# Correlated Insulating States in Twisted Double Bilayer Graphene

## TL;DR

This study combines experiments and theory to explore correlated insulating states in twisted double bilayer graphene, revealing how electric and magnetic fields influence these states at various band fillings.

## Contribution

It provides new insights into how moiré flat bands and external fields induce correlated insulators in twisted double bilayer graphene, highlighting the roles of spin and valley polarization.

## Key findings

- Insulators at integer moiré band fillings 1 and 3, but not 2.
- Electric field enhances insulating states at fractional fillings 1/4, 1/2, 3/4.
- Magnetic field dependence indicates spin and valley polarization effects.

## Abstract

We present a combined experimental and theoretical study of twisted double bilayer graphene with twist angles between 1{\deg} and 1.35{\deg}. Consistent with moir\'e band structure calculations, we observe insulators at integer moir\'e band fillings one and three, but not two. An applied transverse electric field separates the first moir\'e conduction band from neighbouring bands, and favors the appearance of correlated insulators at 1/4, 1/2, and 3/4 band filling. Insulating states at 1/4 and 3/4 band filling emerge only in a parallel magnetic field (B_{||}), whereas the resistance at half band filling is weakly dependent on B_{||}. These findings suggest that correlated insulators are favored when a moir\'e flat band is spectrally isolated, with spin polarization at 1/4 and 3/4 band filling and valley polarization at 1/2 band filling.