Angle Resolved Photoemission Spectroscopy Study On Superstructure,Band Hybridization And Pairing Mechanism In Bi2212 Superconductor

High temperature cupate superconductors exhibit a number of anomalous nor-mal state properties and unusual superconducting properties,bringing great challenges to the microscopic quantum theories of contemporary condensed matter physics.In-vestigations on their electronic structure are a pre-requisite for understanding the ex-otic normal state and unusual superconductivity mechanism to establish a new many-body quantum theory.In this thesis,electronic structure of high temperature cuprate Bi2-xPbx Sr2CaCu2O8+? superconductors are studied by high resolution angle-resolved photoemission spectroscopy(ARPES).The main results of this thesis include the fol-lowing parts1.A brief introduction of the background about superconductors is given.The re-search of high temperature cuprate superconductors is introduced,and the results of the superconducting energy gap,pseudogap,many-body interactions in Bi2212 supercon-ductor investigated by using angle-resolved photoemission spectroscopy is reviewed2.The ARPES principle,the related experimental technique and laser-based ARPES systems are summarized.I mainly introduced our new generation ARToF-ARPES sys-tem.In addition,I also provided a general introduction of the vacuum ultra-violet laser-based ARPES systems,including spin-resolved ARPES system,large-momentum ultralow-temperature ARPES system that have been developed in our lab and some other ARPES systems in the world3.Growth and characterization of high quality Bi2-xPbxSr2CaCu2O8+? single crystal.High-quality single crystals Bi2212 with a wide range of Pb substitution have been successfully grown by the traveling solvent floating zone technique.Their com-position and crystal structure are characterized and the superconducting properties are measured.These lay a solid sample foundation for ARPES study of Bi2212 supercon-ductors.4.A systematic evolution of electronic structure and the superstructure with Pb substitution in Bi2-xPbx Sr2CaCu2O8+? superconductors has been revealed for the first time.The superstructure shows a significant change with Pb substitution and the in-commensurate modulation vector(Q)decreases with increasing Pb substitution.In the meantime,the superstructure intensity from ARPES measurements also decreases dra-matically with increasing Pb concentration.The superstructure in Bi2212 can be ef-fectively suppressed by Pb substitution and it nearly disappears with Pb substitution of x=0.8.We also find that the superstructure bands in ARPES measurements depend sensitively on the photon energy of lasers used;they can become even stronger than the main band when using a laser photon of 10.897 eV.These results provide impor-tant information on the origin of the incommensurate superstructure and its control and suppression in Bismuth-based high temperature superconductors5.The Bogliubov hybridization between the superstructure band and the main band in Bi2212 has been discovered for the first time.ARToF-ARPES has the advan-tage of two-dimensional momentum detection in a single measurement,which enables us to study the delicate energy band change in a small range of momentum.In the 2nd quadrant of the Brillouin zone,the superstructure band and the main band of Bi2212 in-tersect with each other in normal state.In the superconducting state,electron pairing and band back bending cause Bogliubov hybridization.Theoretical simulation shows that Bogliubov hybridization is related to the different energy gap sizes of the two energy bands,which may have some influence on the superconductivity.This phenomenon provides a decisive evidence on the origin of the superstructure band that is intrinsic to the CuO2 planes.6.A new procedure to extract the pairing Eliashberg functions from ARPES mea-surements is proposed and tested.In conventional superconductors like Pb,the extrac-tion of the pairing Eliashberg functions from tunneling experiments provided a smoking-gun-type of evidence that prove the validity of the BCS theory of superconductivity which proposed that phonons are the medium for the electron pairing.Extraction of the pairing Eliahsberg functions in cuprate superconductors will provide important infor-mation in understanding the mechanism of high temperature superconductivity.Using our latest generation of laser-based ARToF-ARPES system,we prosed a new procedure to extract the pairing Eliashberg functions from the ARPES experiments.We have mea-sured the optimally-doped Bi2212 superconductors at different temperatures and found some fine structures at high binding energies that are temperature-and momentum-dependent.Using our new procedures,we have extracted the normal self-energy and pairing self-energy from these experimental data.These have laid a good foundation for further investigations and understanding the mechanism of high temperature super-conductivity in cuprate superconductors.