Probing electronic correlation and topology in twisted multilayer graphene

Abstract

Moiré quantum materials, stacked 2D atomic layers with a small twist angle or a slight lattice mismatch, have become emerging platforms to study the physics of correlation, topology, and the interplay between the two. A plethora of novel states, including unconventional superconductivity, generalized Wigner crystal, orbital magnetism and its associated quantum anomalous Hall effect (QAHE), have been observed within their characteristic flat bands. Moiré quantum materials provide unprecedented opportunities in elucidating the nature of these quantum states of matter because of their high degree of tunability, such as doping, displacement field, pressure, and Coulomb screening.This dissertation presents transport and optical studies on the emergent correlated and topological states in representative twisted multilayer graphene systems. First, we identify correlated metallic states with isospin symmetry breaking in the phase diagram of twisted double bilayer graphene (tDBG). We employ pressure as a dynamic tuning knob to control the electronic correlation in tDBG. Magneto transport further reveals symmetry-broken Chern insulators at high magnetic field and anomalous Hall effect near zero field. In the second part, we study the correlated phase diagram of twisted monolayer-bilayer graphene (tMBG). We reveal abundant orbital magnetic states across a wide range of twist angle, some with exotic isospin polarized ground states. The high field Landau fan exhibits a variety of closely competing Chern insulators and hysterestic first-order phase transitions in between. We discuss these results and their implication regarding the physics of isospin symmetry, orbital magnetism and Hofstadter butterfly spectrum in graphene moiré superlattice. Finally, we propose a new optical probe for twisted graphene by using the Rydberg excitons in an adjacent monolayer WSe2. We demonstrate a new set of moiré Rydberg excitons in the heterostructure of WSe2/twisted bilayer graphene, whose doping tunable moiré potential unravels the underlying flat bands in twisted bilayer graphene.