Tunable Moiré Bands and Strong Correlations in Small-Twist-Angle Bilayer Graphene (1703.00888v1)

Abstract: According to electronic structure theory, bilayer graphene is expected to have anomalous electronic properties when it has long-period moir\'e patterns produced by small misalignments between its individual layer honeycomb lattices. We have realized bilayer graphene moir\'e crystals with accurately controlled twist angles smaller than 1 degree and studied their properties using scanning probe microscopy and electron transport. We observe conductivity minima at charge neutrality, satellite gaps that appear at anomalous carrier densities for twist angles smaller than 1 degree, and tunneling densities-of-states that are strongly dependent on carrier density. These features are robust up to large transverse electric fields. In perpendicular magnetic fields, we observe the emergence of a Hofstadter butterfly in the energy spectrum, with four-fold degenerate Landau levels, and broken symmetry quantum Hall states at filling factors 1, 2, 3. These observations demonstrate that at small twist angles, the electronic properties of bilayer graphene moir\'e crystals are strongly altered by electron-electron interactions.

• The paper reports conductivity minima and unexpected satellite gaps at ±8 electrons per moiré cell, emphasizing strong electron-electron interactions.
• It identifies the flattening of moiré Bloch bands near the magic twist angle, signaling dominant correlation effects.
• The study reveals Hofstadter butterfly spectra and quantum Hall states under magnetic and electric fields, highlighting paths for tunable device applications.

Tunable Moiré Bands in Small-Twist-Angle Bilayer Graphene: An Exploration of Strong Correlations

The research paper "Tunable Moiré Bands and Strong Correlations in Small-Twist-Angle Bilayer Graphene" presents a responsible examination of bilayer graphene when subjected to minor twist angles, specifically less than 1°. This nuanced analysis provides significant insights into the electronic properties of bilayer graphene, highlighting how these are altered by strong electronic correlations and electron-electron interactions.

Key Observations and Results

The researchers employed scanning probe microscopy (SPM) and electron transport techniques to explore the electronic properties of bilayer graphene. Their findings underscore several critical aspects:

1. Conductivity Minima and Satellite Gaps: They observed conductivity minima at charge neutrality, as well as unusual satellite gaps corresponding to ± 8 electrons per moiré unit cell. These gaps deviate from traditional electronic structure theory predictions, which expect gaps at ± 4 electrons. This discrepancy suggests significant roles for electron-electron interactions at small twist angles.
2. Moiré Bloch Bands and Flat Band Formation: By examining the moiré Bloch bands, the paper identifies that for twist angles near the "magic" angle (approximately 1°), the bands become extremely flat. This flattening is pivotal as it signals a regime where interactions play a dominating role, potentially leading to novel quantum states.
3. Hofstadter Butterfly and Quantum Hall States: Under perpendicular magnetic fields, the bilayer graphene reveals a Hofstadter butterfly energy spectrum, with the emergence of four-fold degenerate Landau levels and journal a sequence of quantum Hall states at particular filling factors (± 1, 2, 3). These findings further confirm the profound influence of twist angle on the electronic structure and interaction effects in bilayer graphene.
4. Implications of Transverse Electric Fields: Significantly, the paper reports that the observed electronic properties are robust under large transverse electric fields. This robustness suggests that practical device configurations could exploit these electronic properties, opening opportunities for tunable electronic devices based on moiré band engineering.

Implications and Future Directions

The outcomes of this research mark a step forward in the manipulation and understanding of bilayer graphene's electronic properties via precise twist angle control. The ability to create artificial crystals through moiré engineering could lead to a new class of materials with customizable electronic properties, significant for applications in quantum computing and materials science.

Particularly interesting is the potential for discovering new phases of matter in these two-dimensional systems. As future research extends these studies, it could explore additional factors that influence electronic properties in moiré superlattices, such as different substrate materials or varying environmental conditions. Additionally, theoretical developments could provide a deeper understanding of the intricate interplay between twist angle, electronic interactions, and external fields.

Conclusion

This paper contributes to the growing body of research in the domain of van der Waals heterostructures, offering profound insights into how subtle changes in structural parameters can lead to new physical phenomena. The findings not only challenge existing theoretical models but also propose a practical path towards the realization of advanced electronic and quantum devices using bilayer graphene. The capability to tune moiré bands through small twist angles promises continued exploration and discovery in two-dimensional materials science.