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 Title: Improvement of the dielectric screening model in the Bethe-Salpeter equation approach: From static screening approximation to dynamical screening model Author(s): Zhang, Xiao Director of Research: Schleife, Andre Doctoral Committee Chair(s): Ertekin, Elif Doctoral Committee Member(s): Shoemaker, Daniel Philip; Johnson, Harley T. Department / Program: Mechanical Sci & Engineering Discipline: Mechanical Engineering Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Optical properties Bethe-Salpeter Equation Dielectric screening Abstract: Studying optical properties from first-principles, based on density functional theory and advanced many-body perturbation theory, provide accurate estimation of the dielectric response function of materials. While the underlying ground state DFT approach provides good estimation of structural properties, and reasonable estimation of electronic properties, the electronic excited state, specifically in the context of this thesis, the optical excitation process, which involves interactions between photon, an electron and a hole, is usually described by the Bethe-Salpeter equation (BSE) approach. Rapid advances of computational resources have allowed wide applications of the BSE approach to study different material systems and as well, have allowed the consideration of more accurate physical models. In this thesis, we first show that by applying the state-of-the-arts DFT and BSE approaches, experimentally found phenomena and materials can be studied thoroughly from first principle perspective. We show three examples: electronic properties of ZnO surfaces; structural, electronic and optical properties of experimentally synthesized new material K$_2$Sn$_3$O$_7$; and a study of convex-hull, electronic structures, and optical properties of a family of K/Na-Zn-S/Se materials. In all these studies, we show the power of the state-of-the-arts electronic structure approaches in understanding experimental observations. More importantly in the framework of this thesis research, we highlight that application of the state-of-the-arts BSE approach on these materials allow us to predict the application of new materials found in experiment, and stimulate more experimental measurements to follow. Further, we show that the state-of-the-arts approach of accurate theoretical spectroscopy, the BSE approach, have its limit in studying polar materials and materials with large exciton-binding energies due to the lack of dynamical description of the screening of electron-hole interactions, and we demonstrate our effort in developing the methodology to include dynamical screening effect in the approach. In this thesis work, we studied in detail the dynamical nature of the dielectric screening between electron-hole interaction in the BSE approach, and implemented the approach into our current BSE code. We show that this dynamical electronic screening effect is very strong in large exciton-binding energy material, and needs to be taken into account. Our work also lays the foundation of further studies on the correct model of incorporating dynamical lattice screening to study optical properties of polar materials. Issue Date: 2019-11-22 Type: Text URI: http://hdl.handle.net/2142/106443 Rights Information: Copyright 2019 Xiao Zhang Date Available in IDEALS: 2020-03-02 Date Deposited: 2019-12
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