Frontier exploration of twisted graphene

Abstract

The twisted graphene provides a unique structural model distinct from the traditional Bernal stacking. As a new degree of freedom, the twist angle in graphene leads to the moire superlattice together with the re-constructed electronic band structures, which received considerable attention. Most of the previous reports about twisted graphene focused on theoretical analysis. With the recent development of synthetic methods, the performance of twisted graphene has been heavily investigated. Specifically, the emergent superconductivity and ferromagnetism exhibited by the “magic-angle” twisted bilayer graphene has aroused tremendous attention since these properties have never been proved by carbon-based materials. Moreover, these unpredicted properties indicate challenging puzzles to be explored brought by the twisted graphene. This review majorly focuses on the synthesis methods, electronic structures, characterizations and properties of twisted graphene, especially “magic-angle” twisted bilayer graphene, aiming to provide a comprehensive summary about the progress of twisted graphene and its related research advances. In the early experimental research, twisted graphene is mostly obtained by folding single-layer graphene, or accidental discovered in the graphene samples synthesized by chemical vapor deposition. These uncontrollable methods are soon eliminated because of the rapid development of micro-nano machining related technology. By stacking a single layer graphene onto another at a target angle, the twisted angle between graphene layers can be controlled with a high precision. Then, the theoretical researches about electronic structures of twisted graphene are presented, showing great dependence on the twisted angles especially at low angles. Importantly, the twist in bilayer graphene leads to the re-construction of the Brillouin zone and decreasing of Fermi velocity ( v F), pointing out a series of “magic angles” as v F=0. At these specific angles, twisted bilayer graphene possess flat bands around Fermi level and localized density of states, which has been confirmed by experimental results. Considering the importance of twisted angles, some characterization methods are introduced in this review based on the moire superlattice, the energy gaps between van Hove singularities or the gap-induced insulation state in transport properties of twisted graphene. The latter is majorly applied for fabricating devices containing twisted bilayer graphene packaged with hexagonal boron nitride (hBN). Following, the emergent correlated insulating phase, superconducting phase, topological phases and ferromagnetism in “magic-angle” twisted bilayer graphene are systematically classified. The rich phase diagram of twisted graphene declares a desirable platform for the exploration of many-body physics, gaining lots of efforts to reveal the phase transition and the relationships between different phases. Some possible mechanisms about the emergent unconventional superconductivity and orbital-based ferromagnetism are summarized. The progress of related experiments, characterizations and simulations together with their discussions are interpreted in this article. The results indicate the package layers, mostly hBN, have significant influences on the electronic band structure and performances of twisted graphene. These influences are previously ignored, but now regarded as the key for unveiling the distinct properties of twisted graphene. By tuning the package layers, the superconductivity is achieved at lower twisted angles than other “magic-angle”. At the end of this review, we discuss about the existing challenges and prospects in the related fields, which may give rise to great changes in physics and materials science.