Orderly Iron-based Superconductors In Magnetic And Superconducting Pairing Symmetry Of Angle Resolution Photoelectron Spectroscopy Study

Superconductivity is the most interesting and complex phenomenon in condensed matter physics. After the discovery of superconductivity, intensive studies are focused on understanding its mechanism and increasing the superconducting transition temperature (Tc). The discovery of iron-based superconductor in2008not only ignited another wave for searching new superconductors but also raised great challenges. The highest Tc of iron-based superconductors reaches to about56K so far, indicating unconventional high-Tc superconductivity. Like the cuprates, the parent compounds of iron-based superconductors are intimate related to magnetic orders. With doping, the magnetic order is suppressed and the superconductivity emerges. Therefore, the understanding of the superconductivity in iron-based superconductors could be also helpful for unveiling the long-standing mystery in cupretes.Furthermore, the iron-based superconductors exhibit many distinct behaviors. For example, the parent compounds of iron-based superconductors are metal, which is different from the Mott insulators in cuprates. The magnetic transition is always accompanied by a structural transition, whose transition temperature either coincides or precedes the magnetic transition temperature. The C4rotational symmetry of the system is breaking through this structural transition. For the superconductivity, besides the carrier doping, the chemical pressure or physical pressure could also induce superconductivity by changing the lattice parameters. Therefore, in order to understand these novel phenomena in iron-based superconductors, one crucial step is to understand its electronic structure. Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique to direct probe the electronic structure of materials. In the thesis, I report the systematic ARPES studies on the electronic structure of iron-based superconductors. The main results are shown as follows:1. Distinct characters of the electronic structure in iron-based superconductors. We have measured the electronic structure of almost all the compounds in iron-based superconductors. We found that the Fermi surface and band structure exhibit multi-band and multi-orbital behaviors. The dominating effect of carrier doping on the electronic structure is the Fermi energy shift, while the chemical pressure changes the Fermi surface, orbital character, dimensionality, and correlations in many distinct ways. Our results provide solid experimental foundations for further studies on iron-based superconductors.2. Magnetic and structural transitions in the parent compounds of iron-based superconductors. We first studied the nematicity in NaFeAs with polarization-dependent ARPES. A uniaxial pressure was applied on the sample to overcome the twinning effect in the low temperature C2-symmetric state and obtain a much simpler electronic structure than that of a twinned sample. We found the electronic structure undergoes an orbital-dependent reconstruction in the nematic state, primarily involving the dxy and dyz dominated bands. These findings suggest that the spin fluctuations at high temperatures and their coupling with the orbital degree of freedom could be the dominant force to drive the structural and magnetic transitions here. However, for the iron-chalcogenides, we found that the magnetic transition in Fe106Te is characterized by a massive spectral weight transfer over an energy range as large as the bandwidth. Our observation demonstrates that Fe1.06Te distinguishes itself from other iron-based systems with more local characters and much stronger interactions, and how a magnetic order is formed in the presence of strong correlation.3. Superconducting gap and pairing symmetry in iron-based superconductors. We have measured the superconducting gap of Ba0.6K0.4Fe2As2by ARPES. We found that the superconducting gap on certain Fermi surface sheets shows significant kz dependence. Moreover, the superconducting gap sizes are different at the same Fermi momentum for two bands with different spatial symmetries (one odd, one even). Our results thus reveal the three-dimensional and orbital-dependent structure of the superconducting gap in iron-based superconductors. The superconducting gap distributions in iron-based superconductors are rather diversified. One centul issue is the existence of nodelss and nodal superconducting gap. We have measured the superconducting gap structure of BaFe2(Aso.7Po.3)2, and in particular the direct observation of a circular line node on the largest hole Fermi surface around the Z point at the Brillouin zone boundary. Our findings rule out a d-wave pairing origin for the nodal gap, and establish the existence of nodes in iron-based superconductors under the s-wave pairing symmetry. In AxFe2Se2(A=K, Cs), we found AxFe2Se2(A=K, Cs) is the most heavily electron-doped among all iron-based superconductors. Large electron Fermi surfaces were observed around the zone corners, with an almost isotropic superconducting gap of10.3meV, whereas there is no hole Fermi surface near the zone centre, which demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors. Thus, the sign change in the s±pairing symmetry driven by the interband scattering as suggested in many weak coupling theories becomes conceptually irrelevant in describing the superconducting state here. A more conventional s-wave pairing is probably a better description.