Development Of Angle-Resolved Photoemission Spectroscopy Technique And Its Study On High-Temperature Cuprate Superconductors

The superconductivity mechanism of high temperature cuprate superconductors is one of the most significant problems in condensed matter physics.Angle-resolved photoemission spectroscopy(ARPES)has proven to be a key technique to study on the high temperature cuprate superconductors.This thesis first provides a brief review on the conventional superconductors,BCS theory of superconductivity,unconventional superconductors and high temperature cuprate superconductors.Then the principle of ARPES and the technique are introduced,including the latest development of Laserbased ARPES systems.The scientific research includes two parts: development of new photoemission system and study of high temperature cuprate superconductors by ARPES.The details are described in the following chapters.1.Chapter 1 introduces briefly the history and background of superconductivity research.It starts from introduction of conventional superconductors,and the BCS theory of superconductivity.Then unconventional superconductors and novel superconductors are summarized.Finally,it focuses on high-Tc cuprate superconductors by mainly reviewing the crystal structure,electronic structure and electronic phase diagram of high-Tc cuprate superconductors,particularly the Bi-Sr-Ca-Cu-O family.2.Chapter 2 describes the principle of ARPES,the technique,and the methods of ARPES data analysis.Two laser-ARPES systems were introduced in detail.The first is the ARPES system equipped with the angle-resolved time-of-flight(ARTo F)electron energy analyzer—ARTo F-ARPES system.The second one is the ARPES system that is laser-based with large momentum and ultra-low temperature(<1 K with3 He cooling)—LLL-ARPES system.3.Chapter 3 describes the development of a new generation of dual-mode photoemission system.This system can measure the momentum space by doing ARPES with angle-resolved time-of-flight electron energy analyzer.This system can also measure the real space by doing photoemission electron microscope(PEEM).This system can also measure the electronic structure on a micro-or nano-scale regions,providing a capability of spatially-resolvedARPES with high resolution.The design and simulations are described.Testing facilities are installed,and preliminary testing results are presented.4.Chapter 4 focuses on the ARPES study of the electronic structure and manybody effects in electron-doped Pr1.2-xLa0.8CexCu O4-?(PLCCO)superconductors.We successfully obtained high quality laser-ARPES data on the PLCCO superconductors,establishing the feasibility of performing laser-ARPES measurements on electron-doped cuprate superconductors.The observed Fermi surface is different from the previously-reported ones and not consistent with the “hot-spot” picture;the origin of this new Fermi surface results is discussed.The electron self-energy is extracted from these high-quality laser-ARPES data and its momentum-and temperature-dependences are investigated.5.Chapter 5 focuses on evolution of superconducting gap and electron dynamics with doping in Bi2212 by taking high resolution laser-ARPES measurements.We find that,the Fermi surface region where the superconducting gap follows the standard d-wave form increases with increasing doping in Bi2212.We also find that,the overdoped region shows a different momentum dependence of the observed energy scales from that of the underdoped region.6.Chapter 6 reports high resolution laser ARPES measurements on the bilayersplitting and Fermi surface-dependent superconducting gap and electron dynamics in Bi2212(overdoped,Tc=75 K).Two Fermi surface sheets caused by bilayer splitting are clearly identified with rather different doping levels: the bonding sheet corresponds to a doping level of 0.14 which is slightly underdoped while the antibonding sheet has a doping of 0.27 that is heavily overdoped,giving an overall doping level of 0.20 for the sample.Different superconducting gap on the two Fermi surface sheets are revealed for the first time.The superconducting gap on the antibonding Fermi surface sheet follows a standard d-wave form while it deviates from the standard d-wave form for the bonding Fermi surface sheet.The maximum gap difference between the two Fermi surface sheets near the antinodal region is ?2 me V.These observations provide important information for studying the relationship between the Fermi surface topology and superconductivity,and the layer-dependent superconductivity in high temperature cuprate superconductors.