5,893 research outputs found
How to model quantum plasmas
Traditional plasma physics has mainly focused on regimes characterized by
high temperatures and low densities, for which quantum-mechanical effects have
virtually no impact. However, recent technological advances (particularly on
miniaturized semiconductor devices and nanoscale objects) have made it possible
to envisage practical applications of plasma physics where the quantum nature
of the particles plays a crucial role. Here, I shall review different
approaches to the modeling of quantum effects in electrostatic collisionless
plasmas. The full kinetic model is provided by the Wigner equation, which is
the quantum analog of the Vlasov equation. The Wigner formalism is particularly
attractive, as it recasts quantum mechanics in the familiar classical phase
space, although this comes at the cost of dealing with negative distribution
functions. Equivalently, the Wigner model can be expressed in terms of
one-particle Schr{\"o}dinger equations, coupled by Poisson's equation: this is
the Hartree formalism, which is related to the `multi-stream' approach of
classical plasma physics. In order to reduce the complexity of the above
approaches, it is possible to develop a quantum fluid model by taking
velocity-space moments of the Wigner equation. Finally, certain regimes at
large excitation energies can be described by semiclassical kinetic models
(Vlasov-Poisson), provided that the initial ground-state equilibrium is treated
quantum-mechanically. The above models are validated and compared both in the
linear and nonlinear regimes.Comment: To be published in the Fields Institute Communications Series.
Proceedings of the Workshop on Kinetic Theory, The Fields Institute, Toronto,
March 29 - April 2, 200
Autoresonant control of the many-electron dynamics in nonparabolic quantum wells
The optical response of nonparabolic quantum wells is dominated by a strong
peak at the plasmon frequency. When the electrons reach the anharmonic regions,
resonant absorption becomes inefficient. This limitation is overcome by using a
chirped laser pulse in the autoresonant regime. By direct simulations using the
Wigner phase-space approach, the authors prove that, with a sequence of just a
few pulses, electrons can be efficiently detrapped from a nonparabolic well.
For an array of multiple quantum wells, they can create and control an
electronic current by suitably applying an autoresonant laser pulse and a
slowly varying dc electric field.Comment: 3 page
Fidelity decay in trapped Bose-Einstein condensates
The quantum coherence of a Bose-Einstein condensate is studied using the
concept of quantum fidelity (Loschmidt echo). The condensate is confined in an
elongated anharmonic trap and subjected to a small random potential such as
that created by a laser speckle. Numerical experiments show that the quantum
fidelity stays constant until a critical time, after which it drops abruptly
over a single trap oscillation period. The critical time depends
logarithmically on the number of condensed atoms and on the perturbation
amplitude. This behavior may be observable by measuring the interference
fringes of two condensates evolving in slightly different potentials.Comment: 4 pages, to appear in Physical Review Letters, February 200
A multistream model for quantum plasmas
The dynamics of a quantum plasma can be described self-consistently by the
nonlinear Schroedinger-Poisson system. Here, we consider a multistream model
representing a statistical mixture of N pure states, each described by a
wavefunction. The one-stream and two-stream cases are investigated. We derive
the dispersion relation for the two-stream instability and show that a new,
purely quantum, branch appears. Numerical simulations of the complete
Schroedinger-Poisson system confirm the linear analysis, and provide further
results in the strongly nonlinear regime. The stationary states of the
Schroedinger-Poisson system are also investigated. These can be viewed as the
quantum mechanical counterpart of the classical Bernstein-Greene-Kruskal modes,
and are described by a set of coupled nonlinear differential equations for the
electrostatic potential and the stream amplitudes.Comment: 20 pages, 10 figure
Different Facets of Chaos in Quantum Mechanics
Nowadays there is no universally accepted definition of quantum chaos. In
this paper we review and critically discuss different approaches to the
subject, such as Quantum Chaology and the Random Matrix Theory. Then we analyze
the problem of dynamical chaos and the time scales associated with chaos
suppression in quantum mechanics. Summary: 1. Introduction 2. Quantum Chaology
and Spectral Statistics 3. From Poisson to GOE Transition: Comparison with
Experimental Data 3.1 Atomic Nuclei 3.2 The Hydrogen Atom in the Strong
Magnetic Field 4. Quantum Chaos and Field Theory 5. Alternative Approaches to
Quantum Chaos 6. Dynamical Quantum Chaos and Time Scales 6.1 Mean-Field
Approximation and Dynamical Chaos 7. ConclusionsComment: RevTex, 25 pages, 7 postscript figures, to be published in Int. J.
Mod. Phys.
Variational approach for the quantum Zakharov system
The quantum Zakharov system is described in terms of a Lagrangian formalism.
A time-dependent Gaussian trial function approach for the envelope electric
field and the low-frequency part of the density fluctuation leads to a coupled,
nonlinear system of ordinary differential equations. In the semiclassic case,
linear stability analysis of this dynamical system shows a destabilizing r\^ole
played by quantum effects. Arbitrary value of the quantum effects are also
considered, yielding the ultimate destruction of the localized, Gaussian trial
solution. Numerical simulations are shown both for the semiclassic and the full
quantum cases.Comment: 6 figure
Bose-Einstein condensation of positronium: modification of the s-wave scattering length below the critical temperature
The production of a Bose-Einstein condensate made of positronium may be
feasible in the near future. Below the condensation temperature, the
positronium collision process is modified by the presence of the condensate.
This makes the theoretical description of the positronium kinetics at low
temperature challenging. Based on the quasi-particle Bogoliubov theory, we
describe the many-body particle-particle collision in a simple manner. We find
that, in a good approximation, the full positronium-positronium interaction can
be described by an effective scattering length. Our results are general and
apply to different species of bosons. The correction to the bare scattering
length is expressed in terms of a single dimensionless parameter that
completely characterizes the condensate
Ferromagnetic behavior in magnetized plasmas
We consider a low-temperature plasma within a newly developed MHD Fluid
model. In addition to the standard terms, the electron spin, quantum particle
dispersion and degeneracy effects are included. It turns out that the electron
spin properties can give rise to Ferromagnetic behavior in certain regimes. If
additional conditions are fulfilled, a homogenous magnetized plasma can even be
unstable. This happen in the low-temperature high-density regime, when the
magnetic properties associated with the spin can overcome the stabilizing
effects of the thermal and Fermi pressure, to cause a Jeans like instability.Comment: 4 pages, 1 figur
Simulation of Impedance Discontinuities Resulting from Degradation of Interconnections on Printed Circuit Boards
Numerical simulation of impedance discontinuities resulting from degradation of interconnections on printed circuit board
Theory and applications of the Vlasov equation
Forty articles have been recently published in EPJD as contributions to the
topical issue "Theory and applications of the Vlasov equation". The aim of this
topical issue was to provide a forum for the presentation of a broad variety of
scientific results involving the Vlasov equation. In this editorial, after some
introductory notes, a brief account is given of the main points addressed in
these papers and of the perspectives they open.Comment: Editoria
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