SU(4) symmetry in twisted bilayer graphene - an itinerant perspective

TL;DR

This paper investigates symmetry-breaking phases in twisted bilayer graphene, revealing approximate SU(4) symmetry in Landau functionals and analyzing phase transitions and order parameters at different fillings.

Contribution

It introduces an itinerant perspective on SU(4) symmetry in twisted bilayer graphene, detailing the order parameter manifolds and phase transitions at small and Van Hove fillings.

Findings

First-order transition to SU(3)×U(1) order at small fillings

Weak first-order transition to SO(4)×U(1) order at Van Hove filling

Discussion of particle-hole orders' impact on superconductivity

Abstract

We study symmetry-broken phases in twisted bilayer graphene at small filling above charge neutrality and at Van Hove filling. We argue that the Landau functionals for the particle-hole order parameters at these fillings both have an approximate SU(4) symmetry, but differ in the sign of quartic terms. We determine the order parameter manifold of the ground state and analyze its excitations. For small fillings, we find a strong 1st-order transition to an SU(3)$\otimes$U(1) manifold of orders that break spin-valley symmetry and induce a 3-1 splitting of fermionic excitations. For Van Hove filling, we find a weak 1st-order transition to an SO(4)$\otimes$U(1) manifold of orders that preserves the two-fold band degeneracy. We discuss the effect of particle-hole orders on superconductivity and compare with strong-coupling approaches.