ARPES Study Of Topological Materials SrSn₂As₂,Eu Cd₂As₂ And Iron-Pnictide Superconductor Ba₂Ti₂Fe₂As₄O

The Angular-Resolved Photoemission Spectroscopy?ARPES?is by far the most direct tool for studying electronic structure of solids.The ARPES based on synchrotron radiation has extended photon energy range and more investigating capacity,thus having unique advantages in studying the novel properties of the electronic structure of superconductors as well as the emerging topological materials that are currently the frontier fields in condensed matter physics.In this thesis,we mainly study the following three research topics using synchrotron radiation ARPES and the first-principle calculation based on the density functional theory:Firstly,we discovered that SrSn₂As₂ is a new material with strong topological insulating properties.Our theoretical calculations show that this material is near the critical point of topological phase transition between strong topological insulator,semimetal,and normal insulator.We observed experimentally both the bulk and the surface states,and found the distinct band inversion characteristic in the bulk bands that proves the topologically non-trivial property.This discovery provides a new platform for studying topological quantum phase transition.Secondly,our ARPES results and calculations show that the magnetic material EuCd₂As₂ is topologically non-trivial,which has a linear dispersive Dirac-cone-like band structure near the Fermi energy.This band hybrids with the flat bands formed by the magnetic localized Eu 4f electrons.Our theoretical calculations suggest that in the out-of-plane arranged spins pattern,the Dirac fermion can emerge with time-reversal symmetry breaking below the anti-ferromagnetic?AF?transition temperature?9.5 K?,while in the in-plane arranged pattern,an AF topological insulator state appears instead.Although there is still no difinitive report about the existence of Dirac fermions in the time-reversal symmetry breaking situation,our findings indicate the possibility for finding Dirac fermions in this kind of materials.In addition,this material is an ideal system for investigating the correlation between RKKY interactions and the Kondo effect with Dirac fermions.Finally,we studied the iron-based superconductor Ba₂Ti₂Fe₂As₄O by ARPES.We clearly observed the polaronic states band from the Ti₂O layer appearing at temperature below 120 K.In an earlier study on this material,an abnormal resistance plateau was observed near the temperature 125 K,however,the physical mechanism of this plateau was not understood.Our experiments successfully explain the cause of this abnormal resistance behviour.Additionally,the sample becomes superconducting when temperature decreases to below 21 K,which originates from the FeAs layer.Due to the proximity effect,all Ti₂O layers also open energy gaps on the associated peaks of the polaronic states.Our experiments show that superconducting quasiparticles and polarons can couple with each other,which may play a role in promoting the superconductivity.This material provides a valuable platform for studying the electron-phonon interaction in iron-based superconductors.