image: (a–b) Schematic of the back-gate tunable tMoTe₂ device encapsulated in hexagonal boron nitride (h-BN). A vertical displacement field is applied between the STM tip and the bottom graphite gate. (c) Under zero electric field, the K-valley moiré flat bands exhibit a topologically non-trivial honeycomb lattice structure. (d) Upon application of a displacement field, the interlayer coupling is modified, and the flat bands transform into a topologically trivial triangular lattice structure.
Credit: ©Science China Press
In recent years, twisted two-dimensional materials, particularly twisted bilayer transition metal dichalcogenides (TMDs), have emerged as a forefront platform in condensed matter physics. By adjusting the interlayer twist angle, rich strongly correlated and topological quantum states can be artificially engineered. Among these, twisted bilayer molybdenum ditelluride (tMoTe₂) has garnered significant attention for experimentally exhibiting the fractional quantum anomalous Hall effect at zero magnetic field for the first time. This phenomenon highlights the intricate interplay between topology and non-trivial electron correlations, holding substantial fundamental scientific value. However, direct experimental evidence at the atomic scale regarding the microscopic origin of topological moiré flat bands in tMoTe₂ and their response to electric fields has been lacking. Furthermore, the high susceptibility of few-layer MoTe₂ to degradation in air poses a significant challenge for precise local electronic state measurements.
Recently, a collaborative study by the research groups of Prof. Wang Shiyong and Prof. Li Tingxin from Shanghai Jiao Tong University, along with Prof. Zhang Yang from the University of Tennessee, was published in National Science Review. Titled "Imaging moiré flat bands and Wigner molecular crystals in twisted bilayer MoTe2," this work provides key real-space experimental evidence for understanding the quantum states in this system. The researchers directly observed the electric-field-tuned topological moiré flat bands and the formation of Wigner molecular crystals in tMoTe₂.
To address the material's air sensitivity, the team developed an integrated device fabrication and characterization protocol based on hexagonal boron nitride (h-BN) encapsulation for scanning tunneling microscopy (STM) studies (Fig. 1a, b). This approach effectively protects the sensitive sample from air exposure, enabling atomic-resolution imaging and electronic state probing of the tMoTe₂ moiré superlattice. Using high-resolution scanning tunneling spectroscopy, they directly observed the continuous tuning of the topological moiré flat bands near the K-valley by a displacement field: under zero electric field, the flat bands exhibit a topologically non-trivial honeycomb lattice structure; upon applying a vertical electric field, this structure transitions to a topologically trivial triangular lattice, demonstrating an electric-field-induced topological phase transition (Fig. 1c, d).
Under strong displacement fields, the team further investigated strong electron correlation effects. At an electron filling factor of ν=3, they directly observed a Wigner molecular crystal formed by three electrons self-organizing within a moiré potential well due to Coulomb repulsion (Fig. 2a). By varying the tip-sample distance (thereby tuning the dielectric screening strength), the effective electron-electron interaction could be modulated. This allowed the observation of the crystal's evolution from a closely packed arrangement to an expanded, clearly defined Kagome lattice (Fig. 2b), providing direct real-space evidence for charge-ordered states in strongly correlated systems.
This study provides direct atomic-scale visualization of the microscopic picture of topological moiré flat bands in tMoTe₂ and their electric-field-tuning mechanism, with experimental observations showing excellent agreement with theoretical calculations. The real-space imaging of Wigner molecular crystals offers crucial experimental insight into charge-ordered states in strongly correlated electron systems. Furthermore, the developed h-BN-encapsulated STM technique establishes a reliable experimental protocol for studying other air-sensitive advanced quantum materials. This work deepens the understanding of the synergistic control of topology and correlation in twisted moiré systems and paves the way for the future design and exploration of quantum electronic devices based on twisted materials.
Journal
National Science Review