Wse2 Modulation of Twisted Bilayer Graphene Creates Asymmetric Flat Bands and Three Distinct Atomic Stackings

The subtle interplay between layered materials can dramatically alter their electronic properties, and a team led by Chi Zhang, Shihao Zhang, and Mengmeng Zhang from Hunan University now demonstrates a surprisingly powerful effect in twisted bilayer graphene. They reveal that placing graphene on a substrate of tungsten diselenide (WSe2) significantly modifies its band structure, creating asymmetry within the flat electronic bands crucial for correlated quantum phenomena. This research establishes that the precise atomic arrangement at the interface between these materials, and the resulting strain, acts as a previously unrecognised control mechanism, enabling the simultaneous emergence of competing charge orders. This breakthrough overcomes a fundamental limitation in the field, paving the way for the design of novel quantum materials with tailored properties and potentially unlocking new avenues in electronic device development, alongside Lin He and Qi Zheng.

Graphene-WSe2 Twist and Registry Effects

This research investigates how twist angle and strain influence the electronic properties of twisted bilayer graphene when combined with a single layer of tungsten diselenide. Scientists discovered that the atomic stacking order between graphene and tungsten diselenide significantly impacts the electronic band structure, particularly the formation of flat bands near the material’s Fermi level. Applying strain releases twist-induced stress and broadens the electronic wavefunctions, promoting spatial overlap and homogenization of electronic properties within the stacked regions. By carefully controlling the twist angle and releasing strain, the researchers achieved a more uniform distribution of electronic states, demonstrating greater control over the material’s electronic behavior.

The method involves fabricating heterostructures by stacking graphene and tungsten diselenide, then characterizing them using scanning tunneling microscopy and spectroscopy to map the topography and local density of electronic states. Theoretical calculations using density functional theory simulated the electronic band structure for different stacking configurations and strain levels. This work demonstrates that precise control of twist angle and strain in these heterostructures allows tuning of atomic registry, manipulation of the electronic band structure, and control over correlated electronic states, with implications for developing novel electronic devices based on twisted bilayer graphene and other moiré materials.

Graphene Heterostructures Reveal Electronic Properties

Scientists engineered a method to investigate the electronic properties of magic-angle twisted bilayer graphene by integrating it with atomically flat tungsten diselenide. Researchers fabricated samples by carefully transferring graphene monolayers with precisely controlled twist angles, near 1. 1 degrees, onto mechanically exfoliated tungsten diselenide sheets. Scanning tunneling microscopy revealed a moiré superlattice, where bright spots indicate regions where the graphene layers are directly stacked, and darker areas represent alternating stacking domains within the twisted bilayer graphene. Detailed analysis of these images allowed scientists to accurately determine the twist angle, measured as 1.

19 degrees, and uniaxial heterostrain, quantified as 0. 405 percent. This precise determination, facilitated by the moiré pattern acting as a natural magnifying glass, enabled faithful replication of the moiré structure through theoretical modeling. Simultaneously, scanning tunneling microscopy of the tungsten diselenide revealed a relative twist angle of approximately 17 degrees with respect to the graphene. The study challenged conventional understanding of moiré superlattice periods in heterostructures with significant lattice mismatch, demonstrating that the large relative twist angle results in three distinct atomic stacking configurations.

Researchers meticulously mapped these configurations in real space, revealing interwoven transitions within the moiré unit cell. Crucially, the team demonstrated that these varying stacking configurations significantly modulate the electronic band structure of the twisted graphene, shifting the flat bands and inducing asymmetry within a single stacked region. Spectroscopic mapping and analysis of the local density of states within the stacked regions revealed significant asymmetry near the Fermi level, confirming the breaking of rotational symmetry due to interfacial coupling with the tungsten diselenide substrate.

Atomic Stacking Breaks Symmetry in Twisted Graphene

Scientists revealed a profound influence of atomic arrangement in van der Waals heterostructures on the electronic properties of magic-angle twisted bilayer graphene. Their work demonstrates that the precise stacking of graphene with tungsten diselenide significantly modulates the flat bands within the twisted graphene, creating asymmetry where previously there was none. Experiments using scanning tunneling microscopy and spectroscopy revealed three distinct atomic stacking configurations within the heterostructure, inducing position-dependent potentials that shift the flat bands from hole-like to electron-like behavior within a single region. This symmetry breaking enables the unprecedented coexistence of orthogonal stripe charge orders, a phenomenon previously considered impossible due to fundamental interactions.

The team measured a substantial energy separation of up to 26 meV between the valence and conduction band flat bands at specific locations within the stacked regions. Detailed analysis of the energy profiles across these regions confirms that the flat bands exhibit linear energy shifts, indicative of filling transitions, while remote bands remain largely unaffected by the tungsten diselenide interface. Furthermore, the research demonstrates that interfacial coupling with tungsten diselenide induces asymmetric localization of the flat bands, stabilizing the coexistence of these orthogonal charge orders. Mapping the local density of states revealed that the major axes of the charge order ellipses are roughly orthogonal to each other, ruling out explanations based solely on strain.

Interface Atomic Registry Controls Graphene Band Modulation

Scientists discovered that variations in the stacking order of atoms at the interface between graphene and tungsten diselenide create position-dependent potentials that shift the energy bands within the twisted graphene. This band modulation transforms the behavior of the graphene’s flat bands, creating asymmetry within a single region and enabling the simultaneous existence of charge orders previously thought incompatible due to fundamental interactions. The team’s findings establish that interfacial atomic registry is a critical, previously overlooked factor in controlling the behavior of flat bands, offering a new pathway to engineer correlated quantum states in these complex materials. Detailed analysis using scanning tunneling microscopy and spectroscopy revealed that the interplay between atomic stacking and strain within the graphene significantly alters its electronic structure, leading to antisymmetric flat-band distributions. While the current work focuses on graphene/tungsten diselenide heterostructures, the principles established here likely extend to other van der Waals systems, opening possibilities for designing novel quantum materials.