UC Berkeley

In the last 5 years, the search to understand how correlated phases emerge in condensedmatter systems has shifted away from the cuprate high temperature superconductors and towards the realm of two dimensional materials, with twisted graphene being a particular point of focus. Due to the extremely high level of tunability of both interaction strength and electron kinetic energy in twisted graphene, these systems are an excellent playground to understand the relationship between correlations, electronic structure, symmetry breaking, and quantum phase transitions. This thesis is an examination of the electronic properties of graphene devices in monolayer and twisted geometries, studied through the lens of angle-resolved photoemission spectroscopy (ARPES).

This thesis is organized as follows. Chapter 1 provides a high-level introduction to correlationsand symmetry breaking, followed in Chapter 2 by an introduction to the electronic structure of graphene, twisted bilayer and trilayer graphene. Chapter 3 describes the experimental techniques: first the mechanics of ARPES used to measure electronic structure, and then the methods for fabricating twisted graphene devices with a particular focus on van der Waals heterostrucure samples for ARPES. Chapter 4 presents the ARPES results in monolayer graphene devices, showing how correlations and electron-electron interactions in particular drive electron-hole symmetry breaking. Chapter 5 investigates both the layer- and doping- tunability of electron-electron interactions in bilayer graphene devices at intermediate twist angles, and explores a regime where the signatures of symmetry breaking already present in the system can become further enhanced. Chapter 6 details Future directions for ARPES on twisted trilayer graphene, discussing how periodic strain produced by alignment with an hBN substrate can support certain correlated phases and suppress others.