Quantum Transport in Graphene Heterostructures

Abstract

This thesis investigates quantum transport phenomena in graphene heterostructures, focusing specifically on twisted monolayer-bilayer graphene (tMBG). Graphene, known for its exceptional electrical properties, exhibits unique behaviours when arranged in twisted configurations, giving rise to moiré superlattices. These structures dramatically alter graphene’s electronic band structure, enabling the study of emergent quantum phenomena such as correlated insulating states, quantum anomalous Hall effect and superconductivity. We report detailed studies on the fabrication of high-quality tMBG devices and its characterization. Mechanical exfoliation and dry-transfer methods were optimized to produce clean and uniform graphene layers, which were precisely twisted to specific angles. To ensure accurate control of the heterostructure assembly, we used advanced methods like Atomic Force Microscope (AFM) lithography, facilitating precise patterning and modification of graphene layers. Transport measurements conducted at low temperatures (1.5 K) revealed distinct insulating phases at specific carrier densities, corresponding to the existence of flat electronic bands. Notably, we observed insulating states at complete and partial fillings of the moiré superlattice bands, highlighting strong electron-electron interactions. Furthermore, van Hove singularities were identified and shown to be tunable by the applied displacement field. Additionally, we explored Lateral Force Microscopy (LFM) as a technique for imaging moiré patterns in twisted graphene structures at ambient conditions. Through careful analysis, periodicities consistent with the expected moiré superlattice dimensions were successfully identified, demonstrating the technique’s potential for characterizing local twist-angle disorder. Finally, a quantum point contact (QPC) device using Bernal-stacked bilayer graphene was fabricated, showcasing robustness under high DC bias and setting a foundation for future explorations into quantized conductance channels. This work underscores the rich and tunable physics accessible in graphene heterostructures, providing pathways for further research into quantum electronics and novel device architectures.