Progress in Quantum Matter: Guidelines for Precision Alignment of Atomic Constructs

A clock model demonstrates the rotational correspondence among the hour hand (Top hBN), minute hand (middle graphene), and second hand (Bottom hBN). This amalgamation of top hBN, middle graphene, and bottom hBN culminates in a supermoiré lattice structure at the clock’s nucleus. Credit: National University of Singapore

Researchers in physics have pioneered a method for the accurate alignment of supermoiré lattices, thereby revolutionizing the scope for future advancements in moiré quantum matter.

Scientists from the National University of Singapore (NUS) have designed a method that meticulously governs the alignment of supermoiré lattices. This is accomplished by adhering to a specific set of guidelines termed “golden rules,” thus setting the stage for breakthroughs in next-generation moiré quantum matter.

Supermoiré Lattices: An Overview

Moiré patterns come into existence when either two identical periodic structures are superimposed with a relative twist angle, or when two differing periodic structures are overlaid, either with or without a twist angle. In instances where layered materials like graphene and hexagonal boron nitride (hBN) are combined, the atomic configurations do not align flawlessly, giving rise to interference fringes, or moiré patterns. This subsequently triggers an electronic reconfiguration.

Innovative structures with extraordinary characteristics, such as topological currents and Hofstadter butterfly states, have been synthesized using moiré patterns in graphene and hBN. The stacking of two moiré patterns produces a novel construct known as a supermoiré lattice. This superstructure broadens the horizons of adjustable material traits, enabling its utilization in a broader range of applications compared to traditional, singular moiré materials.

Contributions from the NUS Department of Physics

A team of researchers led by Professor Ariando from the NUS Department of Physics formulated and successfully implemented a technique for the controlled alignment of hBN/graphene/hBN supermoiré lattices. The technique enables the precise superimposition of two moiré patterns. Concurrently, the researchers have coined the “Golden Rule of Three” to serve as a directive for employing their method in the fabrication of supermoiré lattices.

These findings have been formally published in the scientific journal Nature Communications.

Challenges and Their Resolutions

Three primary obstacles exist in the creation of a graphene supermoiré lattice. Firstly, conventional optical alignment is heavily reliant on the straight edges of graphene, which makes the identification of an appropriate graphene flake laborious and time-consuming. Secondly, even when a straight-edged graphene sample is utilized, the probability of achieving a double-aligned supermoiré lattice is merely 1/8, owing to the uncertain nature of its edge chirality and lattice symmetry. Lastly, even if these factors are determined, substantial alignment errors often occur.

Dr. Junxiong Hu, the principal author of the study, remarked, “Our method presents a tangible solution. Researchers have conveyed to me that sample preparation usually takes up to a week. With our technique, not only is the production time significantly reduced, but the accuracy of the samples is also substantially improved.”

Technological Insights

The researchers employed a “30° rotation technique” initially to control the alignment between the top hBN and graphene layers. This was followed by a “flip-over technique” to dictate the alignment between the top and bottom hBN layers. Utilizing these two procedures, they could manage the lattice symmetry and adjust the band structure of the graphene supermoiré lattice. The study successfully produced 20 moiré samples with a precision exceeding 0.2 degrees.

Professor Ariando noted, “We have established three foundational rules that can assist a wide range of researchers in the field of two-dimensional materials. Scientists working on other systems with strong correlations, like magic-angle twisted bilayer graphene or ABC-stacked multilayer graphene, are also expected to benefit from our contributions. I anticipate that our technological advancements will catalyze the evolution of the next generation of moiré quantum matter.”

Future Directions

The research team is currently exploiting this methodology to fabricate single-layer graphene supermoiré lattices and to investigate their unique properties. In addition, they are extending the current techniques to alternative material systems with the aim of uncovering new quantum phenomena.

Reference: “Controlled alignment of supermoiré lattice in double-aligned graphene heterostructures” by Junxiong Hu et al., published on 12 July 2023 in Nature Communications.
DOI: 10.1038/s41467-023-39893-5

Frequently Asked Questions (FAQs) about supermoiré lattices

More about supermoiré lattices

• Nature Communications Journal
• National University of Singapore Department of Physics
• What are Moiré Patterns?
• Introduction to Quantum Matter
• Graphene: Properties and Applications
• Hexagonal Boron Nitride: An Overview