Abstract
By using first-principles calculations based on density functional theory (DFT), we study the electronic structures of several novel materials such as the multiferroic material TbMn2O5, the iron-base superconductor Sr2VO3FeAs, the newly found rhenium-based hexagonal bronze superconductor HgxReO3, as well as the quantum spin Hall material Silicene. For the multiferroic material TbMn2O5, previous calculations based on the generalized-gradient approximation (GGA) gives an electric polarization value two orders of magnitude larger than the experimental data. Taking the on-site Hubbard U (GGA+U) into consideration, we successfully correct the over-binding problem in GGA and gives a much smaller polarization close to the experimental value. The close relations with the charge orbital ordering are also analyzed in detail. On the other hand, among the whole families of the iron-base superconductors, Sr2VO3FeAs has the most complex structure with a unique character that it is a superconductor by itself without any chemical doping. It is of high interests to investigate why the mother compound is already a superconductor whereas all the other Fe-based superconductors require suitable doping. Our GGA calculations demonstrate that the V ions of the VO layers in between the FeAs layers play the role of an electron donor to the FeAs layers, forming the self-doping mechanism. We also found that the VO layers exhibit half-metallic behaviour adjacent to the superconducting FeAs layers. As for HgxReO3, the crystal structure shows a 3D ReO6 network with 1D Hg chains along the c-axis. It was found superconducting in 2011. By studying the electronic structures, we found a spin polarized 2D electron gas (SP-2DEG) on the ReO plan. It is therefore very interesting that we found a SP-2DEG in a 3D+1D structure. This also gives an important indication that a 2DEG can exist in a system without any 2D structure. Finally the silicene, which is a Si version of graphene, has been a very attractive material in the passed few years. A large number of theoretical and experimental groups are studying this material. By applying an out-of-plane electric field, we found the spin degeneracy of the Dirac cone can be lifted. As long as the electric field strength increases, the Dirac cone splits itself into spin up and spin down bands with opposite directions in the occupied and unoccupied states. Consequently the small spin-orbit gap at the Fermi level is closed by fully spin polarized bands, resulting in a 100% spin polarized half-metallic behaviour. The tunable character in both spin-polarization and gap by the electric field would make silicene a very high potential material in spintronics.