Angle-Resolved Photoemission Spectroscopy Study On High Temperature Bi-Based Cuprate Superconductors

High temperature cuprate superconductors has attracted the great research interests of many researchers since its discovery due to a large number of anomaly phenomena.Revealing the electronic structures of high temperature cuprate superconductors is im-portant for understanding the mechanism of high temperature superconductivity and its many anomaly properties,and the angle-resolved photoemission spectroscopy plays an important role in the study of high temperature superconductivity because it can directly probe the electron structures of materials.In this paper,we introduce the characteristics of the four angle-resolved photoemission spectroscopy systems in our laboratory,and use the angle-resolved photoemission spectroscopy systems to study the electron struc-tures of high temperature Bi-based cuprate superconductors systematically.The main contents are as followings:1.Introduce the electron structures of high temperature cuprate superconductors.Summarize the current main research directions of angle-resolved photoemission spec-troscopy in the hole doped high temperature cuprate superconductors.2.Introduce the principle of angle-resolved photoemission spectroscopy as well as the physical meaning of the angle-resolved photoemission spectra.3.Assist building the state-of-the-art ultra low-temperature and vacuum ultra vi-olet laser-based angle-resolved photoemission spectroscopy system of the world,with the lowest temperature to 0.8 K and the energy resolution better than 1 me V.4.Introduce the traveling solvent floating zone method and use this method to grow 11 kinds of Bi₂Sr₂Cu O₆₊single crystals.Using annealing technique to im-prove the single crystal quality and further modify the carrier concentration to all dop-ing levels.Carried out transport measurements of resistance and Hall coefficient for Bi₂Sr₂Cu O₆₊,which laid the foundation for subsequent combination of the angle-resolved photoemission spectroscopy and transport studies.5.The coupling between electrons and boson modes in high temperature cuprate superconductors was studied by the high-resolution laser-based angle-resolved photoe-mission spectroscopy system,and it was found that two kinds of omnipresent coexist-ing sharp electron-boson mode-couplings could be observed in different materials,in the whole momentum space,at different doping levels and at different temperatures.The bosons are attributed to phonons with the evolution of the two mode-couplings.This provides new information on electron-phonon mode-couplings in high tempera-ture cuprate superconductors.6.The particle-hole symmetry of the optimally doped Bi₂Sr₂Cu O₆₊sample in the pseudogap and superconducting states was investigated by 10.897 e V laser angle-resolved photoemission spectroscopy system equipped with a time-of-flight electron energy analyzer,and it was found that the particle-hole symmetry is satisfies in the su-perconducting state,the pseudogap state and even above the pseudogap temperature.This provides important experimental evidence for understanding the origin of pseudo-gap.7.The variation of the Fermi arc length in Bi₂Sr₂Cu O₆₊with doping and tem-perature was investigated by a high-resolution laser-based angle-resolved photoemis-sion spectroscopy system.We found that there exists superconducting fluctuation in the optimally doped Bi₂Sr₂Cu O₆₊sample in a very small temperature range just above the superconducting temperature.The Fermi arc length does not vary with temperature above the superconducting temperature.The electron structure of the Bi₂Sr₂Cu O₆₊sample is coherent in the entire Brillouin zone above the superconducting fluctuation temperature at the higher overdoped region,where the Fermi surface is closed and there is no Fermi arc.In the comparative underdoped to optimally-doped range,the Fermi arc range corresponds to the coherent region of electrons while outside of the Fermi arc range corresponds to the incoherent region of electrons.In this doping range,the Fermi arc length does not vary with doping.In the heavily underdoped to the comparative underdoped range（corresponding to the superconducting temperature of 6 K）,there is a nodal gap,and there is no Fermi arc.In this doping range,as the doping decreases,the coherent momentum space of electrons decreases until it disappears completely.These results suggest that the strange（metallic）state is more intrinsic than the pseudo-gap state,and the Fermi arc is closely related to the strange（metallic）state.This finding contributes to the understanding of the nature of the ground state and pseudogap state in high temperature cuprate superconductors.8.The gaps of Bi-based high temperature cuprate superconductors in the super-conducting state was investigated by a high-resolution laser-based angle-resolved pho-toemission spectroscopy system.We found that at a lower doping level,the gap near the nodal region satisfies a standard(9-wave symmetry in the superconducting state,and de-viates from the standard(9-wave pairing near the antinodal region.With the hole doping increases,the gap along the entire Fermi surface returns to the standard(9-wave symme-try in the superconducting state.The electron pairing intensity starts to decrease with increasing hole dopings,and after a certain doping,it starts to remain constant with dopings which is still larger than that of the weakly coupled(9-wave superconductor.These results indicate that high temperature cuprate superconductors are strongly cou-pled(9-wave superconductors throughout the doping range,while there is competition for superconductivity with other incoherent quantum states near the antinodal region.