Abstract
Two-dimensional (2D) materials provide a rich platform for exploring exotic quantum transport phenomena. In this thesis, two emergent phenomena in 2D systems are mainly focused on: Coulomb drag signatures of quantum solid phases in WSe2 double-layer devices, and the nonlinear Hall effect (NLHE) in a bilayer graphene–black phosphorus (BP) heterostructure.
The thesis first introduces the basic electronic properties of widely studied 2D materials in Chapter 1. After this, the theoretical background and previous experimental work on the Coulomb drag effect and NLHE are reviewed in this chapter. In Chapter 2, a general experimental workflow employed in this thesis is described, comprising the 2D material device fabrication and electronic transport measurements.
Chapter 3 focuses on the Coulomb drag measurements in a thin hBN-spaced WSe2 doublelayer device. By independently tuning the carrier type and density in each layer, multiple quantum solid phases are revealed. In the electron–hole regime, two quantised drag resistance plateaus are observed, attributed to a quantum exciton solid and an exciton solid with an embedded electron solid. The transport in these phases is carried by quantum defects propagating along the sample edges, as confirmed by the contrasting behaviour between the Hall-bar and Corbino geometries. These results demonstrate that TMDC double-layer systems host a rich sequence of strongly correlated quantum solid phases.
Chapter 4 focuses on the NLHE in an artificially designed bilayer graphene–BP heterostructure. The BP substrate breaks the C3 symmetry of bilayer graphene, thereby giving rise to a robust Berry curvature dipole (BCD) induced NLHE. Furthermore, the band alignment is tuned by displacement field into a strong interlayer coupling regime, where a hybridization gap is observed. Near this hybridization gap, giant enhancements of both the second-harmonic and third-harmonic NLHE are observed, which are attributed to the BCD and quantum-metric quadrupole, respectively. These findings demonstrate NLHE as a valuable quantum geometric probe and establish vdW engineering as a powerful strategy for exploring these effects.