Investigation of graphene's properties engineered from proximity effects to twist engineering

Abstract

Two-dimensional materials exhibit unique electronic properties that differ significantly from their bulk counterparts due to their reduced dimensionality. By assembling heterostructures of van der Waals (vdW) crystals, it is possible to engineer new properties and device functionalities that are absent in the individual materials. Graphene has been extensively studied and serves as a model system for exploring band structure modifications through various couplings. In graphene-based heterostructures, spin-orbit and exchange couplings can be induced via proximity effects, which can stabilize superconductivity and potentially host a variety of topological states. Beyond proximity-induced couplings, twist engineering—which controls interlayer couplings—has emerged as another promising approach to create novel properties without relying on different materials. For example, two monolayer graphene sheets rotated by a certain angle form a flat band characterized by strong correlations, leading to the emergence of states not observed in pristine graphene, such as unconventional superconductivity, correlated insulators, and orbital magnetism. This dissertation presents transport studies on three distinct graphene-based structures: graphene-vdW magnets CrX3 (X=I/Br/Cl), graphene-WTe2, and non-magic-angle twisted bilayer graphene. These studies examines the effects of various couplings on the electronic properties of graphene and additionally probe the exotic gap nature of WTe2. In the first part about the graphene-CrX3 structure, we identify several atypical transport features resulting from charge transfer due to work function mismatch. Although modulation doping of graphene is observed under a magnetic field due to magnetic order switching, the exchange coupling strength is found to be weak, as evidenced by the non-split Landau levels and the absence of an anomalous Hall effect. In the second part, we investigate the transport properties of graphene-WTe2 heterostructures and probe the correlated gap behavior of WTe2 with the influence of an adjacent graphene. Finally, in the study of non-magic-angle twisted bilayer graphene, we report an unexpected anomalous Hall effect at half-filling on both the electron and hole sides in two different devices. The anomalous Hall signal and the possible ground state candidates are carefully examined.