The alignment of supermoiré lattices has long been a challenge in the field of quantum materials research. However, a team of physicists from the National University of Singapore (NUS) has recently developed a groundbreaking technique to precisely control the alignment of these lattices using a set of golden rules. This significant advancement paves the way for the next generation of moiré quantum matter, expanding the range of tunable material properties and opening up a plethora of potential applications.
Moiré patterns are formed when two periodic structures, with either a relative twist angle between them or no twist angle, overlap. When layered materials such as graphene and hexagonal boron nitride (hBN) are overlaid on each other, imperfect alignment of their atoms creates interference fringes known as moiré patterns. This electronic reconstruction gives rise to exotic properties, including topological currents and Hofstadter butterfly states.
Supermoiré lattices are formed when two moiré patterns are stacked together. Compared to traditional single moiré materials, these lattices offer a wider range of tunable properties, making them suitable for a variety of applications.
Creating a graphene supermoiré lattice poses three main challenges. First, the optical alignment process is time-consuming and labor-intensive, as it relies on finding a suitable graphene flake with straight edges. Second, even with straight-edged graphene, achieving a double-aligned supermoiré lattice is challenging due to the uncertainty of edge chirality and lattice symmetry. Finally, aligning two different lattice materials accurately is physically demanding.
To address these challenges, the research team led by Professor Ariando developed a technique for controlled alignment. They also formulated the “Golden Rule of Three” to guide the use of their technique in creating supermoiré lattices. This rule provides a systematic approach to overcome the alignment challenges and improve the fabrication process.
The researchers employed two main methods in their technique. Firstly, they used a 30-degree rotation technique to control the alignment of the top hBN and graphene layers. Secondly, they applied a flip-over technique to align the top hBN and bottom hBN layers. These methods allowed them to precisely control the lattice symmetry and tune the band structure of the graphene supermoiré lattice.
Notably, the researchers discovered that the neighboring graphite edge can act as a guide for stacking alignment. By leveraging this insight, they managed to fabricate 20 moiré samples with an accuracy better than 0.2 degrees.
Dr. Junxiong Hu, the lead author of the research paper, highlighted the significance of their technique in solving real-life problems faced by researchers. The fabrication time for samples can be significantly shortened, and the accuracy of the samples can be greatly improved.
Beyond the immediate impact, the established golden rules and improved alignment technique hold promise for researchers working in other fields of quantum materials. Scientists studying magic-angle twisting bilayer graphene or ABC-stacking multilayer graphene are expected to benefit from this technical advancement as well.
The research team at NUS is currently using this technique to fabricate single-layer graphene supermoiré lattices and explore their unique properties. They are also extending the technique to other material systems in the hopes of uncovering novel quantum phenomena.
The National University of Singapore (NUS) physicists’ breakthrough in precisely controlling the alignment of supermoiré lattices marks a significant advancement in the field of moiré quantum matter. Through their technique and the establishment of golden rules, the researchers have overcome longstanding challenges and improved the fabrication process, enabling the development of the next generation of quantum materials. This breakthrough has far-reaching implications and holds promise for various fields within the realm of quantum materials research. As the technique continues to be refined and applied to other material systems, researchers anticipate uncovering novel quantum phenomena and propelling the field forward.
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