Energy Band Structures And Properties Of Electric Materials By DFT Studies

Density functional theory (DFT) is the most important theoretical method in probing and investigating functional materials, which have very important applications. Developed from DFT, the full-potential linear augmented plane-wave (FPLAPW) is the most accurate first-principle method in calculating the electronic states and gaining characteristic parameters. In this thesis, DFT and Local spin density approximation (LSDA) as well as several corrections beyond LSDA, and FPLAPW are introduced. Then we investigate the electronic and magnetic properties of the new compound MgCNi₃ and Gd₂Co₂Al by WIEN2K, which is based on FPLAPW.We study and display the ground state total energy versus volume curve and gain the equilibrium lattice parameters and bulk modulus of MgCNi₃. Optical properties have been studied and we found MgCNi₃ is isotropy, and a lot of free electrons transit between Ni 3d states. Optical absorption is contributed from the inter-band transitions from Ni 3d states and the Ni 3d to C 2s, C 2p and Ni 4s transitions. Two theoretical models are applied to study the variance of superconductivity and magnetism for doping in MgCNi₃. Substitute doping model shows that the high DOS at the Fermi level with vHs and large magnetic fluctuations near the ferromagnetic phase transition point are the important factors for the superconductivity of MgCNi₃. The electron doping moves the Fermi level to the lower DOS side, and translates MgCNi₃ to paramagnetism without superconductivity. The doping of inter-metal compounds of same valence electrons changes the shape of Fermi surface, induced from the hybridization between atoms. The drop of the DOS decreases their superconductivity. The hole doping enhances the DOS at the Fermi level on the peak of vHs, but the large magnetic exchange interactions induce the ferromagnetic order, and the superconductivity disappears abruptly. The virtual crystal approximation model indicates that the total energy and lattice constants of system gradually decrease with the increment of electron doping, but the variety trend of Fermi level and the bulk modulus is opposite. The pressure derivative of the bulk modulus shows fluctuation due to the increment of electron doping. This compound is strongly exchange...