Theory of correlated insulators and superconductivity in twisted bilayer graphene

TL;DR

This paper presents a theoretical model explaining the complex phase diagram of twisted bilayer graphene, including correlated insulators and superconductivity, aligning qualitatively with experimental observations.

Contribution

It introduces a variational mean-field model capturing the interplay of insulating states, superconductivity, and flavor-symmetry breaking in twisted bilayer graphene.

Findings

Robust insulators at even integer fillings

Superconducting domes emerge with phonon-mediated interactions

Phase diagram shaped by interaction details and particle-hole asymmetry

Abstract

We introduce and analyze a model that sheds light on the interplay between correlated insulating states, superconductivity, and flavor-symmetry breaking in magic angle twisted bilayer graphene. Using a variational mean-field theory, we determine the normal-state phase diagram of our model as a function of the band filling. The model features robust insulators at even integer fillings, occasional weaker insulators at odd integer fillings, and a pattern of flavor-symmetry breaking at non-integer fillings. Adding a phonon-mediated inter-valley retarded attractive interaction, we obtain strong-coupling superconducting domes, whose structure is in qualitative agreement with experiments. Our model elucidates how the intricate form of the interactions and the particle-hole asymmetry of the electronic structure determine the phase diagram. It also explains how subtle differences between devices may lead to the different behaviors observed experimentally. A similar model can be applied with minor modifications to other moir\'{e} systems, such as twisted trilayer graphene.