We study the properties of two electrons with Coulomb interactions in a tight-binding model of La-based cuprate superconductors. This tight-binding model is characterized by long-range hopping obtained previously by advanced quantum chemistry computations. We show analytically and numerically that the Coulomb repulsion leads to a formation of compact pairs propagating through the whole system. The mechanism of pair formation is related to the emergence of an effective narrow energy band for Coulomb electron pairs with conserved total pair energy and momentum. The dependence of the pair formation probability on an effective filling factor is obtained with a maximum around a filling factor of 20 (or 80) percent. The comparison with the case of the nearest neighbor tight-binding model shows that the long-range hopping provides an increase of the phase space volume with high pair formation probability. We conjecture that the Coulomb electron pairs discussed here may play a role in high temperature superconductivity.

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Nous avons développé une méthode de cartographie 2D temps réel des champs électromagnétiques par thermo fluorescence. Cette technique est appelée EMVI [1] (Electromagnetic Visible Imaging). Notre expérimentation se base sur deux principaux phénomènes : les pertes magnétiques ou électriques (responsable d'un échauffement du film mince) et la thermo-fluorescence d'un fluorophore, en l'occurrence la rhodamine B (qui est déposée sur ce même film). Dans le cas d'un champ électrique, on utilise un film faiblement conducteur (film « à pertes électriques ») dans lequel des courants induits provoquent des pertes Joule. Dans le cas d'un champ magnétique, l'échauffement peut être dû à des « pertes fer » (hystérésis) pour les basses fréquences (kHz). Pour des fréquences plus élevées (GHz) un phénomène de résonance ferromagnétique s'établit, ce qui dans les deux cas donne lieu à des pertes thermiques (film « à pertes magnétiques ») [2]. Nous présentons ici des cartographies de champ magnétique en champ proche sur deux systèmes :-Le premier système étudié est une bobine plate avec une fréquence de résonance à 172 kHz. Nous avons réalisé des images du champ magnétique rayonné par la bobine sur différents axes.

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The Bethe-Salpeter equation (BSE) formalism is a computationally affordable method for the calculation of accurate optical excitation energies in molecular systems. Similar to the ubiquitous adiabatic approximation of time-dependent density-functional theory, the static approximation, which substitutes a dynamical (\ie, frequency-dependent) kernel by its static limit, is usually enforced in most implementations of the BSE formalism. Here, going beyond the static approximation, we compute the dynamical correction of the electron-hole screening for molecular excitation energies thanks to a renormalized first-order perturbative correction to the static BSE excitation energies. The present dynamical correction goes beyond the plasmon-pole approximation as the dynamical screening of the Coulomb interaction is computed exactly within the random-phase approximation. Our calculations are benchmarked against high-level (coupled-cluster) calculations, allowing to assess the clear improvement brought by the dynamical correction for both singlet and triplet optical transitions.

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This paper describes time-dependent phase modulation experiments using an optical Michelson interferometer. Changes in the path length of the interferometer arms induce phase shifts in time and, thanks to a loudspeaker, make the interferometric signal audible. We produce time-fluctuating refractive indices and mechanical motions of one reflecting mirror. A sinusoidal motion is applied, thus creating a phase modulation of the wave in one of the interferometer arms. Sidebands of the laser frequency appear, which interfere with the other interferometer wave. Our hearing sense is quite developed, and we use it to analyze the non-linear character by the acoustic timbre. Finally, we transfer information by phase modulation. Our setup benefits the visually impaired community as temporal phase changes can be analyzed by the auditory sense, but this unconventional use of an optical interferometer is also attractive to a general audience.

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