66 research outputs found

    Theoretical Aspects of Blunt Body Magnetoaerodynamics

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    Supersonic magnetoaerodynamic flow about blunt body with self contained magnetic field sourc

    Josephson Coupling through a Quantum Dot

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    We derive, via fourth order perturbation theory, an expression for the Josephson current through a gated interacting quantum dot. We analyze our expression for two different models of the superconductor-dot-superconductor (SDS) system. When the matrix elements connecting dot and leads are featureless constants, we compute the Josephson coupling J_c as a function of the gate voltage and Coulomb interaction. In the diffusive dot limit, we compute the probability distribution P(J_c) of Josephson couplings. In both cases, pi junction behavior (J_c < 0) is possible, and is not simply dependent on the parity of the dot occupancy.Comment: 9 pages; 3 encapsulated PostScript figure

    Frictional drag between quantum wells mediated by fluctuating electromagnetic field

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    We use the theory of the fluctuating electromagnetic field to calculate the frictional drag between nearby two-and three dimensional electron systems. The frictional drag results from coupling via a fluctuating electromagnetic field, and can be considered as the dissipative part of the van der Waals interaction. In comparison with other similar calculations for semiconductor two-dimensional system we include retardation effects. We consider the dependence of the frictional drag force on the temperature TT, electron density and separation dd. We find, that retardation effects become dominating factor for high electron densities, corresponding thing metallic film, and suggest a new experiment to test the theory. The relation between friction and heat transfer is also briefly commented on.Comment: 14 pages, 4 figure

    Frictional drag between quantum wells mediated by phonon exchange

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    We use the Kubo formalism to evaluate the contribution of acoustic phonon exchange to the frictional drag between nearby two-dimensional electron systems. In the case of free phonons, we find a divergent drag rate (τD−1\tau_{D}^{-1}). However, τD−1\tau_{D}^{-1} becomes finite when phonon scattering from either lattice imperfections or electronic excitations is accounted for. In the case of GaAs quantum wells, we find that for a phonon mean free path ℓph\ell_{ph} smaller than a critical value, imperfection scattering dominates and the drag rate varies as ln(ℓph/d)ln (\ell_{ph}/d) over many orders of magnitude of the layer separation dd. When ℓph\ell_{ph} exceeds the critical value, the drag rate is dominated by coupling through an electron-phonon collective mode localized in the vicinity of the electron layers. We argue that the coupled electron-phonon mode may be observable for realistic parameters. Our theory is in good agreement with experimental results for the temperature, density, and dd-dependence of the drag rate.Comment: 45 pages, LaTeX, 8 postscript file figure

    OBSERVATIONS CONCERNING MAGNETOAERODYNAMIC DRAG AND SHOCK STANDOFF DISTANCE

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    AXISYMMETRICAL TURBULENT JET- TOLLMIEN'S PROBLEM EXTENDED

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    Magnetogasdynamic re-entry phenomena.

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