Minimum Conductivity and Evidence for Phase Transitions in Ultra-clean Bilayer Graphene

TL;DR

This study demonstrates that ultra-clean bilayer graphene exhibits a disorder-sensitive insulating phase at charge neutrality, indicating a phase transition into a gapped broken symmetry state influenced by disorder, electric field, and doping.

Contribution

It provides evidence for a disorder-dependent phase transition into a gapped broken symmetry state in bilayer graphene, highlighting the role of sample purity and external tuning parameters.

Findings

Insulating state appears only in high-mobility, low-doping devices.

Transition temperature around 5K with a ~3 meV energy gap.

The broken symmetry state is suppressed by weak disorder.

Abstract

Bilayer graphene (BLG) at the charge neutrality point (CNP) is strongly susceptible to electronic interactions, and expected to undergo a phase transition into a state with spontaneous broken symmetries. By systematically investigating a large number of singly- and doubly-gated bilayer graphene (BLG) devices, we show that an insulating state appears only in devices with high mobility and low extrinsic doping. This insulating state has an associated transition temperature Tc~5K and an energy gap of ~3 meV, thus strongly suggesting a gapped broken symmetry state that is destroyed by very weak disorder. The transition to the intrinsic broken symmetry state can be tuned by disorder, out-of-plane electric field, or carrier density.