Twisted bilayer graphene with alignment to hexagonal boron nitride explored with scanning tunnelling microscopy and spectroscopy

When two atomic crystals are stacked on top of each other with a twist, a moiré potential is created which dramatically alters the material's electronic properties. In twisted bilayer graphene (TBG), when the twist between the layers is close to the magic angle, ~1°, nearly flat bands emerge with non-trivial topology, that can host correlated electron phases including superconductivity and Chern insulators. Stacking a TBG sample on hexagonal boron nitride (hBN), introduces a second moiré potential that creates a competing periodicity which further modifies the electronic properties. By using low temperature scanning tunneling microscopy (STM) and spectroscopy (STS) to study the structure and electronic properties of TBG stacked on hBN we discovered a previously unknown class of quasiperiodic crystals, that have long range order but lack translational symmetry. Furthermore, our STS studies of these quasiperiodic crystals revealed that they possess unexpected electronic properties including flat bands and correlation induced gaps suggesting the emergence of correlated quantum states. These findings pave the way to engineering and tuning 2D quasiperiodic crystals and for exploring their unusual electronic properties.