Scanning Tunneling Microscopy Study On Doped Mott Insulators And Kondo Insulators

Strong correlation physics is at the heart of the research in modern condensed matter physics,which is beyond the conventional Fermi liquid theory.Two representative sys-tems of strongly correlated materials are the cuprate high-temperature superconductors and Kondo insulators.Both systems consist of an array of localized magnetic moments,while the latter has itinerant electrons in addition,interacting with local moments.This dissertation presents scanning tunneling microscopy?STM?studies of both sys-tems.For the cuprates as doped Mott insulators,the relationship between superconduct-ing state and other orders,including the Mott insulating phase,the charge order and a novel pair density wave order,is investigated.First,the charge transfer gap?CTG?of par-ent Mott insulators of four different cuprate materials from the Ca_n+1Cu_n O_2nn Cl₂?CCOC?and Bi₂Sr₂Ca_n-1Cu_n O_2n+4+??BSCCO?families are measured.Surprisingly,an anticorre-lation is found between the CTG and the maximum transition temperature(T_c,max)that can be achieved upon charge doping in each material,which renders the important role played by Mottness in cuprates.Then by analyzing the single-particle spectral function probed by scanning tunneling spectroscopy?STS?,a pair density wave?PDW?state is identified in T_c=10 K Bi₂Sr₂CaCu₂O_8+?sample,which is found to be intertwined with the charge order?CO?and superconductivity.These findings will shed important new lights on understanding the mechanism of the unconventional superconductivity.Regarding the Kondo insulators?KIs?,SmB₆as a representative material is studied,where four different surface morphologies are identified.The local electronic structure revealed by STS,which is immune to surface reconstructions,shows a hybridization gap and a coherent in-gap resonance peak.The hybridization gap can be well described by the Kondo lattice model,and its size shrinks upon increasing temperature which is a signature of the KI,distinguished from conventional semiconductors.The gap closes at around 150K,while the in-gap peak disappears at 50 K,which represents a new collective magnetic mode,that is unique to the Kondo lattice system.This work provides valuable clues to clarify the electronic structure evolutions of Kondo lattice systems.