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 Beyond Moore’s technologies: operation principles of a superconductor alternative (2018.01.22)
 Excitonic lasing of strainfree InP(As) quantum dots in AlInAs microdisk (2017.03.20)
 Fast 3C 279 γ flares by a merging medium size black hole jet aligned to the AGN one by tidal torque? (2016.10.17)
 Nanocarbon pixels array for ionizing particles monitoring (2016.08.31)
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 Optimal working conditions for thermoelectric generators with realistic thermal coupling (5641)
 Analytic model of effective screened Coulomb interactions in a multilayer system (4832)
 CV Alexey Kavokin (2010) (3919)
 The Puzzle of Magnetic Resonance Effect on the Magnetic Compass of Migratory Birds (3665)
 Slave bosons in radial gauge: A bridge between path integral and Hamiltonian language (3246)
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MEMBERS' PUBLICATIONS
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ExcitonPolariton Mediated Superconductivity
 Uploaded:
 01.09.10
 Modified:
 12.10.10
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 499 KB
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 excitonpolariton Bose Einstein condensate. We consider a model microcavity where a quantum well with a twodimensional 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 hightemperature superconductivity. 
Barnes slaveboson approach to the twosite singleimpurity Anderson model with nonlocal interaction
 Uploaded:
 13.09.10
 Modified:
 12.10.10
 File Size:
 402 KB
R. Fr´esard, H. Ouerdane and T. Kopp
The Barnes slaveboson approach to the U = ∞ singleimpurity Anderson model extended by a nonlocal Coulomb interaction is revisited. We demonstrate first that the radial gauge representation facilitates the treatment of such a nonlocal 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 twosite cluster. The free energy is also obtained on the same footing. Next, the exact results are compared to their approximations at saddlepoint level, and it is shown that the saddle point evaluation recovers the exact answer in the limit of strong nonlocal Coulomb interaction, while the agreement between both schemes remains satisfactory in a large parameter range.

Photon emission induced by elastic excitoncarrier scattering in semiconductor quantum wells
 Uploaded:
 13.09.10
 Modified:
 12.10.10
 File Size:
 498 KB
H. Ouerdane, R. Varache, M.E. Portnoi and I. Galbraith
We present a study of the elastic excitonelectron (X–e−) and excitonhole (X–h) scattering processes in semiconductor quantum wells, including fermion exchange effects. The balance between the exciton and the free carrier populations within the electronhole plasma is discussed in terms of ionization degree in the nondegenerate regime. Assuming a twodimensional 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 excitonfree 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 quantumwells (QWs) at room temperature and in a low density regime are obtained from Fermi’s golden rule, and studied for midgap and widegap 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 excitoncarrier scattering process. This is confirmed by our analysis of the rates of photon emission induced by electronassisted and holeassisted exciton recombinations.

Slave bosons in radial gauge: A bridge between path integral and Hamiltonian language
 Uploaded:
 13.09.10
 Modified:
 12.10.10
 File Size:
 306 KB
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 twosite 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 autocorrelation 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.

Polaritonpolariton scattering in microcavities: A microscopic theory
 Uploaded:
 13.09.10
 Modified:
 12.10.10
 File Size:
 208 KB
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 polaritonpolariton scattering in semiconductor planar microcavities. Derivations are presented in a simple and physically transparent fashion. A procedure of orthogonolization of the initial and final twoexciton state wave functions is used to calculate the effective scatteringmatrix 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 polaritonpolariton scattering due to the exciton oscillator strength saturation, which we analyze as well.We present a theory of the polaritonpolariton 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. 
Scattering parameters for cold Li–Rb and Na–Rb collisions derived from variable phase theory
 Uploaded:
 13.09.10
 Modified:
 12.10.10
 File Size:
 73 KB
H. Ouerdane and M. J. Jamieson
We show how the scattering phase shift, the swave scattering length, and the pwave 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 alkalimetal 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.

Ultrafast pump–probe dynamics in ZnSebased semiconductor quantum wells
 Uploaded:
 13.09.10
 Modified:
 12.10.10
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 208 KB
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 spinselective excitation and abovebandgap 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 spinflip occurs on a time scale of 30 ps.

Size dependent carrier recombination in ZnO nanocrystals
 Uploaded:
 12.10.10
 Modified:
 12.10.10
 File Size:
 808 KB
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 nonradiative 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. 
Induced currents, frozen charges and the quantum Hall effect breakdown
 Uploaded:
 13.11.10
 Modified:
 13.11.10
 File Size:
 111 KB
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.

Spin relaxation of localized electrons in ntype semiconductors
 Uploaded:
 13.11.10
 Modified:
 13.11.10
 File Size:
 416 KB
K.V. Kavokin
The mechanisms that determine spin relaxation times of localized electrons in impurity bands of ntype semiconductors are considered theoretically and compared with available experimental data. The relaxation time of the nonequilibrium angular momentum is shown to be limited either by hyperfine interaction, or by spin–orbit interaction in the course of exchangeinduced spin diffusion. The energy relaxation time in the spin system is governed by phononassisted hops within pairs of donors with an optimal distance of about 4 Bohr radii. The spin correlation time of the donorbound electron is determined either by exchange interaction with other localized electrons, or by spinflip scattering of free conductionband electrons. A possibility of optical cooling of the spin system of localized electrons is discussed.