Scanning Tunneling Microscopy Study On The Insulating Phases Of Copper And Iron Based High-T_c Superconductors

One of the major puzzles concerning unconventional superconductivity is how some of the most interesting superconductors are related to an insulating phase that lies in close proximity. The most well-known example is the parent compound of cuprates, which is generally regarded as an antiferromagnetically ordered Mott insulator. Recently novel insulating phases are discovered in several Fe based superconductors(FeSCs). Elucidating the electronic structure of these insulating phases could shed new lights on the mechanism of unconventional superconductivity.This thesis presents scanning tunneling microscopy(STM) studies on the insulating phases of unconventional superconductors. For cuprates, we study the atomic-scale electronic structure of the Ca2CuO2Cl2 parent Mott insulator. The full electronic spectrum across the Mott-Hubbard gap is uncovered for the first time, which reveals particlehole symmetric and spatially uniform Hubbard bands. Defect-induced charge carriers are found to create broad in-gap electronic states that are strongly localized in space. We show that the electronic structure of pristine Mott insulator is roughly consistent with the Zhang-Rice singlet model, but the peculiar features of the doped electronic states require further investigations.Regarding the insulating phase of FeSCs, we investigate the local electronic structure of Cu doped NaFeAs across the superconductor to insulator transition. We find that the Cu dopants systematically deplete low-energy electron spectral weight and impose strong local impurity potentials. In the highly insulating regime the electronic spectrum develops an energy gap with diminishing density of state at the Fermi level. The overall line-shape and strong spatial variations of the insulating gap are strikingly similar to that of lightly doped cuprates close to the parent Mott insulator. We propose that the suppression of itinerant electron state and the strong impurity potential induced by Cu dopants together lead to this strongly correlated, insulating state in FeSCs.