Angle Resolved Photoemission Spectroscopy Study On Hole Doped 122 Iron-based Superconductors And Weyl Semimetal Candidate SrMnSb₂

Topological materials and iron-based superconductors are undoubtedly two fron-tier topics in condensed matter physics during the last decade.Potential applications and promotions on the development of modern condensed matter theory have stimu-lated continuous research in these fields.The study on topological materials has been mainly focused on searching for new topological phases as well as topological phase transitions.The ultimate goal of research on iron-based superconductors is to un-derstand the superconductivity mechanism of unconventional superconductors so that it can promote the discovery of superconductors with higher transition temperature even room-temperature superconductors.Angle-resolved photoemission spectroscopy(ARPES)is a powerful experimental technique that can be used to study the electronic structure of materials.It has played a vital and irreplaceable role in the research of these two kinds of materials.In this thesis,I mainly introduce my works in these two fields during my Ph.D.period.The main results are in the following,1.Brief introduction of the background about the discovery and development of iron-based superconductors and related results.2.Discuss theoretical basis and principle of ARPES and the main constituents of an ARPES system.Briefly introduce three ARPES systems in our lab:vacuum ultra-violet(VUV)laser-based ARPES system,spin-resolved ARPES system and large-momentum ultra-low-temperature laser-based(LLL)ARPES system.Mainly introduce the ARPES system based on the angle-resolved time-of-flight(ARToF)electron energy analyzer which I am responsible for testing,maintain-ing and operating during my Ph.D.period.3.Growth and characterization of high quality single crystals of Ba1-xKxFe2As2 iron-based superconductors.Using FeAs self-flux method,I have grown high quality Ba1-xKxFe2As2 single crystals and characterized them well.The avail-ability of these high quality single crystals lays a foundation for my ARPES measurements.4.Study on the superconducting gap and electronic structure in both the super-conducting and normal states of optimally-doped Ba0.6K0.4Fe2As2 single crys-tals with high resolution laser-ARPES(6.994 eV laser).The main results are:(1).There are two different superconducting gaps observed on the two hole-like Fermi surface sheets around the Brillouin zone center.The inner hole-like Fermi pocket exhibits an extremely isotropic superconducting gap with a size of?9.3meV while the outer one exhibits a superconducting gap size of?5meV.Our results are consistent with previous synchrotron-based and heli-um discharge lamp-based ARPES results,but distinct from the previous results that were also performed using 6.994eV laser-ARPES.Our work solves a long-standing controversy on the superconducting gap structure of the optimally-doped Ba0.6K0.4Fe2As2 superconductor.(2).We found that Ba0.6K0.4Fe2As2 exhibits extremely sharp superconducting coherence peaks along the two hole-like Fermi surface sheets in the superconducting state.But there are no well-defined quasiparticles existed along the hole-like Fermi surfaces in the normal state immediately above T,.Our results provide direct spectroscopic evidence that the normal state of Ba0.6K0.4Fe2As2 is a non-Fermi liquid state.Our ARPES results also indicate that Ba0.6K0.4Fe2As2 represents a new typical supercon-ducting system with well-defined Fermi surface but without well-defined quasi-particles along the Fermi surface in the normal state.(3).We discovered that Ba0.6K0.4Fe2As2 exhibits unusual behaviors in the superconducting state.On the one hand,the superconducting gap keeps constant below the superconduct-ing transition temperature and drops to zero abruptly at T,.This behavior de-viates strongly from the temperature evolution of superconducting gap of the BCS theory.On the other band,the spectral weight around the Fermi level is not conserved when Ba0.6K0.4Fe2As2 undergoes from the normal state to the super-conducting state.There is an extra spectral weight gain in the superconducting state.These unusual properties are interesting to promote related theoretical research.5.Study on the electronic structure and superconducting gap of the collinear anti-ferromagnetic phase and C4 magnetic phase of Sr1-xNaxFe2As2 superconduc-tors in the under-doped region.Main results are:(1).There is a new band splitting found at the M point in the C4 magnetic phase of Sr1-xNaxFe2As2 superconductor.The band splitting is different from those induced by the ne-matic phase.The disappearance of band degeneracy at M point caused by the symmetry breaking or emergence of new ordered phase may be responsible for the new band splitting behavior.(2).With high resolution laser-ARPES(6.994eV),we found that the superconducting gap along the inner hole-like Fer-mi surface around Brillouin zone center exhibits anisotropic four-fold symme-try for both the collinear antiferromagnetic phase and the C4 magnetic phase in Sr1-xNaxFe2As2But the Fermi surface angle with the maximum gap size are 45° different between the two samples.The maximum gap sizes are?10meV which are unusually large compared with their Tc-12K.Furthermore,the su-perconducting gap along the outer hole-like Fermi surfaces under both the two phases are basically zero.The superconducting gap structures and their differ-ence between the two distinct magnetic phases provide important information to understand the interplay between magnetic order and superconductivity as well as the high temperature superconductivity mechanism.6.Study on the electronic structure of SrMnSb2 with ARPES.Previous report suggested that SrMnSb2 is a potential time-reversal symmetry breaking Wey1 semimetal.With ARPES experiments,we found:(1).The electronic structure of SrMnSb2 exhibits quasi-2-dimensional characteristic revealed by differen-t photon energies.All the bands around the Fermi level show linear dispersion within?leV energy range.(2).The electronic structure revealed by ARPES is qualitatively consistent with the band structure calculations.There is no topo-logically protected Dirac point or Weyl point found below the Fermi level.(3).Four strong spots are found located along the high symmetry direction of the hole-like pockets around Brillouin zone center.The strong spots cannot be ex-plained by theoretical calculations so far.Further investigations are needed to find out if they are related to the topological phase revealed by quantum oscilla-tion experiments.