Angle-Resolved Photoemission Spectroscopy Study On Mott Insulator Ca₂CuO₂Cl₂ And High-Temperature Superconductor Bi₂Sr₂CaCu₂O_8+?

The mechanism of the cuprate high-temperature superconductivity remains as an unsolved problem in condensed matter physics.To understand the mech-anism of the high-Tc and search for the pairing glue,the first thing to do is to understand the unique electronic structure of the cuprates.Angle-resolved photoemisson spectroscopy has played an important role in the research on cuprates.In this thesis,the development and maintenance of several state-of-the-art ARPES systems will be presented and the research on the Ca2CuO2Cl2 and Bi2Sr2CaCu2O8+? will also be discussed in detail.The main structure of the thesis is as follows:1.A brief introduction is given on conventional and unconventional SC,and the basic properties of the cuprate high-Tc materials.Also described here is the background for my researches.2.The related ARPES principles are discussed.3.The performance and features of the newly developed systems(6.994 eV laser ARPES,spin ARPES,ARTQF and LLL)are presented and discussed.4.The parent compounds of the cuprate high-temperature superconductors are Mott insulators.A natural starting point is to elucidate the basic elec-tronic structure of the parent compound.Here we report high resolution angle-resolved photoemission measurements on the Ca2CuO2Cl2(CCOC)parent Mott insulator of the cuprates.Multiple underlying Fermi surface sheets are clearly revealed for the first time.The high energy waterfall-like band dispersions exhibit different behavior near the nodal and antinodal region.Two distinct energy scales are also identified:a d-wave-like low energy peak dispersion and a nearly isotropic effective lower Hubbard band gap.Our observations provide new information on the electronic structure of the Mott insulator and indicate that the strong electron correlation ef-fect is important to understand the anomalous physical properties of the cuprate parent compound.5.It has been generally agreed that to understand the high-temperature su-perconductivity is to understand the physics of doped Mott insulators.We study the systematic in-situ potassium doping process on Mott insulator CCOC.Our results show that in-situ potassium doping can effectively in-troduce electrons into the surface,and when doped enough electrons,a broad in-gap state feature can be observed.And there is a large spectral weight transfer between the in-gap state and the charge-transfer band.Our experiments show that the strong insulating nature of CCOC prevents low temperature ARPES measurement due to the charging effect and leads to a fast loss of the potassium deposited on the sample surface.And there might be a limit of the potassium deposition for the CCOC surface,which might be the reason of the instability of the in-gap state.Because to make the in-gap state emerge,it needs very large electron doping,thus more potassium depositing on the surface,which may have already gone beyond the limit.6.By performing high resolution ARPES on Bi2212,we find the coexistence of a Fermi-liquid-like excitation and a non-Fermi-liquid-like excitation in mo-mentum space.The Fermi-liquid-like excitation dominates near the nodal region and the non-Fermi-liquid-like excitation dominates near the antin-odal region.For each doping,we use nodal coefficient A0 to represent the FL-like interaction strength,and antinodal coefficient B0 to represent the NFL-like interaction strength.We notice that the A0(doping)/A0(OP91K)increases monotonically with doping,while B0(doping)/B0(OP91K)shows a dome feature,which implies that the linear term,NFL-like excitation,may relate to superconductivity.7.The report of possible s wave pairing symmetry in single-layer CuO2 grown on Bi2212 has attracted much attention.Unlike the single-layer FeSe,the surface of the single-layer CuO2 can't be properly protected,making the sample transfer a huge problem between our partner's MBE system and our ARPES system.By performing high resolution ARPES on single-layer CuO2/Bi2212,we find that the systematic low temperature annealing is an effective way to remove the absorbed Ar gas molecules on sample sur-face.After annealing,ARPES can detect a clear electronic structure,very similar with the electronic structure of the Bi2212 substrate.Successful ARPES measurement on single-layer CuO2 is essential,because it will re-veal the pairing symmetry and Fermi surface topology of the single-layer CuO2,which may provide key information on the mechanism of the high Tc cuprates.