The Study Of Low-dimensional Correlated Systems By Angle-resolved Photoemission Spectroscopy

An unresolved problem in condensed matter physics is the superconducting mech-anism.Angle-Resolved Photoemission Spectroscopy?ARPES?provides rich experi-mental evidence for discussing microscopic mechanisms in strong correlation systems,while Molecular Beam Epitaxy?MBE?provides the possibility to construct complex thin film materials.The update of instrument technology will advance the progress of scientific research.Based on the BCS theory,the thesis introduces the problems that are still unresolved in the high temperature superconducting mechanism,then introduce the principle of ARPES and MBE methods,and introduce the application of two methods in superconductivity.The content of the thesis is divided into two parts,one is the construction of basic equipment,participating in the design and construction of the ozone system-assisted Oxide MBE system,and using Oxide MBE to grow complex oxide films,such as La_2-xSr_xCuO₄and Ba₂CuO₃.Then we do the phase transition studies.The other is the study of the superconducting symmetry and the anomalously large superconduct-ing gap on topological surface state by ARPES combined with in-situ MBE.The main results are as follows:1. In-situ changing the surface doping of the oxide single crystal:We propose two methods for tuning the doping amount of electrons?holes?on the surface of the oxide single crystal by MBE.We provide a PDSS method to in-situ continuously tune doping of Bi₂Sr₂CaCu₂O_8+x?Bi2212?single crystal surface,then the doping amount from 0.08to 0.24 was characterized by in-situ ARPES.The other method is the surface doping of Sr₂IrO₄single crystal by evaporating Al.Inspired by the formation of 2D electron gas at the interface of Al/Si Ti O₃,the same experiment was carried out on the surface of a potential new type of superconductor Sr₂IrO₄.We observed the band shift in Al doped Sr₂IrO₄surface by ARPES,but we didn't observed the metal-insulator transition after doping Sr₂Ir O₄.2. Superconductivity study of CuO₂layer:U-shaped superconducting energy gapobserved in CuO₂/Bi2212 by STS,supporting s wave pairing superconductivity sym-metry.Therefore,we grown CuO_xfilm by MBE and measured the band structure and superconductivity symmetry by in-situ ARPES.The low-energy electronic state did not differ significantly before and after growing film,even in the case of a multi-layered CuO_xfilm.The broad band spectrum show that the CuO_xfilm is an insulating state.No additional low energy states were observed in CuO_x/Bi2212.The superconducting energy gap structure keeps d wave symmetry and is reproducible.But in the system we can't completely rule out the existence of s wave pairing.Our data results are inconsis-tent with the U-band gap observed by STM,possibly because this local state cannot be observed by ARPES.3. Anomalously enhanced topological superconducting surface states:Two super-conducting gaps were observed in the topological superconducting candidate?-Bi₂Pd by STS,and its relatively small gap corresponding to the superconducting gap of single crystal material.We grown the high-quality?-Bi₂Pd film by MBE and measured the?-Bi₂Pd film band structure by ARPES for the first time.The enhanced superconduc-tivity of the topological surface state was observed(???_{T SS}³.8 me V),compared to its bulk gap???_b⁰.8me V.And 2???_{T SS}/k_BT_c¹6.3 is much larger than the BCS value.In order to obtain the difference between single crystal and film,we also measured the band structure of single crystal,the chemical formula moves upward.A possible enhancing mechanism,which is the Dirac-Fermion-mediated parity mixing,was discussed based on our observation of chemical potential shift and the concomitant increasing of surface weight revealed in our first principle calculations.Our experimental results establish a?-Bi₂Pd film as the only example of an enhanced topological superconducting gap in a topological superconductor.It will provide a platform to find enhanced topological surface state superconductivity in a higher T_c superconductor.