Superconductors, Orbital Magnets, and Correlated States in Magic Angle Bilayer Graphene

TL;DR

This study demonstrates that high-quality, uniformly twisted bilayer graphene exhibits a variety of correlated electronic states, including insulators and superconductors, with new phenomena observed near charge neutrality and at different band fillings.

Contribution

The paper reports the fabrication of ultra-uniform twisted bilayer graphene devices revealing correlated insulators and superconductivity across all integer fillings, including new superconducting domes and topological states.

Findings

Insulating states observed at all integer fillings of flat bands.

Superconductivity with critical temperatures up to 3 K near -2 filling.

Discovery of new superconducting domes at lower temperatures near specific fillings.

Abstract

Superconductivity often occurs close to broken-symmetry parent states and is especially common in doped magnetic insulators. When twisted close to a magic relative orientation angle near 1 degree, bilayer graphene has flat moire superlattice minibands that have emerged as a rich and highly tunable source of strong correlation physics, notably the appearance of superconductivity close to interaction-induced insulating states. Here we report on the fabrication of bilayer graphene devices with exceptionally uniform twist angles. We show that the reduction in twist angle disorder reveals insulating states at all integer occupancies of the four-fold spin/valley degenerate flat conduction and valence bands, i.e. at moire band filling factors nu = 0, +(-) 1, +(-) 2, +(-) 3, and superconductivity below critical temperatures as high as 3 K close to - 2 filling. We also observe three new superconducting domes at much lower temperatures close to the nu = 0 and nu = +(-) 1 insulating states. Interestingly, at nu = +(-) 1 we find states with non-zero Chern numbers. For nu = - 1 the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field above 3.6 tesla is applied, consistent with a field driven phase transition. Our study shows that symmetry-broken states, interaction driven insulators, and superconducting domes are common across the entire moire flat bands, including near charge neutrality.