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Ionic irradiation of materials : real-time dynamics of electronic excitations

Abstract : Ionic irradiation damage in condensed matter is central to many technological applications: materials in nuclear plants of course, but also electronics and solar panels in space that are subjected to the cosmic irradiation, living matter treated by radiotherapy to eliminate tumors, etc. For all these subjects, an accurate knowledge of the interaction between the irradiating projectile and the target is crucial. The interaction between the irradiating ion and the target material can be described by a stopping power, defined as the energy transfer from projectile to material per penetration distance. The most important ionic energy loss channels in the irradiation process are the electronic excitations. Therefore, the electronic stopping power is the central quantity in this field. With the advent of time-dependent density-functional theory (TDDFT), it is nowadays possible to provide a complete and realistic quantum-mechanical description of the phenomenon.In this thesis, we have developed a fully ab initio real-time TDDFT (RT-TDDFT) approach in the localized Gaussian basis. This implementation has several appealing advantages, such as the cheap account of core electrons, the ease of using the modern hybrid functionals, the flexibility of the basis set and overall low computational cost. With our tool, we explored the bulk limit, the validity of the projectile impact parameter averaging to obtain the experimental random electronic stopping power. We have proven the importance of core electron excitations in the ionic irradiations. A great care wasalso taken about the Gaussian basis set convergence: the extrapolation of the stopping power based on standard basis sets and the basis set generation scheme were proposed.Finally, we have computed the random electronic stopping power in lithium and aluminum targets for three types of projectiles: protons, antiprotons, and alpha-particles. We have compared our results directly to the experiment as well as to the empirical code SRIM, which is a widely-used database of stopping powers and a de facto standard for experimentalists. The agreement with SRIM is good when the SRIM database contains enough experimental points, whereas we show that the SRIM extrapolation can be hazardous when the underlying experimental data points are too few. Concerning the antiproton irradiation, our RT-TDDFT calculations show that the antiproton stopping power is lower than the proton one, which is in agreement with the general experimental observation (the so-called Barks effect). This effect is out of reach of simpler theories, such as the linear response approximation.
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Ivan Maliyov. Ionic irradiation of materials : real-time dynamics of electronic excitations. Computational Physics [physics.comp-ph]. Université Paris-Saclay, 2019. English. ⟨NNT : 2019SACLS434⟩. ⟨tel-02495699⟩



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