Towards Repeatable Fabrication of moiré heterostructures for orbital magnetism

Moir\'e heterostructures of two-dimensional van der Waals materials and their associated nearly flat electronic minibands have been shown to produce novel correlated electron states, including orbital ferromagnetism. Though these states are robust in the devices in which they are observed, reproducing the same results across devices has proven challenging, limiting our understanding of the nature and origin of these states. Why is this? Some heterostructure parameters, which are known to impact device behavior, are not well controlled. Other impactful device parameters may be unknown to us, and thus also remain uncontrolled. This results in a large variability in moiré devices which are ostensibly created with identical fabrication processes. In this dissertation, I present work aimed at improving repeatable realization of novel states in graphene-based heterostructures, and pursuing measurements beyond transport to directly probe these states in twisted bilayer graphene (TBG) and other moiré materials.

We have developed procedures for controlling graphene-hBN twist angle, a critical parameter for reproducing orbital ferromagnetism in twisted bilayer graphene. We verify this twist angle by directly imaging the moir\'e with AFM-based techniques prior to electron transport measurements. The incorporation of such mid-fabrication characterization enables more direct attribution of behaviors observed in electron transport measurements to the underlying moir\'e structure. We have also adopted the use of inorganic cantilever stamps as a promising platform for clean assembly of van der Waals heterostructures. This provides an additional pathway to improved control over moir\'e structure by reducing moir\'e disorder. Finally, I present our efforts to directly probe the spin degree of freedom in twisted bilayer graphene with a resistively detected electron spin resonance measurement. I propose avenues for further developing this measurement as a tool to map the spin ground state of carriers throughout the phase diagram of twisted bilayer graphene. This work enables future systematic study of the structural requirements for novel electronic phenomena, particularly magnetic ordering, in twisted bilayer graphene and other moir\'e materials.