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C. Farina

Publications and source records attributed to C. Farina.

At least 73 records · Page 4Linked to original sources
arXiv2009

Subtleties on energy calculations in the image method

In this pedagogical work we point out a subtle mistake that can be done by undergraduate or graduate students in the computation of the electrostatic energy of a system containing charges and perfect conductors if they naively use the image method. Specifically, we show that the naive expressions for the electrostatic energy for these systems obtained directly from the image method are wrong by a factor 1/2. We start our discussion with well known examples, namely, point charge-perfectly conducting wall and point charge-perfectly conducting sphere and then proceed to the demonstration of general results, valid for conductors of arbitrary shapes.

physics.class-ph↗
arXiv2009

Dynamical Casimir effect for a massless scalar field between two concentric spherical shells with mixed boundary condictions

We analyze the dynamical Casimir effect for a massless scalar field confined between two concentric spherical shells which impose on the field mixed boundary conditions. We thus complement a previous result [Phys. Rev. A \textbf{78}, 032521 (2008)], where the same problem was considered but in that case the field was submitted to a Dirichlet boundary condition in both moving spherical shells. A general expression for the average number of created particles is deduced for an arbitrary law of radial motion of the spherical shells. This expression is then applied to harmonic oscillations of the shells and the number of created particles is analyzed and compared with the results obtained under Dirichlet-Dirichlet boundary conditions.

quant-ph↗
arXiv2009

Dispersive interaction between an atom and a conducting sphere

We calculate the van der Waals dispersive interaction between a neutral but polarizable atom and a perfectly conducting isolated sphere in the nonretarded case. We make use of two separate models, one being the semiclassical fluctuating-dipoles method, the other using ordinary quantum mechanics.

quant-ph↗
arXiv2009

Magnetic influence on classical dispersion

We discuss the Lorentz model for dispersion and absorption of radiation in dilute, linear and isotropic materials. Initially, with the purpose of making the paper as self-contained as possible, we reproduce the usual calculations concerning the interaction between the charged material oscillators and the electric field of the incident radiation, obtaining the main behavior of the reactive and dissipative electromagnetic properties of the materials. Thereafter, we also include the magnetic contribution of the Lorentz force to the equation of motion of the oscillators up to first order in $v/c$, which leads to some interesting results, like the approximately linear dependence of the refraction index with the radiation intensity and the appearance of a second region of anomalous dispersion around half the natural frequencies of the material.

physics.class-ph↗
arXiv2008

Long-Term Optical Monitoring of Eta Carinae. Multiband light curves for a complete orbital period

The periodicity of 5.5 years for some observational events occurring in Eta Carinae manifests itself across a large wavelength range and has been associated with its binary nature. These events are supposed to occur when the binary components are close to periastron. To detect the previous periastron passage of Eta Car in 2003, we started an intensive, ground-based, optical, photometric observing campaign. We continued observing the object to monitor its photometric behavior and variability across the entire orbital cycle. Our observation program consisted of daily differential photometry from CCD images, which were acquired using a 0.8 m telescope and a standard BVRI filter set at La Plata Observatory. The photometry includes the central object and the surrounding Homunculus nebula. We present up-to-date results of our observing program, including homogeneous photometric data collected between 2003 and 2008. Our observations demonstrated that Eta Car has continued increasing in brightness at a constant rate since 1998. In 2006, it reached its brightest magnitude (V ~ 4.7) since about 1860s. The object then suddenly reverted its brightening trend, fading to V = 5.0 at the beginning of 2007, and has maintained a quite steady state since then. We continue the photometric monitoring of Eta Car in anticipation of the next "periastron passage", predicted to occur at the beginning of 2009.

astro-ph↗
arXiv2008

Estimative for the size of the compactification radius of a one extra dimension Universe

In this work, we use the Casimir effect to probe the existence of one extra dimension. We begin by evaluating the Casimir pressure between two plates in a $M^4\times S^1$ manifold, and then use an appropriate statistical analysis in order to compare the theoretical expression with a recent experimental data and set bounds for the compactification radius.

hep-th↗
arXiv2008

Dispersion forces between an atom and a perfectly conducting wedge

We consider the interaction between an electrically polarizable atom in its fundamental state and a wedge constituted by two semi-infinite perfectly conducting plates. Using a formalism based on a master equation, we compute the dispersion force on the atom for both retarded and non-retarded regimes.

quant-ph↗
arXiv2008

Primitively divergent diagrams in $κ$-deformed scalar field with quartic self-interaction

We obtain the primitively divergent diagrams in $κ$-deformed scalar field in four-dimensional spacetime with quartic self-interaction in order to investigate the effect of the fundamental length $q=1/(2κ)$ on such diagrams. Thanks to $κ$-deformation, we find that the dimensionally regularized forms of the diagrams lead to finite results in the limit of space-time dimension four. The effect of the deformation appears as a displacement of the poles in the complex plane.

hep-th↗
arXiv2007

Atom-wall dispersive forces from master equation formalism

Using the general expressions for level shifts obtained from the master equation for a small system interacting with a large one considered as a reservoir, we calculate the dispersive potentials between an atom and a wall in the dipole approximation. We analyze in detail the particular case of a two-level atom in the presence of a perfectly conducting wall. We study the van der Waals as well as the resonant interactions. All distance regimes as well as the high and low temperature regimes are considered. We show that the Casimir-Polder interaction can not be considered as a direct result of the vacuum fluctuations only. Concerning the interaction between the atom and the wall at high temperature, which show that a saturation of the potential for all distances occurs. This saturated potential coincides exactly with that obtained in the London-van der Waals limit.

quant-ph↗
arXiv2007

Relativistic Green functions in a plane wave gravitational background

We consider a massive relativistic particle in the background of a gravitational plane wave. The corresponding Green functions for both spinless and spin 1/2 cases, previously computed by A. Barducci and R. Giachetti \cite{Barducci3}, are reobtained here by alternative methods, as for example, the Fock-Schwinger proper-time method and the algebraic method. In analogy to the electromagnetic case, we show that for a gravitational plane wave background a semiclassical approach is also sufficient to provide the exact result, though the lagrangian involved is far from being a quadratic one.

hep-th↗
arXiv2007

Particle creation in a Robertson-Walker Universe revisited

We reanalyze the problem of particle creation in a 3+1 spatially closed Robertson-Walker space-time. We compute the total number of particles produced by this non-stationary gravitational background as well as the corresponding total energy and find a slight discrepancy between our results and those recently obtained in the literature

hep-th↗
arXiv2006

Radiative Corrections to Casimir Effect in the $λϕ^{4}$ Model

We calculate radiative corrections to the Casimir effect for the massive complex scalar field with the $λϕ^{4}$ self-interaction in $d+1$ dimensions. We consider the field submitted to four types of boundary conditions on two parallel planes, namely: (i) quasi-periodic boundary conditions, which interpolates continuously periodic and anti-periodic ones, (ii) Dirichlet conditions on both planes, (iii) Neumann conditions on both planes and (iv) mixed conditions, that is, Dirichlet on one plane and Neumann on the other one.

hep-th↗
arXiv2006

Excited state contribution to the Casimir-Polder force at finite temperature

Using the master equation we calculate the contribution of the excited state of a two-level atom to its interacting potential with a perfectly conducting wall at finite temperature. For low temperature, $\hbar ω_0/k_B T = k_0 λ_T\gg 1$, where $ω_0 = k_0 c$ is the transition frequency of the atom and $λ_T$ is the thermal wavelength, we show that this contribution is very small $(\propto e^{-k_0λ_T})$. In the opposite limit $(k_0λ_T \ll 1)$, however, we show that the expression for the interacting potential, for all relevant distance regimes, becomes exactly the same as that for very short distances $(k_0 z \ll 1)$ and with the field in the vacuum state.

quant-ph↗
arXiv2006

A master equation approach for the interaction of an atom with a dielectric semi-infinite medium

We use the master equation approach to calculate the energy level shifts of an atom in the presence of a general dielectric semi-infinite medium characterized by a dielectric constant $ε(ω)$. Particularly, we analyze the case of a non-dispersive medium for which we obtain a general expression for the interaction as well as the asymptotic behaviors for $k_0 z \ll 1$ (non-retarded regime) and $k_0 z \gg 1$ (retarded regime), where $ω_0 = k_0 c$ is the main transition frequency of the atom. The limiting cases $ε\simeq 1$ and $ε\gg 1$ are discussed for both retarded and non-retarded limits. For the retarded limit, we compute the non-additivity contribution of van der Waals forces.

quant-ph↗
arXiv2006

Casimir-Polder forces from density matrix formalism

We use the density matrix formalism in order to calculate the energy level shifts, in second order on interaction, of an atom in the presence of a perfectly conducting wall in the dipole approximation. The thermal corrections are also examined when $\hbar ω_0/k_B T = k_0 λ_T \gg 1$, where ${$ω_0=k_0 c$}$ is the dominant transition frequency of the atom and $λ_T$ is the thermal length. When the distance $z$ between the atom and the wall is larger than $λ_T$ we find the well known result obtained from Lifshitz's formula, whose leading term is proportional to temperature and is independent of $c$, $\hbar$ and $k_0$. In the short distance limit, when $z\llλ_T$, only very small corrections to the leading vacuum term occur. We also show, for all distance regimes, that the main thermal corrections are independent of $k_0$ (dispersion is not important) and dependent of $c$, which means that there is not a non-retarded regime for the thermal contributions.

quant-ph↗
arXiv2005

Dynamical Casimir effect with Robin boundary conditions

We consider a massless scalar field in 1+1 dimensions that satisfies a Robin boundary condition at a non-relativistic moving boundary. Using the perturbative approach introduced by Ford and Vilenkin, we compute the total force on the moving boundary. In contrast to what happens for the Dirichlet and Neumann boundary conditions, in addition to a dissipative part, the force acquires also a dispersive one. Further, we also show that with an appropriate choice for the mechanical frequency of the moving boundary it is possible to turn off the vacuum dissipation almost completely.

hep-th↗
arXiv2005

The Zeta Function Method and the Harmonic Oscillator Propagator

We show how the pre-exponential factor of the Feynman propagator for the harmonic oscillator can be computed by the generalized $ζ$-function method. Besides, we establish a direct equivalence between this method and Schwinger's propertime method.

physics.ed-ph↗