MIFP MEMBERS' PUBLICATIONS

MEMBERS' PUBLICATIONS

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    Exciton-Polariton Mediated Superconductivity

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    01.09.10
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    12.10.10
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    F. P. Laussy, A. V. Kavokin and I. A. Shelykh


    We revisit the exciton mechanism of superconductivity in the framework of microcavity physics, replacing virtual excitons as a binding agent of Cooper pairs by excitations of an exciton-polariton Bose- Einstein condensate. We consider a model microcavity where a quantum well with a two-dimensional electron gas is sandwiched between two undoped quantum wells, where a polariton condensate is formed. We show that the critical temperature for superconductivity dramatically increases with the condensate population, opening a new route towards high-temperature superconductivity.

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    Barnes slave-boson approach to the two-site single-impurity Anderson model with non-local interaction

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    13.09.10
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    12.10.10
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    R. Fr´esard, H. Ouerdane and T. Kopp

    The Barnes slave-boson approach to the U = ∞ single-impurity Anderson model extended by a non-local Coulomb interaction is revisited. We demonstrate first that the radial gauge representation facilitates the treatment of such a non-local interaction by performing the exact evaluation of the path integrals representing the partition function, the impurity hole density and the impurity hole density autocorrelation function for a two-site cluster. The free energy is also obtained on the same footing. Next, the exact results are compared to their approximations at saddle-point level, and it is shown that the saddle point evaluation recovers the exact answer in the limit of strong non-local Coulomb interaction, while the agreement between both schemes remains satisfactory in a large parameter range.

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    Photon emission induced by elastic exciton-carrier scattering in semiconductor quantum wells

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    13.09.10
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    12.10.10
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    H. Ouerdane, R. Varache, M.E. Portnoi and I. Galbraith

    We present a study of the elastic exciton-electron (X–e−) and exciton-hole (X–h) scattering processes in semiconductor quantum wells, including fermion exchange effects. The balance between the exciton and the free carrier populations within the electron-hole plasma is discussed in terms of ionization degree in the nondegenerate regime. Assuming a two-dimensional Coulomb potential statically screened by the free carrier gas, we apply the variable phase method to obtain the excitonic wavefunctions, which we use to calculate the 1s exciton-free carrier matrix elements that describe the scattering of excitons into the light cone where they can radiatively recombine. The photon emission rates due to the carrierassisted exciton recombination in semiconductor quantum-wells (QWs) at room temperature and in a low density regime are obtained from Fermi’s golden rule, and studied for mid-gap and wide-gap materials. The quantitative comparison of the direct and exchange terms of the scattering matrix elements shows that fermion exchange is the dominant mechanism of the exciton-carrier scattering process. This is confirmed by our analysis of the rates of photon emission induced by electron-assisted and hole-assisted exciton recombinations.

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    Slave bosons in radial gauge: A bridge between path integral and Hamiltonian language

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    13.09.10
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    12.10.10
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    Raymond Frésard, Henni Ouerdane, Thilo Kopp

    We establish a correspondence between the resummation of world lines and the diagonalization of the Hamiltonian for a strongly correlated electronic system. For this purpose, we analyze the functional integrals for the partition function and the correlation functions invoking a slave boson representation in the radial gauge. We show in the spinless case that the Green’s function of the physical electron and the projected Green’s function of the pseudofermion coincide. Correlation and Green’s functions in the spinful case involve a complex entanglement of the world lines which, however, can be obtained through a strikingly simple extension of the spinless scheme. As a toy model we investigate the two-site cluster of the single impurity Anderson model which yields analytical results. All expectation values and dynamical correlation functions are obtained from the exact calculation of the relevant functional integrals. The hole density, the hole auto-correlation function and the Green’s function are computed, and a comparison between spinless and spin 1/2 systems provides insight into the role of the radial slave boson field. In particular, the exact expectation value of the radial slave boson field is finite in both cases, and it is not related to a Bose condensate.

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    Polariton-polariton scattering in microcavities: A microscopic theory

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    13.09.10
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    12.10.10
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    M. M. Glazov, H. Ouerdane, L. Pilozzi, G. Malpuech, A. V. Kavokin and A. D’Andrea


    We apply the fermion commutation technique for composite bosons to polariton-polariton scattering in semiconductor planar microcavities. Derivations are presented in a simple and physically transparent fashion. A procedure of orthogonolization of the initial and final two-exciton state wave functions is used to calculate the effective scattering-matrix elements and the scattering rates. We show how the bosonic stimulation of the scattering appears in this full fermionic approach whose equivalence to the bosonization method is thus demonstrated in the regime of low exciton density. We find an additional contribution to polariton-polariton scattering due to the exciton oscillator strength saturation, which we analyze as well.We present a theory of the polariton-polariton scattering with opposite spin orientations and show that this scattering process takes place mainly via dark excitonic states. Analytical estimations of the effective scattering amplitudes are given.

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    Scattering parameters for cold Li–Rb and Na–Rb collisions derived from variable phase theory

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    13.09.10
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    12.10.10
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    H. Ouerdane and M. J. Jamieson

    We show how the scattering phase shift, the s-wave scattering length, and the p-wave scattering volume can be obtained from Riccati equations derived in variable phase theory. We find general expressions that provide upper and lower bounds for the scattering length and the scattering volume. We show how, in the framework of the variable phase method, Levinson’s theorem yields the number of bound states supported by a potential. We report results from a study of the heteronuclear alkali-metal dimers NaRb and LiRb. We explore the mass dependence of the scattering data by considering all isotopomers and we calculate the numbers of bound states supported by the molecular potentials for each isotopomer.

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    Ultrafast pump–probe dynamics in ZnSe-based semiconductor quantum wells

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    13.09.10
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    12.10.10
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    H. Ouerdane, G. Papageorgiou, I. Galbraith, A. K. Kar, and B. S. Wherrett

    Pump–probe experiments are used as a controllable way to investigate the properties of photoexcited semiconductors, in particular, the absorption saturation. We present an experiment–theory comparison for ZnSe quantum wells, investigating the energy renormalization and bleaching of the excitonic resonances. Experiments were performed with spin-selective excitation and above-bandgap pumping. The model, based on the semiconductor Bloch equations in the screened Hartree–Fock approximation, takes various scattering processes into account phenomenologically. Comparing numerical results with available experimental data, we explain the experimental results and find that the electron spin-flip occurs on a time scale of 30 ps.

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    Size dependent carrier recombination in ZnO nanocrystals

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    12.10.10
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    12.10.10
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    G. Pozina, L. L. Yang, Q. X. Zhao, L. Hultman,1 and P. G. Lagoudakis


    Experimental and theoretical studies of fluorescence decay were performed for colloidal ZnO nanocrystals. The fluorescence lifetime reduces from 22 ps to 6 ps with decreasing nanocrystal radius. We postulate that non-radiative surface states dominate the carrier dynamics in small ZnO nanocrystals and perform Monte Carlo simulations incorporating carrier diffusion and carrier recombination to model the experimental fluorescence decay dynamics. The percentage of excitons undergoing nonradiative decay due to surface trapping is as high as 84% for nanocrystals with 8 nm radius, which explains the ultrafast decay dynamics observed in small ZnO nanostructures even at low temperatures.

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    Induced currents, frozen charges and the quantum Hall effect breakdown

    Uploaded:
    13.11.10
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    13.11.10
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    K.V. Kavokina, M.E. Portnoia, A.J. Matthewsa, A. Ushera, J. Gethinga, D.A. Ritchieb, M.Y. Simmonsb

    Puzzling results obtained from torque magnetometry in the quantum Hall effect regime are presented, and a theory is proposed for their explanation. Magnetic moment saturation, which is usually attributed to the quantum Hall effect breakdown, is shown to be related to the charge redistribution across the sample.

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    Spin relaxation of localized electrons in n-type semiconductors

    Uploaded:
    13.11.10
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    13.11.10
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    K.V. Kavokin


    The mechanisms that determine spin relaxation times of localized electrons in impurity bands of n-type semiconductors are considered theoretically and compared with available experimental data. The relaxation time of the non-equilibrium angular momentum is shown to be limited either by hyperfine interaction, or by spin–orbit interaction in the course of exchange-induced spin diffusion. The energy relaxation time in the spin system is governed by phonon-assisted hops within pairs of donors with an optimal distance of about 4 Bohr radii. The spin correlation time of the donor-bound electron is determined either by exchange interaction with other localized electrons, or by spin-flip scattering of free conduction-band electrons. A possibility of optical cooling of the spin system of localized electrons is discussed.

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