STM Study On Iron-based Superconductors,Topological Superconductors And Topological Insulators

Scanning Tunneling Microscopy/Spectroscopy?STM/S?is a powerful experimental method to investigate the sub-nano world in real and energy space.Since1980s,STM/S has solved a lot of problems in the world of Physics.Nowadays,Scanning Tunneling Microscope could be found in every key laboratory of Physics.In this paper,I will introduce the basic information of Scanning Tunneling Microscope and show some interesting discoveries by STM/S.As a very important family of high-Tc Superconductors that discovered in 2008,Iron-based Superconductors?IBSCs?had received a lot of attention.Similar to the Cuprates,they all bear the two-dimensional layered structure and they share similar phase diagrams.Nevertheless,the difference between IBSCs and Cuprates should not be ignored.Most of the parent compounds of Cuprates are insulators,while the parent compounds of IBSCs are poor metals.We've systematically studied iron chalcogenide Fe1+yTe1-xSex?x=0%⁵0%?system by STM/S.Substituting of Se for Te into the parent compound FeTe,the intrinsic antiferromagnetic?AFM?order is suppressed,along with the occurrence of mixture phase of superconductivity and AFM.Continue increasing the content of Se,the AFM phase disappears and a 5.2 meV superconducting gap emerges at x=8.3%.Interestingly,the gap feature exists above T_c?10 K?,which resembles the pseudogap feature discovered in Cuprates.Keep on substituting Se to41%,the gap shrinks to 1.5 meV while Tc rises to 14 K.Along with the rise of T_c,the gap value shrinks while the phase coherence increases?According to the fitting parameter of Dynes function?.The results indicate that the T_c of Fe1+yTe1-xSex?x=0%⁵0%?may be limited by the phase coherence but not the pairing strength.The search for Majorana fermions is one of the most interesting topics in condensed matter physics because the Majorana fermions are predicted to obey non-abelian statistics,which is the basis of fault-tolerance quantum computation.Majorana fermions are predicted to exist at the center of vortex of topological superconductors?TSCs?,which hold superconductivity and topological surface state.With noncentrosymmetric lattice structure,PbTaSe₂ is a promising material to realize TSC not only because it is a superconductor,but the lack of inversion symmetry may help the surface states behave like the surface states of topological insulators.We use STM/S to study the surface states of PbTaSe₂ by quasi-particle interference?QPI?.The results support the topological nature of the surface state,which indicates PbTaSe₂ is a promising candidate for TSCs.We also use STM/S to study the two-dimensional topological insulator?2D TI?candidate HfTe₅.We observe a full energy gap of⁸0 meV near the Fermi level on the surface monolayer of HfTe₅ and conducting in-gap states at the step edge.Remarkably,the in-gap edge states disappear on a one-dimensional stripe scaled by one unit-cell in width,which can be regarded as the key part of the edge of 2D HfTe₅ monolayer.Therefore,the observed in-gap states are not trivial ones caused by translation symmetry breaking at the edge,but topologically protected by the nontrivial bulk properties of the 2D HfTe₅ monolayer.Our results provide fresh and strong experimental evidence for the topological edge states in HfTe₅,suggesting HfTe₅ is a large-gap 2D TI.