady state, and examples are given of supercanonical and subcanonical states. © 2022 Elsevier B.V.Ítem A model for defect formation in materials exposed to radiation(2021-01-01) Davis, Sergio; González-Cataldo, Felipe; Gutiérrez, Gonzalo; Avaria, Gonzalo; Bora, Biswajit; Jalaj Jain, José; Pavez, Cristian; Soto, LeopoldoA simple model for the stochastic evolution of defects in a material under irradiation is presented. Using the master-equation formalism, we derive an expression for the average number of defects in terms of the power flux and the exposure time. The model reproduces the qualitative behavior of self-healing due to defect recombination, reaching a steady-state concentration of defects that depends on the power flux of the incident radiation and the material temperature, while also suggesting a particular time scale on which the incident energy is most efficient for producing defects, in good agreement with experimental results. Given this model, we discuss the integral damage factor, a descriptor that combines the power flux and the square of the irradiation time. In recent years, the scientific community involved in plasma-facing materials for nuclear fusion reactors has used this parameter to measure the equivalent material damage produced in experiments of various types with different types of radiation and wide ranges of power flux and irradiation time. The integral damage factor is useful in practice but lacks formal theoretical justification. In this simple model, we find that it is directly proportional to the maximum concentration of defects. © 2021 Author(s).Ítem Bayesian statistical modeling of microcanonical melting times at the superheated regime(Physica A: Statistical Mechanics and its Applications, 2019-02-01) Davis, Sergio; Loyola, Claudia; Peralta, JoaquínHomogeneous melting of superheated crystals at constant energy is a dynamical process, believed to be triggered by the accumulation of thermal vacancies and their self-diffusion. From microcanonical simulations we know that if an ideal crystal is prepared at a given kinetic energy, it takes a random time tw until the melting mechanism is actually triggered. In this work we have studied in detail the statistics of tw for melting at different energies by performing a large number of Z-method simulations and applying state-of-the-art methods of Bayesian statistical inference. By focusing on a small system size and short-time tail of the distribution function, we show that tw is actually gamma-distributed rather than exponential (as asserted in a previous work), with decreasing probability near tw∼0. We also explicitly incorporate in our model the unavoidable truncation of the distribution function due to the limited total time span of a Z-method simulation. The probabilistic model presented in this work can provide some insight into the dynamical nature of the homogeneous melting process, as well as giving a well-defined practical procedure to incorporate melting times from simulation into the Z-method in order to correct the effect of short simulation times.Ítem Conditional maximum entropy and superstatistics(IOP Publishing Ltd, 2020-11) Davis, SergioSuperstatistics describes nonequilibrium steady states as superpositions of canonical ensembles with a probability distribution of temperatures. Rather than assume a certain distribution of temperature, recently [2020 J. Phys. A: Math. Theor. 53 045004] we have discussed general conditions under which a system in contact with a finite environment can be described by superstatistics together with a physically interpretable, microscopic definition of temperature. In this work, we present a new interpretation of this result in terms of the standard maximum entropy principle using conditional expectation constraints, and provide an example model where this framework can be tested.Ítem Configurational density of states and melting of simple solids(Elsevier B. V., 2023-11-01) Davis, Sergio; Loyola, Claudia; Peralta, JoaquínWe analyze the behavior of the microcanonical and canonical caloric curves for a piecewise model of the configurational density of states of simple solids in the context of melting from the superheated state, as realized numerically in the Z-method via atomistic molecular dynamics. A first-order phase transition with metastable regions is reproduced by the model, being therefore useful to describe aspects of the melting transition. Within this model, transcendental equations connecting the superheating limit, the melting point, and the specific heat of each phase are presented and numerically solved. Our results suggest that the essential elements of the microcanonical Z curves can be extracted from simple modeling of the configurational density of states.Ítem Desarrollo e implementación de métodos de cómputo para densidades de estados, usando ensambles generalizados(Universidad Andrés Bello, 2022) Moreno Muñoz, Felipe Eduardo; Peralta, Joaquín; Davis, Sergio; Faraggi, Alberto; Gutiérrez, Gonzalo; Molina, Francisco; Poblete, Simón; Facultad de Ciencias ExactasEl presente trabajo se enfoca en el desarrollo e implementación de algoritmos de simulación que permitan calcular la Densidad de Estados (DOS) de sistemas de muchas partículas interactuantes usando ensambles generalizados. Métodos para obtener las propiedades de un sistema que se basan en el uso de ensambles canónicos o microcanónicos—como el popular algoritmo de Metropolis—se han usado durante ya un largo tiempo. Sin embargo, el empleo de ensambles generalizados muestra ser una alternativa que permite generar curvas más suaves con menor número de simulaciones, además de evitar ciertos problemas típicos de esos métodos como el tener dificultades para muestrear correctamente sistemas con muchos mínimos locales. En este trabajo se presenta una implementación flexible y modular del algoritmo de Wang–Landau, además de un novedoso algoritmo basado en un método Bayesiano para el cálculo directo de la DOS, los cuales puede ser usados para obtener las propiedades termodinámicas de un sistema con una superficie de energía potencial compleja. Nuestros resultados muestran que los algoritmos son capaces de calcular la DOS con razonable precisión. Hemos estudiado, como aplicación de los métodos anteriormente mencionados, las propiedades termodinámicas de un modelo de Potts de 3 estados modificado, en el contexto de la aproximación de “lattice gas” para partículas cargadas confinadas en un volumen.Ítem Evaluating the Adiabatic Invariants in Magnetized Plasmas Using a Classical Ehrenfest Theorem(MDPI, 2023-11) Tamburrini, Abiam; Davis, Sergio; Moya, Pablo S.In this article, we address the reliance on probability density functions to obtain macroscopic properties in systems with multiple degrees of freedom as plasmas, and the limitations of expensive techniques for solving Equations such as Vlasov’s. We introduce the Ehrenfest procedure as an alternative tool that promises to address these challenges more efficiently. Based on the conjugate variable theorem and the well-known fluctuation-dissipation theorem, this procedure offers a less expensive way of deriving time evolution Equations for macroscopic properties in systems far from equilibrium. We investigate the application of the Ehrenfest procedure for the study of adiabatic invariants in magnetized plasmas. We consider charged particles trapped in a dipole magnetic field and apply the procedure to the study of adiabatic invariants in magnetized plasmas and derive Equations for the magnetic moment, longitudinal invariant, and magnetic flux. We validate our theoretical predictions using a test particle simulation, showing good agreement between theory and numerical results for these observables. Although we observed small differences due to time scales and simulation limitations, our research supports the utility of the Ehrenfest procedure for understanding and modeling the behavior of particles in magnetized plasmas. We conclude that this procedure provides a powerful tool for the study of dynamical systems and statistical mechanics out of equilibrium, and opens perspectives for applications in other systems with probabilistic continuity.Ítem Experimental measurements of high-energy photons in X-rays pulses emitted from a hundred joules plasma focus device and its interpretations(Elsevier B.V., 2020-03) Jain, Jalaj; Moreno, Jose; Davis, Sergio; Boraa, Biswajit; Paveza, Cristian; Avaria, Gonzalo; Soto, LeopoldoIn the present work, efforts are made to identify the presence of high energy photons in X-rays pulses, emitted from a hundred joules plasma focus device, PF-400J. Two different experiments were carried out, with the insertion of a lead piece inside the hollow anode of PF-400J and without insertion of the lead piece. A pair of two photomultiplier tubes (PMTs) was mounted in the axial direction and a similar pair of PMTs was mounted in the radial direction, simultaneously. After establishing a correlation between two PMTs in each pair, one of the PMTs in both directions was blocked by a rectangular slab of the lead of thickness ~17 mm. Linear attenuation coefficient (LAC) of lead was estimated using the PMTs signals. Later, the X-rays energies were interpolated for the estimated LAC values in both cases, with and without insertion of lead piece inside the hollow anode. Interpolated energies reveal the presence of 0.55–0.85 MeV photons in the X-rays pulses in the axial direction, while, in radial direction ranges 0.4–0.9 MeV, for the case without lead inserted inside the hollow anode. Insertion of the lead inside the hollow anode does not change the X-rays energies significantly, nonetheless, it increases X-rays repetition rate per hundred discharges. The presence of high energy photons in the X-rays pulses indicates the existence of relativistic electrons. To explain it, induced electric and magnetic fields were estimated using generalized Ohm’s law. We conclude that the electron acceleration mechanisms might not be the same in the axial and radial directions.Ítem Fluctuating temperature outside superstatistics: Thermodynamics of small systems(Elsevier B.V., 2022-03-01) Davis, SergioThe existence of fluctuations of temperature has been a somewhat controversial topic in thermodynamics but nowadays it is recognized that they must be taken into account in small, finite systems. Although for nonequilibrium steady states superstatistics is becoming the de facto framework for expressing such temperature fluctuations, some recent results put into question the idea of temperature as a phase space observable. In this work we present and explore the statistics that describes a part of an isolated system, small enough to have well-defined uncertainties in energy and temperature, but lacking a superstatistical description. These results motivate the use of the so-called fundamental temperature as an observable and may be relevant for the statistical description of small systems in physical chemistry. © 2021 Elsevier B.V.Ítem Hard X-Ray Emission Detection Using Deep Learning Analysis of the Radiated UHF Electromagnetic Signal from a Plasma Focus Discharge(Institute of Electrical and Electronics Engineers Inc., 2019) Avaria, Gonzalo; Ardila-Rey, Jorge; Davis, Sergio; Orellana, Luis; Cevallos, Benjamin; Pavez, Cristian; Soto, LeopoldoA method to determine the presence of hard X-ray emission processes from a dense plasma focus (205 J, 22 kV, 6.5 mbar H2) using Ultra High Frequency (UHF) measurements and deep learning techniques is presented. Simultaneously, the electromagnetic UHF radiation emitted from the plasma focus was measured with a Vivaldi UHF antenna, while the hard X-ray emission was measured with a scintillator-photomultiplier system. A classification algorithm based on deep learning methods, using two-dimensional convolutional layers, was implemented to predict the hard X-ray signal standard deviation value using only the antenna signal measurement. Two independent datasets, consisting of 999 and 1761 data pairs each, were used in the analysis. Different realizations of the training/validation process using a deep learning model, obtained overall better results in comparison to other machine learning methods like k-neighbors, decision trees, gradient boost, and random forest. The results of the deep learning algorithm, and even its comparison with other machine learning methods, indicate that a relationship between the electromagnetic UHF radiation and hard X-ray emission can be established, enabling the indirect detection of hard X-ray pulses only using the UHF antenna signal. This indirect detection presents the opportunity to have a simple and low-cost diagnostic, compared to the methods currently used to characterize the pulses of X-rays emitted from plasma focus discharges. © 2013 IEEE.Ítem Kappa distribution from particle correlations in nonequilibrium, steady-state plasmas(American Physical Society, 2023-12) Davis, Sergio; Avaria, Gonzalo; Bora, Biswajit; Jain, Jalaj; Moreno, José; Pavez, Cristian; Soto, LeopoldoKappa-distributed velocities in plasmas are common in a wide variety of settings, from low-density to high-density plasmas. To date, they have been found mainly in space plasmas, but are recently being considered also in the modeling of laboratory plasmas. Despite being routinely employed, the origin of the kappa distribution remains, to this day, unclear. For instance, deviations from the Maxwell-Boltzmann distribution are sometimes regarded as a signature of the nonadditivity of the thermodynamic entropy, although there are alternative frameworks such as superstatistics where such an assumption is not needed. In this work we recover the kappa distribution for particle velocities from the formalism of nonequilibrium steady-states, assuming only a single requirement on the dependence between the kinetic energy of a test particle and that of its immediate environment. Our results go beyond the standard derivation based on superstatistics, as we do not require any assumption about the existence of temperature or its statistical distribution, instead obtaining them from the requirement on kinetic energies. All of this suggests that this family of distributions may be more common than usually assumed, widening its domain of application in particular to the description of plasmas from fusion experiments. Furthermore, we show that a description of kappa-distributed plasma is simpler in terms of features of the superstatistical inverse temperature distribution rather than the traditional parameters κ and the thermal velocity vth.Ítem Quantum mutual information, fragile systems and emergence(MDPI, 2022-11) Navarrete, Yasmín; Davis, SergioIn this paper, we present an analytical description of emergence from the density matrix framework as a state of knowledge of the system, and its generalized probability formulation. This description is based on the idea of fragile systems, wherein the observer modifies the system by the measurement (i.e., the observer effect) in order to detect possible emergent behavior. We propose the use of a descriptor, based on quantum mutual information, to calculate if subsystems of systems have inner correlations. This may contribute to a definition of emergent systems in terms of emergent information.Ítem Statistical distribution of thermal vacancies close to the melting point(Elsevier, 2015-01) Pozo, María José; Davis, Sergio; Peralta, JoaquínA detailed description of the statistical distribution of thermal vacancies in an homogeneous crystal near its melting point is presented, using the embedded atom model for copper as an example. As the temperature increase, the average number of thermal vacancies generated by atoms migrating to neighboring sites increases according to Arrhenius' law. We present for the first time a model for the statistical distribution of thermal vacancies, which according to our atomistic computer simulations follow a Gamma distribution. All the simulations are carried out by classical molecular dynamics and the recognition of vacancies is achieved via a recently developed algorithm. Our results could be useful in the further development of a theory explaining the mechanism of homogeneous melting, which seems to be mediated by the accumulation of thermal vacancies near the melting point. © Elsevier B.V. All rights reserved.Ítem Temperature is not an observable in superstatistics(Elsevier B.V., 2018-09) Davis, Sergio; Gutiérrez, GonzaloSuperstatistics (Beck and Cohen, 2003) is a formalism that attempts to explain the presence of distributions other than the Boltzmann–Gibbs distributions in Nature, typically power-law behavior, for systems out of equilibrium such as fluids under turbulence, plasmas and gravitational systems. Superstatistics postulates that those systems are found in a superposition of canonical ensembles at different temperatures, and sometimes the physical interpretation is one of local thermal equilibrium in the sense of an inhomogeneous temperature distribution in different regions of space or instants of time. Here we show that, in order for superstatistics to be internally consistent, it is impossible to define a phase-space function or microscopic observable B(p,q) corresponding one-to-one to the local value of β=1∕kBT. Thus, unlike energy which is defined by a phase-space function H(p,q) (the Hamiltonian), temperature is not a microscopic observable. An important consequence of our proof is that, in Superstatistics, the identification of temperature with the kinetic energy is limited to the expectation of β and cannot be used to measure the different temperatures in local thermal equilibrium or its fluctuations. © 2018 Elsevier B.V.