Electron Properties Of Novel Layered Iron-based Superconductor

This dissertation focus on the electronic properties of new layer iron-based super-conductor using the first principles electronic structure calculation, so as to elucidate the pairing origin of iron-based superconductivity. We research three different types of iron-based superconductor:112-type bulk CaFeAs₂,1111-type CaFeAsH and FeSe monolayer film grown on SrTiO₃ substrate.This dissertation is divided into five chapters as follows:1. Introduction The major part of Introduction includes a brief overview on the concepts of macro-scope and microscope superconducting phenomenon in solid state materials, es-pecially BCS theory. Then charge density wave and spin density wave which are intimately related to FeAs superconductivity is briefly introduced. Finally, I out-line the current research status of iron-based superconductor which is the main topic of this thesis.2. Density Functional Theory This chapter introduces the density functional theory, including the framework of density functional theory, related theorems and important equations, historical development of the local density functional approximation, as well as its current research status and application.3. Magnetism and Electronic Structures of novel layered CaFeAs₂ and Ca_0.75（Pr/La）_0.25FeAs₂ The magnetic and electronic properties of the parent material CaFeAs₂ of new superconductors are investigated using first-principles calculations. We predict that the ground state of CaFeAs₂ is a spin-density-wave （SDW）-type striped an-tiferromagnet driven by Fermi surface nesting. The magnetic moment around each Fe atom is about 2.1 μB. We also present electronic and magnetic struc-tures of electron-doped phase Ca_0.75（Pr/La）_0.25FeAs₂, the SDW order was sup-pressed by La/Pr substitution. The As in arsenic layers is negative monovalent and acts as blocking layers enhancing two-dimensional character by increasing the spacing distance between the FeAs layers. This favors strong antiferromag- netic fluctuations mediated pairing, implying higher Tc in Ca_0.75（Pr/La）_0.25FeAs₂ than Ca_0.75（Pr/La）_0.25Fe2As₂.4. Role of hydrogen in the electronic properties of CaFeAsH-based superconductors The electronic and magnetic properties of the hydride superconductor CaFeAsH, which superconducts up to 47 K when electron doped with La, and the isova-lent alloy system CaFeAsH1xFx are investigated using density functional based methods. The Q= （π,π,0） peak of the nesting function ζ（q） is found to be extremely strong and sharp, and the additional structure in ζ（q） associated with the near-circular Fermi surfaces （FSs） that may impact low energy excitations is quantified. The unusual band introduced by H, which shows strong dispersion perpendicular to the FeAs layers, is shown to be connected to a peculiar vanHove singularity just below the Fermi level. This band provides a three-dimensional electron ellipsoid Fermi surface not present in other Fe-based superconducting materials nor in CaFeAsF. Electron doping by 25% substitution of La or Co has a minor effect on this ellipsoid Fermi surface, but suppresses FS nesting strongly, consistent with the viewpoint that eliminating strong nesting and the associated magnetic order allows high Tc superconductivity to emerge. Various aspects of the isovalent alloy system CaFeAsH1xFx are discussed in terms of influence of incipient bands.5. Electronic coupling between the FeSe monolayer film and SrTiO₃ substrate Several groups have reported superconductivity in single unit cell layers of FeSe on SrTiO₃ and a few other substrates, with critical temperature Tc as high as 65-100 K. Since the Fermi surfaces reported by angle-resolved photoemission spec-troscopy do not have any zone-centered hole pockets, a general supposition is that electron doping has occurred, with possible origins including oxygen vacan-cies, iron deficiency or selenium deficiency, or substrate reconfiguration such as double TiO2 terminating layers. Since the level of doping is of the order of 5% oxygen vacancies, there may be substantial regions where nearly stoichiometric FeSe-SrTiO₃ interfaces persist. We examine more closely the interaction across this interface, finding that the Fermi level of FeSe is pinned by an O px, py sur-face band. To address the observation of replica bands in ARPES studies that seem to be due to emission of substrate optic modes, the influence of zone cen-ter optic modes on the FeSe layer are assessed. We find in the z-polarized LO mode,The Fermi level is pinned in the Fe d bands, which therefore are affected rather modestly. The STO bands are shifted substantially by the internal electric field. The interface Til band remains unoccupied at x=0.10, as it provides the Schottky barrier of about 0.6 eV. Also, since alloying on the Se sublattice is one means of perturbing the system relatively mildly, the effects of substitution of Se and O with S, Te, and N are studied.