Scanning Tunneling Microscopy Study Of Moiré Pattern On PbTaSe2 And Cobalt Doped LiFeAs

Superconducting materials have many spectacle properties,such as complete zero resistance and Meissner effect,making it lots of potential usages.Searching for room temperature superconductor has attracted much attention of both material scientists and physicists.The finding of high temperature superconductors,especially cuprates and iron pnictides encourages the hope of room temperature superconductor.However,the mechanism behind high temperature superconductor is still a mystery.What's more,a lot of new material and artificial composites are being discovery along with many more properties.Superconductivity is now one of the most popular area in condensed matter physics.In this thesis,using our lab built low temperature scanning tunneling microscopy,we present our experimental results of non-centrosymmetric superconducting material PbTaSe₂ and insulating band superconductor Li Fe0.99Co0.01 As.The main content is divided into two parts:1.We study the property of non-centrosymmetric material PbTaSe₂.By comparing the step edge height and crystal structure,we identify the exposed Se and Pb surface with 2x2 reconstructions and 1x1 structures,respectively.We find the electron structures of the two surfaces are quite different except that the superconducting gap remains nearly unchanged.More importantly,we find several superstructure on Se terminated surfaces.By comparing with other experiment and study the energy dependent of the wavelength of the superstructure we identify the superstructure as Moiré pattern.In extend,we find the Moiré pattern is companied by a 1me V energy shift between the bright and dark area of Moiré pattern.Using a simplified model considering space variable interlayer coupling,we try to understand the origin of the energy shift.The LDOS of surfaces with different Moiré pattern shows diverse signature,while the superconducting gap merely getting smaller,this suggests that the superconductivity of Se terminated surface is due to proximity effect of the bulk.2.Exploiting the advantage of STM that can measure both positive and negative energy,we study 1% Co doped Li Fe As,where the insulating band just below Fermi surface opens superconducting gap at low temperature.On contrary to BCS theory prediction of the STM spectra that insulating band superconductivity should have weaker coherent peak and the asymmetry between positive and negative energy?referred as ‘Shadow gap'?,we find the coherence peak remains unchanged comparing to Li Fe As,the ‘shadow gap' is absent,either.We study the electron structure at defects which shows quite different behaver comparing to results on Li Fe As.This suggests the insulating band superconductivity is quite a new phenomenon;its theoretical understanding should be reconsidered.