Correlated Insulators in Twisted Double Bilayer Graphene*

We present a combined experimental and theoretical study of twisted double bilayer graphene with twist angles near 1°. Consistent with moiré band structure calculations, we observe insulators at integer moiré band fillings one and three. Within windows of finite transverse electric fields, the first moiré conduction band is separated from neighboring bands, and we observe correlated insulators at 1/4, 1/2, and 3/4 band filling. The insulators at 1/4 and 3/4 filling emerge in a parallel magnetic field, whereas the resistance at half band filling is weakly dependent on parallel magnetic field. These findings suggest that correlated insulators are favored when a moiré flat band is spectrally isolated, with spin polarization at 1/4 and 3/4 band fillings, and valley polarization at 1/2 band filling.