Novel phases in twisted bilayer graphene at magic angles as a result of van Hove singularities and interactions

TL;DR

This paper models the emergence of superconductivity and insulating phases in twisted bilayer graphene at magic angles, emphasizing the roles of van Hove singularities and electron interactions.

Contribution

It introduces a microscopic model accounting for interactions and van Hove singularities, predicting superconducting symmetries and insulating phases in twisted bilayer graphene.

Findings

Superconductivity with s± symmetry identified.

Insulating phase characterized by charge instability.

Coexistence of metallic and insulating phases.

Abstract

The discovery of different phases as a result of correlations, especially in low-dimensional materials, has been always an exciting and fundamental subject of research. Recent experiments on twisted bilayer graphene have revealed reentrant unconventional superconductivity as a function of doping as well as a Mott-like insulating phase when the two layers are twisted with respect to each other at certain "magic" angles for doping corresponding to two particles per moire unit cell. In this work we propose a microscopic model that takes into account interactions and the van Hove singularities in the density of states of the twisted bilayer graphene at doping corresponding to one particle ($\nu$ =1) per moir\'{e} unit cell and study how superconductivity emerges. We identify the possible symmetry of the order parameter as $s^{\pm}$, while if the inter-valley coupling is negligible the symmetry is $s^{++}$. In addition, we find and characterise the insulating region of the system, as a region with a uniform charge instability where there is coexistence of the metallic and insulating phases.