MIFP MEMBERS' PUBLICATIONS

MEMBERS' PUBLICATIONS

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    The Puzzle of Magnetic Resonance Effect on the Magnetic Compass of Migratory Birds

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    13.11.10
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    13.11.10
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    109 KB

    K.V. Kavokin

    Experiments on the effect of radio-frequency (RF) magnetic fields on the magnetic compass orientation of migratory birds are analyzed using the theory of magnetic resonance. The results of these experiments were earlier interpreted within the radical-pair model of magnetoreception. However, the consistent analysis shows that the amplitudes of the RF fields used are far too small to noticeably influence electron spins in organic radicals. Other possible agents that could mediate the birds’ response to the RF fields are discussed, but apparently no known physical system can be responsible for this effect.

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    Analytic model of effective screened Coulomb interactions in a multilayer system

    Uploaded:
    13.10.11
    Modified:
    13.10.11
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    459 KB

    H. Ouerdane

    The main objective of the present work is the development of an analytically tractable model of screened electron-electron and electron-exciton interactions in layered systems composed of two parallel semiconductor quantum wells separated by a dielectric layer. These systems are promising for superconductivity with excitons-polaritons, and spin manipulation. Polarization effects induced by the dielectric mismatch in the nanostructure are taken into account using the image charge method. The obtained analytic expressions are used to calculate screened electron-electron and electron-exciton interactions; these are compared to results computed using other recently published models.

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    Coulomb singularities in scattering wave functions of spin-orbit-coupled states

    Uploaded:
    13.10.11
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    13.10.11
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    375 KB

    P. Bogdanski1, and H. Ouerdane

    We report on our analysis of the Coulomb singularity problem in the frame of the coupled channel scattering theory including spin-orbit interaction. We assume that the coupling between the partial wave components involves orbital angular momenta such that Δl = 0,±2. In these conditions, the two radial functions, components of a partial wave associated to two values of the angular momentum l, satisfy a system of two second-order ordinary differential equations. We examine the difficulties arising in the analysis of the behavior of the regular solutions near the origin because of this coupling. First, we demonstrate that for a singularity of the first kind in the potential, one of the solutions is not amenable to a power series expansion. The use of the Lippmann-Schwinger equations confirms this fact: a logarithmic divergence arises at the second iteration. To overcome this difficulty, we introduce two auxilliary functions which, together with the two radial functions, satisfy a system of four first-order differential equations. The reduction of the order of the differential system enables us to use a matrix-based approach, which generalizes the standard Frobenius method.We illustrate our analysis with numerical calculations of coupled scattering wave functions in a solid-state system.

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    Error cancellation in the semiclassical calculation of the scattering length

    Uploaded:
    13.10.11
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    13.10.11
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    278 KB

    M.J. Jamieson, and H. Ouerdane

    We investigate the effects of two approximations concerning long range dispersion forces that are made in the derivation of the semiclassical formula for the scattering length of a pair of neutral atoms. We demonstrate numerically, using a published model interaction potential for a pair of Cs atoms in the 3Σ+ u molecular state, that the subsequent long range errors tend to cancel and we show, from an approximate analytical relationship, that the first order errors do indeed largely cancel.We suggest a hybrid method that combines quantum mechanical and semiclassical calculations. We explore its use in finding the scattering lengths of 7Li atoms and 133Cs atoms interacting via the Χ1Σ+ and a3Σ+ molecular potentials and we use it to demonstrate that the semiclassical formula fails for cold collisions of H atoms in the Χ1Σ+g molecular state because of the long range errors rather than because of inadequacies in describing the motion over the potential well semiclassically.

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    Parameters for Cold Collisions of Lithium and Caesium Atoms

    Uploaded:
    13.10.11
    Modified:
    03.09.12
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    419 KB

    Jamieson M. J., Ouerdane H.


    We calculate the s-wave scattering length and effective range and the p-wave scattering volume for 7Li atoms interacting with 133Cs atoms via the Χ1Σ+ molecular potential. The length and volume are found by fitting the log-derivative of the zero energy wave function evaluated at short range to a long range expression that accounts for the leading van der Waals dispersion potential and then incorporating the remaining long range dispersion contributions to first order. The effective range is evaluated from a quadrature formula. The calculated parameters are checked from the zero energy limits of the scattering phase shifts. We comment on ill-conditioning in the calculated s-wave scattering length.

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    Optimal working conditions for thermoelectric generators with realistic thermal coupling

    Uploaded:
    07.11.12
    Modified:
    07.11.12
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    384 KB

    Y. Apertet1, H. Ouerdane, O. Glavatskaya, C. Goupil and P. Lecoeur


    We study how maximum output power can be obtained from a thermoelectric generator (TEG) with non-ideal heat exchangers. We demonstrate with an analytic approach based on a force-flux formalism that the sole improvement of the intrinsic characteristics of thermoelectric modules including the enhancement of the figure of merit is of limited interest: the constraints imposed by the working conditions of the TEG must be considered on the same footing. Introducing an effective thermal conductance we derive the conditions which permit maximization of both efficiency and power production of the TEG dissipatively coupled to heat reservoirs. Thermal impedance matching must be accounted for as well as electrical impedance matching in order to maximize the output power. Our calculations also show that the thermal impedance does not only depend on the thermal conductivity at zero electrical current: it also depends on the TEG figure of merit. Our analysis thus yields both electrical and thermal conditions permitting optimal use of a thermoelectric generator.

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    Thermoelectric internal current loops inside inhomogeneous systems

    Uploaded:
    07.11.12
    Modified:
    07.11.12
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    140 KB

    Y. Apertet, H. Ouerdane, C. Goupil and Ph. Lecoeur


    Considering a system composed of two different thermoelectric modules electrically and thermally connected in parallel, we demonstrate that the inhomogeneities of the thermoelectric properties of the materials may cause the appearance of an electrical current, which develops inside the system.We show that this current increases the effective thermal conductance of the whole system.We also discuss the significance of a recent finding concerning a reported new electrothermal effect in inhomogeneous bipolar semiconductors, in light of our results.

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    Irreversibilities and efficiency at maximum power of heat engines: The illustrative case of a thermoelectric generator

    Uploaded:
    07.11.12
    Modified:
    07.11.12
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    235 KB

    Y. Apertet, H. Ouerdane, C. Goupil and Ph. Lecoeur


    Energy conversion efficiency at maximum output power, which embodies the essential characteristics of heat engines, is the main focus of the present work. The so-called Curzon and Ahlborn efficiency ηCA is commonly believed to be an absolute reference for real heat engines; however, a different but general expression for the case of stochastic heat engines, ηSS, was recently found and then extended to low-dissipation engines. The discrepancy between ηCA and ηSS is here analyzed considering different irreversibility sources of heat engines, of both internal and external types. To this end, we choose a thermoelectric generator operating in the strong-coupling regime as a physical system to qualitatively and quantitatively study the impact of the nature of irreversibility on the efficiency at maximum output power. In the limit of pure external dissipation, we obtain ηCA, while ηSS corresponds to the case of pure internal dissipation. A continuous transition between from one extreme to the other, which may be operated by tuning the different sources of irreversibility, also is evidenced.

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    Efficiency at maximum power of thermally coupled heat engines

    Uploaded:
    07.11.12
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    07.11.12
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    562 KB

    Y. Apertet, H. Ouerdane, C. Goupil and Ph. Lecoeur


    We study the efficiency at maximum power of two coupled heat engines, using thermoelectric generators (TEGs) as engines. Assuming that the heat and electric charge fluxes in the TEGs are strongly coupled, we simulate numerically the dependence of the behavior of the global system on the electrical load resistance of each generator in order to obtain the working condition that permits maximization of the output power. It turns out that this condition is not unique.We derive a simple analytic expression giving the relation between the electrical load resistance of each generator permitting output power maximization. We then focus on the efficiency at maximum power (EMP) of the whole system to demonstrate that the Curzon-Ahlborn efficiency may not always be recovered: The EMP varies with the specific working conditions of each generator but remains in the range predicted by irreversible thermodynamics theory. We discuss our results in light of nonideal Carnot engine behavior.

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    Mesoscopic thermoelectric transport near zero transmission energies

    Uploaded:
    22.04.13
    Modified:
    22.04.13
    File Size:
    913 KB

    YA. Abbout, H. Ouerdane and C. Goupil


    We study the thermoelectric transport coefficients of a one-dimensional (1D) electron waveguide connected to one and then two off-channel cavities, in the presence of dephasing phonons. The model system is that of a linear chain described as a 1Dlattice. For simplicitywe consider single-mode cavities, whichmay be tunedwith external gates. While the presence of only one off-channel cavity yields a nearly symmetric transmission profile, which is canceled around the cavity mode, an additional cavity modifies this profile strongly and results in an asymmetric shape characterized by oscillations. In both cases,we consider electron-phonon interactions in our calculations and analyze their effects on the transmission function around the Fermi energy. Knowledge of the energy-dependent transmission function allows the numerical computation of thermoelectric transport coefficients, including the thermopower. In the presence of a second off-channel cavity, the sign of the thermopower depends on the relative position of this cavity energy level with respect to the Fermi energy: the thermopower is positive when low-energy electrons in the vicinity of the Fermi level are not transmitted and becomes negative when the higher-energy electrons are not transmitted.

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