Topological States In Iron-based Superconductors

Topological states and strongly correlated systems have been the most active research fields in condensed matter physics in recent years.Rencently,the study of Majorana zero-energy modes(MZMs)in iron-based superconductors has linked the two fields and injected vitality into the research of iron-based superconductivity.The ironbased superconductors with strong electron correlation,high Tc and rich topological states successfully avoid the difficulties of the intrinsic topological superconductor and the superconducting heterojunction in the Fu-Kane model,and provide a good platform for studying the properties of MZMs.Topological states have been found in a variety of iron-based superconducting materials,and MZMs have been successfully observed in the vortices of some materials.This opens a new direction for further exploration of MZMs and realization of topological quantum computing.In general,there are two kinds of bound states in superconductors.One is the bound state in the magnetic vortices induced by the applied magnetic field,and the other is the bound state on the magnetic impurities due to the exchange coupling.In this thesis,the magnetic impurity states in iron-based superconductors with topologically nontrivial surface states are studied in detail.We found that the magnetic impurity states were affected both by their own exchange coupling strength and the presence or absence of topological surface states at their locations because of the intrinsic inhomogeneity in FeTe0.55Se0.45.In the topological surface state region,quantum anomalous vortex state(QAV)could be realized when the exchange coupling strength of impurities is strong enough,and the MZMs caused by topological surface states could be observed in it.The hybridization between MZMs can be realized by using the magnetic vortices induced by the applied magnetic field.We also developed a method to control the exchange coupling strength between the impurity and the superconductor surface by adjusting the distance between the STM tip and the impurity,and successfully observed the quantum phase transition of Yu-Shiba-Rusinov(YSR)bound states.Using this method,we can tune YSR bound states to QAV states,which may provide a new way to study the properties of MZMs,including their braiding.In this thesis,we will also focus on the topological phase transition of Fe(Te,Se)system,which is the first example of the topological states observed in iron-based superconductors.Through the measurements of several Fe(Te,Se)samples with different doping and excessive Fe by ARPES,we find that the doping of Te not only affects the pz band,but also the dxz band,which is another key role of band inversion.Whether the band inversion can occur depends on the competition between the two changes.The presence of excess Fe inhibits the occurrence of band inversion,which could explains the absence of MZMs in the early experiments with samples containing excess Fe.Finally,the superconducting properties of heterojunction films composed of topological insulators Bi2Te3 and FeTe are studied.We use Molecular Beam Epitaxy(MBE)to grow high quality 1QL Bi2Te3/FeTe heterojunctions and observe their superconducting transitions with Tc around 11 K,similar to the Tc of the Fe(Te,Se)system.The superconducting energy gap and the vortex bound states were studied by scanning tunneling microscope and the unique anisotropic vortex bound states were observed.The superconductivity of this system should not simply be the superconductivity of FeTe caused by the electron transfer at the interface,and may source from the interface between the two materials.The existence of strange properties may enrich the understanding of topological states in strong correlation systems.In this thesis,the topological states in iron-based single crystals and thin film samples are comprehensively investigated.The magnetic impurity states in FeTe0.55Se0.45,the topological phase transition in Fe(Te,Se)system and the exotic anisotropic superconducting state in 1QL Bi2Te3/FeTe are carefully studied.These studies combine the topological states and superconducting states in iron-based superconductors,and provide a basis for further exploration of topological states in strongly correlated systems and in-depth study of Majorana physics.