The purple arrow shows the direction of the transverse displacement field ( D), ϑ is the twist angle, and d is the distance between the graphene layers which is tuned with the pressure. (b) Illustration of the twisted double bilayer structure. The red arrows represent the pressure p, which modifies the distance between the layers. The sample is contacted by edge contacts (yellow). (a) Schematic of the TDBG (black lattice) with a bottom graphite gate (orange) isolated by a hBN layer (light blue) and a top metallic gate (gray) isolated by a hBN layer and an AlO x layer (blue). Properties of the twisted double bilayer graphene device. Moreover, the correlation effects are often accompanied by nontrivial topology of the bands. (3−13) The ability to control the size of the moiré unit cell size with the twist angle and implicitly the bandwidth of the low energy bands led to the discovery of various correlated phases (3,7−11) such as correlated insulator states, (3−6,8) ferromagnetic phase with a signature of the quantum anomalous Hall effect, (7−9,13) and unconventional superconducting phases resembling high-temperature superconductors (4−6,12) in twisted bilayer graphene (TBG). (1,2) Due to their narrow bandwidth, the electron–electron interaction can become comparable to the kinetic energy and novel, correlated phases form. For small angles, the hybridization between the layers becomes significant, resulting in a reconstruction of the band structure, and for well-defined special, so-called ”magic” angles, the bands flatten out. The rotation of the two layers leads to the formation of a moiré superlattice.
These structures consist of two or more layers of 2D materials, e.g., graphene, placed on top of each other, with a well-defined rotation angle between their crystallographic axes. Twisted van der Waals heterostructures recently opened a new platform to explore correlated electronics phases.
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