Angle Resolved Photoemission Spectroscopy Study On Iron-based Superconductors KCa₂Fe₄As₄F₂ And Ba_1-xK_xFe₂As₂

The discovery of high-temperature superconductors has opened up a new field of great interest.As the second major class of high-temperature superconductors,iron-based superconductors still have many outstanding questions despite long research.The purpose of this paper is to present my research in two systems of iron-based supercon-ductors,KCa₂Fe₄As₄F₂（12442 system）and Ba_1-xK_xFe₂As₂（122 system）,and to con-duct an increasingly deeper exploration of its superconductivity mechanism.This paper is divided into the following sections:1.A brief introduction to the development of superconductivity is given so that readers unfamiliar with this field can understand the main contents of this field as soon as possible.And basic information about iron-based superconductivity is introduced in more detail,leading to the main object of study in this paper.2.The main research method we used,angle-resolved photoelectron spectroscopy（ARPES）,is introduced in detail.This chapter starts from the principle of ARPES,in-troduces the angular-resolved photoelectron spectroscopy technique step by step,and describes the composition of the equipment.After that,it introduces four ARPES de-vices with different functions and characteristics put into use in our laboratory,all of which have played a crucial role in my research.3.Introduction to the growth of Ba_1-xK_xFe₂As₂single crystals.Using the self-solvent method,we have grown a series of high-quality Ba_1-xK_xFe₂As₂single crystal samples and characterized them well,which provide the basis for the later studies.4.A detailed study of the electronic structure of the 12442 system was carried out.As the first”bilayer”structure in iron-based superconductors,its electronic structure is of great interest.We found the first bilayer splitting in Fe-based superconductors;further study of the energy gap revealed that the energy gap in the Fermi surface of the bilayer splitting contains information about the interlayer pairing of Fe As within the bilayer.Through analysis,we find that the interlayer pairing strength is greater than that of other systems in Fe-based superconductors,which may be due to the role of two-dimensionality.Further,we present the study of electron doping and investigate the electron-boson interactions in this system.5.The study of the overdoped Ba_1-xK_xFe₂As₂system is presented in detail.The overdoped Ba_1-xK_xFe₂As₂exhibits a very peculiar evolution,especially the change of its energy gap symmetry,which may change from nodeless to nodal,the mechanism behind which is still unclear;there may even be a time-reversal symmetry breaking in this change region.We have grown a large number of high quality single crystals using the growth method introduced earlier,and have systematically analyzed the evolution of their Fermi surfaces,band structures and energy gaps.6.The electronic structure and energy gap structure studies of KFe₂As₂are pre-sented in detail.We obtained beautiful results of the electronic structure of KFe₂As₂using high quality samples with ultra-high precision energy resolution（better than 0.5me V）and ultra-low temperature environment（below 0.8 K）.We found some discrep-ancies between its Fermi surface structure and the previously thought look,mainly in theband splitting and the strong folding of the energy band.The latter provides us with a perfect opportunity to study its complete gap structure.We have studied its gap structure in detail and found a large anisotropy in some Fermi surfaces and pointed out the possible gap node locations.For theband,its clearly opened energy gap and shal-low band top position imply a potential BCS-BEC cross-over phenomenon.This study is important for understanding the mechanism of iron-based superconductors.