Twisted bilayer graphene, two sheets of graphene stacked with a slight rotational mismatch, has drawn worldwide attention for its exotic electronic behavior near the “magic angle” of about 1.1°, where superconductivity and other strongly correlated states emerge. However, the even smaller twist-angle regime has long remained experimentally inaccessible.
A new study published in National Science Review reports how tiny mechanical strain can fundamentally reshape the electronic landscape of graphene layers twisted by almost imperceptible angles, opening a new route for controlling quantum states in two-dimensional materials. The work is led by Prof. Wei Li of Tsinghua University and Prof. Qi-Kun Xue from Southern University of Science and Technology, in collaboration with researchers from Imdea Nanoscience. Using high-resolution scanning tunneling microscopy, the team performed the systematic experimental investigation of marginally twisted bilayer graphene (m-TBG) with ultra-small twist angles between 0.06° and 0.35°.
At these ultra-small twist angles, the graphene lattice undergoes dramatic relaxation. Perfectly aligned regions (AA stacking) contract, while the more stable Bernal-stacked regions (AB and BA stacking) expand into large triangular domains. These domains are separated by networks of narrow boundaries known as domain walls, which are predicted to host topologically protected one-dimensional quantum channels. The experimental team directly visualized electronic states of these regions. They found that AA-stacked areas exhibit a sharp spectral peak, revealing highly localized electronic states. In contrast, AB regions show smooth, spatially uniform multi-peak spectra, indicating that lattice reconstruction enhances electronic homogeneity.
Most significantly, the study demonstrates that strain can switch the type of the domain walls. The researchers identified two distinct types of domain walls. One type displays a pronounced electronic resonance at −120 meV and corresponds to a shear-type domain wall. The second type lacks this resonance and is instead a mixed shear-tensile structure. Theoretical tight-binding calculations confirm that mechanical strain can drive transitions between these two domain wall states. Domain walls in twisted bilayer graphene are more than structural boundaries—they can act as robust one-dimensional quantum channels and have previously been linked to exotic effects such as topological edge currents. By establishing strain as a key tuning parameter, the new study provides a practical strategy for manipulating these quantum pathways.
By filling a major experimental gap between ultra-small angle and magic-angle twisted bilayer graphene, this work positions marginally twisted graphene as a powerful new platform for engineering topological quantum states. The findings may also inspire future designs of flexible, strain-controlled graphene-based quantum devices.
National Science Review
Experimental study