arXiv subjects

A. S. Pereira

Publications and source records attributed to A. S. Pereira.

10 recordsLinked to original sources

arXiv2025

Constraining tachyonic inflationary β-exponential model with Continuous Spontaneous Localization collapse scheme

In this work, we consider the dynamics of the self-induced collapse of the tachyon wave function in inflationary scenarios. We analyze the modifications on the power spectrum by considering the $β$-exponential potential, whose parameters have updated constraints by the Planck 2018 baseline data and recent results from the Atacama Cosmology Telescope (ACT). Moreover, we show that for this kind of potential, just for a narrow range of $β$-parameter, there is agreement between the theoretical predictions and the current observational data. Considering the proposal for a collapse scheme that leads to the modification of Schrödinger evolution of the inflation wave function from the employment of a Continuous Spontaneous Localization (CSL) approach, we derive the scalar spectral index and tensor-to-scalar ratio. We then obtained the constraints on both collapse and $β$-parameters that, in turn, yield deviations in the $n_{s}$ vs. $r$ plane when compared to the $β$-exponential potential standard estimate. The CSL scheme applied to tachyonic inflation driven by a $β$-potential offers an adequate description of the recent data.

astro-ph.CO↗

arXiv2024

Emergence of squeezed coherent states in Kaluza-Klein cosmology

In this work, we consider a propagating scalar field on Kaluza-Klein-type cosmological background. It is shown that this geometrical description of the Universe resembles - from a Hamiltonian standpoint - a damped harmonic oscillator with mass and frequency, both time-dependents. In this scenario, we construct the squeezed coherent states (SCSs) for the quantized scalar field by employing the invariant operator method of Lewis-Riesenfeld (non-Hermitian) in a non-unitary approach. The non-classicality of SCSs has been discussed by examining the quadrature squeezing properties from the uncertainty principle. Moreover, we compute the probability density, which allows us to investigate whether SCSs can be used to seek traces of extra dimensions. We then analyze the effects of the existence of supplementary space on cosmological particle production in SCSs by considering different cosmological eras.

gr-qc↗

arXiv2023

Squeezed coherent states for a free particle with time-varying mass

We obtain the squeezed coherent states (SCS) for a free particle with exponentially time-varying mass. We write these states in terms of the squeeze and displacement parameters on the time-independent Fock states. Thus, we find a condition on the displacement parameter such that the SCS can be considered semiclassical states. We show that it is possible to obtain the coherent states (CS) for a free particle with minimal uncertainty as long as the mass increases with the time. We analyze the transition probability of a system initially prepared in the time-independent Fock states to the free particle SCS.

quant-ph↗

arXiv2022

Generalized para-Bose states

In this paper, we construct integrals of motion in a para-Bose formulation for a general time-dependent quadratic Hamiltonian, which, in its turn, commutes with the reflection operator. In this context, we obtain generalizations for the squeezed vacuum states (SVS) and coherent states (CS) in terms of the Wigner parameter. Furthermore, we show that there is a completeness relation for the generalized SVS owing to the Wigner parameter. In the study of the probability transition, we found that the displacement parameter acts as a transition parameter by allowing access to odd states, while the Wigner parameter controls the dispersion of the distribution. We show that the Wigner parameter is quantized by imposing that the vacuum state has even parity. We apply the general results to the case of the time-independent para-Bose oscillator and find that the mean values of the coordinate and momentum have an oscillatory behavior similarly to the simple harmonic oscillator, while the standard deviation presents corrections in terms of the squeeze, displacement, and Wigner parameters.

quant-ph↗

arXiv2021

Time-dependent coherent squeezed states in a nonunitary approach

In this work, we have applied the integrals of motion method in a nonunitary approach and so obtained the time-dependent displacement and squeezed parameters of the coherent squeezed states (CSS). On its turn, CSS for one-dimensional systems with general time-dependent quadratic Hamiltonian are constructed. We discuss the properties of these states, in particular, minimization of uncertainty relation and transition probabilities. As an application, we calculate the CSS of an oscillator with a time-dependent frequency and shown that the solution can be obtained from these well-known Mathieu's equation.

quant-ph↗

arXiv2018

Coherent and Semiclassical States of a Charged Particle in a Constant Electric Field

The method of integrals of motion is used to construct families of generalized coherent states of a nonrelativistic spinless charged particle in a constant electric field. Families of states, differing in the values of their standard deviations at the initial time, are obtained. Depending on the initial values of the standard deviations, and also on the electric field, it turns out to be possible to identify some families with semiclassical states.

quant-ph↗

arXiv2017

Coherent and semiclassical states of a charged particle in electromagnetic fields

In the present article we extend our study (BJP 45 (2015) 369) of generalized coherent states (GCS) of a one-dimensional particle considering such important physical system as a 3-dimensional charged particle in electric and magnetic fields. Constructing GCS in many-dimensional case, we meet nontrivial technical complications that make the consideration nontrivial and instructive. The GCS of a system under consideration are constructed. We study properties of these GCS such as completeness relations, minimization of uncertainty relations and so on. We point out which family of the obtained GCS of a charged particle in magnetic field is related with the CS constructed first by Malkin and Man'ko. We obtain conditions under which some of the GCS can be considered as semiclassical states.

quant-ph↗

arXiv2015

Coherent and semiclassical states of a free particle

Coherent states (CS) were first introduced and studied in detail for bound motion and discrete-spectrum systems like harmonic oscillators and similar systems with a quadratic Hamiltonian. However, the problem of constructing CS has still not been investigated in detail for the simplest and physically important case of a free particle, for which, besides being physically important, the CS problem is of didactic value in teaching quantum mechanics, with the CS regarded as examples of wave packets representing semiclassical motion. In this paper, we essentially follow the Malkin-Dodonov-Man'ko method to construct the CS of a free nonrelativistic particle. We give a detailed discussion of the properties of the CS obtained, in particular, the completeness relations, the minimization of uncertainty relations, and the evolution of the corresponding probability density. We describe the physical conditions under which free-particle CS can be considered semiclassical states.

quant-ph↗

arXiv2015

Coherent states of systems with quadratic Hamiltonians

Different families of generalized CS for one-dimensional systems with general time dependent quadratic Hamiltonian are constructed. In principle, all known CS of systems with quadratic Hamiltonian are members of these families. Some of the constructed generalized CS are close enough to the well-known due to Schrödinger and Glauber CS of a harmonic oscillator, we call them simply CS. However, even among these CS there exist different families of complete sets of CS. These families differ by values of standard deviations at the initial time instant. According to the values of these initial standard deviations one can identify some of the families with semiclassical CS. We discuss properties of the constructed CS, in particular, completeness relations, minimization of uncertainty relations and so on. As a unknown application of the general construction, we consider different CS of an oscillator with a time dependent frequency.

quant-ph↗

arXiv2006

Superconductivity in the Nb2SnC compound

Nb2SnC is a member of the large family of lamellar materials that crystallize in the hexagonal structure with space group P63/mmc which are isomorphs with Cr2AlC, also named H-phase. In spite of the great number of compounds which belong to this family, the superconductivity has been reported only for two cases: Mo2GaC and Nb2SC. In this work we show that superconductivity can be observed in Nb2SnC depending on the synthesis method used. The quality of the superconductor is strongly dependent of the synthesis method and the optimal results were reached for samples synthesized at 2.5 GPa and 523 +/- 50oC. This sample showed a critical temperature close to 7.8K, revealed from magnetization and transport measurement, the highest critical temperature reported up to now for an H-phase.

cond-mat.supr-con↗