2D and Moiré Materials | Strong Correlations and Topology | Quantum Transport and Surface Probes
2D Materials
Two-dimensional (2D) materials are those that can be reduced down to single atomic layers (or at least, a few atomic layers). 2D materials are isolated from bulk (3D) materials that have a layered structure, where strong in-plane covalent bonds tightly bind the atoms within a layer together and weak out-of-plane van der Waals forces hold together adjacent layers. These layered materials are often called van der Waals (vdW) materials. Graphite is the canonical example of a vdW material, which can be isolated down to a single atomic layer of carbon atoms, known as graphene, via Scotch tape exfoliation. Graphene is far from the only 2D material, and each material has its own properties: graphene is a (very special) metal, boron nitride is an insulator, molybdenum sulfide is a semiconductor, niobium selenide is a superconductor, etc.
Van der Waals Heterostructures
Owing to the 2D nature of these materials, they can be stacked on top of each other, which allows an unprecedented level of control of structure. The so-called stacking procedure is schematically shown here, where specially-designed polymers are used pick up 2D materials and combine them into structures known as van der Waals heterostructures. VdW heterostructures offer an seemingly endless playground for experimental condensed matter physicists to engineer novel behavior with unique tuning knobs.
Moiré patterns and “Twistronics”
Moiré patterns are long wavelength interference patterns that emerge naturally in 2D systems when there is a small lattice mismatch or misorientation between layers. The image here shows the moiré pattern formed between two hexagonal lattices (both are the same size) when the layers are twisted relative to one another. These moiré patterns give rise to long-range interactions between electrons which can result in a variety of interesting correlated phenomena such as superconductivity in so-called “magic angle” twisted bilayer graphene. Moiré patterns radically alter the relevant structure of the material and can manifest in a wide array of 2D systems like twisted homo-bilayers, hetero-bilayers of semiconductors, or graphene aligned to hexagonal boron nitride, all with their own interesting physics and application potentials.
