VO₂Metal-insulator Phase Transition Studied By First-principles Calculations

The first-principles calculations, without any experience parameters, could analyze the origin of the materials'properties on the electronic level and explain the experimental phenomenon. So in recent decades, researchers pay more and more attention on the first-principles calculations during the material research. In this dissertation, using the first-principles calculations within the framework of density-functional theory and density-functional perturbation theory, we investigated the electronic structure and the phonon dispersion relationship of vanadium dioxide (VO2) and boron carbide (B4C). Based on the calculated results, we confirm that the structure phase transition of both VO2and B4C are triggered by the electron-phonon coupling.The arrangement of this dissertation is as following. In the first chapter, I have given an introduction to the current research condition of VO2and B4C. The second and the third chapters are, respectively, mainly introduce the theoretical approach of the first-principles calculation and the basic physics that will be used in analyzing the calculated results.In the first section of the fourth chapter, we analyzed the calculated results of VO2. The calculated phonon dispersion curves exhibit soft modes which presented in the acoustic branch along the high-symmetry directions ΓM and ΓZ. Analysis of the electronic band structure and Fermi surfaces shown that were Kohn anomalies. The results of this study emphasize the importance of electron-phonon interaction is necessary to trigger the metal-insulator phase transition.In the second section of the fourth chapter, we analyzed the calculated results of B4C. The bending of the C-B-C linear chain was observed with the increase of the unhydrostatic pressure along c axis, which indicated that B4C has had a structure phase transition. In order to seek the origin of the driving forces behind such phase transition, the phonon dispersion relation was carefully analyzed. The phonon dispersion curve exhibits obvious anomalous dips which present in the acoustic branch along the high-symmetry direction FL and ΓZ. Analysis of the energy band and Fermi surface implied such anomalies are Kohn anomaly and this structure phase transition was triggered by the Fermi surface nesting and electron-phonon coupling.