File Download
Supplementary
-
Citations:
- Appears in Collections:
postgraduate thesis: Scanning tunneling microscopy study on superlattice topological materials
Title | Scanning tunneling microscopy study on superlattice topological materials |
---|---|
Authors | |
Issue Date | 2022 |
Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
Citation | Zhang, Y. [張昱]. (2022). Scanning tunneling microscopy study on superlattice topological materials. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. |
Abstract | 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 electron-electron 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. |
Degree | Doctor of Philosophy |
Subject | Superlattices as materials Scanning tunneling microscopy |
Dept/Program | Physics |
Persistent Identifier | http://hdl.handle.net/10722/324411 |
DC Field | Value | Language |
---|---|---|
dc.contributor.author | Zhang, Yu | - |
dc.contributor.author | 張昱 | - |
dc.date.accessioned | 2023-02-03T02:11:41Z | - |
dc.date.available | 2023-02-03T02:11:41Z | - |
dc.date.issued | 2022 | - |
dc.identifier.citation | Zhang, Y. [張昱]. (2022). Scanning tunneling microscopy study on superlattice topological materials. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. | - |
dc.identifier.uri | http://hdl.handle.net/10722/324411 | - |
dc.description.abstract | 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 electron-electron 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. | - |
dc.language | eng | - |
dc.publisher | The University of Hong Kong (Pokfulam, Hong Kong) | - |
dc.relation.ispartof | HKU Theses Online (HKUTO) | - |
dc.rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works. | - |
dc.rights | This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. | - |
dc.subject.lcsh | Superlattices as materials | - |
dc.subject.lcsh | Scanning tunneling microscopy | - |
dc.title | Scanning tunneling microscopy study on superlattice topological materials | - |
dc.type | PG_Thesis | - |
dc.description.thesisname | Doctor of Philosophy | - |
dc.description.thesislevel | Doctoral | - |
dc.description.thesisdiscipline | Physics | - |
dc.description.nature | published_or_final_version | - |
dc.date.hkucongregation | 2023 | - |
dc.date.hkucongregation | 2023 | - |
dc.identifier.mmsid | 991044634606003414 | - |