Angle-resolved Photoemission Spectroscopy Study On The Electronic Structure Of Iron-based Superconductors And MoS₂

Iron-based superconductors,as second-class high-temperature superconductors,have been studied extensively since its discovery.In spite of a lot of theoretical and ex-perimental efforts,the underlying superconductivity mechanism remains unclear.The transition metal dichalcogenides(TMDs)have attracted much attention due to their excellent physical and chemical properties and application prospects.Angle-resolved photoemission spectroscopy(ARPES),as a unique tool to directly probe the energy,momentum and spin of electrons in solid materials,has played a key role in studying iron-based superconductors and TMDs.In this thesis,by carrying out ARPES measure-ments,the electronic structure of the iron-based superconductor(Ba_0.6K_0.4)Fe₂As₂and Mo S₂are studied in detail.The results are summarized as follows:1.Briefly introduce the properties,applications and development history of super-conductors.The crystal structure,phase diagram,electronic structure,superconducting gap and superconductivity mechanism of iron-based superconductors are reviewed.The crystal structure and electronic structure of TMDs are briefly introduced.2.The basic principles and instrumentation of ARPES are discussed in detail.The vacuum ultraviolet laser--based ARPES,spin--resolved ARPES,time-of-flight ARPES and large--momentum ultra--low--temperature ARPES systems in our laboratory are briefly introduced.3.High--resolution ARPES measurements with both Helium lamp and laser light sources are carried out on the optimally--doped(Ba_0.6K_0.4)Fe₂As₂to resolve the detailed electronic structure and superconducting gap structure around?and M.Our ARPES experimental results show:(1).By using Helium lamp and laser,we observe a clear”flat band”feature around the?point in the superconducting state.Our results indicate that this flat band originates from the combined effect of both the superconductivity--induced back--bending of the ? band and the folding of the ? band from M to?.(2).Direct evidence of the band folding between M and?is observed in both the normal and superconducting state.Our resolution of the origin of the flat band around the?point makes it possible to assign the three hole-like bands around?and determine their superconducting gap properly.(3).Near the M point,we observe a tiny electron--like band ? and an M--shaped ? band simultaneously in normal and superconducting states.This makes it possible to correctly determine the superconducting gap around the M point.The obtained superconducting gap for the bands around the M point(?5.5 me V)is significantly different from the previous measurements.Our results resolve a num-ber of prominent controversies concerning the electronic structure and superconduct-ing gap structure in the prototypical iron-based superconductor,the optimally-doped(Ba_0.6K_0.4)Fe₂As₂.They provide important evidence in examining and establishing theories in understanding superconductivity mechanism in iron-based superconductors.4.We discover that the band folding exists commonly in the Fe As-based super-conductors and reveal its origin.(1).We have carried out high-resolution ARPES measurements on Ca KFe₄As₄,KCa₂Fe₄As₄F₂and(Ba_0.6K_0.4)Fe₂As₂superconductors.From the measured Fermi surface and band structures,we have found clear evidence of band folding between the?and M points with a(?,?)wave vector in all the three kinds of superconductors.(2).Our STM measurements on CaKFe₄As₄reveal dom-inant2^1/2×2^1/2 surface reconstruction on the cleaved surface.We propose that such2^1/2×2^1/2 reconstruction,which is also commonly observed in other Fe As--based su-perconductors,provides a general scenario to understand the origin of the(?,?)band folding.These results provide new insights in understanding the electronic structure and superconductivity mechanism in iron-based superconductors.5.The evolution of the electronic structure and superconducting gap of(Ba_0.6K_0.4)Fe₂As₂with potassium doping is studied and the superconducting gap after potassium doping is significantly increased.(1).With potassium doping,the hole-like pockets around?turn smaller while the electron-like pockets around M turn larger.(2).The superconducting gap is increased significantly after potassium doping.These results indicate that the superconducting transition temperature of(Ba_0.6K_0.4)Fe₂As₂may be improved significantly after potassium doping,which needs further experiments to pin down.6.We investigate the band structure evolution of Mo S₂with potassium doping and discover the transition from semiconductor to metal,and finally to insulator.(1).With potassium doping,the transition from semiconductor to metal is observed.The Fermi surface formed by the conduction band becoming larger with the carrier density increasing,and a new band appears below the conduction band.(2).The spectra weight around the Fermi level starts vanishing with continuous potassium doping,resulting in the transition from metal to insulator.