Electronic Properties And Comparative Study Of Magic-angle Twisted Bilayer And Twisted Monolayer-bilayer Graphene

Recent years,twisted bilayer graphene had been become an excellent research system in Condensed Matter Physics that its flat bands induced a variety of novel,abundant strong-correlation phenomena and topological properties.Recently,transport studies have demonstrated the great potential of twisted monolayer-bilayer graphene（t MBG）as a new platform to host moire flat bands with a higher tunability than twisted bilayer graphene（t BG）.However,a direct visualization of the flat electronic bands of t MBG is still lacking and its microscopic localization knowledge,when compared with t BG,remains elusive,which are indispensable for a full understanding of the rich emergent correlation physics in t MBG.In this thesis,via fabricating the coexistence structure of magic-angle twisted bilayer graphene and magic-angle twisted monolayer-bilayer graphene in a single sample,we perform a detailed comparative study of their structure and electrical properties using scanning tunneling microscopy（STM）and scanning tunneling spectroscopy（STS）.The research results of this paper obtained are as follows:（1）First,we show a localized spectroscopic study of the structure and flat-band electronic properties of the twisted bilayer graphene region.The study found that the three-fold（C₃）rotational symmetry is broken,and the flat-band states is localized at the AA site,which are consistent with the previous experiments and theories.In addition,We show that the moiréelectronic structures,including the low-energy flat bands,can sufficiently exist in a complete moiréspot,i.e.,a moirésupercell,right at the edge even the translational symmetry of the moirépatterns is broken in one direction in twisted bilayer graphene region.However,the flat-band characteristic is obviously absent in the incomplete moiréspots that partly terminated by the edge.Our results indicate that a whole moiréspot is necessary for the generation of the effective moiréflat bands in twisted bilayer graphene.The experimental results in this subsection provide fresh microscopic knowledge of graphene moiréflat-band structures and emphasizes the influence of edges on the electronic structures of twisted van der Waals systems.（2）Then,we perform the local spectroscopic studies of structure and flat-band electronic properties of twisted monolayer-bilayer graphene.We show that two-fold rotation symmetry（C₂）is broken of the twisted monolayer-bilayer graphene using STM and STS.We observe a sharp density of states peak near the Fermi energy in tunneling spectroscopy,confirming unambiguously the existence of flat electronic bands in twisted monolayer-bilayer graphene.In addition,by measuring spatially resolved spectroscopy,combined with continuum model calculation,we show that the flat-band states in twisted monolayer-bilayer graphene exhibit a unique layer-resolved localization-delocalization coexisting feature which localization in twisted side while delocalization in bilayer side.The results in this subsection provide a comprehensive study of the local microscopic spectroscopic properties and structural characteristics of twisted monolayer-bilayer graphene,which provide an effective experimental foundation for studying the local microscopic properties of twisted monolayer-bilayer graphene.（3）Finally,we exhibit a comparative study between twisted bilayer graphene and twisted monolayer-bilayer graphene.The bandwidth of the flat-band peak of twisted monolayer-bilayer graphene is found to be slightly narrower than that of twisted bilayer graphene,validating previous theoretical predictions.In addition,the localized characteristic of twisted side of twisted monolayer-bilayer graphene is similar to that of twisted bilayer graphene;compared with previous experimental reports,it is found that delocalized characteristic of bilayer side of twisted monolayer-bilayer graphene is similar to twisted double-bilayer graphene（t DBG）.This result suggests that there may be both t BG-like and t DBG-like electronic strong interaction mechanism in magic-angle twisted monolayer-bilayer graphene.The above results provide important microscopic information for a better understanding of the flat-band-induced strongly correlated physics in graphene moirésystems...