Spin-polarized correlated insulator and superconductor in twisted double bilayer graphene.*

In flat electronic bands, the effect of interaction is elevated, which may lead to the emergence of correlated electronic states. Recently, the discoveries of correlated insulator and interaction-driven superconductivity in the magic-angle twisted bilayer graphene (MATBG) sparked the search of flat band materials enabled by moire superlattices. In this talk, I will present our experimental study of moire flat band in twisted double bilayer graphene (TDBG). In TDBG, the flatness of the band is tunable by perpendicular electric fields, adding another experimental knob and eliminating the requirement of a precise twist angle. Correlated insulators and superconductivity are discovered in TDBG with a much larger energy scale than in MATBG. Intriguingly, these emergent phases are found to be spin-polarized by in-plane magnetic field dependence. Since ferromagnetism and superconductivity typically compete with each other, the finding of spin-polarization in a superconductor carries an implication of exotic paring mechanism and pairing symmetry.