arXiv subjects

Rudnei O. Ramos

Publications and source records attributed to Rudnei O. Ramos.

At least 127 records · Page 7Linked to original sources

arXiv2007

Condensation of Vortex-Strings: Effective Potential Contribution Through Dual Actions

Topological excitations are believed to play an important role in different areas of physics. For example, one case of topical interest is the use of dual models of quantum cromodynamics to understand properties of its vacuum and confinement through the condensation of magnetic monopoles and vortices. Other applications are related to the role of these topological excitations, nonhomogeneous solutions of the field equations, in phase transitions associated to spontaneous symmetry breaking in gauge theories, whose study is of importance in phase transitions in the early universe, for instance. Here we show a derivation of a model dual to the scalar Abelian Higgs model where its topological excitations, namely vortex-strings, become manifest and can be treated in a quantum field theory way. The derivation of the nontrivial contribution of these vacuum excitations to phase transitions and its analogy with superconductivity is then made possible and they are studied here.

hep-th↗

arXiv2006

Critical and Tricritical Points for the Massless 2d Gross-Neveu Model Beyond Large N

Using optimized perturbation theory, we evaluate the effective potential for the massless two dimensional Gross-Neveu model at finite temperature and density containing corrections beyond the leading large-N contribution. For large-N, our results exactly reproduce the well known 1/N leading order results for the critical temperature, chemical potential and tricritical points. For finite N, our critical values are smaller than the ones predicted by the large-N approximation and seem to observe Landau's theorem for phase transitions in one space dimension. New analytical results are presented for the tricritical points that include 1/N corrections. The easiness with which the calculations and renormalization are carried out allied to the seemingly convergent optimized results displayed, in this particular application, show the robustness of this method and allows us to obtain neat analytical expressions for the critical as well as tricritical values beyond the results currently known.

hep-th↗

arXiv2006

Effective Potential and Thermodynamics for a Coupled Two-Field Bose Gas Model

We study the thermodynamics of a two-species homogeneous and dilute Bose gas that is self-interacting and quadratically coupled to each other. We make use of field theoretical functional integral techniques and evaluate the one-loop finite temperature effective potential for this system considering the resummation of the leading order temperature dependent as well as infrared contributions. The symmetry breaking pattern associated to the model is then studied by considering different values of self and inter-species couplings. We pay special attention to the eventual appearance of reentrant phases and/or shifts in the observed critical temperatures as compared to the monoatomic (one-field Bose) case.

cond-mat.other↗

arXiv2006

Inverse Symmetry Breaking in Multi-Scalar Field Theories

We review how the phenomena of inverse symmetry breaking (and symmetry nonrestoration) may arise in the context of relativistic as well as nonrelativistic multi-scalar field theories. We discuss how the consideration of thermal effects on the couplings produce different transition patterns for both theories. For the relativistic case, these effects allow the appearance of inverse symmetry breaking (and symmetry nonrestoration) at arbitrarily large temperatures. On the other hand, the same phenomena are suppressed in the nonrelativistic case, which is relevant for condensed matter physics. In this case, symmetry nonrestoration does not happen while inverse symmetry is allowed only to be followed by symmetry restoration characterizing a reentrant phase. The aim of this paper is to give more insight concerning the, qualitatively correct, results obtained by using one loop perturbation theory in the evaluation of thermal masses and couplings.

cond-mat.other↗

arXiv2006

Incorporating Memory Effects in Phase Separation Processes

We consider the modification of the Cahn-Hilliard equation when a time delay process through a memory function is taken into account. We then study the process of spinodal decomposition in fast phase transitions associated with a conserved order parameter. Finite-time memory effects are seen to affect the dynamics of phase transition at short times and have the effect of delaying, in a significant way, the process of rapid growth of the order parameter that follows a quench into the spinodal region. These effects are important in several systems characterized by fast processes, like nonequilibrium dynamics in the early universe and in relativistic heavy-ion collisions.

hep-ph↗

arXiv2005

Phase Transitions Patterns in Relativistic and Nonrelativistic Multi-Scalar-Field Models

We discuss the phenomena of symmetry non-restoration and inverse symmetry breaking in the context of multi-scalar field theories at finite temperatures and present its consequences for the relativistic Higgs-Kibble multi-field sector as well as for a nonrelativistic model of hard core spheres. For relativistic scalar field models, it has been shown previously that temperature effects on the couplings do not alter, qualitatively, the phase transition pattern. Here, we show that for the nonrelativistic analogue of these models inverse symmetry breaking, as well as symmetry non-restoration, cannot take place, at high temperatures, when the temperature dependence of the two-body couplings is considered. However, the temperature behavior in the nonrelativistic models allows for the appearance of reentrant phases.

hep-th↗

arXiv2005

Abelian Higgs Model Effective Potential in the Presence of Vortices

We determine the contribution of nontrivial vacuum (topological) excitations, more specifically vortex--strings of the Abelian Higgs model in 3+1 dimensions, to the functional partition function. By expressing the original action in terms of dual transformed fields we make explicit in the equivalent action the contribution of the vortex--strings excitations of the model. The effective potential of an appropriately defined local vacuum expectation value of the vortex--string field in the dual transformed action is then evaluated both at zero and finite temperatures and its properties discussed in the context of the finite temperature phase transition.

hep-th↗

arXiv2005

Dynamics of Interacting Scalar Fields in Expanding Space-Time

The effective equation of motion is derived for a scalar field interacting with other fields in a Friedman-Robertson-Walker background space-time. The dissipative behavior reflected in this effective evolution equation is studied both in simplified approximations as well as numerically. The relevance of our results to inflation are considered both in terms of the evolution of the inflaton field as well as its fluctuation spectrum. A brief examination also is made of supersymmetric models that yield dissipative effects during inflation.

hep-ph↗

arXiv2004

Absence of isentropic expansion in various inflation models

Dynamics of the inflaton is studied when it interacts with boson and fermion fields and in minimal supersymmetric models. This encompasses multifield inflation models, such as hybrid inflation, and typical reheating models. For much of the parameter space conducive to inflation, the inflaton is found to dissipate adequate radiation to have observational effects on density perturbations and in cases to significantly affect inflaton evolution. Thus, for many inflation models, believed to yield exclusively isentropic inflation, the parameter space now divides into regimes of isentropic and nonisentropic inflation.

hep-ph↗

arXiv2004

Evaluating Critical Exponents in the Optimized Perturbation Theory

We use the optimized perturbation theory, or linear delta expansion, to evaluate the critical exponents in the critical 3d O(N) invariant scalar field model. Regarding the implementation procedure, this is the first successful attempt to use the method in this type of evaluation. We present and discuss all the associated subtleties producing a prescription which can, in principle, be extended to higher orders in a consistent way. Numerically, our approach, taken at the lowest nontrivial order (second order) in the delta expansion produces a modest improvement in comparison to mean field values for the anomalous dimension eta and correlation length nu critical exponents. However, it nevertheless points to the right direction of the values obtained with other methods, like the epsilon-expansion. We discuss the possibilities of improving over our lowest order results and on the convergence to the known values when extending the method to higher orders.

cond-mat.other↗

arXiv2004

On the Effective Evolution for the Inflaton

The dynamics of the inflaton field is studied in the context of its interaction with bosonic and fermionic fields modeled by a minimal SUSY like model. Our results are based on the observation that in typical multifield inflation models there are parameter regions for which nonlinear and nonperturbative effects can become important and that can lead to important changes in the dynamics of the inflaton field. In those parameter regions feasible to inflation, it is possible the emergence of effective strong dissipative effects that alone can sustain inflation long enough and with observational effects on density perturbations.

hep-ph↗

arXiv2003

Nonequilibrium Dynamics of Coupled Quantum Systems

The nonequilibrium dynamics of coupled quantum oscillators subject to different time dependent quenches are analyzed in the context of the Liouville-von Neumann approach. We consider models of quantum oscillators in interaction that are exactly soluble in the cases of both sudden and smooth quenches. The time evolution of number densities and the final equilibration distribution for the problem of a quantum oscillator coupled to an infinity set of other oscillators (a bath) are explicitly worked out.

hep-th↗

arXiv2003

Asymptotically Improved Convergence of Optimized Perturbation Theory in the Bose-Einstein Condensation Problem

We investigate the convergence properties of optimized perturbation theory, or linear $δ$ expansion (LDE), within the context of finite temperature phase transitions. Our results prove the reliability of these methods, recently employed in the determination of the critical temperature T_c for a system of weakly interacting homogeneous dilute Bose gas. We carry out the explicit LDE optimized calculations and also the infrared analysis of the relevant quantities involved in the determination of $T_c$ in the large-N limit, when the relevant effective static action describing the system is extended to O(N) symmetry. Then, using an efficient resummation method, we show how the LDE can exactly reproduce the known large-N result for $T_c$ already at the first non-trivial order. Next, we consider the finite N=2 case where, using similar resummation techniques, we improve the analytical results for the nonperturbative terms involved in the expression for the critical temperature allowing comparison with recent Monte Carlo estimates of them. To illustrate the method we have considered a simple geometric series showing how the procedure as a whole works consistently in a general case.

cond-mat.soft↗

arXiv2003

Construction of a robust warm inflation mechanism

A dissipative mechanism is presented, which emerges in generic interacting quantum field systems and which leads to robust warm inflation. An explicit example is considered, where using typical parameter values, it is shown that considerable radiation can be produced during inflation. The extension of our results to expanding spacetime also is discussed.

hep-ph↗

arXiv2002

Equilibration Time Scales in Homogeneous Bose-Einstein Condensate Dynamics

We study the nonequilibrium growth of a weakly interacting homogeneous Bose gas after a quench from a high-temperature state to a temperature below the Bose-Einstein critical condensation temperature. We quantitatively characterize the departure from thermal equilibrium and observe the presence of two equilibration time scales. The equilibration times are shown to be inversely proportional to the density.

cond-mat.soft↗

arXiv2002

Noise and Dissipation During Preheating

We study the consequences of noise and dissipation for parametric resonance during preheating. The effective equations of motion for the inflaton and the radiation field are obtained and shown to present self-consistent noise and dissipation terms. The equations exhibit the usual parametric resonance phenomenon, allowing for exponential amplification of the radiation modes inside the instability bands. By focusing on the dimension of the border of those bands we explicitly show that they are fractal, indicating the strong dependence of the outcome in the initial conditions. The simultaneous effect of noise and dissipation to the fractality of the borders are then examined.

hep-ph↗

arXiv2002

Convergent resummed linear delta expansion in the critical O(N) (ϕ_i^2)^2_{3d} model

The nonperturbative linear delta expansion (LDE) method is applied to the critical O(N) phi^4 three-dimensional field theory which has been widely used to study the critical temperature of condensation of dilute weakly interacting homogeneous Bose gases. We study the higher order convergence of the LDE as it is usually applied to this problem. We show how to improve both, the large-N and finite N=2, LDE results with an efficient resummation technique which accelerates convergence. In the large N limit, it reproduces the known exact result within numerical integration accuracy. In the finite N=2 case, our improved results support the recent numerical Monte Carlo estimates for the critical transition temperature of Bose-Einstein condensation.

cond-mat.soft↗

arXiv2002

Unusual Transition Patterns in Bose-Einstein Condensation

We analyze the possible transition patterns exhibited by an effective non-relativistic field model describing interacting binary homogeneous dilute Bose gases whose overall potential is repulsive. We evaluate the temperature dependence of all couplings and show that at intermediate temperatures the crossed interaction, which is allowed to be attractive, dominates, leading to smooth re-entrant phases. At higher temperatures this interaction suffers a sudden sign inversion leading to an abrupt discontinuous transition back to the normal gas phase. This situation may suggest an alternative way to observe collapsing and exploding condensates. Our results also suggest that such binary systems may offer the possibility of observing Bose-Einstein condensation at higher critical temperatures.

cond-mat.soft↗