Since the successful exfoliation of graphene from bulk graphite, the family of layered van der Waals materials has expanded to cover a wide range of physical properties. The so-called van der Waals stacking technique adds an extra dimension to this versatile material platform by allowing any 2D-material crystal to be re-assembled into designer structures. The flexibility of the van der Waals technique allows novel quantum phenomena and physical properties to be stabilized and engineered with unique stacking sequence and special rotational alignment.
For example, distinct quantum phenomena can be achieved by stacking two graphene layers together in different geometries. Novel quantum Hall effect states with exotic topological order is recently demonstrated in Bernal stacked bilayer graphene, as shown in panel a. When two sheets of graphene are rotated by a so-called “magic angle”, a flat superlattice miniband emerges, where electron-electron interactions is strongly enhanced, stabilizing an intriguing superconducting phase, accompanied by correlated insulator and emergent ferromagnetism. The core of graphene double-layer structure consists of two graphene layers separated by a thin layer of hexagonal boron nitride (hBN). Strong Coulomb attraction across the barrier binds electron from one layer could pair with hole from another, forming a composite boson called “exciton”. The ground state of this structure is the Bose-Einstein condensate of excitons.
These discoveries established 2D materials and van der Waals heterostructure as a paradigm platform to investigate the interplay between topology, broken symmetry and electron correlation, and to advance our knowledge of emergent quantum phenomena in low dimensional systems which may enable new classes of technological innovation towards future quantum computation.