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Theoretical description of non-linear process in magnetic materials

Abstract : Second harmonic generation (SHG) is a process in which two photons of energy ω are absorbed by a material and a photon of energy 2ω is emitted. This process is theoretically described by the second order macroscopic susceptibility χ((2). This spectroscopy is used to study the optical properties of materials and it reveals additional information which cannot be accessed with linear optical spectroscopies. Indeed, as the dipolar selection rules prohibit SHG in centro-symmetric materials, it is possible to obtain a structural and electronic characterization of complex systems. In particular, the absence of time inversion symmetry, due to a magnetic order, reveals new contributions in second harmonic generation. For the specific case of antiferromagnetic materials, magnetic symmetry determines the polarization of the material and SHG then reveals the arrangement of spins in the solid. It was shown that it can be used to study ultrafast processes in magnetic materials, such as demagnetization.There are few satisfactory ab initio theoretical descriptions for nonlinear processes in magnetic materials. These theoretical approaches must be able to treat the electron-electron interactions, the effects of local fields (reflecting the microscopic inhomogeneities in the material) and the spin distribution of the electrons on the same footing.The aim of my thesis was to numerically calculate the optical, linear and second order responses for antiferromagnetic materials. I calculated these two answers for a chromium oxide (Cr2O3) as part of an ab-initio formalism, based on TDDFT (Time-Dependent Density Functional). In this approach, the spin distribution was taken into account explicitly and this extension was implemented in the 2light code.The electron-electron interaction is described mathematically by the exchange -correlation kernel fxc which must be approximated. Finding a good approximation for fxc is extremely important because the modeling and the interpretation of experiments is fundamentally based on these approximations. In particular, fxc must be able to describe the excitonic effects (electron-hole interactions) in the optical response. I studied the influence of these approximate kernel on the optical properties of Cr2O3. In the case of absorption spectra, I compared my results to the spectra calculated from the Bethe-Salpeter equation which explicitly takes into account the excitonic effects and I was thus able to demonstrate the presence of a strongly bound exciton.I was finally interested in different possible structures for Cr2O3, differing from each other only in the spin distribution, and I showed that my results can discriminate between these structures.
Keywords : SHG Td-Dft Cr2O3
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Submitted on : Monday, May 9, 2022 - 12:54:28 PM
Last modification on : Wednesday, May 11, 2022 - 3:36:22 AM
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Shalu Rani. Theoretical description of non-linear process in magnetic materials. Physics [physics]. Institut Polytechnique de Paris, 2021. English. ⟨NNT : 2021IPPAX128⟩. ⟨tel-03662475⟩

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