arXiv subjects

T. M. Rocha Filho

Publications and source records attributed to T. M. Rocha Filho.

At least 19 recordsLinked to original sources

arXiv2026

Symplectic Pauli-Schrödinger Equation for the Quark-Antiquark Interaction System

We investigate the quantum behavior of a quark-antiquark bound system under the influence of a magnetic field within the symplectic formulation of quantum mechanics. Employing a perturbative approach, we obtain the ground and first excited states of the system described by the Cornell potential, which incorporates both confining and non-confining interactions. After performing a Levi-Civita mapping in phase space, we solve the time-independent symplectic Pauli-Schrödinger-type equation and determine the corresponding Wigner function. Special attention is given to the observation of the confinement of the quark-antiquark, that is revealed in the phase space structure. Due to the presence of spin in the Hamiltonian, the results reveal that the magnetic field enhances the non-classicality of the Wigner function, signaling stronger quantum interference and a departure from classical behavior. The experimental mass spectra is used to estimate the intensity of the external field, leading to a value that is in order of the transiet magnetic field measured in non-central heavy-ion collisions at RHIC and LHC.

quant-ph↗

arXiv2023

Impulsive Control on Invariant Surfaces

An impulsive feedback-adaptive control is developed in order to drive trajectories of a dynamical system towards an invariant manifold with fixed and spaced impulsive controls. The approach requires the explicit knowledge of the set of equations defining the invariant manifold and is based on the concept of stability exponents of invariant manifolds.

math.DS↗

arXiv2022

COVID-19 denialism in Brazil: a multifactor study

We discuss the relationships between the outcome of the COVID-19 pandemic in Brazil at the municipal level and different health, social, demographic, and economic indices. We obtain significant correlations between the data gathered for each municipalitiy and the proportion of cases and deaths by COVID-19 and the results by municipality of the 2018 Brazilian presidential election. We obtain different estimates for the number of deaths caused by central government denialism of scientific facts and measures for mitigation of the pandemic and its the historical, economic, and social roots.

physics.soc-ph↗

arXiv2022

Reliability of COVID-19 data and government policies

We study how available data on COVID-19 cases and deaths in different countries are reliable. Our analysis is based on a modification of the law of anomalous numbers, the Newcomb-Benford law, applied to the daily number of deaths and new cases in each country. We first revisit the Newcomb-Benford law and show how to avoid false negative compliance of the data. We then compared deviation from this law, to a number of social and economic indices for each country by computing the Spearman rank order correlation between each index and the \c{hi}2 deviation of COVID- 19 data to the modified NB law. A similar analysis for excess deaths for the same countries with sufficient available data was performed. We conclude that in general less democratic, less transparent and more corrupt countries tend to have data of lesser quality. We also discuss the limitations of the present approach.

physics.soc-ph↗

arXiv2020

Slow dynamics and ergodicity in the one-dimensional self-gravitating system

We revisit the dynamics of the one-dimensional self-gravitating sheets models. We show that homogeneous and non-homogeneous states have different ergodic properties. The former is non-ergodic and the one-particle distribution function has a zero collision term if a proper limit is taken for the periodic boundary conditions. Non-homogeneous states are ergodic in a time window of the order of the relaxation time to equilibrium, as similarly observe in other systems with a long range interaction. For the sheets model this relaxation time is much larger than other systems with long range interactions if compared to the initial violent relaxation time.

cond-mat.stat-mech↗

arXiv2019

Relaxation Processes in Long-Range Lattices

The relaxation to equilibrium of lattice systems with long-range interactions is investigated. The timescales involved depend polynomially on the system size, potentially leading to diverging equilibration times. A kinetic equation for long-range lattices is proposed, which explain these timescales as well as a threshold in the interaction range reported in [Phys. Rev. Lett. 110, 170603 (2013)]. Non-Markovian effects are shown to play an important role in the relaxation of systems of up to thousands of particles.

cond-mat.stat-mech↗

arXiv2017

Lyapunov Exponent and Criticality in the Hamiltonian Mean Field Model

We investigate the dependence of the largest Lyapunov exponent of a $N$-particle self-gravitating ring model at equilibrium with respect to the number of particles and its dependence on energy. This model has a continuous phase-transition from a ferromagnetic to homogeneous phase, and we numerically confirm with large scale simulations the existence of a critical exponent associated to the largest Lyapunov exponent, although at variance with the theoretical estimate. The existence of chaos in the magnetized state evidenced by a positive Lyapunov exponent, even in the thermodynamic limit, is explained by the resonant coupling of individual particle oscillations to the diffusive motion of the center of mass of the system due to the thermal excitation of a classical Goldstone mode. The transition from "weak" to "strong" chaos occurs at the onset of the diffusive motion of the center of mass of the non-homogeneous equilibrium state, as expected. We also discuss thoroughly for the model the validity and limits of a geometrical approach for their analytical estimate.

cond-mat.stat-mech↗

arXiv2017

A convergent kinetic equation for gravitational and Coulomb systems

It is well known that due to its divergence at large impact parameters, the Boltzmann collision integral in the kinetic equation for 3D systems of particles interacting through a $1/r$ potential must be replaced by a Balescu-Lenard-like collision term. However, the latter diverges at small impact parameters. This comes from the fact that only weak interactions are considered while strong collisions between close particles are neglected in its derivation. We show that a solution to this dilemma exists in the framework of the BBGKY formulation of statistical mechanics. It is based on a separate treatment of the contribution of the strong interactions from that of the weak interactions. The strong interaction part leads to a new term that involves a fractional Laplacian operator in velocity space while the weak interaction component yields the Balescu-Lenard collision term with an explained lower cut-off at the Landau length. For spatially uniform initial conditions, the fractional Laplacian contribution leads to a long-tailed velocity distribution as long as the spatial inhomogeneity remains small. We present results from molecular dynamics simulations confirming the existence of such long tails.

cond-mat.stat-mech↗

arXiv2017

Ensemble Inequivalence and Maxwell Construction in the Self-Gravitating Ring Model

Equilibrium Statistical Mechanics is undoubtedly a cornerstone for the description of many particle systems. The common interpretation is based on ensemble theory as put forward by Gibbs, alongside the basic assumptions that different ensembles are equivalent, i.~e.\ the properties of the system can equally be obtained in any ensemble with the same results. However, the simplicity of the argument that provides such equivalence, mathematically grounded by the existence of Legendre transformation between the ensembles and the existence of its inverse, may break down for physical systems with long range interactions. In this paper we study the behavior of a simple toy model with a long range interaction and show from first principles, by solving numerically the mechanical equations of motion and Monte Carlo simulations, the inequivalence of ensembles, and discuss in what situations and how the Maxwell construction is applicable.

cond-mat.stat-mech↗

arXiv2016

Long velocity tails in plasmas and gravitational systems

Long tails in the velocity distribution are observed in plasmas and gravitational systems. Some experiments and observations in far-from-equilibrium conditions show that these tails behave as 1/v^(5/2). We show here that such heavy tails are due to a universal mechanism related to the fluctuations of the total force field. Owing to the divergence in 1/r^2 of the binary interaction force, these fluctuations can be very large and their probability density exhibits a similar long tail. They induce large velocity fluctuations leading to the 1/v^(5/2) tail. We extract the mechanism causing these properties from the BBGKY hierarchy representation of Statistical Mechanics. This leads to a modification of the Vlasov equation by an additional term. The novel term involves a fractional power 3/4 of the Laplacian in velocity space and a fractional iterated time integral. Solving the new kinetic equation for a uniform system, we retrieve the observed 1/v^(5/2) tail for the velocity distribution. These results are confirmed by molecular dynamics simulations.

physics.plasm-ph↗

arXiv2016

Non-equilibrium Entropy and Dynamics in a System with Long-Range Interactions

The core-halo approach of Levin et al.\ [Phys.\ Rep.\ {\bf 535}, 1 (2014)] for the violent relaxation of long-range interacting systems with a waterbag initial conditions is revisited for the case of the Hamiltonian Mean Field model. The Gibbs entropy maximization principle is considered with the constraints of energy conservation and of infinite Casimir invariants of the Vlasov equation. All parameters in the core-halo distribution function are then completely determined without resorting to the envelope equation for the contour of the initial state, which was required in the original approach. We also show that a different ansatz is possible for the core-halo distribution with similar or even better results. This work also evidences a link between a parametric resonance causing the non-equilibrium phase transition in the HMF model, a purely dynamical property, and a discontinuity of the (non-equilibrium) entropy of the system.

cond-mat.stat-mech↗

arXiv2015

Microcanonical Monte Carlo Study of One Dimensional Self-Gravitating Lattice Gas Models

In this study we present a Microcanonical Monte Carlo investigation of one dimensional self-gravitating toy models. We study the effect of hard-core potentials and compare to those results obtained with softening parameters and also the effect of the geometry of the models. In order to study the effect of the geometry and the borders in the system we introduce a model with the symmetry of motion in a line instead of a circle, which we denominate as $1/r$ model. The hard-core particle potential introduces the effect of the size of particles and, consequently, the effect of the density of the system that is redefined in terms of the packing fraction of the system. The latter plays a role similar to the softening parameter $ε$ in the softened particles' case. In the case of low packing fractions both models with hard-core particles show a behavior that keeps the intrinsic properties of the three dimensional gravitational systems such as negative heat capacity. For higher values of the packing fraction the ring the system behaves as the Hamiltonian Mean Field model and while for the $1/r$ it is similar to the one-dimensional systems.

cond-mat.stat-mech↗

arXiv2015

Ergodicity in a two-dimensional self gravitating many body system

We study the ergodic properties of a two-dimensional self-gravitating system using molecular dynamics simulations. We apply three different tests for ergodicity: a direct method comparing the time average of a particle momentum and position to the respective ensemble average, sojourn times statistics and the dynamical functional method. For comparison purposes they are also applied to a short-range interacting system and to the Hamiltonian mean-field model. Our results show that a two-dimensional self-gravitating system takes a very long time to establish ergodicity. If a Kac factor is used in the potential energy, such that the total energy is extensive, then this time is independent of particle number, and diverges with $\sqrt{N}$ without a Kac factor.

cond-mat.stat-mech↗

arXiv2015

Scaling of the dynamics of a homogeneous one-dimensional anisotropic classical Heisenberg model with long-range interactions

The dynamics of quasi-stationary states of long-range interacting systems with $N$ particles can be described by kinetic equations such as the Balescu-Lenard and Landau equations. In the case of one-dimensional homogeneous systems, two-body contributions vanish as two-body collisions in one dimension only exchange momentum and thus cannot change the one-particle distribution. Using a Kac factor in the interparticle potential implies a scaling of the dynamics proportional to $N^δ$ with $δ=1$ except for one-dimensional homogeneous systems. For the latter different values for $δ$ were reported for a few models. Recently it was show by Rocha Filho and collaborators [Phys.\ Rev.\ E {\bf 90}, 032133 (2014)] for the Hamiltonian mean-field model that $δ=2$ provided that $N$ is sufficiently large, while small $N$ effects lead to $δ\approx1.7$. More recently Gupta and Mukamel [J.\ Stat.\ Mech.\ P03015 (2011)] introduced a classical spin model with an anisotropic interaction with a scaling in the dynamics proportional to $N^{1.7}$ for a homogeneous state. We show here that this model reduces to a one-dimensional Hamiltonian system and that the scaling of the dynamics approaches $N^2$ with increasing $N$. We also explain from theoretical consideration why usual kinetic theory fails for small $N$ values, which ultimately is the origin of non-integer exponents in the scaling.

cond-mat.stat-mech↗

arXiv2015

Thermofield qubits, generalized expectations and quantum information protocols

Thermofield dynamics (TFD) approach is a real time quantum field method for dealing with finite temperature quantum states in a purified version of usual density operator formalism at finite temperature. In the domain of quantum information, TFD represents a quite promising direction for dealing with qubits under thermal influence and can also be associated to Gaussian states. Here, we propose a generalized TFD mean expectation for the case of thermofield qubits considering the action of gate operators. We propose quantum teleportation protocols involving thermofield states, considering thermal-to-thermal and thermal-to-non-thermal transfering cases. In particular, we discuss the case in which Alice and Bob are at different temperatures. Action of gate operators on the result of the Mandel parameter for thermofields and on Gibbs-like density operators are also discussed. The no-cloning and non-broadcasting theorems in TFD are also considered and cases of superposed thermofield states and maps connecting thermofield vacua at different temperatures are also addressed and associated to metastable and non-equilibrium scenarios.

quant-ph↗

arXiv2014

Scaling of the dynamics of homogeneous states of one-dimensional long-range interacting systems

Quasi-Stationary States of long-range interacting systems have been studied at length over the last fifteen years. It is known that the collisional terms of the Balescu-Lenard and Landau equations vanish for one-dimensional systems in homogeneous states, thus requiring a new kinetic equation with a proper dependence on the number of particles. Here we show that previous scalings described in the literature are due either to small size effects or the use of improper variables to describe the dynamics. The correct scaling is proportional to the square of the number of particles and deduce the kinetic equation valid for the homogeneous regime and numerical evidence is given for the Hamiltonian Mean Field and ring models.

cond-mat.stat-mech↗

arXiv2014

Truncated Lévy Flights and Weak Ergodicity Breaking in the Hamiltonian Mean Field Model

The dynamics of the Hamiltonian mean field model is studied in the context of continuous time random walks. We show that the sojourn times in cells in the momentum space are well described by a Lévy truncated distribution. Consequently the system in weakly non-ergodic for long times that diverge with the number of particles. For a finite number of particles ergodicity is only attained for very long times both at thermodynamical equilibrium and at quasi-stationary out of equilibrium states.

cond-mat.stat-mech↗

arXiv2013

Dynamics and physical interpretation of quasi-stationary states in systems with long-range interactions

Although the Vlasov equation is used as a good approximation for a sufficiently large $N$, Braun and Hepp have showed that the time evolution of the one particle distribution function of a $N$ particle classical Hamiltonian system with long range interactions satisfies the Vlasov equation in the limit of infinite $N$. Here we rederive this result using a different approach allowing a discussion of the role of inter-particle correlations on the system dynamics. Otherwise for finite N collisional corrections must be introduced. This has allowed the a quite comprehensive study of the Quasi Stationary States (QSS) but many aspects of the physical interpretations of these states remain unclear. In this paper a proper definition of timescale for long time evolution is discussed and several numerical results are presented, for different values of $N$. Previous reports indicates that the lifetimes of the QSS scale as $N^{1.7}$ or even the system properties scales with $\exp(N)$. However, preliminary results presented here shows indicates that time scale goes as $N^2$ for a different type of initial condition. We also discuss how the form of the inter-particle potential determines the convergence of the $N$-particle dynamics to the Vlasov equation. The results are obtained in the context of following models: the Hamiltonian Mean Field, the Self Gravitating Ring Model, and a 2-D Systems of Gravitating Particles. We have also provided information of the validity of the Vlasov equation for finite $N$, i. e.\ how the dynamics converges to the mean-field (Vlasov) description as $N$ increases and how inter-particle correlations arise.

cond-mat.stat-mech↗