Pseudospin Paramagnons and the Superconducting Dome in Magic Angle Twisted Bilayer Graphene

TL;DR

This paper develops a theoretical model explaining superconductivity in twisted bilayer graphene as arising from pseudospin paramagnons, highlighting the role of sublattice polarization fluctuations and their impact on superconducting behavior across different filling fractions.

Contribution

It introduces a pseudospin paramagnon model linking sublattice polarization fluctuations to superconductivity in twisted bilayer graphene, emphasizing the effects of interband fluctuations and external fields.

Findings

Superconductivity occurs over a wide range of filling fractions.

Superconductivity is suppressed by sublattice polarizing fields or valley polarization.

The model draws analogies to liquid helium-3 and ferromagnetic metals.

Abstract

We present a theory of superconductivity in twisted bilayer graphene in which attraction is generated between electrons on the same honeycomb sublattice when the system is close to a sublattice polarization instability. The resulting Cooper pairs are spin-polarized valley-singlets. Because the sublattice polarizability is mainly contributed by interband fluctuations, superconductivity occurs over a wide range of filling fraction. It is suppressed by i) applying a sublattice polarizing field (generated by an aligned BN substrate) or ii) changing moir\'e band filling to favor valley polarization. The enhanced intrasublattice attraction close to sublattice polarization instability is analogous to enhanced like-spin attraction in liquid $^3$He near the melting curve and the enhanced valley-singlet repulsion close to valley-polarization instabilities is analogous to enhanced spin-singlet repulsion in metals that are close to a ferromagnetic instability. We comment on the relationship between our pseudospin paramagnon model and the rich phenomenology of superconductivity in twisted bilayer and multilayer graphene.