Untying the insulating and superconducting orders in magic-angle graphene

TL;DR

This study demonstrates how tuning the electronic interactions in magic-angle twisted bilayer graphene by adjusting its proximity to a metallic screening layer can selectively suppress insulating states and promote superconductivity, revealing their independent control.

Contribution

It introduces a method to independently control insulating and superconducting phases in moiré graphene by changing the screening layer separation, challenging the assumed direct relationship between these phases.

Findings

Insulating states are quenched when the screening layer is within 15nm and the twist angle deviates from 1.10°.

Superconducting domes emerge as insulating states vanish, with critical temperatures comparable to strongly insulating devices.

Half-filling insulators reappear under small magnetic fields, showing quantized Hall states with Chern number 2.

Abstract

The coexistence of superconducting and correlated insulating states in magic-angle twisted bilayer graphene prompts fascinating questions about the relationship of these orders. Independent control of the microscopic mechanisms governing these phases could help uncover their individual roles and shed light on their intricate interplay. Here we report on direct tuning of electronic interactions in this system by changing its separation from a metallic screening layer. We observe quenching of correlated insula-tors in devices with screening layer separations that are smaller than a typical Wannier orbital size of 15nm, and with the twist angles slightly deviating from the magic value 1.10 plus(minus) 0.05 degrees. Upon extinction of the insulating orders, the vacated phase space is taken over by superconducting domes that feature critical temperatures comparable to those in the devices with strong insulators. In addition, we find that insulators at half-filling can reappear in small out-of-plane magnetic fields of 0.4 T, giving rise to quantized Hall states with a Chern number of 2. Our study suggests reexamination of the often-assumed mother-child relation between the insulating and superconducting phases in moire graphene, and illustrates a new approach to directly probe microscopic mechanisms of superconductivity in strongly-correlated systems.