Investigating The Local Dopants And High Temperature Electronic Structure Of Cuprate Mott Insulator

The microscopic mechanism of high-temperature superconductivity is one of the most important research areas in condensed matter physics.The high-temperature su-perconductivity problem is closely related to the strong correlation physics,especially for the most representative cuprate superconductors,which originate from doping a Mott insulator.However,so far there is still no rigorous theoretical solutions for the strongly correlated system.Therefore,the study of the microscopic electronic structure of cuprate superconductors will provide important inspirations for understanding the mechanism of high-temperature superconductivity and strongly correlated system.In this thesis,we use scanning tunneling microscope technique to study the evolution of electronic structure of the cuprate Mott insulator with local dopants and temperature.We first study the influence of hole-type dopants on the electronic structure of the Mott insulator phase of cuprates.Ca₂Cu O₂Cl₂,（CCOC）,a prototypical cuprate Mott in-sulator,is selected to study the electronic state of calcium vacancy type hole dopants in it.We found that a single hole-type dopant in CCOC induces a defect electronic state within the charge transfer gap close to the upper Hubbard band.When two hole-type dopants approach each other,the defect state will gradually approach the charge transfer band,so that a V-shape gap similar to the pseudogap appears near the Fermi energy.In addition,we found that the spatial distribution of the defect state is related to the configuration of the two hole-type dopants relative to the crystal lattice.These findings reveal how the elec-tronic structure of cuprate gradually evolves from a charge transfer gap to a pseudogap during the process of doping a Mott insulator.Meanwhile,our experiments give quanti-tative results about the electronic structure of various dopant configurations in the Mott insulator,which can provide important experimental inputs for theoretical calculations of doped Mott insulators.The second part of this thesis is to study the evolution of the electronic structure of cuprate Mott insulators with temperature.Because the scanning tunneling microscope experiment in the high-temperature regime has greater technical difficulties,we select the Ca₃Cu₂O₄Cl₂double-layer sample with a smaller charge transfer gap.We found that as the temperature rises,the upper Hubbard band in Ca₃Cu₂O₄Cl₂gradually moves towards the Fermi energy,while the charge transfer band is almost unchanged.As a consequence the size of the charge transfer gap decreases with the increase of temperature.The rate of decrease with temperature is significantly higher than that of common semiconductors,so its origin is likely to be the strong electron correlation effect,such as the spin fluc-tuations in a Mott insulator.In addition,we found that the electronic structure does not change significantly when crossing the Néel temperature of the antiferromagnetic phase transition.These experimental results reveal the influence of thermal excitation on the electronic structure of cuprate Mott insulator and provide experimental clues for the finite temperature Mott insulator theory relevant to high-temperature superconductivity.