Imaging moiré flat bands in 3D reconstructed WSe2/WS2 superlattices (2007.06113v1)

Abstract: Moir\'e superlattices in transition metal dichalcogenide (TMD) heterostructures can host novel correlated quantum phenomena due to the interplay of narrow moir\'e flat bands and strong, long-range Coulomb interactions1-5. However, microscopic knowledge of the atomically-reconstructed moir\'e superlattice and resulting flat bands is still lacking, which is critical for fundamental understanding and control of the correlated moir\'e phenomena. Here we quantitatively study the moir\'e flat bands in three-dimensional (3D) reconstructed WSe2/WS2 moir\'e superlattices by comparing scanning tunneling spectroscopy (STS) of high quality exfoliated TMD heterostructure devices with ab initio simulations of TMD moir\'e superlattices. A strong 3D buckling reconstruction accompanied by large in-plane strain redistribution is identified in our WSe2/WS2 moir\'e heterostructures. STS imaging demonstrates that this results in a remarkably narrow and highly localized K-point moir\'e flat band at the valence band edge of the heterostructure. A series of moir\'e flat bands are observed at different energies that exhibit varying degrees of localization. Our observations contradict previous simplified theoretical models but agree quantitatively with ab initio simulations that fully capture the 3D structural reconstruction. Here the strain redistribution and 3D buckling dominate the effective moir\'e potential and result in moir\'e flat bands at the Brillouin zone K points.

Analysis of Moiré Flat Bands in 3D Reconstructed WSe$_2$/WS$_2$ Superlattices

The paper presented in this paper provides an in-depth investigation of moiré flat bands in three-dimensional (3D) reconstructed WSe$_2$/WS$_2$ superlattices. These findings are achieved by employing scanning tunneling spectroscopy (STS) combined with ab initio simulations, offering critical insights into the atomically reconstructed moiré superlattice and its electronic properties which are pivotal for understanding novel quantum phenomena.

Experimental Insights and Methods

The research utilizes high-quality exfoliated TMD heterostructure devices for STS measurements to evaluate the moiré electronic structure. The experiments reveal a significant 3D buckling of the WSe$_2$/WS$_2$ heterostructures correlated with large-scale in-plane strain redistribution. The STS imaging displays a notably narrow K-point moiré flat band at the valence band edge. This band, with a remarkable bandwidth of only 10 meV, is expected to underlie observed correlated insulator phenomena and generalized Wigner crystal states. The localization of this band at specific moiré sites challenges previous models based on simplified density functional theory (DFT).

Theoretical Validation

The theoretical analysis through ab initio simulations confirms the experimental findings, contradicting previous simplified theoretical models by accurately capturing 3D structural reconstructions. The analysis identifies that strain redistribution primarily governs the moiré potential, influencing the formation of flat bands at the Brillouin zone K points. Furthermore, the ab initio simulations validate that the Γ-point moiré bands are shaped by interlayer electron hybridization while confirming that the K-point flat bands arise from monolayer deformations.

Significant Findings

• Narrow Flat Bands: The work reports a top-most valence flat band at the K-point with a significantly narrow 10 meV bandwidth, isolated from deeper bands by a 50 meV gap. This characteristic is pivotal in the emergence of correlated phenomena.
• Localization: STS spatial mapping showcases the sharp localization of the K-point flat band, corroborating theoretical predictions.
• Experimental Agreement: The visualization of these localized bands and their behavior is corroborated by DFT simulations using reconstructed moiré superlattices, establishing a firm agreement between experimental observations and theoretical insights.

Implications and Future Directions

The findings provide a fundamental understanding of the interplay between atomic geometry, moiré band structure, and Coulomb interactions in TMD-based moiré superlattices. These insights could pave the way for controlled experimentation and manipulation of novel quantum properties in similar systems. The enhanced understanding of the strain-induced modulations and buckling effects may open avenues for optimizing TMD devices and exploring potential technological applications in quantum computing and advanced materials.

Looking ahead, the research implications extend to the development of quantum materials through precise control of moiré patterns in stacked 2D materials. Future directions may include exploration of other transition metal dichalcogenide systems to observe similar phenomena, thereby broadening the applications of correlated insulator states and potentially realizing advanced electronic and optoelectronic components.

In conclusion, this research significantly advances the understanding of moiré flat bands in 3D reconstructed TMD heterostructures, providing a robust platform to explore and exploit novel quantum phenomena within engineered materials.