Scanning Tunneling Microscopy Studies On Cuprate And Dichalcogenide Mott Insulators

One of the most intriguing phenomena concerning many unconventional superconductors is that the superconductivity exists within a“dome”in the phase diagram and disappears in both the severely underdoped and heavily overdoped limits.There are two valid starting points to address the origin of the superconducting?SC?phase,one from the parent Mott insulator phase and the other from the non-SC Fermi liquid phase.We study two representative doped Mott insulator systems,the 1T-Ta S₂ transition metal dichalcogenide and cuprate high T_c superconductor.In each system,we start from a different limit to understand the evolution of the electronic structure and the ubiquitous charge ordered states.This dissertation presents scanning tunneling microscopy?STM?examination on heavily overdoped?Bi,Pb?₂Sr₂Cu O6+?cuprate across the superconducting dome boundary.Tunneling spectroscopy reveals a continuous distribution of the Van Hove singularity feature and its evolution into the pseudogap phase.Spectroscopic imaging exhibits dispersive quasiparticle interferences,and the Fourier transforms uncover the evolution of momentum space topology.More significantly,we observe nanoscale patches of static char ge order with???×???periodicity in the non-SC phase and apparent phase shift for reverse bias voltages.The fading of this charge order when approaching the SC phase indicates that it is a distinct feature separating the non-SC phase and the SC phase in the overdoped regime.The origin of the charge order and its implications to the superconducting state are discussed.These results shed important new light on the electronic ground state of overdoped cuprate and the emergence of superconductivit y from this limit.Regarding the layered transition metal dichalcogenide 1T-Ta S₂ which also undergoes a Mott-insulator-to-superconductor transition,we use STM to reveal the atomic scale electronic structure of the Mott insulator phase and its evolution to the metallic state upon isovalent substitution of S with Se.In combination with first-principles calculations,we identify two distinct types of orbital textures:one localized and the other extended,demonstrating that the interplay between them is the crucial factor that determines the electronic structure.Particularly,we show that the continuous evolution of the charge gap visualized by STM is due to the immersion of the localized-orbital-induced Hubbard bands into the extended-orbital-spanned Fermi sea,featuring a unique evolution from a Mott gap to a charge-transfer gap.This new mechanism of Mottness collapse revealed here lead to more profound insights into strongly correlated systems.