Thickness-Dependent Topological Phase Transitions and Evolution of Flat Bands in Rhombohedral Multilayer Graphene from Few-Layer to Bulk Limit: A NanoARPES Study

Belangrijkste concepten

The electronic structure of rhombohedral multilayer graphene (RMG) undergoes a thickness-dependent topological phase transition, evolving from a 3D generalization of the Su-Schrieffer-Heeger (SSH) model in thin layers to a topological Dirac nodal spiral semimetal (DNSS) in the bulk limit, as revealed by NanoARPES measurements.

Samenvatting

Bibliographic Information:

Xiao, H. B., Chen, C., Sui, X., Zhang, S. H., Sun, M. Z., Gao, H., Jiang, Q., Li, Q., Yang, L. X., Ye, M., Zhu, F. Y., Wang, M. X., Liu, J. P., Zhang, Z. B., Wang, Z. J., Chen, Y. L., Liu, K. H., & Liu, Z. K. (Year). Thickness-dependent Topological Phases and Flat Bands in Rhombohedral Multilayer Graphene.

Research Objective:

This study investigates the layer-dependent evolution of the electronic structure in rhombohedral multilayer graphene (RMG) from few-layer samples to the bulk limit, aiming to understand the interplay between electron correlations, topological phases, and the emergence of exotic quantum states.

Methodology:

The researchers employed spatially resolved angle-resolved photoemission spectroscopy (NanoARPES) to probe the electronic structure of RMG with varying layer numbers (N = 3, 24, and 48). Density functional theory (DFT) calculations were performed to support and interpret the experimental findings.

Key Findings:

• The NanoARPES measurements revealed the presence of topological surface flat bands (SFBs) and gapped subbands in RMG, consistent with the SSH model.
• As the layer number increased, the number of subbands increased, the subband energy spacing decreased, and the subband gap closed.
• In the bulk limit (N = 48), the gapped subbands transitioned into gapless 3D Dirac cones spiraling around the K/K' points, forming a topological DNSS.
• The SFBs evolved into drumhead surface states with a large number of crossing points in the bulk limit.

Main Conclusions:

The study provides direct experimental evidence for a thickness-driven topological phase transition in RMG, transitioning from a 3D generalization of the SSH model to a topological DNSS as the layer number increases. The observed evolution of the electronic structure and the presence of flat bands highlight the potential of RMG as a platform for exploring the interplay between strong correlations and topological physics.

Significance:

This research significantly contributes to the understanding of the electronic properties of RMG and its potential for hosting exotic quantum phases. The findings have implications for the development of novel electronic devices based on topological materials.

Limitations and Future Research:

Further investigations are needed to explore the correlation effects in RMG and their influence on the observed topological phases. Additionally, studying the effects of external stimuli, such as electric fields and strain, on the electronic structure of RMG would be of interest.

Statistieken

The subband gap in N=3 RMG is about 276 meV.
The subband gap in N=24 RMG is about 83 meV.
The bandwidth of the SFB in N=3 RMG is more than 107 meV.
The bandwidth of the SFB in N=24 RMG is more than 60 meV.
The bandwidth of the SFB in N=48 RMG is greater than 45 meV.

Citaten

"Rhombohedral multilayer graphene (RMG) has recently been demonstrated as a spectacular research platform hosting rich quantum phenomena."
"A critical factor in tuning the properties of RMG is the number of layers, as different quantum phases have been observed only in samples with specific layer counts."
"The intriguing properties of RMG and its layer-tuning capabilities are indebted to its unique electronic structure, characterized by the flat surface bands."
"Our findings establish RMG as a unique topological flatband system to investigate strong correlations and topological physics."

Belangrijkste Inzichten Gedestilleerd Uit

Thickness-dependent Topological Phases and Flat Bands in Rhombohedral Multilayer Graphene

by H. B. Xiao, ... om arxiv.org 11-19-2024

https://arxiv.org/pdf/2411.11359.pdf

Diepere vragen

How might the manipulation of the layer number and the associated topological phase transitions in RMG be applied to the development of novel quantum devices?

The manipulation of layer number in RMG offers a compelling route to engineer its electronic structure and realize novel quantum devices. This stems from the unique layer-dependent topological phase transitions exhibited by RMG, transitioning from a 3D generalization of the Su–Schrieffer–Heeger (SSH) model to a topological Dirac nodal spiral semimetal (DNSS) in the bulk limit. This characteristic presents several promising avenues for quantum device applications:
1. Engineering Topological Transport Channels:

By controlling the layer number, one can precisely tune the energy gap and dispersion of the topological surface flat bands (SFBs). This allows for the creation of well-defined, topologically protected transport channels with tunable properties.
Devices like topological field-effect transistors could be envisioned, where the layer number acts as a gate-tunable parameter to control the conductance through these channels.
2. Harnessing Strong Correlations for Quantum Phenomena:

The increase in SFB flatness with increasing layer number enhances electron-electron interactions, leading to strong correlation effects. This opens possibilities for exploring exotic quantum phases like unconventional superconductivity, fractional quantum Hall states, and other correlated topological phases.
Devices leveraging these phenomena, such as topological superconductor junctions for Majorana fermion-based quantum computing, could be realized.
3. Exploiting the DNSS Phase:

The emergence of the DNSS phase in bulk RMG, characterized by the drumhead surface states and Dirac nodal spiral, offers a unique platform for spintronics and valleytronics applications.
The helical spin texture of the drumhead surface states could be utilized for spin-polarized transport, while the valley-dependent Berry curvature associated with the Dirac nodal spiral enables valley-selective excitations and control.
4. Integration with Other 2D Materials:

RMG's atomically flat nature makes it suitable for integration with other 2D materials, creating van der Waals heterostructures with tailored properties.
Combining RMG with materials exhibiting strong spin-orbit coupling or magnetism could lead to novel topological phases and functionalities, further expanding the device possibilities.
Overall, the ability to manipulate the layer number and the associated topological properties in RMG provides a powerful tool for designing next-generation quantum devices with enhanced functionalities and performance.

Could alternative theoretical models beyond the SSH model provide a more comprehensive understanding of the observed electronic behavior in RMG, particularly in the bulk limit?

While the SSH model provides a valuable starting point for understanding the topological properties of RMG, particularly the emergence of SFBs, it relies on simplifications that might not fully capture the electronic behavior, especially in the bulk limit. Alternative theoretical models incorporating additional factors could offer a more comprehensive understanding:
1. Beyond Nearest-Neighbor Hopping:

The SSH model considers only nearest-neighbor hopping. Including longer-range hopping parameters (γ₁, γ₂, γ₃, etc.) in a generalized tight-binding model could refine the band structure description, particularly the shape and dispersion of the SFBs and subbands.
2. Electron-Electron Interactions:

The SSH model neglects electron-electron interactions, which become increasingly important with increasing layer number and SFB flatness. Incorporating these interactions using methods like density functional theory (DFT) with Hubbard-like corrections (DFT+U) or dynamical mean-field theory (DMFT) could elucidate the role of correlations in shaping the electronic structure and potentially driving exotic quantum phases.
3. Lattice Relaxations and Distortions:

The SSH model assumes a perfectly rigid lattice. However, in reality, interlayer interactions can induce lattice relaxations or distortions, especially at the surface or in the presence of strain. Accounting for these structural changes using first-principles calculations could provide a more accurate description of the electronic structure.
4. Spin-Orbit Coupling:

While generally weak in pristine graphene, spin-orbit coupling can be enhanced by proximity effects in van der Waals heterostructures or by incorporating heavy atoms. Including spin-orbit coupling in the theoretical models could reveal potential spin-momentum locking in the SFBs and its implications for spintronic applications.
5. Beyond Static Descriptions:

The SSH model provides a static picture of the electronic structure. Employing time-dependent methods like time-dependent DFT (TDDFT) or non-equilibrium Green's function techniques could offer insights into the dynamic response of RMG to external stimuli, crucial for understanding its behavior in device settings.
By incorporating these refinements, alternative theoretical models can provide a more complete and accurate description of the electronic behavior in RMG, particularly in the bulk limit where complexities arising from interlayer interactions, electron correlations, and lattice effects become increasingly significant.

What are the potential implications of the observed flat bands and strong correlation effects in RMG for the realization of unconventional superconductivity or other exotic quantum phases?

The observed flat bands and strong correlation effects in RMG, particularly the enhancement with increasing layer number, hold significant implications for realizing unconventional superconductivity and other exotic quantum phases:
1. Enhanced Superconducting Pairing:

Flat bands lead to a high density of states near the Fermi level, creating a favorable environment for electron pairing. This is crucial for superconductivity, as it amplifies any attractive interaction between electrons, potentially leading to higher critical temperatures.
The strong correlations in RMG can mediate unconventional pairing mechanisms, such as those driven by spin fluctuations or charge density waves, leading to unconventional superconductivity with potentially exotic pairing symmetries.
2. Fractional Quantum Hall States:

Flat bands, combined with strong interactions and the topological nature of RMG, provide a fertile ground for realizing fractional quantum Hall states. These states are characterized by fractionalized excitations with anyonic statistics, holding promise for topological quantum computation.
The tunability of the SFBs through layer number and external fields could allow for the exploration of different fractional quantum Hall phases and their properties.
3. Mott Insulators and Quantum Spin Liquids:

Strong correlations can drive the system into a Mott insulating state, where electrons become localized due to strong Coulomb repulsion. This can lead to the emergence of exotic magnetic phases, such as quantum spin liquids, characterized by long-range entanglement and fractionalized spin excitations.
The interplay between the topological nature of RMG and the Mott insulating state could give rise to novel topological phases with unique properties.
4. Density Waves and Charge Ordering:

The interplay of flat bands, strong correlations, and the Fermi surface topology can lead to instabilities towards the formation of density waves, such as charge density waves or spin density waves. These states exhibit periodic modulations of charge or spin density, respectively, and can significantly alter the electronic properties.
The tunability of RMG's electronic structure could allow for the control and manipulation of these density wave states, potentially enabling novel electronic devices.
In conclusion, the presence of flat bands and strong correlation effects in RMG establishes it as a promising platform for exploring unconventional superconductivity and a plethora of other exotic quantum phases. The tunability of its electronic structure through layer number and external fields further enhances its appeal for uncovering and manipulating these fascinating phenomena, potentially paving the way for future quantum technologies.