Preparation Of Copper Oxide Planes In Cuprates And Investigation Of Their Electronic Structures

Synthesis of novel quantum materials lies at the core of modern condensed matter physics.Copper-oxide superconductors,which holds the highest transition temperature T_c at ambient pressure among all superconductors available,is a prototype to reveal the nature of strongly correlated systems and the pairing mechanism of unconventional superconductivity.In this thesis,we have applied molecular beam epitaxy（MBE）technique to prepare successfully the CuO₂ planes-the key building layer of cuprate superconductors,and investigated their properties by using in-situ low-temperature scanning tunneling microscopy（STM）and spectroscopy（STS）.Below are the main research results achieved in this thesis.First we have prepared CuO₂ monolayer on cleaved Bi₂Sr₂CaCu₂O_8+δ（Bi-2212）substrate,and carefully studied their superconducting properties by STS.We found that there exist two different kinds of low-energy excitations,which behave themselves as Fermi level-symmetric U-shaped gap and V-shaped gap,respectively.By investigating their temperature-dependence,we revealed that the V-shaped gap originates from the Bi-2212 substrates,whereas the U-shaped gap is related to the occurrence of superconductivity.Furthermore,by introducing nonmagnetic impurities and investigating their influence on the STS spectra,we showed that the U-shaped superconducting gap is of s-wave pairing symmetry,whereas the V-shaped pseudogap magnitude monotonically increases with potassium doping,indicating its origin from BiO and irrelevance to superconductivity in cuprates.Based on the findings,we proposed a modulation-doping-like charge transfer model,and applied it to well explain the complex phase diagram of cuprates,such as the T_c evolution with chemical doping level,the normal-state strange metal phase and electron-hole asymmetry.Our model can be used for searching high-T_c superconductors.As compared to hole-doped cuprates,the electron-doped counterparts have been less investigated,limited by their poor sample quality and hard cleavage.The infinite layer compound SrCuO₂ has the simplest crystal structure among all cuprates and is terminated with CuO₂ plane,rendering it an ideal system to study the mechanism of high-T_c superconductivity in cuprates.By using MBE,we have grown high-quality SrCuO₂ films on SrTiO₃（001）,and investigated their atomic structures by in-situ STM.We revealed for the first time that the surface oxygen atoms periodically move up and down to compensate the polar catastrophe,leading to a 2?2 reconstruction and the symmetry breaking from C₄ to C₂.We further observed one-dimensional stripe phase by post annealing which leads to high concentration oxygen vacancies on the surface.By varying the growth temperature,we could even obtain the orthorhombic SrCuO₂ phase,which exhibits one-dimensional feature and is characteristic of spin-charge separation.