Superconductivity And Magnectism In The Checkboard Models Of Iron-Pnictides

Since the iron-based superconductors (SCs) were discovered three years ago, the relation between the new superconductors and the high-Tc cuprates has been a central focus of researches. It is highly debated that whether the new superconductors belong to the same category of strongly correlated electron systems in which the cuprates are believed to be. Models based on both strong coupling and weak coupling approaches have been applied to understand the properties of the new materials and their relation to the curpates.In this thesis, we present results for three different tight-binding models for iron-SCs which are constructed using two, three, and four orbitals, respectively, on a checkerboard lattice. Based on the knowledge of exactly solvable four-site problems, we show the phase diagrams of the models in the limit of weakly coupled plaquettes as a function of intra-orbital onsite interaction U, inter-orbital onsite interaction U', Hund's coupling and onsite inter-orbital pairing hopping JH, as well as the next-nearest-neighbour antiferromagnetism (NNN-AFM) coupling J2 which can be generated by super exchange mechanism mediated by As atoms. In all of the models, the leading SC phase is always from the A1g s-wave pairing channel in reasonable parameter regions. The superconductivity is intimately correlated with the development of the NNN-AFM and becomes stronger as J2 increases. Most remarkably, this study also shows that the superconductivity and magnetism are orbital-selective in the three- and four-orbital models:First, in the three-orbital model, the AFM is more pronounced in the dxy, orbital while the superconductivity is more pronounced in the dxz and dyz orbitals. Second, in both three-and four-orbital models, the signs of the intra-orbital pairing order parameters of the dxy and the dxz (or dyz) orbitals are opposite. The sign difference stems from the intrinsic symmetry of the inter-orbital hopping amplitudes between dxy and dxz(yz) This feature could result in the sign-change of the SC order parameters between the hole Fermi pockets at theΓpoint and produce anisotropic or gapless SC gaps in the electron pockets around M point in reciprocal space in the homogeneous limit.