arXiv subjects

J. T. Mendonça

Publications and source records attributed to J. T. Mendonça.

49 records · Page 3Linked to original sources

arXiv2011

Laser scattering by density fluctuations of ultra-cold atoms in a magneto-optical trap

We study the spectrum of density fluctuations in the ultra-cold gas of neutral atoms, confined in a magneto-optical trap. We determine the corresponding amplitude and spectra of laser light scattered by this medium. We derive an expression for the dynamical structure function, by using a test particle method. We propose to use the collective laser scattering as a diagnostic method for the microscopic properties of the ultra-cold matter. This will also allow us to check on the atomic correlations which are mediated by the collective mean field inside the gas.

physics.atom-ph↗

arXiv2010

Plasmon wakefields and dispersive properties of metallic nanostructures

We investigate the excitation of electrostatic wakefields in metallic nanostructures (nanowires) due to the propagation of a short electron pulse. For that purpose, a dispersive (nonlocal) dielectric response of the system is considered, accounting for both the finiteness of the system and the quantum (Bohm) difraction of the conduction electron band, generalizing the results obtained previously in the literature [Phys. Rev. Lett. \textbf{103}, 097403 (2009)]. We discuss on the stability conditions of wakefields and show that the underling mechanism can be useful to investigate new sources of radiation in the extreme-ultra-violet (XUV) range.

cond-mat.mes-hall↗

arXiv2010

Photon Orbital Angular Momentum and Mass in a Plasma Vortex

We analyse the Anderson-Higgs mechanism of photon mass acquisition in a plasma and study the contribution to the mass from the orbital angular momentum acquired by a beam of photons when it crosses a spatially structured charge distribution. To this end we apply Proca-Maxwell equations in a static plasma with a particular spatial distribution of free charges, notably a plasma vortex, that is able to impose orbital angular momentum (OAM) onto light. In addition to the mass acquisition of the conventional Anderson-Higgs mechanism, we find that the photon acquires an additional mass from the OAM and that this mass reduces the Proca photon mass.

physics.plasm-ph↗

arXiv2010

Rossby waves in rapidly rotating Bose-Einstein condensates

We predict and describe a new collective mode in rotating Bose-Einstein condensates, which is very similar to the Rossby waves in geophysics. In the regime of fast rotation, the Coriolis force dominates the dynamics and acts as a restoring force for acoustic-drift waves along the condensate. We derive a nonlinear equation that includes the effects of both the zero-point pressure and the anharmonicity of the trap. It is shown that such waves have negative phase speed, propagating in the opposite sense of the rotation. We discuss different equilibrium configurations and compare with those resulting from the Thomas-Fermi approximation.

quant-ph↗

arXiv2010

Driven collective instabilities in magneto-optical traps: a fluid-dynamical approach

We present a theoretical model to describe an instability mechanism in ultra-cold gases, where long-range interactions are taken into account. Focusing on the nonlinear coupling between the collective (plasma-like) and the center-of-mass modes, we show that the resulting dynamics is governed by a parametric equation of the generalized Mathieu type and compute the corresponding stability chart. We apply our model to typical ranges of magneto-optical traps (MOT) parameters and find a good agreement with previous experimental observations.

cond-mat.quant-gas↗

arXiv2009

Collective oscillations in ultracold atomic gases

Using both fluid and kinetic descriptions, where repulsive forces between near by atoms are included, we discuss the basic oscillations and waves of a cloud of ultra-cold atoms confined in a magneto-optical trap. The existence of a hybrid mode, with properties similar to both plasma and acoustic waves is described in detail. Tonks-Dattner resonances for confined hybrid modes in a spherical cloud are discussed and the prediction of a nonlinear coupling between the dipole resonanc and the hybrid modes is considered. Landau damping processes and quasi-linear diffusion in velocity space are also discussed.

cond-mat.quant-gas↗

arXiv2009

Two-stream instability in quasi-one-dimensional Bose-Einstein condensates

We apply a kinetic model to predict the existence of an instability mechanism in elongated Bose-Einstein condensates. Our kinetic description, based on the Wigner formalism, is employed to highlight the existence of unstable Bogoliubov waves that may be excited in the counterpropagation configuration. We identify a dimensionless parameter, the Mach number at T = 0, that tunes different regimes of stability. We also estimate the magnitude of the main parameters at which two-stream instability is expected to be observed under typical experimental conditions.

cond-mat.quant-gas↗

arXiv2009

A phonon laser in ultra-cold matter

We show the possible excitation of a phonon laser instability in an ultra-cold atomic gas confined in a magneto-optical trap. Such an effect results from a negative Landau damping of the collective density perturbations in the gas, leading to the coherent emission of phonons. This laser instability can be driven by a blue-detuned laser superimposed to the usual red-detuning laser beams which usually provide the cooling mechanism. Threshold conditions, instability growth rates and saturation levels are derived. This work generalizes, on theoretical grounds, the recent results obtained with single ion phonon laser, to an ultra-cold atomic gas, where real phonons can be excited. Future phonon lasers could thus adequately be called phasers.

cond-mat.quant-gas↗

arXiv2009

Stimulated Raman and Brillouin backscattering of collimated beams carrying orbital angular momentum

We study theoretically the exchange of angular momentum between electromagnetic and electrostatic waves in a plasma, due to the stimulated Raman and Brillouin backscattering processes. Angular momentum states for plasmon and phonon fields are introduced for the first time. We demonstrate that these states can be excited by nonlinear wave mixing, associated with the scattering processes. This could be relevant for plasma diagnostics, both in laboratory and in space. Nonlinearly coupled paraxial equations and instability growth rates are derived.

physics.plasm-ph↗

arXiv2008

Quantum Trivelpiece-Gould waves in a magnetized dense plasma

The dispersion relation for the electrostatic waves below the electron plasma frequency in a dense quantum plasma is derived by using the magnetohydrodynamic model. It is shown that in the classical case the dispersion relation reduces to the expression obtained for the well-known Trivelpiece-Gould (TG) modes. Attention is also devoted to the case of solitary waves associated with the nonlinear TG modes.

physics.plasm-ph↗

arXiv2008

Photon orbital angular momentum in a plasma vortex

We study theoretically the exchange of angular momentum between a photon beam and a plasma vortex, and demonstrate the possible excitation of photon angular momentum states in a plasma. This can be relevant to laboratory and space plasma diagnostics; radio astronomy self-calibration; and generating photon angular momentum beams. A static plasma perturbation with helical structure, and a rotating plasma vortex are studied in detail and a comparison between these two cases, and their relevance to the physical nature of photon OAM, is established.

physics.plasm-ph↗

arXiv2008

Detecting orbital angular momentum in radio signals

Electromagnetic waves with an azimuthal phase shift are known to have a well defined orbital angular momentum. Different methods that allow for the detection of the angular momentum are proposed. For some, we discuss the required experimental setup and explore the range of applicability.

astro-ph↗

arXiv2008

Neutrino orbital angular momentum in a plasma vortex

It is shown that an electron-neutrino beam, propagating in a background plasma, can be decomposed into orbital momentum (OAM) states, similar to the OAM photon states. Coupling between different OAM neutrino states, in the presence of a plasma vortex, is considered. We show that plasma vorticity can be transfered to the neutrino beam, which is relevant to the understanding of the neutrino sources in astrophysics. Observation of neutrino OAM states could eventually become possible.

physics.plasm-ph↗