First Principles Study Of High Tciron-based Superconductors

The iron-based high Tc superconductors discovered in2008have attractedpeople’s much attention due to their intrinsic properties. A lot of important discoveriesin iron-based superconductors have been reported on famous journals such as Scienceand Nature. Compared to the high Tc cuprates, the iron-based superconductors havemore advantages, for example, the iron-based superconductors are easily tractablemetals while the high Tc cuprates are intractable ceramics; superconductivity iniron-based materials is isotropic while anisotropic in high Tc ceramics. Thesesuperiorities imply possible wide applications of iron-based high Tc superconductorsin future.The iron-based high Tc superconductors provide a new platform to study themechanism of superconductivity. The parent compounds of high Tc cuprates areantiferromagnetic insulators while those of high Tc iron-based superconductors areantiferromagnetic metals. From the phase diagrams of various iron-basedsuperconductors one can see the superconducting phase appears when theantiferromagnetic order is suppressed by application of pressure or doping, sounderstanding the origin of magnetism in the parent compounds is an essential step touncover the mechanism of superconductivity. But intensive debate persists since thediscovery of iron-based superconductors, on whether the magnetism is originatedfrom itinerant electrons or localized spin. Therefore establishing a unified theory tounderstand the magnetism and superconductivity in iron-based superconductors is inurgent need.In this dissertation, we do a systematic study on the electronic structures,magnetism and properties related with superconductivity of various iron-basedsuperconductors by first principles calculation, tight-binding model and multi-bandHubbard model calculations on the Hartree-Fock level and our work providereasonable explanations for some controversies existing in this area. The dissertationis divided into seven chapters. In the first chapter, we introduce the crystallographicstructures of iron-based superconductors as well as their magnetic properties and theprogress of iron-based superconductors. In the second chapter, we give a briefdescription of density functional theory and the methods of first principles calculation. In the third chapter, we discuss the itinerant origin of the bicollinear magnetic order in FeTe. In the fourth chapter, we uncover the origin of the contrasting low temperature physical properties of the111iron-based materials. In the fifth chapter, we further explore the important roles of the particle-hole excitation away from Fermi level played on magnetism and superconductivity in iron-based superconductors. In the sixth chapter, we discuss the effect of charge transferring among the3d orbitals of iron. In the seventh chapter, we summarized the research contents in this dissertation and point out some problems remaining to be solved as well as the further research directions. The main contents of research work in the dissertation are listed as follows:(1)We for the first time point out that the bicollinear antiferromagnetic state of FeTe can be understood from the itinerant electron picture. Our results show the excess magnetic Fe ions in FeTe stabilize the bicollinear state:the excess Fe contributes electrons to the in-plane Fe, which lift the Fermi level up and lead to strong magnetic instabilities at wave vector (π,0)/(0, π) in dxz/dyz orbitals that are responsible for the bicollinear antiferromagnetic state. Magnetic exchange coupling between excess magnetic Fe ions and in-plane Fe further stabilizes the bicollinear antiferromagnetic order.(2) We show the contrasting low-temperature behaviors observed experimentally among isostructural and isoelectronic materials, like nonsuperconducting and nonmagnetic MgFeGe, magnetically ordered NaFeAs, and superconducting LiFeAs, can be well understood from itinerant electron picture. The strong (π,π) instability appearing in the dx2-y2orbital of NaFeAs is responsible for the occurrence of weak magnetism, while weaker but still prominent (π,π) instability in LiFeAs leads to a superconducting state. In contrast, multiple competing instabilities coexisting in orbital-resolved momentum-dependent susceptibilities, serving as magnetic frustrations from itinerant electrons, may account for the nonmagnetic state in MgFeGe, while poorer Fermi surface nesting leads to a nonsuperconducting state. Based on above findings, we predict a possible way to make MgFeGe a new Fe-based superconductor.(3) LaOFeP is in proximity to an antiferromagnetic quantum critical point due to the competing condensations of intra-orbital contributions to the particle-hole excitations from different orbitals, while further coupling of itinerant d electrons of Fe atoms to the strongly localized f electrons of Ce atoms leads to a heavy Fermion system CeOFeP.(4) We propose that condensations of the particle-hole excitations away from Fermi surface in the momentum space, rather than either the weak coupling theory of Fermi surface nesting or the strong coupling theory of local spin fluctuations, dominate the magnetism and superconductivity in iron-based materials.(5) We emphasize that the inter-layer couplings ignored in most cases are responsible for the larger magnetic moments in AeFe2As2with Ae=Ca, Sr, Ba compared to LaOFeAs. It is found that, though all the iron-based superconductors are the layered materials, the band structures of AeFe2As2compounds are more three dimensional than that of LaOFeAs. As is well-known, the lower the dimensionality, the higher the fluctuations which suppress the ordered state.(6) It is found that the condensation of particle-hole excitations in dx2-y2orbital at q=(π,π) vanishes in BaFe2P2compared with the superconducting LiFeP. Only weak condensations in dxz/dyz orbitals remain at q=(π,π), which may not be enough to support the appearance of either superconductivity or the magnetism in BaFe2P2. Based on the above scenario, we predict that K-doped BaFe2P2can be a new candidate for iron-based superconductor.(7)We unveil the relationship between charge transferring among the3d orbitals of Fe and the electronic structure of iron-based superconductors. While there are always some inconsistencies between the Fermi surface obtained by LDA and ARPES in iron-based superconductors, it is found once the effect of charge transferring among the3d orbitals of Fe is properly taken into account by LDA+U, the Fermi surfaces of BaFe2As2, KFe2As2become consistent with LDA+DMFT.These systematic theoretical studies uncover the relationship between itinerant electrons and magnetic and superconducting properties in iron-based superconductors. It is found the various properties of iron-based superconductors can be explained from the itinerant scenario. It can help us understand the complex observations in experiments more clearly. In addition, our work also provides theoretical foundation for searching for new iron-based superconductors.