Electrically tunable correlated and topological states in twisted monolayer-bilayer graphene (2004.11340v1)

Abstract: Twisted van der Waals heterostructures with flat electronic bands have recently emerged as a platform for realizing correlated and topological states with an extraordinary degree of control and tunability. In graphene-based moir\'e heterostructures, the correlated phase diagram and band topology depend strongly on the number of graphene layers, their relative stacking arrangement, and details of the external environment from the encapsulating crystals. Here, we report that the system of twisted monolayer-bilayer graphene (tMBG) hosts a variety of correlated metallic and insulating states, as well as topological magnetic states. Because of its low symmetry, the phase diagram of tMBG approximates that of twisted bilayer graphene when an applied perpendicular electric field points from the bilayer towards the monolayer graphene, or twisted double bilayer graphene when the field is reversed. In the former case, we observe correlated states which undergo an orbitally driven insulating transition above a critical perpendicular magnetic field. In the latter case, we observe the emergence of electrically tunable ferromagnetism at one-quarter filling of the conduction band, with a large associated anomalous Hall effect. Uniquely, the magnetization direction can be switched purely with electrostatic doping at zero magnetic field. Our results establish tMBG as a highly tunable platform for investigating a wide array of tunable correlated and topological states.

• The paper establishes that tMBG supports electrically tunable correlated states, including ferromagnetism and the quantum anomalous Hall effect.
• Experimental and theoretical analysis shows that reversing the displacement field switches the system between insulating and spin-polarized states.
• The results highlight tMBG's potential for next-generation spintronic applications through precise modulation of its electronic and magnetic properties.

Electrically Tunable Correlated and Topological States in Twisted Monolayer-Bilayer Graphene

The paper presented explores the frontier of condensed matter physics with an investigation into twisted monolayer-bilayer graphene (tMBG). The paper delineates the capability of tMBG to host a variety of correlated and topological states that are electrically tunable. The authors employ a combination of experimental techniques and theoretical calculations to explain the electronic structures emergent in this layered material system at distinct twist angles and interlayer displacement fields.

Key Findings and Numerical Results

The primary finding is that tMBG behaves as an adaptable system for exploring correlated states, like ferromagnetism and the quantum anomalous Hall effect (QAHE), and topological magnetic states. With specific focus on the electric field's vector direction, it is discovered that tMBG can closely approximate the phase diagrams of either twisted bilayer graphene (tBLG) or twisted double bilayer graphene (tDBG). For instance, when the displacement field (D) is directed from bilayer to monolayer graphene, the tMBG system mimics the insulating correlated states observed in tBLG, which are marked by critical perpendicular magnetic fields where insulating states arise. Conversely, reversing the displacement field leads to spin-polarized correlated insulating states akin to tDBG, as demonstrated through a significant anomalous Hall effect observed at a quarter filling of the conduction band.

A standout result is the manifestation of electrically tunable ferromagnetism exhibiting large Hall angles and hysteretic behavior in the absence of an external magnetic field, regulated by adjusting carrier density through gate doping. This discovery suggests tMBG's superior tunability over previously studied vdW heterostructures, which expands the potential for practical applications such as ultra-low-power spintronic devices.

Implications and Future Directions

The research suggests formidable implications for both fundamental physics and application-specific technologies. The observed tunable ferromagnetism and associated magnetic order switching by electrostatic doping signal potential developments in spintronics. This ability to manipulate magnetic states through electronic means alone represents a significant stride toward integrating electronic and magnetic functionalities on a single platform, thereby optimizing device efficiency and miniaturization.

Theoretically, this work enhances the understanding of flat band physics in moiré superlattices, contributing to the broader discussion on the role of symmetry-breaking and electronic correlations in generating novel states of matter. The lack of significant superconductivity reported in this paper offers a unique contrast to observations in other similarly-structured systems, suggesting the influence of tMBG's broken symmetries on suppressing superconductivity—an area warranting further investigation.

Future research avenues might explore variation in twist angle, strain, or the introduction of more complex multi-component systems involving other two-dimensional materials. Additionally, further experimentation could probe the intricate interplay of electric and magnetic field effects on other moiré heterostructures. Such endeavors are poised to enhance the understanding of strongly correlated and topologically nontrivial states, helping to pinpoint conditions where phenomena like fractional Chern insulators could emerge, as predicted for highly correlated systems.

Conclusion

In summary, this examination of tMBG reveals its profound role in the landscape of two-dimensional materials, with extensive tunability and a rich phase diagram featuring correlated and topological states. The adaptability afforded by electric field manipulation posits tMBG as an ideal candidate for next-generation material investigations and quantum device innovations. Its prowess in forming spin-polarized states and unique response to external controls illustrates tMBG's multidimensional utility, positioning it as a keystone in both fundamental research and technological application trajectories.