From Symmetry Breaking to Superconductivity: Unraveling the Hierarchy of Correlated Phases in Moiré Graphene

Author: Kim, Hyunjin

Year: 2026

Degree: Dissertation (Ph.D.)

Advisor: Nadj-Perge, Stevan

Committee Members: Alicea, Jason F.; Hsieh, David; Refael, Gil; Nadj-Perge, Stevan

Option: Physics

Abstract

Magic-angle twisted graphene systems, including bilayer (MATBG) and trilayer (MATTG) structures, constitute a highly tunable platform for exploring strongly correlated electronic phenomena and unconventional superconductivity. Despite extensive studies, the local electronic structure, symmetry-breaking transitions, and their interplay with superconductivity remain elusive. In this thesis, we employ high-resolution scanning tunneling microscopy and spectroscopy, to investigate the evolution, and hierarchy of correlated phases in twisted multilayer graphene as functions of doping, temperature, magnetic field, and twist angle.

In twisted bilayer graphene, we map the evolution of flat electronic bands and detect filling-dependent band flattening, which drives cascades of symmetry-breaking transitions and the emergence of correlated gaps. Correlated gaps that occur at high magnetic fields are identified as Chern insulators, driven by interaction induced degeneracy breaking. In twisted trilayer graphene, we identify a sequence of correlated gaps at the Fermi level, including a robust outer gap associated with intervalley coherence and a more fragile inner gap linked to superconductivity. Atomic-scale reconstruction reveals Kekulé reconstruction indicative of inter-valley coherence, which coexists with moiré-scale translation symmetry breaking.

Our results demonstrate that superconductivity in twisted multilayer graphene emerges from a hierarchy of correlated states, starting from cascade physics, to formation of Kondo resonance, flavor symmetry breaking to superconductivity. Our findings provide an insightful microscopic framework that is relevant to many moir\'e systems and offer guiding principles for engineering correlated and topological states in designer quantum materials.