Investigation Of Majorana Zero Mode In Two Iron Pnictide Superconductors

Majorana zero mode（MZM）is one of the most important research topics in modern condensed matter physics.Due to MZM obeys non-Abelian statistics,MZM is considered as a crucial building block to realize the topological quantum computation.At the same time,as the quasiparticle excitation of Majorana fermion in solid systems,MZM also attracts intensive theoretical and experimental investigations on its novel physical properties.In the past few years,MZM has been reported in many different systems.One of the most important systems is the iron chalcogenide superconductor Fe（Te,Se）.Compared with other systems,Fe（Te,Se）has the advantage of large superconducting（SC）gap,high SC transition temperature,small Fermi energy and intrinsic topological surface state（TSS）close to Fermi level.However,the inhomogeneous distribution of Te and Se as well as the coexistence of topological and non-topological regions on the surface,hamper the Fe（Te,Se）to become the system which has the possibility for practical applications.On the other hand,there are evidences that some iron pnictides also possess TSS.Therefore,whether the iron pnictides can host MZM become an important research issue.To find a better MZM platform than Fe（Te,Se）in iron pnictides is the aim of this dissertation.Thus,by using extreme low-temperature scanning tunneling microscopy/spectroscopy（STM/S）,this dissertation investigates two classes of iron pnictides:Ca KFe₄As₄ and LiFeAs.The innovative results are listed below:1.Investigating the lattice structure and its transition mechanism of cleaved surface of Ca KFe₄As₄ by using the STM atomic resolution.After cleavage,the topography images of Ca KFe₄As₄ show that there are two different types of terraces and the corresponding clusters on the surface.By changing the sample bias of atomic resolution images on flat region,it is found that the lattice structure has a transition from√2×√2 to 1×1 as the bias changes from high to low.First-principle calculations show that this lattice transition originates from the surface buckling of As lattice.Based on experimental and calculated results,it is concluded that the cleaving process happens at Ca/Fe As or K/Fe As layers,and the Ca or K atoms form clusters while As atoms remain intact lattice structure.2.Using STM to investigate the zero-bias peak（ZBP）as well as other discrete peaks in vortex after applying magnetic field perpendicular to the Ca KFe₄As₄ surface.At the region which is clean,flat and homogeneous in SC gap,ZBP and other discrete peaks emerge at the vortex center.The decay behavior of ZBP agrees well with the MZM feature.Multiple peaks fitting demonstrates the bound states in vortex form a integer sequence,which is the strong evidence for MZM and Caroli bound states（CBSs）.The real-space maps of MZM and CBSs intensities are consistent with theoretical simulations.And these results also prove that the Dirac cone is above the Fermi level,which is consistent with angle-resolved photoemission spectroscopy（ARPES）results.3.Investigating the mechanism of MZM disappearance and formation in LiFeAs.There are free vortex and impurity-assisted vortex on LiFeAs surface.Because the Fermi level crosses the TSS twice,causing the two unprotested MZMs fuse into a common fermion.Therefore,no MZM is observed in free vortex.However,impurity induces the local electron doping in impurity-assisted vortex,pushing the Fermi level to the topological Dirac semimetal（TDS）phase.Meanwhile,the impurity breaks the C₄ symmetry,opens a gap for TDS phase and induces a new TSS.Hence,there is a single MZM observed at the vortex core of impurity-assisted vortex.4.Investigating the surface wrinkles of LiFeAs and their influences on superconductivity.Two types of wrinkles are found.Type-I wrinkles enhance the SC gaps and SC transition temperature,while type-II wrinkles suppress the SC gap.The enhancement of superconductivity is reported in LiFeAs the first time.The vortices on wrinkles change from C₄ to C₂ symmetry,suggesting the existence of local strain.The statistics between different orientational wrinkles and the corresponding SC gap size show that there is a jump transition of SC gap size at a certain orientation.Combining the experiment results and the theoretical calculations,it is proposed that the wrinkles can induce the local band shifting,and cause the possible Lifshitz transition.These observations indicate the modulations of band structures and Fermi surfaces would have remarkable influence on the superconductivity of LiFeAs.