Engineering moiré superlattices: structural and electronic properties of twisted bilayer graphene/h-BN heterostructures
Two-dimensional (2D) materials exhibit enhanced physical, chemical, electronic, and optical properties compared to their bulk counterparts, making them highly promising for next generation nanoelectronics. Among these materials, graphene has attracted significant attention due to its exceptional physical and electronic characteristics.
In recent years, the focus was on moiré superlattice formed by stacking two monolayers of graphene with a relative rotation, the so-called twisted bilayer graphene (TBG). At small twist angles, interlayer interactions in TBG can cause strong correlation effects, showing correlated insulating, superconducting, orbital ferromagnetic, and Chern insulating behaviors. At larger twist angles, TBG exhibits quantum Hall states, further enriching its electronic phase diagram.
Hexagonal boron nitride (h-BN) is often used in TBG devices to provide additional control over the graphene layers. Its honeycomb lattice is structurally like that of graphene. The heteroatomic composition and difference lattice constant lead to the fundamental symmetry differences, which significantly affects the electronic properties of TBG—whether the h-BN is aligned or misaligned with the graphene layers.
In her thesis, Ying Wang primarily focuses on the effect of the h-BN substrate on TBG with different twist angles. The TBGs are fabricated on h-BN as field effect transistor devices.
Wang employed low-temperature electronic transport to investigate the symmetry-breaking effects induced by the h-BN substrate in the TBG/h-BN trilayer. By carefully tuning two twist angles in three layers, she observed topologically non-trivial flat bands in TBGs near the magic angle and symmetry-broken quantum Hall states in TBG with larger twist angles.