Scanning Tunneling Microscopy Study On The Electronic Phase Diagram Of Cuprate And Iron-Based High-T_c Superconductors

A major obstacle to the mechanism of high-T_c superconductivity is that the ‘normal' state,which is far from normal.Particularly,the superconducting?SC?state are intertwined with some exotic states in the electronic phase diagram.Therefore the origin of these intertwined states and their relation to superconductivity,is believed to hold the key to unveil the high Tc superconductivity.However,due to the lack of spatial resolution,in the electronic phase diagram there are still a lot of unexplored regimes and controversies,regarding how the SC phase are intertwined intrinsically with other orders,and how the electronic structure evolves from these intertwined phases to SC states.In this dissertation,we use scanning tunneling microscopy to re-examine the electronic phase diagram for both cuprate and iron based superconductors,attempting to resolve several key debates by visualizing the electronic structure and electronic orders at the atomic level.Such intertwined orders in iron based superconductors?FeSCs?are antiferromagnetism?AF?and electronic nematicity.On the atomically resolved FeSe surface of intercalated iron selenide,we observe a well-defined SC gap and the unexpected microscopic coexistence of a charge-density modulation with 2^1/2×2^1/2 periodicity with respect to the original Se lattice.We propose that a possible origin of the pattern is the electronic superstructure caused by the block antiferromagnetic ordering of the iron moments.Meanwhile,the widely expected iron vacancy is not observed,underlying an the exclusive phase separation in the sample,and most importantly indicating the iron vacancy is not necessary ingredient for superconductivity in the intercalated iron selenide.To study the relation between electronic nematicity and Cooper pairing in FeSCs,we systematically investigate the doping dependence of quasiparticle interference?QPI?in NaFe₁-xCo_xAs system.The electronic structure become more and more isotropic and liquid-like as electron are doped into this system.In the parent and underdoped compounds,where four-fold rotational symmetry is broken macroscopically,the QPI patterns reveal strong rotational anisotropy.The electronic nematicity are found to microscopically coexist with SC states.At optimal doping,however,the QPI patterns are always four-fold symmetric.We argue this implies small nematic susceptibility and hence insignificant nematic fluctuation in optimally doped iron pnictides.Since the SC transition temperature is the highest,this suggests nematic fluctuation is not a prerequistite for strong Cooper pairing.In cuprates the intertwined states are AF,charge orders and pseudogap phases at the underdoped regime.These intertwined order together with high temperature superconductivity are widely believed to originate from adding charge carriers into parent Mott insulator.To better elucidate the electronic phase diagram,we investigate the local electronic structure of lightly doped cuprates from insulating AF to underdoped SC regime.We observe a large charge transfer gap?CTG?in the parent Mott state,which can survive in nanoscale domains.We show that the doped charge induces a spectral weight from the high energy Hubbard bands to low energy state within the CTG.With increasing doping,a V-shaped density of state suppression reminiscent of the pseudogap occurs at the Fermi level,which is accompanied by the emergence of checkerboard charge order.It reveals that cuprates first become a charge ordered insulator upon doping,and the Fermi surface and high temperature superconductivity grow out of it upon further doping.