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Multicomponent theory of buoyancy instabilities in magnetized astrophysical plasmas: MHD analysis revisited  [PDF]
Anatoly K. Nekrasov,Mohsen Shadmehri
Physics , 2010,
Abstract: We develop a theory of buoyancy instabilities of the electron-ion plasma with the heat flux based on not the MHD equations, but using the multicomponent plasma approach. We investigate a geometry in which the background magnetic field, gravity, and stratification are directed along one axis. No simplifications usual for the MHD-approach in studying these instabilities are used. The background electron thermal flux and collisions between electrons and ions are included. We derive the simple dispersion relation, which shows that the thermal flux perturbation generally stabilizes an instability. There is a narrow region of the temperature gradient, where an instability is possible. This result contradicts to a conclusion obtained in the MHD-approach. We show that the reason of this contradiction is the simplified assumptions used in the MHD analysis of buoyancy instabilities and the role of the longitudinal electric field perturbation, which is not captured by the MHD equations. Our dispersion relation also shows that a medium with the electron thermal flux can be unstable, if the temperature gradient of ions and electrons have the opposite signs. The results obtained can be applied to ICM and clusters of galaxies.
Multicomponent theory of buoyancy instabilities in astrophysical plasma objects: The case of magnetic field perpendicular to gravity  [PDF]
Anatoly K. Nekrasov,Mohsen Shadmehri
Physics , 2010, DOI: 10.1088/0004-637X/724/2/1165
Abstract: We develop a general theory of buoyancy instabilities in the electron-ion plasma with the electron heat flux based not upon MHD equations, but using a multicomponent plasma approach in which the momentum equation is solved for each species. We investigate the geometry in which the background magnetic field is perpendicular to the gravity and stratification. General expressions for the perturbed velocities are given without any simplifications. Collisions between electrons and ions are taken into account in the momentum equations in a general form, permitting us to consider both weakly and strongly collisional objects. However, the electron heat flux is assumed to be directed along the magnetic field that implies a weakly collisional case. Using simplifications justified for an investigation of buoyancy instabilities with the electron thermal flux, we derive simple dispersion relations both for collisionless and collisional cases for arbitrary directions of the wave vector. The collisionless dispersion relation considerably differs from that obtained in the MHD framework and is similar to the Schwarzschild's criterion. This difference is connected with simplified assumptions used in the MHD analysis of buoyancy instabilities and with the role of the longitudinal electric field perturbation which is not captured by the ideal MHD equations. The results obtained can be applied to clusters of galaxies and other astrophysical objects.
Many particle theory for luminescence in quantum wells
Pereira Jr., M.F.;
Brazilian Journal of Physics , 2002, DOI: 10.1590/S0103-97332002000500020
Abstract: a green's function theory is applied to the description of luminescence and absorption spectra of low dimensional semiconductors. progress in the numerical solution of the bethe salpeter equation for coupled band quantum wells with a t-matrix structure for the polarisation function and carrier-carrier dephasing is given within an approach that satisfy the kubo-martin-schwinger sum rule and eliminates typical artifacts in computed optical spectra.
Many particle theory for luminescence in quantum wells  [cached]
Pereira Jr. M.F.
Brazilian Journal of Physics , 2002,
Abstract: A Green's function theory is applied to the description of luminescence and absorption spectra of low dimensional semiconductors. Progress in the numerical solution of the Bethe Salpeter equation for coupled band quantum wells with a T-matrix structure for the polarisation function and carrier-carrier dephasing is given within an approach that satisfy the Kubo-Martin-Schwinger sum rule and eliminates typical artifacts in computed optical spectra.
Periodic Orbit Theory of the Transition to Chaos in Quantum Wells  [PDF]
E. E. Narimanov,A. D. Stone
Physics , 1996,
Abstract: An analytic theory is developed for the density of states oscillations in quantum wells in a magnetic field which is tilted with respect to the barrier planes. The main oscillations are found to come from the simplest one or two-bounce periodic orbits. We calculate their period and stability analytically and find an infinite sequence of destabilizations followed by restabilizations as the chaos parameter increases. This phenomenon explains the re-entrant frequency-doubling of the density of states peaks observed in recent magnetotunneling experiments.
Theory of Ferromagnetism in Diluted Magnetic Semiconductor Quantum Wells  [PDF]
Byounghak Lee,T. Jungwirth,A. H. MacDonald
Physics , 2000, DOI: 10.1103/PhysRevB.61.15606
Abstract: We present a mean field theory of ferromagnetism in diluted magnetic semiconductor quantum wells. When subband mixing due to exchange interactions between quantum well free carriers and magnetic impurities is neglected, analytic result can be obtained for the dependence of the critical temperature and the spontaneous magnetization on the distribution of magnetic impurities and the quantum well width. The validity of this approximate theory has been tested by comparing its predictions with those from numerical self-consistent field calculations. Interactions among free carriers, accounted for using the local-spin-density approximation, substantially enhance the critical temperature. We demonstrate that an external bias potential can tune the critical temperature through a wide range.
Application of the Genetic Algorithm to Investigate Nanostructures of GaAs/AlGaAs Quantum Wells
H. Arabshahi,D. Ghodsi Nahri
Research Journal of Applied Sciences , 2012, DOI: 10.3923/rjasci.2011.136.139
Abstract: Application of the Genetic Algorithm to the GaAs/AlGaAs quantum wells are presented. We followed a method that is produced by using the Genetic algorithm, Variation method and Monte Carlo integration Scheme (GMV method). We have investigated the effect of the well width on the diamagnetic shift. The effect of the Al doping is also investigated.
Theory of spin waves in diluted-magnetic-semiconductor quantum wells  [PDF]
Diego Frustaglia,Jürgen K?nig,Allan H. MacDonald
Physics , 2003, DOI: 10.1103/PhysRevB.70.045205
Abstract: We present a theory of collective spin excitations in diluted-magnetic-semiconductor quantum wells in which local magnetic moments are coupled via a quasi-two-dimensional gas of electrons or holes. In the case of a ferromagnetic state with partly spin-polarized electrons, we find that the Goldstone collective mode has anomalous $k^4$ dispersion and that for symmetric quantum wells odd parity modes do not disperse at all. We discuss the gap in the collective excitation spectrum which appears when spin-orbit interactions are included.
Resonantly enhanced nonlinear optics in semiconductor quantum wells: An application to sensitive infrared detection  [PDF]
S. F. Yelin,P. R. Hemmer
Physics , 2000, DOI: 10.1103/PhysRevA.66.013803
Abstract: A novel class of coherent nonlinear optical phenomena, involving induced transparency in quantum wells, is considered in the context of a particular application to sensitive long-wavelength infrared detection. It is shown that the strongest decoherence mechanisms can be suppressed or mitigated, resulting in substantial enhancement of nonlinear optical effects in semiconductor quantum wells.
On the buoyancy force and the metacentre  [PDF]
Jacques Mégel,Janis Kliava
Physics , 2009,
Abstract: We address the point of application A of the buoyancy force (also known as the Archimedes force) by using two different definitions of the point of application of a force, derived one from the work-energy relation and another one from the equation of motion. We present a quantitative approach to this issue based on the concept of the hydrostatic energy, considered for a general shape of the immersed cross-section of the floating body. We show that the location of A depends on the type of motion experienced by the body. In particular, in vertical translation, from the work-energy viewpoint, this point is fixed with respect to the centre of gravity G of the body. In contrast, in rolling/pitching motion there is duality in the location of A ; indeed, the work-energy relation implies A to be fixed with respect to the centre of buoyancy C, while from considerations involving the rotational moment it follows that A is located at the metacentre M. We obtain analytical expressions of the location of M for a general shape of the immersed cross-section of the floating body and for an arbitrary angle of heel. We show that three different definitions of M viz., the ?geometrical? one, as the centre of curvature of the buoyancy curve, the Bouguer's one, involving the moment of inertia of the plane of flotation, and the ?dynamical? one, involving the second derivative of the hydrostatic energy, refer to one and the same special point, and we demonstrate a close relation between the height of M above the line of flotation and the stability of the floating body. Finally, we provide analytical expressions and graphs of the buoyancy, flotation and metacentric curves as functions of the angle of heel, for some particular shapes of the floating bodies.
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