Scanning Tunneling Microscopy Studies On Superconductors Ta₄Pd₃Te₁₆ And ?LiFe?OHFeSe

The discovery of superconductivity in iron pnictides/chalcogenides stimulates the research on the origin of high temperature superconductivity.Due to the failure of the electron-phonon coupling based BCS theory in describing the superconductivity in iron based superconductors?IBS?,new microscopic mechanisms are required.As in the case of cuprates,the highest critical temperature IBS always arises at the border of the antiferromagnetic instability.Consequently,the bosonic antiferromagnetic spin fluctuations are naturally regarded as the possible pairing force.Since the wave vector of the antiferromagnetic order connects the hole and electron Fermi surfaces in IBS,the s± pairing symmetry is widely accepted,that is,both the electron and hole pockets have s-wave gaps but with opposite signs.However,the Fermi surface topography in IBS changes with carrier doping levels,such as the extremely electron doped FeSe monolayer thin film with only electron pockets.If the s± pairing symmetry still applies to this extreme case,we will get a unified pairing picture for IBS with or without the hole Fermi pockets.In this thesis,we report detailed scanning tunneling microscopy/spectroscopy re-searches on unconventional superconductors Ta₄Pd₃Te₁₆ and(Li_1-xFe_x)OHFeSe.The result consists of three parts listed as follows.?1?Observation of the anisotropic superconducting gap,elongated vortices and vortex lattice in superconductor Ta₄Pd₃Te₁₆.The one dimensional conducting chan-nels of PdTe2 in Ta₄Pd₃Te₁₆ make a significant anisotropy of electronic structure.By fitting the tunnelling spectrum,we suggest that at least one anisotropic superconduct-ing gap with gap minima or possible node exists in this multiband system.In addi-tion,elongated vortex lattice and elongated vortices are observed with an anisotropy of?||??b/=2.5.Clear Caroli-de Gennes-Matricon states are also observed within the vortex cores.?2?Scrutinizing the double superconducting gaps and strong coupling pairing in(Li_1-xFe_x)OHFeSe.(Li_1-xFe_x)OHFeSe belongs to the extremely electron doped IBS with no hole pockets in the center of Brillouin zone.The shape of tunnelling spec-trum resembles that in the FeSe monolayer thin film and shows double gaps at about 14.3 and 8.6 meV.The calculated 2?max/kbTC?8.7,which demonstrates the strong coupling mechanism in this superconducting material.Further analysis based on the quasiparticle interference?QPI?allows us to assign the larger?smaller?gap to the outer?inner?Fermi pockets?after folding?associating with the dxy?dxz?yz??orbitals.?3?Experimental demonstration of the sign reversed gaps in electron pocket dom-inated(Li_1-xFe_x)OHFe1-yZnySe.We have observed an in-gap impurity state possibly induced by the doped Zn impurity.The applied magnetic field does not change the energy of impurity state but only suppress the height of the resonant peak,which is the feature of a non-magnetic impurity.Then,using a phase-sensitive QPI technique,we show that the superconducting gaps in(Li_1-xFe_x)OHFe1-yZnySe change sign within the electronic pockets.