Large-Angle Twistronics Reveals Topological States and Chiral Coupling in van der Waals

The emerging field of ‘twistronics’ manipulates the properties of layered materials by slightly rotating them relative to each other, but most research concentrates on small rotations, a new study reveals the surprising importance of large-angle twisting in bilayer graphene. Juncheng Li, Cong Chen, and Wang Yao, all from the New Cornerstone Science Laboratory at the University of Hong Kong, demonstrate that significant rotations induce a unique form of electron behaviour governed by ‘Umklapp scattering’, fundamentally altering the material’s electronic structure. Their work shows that these rotations create regions with distinct chiralities, and crucially, boundaries between these regions host robust, previously unseen topological domain-wall states, which behave like protected pathways for electrons. This discovery expands the potential for designing novel electronic devices within twisted materials, offering a new route to control and manipulate electron flow with unprecedented precision.

Intervalley Umklapp scattering plays a vital role in large-angle twisted bilayer graphene, governing its low-energy physics and driving unconventional band topology. Researchers construct symmetry-constrained models to demonstrate how structural chirality imprints distinct electronic responses. The arrangement of the twisted layers, specifically whether they align with hexagon centers or atoms, creates distinct symmetries, categorized as D6 and D3 configurations. The D6 arrangement results in a gapped electronic spectrum, while the D3 configuration allows for semimetallic behaviour with unique quadratic band crossings. Crucially, switching between these configurations, effectively inverting the structural chirality, generates topological domain-wall states, which are absent in untwisted bilayers. These states manifest as counterpropagating pseudospin modes at interfaces between oppositely twisted regions.

Twisted Bilayer Graphene and Correlated Phenomena

Research into twisted bilayer graphene (TBG) and related 2D materials reveals a wealth of correlated electronic phenomena and topological properties. This field explores how the stacking and twisting of atomic layers creates unique electronic behaviours, including correlated insulating states, superconductivity, and novel topological phases. A central theme is the emergence of strong electron-electron interactions, leading to phenomena like Mott insulators and topological insulators. The formation of Moiré patterns, created by the twisting or stacking of layers, is also a recurring topic, as these patterns modify the electronic band structure and give rise to new properties.

Symmetry and topology are key concepts, with researchers investigating how these principles govern the electronic states within these materials. Computational materials science plays a crucial role, with researchers employing first-principles calculations and other computational techniques to predict and understand the electronic structure of 2D materials. Experimental characterization, using techniques like ARPES, STM, and transport measurements, is also essential for verifying theoretical predictions and exploring the properties of these materials. This research extends beyond TBG to other twisted bilayer systems and multilayer graphene structures, broadening the understanding of these phenomena.

Early research established the observation of correlated insulating states and superconductivity in TBG at specific twist angles. Subsequent work focused on developing theoretical models to explain these observations and understanding the nature of superconductivity, including the role of electron-phonon coupling and disorder. Studies also investigated the Mott insulating states in TBG and the effects of electron filling and band structure. This research has expanded to include investigations of topological insulators, Dirac/Weyl semimetals, and other topological phases in 2D materials, exploring the role of spin-orbit coupling and symmetry.

This compilation of research serves as an excellent starting point for a comprehensive literature review. It can help researchers identify current trends and potential areas for future investigation, and provides a valuable resource for students and researchers learning about these topics. The information can also be used to build databases of 2D materials with specific properties and to design new materials with tailored characteristics. This research fosters collaboration between physicists, chemists, materials scientists, and engineers, and supports grant proposals for further investigation.

Large Twist Angles Drive Novel Electron Behaviour

Researchers have discovered that large-angle twisting in bilayer materials fundamentally alters their electronic properties and creates unconventional topological states. While previous studies focused on small twists, this work demonstrates that significant rotations induce a unique form of electron behaviour driven by intervalley Umklapp scattering. This scattering dominates at larger twist angles and fundamentally alters the material’s electronic structure. The team found that the arrangement of the twisted layers, specifically whether they align with hexagon centers or atoms, creates distinct symmetries, categorized as D6 and D3 configurations.

The D6 arrangement results in a gapped electronic spectrum, while the D3 configuration allows for semimetallic behaviour. Importantly, switching between these configurations, effectively inverting the structural chirality, generates topological domain-wall states, which are protected electronic states appearing at the boundaries between differently twisted regions. These domain-wall states emerge through a mechanism similar to that observed in other topological systems, where the change in chirality creates counterpropagating modes within the interface. Detailed calculations confirm that these states are robust, persisting even when the material’s symmetry is slightly disrupted.

This resilience suggests potential for practical applications, as the states are not easily destroyed by imperfections. The discovery of these chirality-driven topological states opens new avenues for engineering materials with tailored electronic properties. By controlling the twist angle and layer arrangement, researchers can design materials with robust, protected pathways for electron transport, potentially leading to advancements in chiral electronics and other novel devices.

Twisted Bilayers Host Robust Topological States

This research demonstrates a crucial role for interlayer rotation in twisted bilayer materials, revealing how large-angle twists give rise to unique electronic properties and topological states. The team discovered that significant twisting creates chiral interlayer coupling, influencing the material’s electronic spectrum and leading to semimetallic or gapped configurations depending on the stacking arrangement. Importantly, the inversion of this chirality at interfaces between differently twisted regions generates topological domain-wall states, which are absent in untwisted materials and manifest as counterpropagating pseudospin modes. These topological domain-wall states are remarkably robust, persisting even when subjected to various symmetry-breaking perturbations, including applied strain and on-site potential variations, as long as the bulk electronic gap remains intact.

The researchers extend this finding, suggesting that these states should be generally observable in twisted bilayer systems beyond ideal configurations, provided the bulk gap is preserved. The strength of the intervalley Umklapp scattering, estimated at around 10 meV, is identified as a key factor governing the formation of these states. While the study focuses on specific conditions, the findings open new avenues for engineering and controlling topological states in twisted materials, potentially leading to novel electronic devices.