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The subject of the thesis focuses on new approximations studied in a formalism based on a perturbation theory allowing to describe the electronic properties of many-body systems in an approximate way. We excite a system with a small disturbance, by sending light on it or by applying a weak electric field to it, for example and the system "responds" to the disturbance, in the framework of linear response, which means that the response of the system is proportional to the disturbance. The goal is to determine what we call the neutral excitations or bound states of the system, and more particularly the single excitations. These correspond to the transitions from the ground state to an excited state. To do this, we describe in a simplified way the interactions of the particles of a many-body system using an effective interaction that we average over the whole system. The objective of such an approach is to be able to study a system without having to use the exact formalism which consists in diagonalizing the N-body Hamiltonian, which is not possible for systems with more than two particles.

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We present the multi-channel Dyson equation that combines two or more many-body Green's functions to describe the electronic structure of materials. In this thesis, we use it to model photoemission spectra by coupling the one-body Green's function with the three-body Green's function and to model neutral excitation by coupling the two-body Green's function with the four-body Green's function . We demonstrate that, unlike methods using only the one-body Green's function, our approach puts the description of quasiparticles and satellites on an equal footing. We propose a multi-channel self-energy that is static and only contains the bare Coulomb interaction, making frequency convolutions and self-consistency unnecessary. Despite its simplicity, we demonstrate with a diagrammatic analysis that the physics it describes is extremely rich. Finally, we present a framework based on an effective Hamiltonian that can be solved for any many-body system using standard numerical tools. We illustrate our approach by applying it to the Hubbard dimer and show that it is exact both at 1/4 and 1/2 filling.

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We present the second release of the real-time time-dependent density functional theory code “Quantum Dissipative Dynamics” (QDD). It augments the first version [1] by a parallelization on a GPU coded with CUDA fortran. The extension focuses on the dynamical part only because this is the most time consuming part when applying the QDD code. The performance of the new GPU implementation as compared to OpenMP parallelization has been tested and checked on a couple of small sodium clusters and small covalent molecules. OpenMP parallelization allows a speed-up by one order of magnitude in average, as compared to a sequential computation. The use of a GPU permits a gain of an additional order of magnitude. The performance gain outweighs even the larger energy consumption of a GPU. The impressive speed-up opens the door for more demanding applications, not affordable before

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We present the multi-channel Dyson equation that combines two or more many-body Green's functions to describe the electronic structure of materials. In this work we use it to model photoemission spectra by coupling the one-body Green's function with the three-body Green's function. We demonstrate that, unlike methods using only the one-body Green's function, our approach puts the description of quasiparticles and satellites on an equal footing. We propose a multi-channel self-energy that is static and only contains the bare Coulomb interaction, making frequency convolutions and self-consistency unnecessary. Despite its simplicity, we demonstrate with a diagrammatic analysis that the physics it describes is extremely rich. Finally, we present a framework based on an effective Hamiltonian that can be solved for any many-body system using standard numerical tools. We illustrate our approach by applying it to the Hubbard dimer and show that it is exact both at 1/4 and 1/2 filling.

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Sujets

Interactions de photons avec des systèmes libres Monte-Carlo Laser Numbers 3360+q Agregats 3115ee Instability Ionization mechanisms Coulomb presssure Fonction de Green Agrégats Collision frequency Hubbard model Photon interactions with free systems Density-functional theory Molecules Méchanismes d'ionisation 3640Cg Molecular irradiation Irradiation moléculaire Hierarchical method Fission Approximation GW GW approximation Clusters Electron correlation Explosion coulombienne Neutronique Modèle de Hubbard Environment Chaos Damping Effets dissipatifs Deposition dynamics Nickel oxide Metal clusters Extended time-dependent Hartree-Fock Inverse bremsstrahlung collisions Théorie de la fonctionnelle de la densité Molecular dynamics CAO Diffusion Instabilité MBPT FOS Physical sciences Density Functional Theory Corrélation forte Méthodes des fonctions de Green 3620Kd Electronic properties of metal clusters and organic molecules Activation neutronique Nuclear Collisional time-dependent Hartree-Fock Embedded metal cluster Hierarchical model Electric field Electron-surface collision Mean-field Electronic excitation Corrélations Deposition Angle-resolved photoelectron spectroscopy Neutronic Green's function Nucléaire High intensity lasers Dynamics TDDFT Metal cluster Au-delà du champ moyen Ar environment Photo-electron distributions Semiclassic Champ-moyen Dynamique moléculaire Aggregates Neutron Induced Activation Dissipative effects Lasers intenses Coulomb explosion Landau damping Electronic properties of sodium and carbon clusters Nanoplasma Atom laser Electron emission Méthode multiréférence Oxyde de nickel Photo-Electron Spectrum Matrice densité Relaxation Optical response Matel clusters Time-dependent density-functional theory Dissipation Electronic emission Correction d'auto-interaction Corrélations dynamiques Energy spectrum Greens function methods Multirefence methods

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