In 2018, the team of Professor Pablo Jarillo-Herrero of the Massachusetts Institute of Technology (MIT) first experimentally observed a series of electronic strong correlation phenomena in magic-angle twisted bilayer graphene, from non-conductive insulating states to very conductive superconducting states. Because its superconducting phase diagram is similar to high-temperature superconductivity, it is expected to realize the understanding of complex high-temperature superconducting mechanism through simple two-dimensional atomic layer stacking. The traditional Moiré superlattice mainly has one Moiré fringe, and the new structure produced by the interaction of two Moiré fringes at the interface is called Supermoiré lattice.
At the Manchester Graphene Conference in 2023, Professor Pablo called this super Moiré lattice composed of two kinds of Moiré fringes “the next generation of Moiré quantum materials”. Compared with the traditional single moiré, this super moiré expands the adjustable dimension of the moiré quantum material and further enriches its physical properties. Taking the super moiré lattice composed of graphene and boron nitride as an example, recent theoretical and experimental studies have shown that even a single layer of graphene can achieve topological flat bands and strong correlations by constructing a super moiré lattice electronic state. Therefore, the construction of super moiré can realize new functions and new phenomena that cannot be realized by traditional single moiré quantum materials.
However, the sample preparation of graphene super moiré lattice mainly faces three technical difficulties: first, traditional optical alignment strongly relies on the straight sides of graphene itself, but it is time-consuming and laborious to find a graphene with straight sides experimentally; Second, although graphene with straight edges has been found, due to the uncertainty of its edge chirality and lattice symmetry, there is only a low probability of 1/8 to obtain a double-aligned super moiré lattice ; finally, even when the edge chirality and lattice are finally determined, experimental alignment errors are often large (>0.5°), since it is challenging to align two different lattice materials stacked together.
A glimpse of results
Based on this, the research group of Professor Ariando of the National University of Singapore proposed a series of technical solutions and successfully realized the controllable preparation of single-layer graphene/boron nitride super moiré lattice. At the same time, to ensure the technology’s productivity and precision, the team came up with a “golden rule” for optical alignment. Finally, the team also applied this technology to other strongly correlated electronic systems, realizing the controllable preparation of multilayer corner graphene super moiré lattice. Related research results were published in Nature Communications under the title of “Controlled alignment of supermoiré lattice in double-aligned graphene heterostructures”.
1. The author first realized the controllable alignment of the top layer of boron nitride and the middle graphene through the “30-degree rotation” technology; secondly, using the “interface flipping” technology to fold the bottom layer of the same boron nitride, a controllable alignment of top boron nitride and bottom boron nitride is achieved. Using these two techniques, the authors successfully resolved the uncertainties posed by interlayer edge chirality and lattice symmetry.
2. The author proves that using the edge of the adjacent graphite block as the main crystal axis to align can greatly improve the yield and sample precision. Using this technique, the authors prepared 20 moiré samples, and the alignment accuracy of each sample was well controlled within 0.2°. To further ensure the reliability and success of these techniques, the authors propose three “golden rules” for their use.
3. Finally, the author extended the technology to other strongly correlated systems, such as low-dimensional corner bilayer graphene and ABC superconducting phase trilayer graphene, and realized the controllable alignment of multilayer corner graphene and boron nitride.
In summary, the author successfully solved the three major problems in the sample preparation process of graphene super moiré through the three core technologies, paving the way for the research on the basic physical properties of graphene super moiré materials. In the van der Waals heterojunction of 2D materials, the uncertainty of edge chirality and lattice symmetry is a common problem, which causes great troubles for interlayer stacking to form moiré materials. Therefore, the technique in this paper can also be used in other moiré materials, such as TMD, magnetic or superconducting two-dimensional moiré quantum systems. The author hopes that these techniques can really help researchers in sample preparation. In particular, the sample preparation of graphene super moiré materials has become simple.
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