Angle Resolved Photoemission Spectroscopy Study On The Electronic Structure And Gap Of Bi2212

High temperature cuprate superconductors have attracted much attention in con-densed matter physics due to their exotic normal state properties and unusual supercon-ducting properties.After extensive research in the past more than thirty years,many prominent issues remain and there is still no consensus on the mechanism of high tem-perature superconductivity.Angle-resolved photoemission spectroscopy?ARPES?is a powerful tool in directly probing the electronic structure of materials in the momentum space?it has played a key role in studying high temperature cuprate superconductors.In this thesis,I carried out extensive ARPES study on the electronic structure and su-perconducting gap of Bi₂Sr₂Ca Cu₂O₈₊?Bi2212?.The main results are summarized in the following chapters:1. A brief introduction to the development,properties and applications of super-conductors.The crystal structure,electronic structure and the phase diagram of high temperature cuprate superconductors are reviewed.The main results and new progress of the ARPES studies on Bi2212 system are summarized.2. The basic principles and techniques of ARPES are discussed,and the four laser-based ARPES systems in our lab are introduced.The time-of-flight ARPES system is focused that is the one I was in charge of maintaining and upgrading.The principle,structure,performance and the working experience are discussed.3. Preparation of high quality Bi2212 single crystals covering a wide doping range from near zero doping parent compound to overdoped?65K superconductor,real-ized by annealing process.?1?.The overdoped samples are obtained by annealing the pristine optimally-doped Bi2212 crystal under high oxygen pressure,with a doping level up to overdoped?65 K??2?.The underdoped samples are obtained by annealing the pristine optimally-doped Bi2212 crystal in vacuum,with a doping level down to un-derdoped?61 K.?3?.The heavily underdoped samples are obtained by annealing Bi₂Sr₂Ca_1-DyCu₂O₈₊?Dy-Bi2212?in vacuum,with a doping level varying from underdoped?50 K to non-superconducting region.4. Discovery of selective band hybridization in Bi2212.High resolution laser-based ARPES measurements are carried out on the Fermi surface and band structure of Bi2212 in the second quadrant of the Brillouin zone.We found that,in the crossing region of one main Fermi surface and one superstructure Fermi surface,FOUR Fermi surface sheets are observed.Such phenomenon is present in Bi2212 samples with dif-ferent doping levels?underdoped,optimally doped and overdoped?,and is observed in both the superconducting state and the normal state.However,when similar ARPES measurements are performed on Bi2201,this phenomenon is not observed.These re-sults indicate that such a phenomenon is Bi2212 specific,and rules out the possibility of spin-related band splitting.The unusual band hybridization can be understood by considering:?1?.the bilayer splitting in Bi2212 that gives rise to bonding and anti-bonding Fermi surface sheets??2?.the selective hybridization of two bands with pe-culiar combinations,i.e.,the main antibonding Fermi surface can only hybridize with the superstructure bonding Fermi surface.Likewise,the main bonding Fermi surface can only hybridize with the superstructure antibonding Fermi surface.But the main bonding?antibonding?Fermi surface will not hybridize with the superstructure bond-ing?antibonding?Fermi surface??3?.the altered photoemission matrix element effects of the hybridized bands.These observations provide strong evidence that the origin of the superstructure bands is intrinsic to the Cu O₂planes.Therefore,understanding phys-ical properties of Bi2212 should consider the complete Fermi surface topology which involves the main bands,the superstructure bands,and their interactions.5. Observation of strong hybridization between the superconductivity-induced Bo- goliubov band and the initial band,and the anomalous momentum dependence of the superconducting gap on the new Fermi surface from the hybridization of the main Fermi surface and the superstructure Fermi surface.We discovered a new approach to detect the phase information of the superconducting gap from ARPES.We found that:?1?.Strong Bogoliubov band hybridization occurs in the superconducting state?it is absent in the normal state??2?.the momentum dependence of the superconducting gap on the reconstructed Fermi surface from the hybridization of the main Fermi surface and su-perstructure Fermi surface deviates obviously from the standard?9-wave form,with an extra gap minima?less than 2 me V?appeared in the hybridization area.The supercon-ducting gap recovers gradually to standard d-wave form when moving away from the hybridization region.To understand these observations,we have done model calcula-tions and can reproduce the Bogoliubov band hybridization and the unusual supercon-ducting gap form when considering the different gap sign of the main Fermi surface and superstructure Fermi surface.These results show the possibility to detect the phase information of the superconducting gap by ARPES measurements for the first time.It provides a new approach for ARPES to study the gap symmetry in superconductors with multiple bands and multiple Fermi surface sheets,such as iron-based superconductors.6. Detailed doping evolution of the electronic structure of Bi2212 is studied by ARPES in overdoped region and in heavily underdoped region.It is the first time to observe the electronic structure evolution from the parent Mott insulator to a super-conductor in Bi2212.The doping level of Bi2212 is tuned by4?9)4)annealing and/or alkali atom deposition on the sample surface during the ARPES experiments,and the doping evolution of the electronic structure is studied.?1?.The overdoped samples are obtained by4)9)4)annealing the optimally-doped Bi2212 sample in ozone while the underdoped samples are obtained by4)9)4)annealing the optimally-doped sample in vacuum.Theis measured by the continuous temperature evolution of the in-gap states near the Fermi level near the antinodal region.The evolution of the Fermi sur-face,superconducting gap and the kink structure in Bi2212 with different doping levels are studied.?2?.With4)9)4)Rb deposition on an underdoped?25 K Dy-Bi2212sample surface in ARPES experiments,the doping level is continuously tuned to nearly zero doping to the parent Mott insulator state.The detailed doping dependence of the Fermi surface,band structure,and the chemical potential is studied.The results indi-cate that,for the parent state with zero doping,the chemical potential locates at about1 e V above the charge transfer band.With hole doping,the chemical potential contin-uously decreases,and the band structure shows a rigid band shift-like behavior.When the chemical potential moves into the charge transfer band,the spectral weight near the Fermi level in the nodal region increases,followed by the emergence of the co-herent quasiparticle peak and the insulator-superconductor transition.This is the first time to observe the detailed evolution of the electronic structure of Bi2212 in heavily underdoped region near zero doping in momentum space.It will help understand the insulator-superconductor transition in doping the parent cuprate compound and provide key information for establishing theories.