Majorana Zero-energy Mode In Iron-based Superconductors And Topological Properties In Two-dimensional Materials

In 1937,an Italian physicist,Ettore Majorana,predicted an elementary particle which is its anti-particle,called the Majorana Fermion.The Majorana Fermion in the condensed materials with non-Abel statistics holds a great promise for the realization of topological quantum computing,which has caused widespread concern.Herein,using scanning tunneling microscope/spectroscopy?STM/S?,we systematically study the properties of Majorana zero modes?MZMs?at vortex core in an iron-based superconductor Fe Te_0.55Se_0.45.These results offer a potential single-material platform for manipulating MZMs at a relatively high temperature.1. Using variable-tunnel-coupled scanning tunneling spectroscopy,we study conductance plateau for MZMs at vortex cores on FeTe_0.55Se_0.45 surface,which is a hallmark for Majorana.Due to its particle-antiparticle equivalence,an MZM exhibits robust resonant Andreev reflection and 2e²/h quantized conductance plateau at low temperature.It is found that only MZMs in vortex cores hold the feature of conductance plateau.In contrast,no such plateau features were observed on other trivial vortices bound states.The conductance plateau feature for MZMs can be observed in different samples and different instruments.The statistical results show that the values of plateau conductance?G_p?are close to or even reaching the 2e²/h quantum conductance.We further found that,both instrumental broadening and quasiparticle poisoning in our system can potentially induce deviation of G_p from the theoretical quantized value 2e²/h.This unique behavior of the zero-mode conductance reaching a plateau strongly supports the existence of MZMs in this iron-based superconductor,which serves as a promising single-material platform for Majorana braiding at a relatively high temperature.2. By comparing the energy level in different vortices on FeTe_0.55Se_0.45 surface,we found the integer quantized and half-odd-integer quantized bound states in the two classes of vortex respectively,and investigate the microscopic mechanism of the MZMs.We further preformed a detailed study for two distinct classes of vortex,including the topological vortices with MZM and the ordinary vortices without MZM.Further measurements show that the spatial patterns of the two types vortex bound states also have significant differences.These signatures indicate that the vortex quasiparticles of the two types of vortices are excited from the topological Dirac surface state and the trivial bulk states,which induces a half-odd-integer quantized energy level shift between two types of vortex bound states.We further perform a statistic for the spatial distribution of the two types of vortices,it is found that the two types of vortices coexist in the sample,and the ratio of the vortex has a strong spatial dependence.Our model calculations fully reproduce the spectra of these two types of vortex bound states,indicating the presence of topological and conventional superconducting regions that coexist within the same crystal.Our findings provide strong evidence for the topological nature of superconductivity in Fe Te_0.55Se_0.45 which have great significance for further research on MZMs.3. Combining the STM measurement and DFT calculation,we report the evidence of one-dimensional topological edge states in buckled antimonene monolayer.We epitaxially grow high-quality buckled antimonene monolayer with nanoislands on Cu?111?substrate and prove its buckled atomically structure with STM and some other measurements.The DFT calculation results show that there are one-dimensional topological edge states in freestanding.The topological protected edge states are robust,but the substrate make it difficult to observe in STM measurements.We observed significantly one-dimensional topological edge states around the rim of the buckled antimonene nanoisland,benefiting from the screen effect of the first antimonene monolayer.We further found that there are two different types of edge states on the opposite edges of the islands.The energy and spatial feature of measured edge states agree well with the theoretical results.Our findings establish monolayer antimonene as a new class of topological monolayer materials hosting the topological edge states,which advances the further studies of physical properties and future applications of antimonene and other two-dimensional topological materials.