UC Berkeley

Two-dimensional (2D) materials host many highly-tunable novel quantum phases of matter that can be readily accessed via surface-sensitive probes. This dissertation—divided into three parts—examines how scanning tunneling microscopy and spectroscopy (STM/STS) reveals rich 2D correlated and topological behavior with unprecedented atomic-scale spatial resolution. The first part provides theoretical and technical background by introducing representative 2D electronic states including charge density waves, exchange-driven correlated insulating states, and Chern insulators (Chapter 1). STM/STS principles, instrumentation, and basic measurement protocols are also described (Chapter 2). Sample preparation techniques such as molecular beam epitaxy and van der Waals device fabrication are additionally discussed.The second part concerns epitaxially-grown transition metal chalcogenide (TMD) thin films hosting novel charge-ordered states that result from a hierarchy of competing interactions and that are directly identifiable via STM/STS (Chapter 3). Single-layer IrTe2 is shown to exhibit a large-gap insulating 2 × 1 dimer ground state in stark contrast to metallic double-layer and bulk forms of this material. 1T-TaTe2, on the other hand, displays a hitherto unobserved √19×√19 superstructure that can be stabilized via either high-temperature annealing or increasing layer thickness. These results establish few-layer TMD films as a unique platform for exploring novel 2D ground states that are difficult to attain otherwise. The final part discusses correlation and topological properties of field-effect transistor devices made from twisted double-bilayer graphene (tDBLG) and twisted monolayer–bilayer graphene (tMBLG). These materials are seen to host spatially-delocalized correlated insulating states at half and three-quarter fillings of moiré mini-bands indicative of exchange-driven spontaneous polarization in the spin–valley space (Chapter 4). The three-quarter-filled tMBLG state furthermore exhibits quantum anomalous Hall insulating behavior with a gate-switchable total Chern number, a functionality that is highly dependent on local structure and that enables direct visualization of spatially-defined topological phase transitions accompanied by chiral interface states (Chapter 5). Our findings are generalizable to related 2D platforms and hold promise for technological applications ranging from magnetic memories and dissipationless transport to quantum computation.