Moiré superlattices and electronic states in twisted multilayer graphene: real-space visualization and transport probes
In his dissertation, Moiré Superlattices and Electronic States in Twisted Multilayer Graphene: Real-Space Visualization and Transport Probes, Pengfei Hou investigated how small rotations between stacked graphene layers affect their structure and electronic properties.
When graphene layers are placed on top of one another with a slight twist, they form larger repeating structures known as moiré patterns. Even a very small change in the twist angle can strongly alter how electrons move through the material. Pengfei studied this relationship using two complementary methods. Conductive atomic force microscopy was used to map the local electrical response and directly visualize the moiré patterns. This revealed local variations in twist angle, stripe-like regions and relaxation of the atomic lattice. Electrical measurements at low temperatures and in magnetic fields were then used to examine the resulting electronic states.
The results showed that twisted trilayer graphene can contain different types of electronic states at the same time. Electrons with strongly different transport properties can therefore contribute to the electrical current in parallel. Magnetic-field measurements also indicated that some electronic states that would normally behave identically had become distinguishable. Near the “magic angle”, where interactions between electrons are especially strong, evidence was observed for an additional state between the usual states associated with integer electron fillings. This feature appeared to provide a new starting point for a sequence of electronic states induced by a magnetic field.
These findings connect the local structure of moiré patterns with measurable electronic behavior. They demonstrate how precisely controlling the alignment of atomically thin layers can produce very different quantum states, which is important for understanding and designing future two-dimensional electronic materials.