| The discovery of high-temperature cuprate superconductors leads to the collap-sion of the tranditional BCS theory,the research on high-temperature superconductivi-tors and its theoretical explanation became one of the most popular topic in condensed matter physics field.As an unique technique to directly detect the electronic structure in materials,ARPES has been playing a major role in understanding the electronic structure of high-temperature cuprate superconductors and to explain the mechanis-m of high-temperature superconductivity.The present dissertation focuses on using tunable laser ARPES to study the Matrix elements effect on ARPES,a reproducible method of growing and annealing to achieve high-quality underdoped high tempera-ture superconductor Bi2201 single crystals with different transition temperatures,and the investigation of superconducting gap and pseudogap in Bi2201 and Bi2212.The main results are as followings:1.Introduced the basic background which the research in this thesis mainly based on.In the superconductor part,we introduced the history of the superconductiv-ity,the basic physical properties of superconductors and the properties of high-temperature cuprate superconductors and its theorical model.In the experimen-tal technique part,we introduce the basic principle of ARPES and the ARPES experimental setups in our lab.We also introduced the previous research results using tunable laser ARPES.2.Using tunable laser,we systematically studied the influence of three variables included in the matrix element effect on Bi2212.From the incident laser po-larization direction,the incident laser photon energy and sample orientation,we study how these three aspects influence the spectroscopy on optimally doped Bi2212,verify that in laser ARPES the sudden approximation assumption stil-1 holds on,and provide informations for the future use of laser angle resolved photoelectron spectroscopy on how to obtain high quality and high reliability of energy spectrum.3.Bi2201 single crystals doped with Bi-Sr and La-Sr doped with high quality were grown,and the annealing conditions and methods were investigated.We use the optical floating zone furnace growing Bi2201 single crystals of a series of differ-ent doping levels,and by annealing in vacuum or in the flowing of oxygen and quenching in nitrogen,finally we can get the high quality of Bi2201 samples,realize the controllable and repeatable preparation of high quality of underdoped Bi2201 crystal with different transition temperatures.By growing and annealing of La-Bi2201 crystal,we can get samples from non superconducting to optimal doping,achieving the full coverage of the underdoped region of the phase dia-gram,which is of great important in studying the physical properties of copper oxide high-temperature superconductors lattice structure and superconducting mechanism.4.The superconducting gap of the optimally-doped Bi2212 is measured by the ultra high resolution laser-based ARPES.A deviation from the standard d-wave form in the superconducting gap from precise measurements and analyses in a nearly optimally Bi2212.Alternately,the observed energy gap as a function of momentum can be well fitted by including a high order term in which the next nearest-neighbor interactions is considered.5.We also study the electronic structure on Bi2212 in the antinodal region,and can not see the similar dispersion change and kf shift with temperature in Bi2201 through the T*,which was proposed as an evidence of phase transition.That means the change in Bi2201 is not a universal phenomenon.This provides strong evidences for studying the nature of the pseudogap,and also provides information for explaining the relationship between the pseudogap and the su-perconducting gap.6.A series of underdoped and optimally doped Bi2201 samples were measured and we systematically study the superconducting gap.We find that the superconduc-tivity gap basically matches a standard d wave form,which is different from the previous results that the superconducting gap deviated from the d-wave form in underdoped Bi2201.The maximum superconducting gap is about 27meV,far less that the 60meV which is reported before.The more precise data on the superconducting gap in Bi2201 provides a wealth of information in the under-standing and study of the underdoped region electronic structure and the nature of the pseudogap. |
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