Scanning Tunneling Microscopy Study on Superlattice Topological Materials

Superlattice materials are crystals with periodic repeating patterns of two or more different crystal components or periodic potential modifications in a more general definition. In this thesis, we use scanning tunneling microscopy to study two kinds of superlattice topological materials. The first one is the superlattice stacked layer by layer through van der Waals interactions, such as intrinsic magnetic topological insulators MnBi4Te7, and MnBi8Te13. The second one is the superlattice in the two-dimensional plane, such as twisted monolayer-bilayer graphene with a moiré pattern.

MnBi4Te7 and MnBi8Te13 are layered van der Waals superlattices, consisting of MnBi2Te4 septuple-layer (SL) and Bi2Te3 quintuple-layer (QL) of stack ratio 1:1 and 1:3 respectively. MnBi2Te4 is an intrinsic A-type antiferromagnetic topological insulator, which exhibits novel topological phases including the axion insulator and Chern insulator phase. The antiferromagnetic interlayer coupling of MnBi2Te4 can be weakened by inserting Bi2Te3 layers between adjacent MnBi2Te4 layers. In our scanning tunneling microscopy study of antiferromagnetic topological insulator MnBi4Te7, we distinguish the distinct surface states of the two terminations of MnBi4Te7. With the assistance of angular-resolved photoemission spectroscopy and theoretical calculation, we identify the hybridization gap on the QL termination surface state and the nearly gapless Dirac cone on the SL termination. By analyzing the quasiparticle interference on the two terminations, we confirm that there exists a helical surface state with hexagonal warping on QL terminations and a strongly canted helical surface state on SL terminations sitting between the Rashba-like splitting state from their neighboring QLs.

MnBi8Te13 has a weak ferromagnetic interlayer coupling. Its long-range ferromagnetic order can open a gap in the topological surface states. In the scanning tunneling spectroscopy study of MnBi8Te13, we observe peaks of local density of states (LDOS) at the Dirac point near the exposed edge of SL terminations. The peaks indicate the existence of edge states. Such peaks disappear as we elevate the temperature above the Curie point where the ferromagnetic order vanishes. The temperature response further confirms the edge state is related to the broken time-reversal symmetry by magnetic moment. No such edge state is observed on the other non-magnetic termination of QL1.

Twisted graphene generates moiré patterns due to the misorientation of the two crystal layers. Such moiré modulation can greatly affect the band structure. By stacking two monolayer graphenes with a magic twist angle, the van Hove singularities, formed by the intersection of Dirac cones, merge into a flat band with strong electron-electron interaction. Magic-angle (~1.1-degree) twisted bilayer graphene hosts novel physics properties including superconductivity, insulating states induced by strong electronelectron interaction, nematicity, and so on. If the twist occurs between a monolayer and bilayer graphene, the crystal symmetry is even lower, with broken C2 and mirror symmetry. In our experiment, We study the magic-angle twisted monolayer-bilayer graphene device using gate-tuning scanning tunneling microscopy. We observe the flat band splits as it crosses the Fermi level at ABB sites, which is possibly due to correlated interaction.

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