First Principles Study About The Electronic Structures And Magnetism Of Fe-based Superconductor Materials

Along with the rapid development of computational technology and with the help of all kinds of numerical calculation, computational physics has become the new idea to recognize the nature people know, and plays an important role in studying natural science. Among them, the first-principles calculations based on the density functional theory have become the most powerful methods in computational physics favored by many researchers of different fields. This paper is mainly based on first-principles BSTATE ?Beijing Simulational Tool for Atom Technology? package to explore the various characteristics of two kinds of transition metal oxides.Transition metal oxides have attracted much attention of physicists due to its intriguing physical properties determined mainly by the unfilled d orbitals of the transition metal elements. These special properties have expanded the field of new physics research. The discoveries of the high temperature superconductors and giant magnetoresistance materials motivate people pay much more attention on the transition metal oxides for their great practical applications and the scientific value as well. This paper adopts the primary principle to explore and analyze the electronic structure, magnetic properties, orbital ordering of the material, specifically, this work mainly includes:?1? A brief overview of research direction on the metal oxide is firstly summarized; and then we briefly introduce the basic theory of the first-principles calculationsand the BSTATE package that we used.?2?We studied the electronic structure and magnetic properties of the studied system. It was found that an antiferromagnetic stripe phase is preferred by LiOHFeSe. The compound includes an insulating layer of LiOH and poor metal layer of FeSe. NM state includes three Fermi surface along the direction of hole type and two along the electrical Fermi surface. When moving along the vector ???=??,?,0?, hole type of Fermi surface will merge with the electron Fermi physiognomy, and the Fermi surface nesting in the un-doped compounds will induce magnetic instability and spin density wave ?SDW?. In addition, the calculated magnetic susceptibility ?Lindhard response function? can be strongly suppressed by electronic doping or holes. On the other hand, superconducting and SDW are usually two competitive states, thus electronic doping and holes will induce superconductivity. That would explain why the electronic doping in superconducting (Li_0.8Fe_0.2)OHFeSe. Because hole doping also suppresses the SDW, we then predict superconducting may also occur in some hole-doped samples, such as in Li_xOHFeSe ?x<1?.?3? Based on the first-principles calculations, we investigate the electronic and magnetic properties of the self-doping compound Ba₂Ti₂Fe₂As₄O. The Fe atoms in the stripe antiferromagnetic configure is more stable than other magnetic states. The spin moment on Fe atom is about 2.2?B, but the moment on Ti atom almost as 0 ?B. So the Fe₂As₂ layers takes major role in the anomaly transition in resistivity and magnetic susceptibility around 125K. We predicate that there will be more than 0.2 electron transfers to the the Fe₂As₂ layers in the superconductive state, and the superconductivity takes place in the Fe₂As₂ layers.