2021 Theses Doctoral

Twisted bilayer graphene probed with nano-optics

Sunku, Sai Swaroop

The discovery of strongly correlated electronic phases in twisted bilayer graphene has led to an enormous interest in twisted van der Waals (vdW) heterostructures. While twisting vdW layers provides a new control knob and never before seen functionalities, it also leads to large spatial variations in the electronic properties. Scanning probe experiments are therefore necessary to fully understand the properties of twisted vdW heterostructures.

In this thesis, we studied twisted bilayer graphene (TBG) with two scanning probe techniques at two twist angle regimes. At small twist angles, our nano-infrared images resolved the spatial variations of the electronic structure occurring within a Moiré unit cell and uncovered a quantum photonic crystal. Meanwhile, with nano-photocurrent experiments, we resolved DC Seebeck coefficient changes occurring in domain walls on nanometer length scales. At larger twist angles, we mapped the twist angle variations naturally occurring in our device with a combination of nano-photocurrent and nano-infrared imaging. Finally, we also investigated different materials for use as nano-optics compatible top gates in future experiments on TBG. Our results demonstrate the power of nano-optics techniques in uncovering the rich, spatially inhomogeneous physics of twisted vdW heterostructures.