Correlated states in twisted double bilayer graphene

TL;DR

This paper reports the discovery of tunable correlated electronic phases in twisted double bilayer graphene, revealing insulating states and spin polarization controlled by displacement fields and magnetic effects.

Contribution

It introduces displacement-field tuning in twisted double bilayer graphene, enabling control over correlated insulating states and spin polarization, expanding the platform for quantum many-body research.

Findings

Insulating states at half-filled conduction band are tunable by displacement field.

Resistance gap increases with in-plane magnetic field, indicating spin polarization.

g factor ~2 suggests spin polarization at half filling.

Abstract

Electron-electron interactions play an important role in graphene and related systems and can induce exotic quantum states, especially in a stacked bilayer with a small twist angle. For bilayer graphene where the two layers are twisted by a "magic angle", flat band and strong many-body effects lead to correlated insulating states and superconductivity. In contrast to monolayer graphene, the band structure of untwisted bilayer graphene can be further tuned by a displacement field, providing an extra degree of freedom to control the flat band that should appear when two bilayers are stacked on top of each other. Here, we report the discovery and characterization of such displacement-field tunable electronic phases in twisted double bilayer graphene. We observe insulating states at a half-filled conduction band in an intermediate range of displacement fields. Furthermore, the resistance gap in the correlated insulator increases with respect to the in-plane magnetic fields and we find that the g factor according to spin Zeeman effect is ~2, indicating spin polarization at half filling. These results establish the twisted double bilayer graphene as an easily tunable platform for exploring quantum many-body states.