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Theoretical study of the many-body electronic states of defects in diamond : the case of the NV center under high pressure

Abstract : The aim of this thesis is to study the influence of the pressure on the optical transitions between multi-determinant ground state and excited states of the NV center from the first-principles.In this work, I study both the neutral NV0 and negatively charged NV- centers.Long-range interactions have a crucial effect in such defects: first, elastic deformations have a long range and need to be accounted for; second, when the defect has a charge, it is important to avoid spurious charge-charge interactions between neighboring supercells caused by the use of periodic boundary conditions. Thus, I study the atomic structure of defect with large supercells by the density functional theory (DFT).The NV center is a deep-center defect, its optical and magnetic properties are related with localized levels in the electronic band-gap. These levels are believed to be built out of the localized orbitals of dangling bonds pointing towards the vacancy, providing strongly correlated electronic states. Thus, an accurate quantum mechanical treatment is needed.DFT is a powerful approach for the calculation of the ground state properties of defects. However, the single Slater determinant nature of the DFT wave function lacks the non-dynamical correlations, that characterize such defects, and does not allow for the calculation of many-body levels. Moreover, exchange and correlation (XC) functionals used in DFT add have a limited accuracy.Therefore, in this PhD work, I first develop a combined DFT + Hubbard model technique. I study the triplet-triplet transition both with the PBE XC functional and the HSE06 one. I confirm that the use of the hybrid XC functional HSE06 improves the description of correlations beyond DFT-PBE and allows for more accurate prediction of optical transitions.An exact diagonalization (or in quantum chemistry language full Configuration Interaction calculations) of the Hubbard Hamiltonian in the many-electron basis constructed of in-gap localized levels, allows to get access to multi-determinant ground and excited states. I benchmark this technique comparing it to the recent state of the art methods.Finally, I apply the developed technique in order to study the effect of the hydrostatic pressure on NV- and NV0 centers. Among many results of my work, I discovered a very interesting effect related to the singlet-singlet transition in the NV-center under hydrostatic pressure. The results I have obtained during my PhD have never been calculated nor observed experimentally. In order to validate the theoretical model, I have compared our results with the measurements that have been obtained by our experimental collaborators for the optical transition in the NV- and NV0. Last but not least, the effect of the electron-phonon interaction was discussed.As a perspective, I developed a new code that can be applied to study other defect systems of interest in the quantum technologies.
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Submitted on : Sunday, January 2, 2022 - 1:20:05 AM
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Mariya Romanova. Theoretical study of the many-body electronic states of defects in diamond : the case of the NV center under high pressure. Condensed Matter [cond-mat]. Institut Polytechnique de Paris, 2019. English. ⟨NNT : 2019IPPAX013⟩. ⟨tel-03506236⟩



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