Spin-polarized Correlated Insulator and Superconductor in Twisted Double Bilayer Graphene

TL;DR

This paper reports the discovery of spin-polarized superconductivity in twisted double bilayer graphene, where ferromagnetic insulating states and superconductivity coexist, tunable by electric fields and magnetic fields, revealing new quantum phases.

Contribution

It demonstrates the realization of a tunable flat band in TDBG that hosts ferromagnetic insulators and spin-polarized superconductivity, a novel combination in moiré superlattices.

Findings

Ferromagnetic insulating states at half and quarter fillings.

Superconductivity emerges upon doping the ferromagnetic insulator.

In-plane magnetic field enhances superconductivity at low fields.

Abstract

Ferromagnetism and superconductivity typically compete with each other since the internal magnetic field generated in a magnet suppresses the formation of spin-singlet Cooper pairs in conventional superconductors. Only a handful of ferromagnetic superconductors are known in heavy fermion systems, where many-body electron interactions promoted by the narrow energy bands play a key role in stabilizing these emergent states. Recently, interaction-driven superconductivity and ferromagnetism have been demonstrated as separate phenomena in different density regimes of flat bands enabled by graphene moire superlattices. Combining superconductivity and magnetism in a single ground state may lead to more exotic quantum phases. Here, employing van der Waals heterostructures of twisted double bilayer graphene (TDBG), we realize a flat electron band that is tunable by perpendicular electric fields. Similar to the magic angle twisted bilayer graphene, TDBG exhibits energy gaps at the half and quarter filled flat bands, indicating the emergence of correlated insulating states. We find that the gaps of these insulating states increase with in-plane magnetic field, suggesting a ferromagnetic order. Upon doping the ferromagnetic half-filled insulator, superconductivity emerges with a critical temperature controlled by both density and electric fields. We observe that the in-plane magnetic field enhances the superconductivity in the low field regime, suggesting spin-polarized electron pairing. Spin-polarized superconducting states discovered in TDBG provide a new route to engineering interaction-driven topological superconductivity.